Avaliação da influência da oxidação devido à deficiência de proteção com gás inerte na soldagem do aço duplex UNS S32205

Abstract

Oil extraction and processing involve the use of equipment and pipelines subject to extreme conditions, such as high temperatures, high pressures and exposure to corrosion, a natural process that can cause serious damage and economic losses. In this scenario, the weldability of duplex stainless steels (DSS) has been widely studied, especially due to its importance in critical applications, such as MIG/MAG welding in offshore environments. In this application, pipeline welding requires strict control of the environment in the root region, and it is essential to perform inert gas purging to protect the weld region and remove atmospheric air. This procedure prevents the formation of undesirable oxides, which can compromise corrosion resistance and the integrity of the welded joint, since the presence of oxygen inside the pipe, combined with the high temperatures reached in the process, can result in oxidation of the weld bead and the Heat Affected Zone (HAZ). Factors such as temperature, time of exposure to the contaminated atmosphere, chemical composition of the material, surface finish and oxygen concentration are determining factors for the formation and characteristics of the oxide layer, including its color and thickness. The focus is on determining the conditions under which the removal of oxidized layers is essential to ensure the integrity of the welded joints, especially with regard to resistance to pitting corrosion. The samples of duplex stainless steel UNS S32205 were subjected to corrosion testing according to ASTM G48, Method A, with the aim of evaluating the pitting corrosion resistance of the material as received, with the shielding gas contaminated with oxygen and without protection. For this purpose, the samples were prepared in two conditions: one without prior treatment and the other subjected to the pickling and passivation processes. The test was conducted in a ferric chloride bath for 24 hours and, as a result, there was no appearance of pits, requiring progressive increases in time and temperature in subsequent studies for obtain more expressive results. However, the data indicated a loss of mass, which confirms that the contamination of the purge gas with oxygen is critical for the corrosion of the duplex steel UNS S32205. The samples without treatment showed the greatest loss of mass. On the other hand, the samples that underwent the pickling and passivation processes did not show significant loss of mass. These results demonstrate the effectiveness of the treatment in mitigating corrosion.