Ferrocyanide-free halogen tin plating process and bath

Incorporating an additive into a tin electroplating bath substantially inhibits soluble ferrous ions, ferric ions, and stannous ions from reacting thus minimizing the formation of stannic tin which is lost in the plating sludge.

FIELD OF THE INVENTION 
The present invention relates to a process for the high-speed tin plating 
of steel and a plating bath composition which is free of ferrocyanides. 
BACKGROUND OF THE INVENTION 
One of several known processes for the production of tinplated steel, 
high-speed halogen electroplating, typically uses plating baths which 
comprise stannous chloride, sodium bifluoride, sodium fluoride, sodium 
chloride and hydrochloric acid together with a grain-refining additive. In 
order to minimize corrosion of the steel as it is plated, however, the 
degree of acidity must be moderate (i.e., pH of between 3 to 4). Moderate 
acidity, in turn, requires that the stannous tin be combined with fluoride 
ions in a chemical complex in order to minimize the reaction of stannous 
tin with oxygen to form stannic tin which precipitates and is lost in the 
plating sludge. Dissolved iron in the plating bath accelerates the 
oxidation of fluoride-complexed stannous ions so that a substantial 
portion of the stannous ions are lost in the plating sludge. Thus, the 
iron, if not removed from the halogen plating bath, de-stabilizes the 
process resulting in off-quality tinplate, low productivity, and high 
costs for replenishment of tin and other chemicals. 
To counteract the effect of iron, ferrocyanide is added in large quantities 
to the bath to remove dissolved iron from the electrolyte by forming 
insoluble compounds which report to the plating sludge. It is normally 
added as the sodium ferrocyanide decahydrate salt and results in the 
immediate and total precipitation of iron ions from the electrolyte before 
they react with oxygen and/or stannous tin. The use of alkali 
ferricyanides or ferrocyanides for this purpose is disclosed in U.S. Pat. 
Nos. 2,402,185 and 2,512,719. 
These ferrocyanide additions result, however, in precipitated iron 
ferrocyanides which, along with insoluble stannic tin in the form of 
sodium fluostannate, become the major ingredients of a heavy sludge that 
accumulates in the plating cells, storage tanks, and throughout the 
recirculating system. Typically, halogen electroplating lines must be shut 
down periodically so that this sludge can be removed. In recent years, 
concern has been growing about the environmental impact of the 
ferrocyanide content of the plating section sludge, slurries, and waste 
waters. 
Thus, there is a need to develop other ways to remove iron and/or stabilize 
a halogen plating bath so that the use of ferrocyanide can be 
discontinued. 
SUMMARY OF THE INVENTION 
The present invention relates to a weakly acidic halogen plating bath 
solution free of ferrocyanides for the electroplating of tin on an 
iron-based substrate. The plating bath solution contains a conductive 
electrolyte, stannous ions, and an effective amount of an additive 
incorporated into the plating bath sufficient to substantially inhibit 
soluble ferrous ions, ferric ions, and stannous ions from reacting in 
solution. This solution minimizes the formation of stannic tin, Sn(IV). 
The additive is preferably selected from the group consisting of 
para-aminobenzoic acid, hydroquinone, gallic acid, catechol, resorcinol, 
salicylic acid, ascorbic acid (L- or D-), citric acid, oxalic acid, formic 
acid, acetic acid, tartaric acid, glycine, diethyl hydroxylamine (DEHA), a 
mixture of citric acid and hydroquinone, and mixtures thereof. 
Also disclosed is a method for minimizing the effect of dissolved iron in 
high-speed tin electroplating without the need for ferrocyanides which 
uses a weakly acidic halogen bath solution containing an additive in an 
amount sufficient to substantially inhibit soluble ferrous ions, ferric 
ions, and stannous ions from reacting in solution, to minimize the 
formation of stannic tin.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention uses chemical additives to stabilize halogen 
electroplating baths so that they may be used without ferrocyanide 
additions in the high-speed halogen tinplating of steel. FIG. 1 shows a 
horizontal, high-speed, halogen line having a cleaning tank 10, a first 
water rinsing unit 20, a surface activation unit 30, a second water 
rinsing unit 40, and a plating section 50. 
