Composition and process for treating tinplate

To impart an excellent corrosion resistance and adherence to the surface of tinplate while avoiding the production of sludge in the treatment bath during continuous treatment, a bath is used that contains phosphate ions, from 0.1 to 5.0 g/L of chelating agent, and tin ions; has a pH from 2.0 to 4.5; is essentially free of oxidizing agent and ferric ion; and has an oxidation-reduction potential of .ltoreq.450 mV more oxidizing than a silver-saturated silver chloride reference electrode.

TECHNICAL FIELD 
The invention relates to a phosphate containing composition (often denoted 
hereinafter as a "bath" for brevity) for treating the surface of tinplate 
(i.e,, tin-plated steel) and to a treatment process for tinplate. More 
specifically, the present invention relates to an improvement of a 
treatment that is already used, prior to the painting or printing of 
surfaces of tinplate sheet, strip, and formed objects, e.g., cans and the 
like, to provide such surfaces with an excellent corrosion resistance and 
paint adherence. In particular, the treatment bath and treatment process 
of the present invention are well adapted for treating surfaces of 
tinplate that has been formed by DI (i.e., drawing-and-ironing) 
processing. Thus, the present invention concerns a novel technology for 
treating tinplate surfaces, a technology that may be used to provide 
tinplate surfaces with an excellent corrosion resistance and paint 
adherence, but which is free or very nearly free of the insoluble salts 
(hereinafter referred to as "sludge") that are produced by the tin ions 
and iron ions that elute into the bath during treatment. This sludge 
reduces the productivity of tinplate surface treatment lines. 
BACKGROUND ART 
The cleaning and surface treatment of tinplate is frequently conducted by a 
spray process. For example, the surface treatment equipment for tinplate 
DI can is generally called a washer. Molded DI can is inverted and 
continuously treated in the washer with a cleaning bath and a surface 
treatment bath. Existing washers normally use 6 steps (pre-cleaning, 
cleaning, water wash, surface treatment, water wash, and wash with 
de-ionized water), and treatment is conducted entirely by spraying. 
Compositions of phosphate ion, tin ion, and oxidizing agent are already 
known as surface treatment baths for tinplate DI can. As discussed by the 
present inventors in Nihon Parkerizing Giho, 89, No. 2, page 6, the 
mechanism of conversion film formation by these components consists of tin 
and iron elution reactions (anodic reactions) and the precipitation of 
insoluble phosphate salts (cathodic reaction). 
Furthermore, in Japanese Patent Application Laid Open [Kokai or Unexamined] 
Number Hei 1-100281 [100,281/1989]), there has already been proposed a 
composition for the purpose of inverting the tin-iron potential in the 
conversion bath, i.e., the tin region becomes the anode and the iron 
region becomes the cathode. This particular invention consists of a 
conversion coating bath for the treatment of metal surfaces. This bath has 
a pH of 2 to 6 and contains 1 to 50 grams per liter (hereinafter often 
abbreviated "g/L") of phosphate ions, 0.2 to 20.0 g/L of oxyacid ions, 
0.01 to 2.0 g/L of tin ion, and 0.01 to 5.0 g/L of condensed phosphate 
ions. Treatment with this conversion treatment bath forms a highly 
corrosion-resistant, highly paint-adherent phosphate film on the surface 
of tinplate DI can. The oxyacid ion is an oxidizing agent that functions 
to oxidatively remove the hydrogen that is produced by the anodic 
reactions. 
When the aforesaid invention is practiced on a continuous basis, it is in 
fact capable of initially providing an excellent surface treatment. 
However, it has been found that the referenced invention gradually 
generates a phosphate salt sludge, which is produced by the reaction of 
the phosphate ions present in the bath with the tin ions and iron ions 
that elute from the tinplate. It has also been determined that iron ions 
elute from the tinplate in the divalent state; that gradual oxidation by 
the oxidizing agent (oxyacid ion, etc.) produces the trivalent state in 
the surface treatment bath at a level of approximately 0.05 g/L; and that 
this is the cause of sludge production. 
This sludge can cause problems by adhering to the tinplate surface and 
degrading the paint adherence. In addition, the sludge can clog the piping 
and nozzles of the spray equipment and can thereby prevent a high quality 
surface treatment. This has necessitated the implementation of periodic 
maintenance in order to clean the piping and nozzles of the spray 
equipment and has resulted in unstable quality characteristics. Since 
productivity enhancements and improvements in quality stability have 
recently become critical issues, a surface treatment bath is desired that 
carries a reduced cleaning burden and that offers stable quality 
characteristics, i.e., that is free of sludge production in the bath even 
during continuous service. 
