ABSTRACT

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.

INTRODUCTION

Ammonium Chloride (NH4Cl) corrosion is prevalent in the refinery and petrochemical industries and its prediction, monitoring and control has been well documented.1,2 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. The resultant deposits absorb water and form a saturated solution, which is acidic and highly corrosive until or unless it is diluted by bulk water condensation. Corrosion mitigation strategies for ammonium chloride corrosion of carbon-steel in the crude overhead units of refineries have been based on the use of amine (neutralizing) and filming corrosion inhibitors.3 Only a few researchers have published the effect of NH4Cl on localized corrosion of CRAs like stainless steels or Ni-base alloys. Farsen et. al4 studied the localized corrosion of stainless steels using electrochemical measurements in aqueous NH4Cl environments with different chloride concentrations and temperatures. They found that based on cyclic potentiodynamic polarization (CPP) curves, Duplex Stainless steels UNS S32750 and UNS S31254 are corrosion resistant to concentrated ammonium chloride solutions at elevated temperatures compared to austenitic stainless steels such as UNS S31803 and UNS S31726. Bastidas et. al5 investigated the localized corrosion performance of highly alloyed stainless steels and Titanium in aqueous ammonium chloride and diethylene amine solutions at elevated temperatures based on gravimetric, polarization and impedance techniques. The ranking of all the studied alloys were based on all these methods and Titanium exhibited the highest corrosion resistance followed by UNS S31254 and UNS S 32550 and the least corrosion resistance was seen in UNS S31726 alloy. Toba et. al6 studied the effect of relative humidity on NH4Cl deposit corrosion at 80°C of eight types of alloys ranging from Carbon steel, Stainless steels to Nickel-base alloys and Grade 2 Titanium. The authors observed based on NH4Cl particle salt, there was remarkable corrosion at 50% or 60% RH on all the alloys except Alloy UNS N10276. These authors also studied the corrosion of carbon steel and various stainless steels alloys in concentrated ammonium chloride solutions at their boiling point and observed that alloys with PREN > 40 were resistant to pitting and cracking.7 Yang et. al8 studied the corrosion of SS and Ni-base alloys in supercritical water at 400°C containing 6000 ppm of NH4Cl under de-aerated (negligible dissolved oxygen) conditions and found that UNS S31600 showed severe general and intergranular corrosion and UNS N06625 alloy exhibited negligible general corrosion due to tenacious Cr-oxides formed, but however exhibited pitting corrosion.

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