Comparison of stress-corrosion cracking susceptibility of mild and low alloy steels in phosphate environments

Comparison between anodic behaviour and susceptibility to stress-corrosion cracking (SCC) in phosphate solutions (NaH2PO4, pH = 4) of two kinds of construction steels - low carbon (0.17% C) and low-alloy (2.0% Cr) steels has been carried out using the potentiodynamic polarization method with slow and fast potential sweep rates (10 and 300 mV/min) as well as slow strain rate techniques. It has been established, that both steels undergo active to passive state transition in phosphate medium, more clearly expressed for the low-carbon steel. For this steel, the susceptibility toward SCC is much higher than that of the low-alloy steel. The occurrence of SCC depends strongly on the potential and it is expressed only within a narrow range of potentials for both steels: -0.3-0.0 V (SCE) for the low-carbon and -0.1+0.3 V (SCE) for the low-alloyed steel. The SCC susceptibility decreases strongly with the increase of temperature and lowering solution concentration. The most severe SCC extent has been registered in 1 N NaH2PO4 solutions at 20 degrees C. It is also shown that under SCC conditions a surface film, containing iron phosphates, is formed on both kinds of steels. The conditions of SCC correspond well to the regions of active-passive state transition in the polarization curves for both materials and are in good agreement with the regions, predicted by the potentiodynamic polarization method with slow and fast potential sweep rates. On the basis of the results obtained, a conclusion is drown for the higher mechano-corrosion stability of the low-alloy steel, compared to that of the low-carbon steel.