Hikozô Endô1, Susumu Morioka1
To clarify the cause of the beneficial effect of copper on the corrosion of steel in a non-oxidizing acid solution, the corrosion test for 12 kinds of copper bearing steels containing 0.03 percent carbon and 0.07-4.75 percent copper was carried out with sulphuric acid solution of various concentrations (1-98%H2SO4). The results obtained are as follows:- (1) In the initial period (untill 20 hours in the present experiment), the rate of corrosion of steels is markedly depressed by an addition of about 0.1-0.5% Cu with varying strength of acid (up to 55% H2SO4), and the addition of copper above these amount has little influence. (2) The steels containing above 0.8% Cu show a remarkable increase in the rate of corrosion with increasing time of testing, and hence a minimum point in corrosibility are shown at about 0.5% Cu after prolonged period (after 3 days in the present experiment). (3) The relation between the corrosibility and strength of acid shows that the steels containing a very small amount of copper (up to about 0.15% Cu) increase the rate of corrosion almost proportional to the strength of acid and time of testing, showing a maximum rate at 50% H2SO4, while the steels containing above 0.3% Cu show a slight increase in the rate of corrosion up to about 40% H2SO4 and above this concentration the increase is fairly great showing a maximum rate at 50% H2SO4 in prolonged period; this maximum rate at 50% H2SO4 is not observed in the initial period. (4) In concentrated sulphuric acid solution above 60% H2SO4, in which the mechanism of corrosion of steels differs from that in the weaker solutions, the beneficial effect of copper is not observed.
The above results are explained by an assumption of high over voltage of the Fe-Cu solid solution for hydrogen and the redeposition of copper and its state in the course of corrosion.
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