Effects of nickel on the chloride stress corrosion cracking(SCC) of low interstitial high chromium ferritic stainless steels in 42%LiCl plus thiourea solution were investigated using a uniaxial constant- load fixture and a constant extention rate tester, incoporating an appropriate corrosion cell assembly. The 29Cr-4Mo alloy and 29Cr-4Mo-2Ni alloy were employed as the prototype for comparison.
The 29Cr-4Mo alloy is immune to SCC at open circuit but becomes suceptible when anodically polarized to a critical potential(-450mV, SCE) that is 20mV noble to the corrosion potential(-470mV, SCE). However, the 2%Ni addition to 29Cr-4Mo alloy increases the corrosion potential in the noble direction without affecting the critical cracking potential(-450mV, SCE) resulting in susceptibility to SCC of 29Cr-4Mo-2Ni alloy at poen circuit.
The 2%Ni addition to 29Cr-4Mo alloy increases the susceptibility to localized corrosion by reducing film breakdown potential, reduces the repassivation rate with or without a sustained load, and increases the susceptibility to hydrogen embrittlement.
Nickel was not observed on Auger Electron Spectroscopic analysis in the passive film on 29Cr-4Mo-2Ni alloy in boiling 42%LiCl plus thiourea solution.
The similarities in SEM fractographs between samples failed by SCC in 42%LiCl plus Thiourea solution and those failed under straining at constant rate during cathodic charging in 1N $H_2SO_4$ plus 100㎎/L $NaAsO_2$ solution suggest that hydrogen embrittlement is involved in the failure process of low interstitial high chromiun ferritic stainless steels in hot chloride solution.
