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National Advisory Committee for Aeronautics, Technical Notes - A Fundamental Study of the Mechanism by Which Hydrogen Enters Metals During Chemical and Electrochemical Processing

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 #130 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.

The detrimental effects of hydrogen in metals are well—known and
are reflected in such physical properties as ductility, tensile strength,
and fatigue strength and in behavior and performance of surface coatings,
both metallic and nonmetallic. The disposition of hydrogen in metals,
particularly steel, has been the subject of many recent papers. various
hypotheses have been advanced relating to the location of the hydrogen
inside the steel and to how hydrogen exerts its damaging effects.

Other papers have reported thermodynamic data relating to the formation of
molecular hydrogen and its availability at metal surfaces. Still other
researches have reported physical conditions of treatment to introduce
hydrogen into metals as demonstrated by subsequently measured changes in
the properties of the metals.