Inconel 625’s performance in wet corrosive conditions

Inconel alloy 625 is known for its high strength, outstanding fabricabiity and weldability and supreme corrosion resistance. The great and versatile corrosion resistance of alloy 625 in a wide range of temperatures and corrosive conditions is the major reason for its wide applications in chemical processing and other processes. This post shows the performance of alloy 625 in various wet conditions.

Hydrogen Embrittlement

The degree of sensitivity to hydrogen embrittlement is measured as ductility loss. Inconel 625 holds moderate ranking as compare to Incooloy 800, SS 316, SS 304, In 825, In X750, In 718 and Monel 400.

Intergranular corrosion

Inconel alloy 625 is stabilized against intergranular corrosion by precipitation of niobium carbides at 927oC to 1038oC annealing temperature. Niobium carbides interact with carbon to make it less available to precipitate as chromium carbides in the grain boundaries. Chromium depletion near the grain boundaries is an effect of intergranular chromium carbide precipitation. This process occurs at specific temperatures and is called as sensitization. Sensitization of an alloy makes it sensitive to intergranular corrosion. Inconel 625 can become sensitive to this corrosion by an inadequate annealing that would prevent the development of niobium carbides by subsequent sensitizing treatment. Corrosion occurs by intergranular attack of Inconel 625 due to sensitization of the grain boundaries.

Annealing samples at different temperatures followed by subjecting them to sensitizing temperatures in the range of 704oC to 871oC for one hour. The samples are tested in boiling 65% nitric acid allowing the precipitation of chromium carbides at the grain boundaries when an alloy is exposed to sensitizing temperatures of 704oC to 871oC. At lower annealing temperatures, stabilizing niobium carbides are precipitated, hence a subsequent sensitizing heat processing of 704oC to 871oC will result into nominal or no precipitation of chromium carbides at the grain boundaries.

Corrosion in bleach plant condition

Pulp and paper bleach plant conditions are studies. Exposures in solutions varying from pH 1.4 to 9.5 with up to 5500 ppm chlorides and 80oC with strong chlorine base oxidizers present were made on 38 samples of 26 different materials. The test included 8 chlorination stage, nine chloride dioxide stage and three hypochlorite stage bleach plant conditions. Inconel 625 ranks high in offering corrosion resistance in bleach plant conditions. High chromium and molybdenum concentration is responsible for Inconel 625’s pitting resistance in this condition.

Marine water corrosion

Inconel alloy 625 stands high in sea water conditions owing again  as it contains high chromium and molybdenum concentration.

The most corrosion resistant alloys are – Hastelloy C, super alloy Hastelloy C276 wire, Inconel 625 and Hastelloy X

Very resistant alloys are – Inconel 718

Alloys resistant while receiving some pitting are – Inconel 600, Inconel X750, Incoloy 800, Incoloy 825, Monel 400 and K500.

Inconel alloy 625 often acts as a cathode when placed in contact with other materials in sea water. Alloy 625 offers good corrosion fatigue strength in sea water.