HIGH TEMPERATURE CORROSION STUDIES ON WELDED DISSIMILAR METALS

Abstract

Dissimilar-metal joints are used widely in various industrial applications due to both technical and economic reasons. The adoption of dissimilar-metal combinations provides possibilities for the flexible design of the product by using each material efficiently, i.e., benefiting from the specific properties of each material in a functional way. Fusion welding is far the most important process used in the fabrication of modem boilers. Components are joined by the formation of a molten pool of metal between them. The production of high quality welds with a high degree of consistency is readily achievable. However, defects are more likely to occur in welds than in wrought material. In all arc welding processes, the intense heat source produced by the arc and the associated local heating and cooling results in a number of consequences in material corrosion behaviour and several metallurgical phase changes occur in different zones of a weldment. Because the occurrence of corrosion is due to electrochemical potential gradient developed in the adjacent site of a weld metal. The materials used for high temperature applications are subjected to hot corrosion and high temperature wear. Hot corrosion can be defined as deposit modified, gas induced degradation of materials at high temperatures. For example, during the combustion stage in heat engines, particularly in gas turbines, sodium and sulphur impurities present either in fuel or in combustion air, react to form sodium sulphate (Na2SO4). If the concentration of the sulphate exceeds the saturation vapor pressure at the operating metal temperature for turbine blades and vanes (700-1100°C), then deposition of the Na2SO4 will occur on the surface of these components. At higher temperatures the deposits of Na2SO4 are molten (m. p. = 884°C) and can cause accelerated attack. This type of attack is commonly called "Hot Corrosion". The accelerated corrosion can also be caused by the other salts, viz. vanadates or sulphates-vanadate mixtures as in oil ash corrosion and in the presence of solid or gaseous salts such as chlorides.