Stress Corrosion Performance of Incoloy 825 Alloy in High Temperature and High Pressure H2S/CO2 Environment (HY-industry technical centre)
Stress corrosion cracking (SSCC) in the wet H2S medium is manifested in each device of the refinery, but it is more prominent in the desulfurization device and the exhaust gas desulfurization device. Corrosion resistant alloys represented by Incoloy 825 alloy are widely used in equipment and pipeline systems in the above-mentioned media to resist stress corrosion (SSCC) caused by hydrogen sulfide, which is very necessary for the safety of equipment operation.
The test plate adopts EF+AOD+ESR smelting process, and the test material specification is Φ266mm×800mm. On a two-ton hammer, a slab of 40mm thick is formed, rolled into a 5mm plate at 1160°C, annealed at 940°C×1h, and then cold rolled into a 2mm thin plate, and then water-cooled at 940°C×1h. A comparative test was carried out using European-sourcing panels.
Sample size: 70×10×(1.5～1.9)mm, polished to 1000# step by step with SiC water sandpaper, cleaned with distilled water, and degreasing with acetone. The H2S/CO2 corrosion test was carried out using a 25MPa Hastelloy alloy dynamic high temperature autoclave, the solution medium was 15% NaCl aqueous solution, and 1g/l elemental sulfur was added to the solution, the test temperature was 175℃, the H2S and CO2 were 3.5MPa each, and the test time It is 720h (30d). Before the experiment, pass high-purity nitrogen for 2h at room temperature to deoxygenate, then pass in H2S gas to reach saturation, raise the temperature to the target temperature to ensure that the H2S partial pressure is 3.5MPa, cool to room temperature, pass in CO2 gas, Then the temperature is raised to the target temperature, so that the partial pressure of CO2 reaches 3.5MPa and the total partial pressure reaches 7MPa. Make sure that the partial pressure of H2S and CO2 gas reaches 3.5MPa at the target temperature.
Corrosion test according to GB/T15970.2-2000 “Corrosion stress corrosion test of metals and alloys Part 2: Preparation and application of curved beam specimens”; metallographic observation, scanning electron microscope observation and composition analysis, surface passivation are carried out as required Membrane structure analysis. The results showed that:
Incoloy 825 sheet in high temperature autoclave H2S/CO2 medium, 30d corrosion at 170℃ is very slight, the corrosion layer thickens only tens of microns, under stress, does not cause cracking, and has good SSCC resistance. , The performance of self-made and imported board is similar.
Incoloy 825 alloy can be self-passivated in the air, and its passivation film is due to the composition of Cr2O3 and NiO. The thickness of the passivation film is greater than 1.5 nm.
In the complex medium under high temperature and high pressure, the passivation film is damaged and loses its protective effect. The basic mechanism is the dissolution of the passivation in the Cl-ion aqueous solution, and the rapid diffusion of high temperature S forms sulfide, which also destroys the dense oxide. At the same time, because Incoloy825 alloy contains high Cr, Ni and the combined effect of more Mo and Cu, and at the same time, because the temperature is relatively low (170℃), the sulfide has little effect, so only a very thin corrosion layer is formed without causing Obvious cracks, so it has little effect on the mechanical properties of Incoloy 825 alloy.