A ferrous substrate, e.g. coiled steel 52, to be plated is first pretreated 
by cleaning and surface activation operations. These are accomplished by 
sequentially passing the substrate 52 through cleaning tank 10, first 
water rinsing unit 20, surface activation unit 30, and second water 
rinsing unit 40. The substrate 52 is then passed through plating section 
50 which comprises a plurality of horizontal plating cells arranged on two 
plating decks, one above the other. The bottom side 54 of the substrate 52 
is plated in the floor-level cells 56, and then the substrate 52 is 
diverted to the second level 58 where it reverses direction and passes 
through additional cells for plating the top side 60, which, because of 
the reversal, is actually the bottom surface in those plating cells. The 
horizontal plating cell geometry and high strip speeds result in a high 
degree of aeration of the plating bath. After exiting the plating section 
50, the steel is given further processing according to well-known 
technology which is not shown, to result in a coil of tin plated steel 
that is ready for shipment to a customer. 
The geometry of the halogen process equipment dictates the chemical 
requirements of the plating bath to be used. When using horizontal plating 
cells the degree of acidity must be moderate (i.e., a pH of from greater 
than zero up to about 6 and preferably between about 3 to about 4) in 
order to minimize corrosion, especially of the top surface of the steel 
strip as it passes through the first tier plating cells. In order to 
perform a high-speed tinplating operation, an acid plating bath, based on 
stannous rather than stannic ions, is required. The moderate acidity of 
the bath requires that the stannous ions (Sn.sup.+2) in the bath be 
combined with fluoride ions in an anionic complex (SnF.sub.3.sup.-1). In 
this form, the reaction between dissolved oxygen resulting from a high 
degree of aeration of the bath and stannous tin is relatively slow, but it 
does result in the formation of tetravalent or stannic tin ion 
(Sn.sup.+4). The stannic ion also combines with fluoride ions to form a 
complex fluostannate anion (SnF.sub.6.sup.-2) which has limited solubility 
and precipitates from the bath as a sodium salt (Na.sub.2 SnF.sub.6). 
It is believed that the effect of iron on the depletion of the stannous 
ions in the halogen bath occurs according to the following series of 
chemical reactions: 
EQU 4Fe.sup.+2 +4H.sup.+ +O.sub.2 .fwdarw.4Fe.sup.+3 +2H.sub.2 O(1) 
EQU Fe.sup.+3 +3Na.sup.+ +6F.sup.- .fwdarw.Na.sub.3 FeF.sub.6 (s).dwnarw.(2) 
EQU 2Fe.sup.+3 +Sn.sup.+2 .fwdarw.2Fe.sup.+2 +Sn.sup.+4 (3) 
EQU 2Sn.sup.+2 +O.sub.2 +4H.sup.+ .fwdarw.2Sn.sup.+4 +2H.sub.2 O(4) 
EQU Sn.sup.+4 +2Na.sup.+ +6F.sup.- .fwdarw.Na.sub.2 SnF.sub.6 (s).dwnarw.(5) 
Reaction 1 illustrates how free ferrous ions (Fe.sup.+2) which enter an 
aerated plating bath react rapidly with the dissolved oxygen to form 
ferric ions (Fe.sup.+3). These ferric ions then follow one of two courses: 
they either react with sodium and fluoride to form insoluble 
sodiumfluoferrate according to reaction 2 or they react with stannous tin 
to produce stannic tin according to reaction 3 (during which ferric ions 
are reduced back to the ferrous ions). Stannic tin will also be produced 
by the direct reaction of stannous tin with oxygen according to reaction 
4. 
In a system where no additional iron is introduced, the effect of reaction 
3 (which occurs very quickly) would quickly dissipate with elimination of 
ferric ions by tile formation of sodium fluoferrate (Na.sub.3 FeF.sub.6) 
which precipitates to the sludge according to reaction 2. 
In a continuous plating line, however, iron contamination of the plating 
bath is persistent. It is believed that the majority of iron enters the 
conventional plating bath in three different ways in a typical halogen 
tinplating process. The first is by drag-in from the surface-activation 
step (acid treatment and water rinse) which precedes the plating process. 
The second is by dissociation of ferrocyanide ions to ferrous ions and 
cyanide ions. Corrosion of the steel strip top surface as the bottom is 
being plated is the third way. 
Corrosion is believed to be the smallest contributor, mainly because iron 
is more noble than tin in the complex chemistry of the halogen bath. As 
soon as any tin is plated on the bottom surface, and especially as the 
deposit wraps around onto the top at the strip edges, the steel is 
galvanically protected and this greatly retards the dissolution of iron 
from the unplated top surface. 
