Unformatted text preview: Industrial Galvanizers Corporation Pty Ltd
Galvanizing Design Manual
2nd edition INTRODUCTION. In this manual, the fundamental design rules that will ensure that steel sections and fabrications are hot dip galvanized to the highest quality standards and at the lowest cost are featured. There is additional supplementary information included in this 2nd Edition of the manual to provide affect information on important aspects of galvanizing that affect its durability in service. Industrial staff Galvanizers' technical staff are available for consultation on detailed aspects of design on specific projects.
GALV FUNDAMENTALS GALVANIZING FUNDAMENTALS
Hot dip galvanizing is an immersion process where steel sections and fabrications undergo the following operations: 1. Hot caustic degreasing (removal of oil, organic materials, mill primers and paint) degreasing Hot 2. Hydrochloric acid pickling (removal of rust and mill scale) Hydrochloric Hydr 3. Rinsing (removal of pickling acid residues) Rinsing 4. Prefluxing in zinc ammonium chloride solution (surface conditioning) Prefluxing Pr . Hot dip galvanizing (at 455-460 degrees C) 5 6. Chromate quenching (passivation of the zinc surface to prevent early oxidation) Chromate Chr 2 IMPORT FACT ACTORS IMPORTANT DESIGN FACTORS
Hot dip galvanizing is a self inspecting process that relies heavily on proper design to achieve a quality result. The major difference between hot dip galvanizing and all paint coatings is that hot dip hot galvanized coatings can only be applied to prepar epared perfectly prepared surfaces. react - The zinc will not react with the steel to form the galvanized coating unless the surface of the steel is perfectly clean . - The hot dip galvanized coating will not form unless the zinc can intimately contact the steel surface. - The hot dip galvanized coating will not form degrees unless the steel is heated to 455-460 degrees C. - Items cannot be galvanized unless the item will fit in the preparation tanks and galvanizing preparation bath. These requirements are the fundamentals of Design for Galvanizing and fall into the four major design categories: Venting, Draining, Dimension instability and Design. A consequence of poor venting; this RHS frame exploded due to moisture entrapment THE PRINCIPLES OF VENTING
While steel is heavier than zinc (molten zinc density 7850kg/m 6620 kg/m 3, steel density - 7850kg/m3), the difference is relatively small. To ensure that the steel fabrication can be immersed in the molten zinc, both the preparation chemicals and the zinc must be able to flow into and onto the item so that all surfaces are contacted. Any air trapped in pockets in or on the item will prevent the preparation chemicals adequately preparing the surface and prevent the zinc contacting the steel and forming the galvanized coating. If the air pockets are large enough, the steel item may have sufficient boyancy to float on the surface of the molten zinc.With hollow sections, a general rule is that if the section contains more than 15% of its internal 15% volume as air because of inadequate venting, the item will not sink in the zinc. The size of vent holes The dimensions of vent holes will be determined by: - the trapped volume of air in the fabrication - the surface area of the steel in the vented area. Each square metre of steel surface produces approximately 200 grams of ash which must be able 200 to escape through the venting holes. The location of the vent holes will be determined by: - the shape of the fabrication - the angle at which it is suspended for galvanizing. 3 BASIC VENTING RULES
1. No vent hole should be less than 8 mm in diameter mm 2. Preferred minimum vent hole size is 12 mm. 12 3. Vent holes should not be located in the centre of end plates and connections. 4. Vent holes should be located at the edges of hollow sections oriented in the same plane as the fabrication. 5. Large hollow vessels require 1250 mm2 of vent hole area for each cubic metre of enclosed volume. This is 1250 equivalent to a 40 mm diameter hole for every cubic metre of volume. 40 6. Hollow sections (pipe, RHS and SHS) require minimum vent holes equivalent to 25% of the sections' 25% diagonal cross section, made up of single or multiple vent holes. The preferred design option is to leave the ends of hollow sections completely open. 7. Hollow sections that are connected require external vent holes as close to the connection as possible. If internal vent holes are used, they should be the same size as the internal diameter of the connecting section. 8. Large seal welded overlapping surfaces will require venting if the enclosed area may contain condensation or allow process chemicals to enter the overlap during the galvanizing process. Overlaps exceeding 40,000 mm2 40,000 should be vented with a 10 mm vent hole. Overlaps under 10,000 mm2 generally do not require venting. 10 10,000 Intermediate sized overlaps should be judged on the basis of weld integrity and residual welding heat in the joint to ensure total dryness at time of sealing. Longer or larger overlapping areas require spaced holes for progressive venting. Very large overlapping areas should be avoided as an undesirable design for galvanizing. SAFETY NOTE: Water or process solutions may enter hollow sections during fabrication or during the galvanizing process. When the item is heated to 450 degrees C, water is converted to steam 450 and undergoes an expansion of approximately 1750 times its original volume. Pressures in the order of 1750 50 MPa can be produced. Adequate hole sizes in sealed hollow sections in the correct locations ensures galvanizing quality. 4 THE PRINCIPLES OF DRAINING
When an item is immersed in process solutions or molten zinc, the liquids have to flow freely into and out of the item. The viscosity of molten zinc and its density are important factors in designing adequate drainage into fabrications. When the item is immersed, good galvanizing practice requires that it is lowered into the galvanizing bath at a steady rate. If drain holes are too small, the zinc will not flow fast enough and the item may become buoyant which may result in the item floating off the dipping equipment, rolling over or otherwise being subjected to erratic immersion. When a hollow vessel or section is withdrawn from the galvanizing bath, the zinc must be able to flow freely from it. Molten zinc has a density of 6620 kg/m3 and excessive dead weight may be applied to the item or the 6620 lifting equipment is molten zinc is retained in the item. With thin walled vessels such as tanks, this can result in mechanical distortion of the item. Good detail design provides adequate natural drainage without requiring holes Closed pipes require at least two diametrically opposed vent and drain holes aligned with the lifting points. The size of drain holes
The size of drain holes will be determined by: - the enclosed volume of zinc in the fabrication - the detail design of the fabrication (size, shape) The location of the drain holes will be determined by: - the shape of the item - the angle of suspension during galvanizing Vessels with large internal volume requires large drain holes. This tank contains over 10 tonnes of molten zinc when immersed 5 BASIC DRAINING RULES BASIC
1. 2. 3. 4. 5. 6. No drain hole should be less than 10 mm in diameter 10 Preferred minimum drain hole size is 25 mm. 25 Drain holes should not be located in the centre of end plates and connections. Drain holes should be located at the edges of hollow sections oriented in the same plane as the fabrication. Large hollow vessels require 10,000 mm2 of drain hole area for each cubic metre of enclosed volume. 10,000 Hollow sections (pipe, RHS and SHS) require minimum drain holes equivalent to 25% of the sections' 25% cross sectional measurement, made up of single or multiple drain holes. The preferred design option is to leave the ends of hollow sections completely open. Hollow sections that are connected require external drain holes as close to the connection as possible. Internal venting is also recommended to ensure that pretreatment chemicals and zinc can flow freely and steam generated from liquids remaining inside can be efficiently vented. 7. Channel frames require at least four and preferably eight vent/drain holes using conventional design By using outward facing channels, no special venting or draining provisions are required NOTE: Water or process solutions enter hollow sections during fabrication or during the galvanizing
process. If the drain holes are not located at the lowest point in the fabrication; - process chemicals will be trapped internally and cause an explosion hazard when immersed in the molten zinc. - zinc will be trapped internally and will freeze in the undrained area. This may interfere with assembly, it will add to the weight of the item in service and it is a waste of zinc which adds to the cost of galvanizing. Vent and drain holes must be located as close to the high and low points of the hollow section as possible to prevent air locks, entrapment of pretreatment chamicals and zinc puddling 6 AV DISTOR ORTION THE PRINCIPLES OF AVOIDING DISTORTION THE
When steel sections or fabrications are immersed in molten zinc, their temperature is raised to that of the molten zinc which is typically 455oC. The rate at 455 which the steel will reach this temperature across its entire surface will depend on; - the thickness of the individual sections making up the item. - the total mass of the item. - the dimension of the item. At galvanizing temperatures, there is no change to steel's metallurgical microstructure and the galvanizing process is not hot enough to have any heat treating effects on the mechanical properties of the steel after galvanizing. However, at galvanizing temperatures, the yield strength of steel is lowered by approximately 50%. If the adjacent steel is not at the same temperature and any stresses exist, the weaker area will be subject to movement by the stronger area. There is a responsibility on the designer, the fabricator and the galvanizer to co-operate in ensuring that distortion risks are minimised or eliminated. The pattern of distortion in this 3 mm floorplate is clearly the result of the welding technique used. Attatching the channels with fasteners after galvanizing would significantly reduce the risk of distorting Basic design rules
1. Design to use uniform thickness sections throughout the fabrication. 2. Ensure welding and assembly techniques minimise stresses in components making up the item. 3. Ensure that venting and draining are adequate. This will allow the item to be immersed in and withdrawn from the molten zinc as quickly as possible. 4. Ensure that the structural design of the item is sufficient to support its own weight at 50% of the 50% steel's specified yield strength. Consider temporary bracing if potential to yeild exists. 5. Avoid using large areas of thin (under 8 mm) flat (under plate. 6. Guillotine cut plate is preferred to oxy cut plate. 7 GALV TO AV DISTOR ORTION GALVANIZING RULES TO AVOID DISTORTION
1. Immerse the item as quickly as possible. 2. Keep the molten metal line as short as possible on the item. 3. Withdraw the work from the galvanizing bath as quickly and as steadily as possible 4. Air cool distortion prone items. Support the item during cooling on level supports. Do not water quench. Classification of distortion prone items 1. Low risk. All hot rolled structural sections, fabrications containing angles, channels and universal hot rolled sections, tube and RHS sections and fabrications, ribbed or corrugated plate sections, grating, heavy plate (over 16 mm). (over 2. Medium risk. Light section roll formed products, long light walled conduit and tubing, fabrications containing assymetrical weldments or steel of significantly different thickness, medium plate, some double dipped items (8-16 mm). (8-16 3. High risk. Thin sheet and plate (under 8 mm depending on shape and area), floorplate, deep web plate girders, platforms containing floorplate, long channel sections with multiple weldments (cleats) on one side of web. 8 GALV BASIC RULES - DESIGN FOR GALVANIZING - Gussets and stiffeners should be cropped prior to assembly for good drainage. - End plates should have vent/drain holes in the corner(s) of the connecting angle, channel or beam. - Outward facing angles and channels in fabri cated frames reduce venting and drainage problems. - Terminating bracing short of adjacent flanges will allow free flow of zinc through the connection and eliminate pockets in service. 9 BASIC DESIGN RULES (CONT) - Consult the galvanizer; understanding how a fabrication will be oriented during galvanizing may simplify detailing. - Avoid connection sections of significantly different thickness in other than short lengths.
