Corrosion Behaviour of NIMONIC-90 Alloy

Investigations were conducted on hot corrosion of Nickel base, NIMONIC-90 at 675°C and 725°C. The tests were carried out on small test coupons coated with a chemical containing Sodium sulphate and Sodium Vanadate which is representative of the corrosive deposit that usually forms during service in the coal fired boiler and the likes operating in marine atmospheres. Corrosion rates were measured by determining weight loss of the specimen after de-scaling. It was observed that there are three distinct stages in hot corrosion of Nimonic-90 under study. In the first stage the rate of corrosion is rapid and this is mainly due to the formation of corrosion products on new surfaces. In the second stage the corrosive reactions have become stagnant. At high temperatures the corrosive deposit fluxes the protective scale to make it possible. It takes some time before the deposit fluxes and penetrates the oxide scale to attack the substrate. This stagnant stage during which there is no attack can be called “Initiation period”. In the third stage, the corrosion rate accelerates again and the rate of corrosion is increased by over two times that of the initial stage. This stage can be called “Propagation Period”. It is found that propagation stage sets in early at 725°C than at 675°C and also corrosion rate both in the first stage and third stage are high at 725°C as can be expected. Microstuctural studies established conclusively that rate of hot corrosion is higher at 725°C than at 675°C.