Investigation of Intergranular Corrosion of Alloy 825 in Sour Service

Abstract Alloy 825, is a nickel base corrosion & heat resistant alloy. This alloy with austenitic matrix has been stabilized with Ti to prevent over production of chromium carbides. However, heating it to the temperature range between 650-800°C, called the “sensitizing” domain, causes chromium carbide precipitation and prepares the alloy for grain boundary corrosion. The present work deals with a case of intergranular attack observed on a tube of a made of alloy 825. This tube was feeding a heater with fuel gas containing H2S. The combustion of this fuel combined with condensation at the surface of the tube created sulfuric acid which promoted the corrosion damage. In this research, the nitric acid test (ASTM A262-C) is used as a standard reference. Optical microscopy and electron microscopy, metallography, tensile and hardness tests were applied to recognize the effect of Cr carbides on steel properties. To stabilize this alloy, heating at 1100°C for 1hour and water quenching are proposed. Finally, it was found that the stabilized sample was not sensitive to intergranular corrosion even after a sensitizing treatment at 670°C for one hour.