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The mechanism of pitting corrosion in carbon steel in a simulated seawater environment was studied by obtaining data on the chemical changes which occur with time in the pit and cathode solutions. Quantitative measurements were made of pH; sodium, magnesium, calcium, iron, chlorine, sulfate concentrations, and weight loss. The solutions used in the analysis were generated by using a recently developed artificial pit which not only provided a sufficient amount of solution but also made it possible to obtain specimens from specific locations in the cell itself. The results show a depletion of magnesium, calcium, chlorine, and sulfate in the cathode region while sodium and hydroxyl ion concentrations increased. Inside the pit the chlorine, iron, and hydrogen ion concentrations rose while the calcium, magnesium, and sodium concentrations dropped. Based on the results of these tests, it is shown that the decrease in pH in the pit is greatly enhanced by the magnesium and calcium reactions occurring at the cathode and by the formation of iron colloids (Fe(OH)4− and Fe(OH)2+).
corrosion, corrosion tests, pitting tests, artificial pit, seawater attack, solution chemistry, carbon steel pitting, evaluation, pitting mechanism
Petersen, C. W.
Senior engineer, Bettis Atomic Power Laboratory, Westinghouse Electric Corporation, West Mifflin, Pa.