This angle bracing is welded toe to toe to the - Bracing angles on trusses and gantries should be connected main members, eliminating the need for any special venting and draining requirements. It also by welding on the toes and not the flats. This facilitates eliminates pockets and horizontal surfaces that drainage, eliminates overlapping surfaces and eliminates allow contaminants to accumulate in service. horizontal surfaces in service for better durability. - Terminating bracing short of adjacent flanges will allow free flow of zinc through the connection and eliminate pockets in service. - Seal weld seams and joints
. Designing for optimum bath dimensions allows large fabrications to be easily galvanized. - Do not mix materials such as mild steel and stainless steel in fabrications. - Avoid severe cold working of steel - punching holes in
thick sections, tight radius bending or rebending to eliminate risk of embrittlement. - Consider the dimensions of the design in relation to galvanizing bath size. Cost and quality benefits arise from co-ordinating item dimensions with galvanizing bath dimensions. - Avoid large overlapping areas (e.g. welded back-to-back
channels). Overlapping surfaces that are seal welded may require venting if there is a risk of moisture entering or being trapped in the overlap.
Two-dimensional items can be handled and transported more economically than threedimensional fabrications. - Consider the overall on-site costs of fabrications rather
than first cost. Two dimensional items can be galvanized and transported more economically than three dimen sional fabrications. - Consider the orientation of the component is service. Good design for galvanizing will generally improve overall anti-corrosion performance through improved drainage. - Moving parts require a design clearance of 2mm 2mm
to accommodate the galvanized coating. 10 10 Hinges and moving parts require adequate clearance to accommodate the galvanized coating on all surfaces. ACTORS GALV SERVICE FACTORS AFFECTING GALVANIZING QUALITY AND SERVICE
There are a number of factors in the nature of steelwork presented for galvanizing that impact on the galvanizers ability to provide a quality product and service. These are: rusty, previously 1. Surface condition of the steel: rusty, painted, previously galvanized. Steel that is badly corroded will be slow to pickle and removal of heavy rust on part of the surface may remain on the surface to cause galvanizing defects. Badly rusted steel should be abrasive blasted too remove heavy rust areas prior to delivery for galvanizing. Steel coated with old paint may not be able to be cleaned effectively in the caustic pre-treatment. Paint remaining on the surface will prevent the acid pickling the surface and galvanizing defects will result. Previously galvanized steel requires complete stripping prior to galvanizing. While this can be done effectively, there is a cost in additional handling and acid consumption that will add to processing cost. For this reason, incorporating pre-galvanized components into fabrications to be galvanized should be avoided. Type product; wrought iron2. Type of product; castings, old wrought ironwork, brazed, soldered or riveted assemblies. soldered Old wrought ironwork may be porous and allow moisture into voids in the castings. Abrasive blasting is the preferred method of surface preparation for this material to minimise immersion in process chemicals. Soldered items must not be galvanized. The solder will melt out at galvanizing temperatures . Steel and iron castings must be sound and free of moulding sand. Any sand that is burned onto the surface will prevent the galvanized coating from forming. Brazed components can be galvanized. The brazing will take on the appearance of a conventional galvanized coating after processing. Rivetted components containing aluminium pop rivets should not be processed. The aluminium will rapidly dissolve in both the caustic tank through sodium hydroxide attack or in the zinc bath. This jig of general work illustrates the variety of conditions that steel is received for galvanizing, including newly manufactured, rusty and previously galvanized. 11 11 ACTORS GALV SERVICE (CONT.) FACTORS AFFECTING GALVANIZING QUALITY AND SERVICE (CONT.)
3. Steel metallurgy The galvanized coating is formed by the steel reacting with the zinc at galvanizing temperature. The metallurgy of the steel combined with its surface condition will affect the appearance and the thickness of the galvanized coating. Steel composition: Most structural steels have low levels of alloying elements that are typically carbon, phosphorous, manganese, silicon and sulphur which total around 1% of the constituents. The balance is iron. 1% The effect of each of these elements is as follows: Iron. 1. Iron. Iron (Fe) is the major component in the zinc-steel reaction that forms the galvanized coating. Pure iron is not highly reactive with zinc. Very low alloy steels will produce below Australian Standard (AS 1650) thick(AS ness coatings with a smooth and shiny appearance. 2. Carbon. Carbon (C) does not have a significant effect on galvanized coating characteristics. High carbon steels with tensile strengths over 1000 MPa may be affected mechanically through hydrogen embrittlement 1000 caused by hydrogen absorption into the steel during pickling. 3. Manganese. Manganese (Mn) is a toughening element used in stee. It has little effect on coating appearance at the levels found in structural steels. Steels with high manganese content may produce galvanized coatings with an uneven brown or gold colouration and may produce thicker coatings that are less impact resistant. 4. Phosphorous. Phosphorous (P) is a very reactive residual element in structural steels that does not present a Phosphorous. problem at residual levels. It is found as an alloying element only in special electrical grades of steel which are rarely galvanized. High phosphorous content will produce thick, brittle coatings. 5. Silicon. Silicon (Si) is the most common reactive alloying element in steel. Most hot rolled structural sections do not have significant reactive silicon content. High silicon levels commonly occur in some plate products and large diameter pipe and RHS which is produced from the same steel source. Thick, grey or unevenly coloured galvanized coatings arise with this type of steel. 6. Sulphur. Sulphur is a reactive residual element in structural steel that does not pose a problem at residual Sulphur. levels. It is found as an alloying element in free machining steels. Threaded sockets and fittings manufactured from high sulphur steel are sometimes incorporated into fabrications for galvanizing. Zinc and acid attack on the steel may be severe with high sulphur steels. 12 12 ACTORS GALV SERVICE (CONT.) FACTORS AFFECTING GALVANIZING QUALITY AND SERVICE (CONT.)
