THE EFFECT OF SALT CONCENTRATION ON NICKEL CORROSION BEHAVIOR IN SLIGHTLY ALKALINE-SOLUTIONS AT DIFFERENT HYDROSTATIC PRESSURES

The hydrostatic pressure effect on nickel corrosion in NaCl solutions (from 5.8 x 10(-2) to 1.2 mol l-1 and in Na2SO4 solutions (from 10(-2) to 10(-1) mol l-1) was investigated with electrochemical and XPS techniques. By increasing the pressure, the average nickel corrosion rate decreases in dilute NaCl and Na2SO4 solutions, whereas it increases in concentrated solutions. This was explained by the different nature of corrosion layers formed in these solutions. Since the percentage of wet nickel oxides decreases by increasing the hydrostatic pressure: (1) in dilute solutions, with insignificant Cl- (or low SO42-) in the corrosion products, the nickel corrosion resistance increases by increasing the pressure; (2) in concentrated solutions, with a greater amount of Cl- or SO4- ions in the corrosion products, by decreasing the pressure, the corrosion resistance decreases. This is likely to be due to the formation of wet oxides at 1 atm of a steric hindrance which inhibits further SO42- or Cl- chemisorption.