I
Zinc-Manganese Alloy Electrodeposition
,f
Ft343-H
by D. R. Gabe, G. D. Wilcox, A. Jamani,
Institute of Polymer Technology and Materials Engineering, Loughborough University of Technology,
Loughborough, Leicestershire, U.K.
and B. R. Pearson,
I.C.I. Paints, Slough, Berkshire, U.K.
inc alloy electrodeposits are
finding increased acceptance as
improved replacements for conventional electrogalvanized finishes.
Zinc-nickel appears to be experiencing
the most widespread use, although
zinc-cobalt and zinc-iron also have
well-defined market shares.
The zinc-manganese system has not
been widely reported; however, some
investigators have examined its corrosion resistance, polarization behavior
during electrodeposition and the range
of possible electrolyte^'-^ and have
found it to have markedly attractive
features. Some of the earliest reported
work was carried out by Sagiyama et
al.1,2They were primarily interested in
evaluating the performance of zincmanganese for use on automotive body
panels. The bath used was based on
zinc sulfate, manganese sulfate and
sodium citrate. It was found that
electrodeposits with a manganese content of 30-50% exhibit excellent corrosion resistance both in the prepainted
and postpainted states. The work and
weldabilty of the alloy were also
assessed and found to be good.
Selvam and Guruviah3also reported
good corrosion resistance with the
zinc-manganese system. Govindarayan
et al.4 carried out similar investigations
using an acetic acid salt spray, and
their data confirmed the superiority of
alloys containing 6-75% manganese
over zinc alone.
One of the main drawbacks of the
zinc-manganese system when electrodeposited from citrate-sulfate electrolytes is its very low cathode current
efficiency. Values of 40% have been
quoted, with improvements to 60%
occurring with the addition of small
amounts of sodium thiosulfate.2 For a
viable industrial process, this is still
considered low; therefore other investigators have examined alternative electrolyte~.~
A fluoroborate bath containing both zinc and manganese fluorobo34
ITEMPERATURE Zl0C
2042. A S 1 t .........
/
101
loo0
1500
2000
2544
-E, mV M MMS
Figure 1. Linear sweep voltammogram for an
iron foil electrode (1 cm2) in a 0.25M zinc
sulfate electrolyte, with and without 0.3M
trisodium citrate: curve 1 = 0.25M
ZnS0,.7HZ0, pH 5.4, 21°C; curve 2 = same
as curve 1 + 0.30M Na,C,H,0,.2HZ0.
rate, boric acid and polyethylene glycol has been reported to achieve cathode current efficiencies of up to
The experimental investigations reported here examine the conventional
sulfate-citrate bath, concentrating on
its stability, and investigate the effect
of high-speed electrodepositionusing a
rotating cylinder electrode (RCE) and
cell to simulate electrolyte flow. The
corrosion resistance of steel panels
coated with electrodepositedzinc-manganese alloys of different compositions
was examined in the “as-plated,”
“phosphated” and “phosphated and
cathodically electrocoated” states in a
conventional 1000-hr continuous neutral salt spray test (ASTM 117 B).
EXPERIMENTAL PROCEDURE
Voltametric and electrodeposition
studies were carried out using the
following sulfate-based electrolyte:
0.25M ZnSO4.7Hz0; 0.25M MnS04.
H,O; 0.60M Na3C,H,0,~2H,O; pH
5.4; 2040°C; anode of 316 stainless
steel; 5-40 A/dm2. Voltametric studies
were conducted in a five-port glass
cell. An iron foil electrode (99.5%)was
used in conjunction with a 316 stainless steel anode. Potential measure0 Copyright Elsevier Science Publishing Co., Inc.
ments were made against a mercury/
mercury sulfate (MMS) reference electrode.
High-speed electrodeposition trials
were made using an RCE and associated rig. The former consisted of a 316
stainless steel cylinder on which had
been fitted a hollow cylindrical copper
sleeve. This could be removed after
experimentation to facilitate electrodeposit analysis.
Corrosion studies were carried out
on a variety of coated and uncoated
zinc-manganese electroplated steel
panels. Pretreatment consisted of degreasing in l ,1, l-trichloroethane, followed by pickling in 50% by volume
(s.g. 1.18) hydrochloric acid. Zincmanganese electrodeposition was carried out under quiescent conditions in
an electroplating cell built for this
purpose.