Profile 4. Surface Profile The rate of reaction between steel and zinc is also affected by the surface profile. Very smooth surfaces such as those found on cold rolled sheet and tube products will have a relatively low rate of reaction and may not produce galvanized coatings that comply with the AS 1650 standard for minimum coating thickness. Also, very smooth steel surfaces on reactive steels may give rise to galvanized coatings that may flake or peel on impact. Hot rolled sections have a natural surface profile arising from the presence of mill scale during rolling. The mill scale is removed by pickling during the galvanizing process leaving a steel surface that will generally produce galvanized coatings in excess of the thickness required by AS 1650. AS Heavy gauge wire used in pool and fence panels may also produce uneven galvanized coatings , typically in a spiral pattern, caused by surface stresses induced in the steel during straightening. While badly rusted steel can be easily regalvanized, the pitted surface condition will be reflected in the appearance of the coating. Weld 5. Weld quality
Weld quality can have a direct impact on galvanizing qualty in both the design of the weld and its execution. Most welding wire is high in silicon and this will cause the weld metal to react more vigorously with the zinc than the parent metal, resulting in thicker coatings on the weld metal. If weld aesthetics are important and welds are required to be flush finished after galvanizing, low silicon welding wire or rods of similar metallurgy to the parent metal should be used. The hot dip galvanized coating reflects the surface condition and metallurgy of the steel. These highly finished heat exchanger tubes produce a very smooth, uniform hot dip galvanized coating. Heavy galvanized coatings may be created deliberately by abrasive blasting, which increased the surface area per unit. Galvanized coatings well above the Australian Standard requirement can be produced on pre-blasted steel and the rougher as-rolled surface on hot rolled merchant sections will also produce thicker galvanized coatings. Old steelwork that has been heavily rusted, and cast iron and steel which has a rough steel surface will also produce galvanized coatings significatly thicker that the minimum specified in AS 1650 . AS Some tube products will generate uneven galvanized coatings characterised by longtitudunal striations that are sharp edged and raised above the base galvanized coating. These ridges are caused by stress areas on the steel surface produced in the tube production process. Weld spatter will be galvanized as well as the steel and should be removed if appearance is a requirement. 13 13 Weld 5. Weld quality (cont) Weld design will be a function of weld location and extent. Unsealed welds will allow preparation chemicals to penetrate joints. Entrapped liquids will boil out and cause surface defects during galvanizing. Residual flux crystals left in joints will absorb atmospheric moisture and cause staining and corrosion problems after galvanizing. Fully sealed welds offer the best performance for galvanizing and in service. The surface tension of molten zinc is such that it will not readily penetrate gaps narrower than 1 mm. mm. Some submerged arc welds may contain small flux particles fused into the surface of the welds which are typically less than 1 mm in diameter. These particles mm are ceramic and unaffected by preparation chemicals and give rise to pinholes in the galvanized coating on the weld bead. Because of their small diameter, these pinholes do not affect the durability of the coating . If the presence of such pinholes is deemed undesirable for aesthetic reasons, abrasive blasting of the welds prior to galvanizing to remove the fused flux is required. MIG welding can leave a thin slag layer on welds. This must be removed prior to galvanizing as it will cause coating defects in the weld area. Weld spatter should be removed as it will remain on the surface and result in an aesthetic defect after galvanizing. New welding technology significantly redices weld spatter. Lincoln Electric's STT technology is an example and virtually eliminates spatter. Weld quality will impact on galvanized coating quality. Poor welds will result in preparation chemicals penetrating the weld and leaching out around the weld perimeter in service. Slag left on welds will prevent the preparation chemicals conditioning the surface and will also prevent the zinc reacting with the weld metal. The galvanized coating will not form on these areas and these defects are beyond the control of the galvanizer. Slag left on welds wil interere with the pretreatment process and will prevent the galvanized coating from forming. The galvanizing process will highlight defective welds like this one. Poor weld penetration has allowed pretreatment chemicals to penetrate the joint. These chemicals leach out after galvanizing a stain the coating. 14 14 6. Dimensions Dimensioning fabrications to best suit available galvanizing bath dimensions will ensure that; a . The item can be hot dip galvanized at the lowest cost and without delay. b . The item can be presented to the molten zinc in a way that optimises venting and draining to pro duce the best possible surface finish. c. The item that can be loaded efficiently into the dipping jigs and produce a better quality finish. are: Some dimensioning rules to consider are: 1. Long or deep items that require double-end dipping will add 30% or more to the galvanizing 30% cost. 2. Three dimensional items have a low mass per unit of volume cannot be processed as efficiently as two-dimensional items. 3. Items that cannot be withdrawn from the molten zinc at sufficient an angle will give rise to heavy zinc buildups and drainage spikes. Industrial Galvanizers larger structural galvanizing plants can process beams around 20 metres in length by double dipping. Three-dimensional fabrications require careful design to ensure proper venting and draining and awkward fabrications like these have low weight per unit of volume, which increases processing cost. These very large portal frames can only be dipped one at a time and also require double dipping. While being galvanized satisfactorily, the handling cost increases the galvanizing cost compared to items designed in smaller modules. 15 15 THREADED PARTS THREADED PARTS
Galvanized fasteners should be used with hot dip galvanized assemblies. When the item to be galvanized contains threaded assemblies, the pitch diameter of the female threads must be increased to permit hand assembly after the addition of a galvanized coating to the male parts. Internal threads and nuts must be tapped oversize after galvanizing to accommodate the thickness of the galvanized coating on the stud or bolt. While the internal threads that are tapped after galvanizing have no galvanized coating, the close contact with the galvanized male threads provide sufficient cathodic protection for adequate durability. Items too long or too large to be centrifuged, such as long threaded rods, may be wire brushed while hot to remove any excess zinc from the threads. Studs welded to assemblies may have to be cleaned after the assembly has cooled. This requires reheating with an acetylene torch and wire brushing to remove excess zinc. Alternatives to welded studs should be considered when possible. Masking to prevent galvanizing threads on pipe or fittings on external threads can be done using glass cloth tape. Internal threads require the application of a high temerature silicone based masking compound or otherwise to clean or tap after galvanizing. Tapped-through holes must be retapped oversize after galvanizing if they are to contain a galvanized bolt after assembly. Tapping of all holes after galvanizing is recommended to eliminate double tapping costs and the possibility of cross threading. Galvanized bolts require the nuts to be tapped oversize to provide clearance for the hot dip galvanized coating. Threaded assemblies can be cleaned by heating and wire brushing after galvanizing. recommended overtapping threads The recommended overtapping for nuts and interior threads is as follows:
Bolt or Stud Size 12 mm and smaller Over 12 mm to 25 mm Over 25 mm Minimum Overtapping of FemaleThreads.* Minimum 0.4 mm 0.53 mm 0.79 mm *Applies to both pitch and minor diameters, minimum and maximum limits.
On threads over 38 mm it is often more practical, if design strength allows, to have the male thread 38 cut 0.79 mm undersize before galvanizing , so a standard tap can be used on the nut. 0.79 On hinges all adjacent surfaces must be ground 0.8 mm on both pieces to allow for thickness increase 0.8 during galvanizing. 16 16 Manufacturers of threaded parts recognize that special procedures must be followed in their plants where certain items are to be galvanized. Following are some examples: 1. Low carbon bars are recommended since high carbon or high silicon causes a heavier, rougher galvanized coating on the threads. 2. Hot formed heading or bending requires cleaning at the manufacturing plant to remove scale before threading. Otherwise, over-pickling of threads will result during scale removal. 3. Sharp manufacturing tools are mandatory. Ragged and torn threads open up in the pickling and galvanizing processes. Worn tools also increase bolt diameters. Frequent checking is necessary on long runs. 4. Standard sized threads are cut on the bolt, while standard sized nuts are retapped oversize after galvanizing. ARTS MOVING PARTS MOVING PAR
When a galvanized assembly incorporates moving parts (such as drop-handles, shackles and shafts), a radial clearance of not less than 1.6 mm must be allowed to 1.6 ensure full freedom of movement after the addition of zinc during galvanizing. Designs should provide additional clearance for moving parts to allow for the pickup of zinc during galvanizing. It is recommended that, whenever possible, work be designed so that hinges can be bolted to frames, covers, bodies, and the like after galvanizing. Hinges should be galvanized separately and assembled after galvanizing. All hinges to be galvanized should be of the loose pin type. Before galvanizing, any adjacent edges should be ground to give at least 0.8 mm clearance. The pin 0.8 holes can be cleared of excess zinc at time of assembly. After hinges are galvanized, it is recommended that an undersized pin be used to compensate for the zinc picked up during the galvanizing process. If desired, the pin holes in the hinges may be reamed 0.8 mm 0.8 after galvanizing to permit the use of regular size pins. 17 17 MARKING FOR IDENTIFICATION
Identification markings on fabricated items should be carefully prepared before galvanizing, so they will be legible after galvanizing. Do not use paint to apply addresses, shipping instructions, and job numbers on items to be galvanized. Oil based paints and crayon marks are not removed by the pickling acids. This results in extra work and extra charges by the galvanizer to properly prepare the steel for galvanizing. For temporary identification, use heavily embossed metal tags wired to the work or a water soluble marker such as Dymark PB80 s hould be specified. For Dymark permanent identification use heavily embossed, punched or welded lettering. Where permanent identification is needed there are suitable alternatives for marking steel fabrications to be hot dip galvanized. Each will enable items to be rapidly identified after galvanizing and at the assembly site. The three marking alternatives are: 1 Fixing a deep stamped steel tag (minimum thick (minimum ness - 2.5 mm) to the fabrication by seal-welded directly to the item to be galvanized. 2. Stamping the surface of the item using die cut deep stencils or series of center punch marks. These marks should be placed in a standard position on each of the members. They should be a minimum of 12 mm 12 high and 0.80 mm deep to ensure readability after 0.80 galvanizing. This method should not be used to mark fracture critical members. REPAIR OF DAMAGED GALVANIZED SURFACES
Where significant coating damage has occurred to the galvanized coating through cutting, welding or impact, there are a number of methods of repair that are intended to provide a repaired area of equivalent performance to the undamaged coating. The three main repair materials are: 1. Zinc rich paints 2. Zinc metal spray 3. Zinc alloy repairs sticks. Zinc rich paints used for repairing damaged hot dip galvanized coatings should conform to AS2204 (ZincAS2204 rich organic priming paint). High quality two-pack epoxy zinc rich paints, applied to the damaged area to a minimum dry film coating thickness of 100 microns 100 microns are most commonly used. Single pack epoxy zincs are also suitable for galvanizing repair applications. Zinc metal spray will provide a metallic repair coating of equal performance to the hot dip galvanized coating. 18 18 Zinc metal spray is not well suited to the repair of small areas and must be applied to a Class 3 (AS 1624.4) Class abrasive blasted surface. Zinc alloy repair sticks are best suited for repairing weld areas in the downhand position while they are still hot, as the surface needs to be heated to the melting temperature of the zinc alloy for application. STORAGE STAIN WET STORAGE STAIN PREVENTION
When steel is freshly galvanized, the surface of the zinc is succeptible to reaction with rainwater and dew on prolonged exposure in poorly ventilated conditions. The stable oxidising carbonate films that form on the surface of the zinc require the presence of carbon dioxide for their formation. Galvanized steel that is stacked or nested in wet conditions will inhibit the formation of these stable films. Wet storage stain or `white rust' occurs under these conditions. Considerable coating damage may occur with prolonged exposure to wet, poorly ventilated environments. Normal galvanizing practice is to quench the galvanized steel in a very weak solution of sodium dichromate to passivate the zinc surface during the early stages of atmospheric exposure. Other post treatments such as oil, wax or polymer coatings can be applied for applications that are considered to have a high risk of wet storage staining. Stacking of galvanized work in storage to provide good ventilation and drainage will ensure that white rusting problems are eliminated. 2. Heavy staining Brush off bulky oxidation products with a wire brush or stainless steel pot scourer. Treat the affected surface with a solution of 5% sodium dichromate, 0.1% sulphuric acid, brushing with a stiff nylon bristle brush for 30 seconds before thorough rinsing of the surface. If the surface has been dulled and still has sufficient galvanized coating present; to restore a bright finish, apply a small amount of aluminium paint to a cloth pad and rub over the surface to blend the damaged area with the adjoining galvanized coating. These freshly galvanized sections show the reaction of rainwater on areas of the coating that have been buffed. The buffing removes the passivation film and exposes pure zinc to oxidation until new passivation films form. This guardrail bullnose section has been badly affected by white rust through being stored in a pack for a long period after exposure to rainy weather shortly after galvanizing. 3. Severe staining If severe staining has occurred to the extent that most of the galvanized coating has been removed, it is desirable to treat the affected area as damaged. The surface should be prepared by buffing or wire brushing and repaired with a high quality two-pack epoxy zinc rich paint, applied to the damaged area to a minimum dry film coating thickness of 100 microns. If extensive severe white storage staining has occurrs, regalvanizing of the item may be required. 19 19 Removal of 'white rust' stain.
