Difference of Corrosion Characteristics of the Welding Zone between Laser and TIG Welding

Abstract

Two kinds of welding methods were performed on 22APU stainless steel: laser welding and TIG welding. In this case, the differences of the corrosion characteristics of the welded zones between the two welding methods mentioned above were investigated with electrochemical methods such as the measurement of corrosion potential, polarization curves, cyclic voltammogram, etc. The Vickers hardness of all welded zones (WM:Weld Metal, HAZ:Heat-Affected Zone, BM:Base Metal) was relatively higher for the laser welding than for the TIG welding. Furthermore, the laser welding method’s corrosion current densities in all welding zones were also observed to have a lower value compared to TIG welding. In particular, the corrosion current density of BM, regardless of the welding method, was the lowest value among all other welding zones. Intergranular corrosion was not observed at the corroded surface of all laser-welded welding zones; however, it was observed at the TIG-welded WM and HAZ welding zones, which suggests that chromium depletion due to the formation of chromium carbide occurs on the WM and HAZ which are in the range of sensitization temperatures, therefore the zones can easily be corroded with a more active anode. Consequently, we can see that corrosion resistance of all welding zones of 22APU stainless steel may be improved by the use of laser welding. Keywords: Laser welding, TIG welding, Corrosion potential, Weld metal, Heat affected zone, Polarization curves, Chromium depletion1.Introduction In recent years, use of austenitic stainless steel, which has a high corrosion resistance, has been increasing due to the development of industries, such as atomic energy, aerospace, petro chemical, etc. When stainless steel was welded for numerous kinds of structures, intergranular corrosion would often be observed at the area surrounding the welding zone due to chromium depletion; in addition, there are numerous papers which have investigated both general corrosion and intergranular corrosion[1-6]. However, there are few experimental results on the effect of corrosion control at the welding zones when laser or TIG welding are used for the purpose of constructing heat exchangers with 22 APU stainless steel. Although laser welding is more expensive than TIG welding, laser welding is often used instead of TIG welding for the production of heat exchangers. Consequently, it has been suggested that, from a long-term point of view, laser welding is more economic than TIG welding. In this study, when TIG and laser welding are performed on the stainless steel, the differences of the corrosion characteristics in the welding zone was investigated with electrochemical methods. The experimental results are therefore expected to provide useful reference data for the appreciation of mechanical and corrosion characteristics in the welding zones.