Inconel alloy 625 performance in aqueous corrosion media

Inconel alloy 625 is an austenitic solid solution strengthened NiCrMo alloy comprising of niobium. The high concentration in this alloy enables it to withstand a wide range of vigorous corrosive conditions. In mild conditions for example ambient condition, fresh and seawater, neutral salts and alkaline media, it is resistant to corrosion. In more severe corrosive media, chromium offers resistance to oxidizing media whilst the high nickel and molybdenum concentrations make it resistant to nonoxidizing conditions.

High molybdenum content also makes alloy Inconel 625 wire highly resistant to pitting and crevice corrosion. Added niobium acts to stabilize the alloy against sensitization during welding, hence preventing subsequent intergranular corrosion. Additionally, high nickel concentration prevents chloride ion stress corrosion cracking. These characteristics make this alloy one of the most popularly used material in chemical and aerospace industries.

Aqueous Corrosion

Uniform Corrosion

Uniform corrosion is a type of corrosion that is evenly distributed and proceed at about the same rate over the whole meal surface. Tess are made to evaluate the alloy’s resistance to uniform corrosion primarily aids in the selection of materials. These tests are normally conducted in different acids to take into account various situations that may occur. Some alloys are good in non-oxidizing acids may not be good in oxidizing acids. Although Inconel alloy 625 is among the most resistant alloys to uniform corrosion and can be used in the non-oxidizing acids and oxidizing conditions.

The applicability of Inconel 625 corrodes at a rate of less than 20 mpy in all concentrations of sulfuric acid at different temperatures.

Localized Corrosion

Pitting is a localized form of corrosion confined to a small region compared with the whole surface. Pitting and crevice corrosion or localized corrosion data to ppm chloride concentration.

Chromium and molybdenum have been found to be advantageous in enhancing pitting resistance. Inconel alloy 625 has a high chromium and molybdenum concentration accounting for its superior pitting resistance. An empirical relationship between the critical temperature for pitting to begin and chromium and molybdenum contents of alloy. Inconel alloy 625 and Hastelloy C276 offer high percentages of chromium and molybdenum and hence rank the highest for pitting resistance is shown.

Chloride stress corrosion cracking

Nickel and nickel base alloys are in general not subjected to stress corrosion cracking. Alloys containing more than 42 to 45% nickel will not crack in this condition. Inconel alloy 625 contains 58% nickel hence it will not crack in this condition. Among the conditions in which stress corrosion cracking can occur in Inconel alloy 625 are high temperature water and chlorides. Inconel alloy 625 and different alloys were analyzed in an accelerated wet oxidation condition that comprised of a synthetic sewage with chlorides and fluorides, pH 3.2 and a temperature of 292oC or 560oF. Inconel alloy 625 containing more than 34% nickel is highly resistant to stress corrosion cracking in this condition.

Sulfide stress cracking

Sulfide stress cracking is an issue in the oil and gas industry It often results in a brittle failure of material in stress in water contaminated with hydrogen sulfide. Inconel 625 offers significant performance in this condition.