1. Light staining The corrosion products formed when white strorage staining occurs are bulky and powdery when dry. Light white rust films that have stabilised will quickly weather off in service and have little effect on the performance of the galvanized coating. PAINTING GALVANIZED STEEL Galvanized steel surfaces are painted easily and satisfactorily using established and proven paint systems ranging from simple self-priming finishes to complex systems suitable for the most demanding service requirements. Powder coatings over galvanized steel also offer advantages in architectural applications. Paint systems for galvanized steel Different procedures and paint types are necessary from those used in painting uncoated steel. Two factors are critical to the satisfactory performance of paint coatings applied over galvanized coatings: 1. Initial adhesion 2. Long term adhesion Initial adhesion of the paint system is achieved by use of a recommended pretreatment primer or self priming finish on an uncontaminated surface, which provides a base for subsequent coatings. Long term adhesion depends on compatibility of the pretreatment primer or finish coats with the galvanized coating. Use of incompatible paint systems, or direct application of unsuitable finishes without the correct primer or pretreatment will result in premature paint failure. Preparation of galvanized steel surfaces for painting As in all painting operations, the surface to be painted should be thoroughly clean, free from grease and oil, and dry. Galvanized steel surfaces are clean and oil free as manufactured but should they become contaminated during transport*, storage or fabrication the following cleaning methods should be used: 1. Non-oily soils and dirt should be removed by brush ing or scrubbing. Detergent washing, followed by thorough clean water rinsing is satisfactory, provided the detergent is of the non-ionic type. Other types should be avoided as their residues may interfere with paint adhesion. 2. Grease and oil may also be removed by swabbing generously with a suitable hydrocabon solvent such as white spirit or mineral turpentine, using several clean swabs on each area. 3. Thoroughly dry areas which have been cleaned. 20 20 *NOTE: Diesel fumes falling onto surfaces from truck exhausts are a major cause of surface contamination. Clean galvanized work being transported to painting facilities should be covered in transit. Pretreatment systems for galvanized steel Many specialised paint systems require pretreatment of the galvanized surface to obtain adequate long term adhesion and optimum paint performance. Manufacturers’ instructions should be followed exactly from product data sheets. Mechanical preparation using abrasive blasting techniques should be done using the following parameters: blast pressure 40 psi (280 MPa) maximum abrasive grade 0.2-0.5 mm - ilmenite blasting angle - 45 degree angle to surface distance from surface 300-400 mm nozzle type - minimum 10 mm venturi This will ensure that the blasting causes minimum damage to the galvanized coating and removes no more than 10 microns of the coating Chemical etching: of galvanized surfaces prior to painting is not recommended because of inconsistency of application and entrapment of corrosive residues. Painting over galvanizing provides one of the best steel protection systems available with 50 year coating life achievable with properly specified coatings. Weathering: Leaving galvanized surfaces to weather as a pretreatment for painting is not recommended. While weathering will establish stable carbonate films on the surface, atmospheric contaminants will also accumulate. Chemical conversion pr etreatments: A range of pretr etreatments: chemical conversion treatments is available which convert the galvanized surface to an insoluble complex to provide good adhesion for subsequent paint finish. treatments: Phosphate treatments: Many specialised phosphating systems are available for use with galvanized steel. Phosphate conversion coatings give excellent, permanent adhesion to suitable paints and are ideal for the preparation of galvanized steel surfaces. Cold phosphating: Treatments such as Prep Galv and Lithoform No 2 are suited to on-site applications. Their use will generally upgrade the performance of suitable paint systems on galvanizing, improving initial adhesion when used before etch primers. The instructions of the phosphating treatment manufacturer must be followed exactly. Chromate treatments: The use of chromate treatments on galvanized steel before painting may interfere with paint adhesion if no other pre-treatment is undertaken prior to application. Specialised primers for galvanized steel Several primer types may be applied directly to galvanized steel without pretreatment other than cleaning and degreasing. These include specific formulations of the following types: 1. Modified acrylic water-borne primers 2. Certain water borne self priming finishes 3. Etch primers Modified acrylic water-borne primers: Specialised primers suitable for direct application to galvanized steel without preparation other than degreasing, include Taubmans Duraquaprime and Rust Proof Etch Primer, and Dulux Galvanized Iron Primer. Water-borne self priming finishes: Many waterborne self priming finishes of the 100% acrylic type give excellent performance on new galvanized steel and are ideal for decorative and protective coatings on new galvanized steel used for exterior walls, guttering and roofing. Gloss and matt finishes are available. Twocoat application direct to new clean galvanized steel surfaces will ensure a minimum of 10 years service without loss of adhesion. 21 21 Maintenance painting is normally carried out using the same type of water-borne finish. Water-borne finishes designed specifically for galvanized surfaces may be top coated with oil or synthetic vehicle systems. Etch primers Single pack or two-pack etch primers or wash primers are ideal for priming galvanized steel surfaces. Thin coatings, typically no more then 12-15 microns should 12-15 microns be applied, preferably by spray and strictly in accordance with the manufacturers' instructions Finish coats for galvanized steel Selection of paint systems including pretreatments, primers and finish coats should be made from manufacturer's recommendations depending on the type of galvanized steel surface and the demands of subsequent exposure and service.Advice as to suitability of particular finishes for specific products and environments is available from paint manufacturers. Powder coating galvanized steel The polyester powder coating of galvanized steel greatly enhances the durability of galvanized coatings under atmosheric exposure conditions. A wide range of colours and finishes is available. Powder coating should be performed soon after galvanizing, preferable within 12 12 hours and work be kept dry in the period prior to coating. Special preheating teatment combined with special grades of polyester powder is required to produce a high quality powder coating on hot dip galvanizing. Galvanized steel intended for powder coating is best not chromate quenched. For normal exposure conditions, the freshly galvanized steel surface is zinc phosphate or iron phosphate pretreated, and then powder coated. For heavy duty applications where maximum adhesion, salt spray resistance and durability is required, zinc phosphate pretreatment is recommended. Industrial Galvanizers installed a purpose built powder coating facility in conjunction with its Newcastle (NSW) operations to apply high performance polyester powder coatings to hot dip galvanized products like these pool fence panels. Polyester powder coating over hot dip galvanizing provides a high performance architectural finish that can stand up to UV exposure and wear. Pedestrian railings like this have been in service for nearly 10 years without maintenance. 22 22 GLOSSARY GALVANIZING GLOSSARY
Acid pickling: Hydrochloric acid is used to remove rust and mill scale from steel prior to galvanizing. Alloy layers: The hot dip galvanized coating consists of a series of alloy layers of zinc-iron alloys, coated with a layer of zinc. The alloy layers enhance the abrasion resistance and allow thicker coating to be applied. Ash: Zinc oxidation products formed from the molten zinc’s reaction to air and the flux on the steel surface float on the bath’s surface and is skimmed off as zinc ash which can then be processed to recover zinc metal and compounds. Bare spots: Defects in the galvanized coating due to inadequate cleaning prior to galvanizing. Beam work: Dipping beams are used in the galvanizing process to support items suspended from the beam from hooks or wire to minimise the non-productive steel entering the bath . Brush blasting: Light abrasive blasting to prepare galvanized surfaces for painting. Mild abrasives such as illmenite are required at low nozzle pressures (40 psi) to prevent damage to the galvanizing. (40 Centrifuge work: Small parts are galvanized by spinning or centrifuging in a bucket to throw off excess zinc. Used for fasteners, washers, chain, brackets and bolts. Cathodic protection: Zinc is more electrochemically reactive than steel and will corrode preferentially of steel is exposed through the galvanized coating through cutting or damage. Caustic degreasing: All work to be galvanized is first treated in a hot caustic bath to remove grease, oil, some types of paint and organic material so that the steel can be pickled . Acid will not remove rust and oxides if organic contamination is present on the surface. Chain work: Large, long or awkward items dipped by suspending them on chains are classified as chain work in the galvanizing process.
from Clockwise from top: 1. Alloy layers are shown in this micrograph of a galvanized coating on high silicon steel. 2. Ash forms on the surface of the galvanizing bath and is skimmed off periodically. 3. Bare spots occur when pretreatment is inadequate or air locks prevent the molten zinc contacting the stee; the cause of this particular defect. 4. Chain work is work that is too large to fit onti jigs or beams and is handles through the galvanizing process on chains as individual items. 23 23 GALV GLOSSARY (CONT.) GALVANIZING GLOSSARY (CONT.)
