The effect of temperature, pressure and impurities present in carbon dioxide gas on the corrosion of magnesium alloys for gas cooled reactors were studied. Test alloys were magnesium alloy with a small addition of aluminium and beryllium, i. e. so-called magnox AL-80 alloy.
The results of tests conducted in carbon dioxide gas at the temperature of 400-600°C, at the pressure of 1-100kg/cm
2, under the stress of 0-0.146kg/mm
2 and at the flow rate of 0-10
4cc/min, were summerized as follows:
1) The penetration rati of magnox alloy in static carbon dioxide gas was about 6-8×10
-4mm/year and 1-4×10
-3mm/year at a temperature of 400°C and 500°C respectively, and the rate increased with the increment of testing gas pressure from 1kg/cm
2 to 50kg/cm
2.
2) The activation energy of the oxidation reaction of the alloy in carbon dioxide at the pressure of 50kg/cm
2, calculated from an Arrhenius' plot of the experimental data, was 21, 580cal/mol.
3) The presence of oxygen, water vapour up to 2% and/ nitrogen up to 5% in carbon dioxide did not affect the corrosion behavior of the test specimen, however, the increase in corrosion rate and occurance of blister type corrosion was noted when the amount of water vapour exceeded 5% of carbon dioxide.
4) The formation of intergranular cavities or oxidation were not observed in the magnox alloy when heated in carbon dioxide gas at the temperature of 500°C even under stressed condition.
5) The corrosion rate of the alloy in flowing carbon dioxide at the flow rate of 100-500cc/min was about 2 times faster than that of in static gas. This effect cannot be solely attributed to the production of carbon monoxide during reaction, because the equibulium constant for the reaction of Mg+CO
2=MgO+CO at the temperature of 400°C is about 5.46×10
23 and hence the minimum percentage of CO
2 in CO+CO
2 mixture necessary to oxidize magnesium is estimated as only 5.5×10
-22%. It is rather suspected that the vaparization of magnesium may play a role for the increase of metal loss of specimens at a high flow rate.
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