Uniform, Localized, and Under-Deposit Corrosion
General and galvanic corrosion
Uniform or general corrosion removes metal broadly and predictably; it is managed with corrosion allowance, and rates come from coupons, probes, and thickness surveys. Galvanic corrosion occurs where dissimilar metals contact in an electrolyte; severity increases with potential difference, cathode-to-anode area ratio (a small anode to a large cathode is worst), and conductivity.
Localized and under-deposit corrosion
Localized mechanisms remove metal in small areas at rates far above the general rate. Pitting is autocatalytic, often driven by chlorides and oxygen on stainless steel or by oxygen and deposits on carbon steel, and it penetrates rather than spreads. Crevice corrosion needs a tight geometry - gaskets, lap joints, under washers, tube-to-tubesheet crevices - that traps stagnant liquid. Under-deposit corrosion is the same chemistry driven by a deposit: sludge in boiler waterwalls, sediment in cooling water exchangers, coke in cokers, scale in tanks. The deposit shields the surface from oxygen, creating a differential aeration cell; the shielded area becomes anodic, and chlorides and acidic species concentrate there. Corrosion under insulation is a related special case on carbon steel at roughly 10 to 350 deg F wherever water can enter the insulation.
Microbiologically influenced corrosion produces pitting, tubercles, and slimes, typically in stagnant low-velocity water; confirm it with deposit and bacteria analysis rather than appearance alone. Stray current and soil corrosion matter mainly on buried piping and tank bottoms.
Trap and rule of thumb
Questions mix crevice with under-deposit corrosion, or assume a low general rate rules out pitting. Rule of thumb: for localized threats, do not use a general-rate coupon as acceptance - inspect the surfaces and remove the deposit source with velocity, filtration, biocide, and chemical treatment.