Hot corrosion behavior of super alloys

Different tests have been conducted to evaluate hot corrosion. Immersion testing that was the initial lab test method, is not considered reliable for simulating the gas turbine conditions. The salt coated method is very popular to study corrosion mechanisms. Engine manufacturers although use the burner rig test system to determine relative alloy functional ranking. The rig burns fuel with extensive air to develop combustion gases with continuous injection of a synthetic sea salt solution. This kind of test system shows the best lab system for simulating the gas turbine environment.

Hot corrosion resistance of different nickel and cobalt base alloys at temperatures from 870oC to 1040oC with 5ppm sea-salt injection. A good correlation between alloy functionality and chromium concentration has been seen. With increase in chromium content in alloy resistance to hot corrosion is improved. Alloys with 15% chromium or less are prone to hot corrosion. Cobalt based alloys are normally better than nickel based alloys. It may be because of higher chromium concentrations in cobalt base alloys. Hastelloy alloy wire with chromium content similar to cobalt base alloys was noticed to behave similarly to cobalt base alloys.

Burner rig tests were performed at 900oC or 1650oF on various wrought super alloys and nickel aluminides. The combustion gas stream was produced by using fuel oil comprising of 0.4 wt% sulfur with an air to fuel ratio of 35 : 1 and injection of 5 or 50 ppm sea salt in the combustion gas stream. The samples were loaded in a carousel that moved at 30 rpm during testing to ensure that all samples were subjected to the same test condition. The samples were cycled out of the combustion gas stream once every hour for two minutes, during which time the samples were quenched by forced air to less than 205oC or 400oF.

Superalloys analyzed were Hastelloy X, alloy 25 and alloy 150. The test results at 900oC for 200 hours with 50 ppm sea salt suffered extensive hot corrosion attack after 200 hours at 900oC with 50 ppm sea salt being injected into the combustion gas stream. Scanning electron microscropy with energy dispersive x- ray spectroscopy analysis described that nickel aluminides attained porous nickel or nickel rich oxides with nickel sulfide penetrating through the remaining metal. A secured chromium rich oxide layer was developed on alloy X.

Alloy 25 attaining nominal weight change, showed proof of initial cracking of chromium –rich oxide layer. SEM/EDX studies showed the development of cobalt-rich oxide nodules on the outer oxide layer on alloy 25. This showed the beginning of the breakway corrosion for alloy 25 after 200 hours at 900oC with 50 ppm sea salt. Prolong test outcomes under the same test condition clearly described that alloy 25 experienced extensive hot corrosion in excess of 200 hours of testing.

High temperature or hot corrosion normally occurs in the temperature limit of 800 to 950oC. It is trusted that molten sodium sulfate deposit is needed to begin hot corrosion attack. Another kind of hot corrosion is featured by pitting attack with little or no internal corrosion underneath the pit. Cobalt base alloys are more sensitive to such corrosion that normally includes Na2SO4 and CoSO4. Hastelloy X showed excellent performance in these media.