Effect of Alloy Element in Low Alloy Steels on Corrosion Behavior

The corrosion characterization of the low alloy steel under seashore environment depends on the alloy elements. From the viewpoint of saving resource, it is important to decrease the alloy element. Rusting process and corrosion behavior of pure iron, Ni additive alloy steels and Cr additive alloy steels in atmospheric corrosion environment were investigated by the corrosion cycle test and the polarization curve measurement. The alloy elements, Ni and Cr concentrations are 1, 3, 5, 9 mass%. The corrosion cycle test shown in Fig. 1 simulated the atmospheric corrosion by wet/dry cycle revealed that Ni additive alloy steels and Cr additive alloy steels showed smaller weight change than pure iron, as shown in Fig 2. In an atmospheric corrosion environment, Ni or Cr is effective alloy element for decreasing the corrosion rate. The corrosion rate of Ni additive alloy steels was lower than the rate of Cr additive alloy steels during the tests. The polarization curves of bare steels and rusted steels were measured in 0.5M NaCl solution. An anodic current in the polarization curve was suppressed by adding Ni and Cr to low alloy steel. The corrosion potential of the steels shifted to noble potential with increasing the additional value of Ni or Cr. In Fig.3 and Fig.4, polarization curves of Fe-9mass%Ni and Fe-9mass%Cr was shown, respectively. In the case of both two types of alloys, the cathodic current density of rusted steels was increased comparing with the current of bare steels. The increasing of the current density was induced by the reduction of rust layer. On the other hand, the anodic current density of rusted steels was decreased against bare steels because of the barrier effect of the rust layer for the atmospheric corrosion. From these results, it become clear that small amount of Ni and Cr addition remarkably improved the corrosion resistance comparing with the case of a pure iron. Drying / Wetting