A site investigation was conducted on corrosion damage to stainless steel alloy 316 Ti (UNS S31635) motor pump casings installed in deep fresh water wells. The damage consists of full-wall perforation, localized pitting and crack like attack. The corrosion attack was observed in the weld area and the heat affected zone (HAZ) of the motor casing longitudinal seam weld. The investigation revealed that small anodic sites were formed on the motor casing surfaces close to the seam weld as a result of poor welding and lack of post treatment. This is confirmed through a comparison of the same material welded using a good welding practice and adequate post treatment and installed in a similar environment. This paper presents the corrosion failure investigation and outlines measures to avoid such failures including precautions to be taken during welding of stainless steel alloys.
The corrosion-resistant nature of stainless steels (SS) is attributed to its inherent ability to form a protective oxide film. The formation of this passive film is instantaneous in an oxidizing atmosphere such as air, water, or many other fluids that contain oxygen. The passive film on the SS is invisible and it is formed due to the reaction of oxygen with the chromium in the SS1. General corrosion over large areas of the surface is not usually encountered on SS. However, localized corrosion can take place at discrete sites due to the breakdown of the passive layer by chlorides. SS are usually used because of the corrosion resistance of their surfaces. This excellent corrosion resistance can only be achieved if proper cleaning and finishing operations are carried out after any fabrication process which has damaged the surface condition. SS are pickled in order to remove certain surface defects and contaminations, such as welding oxide, heat tint or scale, formed during fabrication. If left on the final product surface, these oxides will reduce the corrosion resistance of the SS surface. The most important methods used for SS surface treatments are passivation, pickling, electropolishing and in some circumstances, mechanical cleaning2. Alloying additions in SS vary significantly and can have a major effect on weldability and the aswelded microstructure. Austenitic SS have high thermal coefficient of expansion, higher electrical resistance, and lower thermal conductivity than mild-carbon steels. Therefore, high travel speed welding is recommended, which will reduce heat input and carbide precipitation, and minimizes distortion. Proper weld or filler rod selection is important to achieve a weld metal with the desired corrosion resistance and strength characteristics. Sensitization begins to appear during welding and stress relieving of austenitic SS (the 300 series) between 400 to 850°C; the carbon in the alloy diffuses rapidly to the grain boundaries and reacts with chromium to form chromium carbides3. Therefore, the chromium near the grain boundaries is tied up and no longer forms the protective oxide film (chromium oxide). Consequently, the grain boundaries are susceptible to intergranular corrosion and the stainless steel is no longer in the passive state.