On the other hand, the effect of the dissociation of ferrocyanide ions is 
believed to be significant, depending on pH, temperature, and ferrocyanide 
concentration. In a ferrocyanide-free electrolyte, however, such 
dissociation would not be a source of iron. Thus, by discontinuing 
ferrocyanide additions, the most significant source of iron will be 
drag-in from the pre-treatment section. Although improvements can be made 
to reduce the amount of iron dragged into the bath, the problem cannot be 
totally avoided. As a result, reactions 1 and 3 take place continuously at 
rapid rates to produce the stannic tin ion. 
Stannic tin ions are also produced at lower rates according to reaction 4. 
The stannic ions ultimately precipitate to the sludge as sodium 
fluostannate according to reaction 5. This continuous precipitation leads 
to a rapid decrease in the stannous concentration and a consequent 
decrease in the limiting current density of the plating process. Without 
the use of ferrocyanide to immediately precipitate iron, the stannous 
concentration may drop to a level where it is difficult or impossible to 
produce a high-quality tin coating. Moreover, to compensate for the 
precipitation of sodium fluostannate, higher tin and chemical 
replenishments must be made to the plating bath, thus resulting in higher 
operating costs. 
Thus, according to the present invention, in order to negate the 
deleterious effects of iron on the tinplating process without using 
ferrocyanide addition to the bath, specific organic compounds and 
combinations thereof have been evaluated as additives to the halogen bath 
which slow down the depletion of divalent tin (i.e., stannous ions) from 
the electroplating process. These chemical additives either stabilize the 
ferrous ions in solution so that they are more slowly oxidized to the 
ferric ion (i.e., reaction 1 is retarded) or stabilize the stannous ions 
so that they react more slowly even in the presence of ferric ions (i.e., 
reactions 3 and 4 are minimized). 
The additives found particularly effective in accomplishing the above 
include para-aminobenzoic acid, hydroquinone, gallic acid, catechol, 
resorcinol, salicylic acid, ascorbic acid (L- or D-), citric acid, oxalic 
acid, formic acid, acetic acid, tartaric acid, glycine, diethyl 
hydroxylamine (DEHA), mixtures of citric acid and hydroquinone, and 
mixtures thereof. 
A method for minimizing the effect of dissolved iron in high-speed tin 
electroplating comprises incorporating the chemical additives discussed 
above in a weakly acidic halogen bath solution in an amount sufficient to 
substantially inhibit soluble ferrous ions, ferric ions, and stannous ions 
from reacting in the plating bath. As a result, these additives minimize 
the formation of stannic tin without the need for ferrocyanide additions. 
The following examples illustrate the present invention. Halogen bath tin 
oxidation studies were performed in which oxygen was bubbled through 150 
mL of a halogen bath plating solution containing 19.6 gms/l NaHF.sub.2, 
26.5 gms/l NaF, 12.68 gms/l NaCl, and 33.0 gms/l SnF.sub.2, together with 
1 ml/l of a commercial grain refining additive known in the trade as Agent 
20, in a 250 mL graduated cylinder immersed in a water bath at 140.degree. 
F. The electrolytes used for these experiments were typical of halogen 
tinplating baths and contained 25 g/l of total tin to initially yield 
20-24 g/l of stannous ions. The mole ratio of fluoride to total tin was 
8:1 and the initial pH of the solution was 3.4-3.5. The concentration of 
the addition agents in the experimental baths was set at 0.1 molar unless 
otherwise specified. Iron (1 g/l) was added at the beginning of the test 
as ferrous sulfate crystals (FeSO.sub.4.7H.sub.2 O). To accelerate the 
oxidation process, either air or pure oxygen gas was sparged from the 
bottom of the container at 120 cc/min. The concentration of stannous ions 
was determined by titration every thirty or sixty minutes during a three 
or eight hour test period. 
