Effect of chloride and phosphoric acid on the corrosion of alloy C-276, UNSN08028, and UNSN08367

The effects of chloride and phosphoric acid (H3PO4) on the corrosion of Alloy C-276 (UNS N10276), UNS N08028, and UNS N08367 have been studied using electrochemical techniques. All of these alloys are widely used in the chemical process industries because of their excellent corrosion resistance in the severe service conditions involving high temperature and stresses. Tests were performed in deaerated solutions, at two different concentrations of sodium chloride (NaCl) and H3PO4 at a fixed temperature of 95 degrees C. Potentiodynamic polarization scans in NaCl solution have been carried out to determine the pitting and the repassivation potentials for UNS N08028 and UNS N08367 and the range of passivity of Alloy C-276. These tests help rank the materials based on resistance to localized corrosion. All the alloys tested show a range of passivity. Alloy C-276 showed the best corrosion resistance under all test conditions. UNS N08367 performed better than UNS N08028. Alloy C-276 had the highest range of passivity followed by UNS N08367 and UNS N08028. Under all test conditions Alloy C-276 did not exhibit pitting but showed transpassive dissolution of chromium. In the most aggravated condition Alloy C-276 showed a passivity range of about 800 mV (-100 mV to 700 mV)followed by UNS N08367, with a passivity value of about 300 mV. The pitting potential of UNS N08367 was 300 mV higher than UNS N08028. The passive current density of Alloy C-276 was lower than UNS N08028 and UNS N08367 in most conditions. Alloy C-276 and UNS N08367 were tested in the sensitized conditions. For both alloys sensitization decreased the resistance to localized corrosion. The effect of the carbide precipitation and intergranular corrosion are also discussed. The results show that Alloy C-276 performed better than UNS N08367 and UNS N08028 in both the sensitized and the solution-annealed conditions.