DISCLOSURE OF THE INVENTION 
Problem(s) to Be Solved by the Invention 
Accordingly, the present invention takes as its object the introduction of 
a bath and process for treating tinplate surfaces that solves the problems 
described above and that enhances quality stability and leads to 
improvements in productivity (easy maintenance and the like). 
SUMMARY OF THE INVENTION 
As a result of extensive research into the problems described above, it was 
determined that sludge production is particularly significantly influenced 
by the oxidation state (divalent or trivalent) in the treatment 
composition of the iron ions present therein, which normally elute from 
the tinplate during treatment with the composition. With respect to a bath 
for treating tinplate surfaces that comprises, preferably consists 
essentially of, or more preferably consists of, water, acidity, phosphate 
ions, chelating agent, and tin ions, it was also determined that an 
excellent corrosion resistance and paint adherence could be obtained 
without sludge production--even during continuous treatment--by such a 
bath for treating tinplate surfaces that has a pH in the range of 2.0 to 
4.5 and a concentration of chelating agent in the range of 0.1 to 5.0 g/L 
and that essentially does not contain ferric iron or an oxidizing agent 
sufficiently strong to oxidize ferrous to ferric ions. The present 
invention was achieved based on these findings. 
In addition, the iron ions eluting from tinplate often undergo spontaneous 
oxidation to the trivalent state when the surface treatment process 
employs the surface treatment bath on a continuous basis. With the 
objective of maintaining the iron ions in the divalent state, the use of 
the oxidation-reduction potential to monitor the oxidation state of the 
iron ions was therefore examined. As a result, with respect to the 
treatment of tinplate surfaces by contacting tinplate with an acidic 
surface treatment bath that contains at least phosphate ion, chelating 
agent, and tin ion, a method for treating tinplate surfaces was discovered 
whose characteristic features are a pH in the surface treatment bath in 
the range of 2.0 to 4.5 and control of the oxidation-reduction potential 
of the surface treatment bath to .ltoreq.450 mV by the addition of 
reducing agent on an as-required basis. The present invention was also 
achieved based on this discovery. The structure of the present invention 
is explained in detail below. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
Phosphoric acid (H.sub.3 PO.sub.4), sodium phosphate (Na.sub.3 PO.sub.4), 
and the like can be used to provide the phosphate ion, and this component 
should be used in quantities sufficient to bring about tin phosphate 
precipitation. The reactivity is low when phosphate ion is present at less 
than 1 g/L, and this prevents satisfactory formation of the coating under 
ordinary treatment conditions. While a good quality coating is formed at 
values in excess of 30 g/L, the corresponding high cost of the treatment 
bath becomes economically disadvantageous. Thus, the phosphate ion is 
present preferably in the range of 1 to 30 g/L and more preferably in the 
range of 4 to 8 g/L. 
The present invention requires that the bath contain chelating agent in a 
quantity sufficient to bring about a satisfactory etching, selective 
conversion film formation on exposed iron regions, and a satisfactory tin 
ion stabilization. Preferred chelating agents that meet these requirements 
are exemplified by condensed phosphate ions, tartaric acid, oxalic acid, 
and citric acid. Particularly preferred chelating agents comprise at least 
one selection from the condensed phosphate ions. This is because the 
condensed phosphate ions gradually decompose to phosphoric acid and 
therefore have little to no adverse effect on waste water treatment. The 
acid or salt can be used to provide condensed phosphate ion. For example, 
pyrophosphoric acid (H.sub.4 P.sub.2 O.sub.7), sodium pyrophosphate 
(Na.sub.4 P.sub.2 O.sub.7), and so forth can be used to provide 
pyrophosphate ion. The etching activity is weak and film formation is 
unsatisfactory at a chelating agent concentration of less than 0.1 g/L. On 
the other hand, the etching activity is too strong and the film-formation 
reactions are inhibited at more than 5 g/L of chelating agent. The 
chelating agent content therefore preferably falls in the range of 0.1 to 
5 g/L and particularly preferably falls in the range of 0.2 to 1.0 g/L. 
Since tinplate DI can has been subjected to DI processing, its surface 
presents both tin-plated regions and iron regions that have been exposed 
by the processing, and the corrosion resistance is generally poor when 
large areas of iron are exposed. For this reason, the generation of 
uniform coverage of the exposed iron regions by the conversion coating is 
a crucial issue from the standpoint of improving the corrosion resistance. 
Because the surface treatment bath of the present invention contains a 
chelating agent, it is able to selectively and uniformly cover the exposed 
iron regions with a conversion coating, whereas a very poor conversion is 
produced at these exposed iron regions in the absence of chelating agent. 
