Conventional 18Cr-8Ni austenitic stainless steels with high molybdenum content such as UNS S31603 (316L) and UNS 31703 (317L) are effective for use in most naphthenic acid corrosion (NAC) environments. However, type 316L or 317L have the risk to polythionic acid stress corrosion cracking (PTA SCC) due to sensitization during long-term service at high temperatures. Although the stabilized austenitic stainless steel such as UNS S34700 (347) or UNS S32100 (321) resist sensitization up to higher temperatures, especially with post weld heat treatment (PWHT), they have less NAC resistance than type 316L and 317L.

In this study, the effect of Mo, Cu and W contents in stainless steels on both NAC and PTA SCC resistance are investigated. The purpose was to optimize a proprietary version of UNS S34751 (TP347LN) with excellent PTA SCC resistance. This research found that adding 3% of Mo to TP347LN provided good resistance to NAC and PTA SCC at the thickness tested without PWHT and neutralization treatments.


The usage ratio of the crude containing high total acid number (TAN) continues to increase by state- of-art crude refining technologies. These crudes can cause NAC in crude oil refinery piping and equipment operating at high temperatures. Typical locations are atmospheric distillation units, vacuum distillation units and some downstream units. It has been known that increasing the Mo content is effective to improve NAC resistance.1

The protective mechanisms to NAC by increasing Mo content are reported in previous papers.2 Although Mo is homologous to Cr (both are Group VI elements), the metal activity of Mo is weaker than that of Cr, so the affinity of Mo with the RCOO- dissociated ingredient from naphthenic acid is much weaker than Fe, and even Cr. Therefore increasing Mo content will reduce the active spots of Fe in the steels that affect the NAC resistance of steels. However, few research studies have been carried out regarding the effect of W, which is also a homologous to Cr and Mo, on 18Cr-8Ni stainless steels (SS).

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