Nickel-alloys containing optimum amounts of chromium (Cr), molybdenum (Mo) and tungsten (W) are widely used in industries because of its high corrosion resistance and mechanical strength. The corrosion-resistant Ni-Cr-Mo and Ni-Cr-Mo-W alloys are designed for applications at temperature below 1000 0F, however, the material may be subjected to high temperatures during welding or heat treatment operations. High temperature phase formation following welding operations or incorrect heat treatment can lead to intergranular corrosion, as a result of sensitization, when exposed to the corrosive solutions. A most common approach to detect the susceptibility of Ni-Cr-Mo and Ni-Cr-Mo-W alloys to intergranular corrosion is by using ASTM G-28A test methodology. However, the corrosion rate obtained though G-28A technique is a sum of both uniform and intergranular corrosion, a criticism of this technique.
In the present study, mill-annealed coupons of UNS N10276, N06022 and N06035 alloys were heat-treated for few mins to couple of hours at different temperatures in the temperature range of 1000-1900 0F. The heat-treated specimens were tested in ASTM G-28A test solution followed by internal attack measurement through optical microscope. Furthermore, electrochemical tests were conducted to evaluate the sensitization mechanism in more detail.
To understand the effect of sensitization on the uniform and localized corrosion resistance performance of the alloys, the heat-treated specimens were tested in various acidic solutions, like hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3) and acidified ferric chloride (ASTM G-48C).
Key Words: Ni-Cr-Mo Alloys, Ni-Cr-Mo-W Alloys, Sensitization, ASTM G-28A, ASTM G-48C, Potentiodynamic Polarization Technique, Double-Loop Electrochemical Potentiokinetic Reactivation Technique (DL-EPR), Hydrochloric Acid, Sulfuric Acid, Nitric Acid