This makes possible the production of a highly corrosion-resistant 
conversion film. Moreover, the chelating agent and particularly the 
condensed phosphates function to stabilize the eluted tin ions in the bath 
and therefore also act to inhibit sludge production. 
The tin ions can be supplied by tin metal or a tin salt, for example, tin 
chloride, but the tin source is not specifically restricted. In the case 
of continuous treatment, supplemental additions are not specifically 
required due to elution of tin ion from the tinplate. The tin ion content 
should be selected so as to yield the formation of a satisfactory tin 
phosphate coating, and preferably falls into the range of 0.01 to 2.0 g/L, 
more preferably into the range of 0.1 to 1.0 g/L, and particularly 
preferably into the range of 0.2 to 0.6 g/L. The range of 0.01 to 2.0 g/L 
yields a highly corrosion resistant film and avoids the precipitation of 
sludge. 
The pH of the treatment bath should be maintained at 2.0 to 4.5. Strong 
etching and an inhibition of film formation are obtained at below 2.0. The 
anodic reaction conditions suffer from substantial deterioration when the 
pH exceeds 4.5 because the development of the anodic reactions is 
inhibited due to the essential absence of oxidizing agent from the 
treatment bath in accordance with the present invention. Accordingly, the 
pH must be held in the range of 2.0 to 4.5, and is preferably held in the 
range of 2.5 to 3.5 and more preferably in the range of 2.7 to 3.3. The pH 
may be adjusted through the use of an acid such as phosphoric acid, 
sulfuric acid, and the like or through the use of an alkali such as sodium 
hydroxide, sodium carbonate, ammonium hydroxide, and the like. 
A characteristic feature of the treatment bath in accordance with the 
present invention is that essentially it contains neither ferric iron ions 
nor any oxidizing agent that will oxidize any substantial amount of 
ferrous iron ions to ferric iron ions. Preferably, the concentration of 
ferric ions in any surface treatment bath according to this invention is 
not greater than 7 mg/L, more preferably not greater than 3 mg/L, still 
more preferably not greater than 2.0 mg/L, or most preferably not greater 
than 1.1 mg/L. 
Although prior surface treatment baths have contained oxidizing agent, the 
surface treatment bath in accordance with the present invention 
essentially does not contain an oxidizing agent such as oxyacid ion or the 
like, that is, does not contain oxidizing agent which substantially 
removes the hydrogen produced by anodic reactions. Given that trivalent 
iron ion facilitates the occurrence of sludge precipitation, the reason 
for omitting the oxidizing agent is that the presence of oxidizing agent 
leads to a condition in which both divalent and trivalent iron ions are 
present. 
The absence of oxidizing agent from tinplate surface treatment baths has 
heretofore resulted in Unstable conversion characteristics and in 
particular in an inability to obtain a uniform conversion at exposed iron 
regions, and for these reasons the absence of oxidizing agent has 
heretofore been considered undesirable. However, the continuous execution 
of conversion while still maintaining a good quality conversion film is 
made possible even in the absence of oxidizing agent by holding the pH and 
chelating agent concentration within the ranges specified above. 
Another crucial point in the treatment process in accordance with the 
present invention is that the oxidation-reduction potential of the 
treatment bath is to be controlled to .ltoreq.450 mV during treatment. No 
specific restrictions apply to the electrodes used to measure the 
oxidation-reduction potential. The potentials provided in the present 
invention were obtained using a platinum electrode as the 
oxidation-reduction electrode and a silver-saturated silver chloride 
electrode as the reference electrode. When the oxidation-reduction 
potential is .ltoreq.450 mV during this measurement, the iron ion is 
present almost entirely in the divalent state and the production of sludge 
is inhibited. 
In addition to deliberately added oxidizing agent, atmospheric oxygen also 
can oxidize the divalent iron ions in the treatment bath. The tendency for 
the divalent iron ions to be oxidized by atmospheric oxygen varies as a 
function of the precise nature of the equipment, the spray conditions, and 
the like. The oxidation-reduction potential may in some cases exceed 450 
mV when the present invention is implemented on a continuous basis under 
conditions in which air tends to be taken up and the difficult-to-avoid 
removal of bath by the treatment substrate requires only minor renewal of 
the surface treatment bath. Because sludge will be produced under such 
circumstances and quality and equipment maintenance will then again become 
problematic, reducing agent must be added on a preliminary basis or when 
the oxidation-reduction potential becomes elevated in order thereby to 
maintain the oxidation-reduction potential at .ltoreq.450 mV. No specific 
restrictions apply to this reducing agent, but substances that inhibit 
conversion film formation on the tinplate by the surface treatment bath 
should be avoided. Viewed from this perspective, phosphorous acid and 
hypophosphorous acid are preferred as reducing agents, because the main 
component of the surface treatment bath is phosphate ion and both 
phosphorous acid and hypophosphorous acid are converted into phosphate ion 
in fulfilling their function as reducing agent. Thus, adverse effects due 
to an accumulation of their decomposition product are completely avoided. 
