Process not using chlorate or nitrite for the production of nickel and manganese containing zinc phosphate films

The invention concerns a process, not using chlorate or nitrite, for the production of nickel and manganese containing zinc phosphate films on steel, zinc, and/or zinc alloys by spraying, spray-dipping, and/or dipping, using an aqueous solution. An organic oxidizing agent is added, primarily to depolarize the nascent hydrogen produced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for phosphating metal surfaces and, 
more particularly, to a process for the production of manganese- and 
nickel-containing zinc phosphate coatings on steel, zinc, and/or alloys 
thereof. These nickel- and manganese-containing zinc phosphate coatings 
are applied by spray coating, spray/dip coating and dip coating with 
aqueous solutions. 
Processes for phosphating surfaces of iron, steel, zinc and alloys thereof 
and also aluminum have long been known (Ullmanns Encyklopadie der 
technischen Chemie, 4th Edition, Vol. 15, pages 686 and 687). The object 
of phosphating the surfaces mentioned is to increase the adhesive strength 
of paint films and to improve protection against corrosion. 
Acidic zinc and alkali metal phosphate solutions have acquired the greatest 
significance for phosphating processes. Zinc phosphating baths may 
contain, for example, monozinc phosphate, free phosphoric acid, zinc 
nitrate and oxidizing agents as their principal components. The pH value 
of such solutions is typically in the range from 2.8 to 3.4. The process 
essentially comprises two reactions, namely: the pickling reaction and the 
formation of a zinc phosphate coating on the surface to be phosphated. 
Manganese-modified zinc phosphate coatings as a primer for modern paints 
are known from W.A. Roland and K.-H. Gottwald, Metalloberflache, Vol. 42, 
1988/6. According to this publication, the use of manganese ions in 
addition to zinc and nickel ions in low-zinc phosphating processes 
demonstrably improves protection against corrosion, particularly where 
surface-treated fine metal sheets are used. The incorporation of manganese 
in the zinc phosphate coatings leads to relatively small and relatively 
compact crystals showing increased stability to alkalis. At the same time, 
the operating range of phosphating baths is increased; even aluminum in 
conjunction with steel and galvanized steel can be phosphated to form 
coatings for which the quality standard generally reached is guaranteed. 
Phosphating with addition of oxidizing agents has long been practiced in 
the automotive industry. By contrast, in the cold-forming field, several 
processes are operated on the iron side, i.e. with an increased content of 
iron(II) and no addition of oxygen-yielding accelerators. 
The disadvantage of conventional phosphating baths lies in particular in 
the use of nitrites and/or chlorates as accelerators for the phosphating 
reaction. The nitrous gases formed where nitrites are used are known to 
have adverse effects on the usefulness of such baths. 
2. Description of the Invention 
The problem addressed by the present invention was to provide a 
nitrite-free and also chlorate-free phosphating process which, in 
particular, causes no so-called "fish eyes". Fish eyes are understood on 
the one hand to be buildups of zinc salts which are known in English as 
"white staining" and which are caused by the cleaning solution. On the 
other hand, crater-like openings known in English as "nubbing" are formed 
during phosphating. The systems normally used contain chlorate, bromate, 
nitrate, nitrite, peroxide and/or organic nitro compounds, such as 
3-nitrobenzene sulfonate, as accelerators. Hitherto, 3-nitrobenzene 
sulfonate has generally been used together with chlorate and/or nitrite 
accelerators. In the presence of nitrate, however, the system normally 
used, 3-nitrobenzene sulfonic acid/chlorate, produces the above-mentioned 
fish eyes on electrolytically galvanized steel, so that phosphating 
normally has to be carried out in the absence of nitrate. 
SUMMARY OF THE INVENTION 
The problem stated above has been solved by a chlorate-free and 
nitrite-free process for the production of nickel- and 
manganese-containing zinc phosphate coatings on steel, zinc and/or alloys 
thereof by spray coating, spray/ dip coating, and/or dip coating with an 
aqueous solution containing 0.3 to 1.5 g/1 zinc(II), 0.01 to 2.0 g/1 
manganese(II), 0.01to 0.8 g/1 iron(II), 0.3 to 2.0 g/1 nickel(II), 10.0 to 
20.0 g/1 phosphate ions, 2.0 to 10.0 g/1 nitrate ions and 0.1 to 2.0 g/1 
of an organic oxidizing agent, the aqueous solution having a content of 
free acid of 0.5 to 1.8 points and a total acid content of 15 to 35 points 
and Na.sup.+ being present in the quantity required to establish the free 
acid content.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Accordingly, the present invention provides a low-zinc process which is 
characterized by a defined content of iron(II). Through the contact of the 
bath solution with atmospheric oxygen during circulation and spraying, the 
iron(II) is partly oxidized to iron(III) and removed from the system as 
iron phosphate sludge. 
If, in certain types of installation, the iron(II) content rises beyond the 
claimed level of 0.8 g/1, the desired iron(II) content can be established 
by the oxidation of iron(II) to iron(III) through the controlled addition 
of hydrogen peroxide and/or potassium permanganate in the sludge 
elimination system. 
