Abstract
The Influences of five common tramp elements P, S, Sn, Cu, and Ni at typical residual levels on the intergranular stress corrosion cracking (IGSCC) of a typical mild steel. (0.15%C, 0.8%Mn and 0.05%Si), are studied in nitrate solutions and a mixture of carbonate and bicarbonate solutions. The relative effects of these impurities are examined in terms, of the electrochemical properties of unstressed material, of the IGSCC susceptibility in a constant strain rate test, and of the grain boundary and surface oxide compositions via surface analysis. Results show that in general only phosphorus can be detrimental. A slight beneficial effect is apparent for Cu in both electrolytes. Phosphorus has small adverse effects in certain narrow potential regions in the NH[4]NO[3] electrolyte. These occur just above the free corrosion potential and above the active-passive transition (1100 mV). There are no detrimental effects due to the impurities in the carbonate/bicarbonate electrolyte. The negligible effects of impurities occurs in spite of impurity concentrations at the grain boundaries being significantly enhanced above the bulk to 10%. as confirmed by Auger electron spectroscopy. Hence, the IGSCC susceptibility is shown to be dominated by the carbon present in the steel. XPS measurements indicate that carbon is present throughout the oxide, and importantly up to the electrolyte interface. IGSCC susceptibility is thought to occur principally by a breakdown in the oxide integrity, which leads to prolonged chemical attack. This mechanism is attibuted to the action of carbon and possibly the impurities, however under certain conditions some secondary mechanisms are needed to account for ail the effects due to the impurities. These include improved passivation by reaction products and a remedial influence on the detrimental effect of carbon.