High-temperature aqueous corrosion of Alloy 400: Influence of pH and dissolved oxygen

This study investigates the high-temperature aqueous corrosion of Alloy 400 using immersion and electrochemical experiments at 200 °C across a range of pH values under both deaerated and dissolved oxygen (DO)-containing conditions. Under DO-containing conditions, the highest corrosion rate occurred at pH 200 °C =1.1 (>1000 µm/year), where nickel dealloying resulted in a porous copper-rich layer (~20 µm thick), and an underlying intergranular attack (IGA) was detected. At pH 200 °C =5.4 (mildly acidic at 200 °C), corrosion was less severe but remained dominated by IGA. Combining electrochemical measurements performed at 200 °C with E–pH diagrams constructed at the same temperature showed that in mildly acidic solutions, DO shifts the corrosion potential to more noble values, promoting selective nickel dissolution along grain boundaries (IGA). At pH 200 °C =6.6 (mildly alkaline at 200 °C), the alloy showed the greatest resistance (18.4 µm/year), attributed to the stability of Ni(OH) 2 and Cu 2 O, with no detectable signs of corrosion. At pH 200 °C =10, corrosion rate increased, characterized by copper dissolution and the formation of NiO/Ni(OH) 2 . The results emphasize the critical role of pH and DO in governing the corrosion behavior of Alloy 400 in high-temperature aqueous solutions, with mildly alkaline environments minimizing degradation. Alloy 400 shows no sign of corrosion in a dissolved oxygen-containing 0.1 M sodium acetate solution at pH 200 °C =6.6 and 200 °C, whereas under highly acidic pH it undergoes nickel dealloying, and under highly alkaline pH it shows copper dissolution. • Aqueous corrosion of Alloy 400 was investigated across a wide pH range at 200 °C. • Nickel dealloying was observed at pH 200 °C =1.1 under oxygenated conditions. • Sulfate reduction to sulfide and metal-sulfide formation occurred at pH 200 °C =1.1. • The best corrosion resistance was obtained at mildly alkaline environment. • Copper dissolution occurred at oxygenated highly alkaline pH.