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Effect of Backing Gas Composition on Corrosion Behavior of Conventional Duplex Stainless Steel Weldments

Picture for Effect of Backing Gas Composition on Corrosion Behavior of Conventional Duplex Stainless Steel Weldments
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Product Number: 51321-16598-SG
Author: Ricardo Hernández Soto/ Abdullah M. Al-Rumaih
Publication Date: 2021
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Duplex Stainless Steels (DSSs) are an attractive material solution in industry environments where the presence of chlorides and ammonium salts prevent the use of standard austenitic stainless steels, since these contaminants promote severe localized corrosion processes. In refining, as an example, UNS S32205 duplex stainless steel has been widely used in the past for some sections of Reactor Effluent Air-cooler (REAC) in hydrotreating plants, though it is currently being displaced by nickel-based alloys. One of the main reasons for such upgrading is the difficulties to keep corrosion properties of DSS during fabrication, which is assessed through standard tests such as ASTM(1) A - 923 Method C during welding procedure qualification. As a matter of fact, welding affects the original base metal ferrite-to-austenite balanced ratio and therefore the corrosion performance of the weldment. The role of variables like welding heat input, shielding gas composition, and filler metal are well known and recorded in the technical literature. However, backing gas composition has not received enough attention. This paper details the backing gas composition effect on the corrosion resistance of circumferential welds made on ASTM A - 790 S32205 pipes of NPS 4 (114.3 mm) diameter, wall thickness schedule 120 (11.13 mm). Four different gas mixtures were used. Standard tests require in any procedure qualification for DSSs were performed: weld metal and heat affected zone (HAZ) ferrite-to-austenite balance through metallographic examination and corrosion performance by means of standard ASTM A - 923 methods. Conclusions were drawn on the optimum backing gas that is recommended for the fabrication of DSS piping systems.

Key words: Welding, Stainless steel, Metallurgical failures, Pitting, Corrosives in refining process

Duplex Stainless Steels (DSSs) are an attractive material solution in industry environments where the presence of chlorides and ammonium salts prevent the use of standard austenitic stainless steels, since these contaminants promote severe localized corrosion processes. In refining, as an example, UNS S32205 duplex stainless steel has been widely used in the past for some sections of Reactor Effluent Air-cooler (REAC) in hydrotreating plants, though it is currently being displaced by nickel-based alloys. One of the main reasons for such upgrading is the difficulties to keep corrosion properties of DSS during fabrication, which is assessed through standard tests such as ASTM(1) A - 923 Method C during welding procedure qualification. As a matter of fact, welding affects the original base metal ferrite-to-austenite balanced ratio and therefore the corrosion performance of the weldment. The role of variables like welding heat input, shielding gas composition, and filler metal are well known and recorded in the technical literature. However, backing gas composition has not received enough attention. This paper details the backing gas composition effect on the corrosion resistance of circumferential welds made on ASTM A - 790 S32205 pipes of NPS 4 (114.3 mm) diameter, wall thickness schedule 120 (11.13 mm). Four different gas mixtures were used. Standard tests require in any procedure qualification for DSSs were performed: weld metal and heat affected zone (HAZ) ferrite-to-austenite balance through metallographic examination and corrosion performance by means of standard ASTM A - 923 methods. Conclusions were drawn on the optimum backing gas that is recommended for the fabrication of DSS piping systems.

Key words: Welding, Stainless steel, Metallurgical failures, Pitting, Corrosives in refining process

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