Characterization of Corrosion Related Failures

Abstract

This paper deals with the characterization corrosion rel-ated failures of structural steel components in the various environments of petrochemical industries. The case studies, pertaining to the failures due to, stress corrosion cracking in co2 environment, catastrophic carb-urization(metal dusting corrosion) in hydrocarbon envi-ronment, and corrosion fatigue in steam environment have been discussed. The cracking in the linepipe (conforming to IS 1239) was due to grain boundary segregation of Mn andSi leading to hardening in the region just outside the outermost area of HAZ, i.e., SCHAZ and in the parent metal. The service cracking as well as tensile test fracture always occurred in parent metal with different degree of intergranular brittle fracture-The factors that directly affect the occurrence of cracks are due to; segregation of Mn & Si outside the HAZ, longitudinal stress as well as residual stress due to weld and corrosive nature of C02-CO-l H2O system. These three factors satisfies the occurrence of SCC. A failed reactor tube of I.25Cr-0.5Mo steel have been investigated. The characterization of corrosion products revealed the presence of wustite, magnetite, Cr2C, M23C6, rounded metal particles & carbon soot. The morphological feature in the subsurface region revealed highly carb-urized inner tube metal surface. This carburized layer was indicative of non-protective behavior of oxide scale in the operative temperature and environment. In the thi-cker region of tube wall, thinning in the form of rounded pits whereas in the thinner region of tube, metal loss in the form of uniform thinning was encountered. All these observations indicate that the metal wastage was due to carburizing condition. The microstructural changes occurred in the steam drum nozzle weld its well as vortex hrcaker weld, made of a commercial HSLA steel (DIN I5NiCuMoNh5) have been invest-igated. The welds showed predominantly an accicular ferri-tic microstructure with dislocation substructure which is beneficial for weld toughness. Various types of intra-granular and intergranular precipitates have been obse-rved. However, the e-Cu precipitates of about 40 nm were observed which is much higher than the 20 nm size often observed in this class of steel, indicating an overaging effect. Large number of spherical aluminosilicate inclu-sions containing titanium have also been found. The cracks were revealed by SEM which infers the crack propagation direction. The features and service conditions are indic-ative of corrosion fatigue cracks. The corrosion fatigue cracking resulted from combination of, i) vibration, ii) susceptible microstructure, and iii) corrosive environment.