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Oawa, January 22, 2014 Forms of corrosion: recogni:on and preven:on Corrosion problems can be divided into 8 categories based on the appearance of the corrosion damage or the mechanism of aack: -‐ Uniform or general corrosion -‐ PiHng corrosion -‐ Crevice corrosion -‐ Galvanic corrosion -‐ Erosion-‐corrosion -‐ Intergranular corrosion -‐ Dealloying -‐ Environmentally assisted cracking (stress-‐corrosion cracking, corrosion fa:gue and H damage) 1 Forms of corrosion: recogni:on and preven:on Uniform corrosion: general descrip2on Uniform corrosion is characterized by a fairly uniform penetra:on (or thinning) over the en:re exposed metal surface. Mul:ple anodes and cathodes form and con:nue to move about on the surface, resul:ng in uniform corrosion. -‐ Uniform corrosion represents the greatest destruc:on of metal on a tonnage basis, but is not of too great concern from a technical standpoint. -‐ Uniform corrosion oTen results from atmospheric exposure, exposure in water, soils and chemicals. 2 Forms of corrosion: recogni:on and preven:on Uniform corrosion: metals affected All metals are affected by uniform corrosion, although passive materials, such as SS and N-‐Cr alloys, are normally subjected to localized forms of aack. Copper In metals such as steel, uniform corrosion removes a substan:al amount of metal, which either dissolves in the environment or reacts with it to produce a loosely adherent, porous coa:ng of corrosion products. 3 Steel Forms of corrosion: recogni:on and preven:on Uniform corrosion: rus2ng of Fe 4 Forms of corrosion: recogni:on and preven:on Uniform corrosion: preven2on -‐ Proper materials selec:on -‐ Use of coa:ngs or inhibitors -‐ Cathodic or anodic protec:on Uniform corrosion is oTen treated by building a corrosion allowance into the structure, es:mated from -‐ Prior condi:ons in similar service -‐ Data presented in the corrosion literature -‐ Experimental data determined from coupon exposures 5 Forms of corrosion: recogni:on and preven:on Pi9ng corrosion: general descrip2on PiHng is a highly localized form of corrosion that produces sharply defined holes, generally small. It occurs when one area of a metal becomes anodic with respect to the rest of the surface or when highly localized changes in the corrosive solu:on in contact with the metal cause accelerated localized aack. PiHng is one of the most insidious forms of corrosion, oTen hard to detect. 6 Forms of corrosion: recogni:on and preven:on Pi9ng corrosion: stages of pi9ng 1. Rapid dissolu:on within the pit and reduc:on of oxygen on adjacent surfaces (self-‐propaga:ng process) 2. Excess of posi:ve charges causing migra:on of Cl ions into the pit (high concentra:on of M+Cl-‐) 3. Hydrolysis and increase of concentra:on of H+ ions 4. H+ and Cl-‐ ions s:mulate the dissolu:on of metals 5. No reduc:on of oxygen occurs within the pit, thus pits cathodically protect the surrounding metal surface 7 Immediately aTer a pit has ini:ated, the local environment and any surface films on the pit-‐ini:a:on site are unstable, and convec:on currents are cri:cal to determine its fat