Effect of Co and Ni on the corrosion behavior of low alloy steels in wet/dry environments

The iron rust phase formed on low alloy steels has been analyzed by using EPMA, XPS, TEM and alternating current (AC) impedance methods after wet/dry corrosion tests using 0.5 mass% NaCl solution. Steel containing Co or Ni from 1 to 3 mass% showed higher corrosion resistance than carbon steel in the test. Cobalt was identified in its trivalent state (Co³⁺) in the rust of Co-bearing steel by EPMA and TEM, which means that Co was incorporated mainly into FeOOH. On the other hand, nickel was identified in its bivalent state (Ni²⁺) in the rust of Ni-bearing steel by XPS and TEM, which implies that Ni was involved in the formation of spinel oxide in rust. AC impedance demonstrated that the resistance of rust (R(rust)) corresponds to the structural factor of the rust. The high value of R(rust) for Co-bearing steel indicated high resistance of FeOOH-type rust against the penetration of Cl ion. The high R(rust) value was observed at the early stage of corrosion cycles. The R(rust) value of Ni-bearing steel increased as the amount of rust on the steel increased, implying that Ni helped to convert spinels into a dense and fine structure, and that its effectiveness increased after the formation of the inner rust layer. (C) 2000 Elsevier Science Ltd. All rights reserved.