Tables 1 and 2 below tabulate the results of the oxidation studies using 
the additives of the present invention with Table 2 focused on ascorbic 
acid as a preferred additive. The effect of L- and D- ascorbic acid 
additives upon the oxidation of stannous ions was evaluated both with and 
without the addition of ferrous ions in the bath. Generally, the data show 
the depletion of stannous ions by oxidation in both cases. This oxidation 
is exacerbated, however, when ferrous ions are added to the bath. In order 
to determine the effectiveness of the additives, comparative examples were 
also run on baths free of addition agents both with and without ferrous 
ions in the bath. Also included were examples which included ferrocyanide 
additions. Table 3 tabulates results of oxidation studies on a halogen 
bath where either no additives, sodium ferrocyanide, or nitrogen were 
introduced into the bath. 
In comparing Tables 1 and 2 with Table 3, the loss of stannous ions is seen 
to be lower when the additives of the present invention are incorporated 
into the bath than when no additions are made at all. In particular, the 
use of ascorbic acid as an additive is particularly effective. A 
comparison of run 3 of Table 2 with run 20 of Table 3 shows that a low 
concentration (e.g., 0.005 M) of L-ascorbic acid results in more than four 
times the concentration of stannous ions in the bath after 180 minutes of 
oxygen injection in the presence of iron added to the bath at an initial 
concentration of 1 g/l. An increase in the L-ascorbic acid concentration 
to 0.05 M (run 25 of Table 1) increases the amount of stannous ions to 
almost six times after 180 minutes of oxygen injection. Similarly, 
D-ascorbic acid, when used at a concentration off 0.05 M (run 27 of Table 
1), increases the amount of stannous ions to over 6 times after 180 
minutes of oxygen injection. 
The additives used in Tables 1 and 3 have been abbreviated as follows: 
PDHB=hydroquinone; MDHB=resorcinol; 
ODHB=catechol; THBA=gallic acid; 
AAA=glycine; DEHA=diethyl hydroxylamine; 
PABA=para aminobenzoic acid; EDTA=ethylene diamine tetra acetic acid; and 
AA-55 is known in the art as the designation for sodium ferrocyanide. 
TABLE 1 
__________________________________________________________________________ 
PRESENT INVENTION 
Fe.sup.+2 initial 
final 
Stannous Ion Concentration (g/l) 
Run 
Addition Agents 
added g/l 
Oxidant Temp. F. 
pH pH 0.0 
30.0 
60.0 
90.0 
120.0 
150.0 
180.0 
(Min.) 
__________________________________________________________________________ 
1 0.1M oxalic acid 
0.000 
O.sub.2 120 ml/min 
145 3.5 -- 19.3 
17.4 
15.7 
13.8 
12.1 
10.5 
8.4 
2 0.1M oxalic acid 
0.040 
O.sub.2 120 ml/min 
145 3.5 -- 18.6 
13.3 
11.4 
9.3 
7.6 
6.2 
4.6 
3 0.1M citric acid 
0.000 
O.sub.2 120 ml/min 
145 3.5 -- 20.0 
18.6 
16.8 
15.0 
13.2 
11.7 
10.0 
4 0.1M citric acid 
1.000 
O.sub.2 120 ml/min 
145 3.5 -- 18.8 
14.0 
12.4 
10.7 
9.3 
7.9 
6.4 
5 0.1M PDHB 1.000 
O.sub.2 120 ml/min 
140 -- 4.0 
19.0 
15.5 