RESULTS AND DISCUSSION
Polarization Characteristics
The voltametric investigations illustrated the complex nature of the process electrochemistry. Figure 1 illustrates the effect of the citrate complexing agent on the zinc electrodeposition
process from a sulfate electrolyte.
Without the complexant (curve I), the
plot illustrates a rapid rise in current,
with no indication of mass transport
limitations; however, with the addition
of 0.3M trisodium citrate, a clear
plateau current is discernible followed
by an increase in current due to
combined metal electrodeposition and
increasing hydrogen evolution. The
actual onset of zinc electrodeposition
appears to occur at more negative
potentials, illustrating the complexing
action of the citrate. Thus, with a
greater driving force for deposition, the
number of nucleation sites will be
correspondingly larger, hence the onset
of mass transport limitation in the form
METAL FINISHING
AUGUST 1993
T
Jane Sellu of the SatisfactoryPlating Company asked Dave,
onight’s guest speaker was re
“What special equipment is used to produce the hardDr. Eddy Cated’s class on
anodized coating?” Dave replied that the most expensive
speaker was Mr. Dave K
equipment in his plant is a 30-tw chiller that circulates cold
Processing Inc., Seattle. Mr. Kelly,
nless steel coils in the tank.
Dave, was going to discuss hard a
cy
of 15 years. The cost of
First, Dr. Eddy discussed anodi
into the price of all hard
process the aluminum parts are made w
as Dave told the class, after
conductive solution. Two reactions take
le for a replacement chiller.
num is converted to aluminum
he Cause-Effect Plating Company asked
coating is dissolved. When the fo
aration is required before hard anodizing.
equals the dissolution of the coating, the
at the usual aluminum cleaning process is
remains constant. If the operatin
al. Receiving inspection
lower the dissolution of the co
ill meet required final
coating results. This is called hard anodizing.
e used cautiously, if at
Dr. Eddy recalled advising
um base metal. Parts
installation. The client had cal
riate
gauges are not
few weeks and complained
e satisfactory finished
formed. Finally, Dr. Eddy a
/ parts achievable.
work was connected to the positive side or the nega
Rhoda Silver of the Precious Metal Plating Company
of the rectifier. The client h
asked
Dave what post-hard-anodizing treatments were
cathode side, of course, like
available.
Dave said that only black dye is available. If black
“Why do they call it anodizing?” The client said, “I’ll,
dye is used, it should be kept at room temperature so that
you back later, Dr. Eddy.” Later, the client reported tha
had now gotten the coating to form. That night, as .he was \, abrasion resistance will not be lowered. If sealing is
required, abrasion resistance will also be reduced. Hardenjoying his small daily additions, Dr. Eddy thought that he
anodized surfaces are sometimes impregnated with materi.
shouldn’t rely on telephone consultations.
s such as waxes or silicone fluids when special surface
Sonny Line of the Cause-Effect Plating C6mpany asked
ch
acteristics are required.
Dave, “What is hard anodizing?” Dave,hplied that hard
%ve
summarized hard-anodized coatings by stating that
anodizing is a process developed to field abrasion and
these
‘qoatings
are thicker, harder, less porous and more
corrosion resistance for aluminum parts: Lower temperature,
abrasion
and
corrosion
resistant than conventional or
higher current density and so1ution”additions are used to
decorative
anodizing.
They
are produced with higher current
lower the rate of dissolution o f / h e anodic coating and
densities, lower solution temperatures and solution addiincrease the coating thickness.
tives. For some processes, AC is superimposed on the DC
Dr. Eddy thought that Da was fortunate to have his
normally used.
company in Seattle. He told e class that he always enjoyed
Dr. Eddy recalled how this superimposed AC current hx!
the vie.v nf the ships and b j x s sai!ing the Semk waierfrmi.
cost
him some money. His technician, when calibrating
Dave told Dr. Eddy that h&d anodizing was used for critical
meters
on a superimposed AC current source, had overaluminum parts on his and most other pleasure boats.
loaded Dr. Eddy’s standard meter and the client’s meter. Dr.
