Inconel 625 is a popular nickel based superalloy for its durability, fabricability, weldability and excellent corrosion resistance. Its supreme and versatile corrosion resistance under diverse temperature and corrosive conditions is the main reason for its wide acceptance in chemical and other plant applications.

Inconel alloy 625 is an austenitic solid solution strengthened NiCrMo alloy that can withstand diverse corrosive conditions. In the mild conditions like ambient atmosphere, fresh and brackish water, neutral salts, alkaline media, it is immune to corrosion. In the vigorously severe corrosive conditions, chromium resists oxidizing chemicals whilst high nickel and molybdenum concentrations make this alloy extremely resistant to pitting and crevice attack. Niobium present in this alloy stabilizes it against sensitization during welding, hence preventing after intergranular corrosion. Additionally high nickel concentration prevents chloride induced stress corrosion cracking. These corrosion resistant properties of this alloy have made it one of the commonly used superalloy in the chemical and aerospace industries.

Versatility of Inconel 625 as a significant material in different corrosive media is evaluated.

Uniform Corrosion

Uniform corrosion also referred as corrosion attack is uniformly distributed corrosion that excels at about the same rate throughout the metal surface. Alloy 625 is one of the most resistant alloys to uniform corrosion, it can be used in both non-oxidizing acids and oxidizing acidic conditions.

Localized corrosion

Pitting attack is a type of confined corrosion, it is related to ppm chloride concentration and pH. Inconel 625 offers superior performance against localized corrosion. Chromium and molybdenum elements offer beneficial performance in enhancing pitting resistance. Inconel 625 contains high chromium and molybdenum concentrations that are responsible for its excellent pitting resistance. Inconel 625 and Hastelloy C276 possess high concentrations of chromium and molybdenum, hence they show the best performance in pitting conditions.

Chloride stress corrosion cracking

Nickel and nickel based alloys are not prone to chloride stress corrosion cracking unlike to stainless steels. The advantageous effect of nickel on resisting stress corrosion cracking in boiling 42% MgCl2 is significant. It is found that alloys containing above 42 -45% nickel resist corrosion in this condition. Inconel 625 contains 58% nickel, hence it is not affected. Stress corrosion cracking conditions that can affect alloy 625 are hot water and chlorides with other conditions causing cracking. Inconel 625 and other alloys were evaluated in an accelerated wet oxidation condition that comprised of a synthetic sewage with chlorides and fluorides, pH 3.2 and at temperature 292oC. Performance of Inconel 625 wire containing more than 34% nickel is outstanding against stress corrosion cracking in this condition.

Sulfide stress cracking

Sulfide stress cracking is an issue in oil and gas plants. It often causes brittle failure of material due to stress in water contaminated with hydrogen sulfide. The tests were performed at room temperature under constant load in a mixture of oxygen free water comprising of 3000 ppm dissolved H2S, 5% NaCl and 0.5% acetic acid. The threshold stress of Inconel 625 is approximate to the yield strength in annealed and cold rolled conditions.