| Abstrakt: |
Up to now, inadequate attention has been paid to the mechanism whereby metallurgical inhomogeneities in steels and electrochemical factors affect static corrosion and cyclic corrosion cracking under the conditions encountered in hot aqueous media. In particular, it has not been decided how inclusions of manganese sulfide affect the rates of crack growth in the medium. In this paper, measurements are reported on the dependence of the electrode potentials of freshly exposed and passivated surfaces on temperature in the range 20-300‡C and on the repassivation kinetics of freshly cleaned surfaces of 15Kh2NMFA and 08Kh18N10T steels. The potential of the manganese sulfide relative to a silver chloride reference electrode has been determined along with the differences in potential between the manganese sulfide and the surface of 15Kh2NMFA steel in the same temperature range. The potentials of the MnS inclusions relative to 15KhNMFA steel are negative up to 80‡C, but beyond that become positive. The height of the jump is about 300 mV. At 300‡C, the potential difference between the MnS and the steel is 390 mV. Consequently, the MnS inclusions on the steel surface act as hydrogenation sources. The hydrogen in the zone of prefailure arises from electrochemical reactions at the tip of the crack, and above 200‡C arises from the manganese sulfide, iron carbides, and other inclusions, which leads to weakening in the grain boundaries and the formation of an internal gas pressure in the micropores and thus facilitates the cracking. The differences in behavior and in physical models for the effects of inclusions on the cracking are examined for low- alloy steels at high and low temperatures. [ABSTRACT FROM AUTHOR] |
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