Table 5-1. Metal and alloy classification
(3) A metal's tendency to corrode in a galvanic cell is determined by its metal compatability
position in the metal and alloy galvanic series shown in Table 5-2. The metal compatibility order listed
is only appropriate for seawater at 77€F. Both temperature and the makeup of an electrolyte (water or
soil) may vary the order in which the metal is listed. For example, in fresh water at a temperature above
150€F, iron may become anodic (more corrosive) with respect to zinc. The less noble (anode) the metal
is, the more it suffers from an accelerated corrosion attack. The more noble (cathode) the metal is, the
greater it is cathodically protected by a galvanic current. The more noble the metal or alloy, is the more
chemically inert or inactive it is, especially toward oxygen.
c. Pitting. This corrosion occurs in most alloys, but it is most common in aluminum and
magnesium. It is first noticeable as a white or gray powdery deposit, similar to dust, which blotches the
surface. When the deposit is cleaned away, tiny pits or holes are seen in the surface. Pitting corrosion is
a localized form that begins at a break in the passive (protective) film. A cell is formed between the
exposed metal and the passive metal where the film is broken. Such breakdowns in the protective
coating can occur at a rough spot, machining mark, scratch, or other surface flaw. Pitting corrosion can
also occur under a small deposit (weld spot or dirt particle) that prevents the access of oxygen to the
metal. Pitting corrosion proceeds at a rapid rate if the products of corrosion are conductive.
EN0562
5-6
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