This study evaluated the crevice and pitting corrosion characteristics of the Gas Tungsten Arc (GTA) weldment of N-added 4.5% Mo austenitic stainless steels(UNS $34565). Both the Critical Crevice Temperature (CCT) and Critical Pitting Temperature (CPT) of GTA weldments were evaluated in a 6%FeC13+1%HC1 solution simulating a high chloride content and low pH-value condition in which the alloy is used, such as flue-gas desulphurization (FGD) systems incorporated into a power plant. Considering the importance of the geometric factor in the crevice and pitting corrosion behavior of the as-welded structures, the local corrosion characteristics of weldments including the Heat Affected Zone (HAZ) were electrochemically evaluated by using a potentiostat employing the cyclic polarization method. For this purpose, a selected area of the as-welded specimen was exposed to the corrosion media for the potentiostat scanning, which provided a better understanding of the corrosion susceptibility of the weldment. The microstructural change in the weldment was also evaluated to study its effect on the local corrosion characteristics of the weldments.
Use of austenitic stainless steels having proper strength and high corrosion resistance is largely increasing in power plant systems and various chemical process equipment. Particularly, recent trends of strict environmental regulations regarding the maximum amount of allowable sulphur oxide exhaust has accelerated installation of Flue Gas Desulfurization(FGD) plants around the world. The FGD plants remove SOx gas generated from the fossil fuel fired power plants via chemical reactions. Considering severe corrosion environments containing SO2 and CI2 in the FGD plant, high-alloyed and highly corrosion resistant austenitic stainless steels, such as UNS N08926 and UNS N08367 or Ni alloys, such as UNS N10276 have been widely used for absorbers, ducts, dampers, etc [1,2].
The pitting and crevice corrosion resistance of austenitic stainless steels can be evaluated by PREN(Pitting Resistance Equivalent Number, PREN = %Cr + 3.3 x %Mo + 30x %N). Among these alloy elements, recently, the importance of N as a corrosion resistant element has been recognized. It was reported that N-added austenitic stainless steels(UNS $34565) with less Mo content than other super austenitic stainless steels presented excellent local corrosion resistance such as pitting and crevice corrosion in chloride containing corrosion environments [1~3]. The application of N-added austenitic stainless steels is gradually increasing in FGD plants as well as piping systems for heat exchangers in sea water desalination systems.
