AWS PRGC:1999 pdf download.THE PRACTICAL REFERENCE GUIDE for CORROSION OF WELDS - CAUSES AND CURES
Pitting Corrosion
Pitting corrosion differs from general corrosion pri-marily in that pitting corrosion damage is quite lo-calized rather than covering the entire surface. It is defined as "Extremely localized corrosion, resulting eventually in holes in the metal." The total area based on the pits' diameters is usually quite small com-pared to the total exposed surface area of the com-ponent. Pitting damage can occur at a very few locations on a large part, causing complete perfora-tion of the part, and render it useless even though the majority of the component has suffered little or no corrosion damage. A good example of this is the hot water heater found in our homes. The majority of the heater interior may be in very good condition,but the heater may leak due to a single pit that ex-tends entirely through the pressure containing shell.
Avoiding Pitting Damage
To avoid pitting damage, several methods can be used. Protective coatings are very effective pro-vided the coatings are completely continuous over the entire surface and contain no breaks. Quite of-ten, when organic coatings are used to avoid pit-ting, and the coating contains scratches or coating breaks, pitting not only occurs, but also often at a much higher rate than if there were no coating at all. This increased pitting rate is due to the rela-tively small area of metal actually exposed to the pitting agent's inherent driving force; most of the surface is protected but a very small area is subject to more severe pitting.
Alloy upgrades are another common solution; if carbon steel is failing by pitting in water services, a switch to a copper-based alloy may solve the prob-lem. If 304 stainless steel (304 sst) pits in an environ-ment, an upgrade to the 316 sst grade containing 2- 3% molybdenum (Mo) often is the answer since its addition significantly improves pitting resis-tance in the 300 series. Electrochemical protection methods are also effective in reducing or eliminat-ing pitting damage, and corrosion inhibitors may
reduce the pitting attack.
Intergranular Corrosion
This form of corrosion is defined as "Localized attack at, and adjacent to grain boundaries, with relatively ltle corrosion of the grains." Metal structures are com-posed of very small grains, or crystals, with a par-ticular structure, and arranged in random patterns separated by grain boundaries. The grains in metals have one of several geometric crystal structures such as body centered cubic, body centered tetrago-nal, face centered cubic, or hexagonal close packed.The individual grains grow during solidification of the metal, and where two grains come together at different orientations, the line or irregular plane of contact is called the grain boundary. Examination of the metal structure at high magnification will show these individual grains and grain boundaries.AWS PRGC pdf download.