Corrosion

Corrosion - Corrosion CE 482 Definitions (Webster’s Ninth...

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Corrosion CE 482
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Definitions (Webster’s Ninth New Collegiate) Corrosion: The action, process, or effect of corroding A product of corroding Corrode: To eat away by degrees as if by gnawing; esp: to wear away gradually usually by chemical action To weaken or destroy gradually
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Example - Rust
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Corrosion - Basics Corrosion is the primary means by which metals deteriorate Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals Metals will also corrode when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing gases Corrosion specifically refers to any process involving the deterioration or degradation of metal components The best known case is that of the rusting of steel Corrosion processes are usually electrochemical in nature, having the essential features of a battery
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Corrosion - Basics When metal atoms are exposed to an environment containing water molecules they can give up electrons, becoming themselves positively charged ions, provided an electrical circuit can be completed This effect can be concentrated locally to form a pit or, sometimes a crack, or it can extend across a wide area to produce general wastage Localized corrosion that leads to pitting may provide sites for fatigue initiation and, additionally, corrosive agents like seawater may lead to greatly enhanced growth of the fatigue crack Pitting corrosion also occurs much faster in areas where microstructural changes have occurred due to welding operations.
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Example – Hot Water Pipes
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Example – Hot Water Pipes
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Example – Leaking UST
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Example – Ductile Iron Pipe
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Example – Condo Steel Frame @ Sill Plate
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Corrosion - Basics The corrosion process (anodic reaction) of the metal dissolving as ions generates some electrons that are consumed by a secondary process (cathodic reaction) These two processes have to balance their charges
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Corrosion - Basics Rusting of iron consists of the formation of hydrated oxide, Fe(OH)3, FeO(OH), or even Fe2O3.H2O. It is an electrochemical process which requires the presence of water, oxygen and an electrolyte In the absence of any one of these rusting does not occur to any significant extent In air, a relative humidity of over 50% provides the necessary amount of water and at 80% or above corrosion of bare steel is worse
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Corrosion - Basics When a droplet of water containing a little dissolved oxygen falls on an steel pipe, the solid iron or Fe(s) under the droplet oxidizes: Fe(s) --> Fe2+(aq) + 2e- The electrons are quickly consumed by hydrogen ions from water and dissolved oxygen at the edge of the droplet to produce water: 4e- + 4H+(aq) + O2(aq) --> 2H2O(l) More acidic water increases corrosion. If the pH is very low the hydrogen ions will consume the electrons anyway, making hydrogen gas instead of water: 2H+(aq) + 2e- --> H2(g)
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Corrosion - Corrosion CE 482 Definitions (Webster’s Ninth...

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