Chromate treatment: After steel is galvanized, it is quenched in water containing a small amount of sodium dichromate which passivates the new zinc surface and prevents early oxidation of the surface. Coating thickness: hot dip galvanized coatings are thicker on heavier sections like these lighting columns which have 200 micron coatings applied. Double dipping allows large and complex fabrications to be satisfactorily galvanized. Coating thickness: The hot dip galvanized coating thickness is determined by the metallurgy , surface condition and section thickness of the steel. Australian Standard AS1650-1989 defines minimum coating thickness standards for hot dip galvanizing. Continuous galvanizing: Sheet, wire and some tube sections are continuously galvanized. The coating formed is relatively thin and soft. Corrosion rate: The corrosion rate of galvanized coatings is predictable so the life of the coating can be accurately estimated in any known environment. Double-dipping: Long or wide fabrications or sections can be galvanized by dipping each end or side sequentially. Dross: Steel reacting with the zinc in the galvanizing bath forms dross which is a zinc-iron crystal that is heavier than zinc and has a higher melting point. Dross must be periodically removed from the bottom of the galvanizing bath to maintain operating depth. Draining: All items to be galvanized must allow zinc to flow off or out of the items during galvanizing. Duplex coating: When galvanized coatings are painted, duplex coating systems are formed which have durability higher than that of the sum of the coatings used separately. Electroplating: Thin zinc coating are applied by electroplating. Used for coating small parts and fasteners with a smooth, bright coating but are unsuitable for exterior exposure. Embrittlement: Embrittlement can be induced in some steel items during galvanizing due to excessive cold working (strain ageing) or acid pickling (hydrogen embrittlement) of high strength steel. Dross is a mushy zinc-iron alloy that forms during the galvanizing process and must be periodically removed from the galvanizing bath. Electoplating (centre) is characteristically bright and shiny compared to hot dip galvanized coatings (l and r). Electroplated coatings are very thin and are unsuitable for exterior applications. 24 24 GALV GLOSSARY (CONT.) GALVANIZING GLOSSARY (CONT.)
Etch primers: Some galvanized coating primers contain acid etching components to condition the zinc surface for painting. These primers require careful application to be successful. Fluxing: Prior to entering the galvanizing bath and after caustic degreasing and acid pickling, steel is fluxed in a hot zinc ammonium chloride solution to condition the surface for galvanizing. Galvanizing: Coating steel by immersing it in molten zinc either as a batch or continuous process. Grey coatings: Certain types of steel can produce dull grey galvanized coating. These coatings have no free zinc on their surface and tend to be thicker and less impact resistant than shiny coatings. Hardness of galvanized coatings: The zinc component of the galvanized coating is about half the hardness of 250 grade steel. The alloy layers in a hot dip coating are about twice as hard as 250 grade 250 steel. Hydrogen embrittlement: Steel over 1000 mPa 1000 yield strength may be prone to embrittlement from hydrogen entering the steel crystals from acid pickling. Inclusions: Hot dip galvanized coatings may have inclusions in the coating formed by dross crystals floating in the molten zinc. They have no effect on the coating’s durability. Jig: A steel frame which supports work during the galvanizing process. Special jigs are designed for specific products to optimise quality and productivity. Magnetic testing: Galvanized coating thickness is measured by magnetic flux instruments which measure the distance from the surface of the coating to the surface of the steel. Metallising: Zinc wire or powder is melted in an oxyacetylene flame and sprayed onto a Class 3 Blast Class steel surface. Metallising is a recommended repair method for large damaged or uncoated areas of a galvanized item. Moving parts: The hot dip galvanized coating thickness on small parts is typically around 100 100 microns. Clearances on moving parts should accommodate this thickness on both surfaces plus an allowance for irregularities in the coating. Normalising: The galvanizing temperature (455 degrees C) is not high enough to effect the strength or temper of steel but will have stress relieving effect in welded items. Passivation: Galvanizing is passivated by quenching in a weak sodium dichromate solution to prevent early oxidation of the zinc surface if exposed to dew or rainwater. Grey coatings are caused by metallurgical differences in the steel which increases the rate of reaction between zinc and steel during galvanizing. The are most commonly arise on plate products like these brackets. Jigs are used in galvanizing to support work through the process. This jig is used for galvanizing flat bar to minimise touch marks and distortion during galvanizing. Magnetic thickness guages are used universally for the measurement of galvanized coatings and provide accurate and reproducible results 25 25 GALV GLOSSARY (CONT.) GALVANIZING GLOSSARY (CONT.)
Phosphorous: Steels containing high levels of phosphorous (in conjunction with silicon) are very reactive with molten zinc and will form thick, grey coatings. Powder coating: Polyester powder coating over galvanizing is done by phosphate pretreating the galvanizing then electrostatically applying polyester powder and then fusing it in an oven. Repairs: Damage to galvanized coatings can be repaired with recommended zinc rich paint touch up systems , with zinc repair sticks or by zinc metal spraying. Runs: When molten zinc drains off items as the work emerges from the zinc bath, the zinc may freeze on the surface to form runs or drainage spikes. Silicon steel: Some steels with high levels of silicon are very reactive with molten zinc and can form very thick coating many times thicker than a standard coating. The coatings thus formed are duller and more susceptible to mechanical damage but will provide extremely long service life. Strain ageing: Severely cold-worked steel may become brittle during galvanizing as a result of the heat of the process accelerating the stress effects of the cold work. Venting: All hollow section must be correctly vented to allow air and steam to escape during immersion in the molten zinc. Items must allow zinc to flow in and air to flow out to ensure that the item will sink in the zinc and that molten zinc can contact all surfaces during immersion. Welding: Welding of galvanized steel requires correct ventilation and the use of correct welding electrodes and techniques. White rust or wet storage stain: When steel is freshly galvanized, the zinc is free of any protective oxide film. If pure water (dew or rain) is in prolonged contact with the zinc in this condition, the zinc will react with the water to for zinc hydroxide; a bulky white oxide deposit. Passivation after galvanizing along with good ventilation and drainage will prevent white storage staining. Zinc carbonate film: This oxide film provides zinc’s excellent atmospheric corrosion resistance and as this coating thickens with weathering, the galvanizing develops its characteristic soft grey appearance. Runs are caused by molten zinc freezing as it drains off the steel. These drainage spikes are removed during dressing and inspection after galvanizing. Welding of galvanized steel requires the use of appropriate ventilation and the correct electrodes. Repairs of welds is best done as soon as possible after completion of welding. White rust:Hot dip galvanized product such as guard rail should be stacked to allow rainwater to drain out as storage of nested product in wet conditions will accelerate white rust problems. 26 26 GALV EMBRITTLEMENT AND GALVANIZING
With mild steels that are produced by conventional methods, hot dip galvanizing has no significant effect on their properties or performance other than enhancing the steel's durability and providing a low level of stress relief from fabrication stresses through the item’s being heated to the galvanizing temperature. With some types of steels and with some fabrication techniques which involve severe cold working of the steel prior to galvanizing, embrittlement problems can arise that can result in the performance of the item in service being affected. There are three significant types of steel embrittlement that can be associated with the hot dip galvanizing process. These are: - liquid metal embrittlement - hydrogen embrittlement - strain age embrittlement Liquid metal embrittlement: L iquid metal embrittlement is caused by the attack of the molten metal (zinc in the case of galvanizing) on susceptible steels. The most common liquid metal embrittlement problems associated with hot dip galvanizing are with stainless steel. Attaching stainless steel fittings to mild steel items prior to galvanizing should be avoided for this reason as the molten zinc may affect the mechanical properties of the stainless steel. Hydrogen Hydrogen embrittlement. When atomic hydrogen diffuses into the structure of susceptible metal such as high strength steel, some mechanical properties can be seriously impaired. Sustained tensile stress can thus lead to failure. Dynamic and static laboratory testing can detect losses of tensile or torsional ductility. Hydrogen embrittlement is caused by the presence of hydrogen atoms within the crystal lattice structure of a metal or alloy. In the galvanizing process, hydrogen may be absorbed in the steel during the pickling process through contact with the hydrogen ions present in the hydrochloric acid. Steels with a tensile strength in the order of 1000 MPa 1000 or higher or with an equivalent surface hardness of 30 30 Rockwell C or higher are considered to be most susceptible to hydrogen embrittlement. 27 27 The recommended method of processing high strength steels for galvanizing is to eliminate the acid pickling process and use mechanical cleaning methods for preparation of the surface prior to hot dip galvanizing. Abrasive blast cleaning to Class 2 1/2 immediately Class prior to galvanizing will ensure that the steel is adequately cleaned and that a satisfactory hot dip galvanized coating will be produced. Australian Standard AS 1214-1973, Appendix C AS Appendix states the following with respect to hydrogen embrittlement of high strength bolts, which are the most commonly encountered high strength steel requiring to be galvanized. The hot dip galvanizing processes throughout Australia use hydrochloric acid at ambient temperature almost exclusively for pickling prior to galvanizing. Acid concentration is typically 10-15% HCl 10-15% The majority of steel hot dip galvanized is generally in the range of 200-450 MPa so is not subject to hydrogen 200-450 embrittlement problems. Higher strength steels such as the quenched and tempered Bisalloy steels are appearing in the structural area and special consideration must be given to these types of steels if they are required to be hot dip galvanized. hydrogen Avoiding hydrogen embrittlement The requirement to galvanize high strength steels is a very small one in comparison to the volume of lower strength product that is routinely processed through galvanizing plants. High strength steels can be galvanized satisfactorily provided the necessary precautions are taken in the galvanizing process. Where additional sqfeguard is sought (eg. For bolts of Grade 10.9 or higher cleaned by acid pickling), Grade Fasteners should be baked at a temperature of 200 200 degrees C + 10 degrees C, for a time found on the basis qf experience to be adequate (for guidance, a time of 30 minutes before galvanizing, or 4 hours 30 hours immediately after galvanizing, might prove satisfatory). GALV (CONT.) EMBRITTLEMENT AND GALVANIZING (CONT.)