15.0 
13.6 
12.6 
11.7 
10.7 
6 0.1M PAA 1.000 
O.sub.2 120 ml/min 
140 -- 4.5 
18.2 
15.5 
12.0 
8.1 
4.3 
2.0 
0.0 
7 0.1M EDTA 1.000 
O.sub.2 120 ml/min 
140 3.5 4.0 
21.9 
13.6 
10.7 
8.1 
6.0 
4.1 
1.9 
8 0.02M PDHB 
1.000 
O.sub.2 120 ml/min 
140 3.5 4.4 
21.9 
15.7 
14.3 
12.6 
11.2 
9.5 
7.9 
9 0.05M PDHB 
0.000 
O.sub.2 120 ml/min 
140 3.5 4.3 
21.7 
20.7 
20.2 
19.3 
18.3 
17.6 
16.9 
10 0.05M PDHB 
1.000 
O.sub.2 120 ml/min 
140 3.5 4.5 
21.7 
17.6 
16.7 
15.2 
14.0 
12.6 
11.4 
11 0.05M PDHB 
0.500 
O.sub.2 120 ml/min 
140 3.4 4.4 
24.5 
21.4 
19.8 
19.0 
17.6 
16.9 
16.0 
12 0.05M PDHB 
0.200 
O.sub.2 120 ml/min 
140 3.4 4.4 
24.5 
21.7 
20.7 
19.8 
19.0 
18.1 
17.1 
13 0.05M MDHB 
0.000 
O.sub.2 120 ml/min 
140 3.4 4.5 
25.0 
23.8 
22.9 
21.2 
19.8 
18.3 
17.4 
14 0.05M MDHB 
1.000 
O.sub.2 120 ml/min 
140 3.4 4.5 
25.0 
18.3 
16.9 
15.0 
13.6 
11.9 
11.0 
15 0.05M ODHB 
0.000 
O.sub.2 120 ml/min 
140 3.4 4.0 
24.5 
22.9 
22.4 
21.2 
20.5 
19.8 
19.0 
16 0.05M ODHB 
1.000 
O.sub.2 120 ml/min 
140 3.4 4.3 
24.5 
18.3 
17.4 
16.7 
16.0 
13.1 
12.6 
17 0.05M 3,4,5 THBA 
0.000 
O.sub.2 120 ml/min 
140 3.4 4.1 
24.5 
23.6 
22.6 
21.4 
20.7 
19.5 
18.3 
18 0.05M 3,4,5 THBA 
1.000 
O.sub.2 120 ml/min 
140 3.4 4.3 
24.5 
19.8 
18.8 
17.1 
15.2 
14.3 
13.1 
19 0.1M EDTA 0.000 
O.sub.2 120 ml/min 
140 3.4 4.1 
24.5 
22.6 
20.9 
19.5 
17.6 
16.4 
14.8 
20 0.05M AAA 0.000 
O.sub.2 120 ml/min 
140 3.4 4.4 
24.5 
22.9 
21.2 
19.8 
17.9 
16.2 
14.5 
21 0.05M AAA 1.000 
O.sub.2 120 ml/min 
140 3.4 4.7 
24.5 
15.0 
13.8 
12.6 
11.9 
11.2 
10.0 
22 0.10M PDHB 
0.000 
O.sub.2 120 ml/min 
140 3.4 3.7 
24.5 
23.6 
23.1 
22.6 
22.4 
21.9 
21.7 
0.10M citric acid 
23 0.10M PDHB 
1.000 
O.sub.2 120 ml/min 
140 3.4 3.7 
24.5 
21.4 
20.9 
20.5 
19.5 
19.3 
18.8 
0.10M citric acid 
24 0.05M L-ascorbic 
0.000 
O.sub.2 120 ml/min 
140 3.4 4.2 
22.4 
23.8 
23.6 
22.9 
21.7 
21.5 
21.0 
25 0.05M L-ascorbic 
1.000 
O.sub.2 120 ml/min 
140 3.4 4.2 
22.4 
22.4 
21.3 
20.8 
19.8 
19.1 
18.4 
26 0.05M D-ascorbic 
0.000 
O.sub.2 120 ml/min 
140 3.4 3.7 
24.2 
26.1 
25.4 
25.2 
25.0 
24.2 
24.0 
27 0.05M D-ascorbic 
1.000 
O.sub.2 120 ml/min 
140 3.4 3.7 
24.2 
23.5 
23.4 
22.8 
22.3 
21.0 
20.9 
28 0.05M PABA 
0.000 
O.sub.2 120 ml/min 
140 3.4 4.1 
24.1 
22.4 
20.7 
19.3 
17.8 
16.1 
14.6 
29 0.05M PABA 
1.000 
O.sub.2 120 ml/min 
140 3.4 4.4 
24.1 
19.1 
17.5 
16.3 
15.2 
13.9 
12.9 
30 0.05M salicylic 
0.000 
O.sub.2 120 ml/min 
140 3.4 4.2 
24.1 
22.1 
19.8 
17.7 
15.1 
13.5 
12.0 
31 0.06M salicylic 
1.000 
O.sub.2 120 ml/min 
140 3.4 4.4 
24.1 
16.5 
14.4 
12.5 
10.4 
9.2 
7.6 
32 0.05M DEHA 
0.000 
O.sub.2 120 ml/min 
140 3.4 4.5 
22.2 
20.7 
19.7 
18.4 
17.5 
16.9 
15.8 
33 0.05M DEHA 
1.000 
O.sub.2 120 ml/min 
140 3.4 4.7 
22.2 
15.8 
15.4 
14.5 
13.7 
13.2 
12.9 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Fe.sup.+2 
Addition added Temp. 
initial 
final 
Stannous Ion Concentration g/l) 
Run 
Agents 
g/l Oxidant F. pH pH 0 60 120 
180 
240 
300 
360 
420 
480 
(Min.) 
