Corrosion Resistance of AISI 316 (UNS S31600) Stainless Steel at Near Deaerated-Seawater Systems under Stagnant and Dynamic Flow Conditions

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UNS S31600 is usually selected for pipes to be applied under conditions where cleanliness is an essential factor. As the lines around Sulfate Removal Unit (SRU) have this requirement (considering the membranes sensibility to any debris), it is common to find this alloy as the preferred one in the material selection criteria for systems like this in Floating Production Storage and Offloading units (FPSO) where the deaerator tower is upstream the SRU, with the role to limit the oxygen content up to 10 ppb in the seawater.

Although the selection of UNS S31600 agrees with ISO 21457 and NORSOK M-001 for this part of seawater treatment system (Figure 1), in 2019 several pits were noticed at pipes downstream the SRU in a FPSO being operated at Brazil Santos basin.

Product Number: 51323-19283-SG

Author: Gustavo Brandolin, Matheus Biava, Nathalia Mota, Daiana Souza, Gabriela Souza, Rodrigo Landim, Luis Rodrigues, Arthur Azevedo, Javier Velasco

Publication Date: 2023

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The detrimental effect of natural seawater on corrosion resistance of stainless steels has been studied over the years by the Oil and Gas industry. Based on past studies, it is known that oxygenated-seawater systems limit the material selection, where only high Pitting Resistance Equivalent number (PREN above 40) CRAs are technically feasible for such environments. Oxygen scavengers have been used to treat seawater at certain levels of dissolved oxygen (DO), allowing the use of UNS(1) S31600 steel on treatedseawater
systems.

The aim of this paper is to define practical safe limits for UNS S31600 (i.e., operating standpoint) of dissolved oxygen in seawater-carrying systems for flowing conditions, as well as identify the effectiveness of the proposed preservation solutions in stagnant flow under different bisulfite additions and periods of immersion. Immersion tests per ASTM G31, creviced and non-creviced samples, base metal and welded joints, were performed to evaluate the corrosion performance of the alloy under near-deaerated stagnant condition for 1 and 3 months, simulating the preservation of piping systems when not in normal operation. In addition, customized test under dynamic flowing condition in a piping loop were also performed in near-deaerated environment, simulating the operating scenario, where the parameters dissolved oxygen, pH and temperature were automatically monitored for 3 months (test duration). The results showed that localized corrosion was higher in dynamic flow condition in comparison to stagnant flow condition. However, crevice corrosion seems to be more detrimental to the alloy (especially for welded joints), considering specific test scenarios.