Strain Age Embrittlement Strain ageing is associated with strain that results from plastic deformation which is more commonly known as cold working. Steel is an alloy of iron and carbon and contains other alloying elements which provide it with specific performance characteristics. Severe cold working of steel causes the migration of carbon atoms in the iron crystals and the segregation of these atoms at dislocations in the steel causes a reduction in ductility of the steel. The ageing process is a function of temperature and time and occurs very slowly at ambient temperature but very rapidly at the 450-460oC temperatures of the 450-460 galvanizing process. Severe cold working of steel can be caused by hole punching in thicker sections, tight radius bending or rebending. It should be noted that it is not the hot dip galvanizing that is the cause of accelerating the strain ageing of the steel, but the heat of the process, so strain age embrittlement can be induced in any severely cold worked steel by heating and the tendency to embrittlement by strain ageing will always be present and its manifestation will simply be a matter of time. Avoiding strain age embrittlement To avoid the risk of strain ege embrittlement, the following design criteria should be followed: use bend radii at least 3 x section thickness; · hot bend if bend radii under 3 x requir equired; section thickness is required; anneal at 650-815°C prior to galvanizing; remove severely ream punched holes to remove severely cold worked material from surface prior to from galvanizing. - - INFORMATION GALV COATINGS GENERAL INFORMATION ON GALVANIZED COATINGS
provided products This information has been provided to assist in the selection of zinc coated products and materials for products. use in conjunction with hot dip galvanized products. COATING GALV COATINGS. COATING CHARACTERISTICS OF GALVANIZED COATINGS. Most galvanized coatings involving the immersion of steel in molten zinc are made up of a series of zinc-iron alloy layers that are formed through the reaction of the zinc with the steel at 450 degrees centigrade. 450 degrees The alloy layers are generally covered by a layer of free zinc. The alloy layers have a characteristic pale grey appearance while the free zinc is shiny when first galvanized. Some galvanized coatings will have a spangled appearance due to trace elements in the zinc coating. The combination of alloy layers and free zinc and their proportion in the coating are important in determining the flexibility and durability of the coating. The free zinc is relatively soft (about 70 Diamond 70 Pyramid Hardness). The alloy layers range in hardness up to 240 DPN in the thicker Delta layers 240 which contain 7 to 12% iron. This is considerably iron to harder than 250 Grade steel, which is typically 160 250 160 DPN. The continuously galvanized coatings have very thin alloy layers and are thus capable of being heavily deformed without delamination of the coating but the coating is not nearly as abrasion resistant as hot dip galvanizing. The general galvanized coatings, with their thicker alloy layers, are not as flexible but have a much higher abrasion resistance (5X t hat of continuously galvanized coatings) and are also typically 4-5 times 4-5 as thick . 28 28 GALV COATING GALVANIZED COATING METHODS
There are a number of methods of applying zinc coatings and each will determine the coating ‘s thickness and its ultimate durabilty in a specific environments The most commonly encountered types of zinc coatings are: electroplating 1. Zinc electroplating 2. Mechanical plating 3. Sherardising 4. Continuously galvanized sheet wire 5. Continuously galvanized wire 6. Galvanized pipe and tube 7. General or hot dip galvanizing 8. Zinc metal spraying A brief description of each application process and the characteristics of the coating formed is as follows: ELECTROPLATING 1. ZINC ELECTROPLATING involves immersion of the items to be coated in a solution containing zinc ions and applying an electric current to uniformly coat the surface. Coating characteristics: Zinc electroplated coatings are bright coatings that are thin - typically around 5-10 5-10 microns microns and are not suitable for exterior use where durability is required. Heavy chromate coatings are frequently applied to zinc platings to improve their durability, especially for fastener applications. The coating is all pure zinc and lacks the hard alloy layers of the hot dipped coatings. PLATING 2. MECHANICAL PLATING involves tumbling the items to be coated in zinc powder with glass beads and special reducing agents to bond the zinc particles to the steel surface. Coating characteristics: The mechanical plating processis used to apply zinc or alloy coatings to fasteners and small parts. The zinc particles are in lamellar form and durability equivalent to hot dip coatings can be achieved in a uniform coating that is particularly suited to threaded fasteners and hardened TEK type screws that are unsuitable for hot dip galvanizing. These coatings are typically 15 - 20 microns thick. microns 15 3. SHERARDIZING involves heating the articles to be coated in zinc powder to approximately 400oC at 400 which temperature diffusion bonding of the zinc with the steel occurs. Coating characteristics: Sherardised coatings are diffusion coatings whose thickness can be varied considerably up to over 300 microns and whose 300 microns constituents can be modified by adding other metal or inorganic compounds to the zinc powder. The sherardized coatings are almost entirely made up of ironzinc alloy phases. The long cycle times for the process make application costly. It is now rarely used. GALV 4. CONTINUOUS STRIP GALVANIZING involves passing coil steel through a bath of molten zinc in a controlled reducing atmosphere at high speed (180 m/ (180 min). Coating characteristics: The zinc coating thickness is closely controlled in the manufacturing process by air wiping of the sheet as it emerges from the galvanizing bath. The coating thickness varies from an average of microns ZI00 7 microns on ZI00 sheet to 42 microns on the heaviest 42 microns Z600 sheet. The coating has a very thin zinc-iron alloy layer which gives it its flexibility for pressing and forming. 5. CONTINUOUSLY GALVANIZED WIRE is CONTINUOUSLY GALV produced by passing cleaned steel wire through a lead/ zinc bath at high speed ( 180 m/min). 180 Coating characteristics: S imilar to those of continuously galvanized sheet. Coating thickness varies depending on the diameter and coating grade of the wire from 3 microns in the thinnest standard gauge to 43 microns micr 43 microns microns in the thickest (8 mm) heavy galvanized grade. (8 29 29 GALV COATING (CONT.) GALVANIZED COATING METHODS (CONT.) GALV 6. GALVANIZED PIPE AND TUBE is produced by two methods; one is semi-continuous where stock lengths of tube are cleaned and passed continuously through a bath of molten zinc at 450 degrees centigrade. 450 degree The other method is continuous where strip is formed into tube from coil and the tube then passed through a bath of molten zinc at 450 degrees centigrade. This 450 degrees second method coats the exterior of the tube only. Coating characteristics: The semi-continuously applied coating is a conventional galvanized coating having a coating thickness typically around 65 microns 65 microns which consists largely of zinc-iron alloy layers as the free zinc layer is largely removed through air wiping during the process. The continuous tube galvanizing process produces a bright coating which is almost all free zinc with very thin aloy layers, giving the product good forming properties. Coating thickness is typically 12-25 microns on the exterior of the tube only. microns 7. GENERAL OR HOT DIP GALVANIZING involves preparing work by acid pickling in batches or on jigs and then dipping the work into a bath of molten zinc. Coating characteristics: The typical general galvanized coating ranges from 65 microns to over 300 microns 65 microns 300 microns depending on the steel analysis, thickness of material and immersion time in the galvanizing bath. Typical coating thickness on most general galvanized products is 80-100 microns. 80-100 microns. METAL SPRAYING 8. ZINC METAL SPRAYING requires the steel surface to be cleaned to a Class 3 level and then zinc Class wire or zinc powder is sprayed onto the surface with an oxy-acetylene or plasma flame gun. Coating characteristics: Zinc metal spraying produces a relatively porous coating that is able to be applied in any desired thickness but is typically 75-200 microns. 75-200 microns It is used where the size or shape of the article make it unsuitable for hot dip galvanizing. The availability of larger galvanizing baths has resulted in it being little used for other than repairs to galvanized coatings.