Phosphorous acid and hypophosphorous acid can be added as the acid or salt. 
The quantity of addition will vary as a function of the treatment 
conditions, but is preferably as small as possible from the standpoint of 
economics. Thus, the presence or addition of the minimum quantity that 
maintains the oxidation-reduction potential at .ltoreq.450 mV is 
sufficient. In other words, the quantity of addition of the reducing agent 
can be regulated based on the oxidation-reduction potential. When the 
reducing agent is supplied so as to maintain the oxidation-reduction 
potential at .ltoreq.450 mV, substantially all of the iron ions in the 
composition are maintained in the divalent state and the production of 
sludge in the surface treatment bath can be prevented even during 
continuous treatment over long periods of time. 
The conversion film that is formed will now be briefly considered. The 
conversion film that is formed by a phosphate surface treatment bath for 
tinplate is generally a phosphate salt whose principal component is tin 
phosphate, and the basic mechanism for its formation is believed to be the 
same even for the present invention. Thus, the tinplate substrate is 
etched by the phosphate ions and chelating agent (particularly condensed 
phosphate ions); a local increase in the pH at the interface occurs at 
this time; and a phosphate conversion film (principally of tin phosphate) 
precipitates on the surface. 
One difference between prior phosphate films and the phosphate film of the 
present invention is the fact that the prior films are produced in the 
presence of chelating agent and oxidizing agent while in the present 
invention production occurs in the presence of chelating agent and 
(optionally) reducing agent, i.e., the iron ions are only in the divalent 
state and production occurs essentially in the absence of trivalent ferric 
ions. A second difference is that the "sludge skin" is then presumably 
negligible for the film of the present invention. "Sludge skin" refers to 
the adhesion of a relatively poorly adherent, sediment-like substance in 
the vicinity of the tin phosphate film proper. Moreover, because the 
phosphate film formed on tin-plated steel sheet in the case of tinplate DI 
can is usually extremely thin, approximately 10 to 20 .ANG.ngstroms, in 
both the tin-plated regions and the exposed iron regions, the sludge skin 
is not susceptible in this case to visual evaluation, in contrast to 
ordinary zinc phosphate films, for which the areal density is 
approximately 1 to 10 g/m.sup.2 and the corresponding thickness from 1,000 
to 8,000 .ANG.ngstroms. The exact situation has therefore yet to be 
elucidated. 
The treatment of tinplate using the surface treatment bath of the present 
invention is briefly explained below. The treatment bath of the present 
invention is used, preferably as part of the following sequence, which is 
provided as a preferred example: 
Tinplate cleaning: degreasing (a weakly alkaline degreaser is typically 
used) 
Water wash 
Surface treatment (application of treatment bath of the present invention) 
Treatment temperature: 30.degree. C. to 70.degree. C. 
Treatment technique: spray or immersion 
Treatment time: 2 to 40 seconds 
Water wash 
Wash with de-ionized water 
Drying. 
The treatment temperature with the surface treatment bath of the present 
invention is preferably 30.degree. C. to 70.degree. C., and heating the 
bath generally to 40.degree. C. to 60.degree. C. for use is particularly 
preferred. The preferred treatment time is 2 to 40 seconds. At below 2 
seconds, the reaction is inadequate and a highly corrosion-resistant film 
will not normally be formed. On the other hand, the performance does not 
improve at treatment times in excess of 40 seconds, and therefore optimal 
treatment times fall in the range of 2 to 40 seconds. 
While the treatment technique can be either immersion or spray, as 
discussed above the present invention gives particularly good effects when 
used with spray equipment. 
As discussed hereinbefore, the oxidation state of the iron ions that have 
eluted from the tinplate significantly affects sludge production. Iron 
ions are believed to elute from the tinplate as divalent ferrous ions. In 
the treatment bath in accordance with the present invention, the iron ions 
are typically present as ferrous ions at a concentration of about 0.005 to 
about 0.025 g/L when the line is running, while ferric ions are 
essentially not present. In contrast to this, the ferrous ions are almost 
entirely oxidized in prior art treatment baths to yield ferric ions or 
colloid in a concentration typically on the level of 0.05 g/L. Sludge is 
produced because this ferric ion and the phosphate ion form an insoluble 
salt that also traps the tin and phosphate ions that are present. In other 
words, sludge production in the surface treatment bath can be suppressed 
by maintaining the iron ion eluted from the tinplate in the divalent 
state. 
