Industrial Corrosion and Protection--08 Crevice Corrosion

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Basic concept of crevice corrosion: Corrosion of metal within crevices, which occurs when gaps are formed on the metal surface due to the presence of foreign substances or structural factors, making it difficult for substances in the solution within those gaps to move, is known as crevice corrosion. Almost all metals can suffer from crevice corrosion, but those that rely on passivation for corrosion resistance are most prone to this type of corrosion. Crevice corrosion is mostly macroscopic cell corrosion. The corrosion patterns range from pitting of the metal within the cracks to general corrosion. Crevice corrosion can undermine the integrity of mechanical connections and the sealing properties of equipment, causing serious obstacles to its proper operation, leading to failure or even destructive accidents. Crevice corrosion is not limited to stainless steel; it occurs in many alloy families as well, including alloys of titanium, aluminum, and copper. Special attention should be paid to crevice corrosion in marine environments. In addition to crevice corrosion that occurs in solutions, there is another type of atmospheric corrosion that commonly occurs on painted metal products such as steel, aluminum, magnesium, and zinc; it is known as filiform corrosion. This type of corrosion has been observed on steel parts that are nickel-plated, chromium-plated, or enamel-coated; it develops particularly severely on metal beneath varnish or enamel coatings. Since it occurs mostly beneath the paint film, it is also known as underfilm corrosion. Although the metal loss caused by this type of corrosion is not significant, it damages the appearance of metal products. It sometimes starts as filiform corrosion and can develop into crevice corrosion or pitting, and it may also lead to stress corrosion. It has been reported that on aluminum, the phenomenon of filiform corrosion progressing to pitting and intergranular corrosion has been observed. There are also examples of pitting induced by filiform corrosion on magnesium products, leading to perforation of those magnesium items. Conditions for occurrence: Crevice corrosion typically occurs in crevices or shielded surfaces where certain electrolyte solutions become stagnant. Almost all corrosive media (including fresh water) can cause metal intergranular corrosion, but solutions containing Cl are the most likely to induce this type of corrosion. The width of the gap must be narrow enough to make it difficult for substances to migrate between the solutions on the inside and outside of the gap, but it should still be wide enough to allow the solution to enter the gap. In other words, only a gap of a certain width can cause crevice corrosion. Tests have shown that this width is generally in the range of 0.025 to 0.1 mm. Such gaps are common in practice, which is why metal intergranular corrosion is widespread. Crevice corrosion tends to occur at the flange connections in stainless steel equipment, as well as at points of contact with rivets, bolts, gaskets, washers (especially rubber washers), valve seats, loose surface deposits, attached organisms, and at the expansion joint gaps in shell-and-tube heat exchangers. Crevice corrosion generally occurs only in localized areas where there are gaps in the equipment or components, rather than on the entire surface. Typically, ulcer-like grooves or pitting of a certain shape (depending on the shape of the gap) appear in patches. Filamentary corrosion is a shallow type of subfilm corrosion. Once it forms, it grows rapidly, eventually creating a dense network of patterns on the metal surface; this causes bulges in the paint coating on the metal surface without any visible damage, thereby rendering the protective coating ineffective. It is characterized by corrosion products in the form of a fibrous network; the corrosion that occurs along the lines etches out a visible small groove in the metal, with a depth of about 5–8 um, and there is a deeper hole at regular intervals along this groove. On iron, the corrosion product appears as red filaments, about 0.1~0.5 mm in width. Filamentous corrosion generally consists of a blue, V-shaped head and a brownish, filament-like body (or tail). Corrosion occurs at the active head, while the body (or tail) is inactive.