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Super Duplex Can Corrode Source: LMATS (Laboratories for Materials Advanced Testing Services) This short investigation demonstrates how poor heattreatment during initial manufacture can still have detrimental effects years later.
at various positions around the circumference. The intensity of the metal loss was evident, with very deep pitting and severe loss of material.
Introduction
Macrosections were cut through some of the areas of corrosion, which revealed that extensive corrosion had propagated the whole depth of the sections.
Leaking was found to be occurring from the ends of super duplex (UNS S32750) pipes. It was reported that the pipes had been in operation for 10 years and the pipework was connected together using mechanical pressure joints. The affected material was similar to material that had been in operation for two years longer under the same conditions, which showed none of these failures. Two rings, which had been cut from the ends of two 425mm diameter pipes, were supplied for examination. The reported operating conditions for both rings were: • Pressure of 60Ba • Temperature of 13-180oC • Approximate pH of 8.1 Ring 1 carried brine with a salinity of 51,200ppm. Ring 2 carried seawater with a salinity of 32,640 to 34,560ppm. Ring 1 – Brine Severe corrosion was found on the end face of the supplied ring in three positions around the circumference. The rest of the end face was relatively unaffected, but red rust staining was prevalent. The intensity of the metal loss showed that the metal of the ring had practically disintegrated.
Macrosections though the Rings showing the extensive internal corrosion.
Hardness, Chemical and % Ferrite The chemical analysis for both samples met the requirements of UNS S32750, a superduplex stainless steel except for the slightly lower Nitrogen values. Both meet the requirements of NORSOK M-630 (MDS D51/D52(5)) for a PREN > 40. PREN = CR% + (3.3 x Mo%) + (16 x N%) 2251/01 = 24.4 + (3.3 x 4.1) + (16 x 0.15) = 40.33
The as received Ring 1 showing extensive corrosion on the end face.
Close up of Ring 1 showing extensive corrosion on the end face. The yellow paint marks the position of the microsection.
Ring 2 - Seawater Severe corrosion was also found on the end face of the Ring 2
2251/02 = 24.7 + (3.3 x 4.2) + (16 x 0.21) = 41.93 The percentage ferrite was measured using a Ficher Ferritscope. The percentage ferrite met the requirements of DNV-OS-F101 with ferrite content of 35 – 55%. The hardness values of 255Hv (Ring 1) and 258Hv (Ring 2) met the requirements for UNS 32750 of < 326Hv in the solution annealed condition. ASTM G48 Corrosion Test The corrosion test, ASTM G48 Method A, was conducted at 500oC for 24 hours. An acceptance criteria of a maximum weight loss of 4g/m2 is specified in DNV-OS -F101 for superduplex stainless steels. Pitting was observed in both samples after testing with weight losses of 867g/m2 for Ring 1, and 670g/m2 for Ring 2.
The as received Ring 2 showing corrosion on the end face.
38 | JUNE 2022
Close up of Ring 2 showing corrosion on the end face.
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