By essentially omitting the oxidizing agent that has been used in prior-art 
treatment baths, the iron ions in the present invention consist almost 
completely of divalent iron ions. It is thought that this occurs because 
both divalent tin ions and tetravalent tin ions are present and the 
divalent tin ions rapidly reduce trivalent iron ions to divalent iron 
ions. 
The oxidation-reduction potential of a composition is measured by the 
equilibrium electrode potential of an inert oxidation-reduction electrode 
in contact with the composition, and it represents the magnitude of the 
oxidizing power or reducing power of the composition. The following 
equation gives the oxidation-reduction potential E.sub.e for the 
half-reaction oxidation of ferrous ion to ferric ion according to the 
chemical equation Fe.sup.2+ .fwdarw.Fe.sup.3+ +e.sup.-. 
EQU E.sub.e =E.sub.o --(RT/ In([Fe.sup.+2 ]/[Fe.sup.+3 ]), 
where R=the gas constant, T=the absolute temperature, =Faraday's constant, 
square brackets indicate activities of the chemical species within the 
brackets, and E.sub.o =the standard electrode potential for the reaction. 
Larger values of E.sub.e correspond to a higher oxidizing power and thus 
to a higher ferric ion/ferrous ion ratio; smaller values of the 
oxidation-reduction potential indicate fewer ferric ions. Accordingly, the 
average oxidation state of the eluted iron ions can be controlled by 
controlling the oxidation-reduction potential.

EXAMPLES 
The utility of the surface treatment bath of the present invention is 
explained below through a comparison of several working examples with 
comparison examples. In these examples, the tinplate substrates consisted 
of tinplate DI cans fabricated by the DI processing of tin-plated steel 
sheet. The corrosion resistance after surface treatment was evaluated 
using the iron exposure value ("IEV"). The IEV was measured in accordance 
with U.S. Pat. No. 4,332,646. Lower IEV values correspond to a better 
corrosion resistance, and values .ltoreq.150 generally correspond to an 
excellent corrosion resistance. 
The paint adherence was evaluated through the peel strength. An epoxy/urea 
can paint was coated on the surface of the treated can to a paint film 
thickness of 5 to 7 micrometers (".mu.M") followed by baking for 4 minutes 
at 215.degree. C. Each can was subsequently cut into 5.times.150 mm 
strips, and a test specimen was prepared by hot pressing polyamide film 
onto a strip. The test specimen was then peeled in a 180.degree. peel test 
and the peel strength was measured. In this case, larger peel strength 
values indicate a better paint adherence, and values of 1.5 kilograms 
force ("kgf")/5 mm-width or more are generally regarded as excellent. 
Sludge production was evaluated as follows. 0.05 g/L of iron ions from 
ferrous chloride was added to the particular surface treatment bath as 
described in the working or comparison example, the pH was adjusted, the 
bath was allowed to stand for 1 day, and the status of the bath was then 
inspected. A bath that was transparent and free of precipitate or the like 
was judged as essentially free of ferric ion. The oxidation-reduction 
potential was measured after standing using a platinum electrode as the 
oxidation-reduction electrode and a silver-saturated silver chloride 
electrode as the reference electrode. 
In order to evaluate sludge production during continuous treatment, a 
continuous treatment was run using freshly prepared surface treatment bath 
as reported in the particular example or comparison example. The 
continuous treatment used 2 liters ("L") of treatment bath, and a 
30-second treatment was conducted on a total of 360 cans. The bath 
quantity and pH were maintained at their initial values through the 
addition of the particular surface treatment bath and phosphoric acid, 
respectively. The bath status and oxidation-reduction potential ("ORP") 
were evaluated after the continuous test. 
A bath that was transparent and free of precipitate or the like was judged 
to be essentially free of ferric ion. In addition, the iron ion 
concentration in the treatment bath after continuous treatment was 
measured by atomic absorption. When a precipitate had been produced, 
analysis was run by dissolving the precipitate by the addition of 
hydrochloric acid. 
