Several known methods of controlling the entry and exit of hydrogen in steel were correlated with the known chemical behavior of atomic and molecular hydrogen. Through this correlation, suitable reagents were found which increase or, for the first time, significantly decrease hydrogen permeability and embrittlement of SAE 4l30 and other steels during cathodic pickling and of spring steel during zinc and cadmium plating without lessening the apparent quality of the pickling or the plating. This successful correlation was an important step toward arriving at the mechanism of hydrogen entry into steel. The same data showed that the diffusion of hydrogen in steel and the freedom of exit of hydrogen are also important in determining the extent of embrittle¬ment. Heretofore, the separate importance of entry and exit has not been clearly recognized and subjected to independent control. The phenomena are chemical and not mechanical. Entry is through the direct formation of metal hydrides (intermetallic compounds) or solid solutions of hydrogen in metal at the instant of hydrogen discharge. Exit is the decomposition of metal hydrides or of solid solutions of hydrogen in metal. The rate of accumulation of hydrogen in the metal is dependent on the relative speeds of entry, diffusion, and exit under whatever environmental conditions prevail.
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