A study on preferential weld metal corrosion in UNS S31600 austenitic stainless steels in HCl containing environment

• Published in: NACE International Corrosion Conference Proceedings pp. 1 - 8
• Main Authors: Park, Hyunjoon, Jang, Sungyong, Park, Ihho, Lee, Hyungjin, Cho, Sungsu
• Format: Conference Proceeding
• Language: English
• Published: Houston NACE International 01.01.2019
• Subjects: Aeration; Austenitic stainless steels; Base metal; Chromium; Corrosion mechanisms; Corrosion resistance; Corrosion resistant steels; Cracking (fracturing); Delta ferrite; Dissimilar metals; Filler metals; Galvanic corrosion; Hydrochloric acid; Laboratories; Matching; Metals; Microscopy; Microstructure; Molybdenum; Morphology; Nickel; Solidification; Stainless steel; Weld metal; Welded joints

The obvious example is a dissimilar weld metal couple, which is prone to galvanic corrosion between different weld metal and base metal.1 The weld metal itself may offer a microscopic galvanic effect due to microstructural segregation and partitioning of elements resulting from heating cycle during welding.2 Micro-segregation of Cr and Mo during solidification and cooling in UNS S31600 type weld metals has been studied by a number of investigators.3"5 It has been also reported that the corrosion resistance of welds made of matching filler metals is inferior to that of the base metal.6 Especially, as commonly employed in industrial practice, 2 to 10% delta-ferrite is considered as a critical area fraction range to avoid solidification cracking and micro-fissuring in weld metals.7 Although the range of delta-ferrite in weld metal is within the critical range, delta-ferrite still undergoes corrosion attack in a certain corrosive media, such as hydrochloric acid, leading to component failures.8 Hence, the behavior of preferential weld metal corrosion in the specific environment, i.e. aerated hydrochloric acid solution, is not fully understood, therefore, it is necessary to investigate the origin of preferential corrosion in weld metal within 2-10% delta-ferrite. [...]during service, the temperature near welded joint had cooled down by accident. [...]liquid hydrochloric acid was ultimately formed by the condensation of water, which led to aggressive corrosion. Considering the contents of Ni, Cr and Mo in the base and weld metal, it was clear that the weld metal was made of matching filler metal of UNS S31600. [...]it was concluded that the preferential weld metal corrosion was not caused by macroscopic galvanic corrosion between base metal and weld metal. Considering the fact that a delta-ferrite fraction ranging from 2% to 10% is recommended to avoid hot cracking in the welded austenitic stainless steels, it shows typical microstructure of UNS S31600 weld metal in terms of deltaferrite fraction. [...]it was necessary to find out the mechanism of delta-ferrite corrosion in order to understand the preferential weld metal corrosion.