EXAMPLE 1 
Tinplate DI cans (fabricated by the DI processing of tin-plated steel 
sheet) were (1) thoroughly cleaned using a hot 1% aqueous solution of a 
weakly alkaline degreaser (FINECLEANER.TM. 4488 from Nihon Parkerizing 
Company, Limited); (2) sprayed for 20 seconds with surface treatment bath 
1 heated to 60.degree. C.; (3) washed with tap water; (4) sprayed with 
deionized water (with a specific resistance .gtoreq.3 Mohm-cm) for 10 
seconds; and (5) dried in a hot-air drying oven for 3 minutes at 
180.degree. C. The treated cans were evaluated for corrosion resistance 
and paint adherence, and surface treatment bath 1 was evaluated for sludge 
production. 
______________________________________ 
Surface treatment bath 1 
______________________________________ 
75% phosphoric acid (H.sub.3 PO.sub.4) 
10.0 gL (PO.sub.4.sup.3- : 7.2 g/L) 
Sodium pyrophosphate 
1.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.4 gL) 
(Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O) 
SnCl.sub.4.5H.sub.2 O 
0.6 g/L (Sn.sup.3+ : 0.2 g/L) 
FeCl.sub.3.6H.sub.2 O 
4.8 mg/L (Fe.sup.3+ : 1.0 mg/L) 
Phosphorous acid (H.sub.3 PO.sub.3) 
0.01 g/L 
pH 3.0 (adjusted with sodium carbonate) 
______________________________________ 
The ferric chloride was added in order to examine the effect of trivalent 
iron ion on sludge production. 
EXAMPLE 2 
Tinplate DI can was cleaned using the same conditions as in Example 1, 
sprayed for 10 seconds with surface treatment bath 2 heated to 40.degree. 
C., and then washed with water and dried under the same conditions as in 
Example 1. The treated can was evaluated for corrosion resistance and 
paint adherence, and surface treatment bath 2 was evaluated for sludge 
production. 
______________________________________ 
Surface treatment bath 2 
______________________________________ 
75% phosphoric acid (H.sub.3 PO.sub.4) 
5.0 gL (PO.sub.4.sup.3- : 3.6 g/L) 
Sodium pyrophosphate 
2.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.8 g/L) 
(Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O) 
SnCl.sub.4.5H.sub.2 O 
1.2 g/L (Sn.sup.4+ : 0.4 g/L) 
pH 2.8 (adjusted with phosphoric acid) 
______________________________________ 
EXAMPLE 3 
Tinplate DI can was cleaned using the same conditions as in Example 1, 
sprayed for 40 seconds with surface treatment bath 3 heated to 60.degree. 
C., and then washed with water and dried under the same conditions as in 
Example 1. The treated can was evaluated for corrosion resistance and 
paint adherence, and surface treatment bath 3 was evaluated for sludge 
production. 
______________________________________ 
Surface treatment bath 3 
______________________________________ 
75% Phosphoric acid (H.sub.3 PO.sub.4) 
5.0 g/L (PO.sub.4.sup.3- : 3.6 g/L) 
Sodium pyrophosphate 
2.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.8 g/L) 
(Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O) 
SnCl.sub.4.5H.sub.2 O 
0.10 g/L (Sn.sup.4+ : 0.03 g/L) 
Hypophosphorous acid (H.sub.3 PO.sub.2) 
0.01 g/L 
pH 4.0 (adjusted with sodium hydroxide) 
______________________________________ 
EXAMPLE 4 
Tinplate DI can was cleaned using the same conditions as in Example 1, 
sprayed for 10 seconds with surface treatment bath 4 heated to 40.degree. 
C., and then washed with water and dried under the same conditions as in 
Example 1. The treated can was evaluated for corrosion resistance and 
paint adherence, and surface treatment bath 4 was evaluated for sludge 
production. 
______________________________________ 
Surface treatment bath 4 
______________________________________ 
75% Phosphoric acid (H.sub.3 PO.sub.4) 
15.0 g/L (PO.sub.4.sup.3- : 10.8 g/L) 
Sodium pyrophosphate 
2.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.8 g/L) 
(Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O) 
Sodium tripolyphosphate 
1.0 g/L (P.sub.3 O.sub.10.sup.5- : 0.6 g/L) 
(Na.sub.5 P.sub.3 O.sub.10) 
SnCl.sub.4.5H.sub.2 O 
1.2 g/L (Sn.sup.4+ : 0.4 g/L) 
Phosphorous acid (H.sub.3 PO.sub.3) 
0.01 g/L 
Hypophosphorous acid (H.sub.3 PO.sub.2) 
0.01 g/L 
pH 3.0 (adjusted with sodium carbonate) 
______________________________________ 
EXAMPLE 5 
Tinplate DI can was cleaned using the same conditions as in Example 1, 
sprayed for 30 seconds with surface treatment bath 5 heated to 50.degree. 
