Duplex spool corrosion test failure after a purge change
A chemical plant fabricates eight 8-inch Sch 40 spools in UNS S32205 duplex stainless steel. The qualified procedure is GTAW for the root (ER2594, 2.4 mm) and GMAW for fill and cap, with a pure argon backing purge. During production the welder raises heat input to 2.6 kJ/mm to improve the bead profile and allows interpass temperature to reach 210 °C. The purge stays pure argon because the nitrogen cylinder was empty. Root ferrite readings are 60–65% by ferrite scope; the fill passes read about 30%. A production coupon tested to ASTM A923 Practice C gives a corrosion rate above the acceptance limit, and metallography shows chromium nitride precipitation in the HAZ. The spools are due for hydrotest in two weeks and eight girth welds have already been completed.
1. Interpret the ferrite readings and the corrosion failure. What is the root cause, and is it confined to the root?
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The evidence points to nitrogen loss combined with excessive heat input, not to the filler metal. A pure argon purge removes the nitrogen that stabilises austenite in duplex, so the root ferrite climbs to 60–65% instead of the 35–55% normally targeted. The high heat input of 2.6 kJ/mm and interpass at 210 °C then slow the cooling through roughly 700–1000 °C, where chromium nitrides and sigma phase form. Those chromium-rich phases deplete the surrounding matrix of chromium and molybdenum, which is exactly why the ASTM A923 Practice C corrosion rate fails even though the fill-pass ferrite of 30% looks acceptable on the gauge. It is not confined to the root: the HAZ of every pass made with this practice is exposed to the same thermal cycle, so the eight completed welds must be treated as suspect. A hydrotest will not detect loss of corrosion resistance; it only proves mechanical integrity. Note also that a ferrite scope reading is influenced by instrument calibration and by the phase balance at that location, so it is supporting evidence, not the primary finding.
2. What corrective actions would you take, and how would you dispose of the eight completed welds?
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Correct the procedure first, then deal with the product. Change the backing purge to argon with 2–5% nitrogen, or use a pre-nitrified root technique; set heat input in the 0.5–1.5 kJ/mm range; cap interpass at 150 °C and measure it with a contact thermometer, not by hand; and control ferrite with a calibrated instrument aiming for 35–55%. Re-qualify the WPS with the nitrogen purge and the revised heat input, and repeat ASTM A923 Practice A and Practice C on the new coupon so the corrosion resistance is demonstrated, not assumed. Do not add nitrogen blindly: excessive nitrogen can cause porosity and affect toughness, so the purge mixture should be justified on the qualification coupon. For the eight completed girth welds, the conservative disposition is to cut out and re-weld them, or at minimum to remove and replace the root and the affected HAZ, because the corrosion test failure is a property of the joint, not a surface condition. Re-test the replacements, then hydrotest. Add ferrite checks, heat-input recording and interpass checks to the inspection and test plan so this is controlled in production, not discovered at the coupon stage.