__________________________________________________________________________ 
1 0.010M 
1.000 
O.sub.2 120 ml/min 
140 -- 4.6 
25.2 
19 17.7 
16.9 
15.4 
14.3 
13.1 
11.9 
10.5 
L-ascorbic 
2 0.025M 
1.000 
O.sub.2 120 ml/min 
140 -- 4.4 
25.2 
21.4 
20 18.8 
17.3 
16.4 
14.7 
13.3 
11.9 
L-ascorbic 
3 0.005M 
1.000 
O.sub.2 120 ml/min 
140 -- 5 23.8 
17.3 
15.8 
14.3 
12.7 
11.4 
10 8.8 
6.7 
L-ascorbic 
4 0.05M 1.000 
O.sub.2 120 ml/min 
140 3.4 4.2 
22.4 
21.3 
19.8 
18.4 
-- -- -- -- -- 
L-ascorbic.sup.1 
5 0.05M 1.000 
O.sub.2 120 ml/min 
140 3.4 3.7 
24.2 
23.4 
22.3 
20.9 
-- -- -- -- -- 
D-ascorbic.sup.2 
6 none 0.000 
O.sub.2 120 ml/min 
140 3.4 4.7 
23.8 
20.9 
18.1 
15.9 
13.4 
11.3 
10.1 
9.4 
9 
7 none 1.000 
O.sub.2 120 ml/min 
140 3.4 4.6 
23.7 
12.1 
7.1 
3.3 
-- -- -- -- -- 
__________________________________________________________________________ 
.sup.1 Run 25 Table 1 
.sup.2 Run 27 Table 1 
TABLE 3 
__________________________________________________________________________ 
Fe.sup.+2 initial 
final 
Stannous Ion Concentration (g/l) 
Run 
Addition Agents 
added g/l 
Oxidant Temp. F. 
pH pH 0.0 
30.0 
60.0 
90.0 
120.0 
150.0 
180.0 
(Min.) 
__________________________________________________________________________ 
1 none 0.000 
O.sub.2 120 ml/min 
145 4 -- 20.8 
19.0 
17.6 
15.5 
13.3 
11.4 
9.5 
2 none 1.000 
O.sub.2 120 ml/min 
145 4 -- 20.0 
11.4 
9.0 
6.4 
3.8 
2.4 
1.4 
3 3 g/l AA-55 
0.000 
O.sub.2 120 ml/min 
140 3.4 4.7 
24.5 
23.0 
21.9 
20.7 
19.5 
17.9 
17.1 
4 3 g/l AA-55 
1.000 
O.sub.2 120 ml/min 
140 3.4 4.8 
24.5 
17.1 
16.0 
14.3 
13.1 
12.4 
11.2 
5 none 0.000 
O.sub.2 120 ml/min 
140 3.4 4.5 
24.8 
22.6 
20.5 
18.6 
17.1 
16.2 
13.1 
6 none 0.200 
O.sub.2 120 ml/min 
140 3.4 4.7 
24.8 
19.3 
17.6 
16.0 
14.0 
11.7 
10.2 
7 none 0.000 
AIR 120 ml/min 
140 -- 3.7 
24.0 
24.0 
23.8 
23.3 
23.3 
22.9 
22.6 
8 none 0.056 
AIR 120 ml/min 
140 -- 3.8 
24.0 
23.3 
22.9 
22.6 
22.1 
21.7 
21.2 
9 none 0.113 
AIR 120 ml/min 
140 -- 3.8 
24.0 
22.9 
22.4 
21.7 
21.4 
20.7 
20.5 
10 none 0.565 
AIR 120 ml/min 
140 -- 3.9 
24.0 
20.7 
19.8 
19.3 
18.6 
18.6 
17.9 
11 none 0.000 
AIR 120 ml/min 
140 -- 4.1 
24.3 
24.0 
24.0 
23.8 
23.6 
23.1 
23.1 
12 none 1.000 
AIR 120 ml/min 
140 -- 4.4 
24.3 
19.5 
18.6 
17.9 
17.1 
16.9 
16.4 
13 none 0.000 
AIR 120 ml/min 
140 3.4 3.7 
23.9 
23.6 
23.5 
23.4 
23.0 
22.9 
22.8 
14 none 0.056 
AIR 120 ml/min 
140 3.4 3.8 
23.9 
22.0 
21.0 
21.0 
20.7 
20.7 
20.6 
15 none 0.113 
AIR 120 ml/min 
140 3.4 4.0 
23.9 
20.6 
20.2 
20.2 
19.6 
19.6 
19.3 
16 none 0.226 
AIR 120 ml/min 
140 3.4 4.2 
23.9 
22.0 
20.4 
19.8 
19.8 
19.7 
19.6 
17 N.sub.2 120 ml/min 
0.000 
none 140 3.4 3.4 
23.0 
22.9 
22.9 
22.9 
22.9 
22.9 
22.9 
18 N.sub.2 120 ml/min 
0.565 
none 140 3.4 3.5 
22.9 
22.2 
22.2 
22.2 
22.2 
22.2 
22.2 
19 N.sub.2 120 ml/min 
0.565 
none 140 3.4 3.6 
22.9 
21.6 
21.6 
21.7 
21.8 
21.7 
21.8 
Fe.sup.+3 
20 none 1.000 
