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Wengler, John (ed.) / Wisconsin engineer
Volume 87, No. 3 (February, 1983)

Christopherson, Penny J.
A walk on the dean's side,   pp. 10-[13]


Page [12]


                          0        0
T h e C h r o m i u m _   M e c h ai s m
The first comprehensive explanation of electrochemical activity
during the plating of chromium has recently been formulated at
the General Motors Research Laboratories. This understanding
has aided in transforming chromium plating into a highly
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   IOR MANY industrial applica-
   tions, chromium  coatings of
more than 0.2 mil thickness are
required for wear and corrosion
resistance. But the conventional
method of plating chromium is
neither fast nor efficient. Nor, utn-
til the recent work of a GM
researcher, had the steps involved
in the century-old plating process
been explained in detail. Through
a combination of theory and
experiment, Dr. James Hoare has
devised the first comprehensive
mechanism for chromium plating.
This increased understanding has
helped electrochemists at the Gen-
eral Motors Research Laboratories
develop a system that plates chro-
mium sixty times faster than the
conventional method, while
improving energy-efficiency by a
factor of three.
     The electrolyte for plating is
C
a chromic acid solution which
contains various chromate ions:
chromate, clichromate and
trichromate. From a series of
steadly-state polarization exxperi-
ments, I)r. Hoare concluded that
trichromate is the ion important in
chromium (lelposition.
     Sulfuric acid haes been recog-
nizecl as essential to chromium
plating and has been assumed by
some to be a catalyst for the proc-
ess. In this strongly acidic solu-
tion, sulfate should be mostly
present as the bisulfate ion (HS04)-
I)r. Hoare found, contrary to
expectations, that the addition of
sulfuric acid to the plating bath
decreased the conductivity of the
solution.
     Combining these findings
with the results of previous inves-
tigations, I)r. Hoare conclude(l that
the electroactive species was a
trichromnate-bisulfate complex (see
Figure 1). From equilibrium con-
siclerations, he theorized that the
maximum concentration of this
species occurredl at a 100-to-1 chro-
mic aci(l/sulfuric acid ratio. The
observation that the maximum rate
of chromium deposition also
occurred at this ratio supports the
conclusion that this trichromate-
bisulfate complex is the electroac-
tive species.
     I)uring the plating process,
the complex cliffuses from the bulk
solution toward the cathode (see
Figure 2). Electron transport takes
place by quantum mechanical tun-
neling through the potential
energy barrier of the Ilelmholtz
double layer and the unprotected
chromium in the complex (Cr atom
A
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