Corrosion of metals which would ordinarily be resistant to the environment may sometimes take place by electrochemical means as a result of a particular method of equipment design or use. Typical corrosion processes which may lead to severe localized attack include the formation of metal-ion concentration cells, oxygen concentration cells, galvanic action and the action of stray electrical currents. The first type of corrosion mentioned may occur as results of a construction method which leaves crevices and cracks in the structure. Small openings of this type restrict the free access and circulation of solutions and are likely to produce localized high concentrations of dissolved metal ions in the trapped solution. When the metal in this area is electrically connected to the open area, a difference in electrical potential will develop between the two areas and the resulting current flow will corrode the metal in the area of lower dissolved ion concentration. Normally, the attack will be in areas immediately adjacent to the crevice. This type of corrosion may also occur as a result of different rates of solution flow across connected metal surfaces causing localized differences in dissolved ion concentrations in the solution adjacent to the metal.
In the case of corrosion by oxygen concentration cells, the solution potential and the tendency for a metal to dissolve or corrode may be influenced by differences in oxygen concentration in the solution in contact with different points in the metal structure. This will cause electrical current to flow so that an increased corrosion rate will result in local areas of depleted oxygen content. This effect may occur either in crevices or underneath deposits of any type of metal surface that tends to restrict oxygen access. The oxygen concentration cell causes attack within the restricted area, as opposed to corrosion of exterior areas by the action of metal-ion concentration cells. Opposite current flow occurs in these two corrosion processes and the two may in some cases cancel each other. Nevertheless, metals of different types are likely to be more susceptible to one type of corrosion than to the other. The stainless steel and metals that passivate due to the formation of oxidation films are more likely to develop pitting and crevice corrosion from oxygen concentration cells.
By galvanic action, the electrochemical effects of different metals in electrical contact with each other and immersed in the same solution are well know. In such system the material with the most negative potential will be corroded, except that the extent of the reaction will be greatly influenced by the individual metals and by the solution chemistry. Basically, any construction that tends to include of dissimilar electrical potential may cause corrosion and must be tested before use. About this, it must be mentioned that a considerable difference in potential may exist between the active and passive states of several metals and alloys, including stainless steel, nickel, hastelloy C and others. This may produce rapid acceleration of corrosion rate in a small area of an exposed metal from which the passivated surface film has been lost. About stray electrical currents, improper or insufficient grounding of equipment in some cases lead to corrosion as a result of the passage of electrical currents through the system. This type of corrosion is normally associated with direct current flow and occurs where the positive side of the current is connected through the solution to ground.
Pitting corrosion of mild steel has been attributed to attack of the oxidation-concentration cell type. Damage may occur in alternately wetted areas and in zones where poor solution circulation may occur. In each case the corrosion is initiated under layers of wet ore, salt crusts, or an initial light layer or ordinary iron rust. An oxygen concentration cell is formed between the covered area, in which the oxygen concentration is low, and electrically connected areas in contact with solution containing dissolved oxygen. As a point corrosion occur, the generation of further corrosion products intensifies the action and severe pitting occur, which may in severe cases produce perforation of the metal. High sulphate content in the solution apparently contributes to the effect.
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