Welded joints are often the weakest link in corrosion resistance. Three types dominate failures in piping and structural fabrication: galvanic corrosion, crevice corrosion, and stress corrosion cracking (SCC).
Galvanic corrosion occurs when dissimilar metals are welded, such as carbon steel to stainless steel. The more noble metal accelerates attack on the less noble one. Per NORSOK M-001, the area ratio of cathode to anode must be controlled to avoid rapid localized attack. Using buttering layers with Inconel 625 filler (ERNiCrMo-3) is a common mitigation.
Crevice corrosion thrives in tight spaces like incomplete fusion at weld roots or under deposits. For duplex stainless steels, the critical crevice temperature (CCT) is around 25-35?C in chloride environments. Per ASTM A923, acceptance testing for 22Cr and 25Cr duplex includes a ferric chloride corrosion test to verify resistance.
SCC is the most insidious. It requires tensile stress, a susceptible microstructure, and a corrosive environment. In carbon steel welds, residual stresses from welding often exceed yield strength locally. PWHT per ASME Section VIII Div 1 reduces residual stress below 20% of yield. For austenitic stainless steels, sensitized HAZ (chromium carbide precipitation) is the classic SCC trigger. Using low-carbon grades like 316L or stabilized grades (321, 347) prevents sensitization.
Source: NORSOK M-001
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