CORROSION BEHAVIOR OF Nickel - BASED ALLOY 600, 800H, 625, 825 AND 840

Nickel based alloys with high mechanical strength and good resistance to corrosion at high temperature are being widely used in high temperature and corrosive environment. However, when this alloy is exposed to thermal treatment like welding in the temperature range of550to800℃, its intergranular corrosion resistance will probably decline as a result of the segregation of chromium carbides, which is the so-called sensitization. Alloy after sensitization are likely to intergranular corrosion and intergranular stress corrosion cracking in a corrosive environment, these failures have been identified as the main cause of premature failure of the structure member. Therefore, investigation on the corrosion property of nickel based alloys is of importance. Based on the above background, in this paper, intergranular corrosion resistance and pit corrosion resistance of several kinds of nickel based alloy, Incoloy800H, Inconel825, Inconel600, Inconel625and Incoloy840was studied, the main contents:Optimized the experiment parameters of the double loop electrochemical potentiokinetic reactivation (DL-EPR) method to evaluate the intergranular corrosion property of alloy825, and the optimized method was used to study effects of heat treatment temperature and time on the intergranular corrosion resistance of alloy825. The results showed that the intergranular corrosion resistance of720℃treated alloy825is the worst, and the Ra of15min treated alloy825is higher than5%. Therefore720℃was conjectured to be the nose-tip temperature, and15min the nose-tip temperature time.According to the ASTM A262standard, intergranular corrosion property of welded joints of these four Ni-based alloys was evaluated by boiling nitric acid immersion method. The results showed that, the corrosion resistance of these four kinds of welded joint is in the order of825>625>800H>600, and for600and800H welded joint sample, the parent metal zone corrode preferentially.Critical pit corrosion temperature (CPT) test and polarization curve were used to investigate the effect of the concentration of Cl-and processing on the pit corrosion property of alloy800and alloy840, as well as800and840tube. The result showed that pit corrosion of both the alloys and the tubes deteriorated as the concentration of Cl-increased. Compared to the tube, pit corrosion property of the alloy is better. Susceptibility to intergranular corrosion of the alloys and the tubes were evaluated by 10%electrolytic etching in oxalic acid, the result showed that processing has no effect on the intergranular corrosion property of the two kinds of alloy, both SEM and EDX analysis observed TiN impurity on the sample surface.