General or hot dip galvanizing is largely applied after fabrication and the immersion process ensures that all internal and external surfaces are heavily coated with zinc. Most larger diameter pipe products are batch hot dip galvanized. Smalller sections are galvanized in semi-continuous or continuous processes. Many building products are manufactured from contunuously galvanized sheet steel. While this facilitates manufacture, light zinc coatings (typically 20 microns) and exposed steel at cut edges reduce durability to less than 1/3 that of hot dip galvanized components. 30 30 BOLTING GALV FRICTION-GRIP BOLTING GALVANIZED STEEL Bolting of steel structures is well suited for use with hot dip galvanized coatings. Apart from the need to ensure that the bolts used are hot dip galvanized to the same standard as the structural steel, designers are provided with various Australian standards associated with bolting, including AS1214 - 'Hot dip galvanized AS1214 coatings on threaded fasteners', AS 1252 - `High AS strength steel bolts with associated nuts and washers.', AS 1559 - `Fasteners -bolts, nuts and washers for tower construction.', AS 1111 - 'ISO metric commercial bolts AS 11 and screws' and AS 1112 - `ISO metric hexagon nuts...' AS 11 and AS4100 - `Steel structures'. AS4100 Australian Standard 4100 - `Steel structures' assumes a slip factor of 0.35 for clean as-rolled steel surfaces 0.35 with tightly adherent mill scale. In friction-type bolted joints, all loads in the plane of the joint are transferred by friction between the mating surfaces. The load can be transmitted by a friction type joint is dependent on the clamping force applied by the bolts and the slip factor of the mating surfaces. AS 4100 p ermits the use of applied finishes to connecting surfaces of friction type joints provided that the slip factor used in design calculations is based on test evidence. Industrial Galvanizers has undertaken extensive test work in association with the University of Newcastle's TUNRA research organisation to establish parameters for post treatment of hot dip galvanized structural steelwork that will ensure that the coefficient of friction will meet or exceed 0.35. 0.35. Where friction grip bolting of structures is required, consultation with Industrial Galvanizers will allow the required quality assurance procedures to be implemented to ensure that design requirements are satisfied. The test work undertaken by TUNRA established that two techniques will produce coefficients of friction on hot dip galvanized surfaces exceeding the 0.35 0 .35 minimum. These are: 1. Buffing the connecting surfaces during final inspection to roughen the surfaces. 2. Galvanizing at higher temperature or deferring quenching to produce fully alloyed `grey' coatings. Steel metallurgy and section thickness will impact on the effectiveness of this technique. A large number of structures such as this have been been friction grip bolted satisfactorily after galvanizing. Controlled galvanizing techniques combined with post galvanizing treatment of the connecting surfaces by the galvanizer ensures that coefficients of friction in excess of 0.35 are obtained at the galvanized connections. 31 31 RATES GALV COATINGS CORROSION RATES OF GALVANIZED COATINGS
In most environments, the corrosion rate of of galvanized (and zinc) coatings is proportional to its thickness. When selecting components to be used in conjunction with hot dip galvanized coatings, it is essential to ensure that their coating thickness is equivalent to that of hot dip galvanizing. All pre-galvanized products have much thinner zinc coatings than their hot dip galvanized equivalents. The following basic rules should be applied: 1. Do NOT use zinc plated fasteners for heavy duty applications. 2 Specify hot dip galvanized purlins and girts in hot dip galvanized structures. Z300 galvanized coatings on roll-formed structural sections are typically only 20 microns in thickness and have exposed steel at cut edges. Coating life is typically 20% t hat of equivalent section hot dip galvanized products. 3. Do NOT use continuously coated tube and sections for heavy duty applications. These coatings are typically 12 - 25 microns in thickness and the hollow sections are not internally coated. In non-atmospheric exposures (contact with chemicals, buried in soil, immersion in potable and ground waters), the corrosion rate of zinc will depend on the following factors: 1. pH of the liquid (galvanized/zinc coatings perform best in the pH range from 6-10) 6-10 2. Time of wetness ( prolonged periods of wetness with rainwater or condensation will increase corrosion rate by 2X or more). 2X 3. Contact with electrochemically incompatible metals (copper, brass, stainless steel) will result in very high zinc corrosion rates. 4. Splash zone exposure in both marine and fresh water prevents the formation of stabilizing oxide films and will result in higher zinc corrosion rates. NOTE: For any specific environment, the suitability of hot dip galvanizing can be quickly determined through a site audit of environmental and chemical exposure conditions, and evaluation of process water quality. GALV COATING RATES ATMOSPHERIC GALVANIZED COATING CORROSION RATES IN ATMOSPHERIC ENVIRONMENTS Atmospheric Environment Corrosion rate range - microns per year
1-2 microns 2-3 microns 5-8 microns 3-5 microns 5-8 microns 5-8 microns 8-15 microns Rural - dry, 600 mm annual rainfall or lower Residential - non-industrial, over 600 mm annual rainfall Tropical - over 1000 mm annual rainfall, high average humidity Industrial - External Industrial - Internal (fumes, vapour, steam) Coastal - 100 to 500 metres from ocean surf Marine - oceanfront / surfside - 32 32 SURFACE TABLES SURFACE AREA TABLES While hot dip galvanizing is usually priced in dollars per tonne, it is desirable to also convert this to dollars per square metre to allow comparison with alternative coatings. In addition, the conversion to square metres allows accurate estimation of weight increase through the addition of the hot dip galvanized coating. The surface area per tonne can also be calculated using the following formula: area Surface area per tonne = ________ 255________ ________ Section thickness in mm Mass per square metre of steel can be calculated using the following formula: square metre Mass per square metre (kg/m2) = section thickness in mm x 7.85. Section thickness mm 1 mm mm 2 mm 3 mm 4 mm 5 mm 6 mm 8 mm 10 mm 12 mm 15 mm 20 mm 25 mm area/tonne Surface area/tonne m2/tonne Mass/m 2 kg/m2 Min coating thickness microns microns -per AS 1650* increase Mass increase %** 255 127 64 85 51 42 32 25 21 17 13 10 7.85 15.70 23.55 31.40 39.25 47.10 62.80 78.50 94.20 117.75 117.75 157.00 196.25 microns 45 microns (320 g/m2) microns 55 microns (390 g/m2) 55 microns (390 g/m2) microns microns 70 microns (500 g/m2) microns 70 microns (500 g/m2) microns 70 microns (500 g/m2) microns 85 microns (600 g/m2) microns 85 microns (600 g/m2) microns 85 microns (600 g/m2) microns 85 microns (600 g/m2) microns 85 microns (600 g/m2) microns 85 microns (600 g/m2) 4.10% 2.85% 1.65%. 1.59% 1.25%. 1.05% 0.95% 0.75%. 0.65% 0.50% 0.40% 0.30% * Hot rolled steel sections and heavier steel sections will generate galvanized coatings considerably thicker than required by the AS 1650 Standard. To convert coating thickness in microns to equivalent AS coating mass in grams per square metre (g/m2), use the following formula: Coating mass (g/m2) Coating (microns) Coating thickness (microns) = = microns Coating thickness in microns x 7.05 Coating mass (g/m 2) x 0.14 ** Actual zinc pickup after galvanizing will depend on average coating thickness and section design. Poor drainage, zinc entrapment and large horizontal surfaces will result in higher zinc pickup. NOTE: Actual zinc usage in hot dip galvanizing is significantly higher than physical zinc pickup. Zinc usage in hot dip galvanizing is typically 5-7% of the mass of steel dipped because of zinc consumed 5-7% on jigs and in generating zinc ash and zinc dross in the galvanizing process. 33 33 GALV INSPECTION OF GALVANIZED PRODUCTS INSPECTION
Australian Standard AS1650, Section 1.6 - Appearance (Page 5) defines the requirement for hot dip galvanized AS1650, coatings as follows:
The galvanized coating shall be continuous, as smooth and evenly distributed as possible, and free from defects that are detrimental to the stated use of the coated article. Methods recommended for the renovation of damaged galvanized coatings or uncoated areas are given in Appendix F (of AS 1650). Notes: 1. Defects cannot be completely quantified. When the presence, size or frequency of any defects in the coating are considered manufacturer.This to be of concern, appropriate arrangements should be made between the purchaser and the manufacturer.This may be achieved by the provision of acceptable samples or methods of test. Where defects are present and the product is submitted for acceptance, the manufacturer should be able to demonstrate fitness for purpose. thicker, 2. A thicker, less smooth coating is obtained on job galvanized articles compared with continuously galvanized sheet or wire. 3. (Not applicable to general hot dip galvanized products) 4. The finish of the coated object may be partly or wholly grey in colour for steels of certain composition or articles that are slowly cooled after galvanizing. Provided that such a coating has adequate adhesion, the grey finish is not detrimental, although premature staining may occur in service. 5. Advice on the transport and storage of galvanized articles is given in Appendix G (of AS 1650) GALV COATING GALVANIZED COATING APPEARANCE AND DEFECTS
There are a number of common types of defects arising from the hot dip galvanizing process. An explanation of the causes of defects and variations in appearance follows: areas. 1. Ungalvanized weld areas. Coating misses on weld areas are caused by the presence of welding slag on the welds. All welding slag must be removed by the fabricator prior to despatch to the galvanizer. These areas require repair. Unsealed welds where preparation chemicals penetrate the overlap cause blowouts which cause surface contamination and subsequent coating defects. Welding slag left on or in welds will not beremoved by the galvanizing process and will result uncoated areas on welds. 2. Dark staining adjacent to welds. Preparation chemicals entering unsealed overlaps or through poor quality welds boil out of the connection during galvanizing and cause surface contamination and coating misses during galvanizing. Also, anhydrous fluxing salts left in the connection will absorb atmospheric moisture and leach out onto the adjacent galvanized surface. Leaching of these salts will eventually reach equilibrium. Affected area should be washed clean to remove slightly corrosive leachate. 34 34 Ash is a by-product of the galvanizing process that floats on the surface of the bath. Ash should be brushed off during inspection and dressing. GALV (CONT.) INSPECTION OF GALVANIZED PRODUCTS (CONT.) INSPECTION
3. Dull gray or mottled coatings. Reactive steels will generate thicker galvanized coatings that are duller than standard coatings. These coatings have longer life because of their greater thickness and their appearance is a function of steel metallurgy and generally beyond the control of the galvanizer. Dross 4. Dross pimples/inclusions. Dross is formed in the galvanizing process in the form or zinc-iron crystals (approx 95% zinc - 5% iron) with a higher melting iron 95% point that the metal in the zinc bath. Dross trapped in the galvanized coating may give the coating a rough or gritty appearance. The presence of dross inclusions in the coatings is not detrimetal to the coating's performance as the corrosion resistance of zinc dross is identical to that of the galvanized coating 5. White storage staining. After galvanizing, items stored or stacked in wet, poorly ventilated conditions will react with atmospheric moisture to form bulky white zinc hydroxide deposits on the surface of the galvanized coating. 6. Ash staining. Zinc ash is formed in the galvanizing process as the work is immersed in the zinc. The ash formed is skimmed off the surface of the molten zinc prior to withdrawing the work from the galvanizing bath. Sometimes, ash is trapped inside inaccessible areas and sticks to the outside of the coating as the work exits the bath. Ash may leave a dull surface appearance or a light brown stain after removal. It does not affect the performance of the galvanized coating. irregularities. 7. Striations and general surface irregularities. Ridges and lines thicker than the adjacent galvanized coating are caused by different rates of reaction of the zinc with the steel surface due to stress areas on the steel surface or the presence of weld areas or weld metal with modified metallurgy to the parent metal. This phenomenon is most commonly encounted on pipe and tube products.Coating performance is unaffected 8. Runs, drainage spikes and puddling. These defects are unavoidable in the hot dip galvanizing of general items and are acceptable as long as they do not interfere with the assembly of the function of the item or present a safety hazard in handling or service. Bare 9. Bare patches. Uncoated areas on the surface of galvanized work are due to poor surface preparation; inadequate pretreatment in degreasing, pickling and prefluxing. These areas must be repaired using a recommended repair method or the item regalvanized if the defect is of sufficient size.
