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Localized Corrosion of UNS S31603 in Aqueous Ammonium Chloride Environments at Elevated Temperatures and Low Dissolved Oxygen

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Ammonium Chloride (NH4Cl) corrosion is prevalent in the refinery and petrochemical industries and its prediction, monitoring and control has been well documented. In Chemical plants, when NH3 and HCl are present in a gas-mixture and the temperature drops, NH4Cl can desublime. In the presence of water-vapor, NH4Cl desublime within components like heat-exchanger tubes, creating saturated wet salt deposits, causing general and localized corrosion.

Product Number: 51323-19125-SG
Author: Guru Prasad Sundararajan, Nicholas J. Laycock, Nikhil Amin, Wouter Hamer, Sami Ahmed
Publication Date: 2023
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This work assesses the extent of localized corrosion of UNS S31603 (316L) in ammonium chloride solutions at low dissolved oxygen concentrations that could be a precursor to Stress Corrosion Cracking (SCC). The localized corrosion testing for UNS S31603 were conducted under de-aerated conditions with a maximum dissolved oxygen content of ~ 25 ppb in NH4Cl solutions of concentrations ranging from 5 wt.% to 40 wt.% (saturation) at temperatures of 65°C, 80°C and 95°C. Open-circuit potential transients were measured over a 48-72 hr. period under de-aerated conditions and Repassivation potentials of UNS S31603 were determined using Tsujikawa-Hisamatsu Electrochemical method based on ASTM G192 standard. It was seen that UNS S31603 is susceptible to pitting corrosion in NH4Cl solutions even under very low DO concentrations (~ 1 ppb) and were corroborated by pit-density and depth measurements through optical microscopy. The Galvele criteria of stable pits based on corrosion current agreed with measured pit-depths and repassivation abilities. The implications of pit size and shape on SCC propensity of UNS S31603 alloy due a thin film electrolyte formed under moist NH4Cl deposits have been discussed.

This work assesses the extent of localized corrosion of UNS S31603 (316L) in ammonium chloride solutions at low dissolved oxygen concentrations that could be a precursor to Stress Corrosion Cracking (SCC). The localized corrosion testing for UNS S31603 were conducted under de-aerated conditions with a maximum dissolved oxygen content of ~ 25 ppb in NH4Cl solutions of concentrations ranging from 5 wt.% to 40 wt.% (saturation) at temperatures of 65°C, 80°C and 95°C. Open-circuit potential transients were measured over a 48-72 hr. period under de-aerated conditions and Repassivation potentials of UNS S31603 were determined using Tsujikawa-Hisamatsu Electrochemical method based on ASTM G192 standard. It was seen that UNS S31603 is susceptible to pitting corrosion in NH4Cl solutions even under very low DO concentrations (~ 1 ppb) and were corroborated by pit-density and depth measurements through optical microscopy. The Galvele criteria of stable pits based on corrosion current agreed with measured pit-depths and repassivation abilities. The implications of pit size and shape on SCC propensity of UNS S31603 alloy due a thin film electrolyte formed under moist NH4Cl deposits have been discussed.

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