A STUDY OF THE STRESS CORROSION CRACKING OF SENSITIZED 1Cr18Ni9Ti STAINLESS STEEL IN HIGH TEMPERATURE HIGH PRESSURE WATER

The corrosion behaviour of sensitized 1Cr18Ni9Ti stainless steel in high tem-perature water was nvestigated from the environment angle. Stress corrosion crack-ing would occur in an acidic or neutral high emperature water in the presenceof a definite concentration of Cl~- and/or O_2. SEM electron fractographic nalysisshowed that in Cl~- containing water cracking was intercrystalline, while in O_2bearing water, racking was of a mixed type, mainly as cleavage. The apparentand real corrosion kinetic curves of Cr18Ni9Ti n high temperature water weregiven. It was shown that corrosion consisted of both dissolution and wet xida-tion, chiefly the latter. The correlation between the structure of the oxide films andcorrosion has een studied by means of Mossbauer internal conversion electronspectroscopy. In the case of general orrosion, the film consisted predominantlyof Fe_3O_4, while in the case of stress corrosion, the film onsisted of a multi-layerstructure with Fe_2O_3 as a predominating layer overlapped with layers containingFe_2O_3 and Fe_3O_4. In a boron containing water, FeBO_3 phase might appear in theoxide film ocally, but it neither caused nor inhibited stress corrosion. This "inert"behaviour of boron is related to he non-stoichiometry of Fe_3O_4 in the film tomake it anion-selective in nature. Based on corrosion-electro hemistry, it may beconcluded that stress corrosion takes place between the passive and transpassive regions n the polarization curve. Increasing Cl~- or O_2 content shifts the potentialof the alloy toward the ranspassive region. The action of Cl~- and O_2 are tochange the anodic and cathodic polarization curve espectively. It was suggestedthat in the Cl~- containing water the rapid dissolution of TiC along boundary sthe main cause of stress corrosion, while in the O_2 bearing water, the selectivedissolution of Cr in the lloy is critical. After thoroughly reviewing the charac-teristics of stress corrosion, the authors were nclined to think that stress not onlyis one of the causes for the serious corrosion pits, but also induced ctive pathswhich accomodate, accelerate and help guide the metallic atoms in the pit in dis-solving egularly. Finally, a preliminary model of stress corrosion cracking of1Cr18Ni9Ti in high temperature high ressure water has been proposed.