C., and then washed with water and dried under the same conditions as in 
Example 1. The treated can was evaluated for corrosion resistance and 
paint adherence, and surface treatment bath 5 was evaluated for sludge 
production. 
______________________________________ 
Surface treatment bath 5 
______________________________________ 
75% Phosphoric acid (H.sub.3 PO.sub.4) 
1.0 g/L (PO.sub.4.sup.3- : 0.7 g/L) 
Sodium pyrophosphate 
2.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.8 g/L) 
(Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O) 
SnCl.sub.4.5H.sub.2 O 
1.2 g/L (Sn.sup.4+ : 0.4 g/L) 
Phosphorous acid (H.sub.3 PO.sub.3) 
0.01 g/L (H.sub.3 PO.sub.3 : 0.01 g/L) 
pH 3.0 (adjusted with phosphoric acid) 
______________________________________ 
EXAMPLE 6 
Tinplate DI can was cleaned using the same conditions as in Example 1, 
sprayed for 20 seconds with surface treatment bath 6 heated to 50.degree. 
C., and then washed with water and dried under the same conditions as in 
Example 1. The treated can was evaluated for corrosion resistance and 
paint adherence, and surface treatment bath 6 was evaluated for sludge 
production. 
______________________________________ 
Surface treatment bath 6 
______________________________________ 
75% Phosphoric acid (H.sub.3 PO.sub.4) 
5.0 g/L (PO.sub.4.sup.3- : 3.6 g/L) 
Sodium pyrophosphate 
2.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.8 g/L) 
(Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O) 
Tin (by dissolution of tin metal) 
0.2 g/L (Sn.sup.2+ : 0.2 g/L) 
Phosphorous acid (H.sub.3 PO.sub.3) 
0.01 g/L (H.sub.3 PO.sub.3 : 0.01 g/L) 
pH 3.0 (adjusted with phosphoric acid) 
______________________________________ 
EXAMPLE 7 
Tinplate DI can was cleaned using the same conditions as in Example 1, 
sprayed for 2 seconds with surface treatment bath 7 heated to 70.degree. 
C., and then washed with water and dried under the same conditions as in 
Example 1. The treated can was evaluated for corrosion resistance and 
paint adherence, and surface treatment bath 7 was evaluated for sludge 
production. 
______________________________________ 
Surface treatment bath 7 
______________________________________ 
75% Phosphoric acid (H.sub.3 PO.sub.4) 
30.0 g/L (PO.sub.4.sup.3- : 21.6 g/L) 
Sodium pyrophosphate 
2.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.8 g/L) 
(Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O) 
Sodium tripolyphosphate 
1.0 g/L (P.sub.3 O.sub.10.sup.5- : 0.6 g/L) 
(Na.sub.5 P.sub.3 O.sub.10) 
SnCl.sub.4.5H.sub.2 O) 
1.2 g/L (Sn.sup.4+ : 0.4 g/L) 
Phosphorous acid (H.sub.3 PO.sub.3) 
0.01 g/L 
Hypophosphorous acid (H.sub.3 PO.sub.2) 
0.01 g/L 
pH 2.0 (adjusted with phosphoric acid) 
______________________________________ 
Comparison Example 1 
Tinplate DI can was cleaned using the same conditions as in Example 1, 
sprayed for 30 seconds with surface treatment bath 8 heated to 40.degree. 
C., and then washed with water and dried under the same conditions as in 
Example 1. The treated can was evaluated for corrosion resistance and 
paint adherence, and surface treatment bath 8 was evaluated for sludge 
production. 
______________________________________ 
Surface treatment bath 8 
______________________________________ 
75% Phosphoric acid (H.sub.3 PO.sub.4) 
10.0 g/L (PO.sub.4.sup.3- : 7.2 g/L) 
SnCl.sub.4.5H.sub.2 O 
0.6 g/L (Sn.sup.4+ : 0.2 g/L) 
Phosphorous acid (H.sub.3 PO.sub.3) 
0.01 g/L 
pH 3.0 (adjusted with sodium carbonate) 
______________________________________ 
Comparison Example 2 
Tinplate DI can was cleaned using the same conditions as in Example 1, 
sprayed for 30 seconds with surface treatment bath 9 heated to 50.degree. 
C., and then washed with water and dried under the same conditions as in 
Example 1. The treated can was evaluated for corrosion resistance and 
paint adherence, and surface treatment bath 9 was evaluated for sludge 
production. 
