Effect of Co and Ni on the electrochemical behavior of rust formed on the low alloy steels in cyclic wet and dry environment

The iron rust phase formed on low alloy steels has been analyzed by using EPMA, XPS, TEM and alternating current (AC) impedance methods after cyclic wet and dry corrosion test using 0.5 mass% NaCl solution. The steel containing Co or Ni from 1 to 3 mass% showed higher corrosion resistance than carbon steel in the test. Cobalt was identified as its trivalent state (Co³⁺) in the rust of Co-bearing steel by EPMA and TEM, which means that Co is incorporated mainly into FeOOH. On the other hand, nickel was identified as bivalent state (Ni²⁺) in the rust of Ni-bearing steel by XPS and TEM, which implies that Ni is involved in the formation of spinel oxide in rust. By AC impedance it is demonstrated that the resistance of the rust (R_rust) corresponds to the structural factor of the rust of steels. The high value of R_rust of Co-bearing steel indicated resistance of its FeOOH-type rust against the penetration of Cl ion. The high R_rust value was observed mainly at the early stage of corrosion cycles. The R_rust value of Ni-bearing steel increases with increasing amount of rust on steel, which implies that Ni takes part in the conversion of spinels into dense and fine structure in the inner layer of rust layer.