SCC Mechanism Near Fusion Line of Low C-13%Cr Welded Joints

Abstract

Low Carbon-13%Cr martensitic stainless steels have been widely used for line pipe application because of their high strength and excellent corrosion resistance in corrosive conditions. Recently, both laboratory and field experiences related to cracking near fusion line of these steels weld joints in hot acid environments have been published. In this paper, SCC (Stress Corrosion Cracking) mechanism near fusion line of low C-13%Cr welded joints is discussed. Especially, initiation process is focused. Mainly electrochemical measurement using solution flow type micro-droplet cell and surface analysis of weld joints are conducted. In the evaluation of electrochemical behavior of low C-13%Cr welded joints with and without PWHT (Post Welded Heat Treatment) by the use of a solution flow type micro-droplet cell, PWHT leads to more noble and stable potential in HAZ (Heat Affected Zone) compared to as-welded condition. In the HAZ portion of the as-welded joint, Cr depleted layer was detected under welding scale with the use of GDS (Glow Discharge Spectrometer). PWHT was confirmed to eliminate Cr depleted layer under welding scale. This can be an initiation of SCC near girth welded joint in hot acid environment. Finally, mechanism of SCC initiation near fusion line of as-welded joint was proposed. Localized corrosion would start at Cr depleted layer under welding oxide scale. This dissolution was also accelerated by a galvanic effect due to a large cathode area of base metal.