Effects of exposure time on the anodic dissolution of Monel-400 in aerated stagnant sodium chloride solutions

The anodic dissolution of Monel-400 (63.0% Ni, 28-34% Cu) alloy after its immersion in freely aerated stagnant 3.5% NaCl solutions for 0, 24, and 72 h has been investigated. The study was carried out using a variety of electrochemical techniques and gravimetric measurements after varied exposure periods (5-160 days). The work was complemented by scanning electron microscopy and energy dispersive X-ray analyzer (SEM/EDX) investigations. The electrochemical measurements showed that Monel suffers both general and pitting corrosion. The severity of uniform corrosion decreased, while pitting one increased with increasing the immersion time to 24 h and further to 72 h before measurements. Gravimetric data indicated that the weight loss increased, while the corrosion rate decreased for Monel with time. SEM images and EDX profile analyses confirmed that the corrosion of Monel after 160 days immersion in NaCl solutions occurs due to the selective dissolution of nickel.