Tunneling Corrosion Mechanism of 25% Chromium-20% Nickel-Niobium Stainless Steel in Highly Oxidizing Nitric Acid

Abstract

Austenitic stainless steel (SS) is used commonly in chemical plants dealing with nitric acid (HNO 3 ) because of its high corrosion resistance. However, even SS tends to corrode intergranularly in highly oxidizing HNO 3 containing oxidizing chromium or cerium ions, such as Cr 6+ or Ce 4+ . Furthermore, pitting corrosion, so-called tunneling corrosion or end-grain attack, occurs on the forgings in highly oxidizing HNO 3 . The mechanism of tunneling corrosion and its countermeasures for a 25% Cr-20% Ni-Nb steel (UNS S31040) in a mock reprocessing plant were studied. Results indicated tunneling corrosion was a type of pitting corrosion with general and intergranular attack on pit surfaces. It propagated along metal flows with penetrations up to 2 mm (0.079 in.) in diameter and 6 mm (0.236 in.) in depth. Rates of tunneling corrosion were 3 to 13 times faster than general corrosion. Tunneling corrosion initiated and propagated parallel to metal flows on the local parts where chromium content was depleted. This method of corrosion was caused both by preferential corrosion of a low-chromium part and corrosion accelerated by the galvanic action between a low-chromium part as an anode and a high-chromium part as a cathode. The electroslag remelting process completely resolved this problem by making ingots free from continuous chromium segregation.