O.sub.2 120 ml/min 
140 3.4 4.6 
23.7 
14.9 
12.1 
9.2 
7.1 
4.8 
3.3 
21 none 0.000 
O.sub.2 120 ml/min 
140 -- 3.8 
25.2 
22.0 
19.1 
16.6 
14.7 
14.0 
13.8 
22 none 0.000 
O.sub.2 120 ml/min 
140 -- 3.7 
25.1 
23.5 
21.9 
20.6 
19.3 
17.7 
16.4 
__________________________________________________________________________ 
From the data in Table 1, it has been determined that when the stannous ion 
concentration is in the range of 8 to 25 g/l the stannous ion oxidation 
rate is independent of the stannous ion concentration. Thus the change in 
stannous ion concentration, .DELTA.Sn.sup.+2, of the data is discussed 
below and graphically displayed in FIGS. 2-7 in order to eliminate the 
need to consider small variations in the starting stannous ion 
concentrations. It should be noted that, for all of the additives of the 
present invention, the .DELTA.Sn.sup.+2 versus time curves shown in FIGS. 
3-7 exhibit basically the same shape, i.e., a sudden decrease in the 
stannous ion concentration in the first thirty minutes (represented in the 
graphs by a sharp increase in the slope of the .DELTA.Sn.sup.+2 versus 
time curves) followed by a linear region of decreasing stannous ion 
concentration (represented in the graphs by a continued increase in the 
slope of the .DELTA.Sn.sup.+2 versus time curves but at a more gradual 
rate) similar to that which results in the absence of iron. 
It is believed that this initial deleterious effect of iron in increasing 
the oxidation rate of stannous ions corresponds to reaction 3 discussed 
above and diminishes quickly as the stable sodium fluoferrate compound is 
formed according to reaction 2 above. It is also theorized that the linear 
region of decreasing stannous ion concentration corresponds to the 
continued direct oxidation of stannous ions in solution according to 
reaction 4 above. The data of Tables 1, 2, and 3 were used to prepare the 
graphs of FIGS. 2-7 which serve to further explain the present invention 
with regard to the effect of the additives of the present invention upon 
stannous ion depletion by both of these mechanisms. 
FIG. 2 shows, as a comparative baseline, the effect of oxygen on the 
stannous tin concentration, without iron added and with iron added in 
concentrations of 0.2 g/l (200 ppm) and 1.0 g/l (1,000 ppm) as solid 
ferrous sulfate (FeSO.sub.4.7H.sub.2 O) at the beginning of the 
experiment. The effect of iron on accelerating the oxidation of Sn.sup.+2 
is very strong and occurs during the initial minutes of oxygenation (i.e., 
during the first 30 minutes of oxygenation), after which the oxidation 
rate is essentially the same as without iron. Furthermore, in the case of 
iron additions, a white powdery precipitate formed during the initial 
minutes. This powder was identified as the complex ferric salt, sodium 
fluoferrate (Na.sub.3 FeF.sub.6), by X-ray diffraction and by Energy 
Dispersive Spectroscopy. 
