The Quantitative Effect of Mo and Cu on the Stress Corrosion Cracking and Pitting Corrosion Behavior of Ultra-Pure Ferritic Stainless Steels

The quantitative effect of Mo and Cu on the stress corrosion cracking and the pitting corrosion of ultra-pure ferritic stainless steel has been studied by immersion tests in 42 wt.% MgCl2 boiling solution, cyclic salt spray test, polarization curves and FeCl3 immersion test. The results showed that the steels could be immune to the stress corrosion cracking failure with Mo and Cu content both less than 0.4 wt.%. The specimens containing more than 0.8 wt.% Mo and 0.2 wt.% Cu were sensitive to the stress corrosion cracking. The surface morphology of the cracked steels after stress corrosion cracking tests was featured by a mixture of intergranular and transgranular fracture, but mostly transgranular fracture. With the more addition of Mo and Cu, the stress corrosion cracking sensitivity grew higher. The quantitative effect of Mo and Cu addition on the stress corrosion cracking of ultra-pure ferritic stainless steel was given in a whole graph and the synergetic effect of Mo and Cu elements on the stress corrosion cracking was discussed. Besides, it was found that the Mo element increased the pitting corrosion resistance in the chloride containing environment. However, the addition of Cu reduced the pitting corrosion resistance of ultra-pure ferritic stainless steel, which might be attributed to the precipitation of epsilon-Cu phase.