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Stress Corrosion Cracking (SCC) and Electrochemical Corrosion Study of Cold Formed Alloy 625 (UNS N06625) In Extremely Low pH and High Chloride Conditions

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Alloy 625 (UNS N06625) is an austenitic solid solution strengthened nickel-chromium-molybdenum alloy containing niobium. The high alloy content of alloy 625 enables it to withstand a wide variety of severe corrosive environments. In mild environments, such as ambient atmosphere, fresh and seawater, neutral salts and alkaline media, there is almost no attack.

Product Number: 51322-17782-SG
Author: Suresh Divi, Sri Krishna Chimbli
Publication Date: 2022
Industries: Corrosion Monitoring and Control , Coatings and Linings , Coatings
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Corrosion-resistant alloy (CRA) such as alloy 625 (UNS N06625) provides excellent corrosion resistance and chloride stress-corrosion cracking (SCC) resistance than stainless steels in many corrosive environments. Literature shows that under annealed condition alloy 625 is immune to chloride stress-corrosion cracking in high chloride environments and provide excellent localized corrosion resistance in high chloride concentrations. However, the literature shows that when the alloy is heavily cold formed it could become more sensitive to high chloride levels and susceptible to SCC.

In order to understand the sensitivity of alloy 625 under cold-worked conditions, SCC tests were performed on annealed and cold-formed alloy 625 in boiling high concentrations (50,000 ppm and 80,000 ppm) of chlorides (NaCl) and at very low pH (1.0). Electrochemical, linear polarization resistance (LPR) and cyclic potentiodynamic polarization (CPP) scans at 40oC were also performed to study the corrosion and passivation behavior under the above-mentioned chloride concentrations.

The test results show that under cold-formed conditions alloy 625 is not susceptible to SCC in very high concentrations (50,000 ppm and 80,000 ppm) of chlorides and under low pH (1.0) conditions and CPP scans at 40oC demonstrates that under cold-formed condition, the alloy 625 is not susceptible to pitting corrosion.

Corrosion-resistant alloy (CRA) such as alloy 625 (UNS N06625) provides excellent corrosion resistance and chloride stress-corrosion cracking (SCC) resistance than stainless steels in many corrosive environments. Literature shows that under annealed condition alloy 625 is immune to chloride stress-corrosion cracking in high chloride environments and provide excellent localized corrosion resistance in high chloride concentrations. However, the literature shows that when the alloy is heavily cold formed it could become more sensitive to high chloride levels and susceptible to SCC.

In order to understand the sensitivity of alloy 625 under cold-worked conditions, SCC tests were performed on annealed and cold-formed alloy 625 in boiling high concentrations (50,000 ppm and 80,000 ppm) of chlorides (NaCl) and at very low pH (1.0). Electrochemical, linear polarization resistance (LPR) and cyclic potentiodynamic polarization (CPP) scans at 40oC were also performed to study the corrosion and passivation behavior under the above-mentioned chloride concentrations.

The test results show that under cold-formed conditions alloy 625 is not susceptible to SCC in very high concentrations (50,000 ppm and 80,000 ppm) of chlorides and under low pH (1.0) conditions and CPP scans at 40oC demonstrates that under cold-formed condition, the alloy 625 is not susceptible to pitting corrosion.

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Stress relaxation cracking (SRC) is a failure mechanism known to occur in austenitic stainless steels and nickel alloys operating at moderate to high temperatures.   

Typically, SRC failures tend to occur under the following conditions: 1-6  1. Susceptible material: 800H, 347H, 617, etc. (typically materials with low creep ductility)  2. High residual stresses: Hardness > 200 HV (welded thick section) 3. Specific temperature range: usually between 500 °C (932 °F) and 750 °C (1382 °F). 

Under these conditions, component stresses are relieved by time dependent inelastic deformation.3 In susceptible materials, this process occurs by intergranular cracking and is essentially a creep mechanism.2-6 In this respect, materials with low creep ductility tend to be prone to this type of damage mechanism. On the other hand, materials that have good creep ductility can tolerate the inelastic strains due to relaxation without cracking.3 
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