Stress Corrosion Cracking of Stainless Steel Super Heater Tubes in 200 MW Boiler

Abstract

A few cases of super heater tube failures by corrosion were noticed in a boiler of 210 MW units of NTPC stations. The failure investigation was carried out at the Research and Development centre of NTPC. The failed tubes were examined by using Optical, scanning Electron Microscope and conventional methods. The material of the tube was stabilised grade austenitic stainless steel. The micro-scopic examination indicated the presence of a large number of transgranular cracks with branches. The surface analysis by EDAX revealed the presence of chloride in the corrosion products. The reason of failure has been attributed to chloride induced stress corrosion cracking. Chloride ions had generated supposedly due to leaching of residual contaminants carried over from manufacturing stage. It was suggested to check the cracks already gene-rated by NDT methods and wash the tube panels which were already installed. After implementation of the recomm-endations, no such case of tube failure was reported. The present paper describes the investigations carried out and the remedial measures to prevent chloride induced stress corrosion cracking in supper heater tubes.