Galvanic Corrosion II. Effect of pH and Dissolved Oxygen Concentration on the Aluminum-Steel Couple

The effect of pH, dissolved gas composition, and deaeration on the galvanic characteristics of the aluminum-steel couple was investigated. High-purity a luminum cathodically protects steel completely in chloride solutions within the pH range 0.2-14.0. The galvanic urrents and weight losses of the a luminum anodes are at a min imum from pH 4.0 to 10.0. Within this pH range the rate of galvanic corrosion is controlled by oxygen depolarization at the steel cath-odes. The galvanic urrent fluctuates because the oxide film on the steel cathode is thickening with time in an erratic manner. At higher and lower pH the rate of galvanic corrosion is much greater and is independent of the oxygen concentration i solution. The reaction occurring on the steel cathodes is the vigorous evolution of gaseous hydrogen. Any oxide films on the steel cathodes are subjected to rapid reductive dissolution to solu-ble ferrous ions. Consequently the galvanic current is extremely steady and reproducible