The possibility of replacing lighter alloys such as 7075-T6 aluminum alloy with gray cast iron was investigated in current research. 7075-T6 aluminum alloy can be replaced with gray cast iron used in hydrogen gas compression compressors due to its properties such as lightness, reducing the wear rate of rider rings, and increasing the durability of equipment. Recent research has indicated the sensitivity of 7075-T6 aluminum alloy to degradation caused by hydrogen penetration in wet conditions. In current research, the change of mechanical properties of 7075-T6 aluminum alloy due to its placement in hydrogen gas atmosphere at high pressure (about 30 bar) and temperature of 115 °C (using autoclave) has been investigated. Also, the effect of the presence of moisture, hydrochloric acid, hydrocarbon, and green oil (100 ppm) in hydrogen gas, as existing impurities, was eva‎luated. The results showed that hydrochloric acid leads to a slight drop in yield strength and ultimate tensile strength of the alloy, as well as a slight drop in alloy hardness (6 %). However, if the hydrocarbon was accompanied by acid, the negative effect of the acid is neutralized and the tensile parameter values were equal to the parameters obtained in the atmospheres of pure hydrogen gas and hydrogen gas containing water, which is equivalent to the reference conditions (argon gas). The presence of green oil in the hydrogen gas atmosphere does not affect the tensile properties of the alloy, but if this alloy is in direct contact with liquid green oil, the yield strength and ultimate tensile strength will decrease. The fatigue resistance of the alloy at high stresses (326 and 342 MPa) in the presence of pure hydrogen gas and also in the presence of impurities such as water, hydrochloric acid along with hydrocarbon compounds or direct contact with liquid green oil did not show significant changes, but at a higher stress value of 342 MPa and in the presence of only acid, a significant drop (45 % reduction) was observed in the tolerable cycle number of 7075-T6 alloy. By analyzing the results of mechanical tests and examining images of the fatigue fracture surface of 7075-T6 alloy, it was found that the mechanical properties of this alloy (except the compressive yield strength) after exposure to hydrogen atmosphere at the temperature of 115 °C are comparable with gray cast iron pistons. But due to the severe drop in the mechanical properties of this alloy at temperatures above 115 °C, which cannot be avoided in real conditions, especially the fatigue resistance at high stresses (drop 85 % at 150 °C), this alloy is not recommended to be replaced with gray cast iron in the piston of such compressors.

كيوان رئيسي

فخرالدين اشرفي زاده , احمد رضائيان