Failure analysis of titanium heater tubes and stainless steel heat exchanger weld joints in nitric acid loop

Abstract A 400 l capacity nitric acid loop (NAL) test facility simulating inactive plant operating conditions was designed, constructed and evaluation of candidate materials was carried out for nuclear spent fuel reprocessing applications. Austenitic stainless steel coupons were exposed to flowing nitric acid medium at various temperatures in NAL. Assessment of critical components of NAL after 10,000 h operation revealed failure of titanium heater tubes and stainless steel heat exchanger tube sheet welds. Failure analysis and detailed characterisation of corrosion products on the failed tubes were carried out using SEM, EDX and XRD techniques. The brown colour corrosion product on titanium heater tubes comprised iron and small amounts of chromium indicating that failure of titanium tubes was due to deposition of corrosion products of stainless steel, leading to galvanic corrosion. Cracks observed on titanium heater tubes were attributed to hydrogen-induced cracking, resulting in severe degradation. Surface morphology and EDX analysis of the corrosion product on stainless steel heat exchanger tubes revealed dislodged grains from 304LSS and XRD analysis confirmed the deposit as stainless steel. Grain dropping occurred due to intergranular corrosion of 304L SS in nitric acid and deposition of the dislodged grains at the seal weld joints resulted in localised corrosion and failure. Based on the studies carried out, surface heaters instead of immersion heaters and periodic inspection and frequent removal of corrosion products from the nitric acid loop were suggested to mitigate such failures. Based on the results of the investigation, repair and refurbishment of the nitric acid loop was undertaken.