Dross pimples are formed when dross crystals present in the molten zinc are caught up in the coating, giving the surface a grity appearance.(lower section). The textures and striated galvanized surface on the upper steel component is caused by the surface metallurgy of the steel producing differing micro reactive zones on the surface. Chain marks are unavoidable when galvanizing large items. These defects are normally buffed off during dressing and inspection Drainage spikes are formed when moltez zinc freezed while draining from horozontal surfaces. Most 3 dimensional sections will always generate drainage spikes in one plane. These are removed by buffing during dressing and inspaction. 35 35 Galvanizing appearance and defects (cont)
10. Rust staining. U ncoated steel in contact with galvanized coatings will accelerate corrosion of the coating and stain the coating brown in the area of contact. This can be removed by wire brushing. 11. Delamination. Very heavy galvanized coatings (over 250 microns thick) may be brittle 250 and delaminate from the surface under impact and require more careful handling in transport and eraction. Thin, cold rolled items with very smooth surface finish and manufactured from reactive steel may also give rise to coating delamination. 12. Black spots. S cattered black spotting is due to residual galvanizing flux crystalising on the surface of the work and is generally due to poor rinsing after galvanizing or flux contaminated rinse water. This defect is usually encountered from galvanizing baths using the `wet' galvanizing process where the flux is on top of the molten zinc. Excess aluminium in the galvanizing bath can also give rise to this defect. 13. Spangled coatings. S ome hot dip galvanized coatings exhibit a high level of `spangling' caused by zinc crystal patterns on the surface. This phenomenon arises with galvanizing alloys produced in particular smelting processes and these alloys are commonly used for hot dip galvanizing. There is no difference in coating performance. Hot dip galvanized coatings will be stained by contact with rusty steel or timber in outdoor exposure conditions. Contact with scrap steel will accelerate local consumption of the galvanized coating. Timber staining can be avoided by using seasoned timber of the right variety. Industrial Galvanizers Services
Industrial Galvanizers Corporation has a network of hot dip galvanizing operations throughout Australia and in South East Asia and the USA. The group processes over 100,000 tonnes of steel annually and offers a range of specialised services associated with the efficient provision of hot dip galvanized coatings to steel. Some of the groups hot dip galvanizing capabilities include: 24 hour turnaround on negotiated contract galvanizing. Heavy lift galvanizing (exceeding 20 tonnes) in specific plants. Large bath sizes in most major centres to accommodate long or wide fabrications Project management capabilities to co-ordinate steel to construction site. Design assistance in detailing project steelwork to facilitate galvanizing and reduce coating time and cost. 36 36 - - - - This Hunter Valley (NSW, Australia) coal mine construction illustrates Industrial Galvanizers ability to supply widely based projects. Steel for this project was galvanized in Industrial Galvanizers facilities in Brisbane, Newcastle, Sydney and Melbourne. GALV CAPABILITIES INDUSTRIAL GALVANIZERS CAPABILITIES
The galvanizing plants within the Industrial Galvanizers Group galvanizers a wide variety of fabricated steelwork. These examples illustrate the capabilities within the group. With the world's largest galvanizing bath in Newcsatle, NSW, very large items can be galvanized within the IG Group. Galvanizing Industries, Industrial Galvanizers Melbourne Vic plant can galvanize sections over 22 metres in length . Stadium lighting tower segments, manufactured by Riverton Engineering in Perth , WA were able to be galvanized in NSW and Qld plant to supply a Qld project. All of Industrial Galvanizers plants have a large customer base providing a wide variety of products. Accurate work tracking systems are in place to account for this diversity of products. Above: This marine lead light tower was hot dip galvanized and painted with Industrial Galvanizers administering the application of the total coating system. Left: Industrial Galvanizers pioneered the hot dip galvanizing of railway bridge beams, each weighing over 9 tonnes 37 37 GALV GALVANIC CORROSION
While hot dip galvanized coatings provide envelope protection for steel, the zinc in the coating will also protect the steel cathodically if the coating is damaged and the bare steel exposed. The rate of consumption of the galvanized coating will depend on the size of the cathode (the bare steel) and the size of the anode) the galvanized coating). It is essential that exposure of galvanized coatings to cathodic metals and materials be minimised to prevent accelerated corrosion of the galvanized coating. The following table lists metals in order of their relative reactivity with each other and when considering coatings and materials for maximim durabality, contact between dissimilar metals should be avoided where electolyte forming moisture is likely to be present. INFORMATION GENERAL INFORMATION ABOUT ZINC AND STEEL Zinc
Atomic weight rolled Density - rolled - cast - liquid Melting point Boiling point Thermal conductivity 65.37 7192 kg/m3 6804 kg/m3 6620 kg/m3 419.5oC 907oC 113 W/m.K Steel
Atomic weight 55.85 Density 7850 kg/m3 Melting point 1540 oC Boiling point 2900 oC Thermal conductivity 57 W/m.K 38 38 PERTINENT GALVANIZING STANDARDS
Standards AS 1650 AS 1214 AS 1250 AS 1394 AS 1074 AS 1397 AS 2312 AS 2331.1.3 Australia Standards Hot Dip Galvanized Coatings on Ferrous Articles Hot Dip Galvanized Coatings on Threaded Fasteners SAA Steel Structures Code Round Steel Wires for Ropes Steel Tubes and Tubulars Steel Sheet and Strip - Hot-dipped Zinc Coated and Alimimnium/zinc Coated Steel Guide to the Protection of Iron and Steel in Atmospheric Environments Methods of Test for Metallic and Related Coatings - Magnetic Method New Zealand Standards BS 729* BS 443* Ministry of Works Ministry of Works * British Hot Dip Galvanized Coatings on Iron and Steel Articles Galvanized Coatings on Wire MOW CD306 Specification for Hot Dip Galvanizing on Structural Steel Work. MOW CD307 Specification for Protection of Structural Steel Work Standards Society for Testing and Material American A90 A123 A143 A153 A325 A384 A385 A394 A780 B6 E376 Test Methods for Weight of Coatings on Zinc Coated (Galvanized) Iron or Steel Articles Zinc (Hot Dip Galvanized) Coatings on Iron and Steel Products Recommended Practice for Safeguarding Against Embrittlement of Hot Dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement Zinc Coating (Hot Dip) on Iron and Steel Hardware High-Strength Bolts for Structural Steel Joints, including Suitable Nuts and Plain Hardened Washers Recommended Practice for Safeguarding Against Warpage and Distortion During Hot Dip Galvanizing of Steel Assemblies Recommended Practice for Providing High Quality Zinc Coatings (Hot Dip) on Assembled Products Galvanized Steel Transmission Tower Bolts and Nuts Practice for Repair of Damaged Hot-Dip Galvanized Coatings Zinc Metal (Slab Zinc) Recommended Practice for Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Test Methods Standards Association Canadian G164-M Hot Dip Galvanizing of Irregularly Shaped Articles. 39 39 ...
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This note was uploaded on 03/17/2010 for the course ME ME78212 taught by Professor Prof.sulis during the Spring '10 term at Institut Teknologi Bandung.
- Spring '10