Corrosion behavior of Inconel 625 in various environments

Hydrogen Embrittlement

Inconel 625 is intact from intergranular corrosion due to precipitation of niobium carbides at 927oC to 1038oC annealing point. Niobium carbides hold carbon, 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 temperature limits called as sensitization.

Sensitization of an alloy makes it sensitive to intergranular corrosion. Inconel alloy 625 can be made sensitive to this type of corrosion through insufficient annealing that would prevent the development of niobium carbides hence availing more carbon to precipitate intergranularly as chromium carbides by a subsequent sensitizing treatment. Corrosion rate data in mpy produced from the intergranular corrosion of Inconel alloy 625 due to sensitization of the grain boundaries. It is received by annealing alloy samples at different temperature after subjecting them to sensitizing temperatures in the range of 704oC to 871oC for 60 minutes. The samples were then evaluated in boiling 65% nitric acid allowing the subsequent precipitation of Chromium carbides at the grain boundaries when the alloy is subjected to sensitizing temperatures of 704oC to 871oC or 1300oF to 1600oF. At lower annealing limits, stabilizing Nb carbides are precipitated, hence an after sensitizing heat processing of 704oC to 871oC will result into nominal or no precipitation of Chromium carbides at the grain boundaries.

Corrosion in Bleach Plants

Pulp and paper bleach plant corrosion is studied. Alloy is exposed to solutions of pH 1.4–9.5 containing 5500ppm chlorides and 80oC with strong chlorine base oxidizers present, were made on 38 samples. There were 8 chlorination stage, 9 chloride dioxide stage and three hypochlorite stage bleach plant conditions. High chromium and molybdenum concentration of Inconel alloy 625 is responsible for Inconel 625 wire ’s pitting resistance in this condition and pitting resistance is evaluated.

Marine water corrosion

Inconel 625 is found to be one of the best alloys that withstand in marine conditions for containing high chromium and molybdenum concentration. It often acts as cathode in presence of other materials in sea water. Alloy 625 shows good corrosion fatigue strength in sea water.

High temperature corrosion

However Inconel alloy 625 is a popular material for resisting aqueous corrosion and also prevent oxidation and scaling at high temperatures. An oxidation analysis was performed in air containing 5% water vapor running at the rate of 500 cc/min at 1000oC for 1008 hours. The water was added by bubbling air through water controlled at 33oC or 91oF to develop 5% water vapor. Periodically the sample were pulled of the furnace and weighed after quenching to room temperature to receive mass change following the time. It is found that Inconel 625 is comparable to Inconel 600 and Incoloy 800 and superior than Hastelloy C276 in oxidation at 1000oC.

A cyclic oxidation test is often performed on high temperature alloys to determine their scale adherence. The test conducted at 982oC in which the samples were heated to temperature in 15 minutes and quenched in air for 5 minutes. Samples were weighed after 100 cycles to receive the mass change. Inconel 625 offered outstanding resistance to cyclic oxidation.