316L filtrate header pitting in a D0 washer
A D0-stage washer filtrate header, 6-inch schedule 40, 316L, carries 120 m3/h at 80 C, pH 2.5, chloride 650 mg/L, and 200 mg/L residual ClO2. After 18 months, internal inspection finds shallow pitting under a 4-mm-thick deposit and one 4-mm-deep pit in the weld heat-affected zone. The line is isolated and can be replaced in a 3-day window; 2205 duplex is in the warehouse, while 904L would take six weeks. The mill has a 316L spool that could be installed now as a temporary patch. A post-inspection pressure test held at 1.5 times design with no leakage. Operations wants to return to service for six months until the annual outage. The line has a history of two similar pits in the same service, and the mill corrosion engineer is on leave.
1. Identify the likely damage mechanism and explain why a 316L patch is not acceptable.
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The combination of pH 2.5, 650 mg/L chloride, 80 C, and residual ClO2 is aggressive to 316L. ClO2 raises the corrosion potential and promotes pitting; the deposit creates a crevice with a low-pH, high-chloride solution. The 4-mm pit in the weld HAZ is consistent with pitting or crevice corrosion and possibly chloride stress-corrosion cracking initiation; at 80 C, 316L is above its practical chloride-SCC threshold. A hydrotest at 1.5 times design only proves no gross leak; it does not prove the pit is not a crack or that the remaining wall is adequate. A 316L spool would repeat the failure. The prior pits confirm the service is outside 316L's range.
2. Recommend the material and installation scope, including welding and quality controls.
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Install 2205 duplex now because it is available and has a PREN near 35, well above 316L's roughly 25; it also has higher chloride-SCC resistance and yield strength. Verify against the mill corrosion chart for 650 mg/L chloride at 80 C; if the chart flags SCC risk, use 904L or a lined pipe instead. Use ER2594 filler, a qualified WPS, heat input 0.5-1.5 kJ/mm, interpass below 150 C, no preheat, purged root, and check ferrite 35-55%. Do not use 316L as a temporary patch. Replace the full spool from flange to flange, including the pit in the HAZ; a local weld patch would leave the cracked HAZ in service. Pickle and passivate the welds.
3. What non-destructive examination would you require before return to service, and would you accept the existing pit?
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PT the root and cap, UT or RT the butt welds, check ferrite, and take a hardness survey. For the existing pit, grind out and PT; if it extends as a crack, cut out the section. Do not accept the pit on the strength of a hydrotest alone. UT the wall thickness around the replacement and perform positive material identification. Require a weld map and a final visual check for crevices at supports and flanges. Because the pit is in a weld HAZ in a known aggressive service, the safe call is replacement of that spool, not blend-and-run.