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.