FIG. 3 shows the effect of 0.10 molar concentrations of carboxylic acids, 
namely acetic, oxalic, citric, formic, and tartaric acids, on the 
oxidation of stannous tin. Citric acid reduced the initial effect of iron 
by almost 50% and also reduced the continued direct oxidation of stannous 
ions, as evidenced by the change in slope of the linear portion of the 
curve. Oxalic acid was nearly as effective in mitigating the iron effect, 
but did not have as strong an effect on continued direct stannous 
oxidation. The impact of the other carboxylic acids, although not as 
great, still shows an improvement over using no addition agent at all. 
FIG. 4 shows the effect of 0.05 molar concentrations of six aromatic 
compounds, namely, gallic acid, hydroquinone and its two isomers 
(resorcinol and catechol), para-aminobenzoic acid, and salicylic acid. All 
of these additives reduced the initial iron effect to a certain extent and 
most had a strong impact on the continued direct oxidation of stannous 
ion. Hydroquinone had a very strong effect in reducing both the initial 
impact of iron and the continued direct oxidation of stannous ions. Gallic 
acid and para-aminobenzoic acid were also nearly as effective as 
hydroquinone. Catechol, resorcinol, and salicylic acid were also found to 
have the effect of lowering the oxidation rate of stannous ions. Thus, 
aromatic compounds were found to be effective in both reducing the initial 
effect of iron and lowering the continued direct oxidation rate of 
stannous ions in the plating bath. 
FIG. 5 shows the profound effect 0.05 M L-ascorbic acid have on both the 
initial effect of iron and the continued direct oxidation of stannous ions 
in the plating bath. It can be seen, by the negative slope in the curve in 
the case where no iron is added to the bath, that the stannous ion 
concentration increases during the first thirty minutes of oxygenation 
when using L-ascorbic acid. It is believed that ascorbic acid actually 
reacts with up to an equivalent weight of stannic ions which may be 
present in the bath, reducing them back to stannous ions. When iron is 
added to the bath, the ascorbic acid eliminates the initial deleterious 
effect and also has a very strong impact on the continued direct oxidation 
of stannous tin. Moreover, as shown in the tables it should be noted that 
the addition of 0.05 M L- or D-ascorbic acid makes a bath with 1.0 g/l of 
iron more stable than even a bath containing no iron and no additives. 
FIG. 6 shows the effect of aliphatic compounds, namely, 0.05 M additions of 
glycine (aminoacetic acid), 0.10 M ethylenediaminetetraacetic acid (EDTA), 
and 0.05 M N, N diethylhydroxylamine (DEHA). DEHA reduced both the initial 
iron effect and the continued direct oxidation of the stannous ion in the 
plating bath. Although glycine was found to slightly increase the initial 
iron effect, this compound was found to be effective in reducing the 
continued direct oxidation of stannous ions. EDTA did not greatly impact 
the initial iron effect or the continued direct oxidation of the stannous 
ions in the bath. 
FIG. 7 shows the combined effects of citric acid, the most effective of the 
carboxylic acids tested above, and hydroquinone, the most effective 
aromatic compound tested above. The effects of adding a combination of 0.1 
citric acid and 0.1 hydroquinone are a very sharp decrease in the initial 
effect of iron and a large reduction in the continued direct oxidation of 
stannous ions in the bath. 
Although the additives (and methods for using them) disclosed above are 
effective when used alone or in combination to stabilize the stannous tin 
concentrations in halogen plating baths, they may also be used in 
conjunction with other iron removal technologies. It is contemplated that 
these addition agents, when used in this manner, may stabilize iron ions 
in the plating bath for a time sufficient to permit circulation of the 
bath through an iron removal apparatus, such as ion exchange columns or 
molecular recognition technology, to remove them from a plating bath. 
Because these iron removal systems require time to process the bath, it is 
believed that the addition agents disclosed will permit the use of these 
slower iron removal systems in a high-speed plating line which in the past 
required that iron be removed rapidly from the plating bath (i.e., with 
sodium ferrocyanide). 
It is also within the scope of the present invention to use the additives 
to stabilize non-halogen tin plating baths. 
While the invention has been described herein with reference to specific 
embodiments, it is not limited thereto. Rather it should be recognized 
that this invention may be practiced as outlined above within the spirit 
and scope of the appended claims, with such variants and modifications as 
may be made by those skilled in this art.