Dee Line had noticdd earlier in the day that the decorative
Eddy paid all the costs, even though there was no sign
anodized coating had worn off the aluminum slides of Ada
advising superimposed AC current. Dr. Eddy told the class
Line’s preschool playground. Dee asked Dave, “What are
that, in the long run, it pays to say “the customer is always
some of the uses of parts that are hard anodized?” Dave
right.”
responded that these uses include
After the class, Dr. Eddy and Dave went to the Profitland
Lounge and superimposed small daily additions.
Maritime-pleasure boats, both sail and power
Appreciation is expressed to Dave Kelly, Asko Process0
Aircraft-hydraulic cylinders
ing Inc., Seattle. Asko is in its 26th year of metal finishing.
Oceanographic-submarines used for oceanographic research
Milton Weiner is an independent chemical engineer in Santa Fe
Tools and gauges-for measuring equipment.
MF
Springs, Calif.
igf
,’
rx”
METAL FINISHING
AUGUST 1993
0 Copyright Elsevier Science Publishing Co., Inc.
33
Table 1. Alloy Compositions for Electrodeposits Formed from the “Standard Electrolyte”
Deposited at 7 Ndm2 and 40°C on a Rotating Cylinder Electrode
20(
-
TEMPERATURE 21°C
0
lo00
lsoo
/
m
Figure 2. Linear sweep voltammogram for an
iron foil electrode (1 cm2) in the ’standard“
sulfate-citrate- based zinc-manganese bath:
0.25M ZnS04.7H20; 0.25M MnS0,.2H20;
0.60M Na,C,H,0,.2H20; pH 5.4,21 “C.
of a limiting current.
Figure 2 illustrates a curve from the
“standard sulfate” zinc-manganese
bath. The onset of metallic electrodeposition occurs at approximately 1550 mV versus MMS. Again, a
limiting current is prominent before the
initiation of hydrogen evolution. The
reduction of both metals is not evident
on the curve; the initial peak (at
approximately -1650 mV versus
MMS) is probably due to zinc reduction. The manganese electrodeposition
is not clearly definable, occurring at
more negative potentials than zinc and
probably being “masked” by the hydrogen evolution process.
Bath Stability
Problems with bath stability have
been reported by Sagiyama et a1.2 and
Brenner.6 A precipitate is said to form
at pH 3.0-5.4 in a bath stored at 50°C.2
The precipitate is thought to be a
trivalent manganese citrate complex.
The addition of metallic zinc or manganese to the bath is said to alleviate
the problem.
Preliminary trials were carried out
&;ng 8 vwiety of ab&Gn &gcnistii
elucidate their effect on the presence of
the precipitate. It was found that small
amounts of ascorbic acid (typically 2.0
g/L) were successful in inhibiting the
appearance of the precipitate during
bath storage at 50°C.
0.
High-speed Electrodeposition
The effects of high mass transfer
created by forced convection are particularly important when considering
an alloy system for possible highspeed electroplating purposes. In these
METAL FINISHING
(ww
4
(wry
(wt%)
Cathode Current
Efficiency (%)
12.5
13.4
15.2
22.9
35.7
13.5
13.6
13.3
8.1
3.3
74.0
73.0
71.5
69.0
61.O
26
27
29
31
39
Zn
0
50
100
300
500
wx)
-E, mV va MMS
.
~
Mn
Rotation
Speed(rp”
AUGUST 1993
investigations, laboratory simulation
of a high-speed process was achieved
using an RCE and cell. The parameters
or rotation speed, pH and electrolyte
composition were examined.
Table I illustrates the change in
percentage alloy composition with rotation speed and the cathode current
efficiency (CCE) as well. As can be
seen, above approximately 100 rpm the
manganese content fails rapidly with
increasing rotation speeds; this is combined with a steady rise in CCE. These
results suggest that zinc electrodeposition from this electrolyte is predominantly mass transport limited, whereas
manganese reduction clearly is not.
The diminishing manganese content
combined with the increasing CCE
suggests that as the rotation speed is
increased, it becomes more favorable
to reduce hydrogen ions on the manganese than to reduce manganese species
to the metallic state. The limiting
process for manganese electrodeposition is unclear; because it is in all
probability complexed by the citrate,
the shedding of the ligands may well
be the rate-determining step.
deposited on each specimen.
