Stress Corrosion Cracking of Electroless Nickel-Plated Low-Carbon Steel in Hot Concentrated NaOH Solutions

The stress corrosion cracking (SCC) behavior of electroless nickel (EN)-plated, low-carbon steel (LCS) in boiling concentrated sodium hydroxide solutions has been studied. The effects of NaOH concentration and heat-treatment of the EN layer on the SCC susceptibility were evaluated by constant load tests, with an initial applied stress of 120% yield strength. The results of polarization curves showed that the anodic current densities of LCS with EN coating in hot caustic solutions were almost two orders of magnitude higher than those of bare LCS. Heat-treatment at 400°C for lh or 650°C for 4h, respectively, could enhance the passive current density of EN coatings in the environments studied. The present results also showed that the as-plated EN coating improved the SCC resistance of the LCS in boiling 20 and 50 weight percent (w/o) NaOH solutions, while a detrimental effect (reducing the time to failure drastically) of the as-plated EN coating was found in boiling 40% NaOH solution by promoting intergranular SCC of the steel. Heat-treatment at 400°C for lh or 650°C for 4h could further increase the SCC susceptibility of the EN-coated LCS. Microstructural change, namely, recrystallization and grain coarsening, could be one of the main factors affecting the corrosion and cracking behavior of the EN-coated LCS.