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 40°C 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 40°C demonstrates that under cold-formed condition, the alloy 625 is not susceptible to pitting corrosion.
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.1-3 In mild environments, such as ambient atmosphere, fresh and seawater, neutral salts and alkaline media, there is almost no attack. In more severe corrosive environments, chromium provides resistance to oxidizing chemicals, whereas the high nickel and molybdenum contents make the alloy resistant to nonoxidizing environments and also makes this alloy very resistant to pitting and crevice corrosion. The added niobium (3 to 4%) acts to stabilize the alloy against sensitization during welding, thereby preventing subsequent intergranular attack.
Nickel and nickel-base alloys such as alloy 625 are in general not subject to chloride stress-corrosion cracking SCC) as compared to stainless steels. High nickel content in alloy 625 is beneficial in resisting the SCC in high chloride aqueous environments.4 However, the SCC resistance in high chloride environments under cold formed conditions is not well known.