As the plating current density increases, the weight percent of zinc
decreases rapidly; however, manganese behaves in a slightly different
manner in that a peak in weight percent
is measured at 20 A/dm2, followed by
a reduction at higher current densities.
This suggests that as the current density (and hence the potential) at the
electrode increases, it gradually becomes more favorable to electrodeposit
manganese and reduce hydrogen ions
than it does to electrodeposit zinc. A
further increase in current density (i.e.,
20-40 A/dm2) illustrates the then increasing tendency for hydrogen evolution to become more favorable at the
expense of manganese electrodeposition. This indicates that in purely
deposit composition terms (Le., discounting the contribution made by
hydrogen evolution), higher current
densities promote coatings richer in
manganese. In effect this confirms the
mass transfer-controlled nature of the
zinc electrodepositionprocess in that at
higher current densities, its limitation
is highlighted through decreasing zinc
content and lower CCE.
Effect of Current Density
Table 11 illustrates the effect of
increasing the current density on the zinc
and manganese content of the electrodeposits. All samples were produced using
the “standard electrolyte” and conditions, with rotation speed of 500 rpm. A
nominal coating thickness of 10 pm was
Effect of Electrolyte pH
Zinc-manganesealloy electrodeposition was carried out with the standard
solution at a current density of 20
A/dm2 and a bath temperature of 40°C.
The pH of the electrolyte was varied
between 3.5 and 6.5, and the RCE was
Table 11. Alloy Compositions for ElectrodepositsFormed from the “Standard Electrolyte”
Deposited at 500 rpm, 40°C and pH 5.4 at Various Current Densities
Current Density
Zn
(wt%)
Mn
(wt%)
(M%)
H2
Cathode Current
Efficiency (%)
5.0
7.0
10.0
15.0
20.0
25.0
40.0
44.1
35.7
31.5
26.6
18.4
8.6
4.1
1.5
3.3
3.8
7.4
11.1
9.5
6.4
54.3
61.O
64.7
66.0
70.5
ai .9
89.5
45.7
39.0
35.3
34.0
29.5
18.1
10.5
(A!d&‘)
35
\
Table 111. Alloy Compositions for Electrodeposit Formed from the “Standard Electrolyte”
Deposited at 20 Aldm2; 40°C and 500 rpm at Varying pH Values
-
PH
3.5
4.5
5.0
5.3
5.4
5.8
6.5
(ut%)
(W%)
(wry
Y
Cathode Current
Efficiency (%)
76.1
50.7
27.6
22.7
18.4
13.8
7.4
1.6
3.8
8.1
10.4
11.1
9.7
15.1
22.3
45.5
64.3
66.9
70.5
76.5
77.5
77.7
54.5
35.7
33.1
29.5
23.5
22.5
Zn
maintained at 500 rpm.
Table I11 illustrates the effect of pH
on the high-speed electrodeposition
process, together with the CCE. As can
be seen, an increasing pH brings about a
fall in the zinc content of the electrodeposits. This is mirrored by a rapid
decrease in CCE (77.7-35.7%) between
pH 3.5 and 5.0. One possible explanation for these trends is that as the bulk
pH of the solution is increased, the pH
at the cathode surface is even higher,
suggesting that the onset of the precipitation of zinc hydroxide is quite possible at much lower bulk pH values than
would have been expected. Thus, zinc
in the diffusion layer is less likely to be
reduced to the metallic state and increasingly forms zinc hydroxide. Because the electrodeposition of manganese in its metallic form is not a
particularly efficient process, hydrogen
evolution becomes the more kinetically
favorable reaction as the potential increases and is coupled with a progressively increasing manganese electrodeposition reaction.
The results presented here illustrate
the generally unfavorable CCEs available from standard, citrate-complexed
sulfate electrolytes. Of particular concern is the very low efficiency possible
when a 50-weight percent manganese
alloy is required to afford optimum
corrosion resistance.
