Low-alloy steels show more noble corrosion potentials than carbon steel in sea water and require more current to be cathodically polarized to a given potential. When coupled to an insufficient galvanic anode, a low-alloy steel is polarized less but gives a better protection efficiency. A low-alloy steel has an advantage over carbon steel at a position far from anode or where fluctuation of the environment tends to produce incomplete cathodic protection conditions. The phenomenon is explained by electrochemical considerations.

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An optical measuring method for corrosion patterns is newly developed, which is based on an application of the Moire phenomena. A specially designed grided plate is set just above a corroded surface. The shadows of grids, which are formed on the corroded surface by the light from a point source, superpose on the grids themselves and make a Moire pattern, in which black stripes exactly correspond to the constant range of depth beneath them. The range of depth which is darkened in a photograph can be controled to any value. It is easy to record the position and depth of the most deeply corroded location. The distribution of corrosion loss on an unevenly corroded surface may also be determined by taking many photographs under different conditions and by analysing them using an electronic image analyzer.

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