1844
MEDVEDEV, MAKRUSHIN
curves 3, 5) shows that these parameters are corre-
L, %
lated: the higher the extent of leveling, the stronger
the luster. Thus, leveling of submicro- and microir-
regularites on the cathode surface occurs in the course
of electrolysis in a tin-plating electrolyte containing
Synthanol, formalin, and benzyl alcohol.
On the basis of the performed investigations, a sul-
fate electrolyte of the following composition was
1
developed for obtaining lustrous tin coatings (g l ):
SnSO 5 50, H SO 90 100, Synthanol DS-10 2 3;
4
2
4
1
formalin 37% solution 6 8, benzyl alcohol 6 8 ml l ;
2
deposition mode: i = 1 12 A dm , CE = 65 98%.
c
The process is carried out with mechanical stirring of
the electrolyte. To obtain high-quality lustrous coat-
ings, it is necessary to use anode made of pure tin. In
order to prevent electrolyte contamination with sludge,
the anodes are to be placed in sheaths made of poly-
propylene, before being submerged in the electrolyte.
The electrolyte temperature is 20 25 C. At higher
temperatures, the electrolyte rapidly turns turbid, and
a large amount of precipitate is formed at the bath
bottom, which impairs the coating quality. Long-term
tests with the electrolyte demonstrated its high stabil-
ity in operation. However, it should be noted that, dur-
ing prolonged operation, a light yellow precipitate im-
pairing the coating quality is formed on the bath bot-
tom. The precipitate should be filtered off at regular
intervals. The adjustment of the SnSO , H SO , and
formalin content relies upon the results of chemical
analysis [5]. The adjustment of the Synthanol content
of the electrolyte should be done after passing 100
A h l 1 of electricity, by introducing 1 g l 1 of the
additive into the bath. Since there is no technique for
determining the concentration of benzyl alcohol in a
tin-plating electrolyte, a spectrophotometric method
was developed for this purpose. A 50-ml sample of
a tin-plating electrolyte containing Synthanol, forma-
lin, and benzyl alcohol was extracted with octanol
(50 ml) under vigorous stirring in the course of
10 min. During this time, complete extraction was
achieved. Part of the obtained organic phase was
placed in a cell 10 mm thick, and spectra were re-
corded in the optical density wavelength coordinates
with an SF-26 spectrophotometer in the range 220
310 nm. As blank sample was used a solution ob-
tained by extraction with octanol of a tin-plating elec-
trolyte containing no benzyl alcohol. It was found that
for all of the solutions studied the peak of the absorp-
tion band is observed at 252 nm, with the peak grow-
ing in height with increasing concentration of benzyl
alcohol. It should be noted that the absorption is zero
or very low in the employed wavelength range for all
other components.
ic, A dm 2
C, ml l 1
Fig. 3. Leveling power P vs. (1 3) current density i and
c
(
4) benzyl alcohol concentration C, and (5) coating luster L
vs. current density for tin-plating electrolyte. (1) Electro-
lyte + Synthanol, 2 g l ; (2) 1 + formalin, 6 ml l
1
1
;
1
(
3, 5) 2 + benzyl alcohol, 6 ml l
.
of metals becomes stronger with increasing rate of ad-
ditive diffusion toward the cathode (high speed of
rotation), this effect is more pronounced at micro-
projections, and less so at microdepressions, which
leads to nonequilibrium distribution of the electro-
deposition rates.
4
2
4
Analysis of the polarization curves measured in
electrolytes with organic additives (Fig. 2, curves 2 4)
shows that the cathodic polarization curves measured
at different speeds of rotation qualitatively model the
distribution of the tin electrodeposition rate over the
surface microprofile [3]. With increasing i , the level-
c
ing power first grows and then, at comparatively high
current densities, the surface concentration of the addi-
tive and its inhibiting action decrease even at micro-
projections, which makes the leveling effect weaker.
At too low content of additive in the electrolyte, when
there occurs weak inhibition of tin electrodeposition,
one cannot expect a pronounced leveling effect. At the
same time, at too high concentrations of additive, the
leveling power decreases because of the termination of
the diffusion control over the rate of consumption of
the additive and its inhibiting action. The phenome-
non of leveling is related to the luster of electroplated
coatings, since luster formation is also governed by
leveling of submicrometer surface irregularities. Ac-
cording to [4], large microirregularities, from 0.2 to
1
00 m and more in size, are eliminated in leveling,
and very fine submicroirregularities of about 0.15 m
and less, in luster formation. Comparison of samples
differing in luster and extent of leveling (Fig. 3,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 11 2001