When should I use Inconel 625 instead of Incoloy 825?

Alloy 825 (UNS N08825) is about 38 to 46% nickel with 2.5 to 3.5% molybdenum, 1.5 to 3% copper and a titanium stabiliser. The copper addition is specifically there for reducing acids — sulphuric and phosphoric — and the alloy resists stress corrosion cracking in chlorides far better than 316L at a fraction of the cost of 625.

Alloy 625 (UNS N06625) is 58% nickel minimum with 20 to 23% chromium, 8 to 10% molybdenum and 3.15 to 4.15% niobium. The niobium gives solid solution and precipitation strengthening, so 625 holds strength to 800 C and higher, resists seawater pitting and crevice attack almost completely, and handles oxidising acids and hot chlorine-bearing gas.

Choose 825 for: sulphuric acid pickling plant, phosphoric acid service, sour gas where cost matters, heat exchanger tubing in acid duty, and tank internals.

Choose 625 for: seawater and brine injection hardware, flare tips and hot gas components, bellows and expansion joints, downhole and subsea components, and anything that must carry load above 600 C.

For hydrogen sulphide service, both can be qualified to NACE MR0175 / ISO 15156, but the hardness and cold work limits differ — state the partial pressure of H2S, chloride level and temperature when enquiring and the grade can be confirmed against the standard rather than guessed.