Preliminary Corrosion Trials
From previous investigations? it was
suggested that an approximate alloy
composition of 50 weight percent manganese provided the optimum corrosion
resistance to conventional neutral saltspray testing. Utilizing this alloy composition, a series of steel specimens
were coated with thicknesses of 2.5,5.0,
7.5 and 10.0 pm of zinc-manganese
electroplate. These panels were subsequently phosphated and cathodically
electrocoated (approximately 25ym).
Also included in the tests were two
36
Mn
other zinc-based coatings, namely, electroplated zinc (7.5 pm) and electroplated zinc-13% nickel (6.0 pm). Both
of these systems and the control bare
steel samples were phosphated and
electrocoated. Before commencement
of testing, all panels were scribed to
allow substrate exposure.
Table IV illustrates the corrosion
data from the 1000-hr continuous salt
spray trials. As can be seen, the
zinc-manganese-coated samples appear to generally convey the greatest
degree of protection. The other zincnickel alloy system had an approximately equivalent corrosion resistance
to that of 2.5 pm of the zinc-manganese alloy. Samples having a greater
thickness of zinc-manganese withstood
the corrosion test with, at worst, only
minor corrosion product evident.
Mechanism of Protection
The mechanism of protection of
steel by zinc is dependent on the
galvanic sacrificial nature of zinc toward steel, which it can retain in many
corrosion conditions. The protection
can be affected if zinc becomes noble
toward steel by the effect of ions in
solution, the formation of a passive
film or by increased temperature. The
protection afforded by zinc alloys must
also be governed by these principles.
Zinc-nickel, zinc-cobalt and zinc-tin
alloys all become increasingly noble as
the alloy content increases and at a
critical content will cease to sacrificially protect steel. They also become
intrinsically slower to dissolve, and
this is another protection factor. Consequently, a critical alloy content must
exist (e.g.. 12% nickel in zinc) for
maximum protective ability.
Zinc-manganese alloys become increasingly base as the manganese content increases, and clearly a mechanism
is appropriate to explain the markedly
improved service life observed. This is
attributed to the tendency to form a
passive film, probably of manganese(II1) oxide or manganese dioxide (a
black rather than white rust for this
alloy), which allows the galvanic polarity to be maintained while acting
itself as a kinetic barrier. Such behavior
may be analogous to the “weathering
steels” when iron, in the presence of
certain alloying elements, can produce a
film of &FeO.OH, which offers stable
passive film protection. The 50%
threshold for manganese content of zinc
alloys then represents the ability to form
an oxide film of this type.
References
1. Sagiyama, M. et al., SAE Technical
Paper 860,268; Society of Automotive
Engineers, Warrendale, Pa.; 1986
2. Sagiyama, M. et al. Plating and Surface
Finishing, 74:77; 1987
3. Selvam, M. and S . Guruviah, CECRI
Bulletin of Electrochemistry, 5:352;
1989
4. Govindarajan, G. et al., CECRI Bulletin
of Electrochemistry, 5:422; 1989
5. Sugimoto, Y. et al., Proceedings of the
179th Meeting of the Electrochemical
Society; Washington, D.C., May 5-10;
1991
6. Brenner, A. Electrodeposition of Alloys,
Vol. 2; Academic Press, New York;
1963
7. Jamani, A.J. et al., Proceedings of the
179th Meeting of the Electrochemical
Society; Washington, D.C.,May 5-10;
1991
MF
Table IV. ASTM 117B Salt Spray Data for Zinc-Manganese and Rival Systems
Samde
Bare steel
7.5 pm Zn
6.0 pm Zn-Ni
2.5 pm Zn-Mn
5.0 pm Zn-Mn
7.5 pm Zn-Mn
10.0 pm Zn-Mn
-
Corrosion
Performance
Electrocoat
Performanceb
Highlred
Medlredtwhite
Medlwhite
Lowhlack
None
None
None
2
5
2
2
0
0
0
Comment
Unacceptable
Unacceptable
Unacceptable
Unacceptable
Very good
Very good
Excellent
of rust: WhitelRedlBlack = type of rust.
no undercutting; 1 = up to 1 mm; 2 = up to 2 mm; 3 = up to 3 mm; 4 = up to 4 mm; 5 = >4 mm (delamination).
a LowlMediumlHigh = degree
METAL FINISHING
AUGUST 1993
~~