The initial cost of a structural stainless steel product is considerably higher than that of an equivalent carbon steel product, depending on the grade of stainless steel. However, savings will arise from the omission of surface coatings at regular (repeated) intervals in time. The excellent corrosion resistance of stainless steel can offer many benefits including: reduced inspection frequency and costs, reduced maintenance costs, long service life. Stainless steel has a high residual value (i.e. value at the end of a structure's life), though this is rarely a deciding factor for a structure with a long projected life (for instance over 50 years). Life cycle costing uses the standard accountancy principle of discounted cash flow to reduce all those costs to present day values. The discount rate encompasses inflation, bank interest rates, taxes and, possibly, a risk factor. This allows a realistic comparison to be made of the options available and the potential long term benefits of using stainless steel to be assessed against other material selections.
23 3.6 Selection of materials 3.6.1 Grades In the great majority of structural applications utilising stainless steel, it is the metal's corrosion resistance, which is being exploited, whether this be for reasons of aesthetics, minimal maintenance or long-term durability. Corrosion resistance must therefore be the primary factor in choosing a suitable grade. Stainless steels derive their corrosion resistance from the presence of a passive surface film which, given adequate access to oxygen or suitable oxidising agents, tends to be self-healing when damaged. This oxide film is primarily a consequence of the chromium content of the steel, though the addition of nickel and other alloying elements can substantially enhance the protection offered by the film. In particular, a small percentage of molybdenum is used to improve the pitting resistance of the steel (see Section 3.7.2). It is when the surface oxide film is damaged, possibly by electro-chemical attack or by mechanical damage, that corrosion might initiate. Careful design should ensure trouble-free performance, but designers should be aware that even stainless steels may be subject to various forms of corrosion under certain circumstances. Notwithstanding the existence of these degradation effects, it is perfectly possible to employ stainless steels extremely effectively, provided that a few elementary principles are kept in mind. It is only when these materials are used without consideration for the principles behind their corrosion properties that problems might be encountered. The selection of the correct grade of stainless steel must take into account the environment of the application, the fabrication route, surface finish and the maintenance of the structure. It might be noted that the maintenance requirement is minimal: merely washing down the stainless steel, even naturally by rain, will markedly assist in extending the service life.
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- Winter '19
- Professor Robert
- Test, Tensile strength, Stainless steel