KINETICS OF WETTING IN WASHOUT OF DUST
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Habs + H2Oaas ™~ Ho gas + OH ads
(4)
The first mechanism should be operative on hydrogen-replacing met-
als. The second mechanism should be operative on any metal containing
dissolved hydrogen, whether the metal itself is hydrogen-replacing or
noble.
There is an important difference between the two mechanisms.
The reaction given as Eq. 3 requires that there be a free adsorption
site next to that holding the H,O; the reaction given as Eq. 4 does not.'!
Thus the rates of the two reactions are
HOH] - i, [0] (1-e)
(5)
SOR! — k, [H,0] {H)
(6)
and
In these formulas [OH] and [H,0] denote surface coverages, {H} denotes
volume concentration of dissolved hydrogen, © denotes the total coverage, and k; and k, are constants. The total coverage may be given as
© = [OH] + [H,O]
(7)
The rate (Eq. 5) is zero when O= 1.
When a water drop is pressed against a metal surface, one may
assume[H,O] to be very large, close to unity. The first mechanism
(Eq. 3) then contributes little to the formation of OH on the surface.
However, in the transient state preceding this high coverage by H,O,
some OH,,, should be formed. Hence wetting may occur in this case by
bonding between the water drop and the surface OH. Physically ad-
sorbed H,O apparently does not hold on strongly enough to metal to
cause wetting of the metal. However, if the H,O,,, is bonded to the water
in the drop, the OHads should be also.
A steel that contains much hydrogen, e.g., steel A or steel B after
pickling in 2N H,SO,, should be expected to be wetted by the second
mechanism (Eq. 4). The fact that the rate of wetting is independent of
the voltage applied shows that the reorientation of bonds at the surface
of the water drop as caused by the applied field takes no partin the
rate-determining step. The process is rapid. It is much more rapid at
low voltages than in the case of a pair of water drops. It follows that
the reorientation of the bonds is caused by the reaction itself; i.e., the
H,Oads and OHads in the formula remain bonded to the H,O in the drop.
This should apply equally to the first mechanism (Eq. 3) in the transient state. Consequently the fact that no wetting of steel A occurs in