______________________________________ 
Surface treatment bath 9 
______________________________________ 
75% Phosphoric acid (H.sub.3 PO.sub.4) 
10.0 g/L (PO.sub.4.sup.3- : 7.2 g/L) 
Sodium pyrophosphate 
1.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.4 g/L) 
(Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O) 
SnCl.sub.4.5H.sub.2 O 
0.6 g/L (Sn.sup.4+ : 0.2 g/L) 
Phosphorous acid (H.sub.3 PO.sub.3) 
0.01 g/L 
pH 4.6 (adjusted with sodium hydroxide) 
______________________________________ 
Comparison Example 3 
Tinplate DI can was cleaned using the same conditions as in Example 1, 
sprayed for 30 seconds with surface treatment bath 10 heated to 50.degree. 
C., and then washed with water and dried under the same conditions as in 
Example 1. The treated can was evaluated for corrosion resistance and 
paint adherence, and surface treatment bath 10 was evaluated for sludge 
production. 
______________________________________ 
Surface treatment bath 10 
______________________________________ 
75% Phosphoric acid (H.sub.3 PO.sub.4) 
1.33 g/L (PO.sub.4.sup.3- : 0.97 g/L) 
Sodium pyrophosphate 
1.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.4 g/L) 
(Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O) 
SnCl.sub.4.5H.sub.2 O 
0.6 g/L (Sn.sup.4+ : 0.2 g/L) 
FeCl.sub.3.6H.sub.2 O) 
48 mg/L (Fe.sup.3+ : 10 mg/L) 
pH 4.0 (adjusted with sodium carbonate) 
______________________________________ 
Comparison Example 4 
Tinplate DI can was cleaned using the same conditions as in Example 1 and 
was then sprayed for 30 seconds with a 4% aqueous solution (heated to 
50.degree. C.) of a commercial tinplate DI can surface treatment agent 
(FOS.TM. K3466 from Nihon Parkerizing Company, Limited). This was 
followed by washing with water and drying under the same conditions as in 
Example 1. The treated can was evaluated for corrosion resistance and 
paint adherence, and the treatment bath was evaluated for sludge 
production. 
Comparison Example 5 
Tinplate DI can was cleaned using the same conditions as in Example 1 and 
was then sprayed for 30 seconds with a 4% aqueous solution (heated to 
50.degree. C.) of a commercial tinplate DI can surface treatment agent 
(FOS.TM. K3482 from Nihon Parkerizing Company, Limited). This was 
followed by washing with water and drying under the same conditions as in 
Example 1. The treated can was evaluated for corrosion resistance and 
paint adherence, and the treatment bath was evaluated for sludge 
production. 
The results are reported in Table 1. 
Benefits of the Invention 
As discussed in the preceding, treating the surface of tinplate (tin-plated 
steel) sheet, strip, or shaped objects (cans or the like) with the surface 
treatment bath of the present invention accrues the highly desirable 
effects of imparting an excellent corrosion resistance and adherence to 
the tinplate surface and avoiding sludge production in the treatment bath 
when treatment is run on a continuous basis. 
TABLE 1 
__________________________________________________________________________ 
Example ("E") 
or Com- 
parison Conversion Film Quality 
Sludge Production Results 
Example Corrosion With Direct Addition of Iron 
After Continuous Use for Treatment 
("CE") Resistance 
Adhesion, 
Bath Bath 
Number (IEV Value) 
kgf/5 mm 
Appearance 
ORP, mV 
Appearance 
ORP, mV 
Iron, ppm 
__________________________________________________________________________ 
E 1 100 3.0 Transparent 
370 Transparent 
170 18 
E 2 100 3.0 Transparent 
430 Transparent 
260 9 
E 3 120 2.5 Transparent 
420 Transparent 
210 10 
E 4 100 3.0 Transparent 
400 Transparent 
200 8 
E 5 100 3.0 Transparent 
390 Transparent 
200 12 
E 6 100 3.0 Transparent 
400 Transparent 
210 18 
E 7 150 2.0 Transparent 
430 Transparent 
230 25 
CE 1 500 1.5 Tur. W. Ppt. 
450 Tur. W. Ppt. 
650 80 
CE 2 300 1.5 Transparent 
400 Transparent 
350 8 
CE 3 300 1.5 W. Turbidity 
550 Tur. W. Ppt. 
650 30 
CE 4 100 3.0 Tur. W. Ppt. 
700 Tur. W. Ppt. 
700 60 
CE 5 100 3.0 Tur. W. Ppt. 
700 Tur. W. Ppt. 
700 25 
__________________________________________________________________________ 
Notes for Table 1 
"ORP" means "oxidationreduction potential", which was measured against a 
silversaturated silve chloride reference electrode; "IEV" means: "iron 
exposure value", as described in the main text; "Tur. W. Ppt." means 
"Turbid White Precipitate"; "W. Turbid." means "white turbidity".