Pitting Corrosion Evaluation of Post-Weld Cleaning Methods for Stainless Steel Welds

Abstract: Welding stainless steel creates an oxide film, heat tint, on the surface and around the joint. It disrupts passivation and influence the corrosion resistance for the material. The result is often a decrease in pitting corrosion resistance for the joint and surrounding area. The heat tint is therefore commonly removed by post weld cleaning. Chemical pickling is traditionally performed on stainless steel welds as a mean to ensure passivation. The method is however hazardous both for the environment and from a safety aspect, because of the harsh chemicals used. It is also increasingly hard to perform on high alloyed stainless steel due to the need for a longer application time and more concentrated chemicals. The problems have led to an interest in alternative methods, for example electrolytical cleaning and laser ablation. Two varieties of electrolytical methods and laser ablation have been investigated in comparison to chemical pickling for five differently alloyed stainless steels. They have been ranked based on the critical pitting temperature (CPT) in reference to un-welded and welded samples and further examined microstructurally. An increase in CPT was generally seen for the cleaning methods, compared to the as-welded samples, in some cases obtaining the same CPT as the base material. The difference between the cleaning methods was usually not major and varied depending on material, therefore it is hard to clearly rank one method as superior. Chemical pickling did however tend to result in smaller or fewer pits at CPT compared to the other methods. Process defects or unfavourable structural change caused by welding was ruled out based on microscopy, microstructural analysis and ferrite measurements. SVET analysis was performed on one material for the cleaning methods and an as-welded sample. It was indicated that the sample with heat tint remaining and the laser ablated specimen were prone to pitting in the area closest to the weld. The chemically pickled sample showed some activity in the heat affected zone, while the two varieties of the electrolytical methods were passive. The activity was, however, not investigated for the weld metal. It is concluded that chemical pickling can be used as an effective cleaning method, but that an electrolytical method or laser ablation is much faster, less harmful and also tends to facilitate passivation. An electrolytical method or laser ablation might therefore be more beneficial, compared to chemical pickling, depending on the application requirements.