Effect of Ni Content on the Initial Corrosion Resistance of Steel in Atmospheric Environment

Abstract

Weathering steels with a protective rust layer are well known to exhibit high corrosion resistance in mild corrosive environment for a long time even if it is used without paint. However, they do not generally show high corrosion performance in coastal area. Therefore, there have been numerous reports on the corrosion resistance and corrosion mechanism of weathering steel by exposure tests, electrochemical measurements, and rust analysis. However, the detailed effects of additive elements such as Ni and Cr on corrosion resistance have not been clarified. In this study, we focused on Ni as an additive element, and investigated the effect of Ni addition on the initial corrosion resistance of Fe-Ni alloy with different amounts of Ni by corrosion monitoring and electrochemical measurement under atmospheric corrosion environment. Fe (Fe : 99.7%), Fe-3mass% Ni, and Fe-5mass% Ni (Fe, 3Ni, and 5Ni) were used as the samples. The concentric ring type corrosion sensors (corrosion sensor) and the electrodes embedded in an epoxy resin were prepared for an atmospheric corrosion monitoring and for an electrochemical measurement, respectively. The wet and dry cyclic corrosion test was conducted in the chamber set at 25°C and 60% RH after adding 100 µL droplet of a 0.01 mol dm-3 NaCl solution. During the corrosion test, the atmospheric corrosion behavior was monitored by continuously measuring the impedance at low frequency (10 mHz) and high frequency (10 kHz) by a corrosion monitor. On the other hand, polarization curves were measured with a three-electrode system from the immersion potential. The scanning rate was set at 0.5 mV / s. In addition, the electrochemical impedance measurements were performed for the samples with and without passive film, in order to investigate the characteristic of the sample surface. The impedances were measured at 10mV of applied potential with the frequency range from 100 kHz to 10 mHz. The corrosion rate of the steel content Ni in the first cycle was less than one-tenth that of Fe, and was almost the same for 3Ni and 5Ni. However, the corrosion rate after the second cycle decreased with increasing Ni content, and 5Ni showed a low value for a long time. This indicates that the Ni-content steel shows high corrosion resistance in the initial corrosion process. Almost the same oxygen diffusion limited current was observed in the cathodic polarization curves of all samples. On the other hand, the anodic currents of 3Ni and 5Ni, which are considered to correspond to the dissolution of Fe, were about one digit smaller than that of Fe. The impedance of Ni-content steels was larger than that of Fe. However, this result was not agreement with the corrosion monitoring results. Furthermore, the result of surface alalysis will be introduced in our poster.