V.R. Raju

Corrosion assessment of type 304L stainless steel in nitric acid

Abstract AISI type 304L stainless steel (SS) was assessed for its corrosion resistance in nitric acid solution. The effect of variation in nitric acid concentration (1 to 10N) and temperature (298 K, 313 K, 333 K, 348 K) on the corrosion resistance of 304L SS was investigated using potentiodynamic polarisation studies. The corrosion resistance of the alloy was also studied in 1N nitric acid containing different concentrations of chloride ion (1000, 5000, and 10 000 ppm) in order to understand the tendency to pitting corrosion. The results show that the corrosion resistance of the material deteriorated for acid concentrations greater than 4N. The temperature of the acid was found to have a profound effect on the corrosion resistance, with corrosion becoming more prominent at temperatures of 333 K and above. Chloride ions up to a concentration of 10 000 ppm were found to have no effect on the pitting resistance of the material studied.

Preparation and characterisation of platinum and platinum-iridium coated titanium electrodes

Abstract Metallic coatings of Pt and Pt–Ir were prepared by the thermochemical glazing process on titanium substrates for application as electrodes in the electrolytic dissolution of nuclear fuels. Titanium substrates with a RuO 2 –TiO 2 single layer were applied with Pt and Pt–Ir chemical solutions and heat treated in vacuum at different temperatures between 973 and 1473 K for 1 h. The physical characterisation by SEM/EDX, XRD and SIMS indicated the presence of adherent and smooth metallic coatings of Pt–Ti and Pt–Ir containing intermetallic compounds of Ti, Pt and Ir. Electrochemical polarisation studies carried out in 1 M HNO 3 at room temperature showed that the specimen prepared at 1373 K had a better electrochemical performance in comparison with Pt sheet electrodes. Significant changes in the open circuit potential (OCP) and the polarisation behaviour of the electrodes were noticed. The life assessment in a simulated reprocessing test solution of boiling 10 M HNO 3 showed excellent performance of the Pt–Ir layered electrode in comparison with conventional mixed oxide coated titanium (MOCTA) electrodes.

ELECTROCHEMICAL NOISE TECHNIQUE FOR CORROSION ASSESSMENT - A REVIEW

This review details the electrochemical noise technique as applicable to corrosion assessment with emphasis on the fundamental principles, theoretical aspects, data analysis and applications. The use of this technique in understanding the mechanistic aspects of corrosion and corrosion monitoring has been discussed. The important applications of this technique in corrosion processes like uniform corrosion, localized corrosion (pitting, crevice corrosion and stress corrosion cracking), and evaluation of coatings are highligted. Miscellaneous applications are also explained.

Failure analysis of carbonate reboiler heat exchangers

Unacceptably high leakage rates from a large number of type 304 stainless steel tubes forced the scrapping of two carbonate reboiler heat exchangers from the carbon dioxide removal system of a fertilizer plant. Extensive failure investigation carried out on the failed heat exchangers established that the main factor responsible for premature failure of the heat heat exchangers was partial expansion of the stainless steel tubes into the carbon steel tube sheet as against the design specification for the expansion of almost the full length of the tubes inside the tube sheet. Partial expansion of the tubes into the tube sheet left undesirable tube-to-tube sheet gaps, which subsequently acted as crevices and led to localized corrosion of the tube sheet as well as the tubes.

Pitting induced failure of austenitic cryogenic condenser coil for acetone service

Abstract A vertical condenser made of type 304 SS material and 304L SS finned coils for acetone–methanol vapour condensation using brine solution (70:30 methanol–water) developed leakage through pinholes formed in the coils. Detailed investigations of the failed coil were made by visual and dye penetrant examination, stereomicroscopic and scanning electron microscopic observation, XRD analysis of the corrosion products, and intergranular corrosion and electrochemical investigations on the coil specimens in brine solution containing chloride. The analysis indicated that carbon steel parts in the process streams corroded and transportation of rust from them lead to the deposition of corrosion products on the stainless steel coil. Under-deposit attack in the presence of chloride ions led to severe pitting corrosion and subsequent failure. Wrong use of carbon steel in the system and ingress of chloride ions resulted in the failure. High temperatures at the bottom of the condenser led to more failures of the coils at such locations.