The particular advantage of the process according to the invention is that, 
providing the maximum limits mentioned for iron(II) are observed and a 
single accelerator, namely an organic oxidizing agent, is used, no nitrous 
gases are formed. In addition, the serious problems normally involved in 
the use of nitrate-free phosphating baths do not occur where the process 
according to the invention is applied. In the application of the process 
according to the invention, the iron(II) concentration is determined 
continuously and/or discontinuously. Analytical determinations such as 
these are known to the expert. By using oxidizing agents, particularly in 
the sludge elimination system, an excessively high iron(II) concentration 
can be regulated by formation of iron(III). Accordingly, exact control of 
the iron(II) content is a highly critical aspect of the present invention. 
Accordingly, the present invention provides a zinc phosphating process 
which may be used in particular in the low-zinc range. Phosphate coatings 
containing nickel and manganese in addition to zinc and iron as cations 
are produced by this process. 
According to the invention, the organic oxidizing agent to be used is 
selected so that it makes very little contribution, if any, to the 
oxidation of iron(II) to iron(III). Instead, it is mainly used to 
depolarize the nascent hydrogen. 
In one preferred embodiment of the present invention, the chlorate- and 
nitrite-free process for the production of zinc phosphate coatings on 
steel, zinc and/or alloys thereof by dip coating, spray/dip coating and/or 
dip coating with an aqueous solution is modified by the use of an aqueous 
solution containing 0.8 to 1.0 g/1 zinc(II), 0.8 to 1.0 g/1 manganese(II), 
0.2 to 0.4 g/1 iron(II), 0.5 to 0.7 g/1 nickel(II), 12.0 to 16.0 g/1 
phosphate ions, 3.0 to 6.0 g/1 nitrate ions and 0.3 to 0.8 g/1 of an 
organic oxidizing agent. The free acid content and the total acid content 
correspond to the values mentioned above, as does the quantity of sodium. 
In one preferred embodiment of the present invention, 3-nitrobenzene 
sulfonic acid is used as the organic oxidizing agent. 
Another preferred embodiment of the invention is characterized in that the 
upper limit to the concentration of iron(II) in the aqueous solution is at 
or below 0.3 g/1. Atmospheric oxygen is mainly used for this purpose, 
although other oxidizing agents, such as hydrogen peroxide, oxygen, and/or 
potassium permanganate, may also be used in principle. 
The sodium salt of 3-nitrobenzene sulfonic acid is preferably used as the 
organic oxidizing agent. 
EXAMPLES OF BATH COMPOSITIONS 
Within the usual sequence of process steps, namely: 
1. Cleaning and degreasing: 
Use of surfactant-containing alkaline cleaning solutions applied by 
spraying and/or dipping (for example RIDOLINE C1250) for 1 to 5 minutes at 
50.degree. to 60.degree. C. 
2. Rinsing 
3. Activation: 
Use of preparations containing titanium salts (for example FIXODINE C9112) 
applied separately by spraying or dipping for 30 to 180 s at 20.degree. to 
40.degree. C. The activation stage may be omitted if this activating agent 
is added to the cleaning stage. 
4. Phosphating: 
For composition, see Table 1. 
5. Rinsing 
6. After-passivation: 
Use of chromium-containing or chromium-free post-passivating agents (for 
example DEOXYLYTE 41 or 80) applied by spraying or dipping for 30 to 180 s 
at 20.degree. to 50.degree. C. 
7. Rinsing with deionized water, surface treatment was carried out under 
the following conditions: 
TABLE 1 
______________________________________ 
Phosphating 
Method of application 
Spray Spray/dip Dip 
coating coating coating 
Bath parameters 
(A) (B) (C) 
______________________________________ 
FA.sup.1) (points) 
0.7 1.1 1.3 
TA.sup.2) (points) 
20 22 26 
Zn.sup.2+ gl.sup.-1 
0.8 0.9 1.0 
Mn.sup.2+ gl.sup.-1 
0.8 0.8 0.8 
Ni.sup.2+ gl.sup.-1 
0.6 0.6 0.6 
Fe.sup.2+ gl.sup.-1 
0.2 0.2 0.3 
PO.sub.4.sup.3- gl.sup.-1 
12.6 14.0 15.8 
NBS.sup.3) gl.sup.-1 
0.3 0.4 0.5 
Temp. .degree.C. 
50 52 54 
Time s 90 30 S/180 D 
180 
______________________________________ 
.sup.1) FA = free acid 
.sup.2) TA = total acid 
.sup.3) NBS = 3nitrobenzene sulfonic acid, Na salt 
Weights per unit area of the phosphate coating of 1.3 to 2.5 gm.sup.-2 were 
produced with the above-mentioned variants of spray coating (A), spray/dip 
coating (b) and dip coating (c). In method of application (B), dip coating 
was carried out for 180 s after spraying for 30 s. 
The iron(II) content in the bath solution was kept below the values shown 
in Table 1 by means of oxidizing agents, such as hydrogen peroxide, 
potassium and/or sodium permanganate, ozone, oxygen and/or atmospheric 
oxygen, which were added continuously or discontinuously in the quantities 
required to adjust the iron(II) concentration. 
Steel plates (quality St.1405) treated by method of application B show the 
following layer composition: 
52% hopeite (including Zn.sub.2 Mn(PO.sub.4).sub.2 .times.4 H.sub.2 O) 48% 
phosphophyliite. Metal sheets treated by methods (B), spray coating, and 
(C), dip coating, were subjected to corrosion tests in accordance with the 
VW standard P 1210; alternating climate, test period 30/60 days: (The 
paint used was the standard cathodic electrodeposition primer FT 85 7042 
manufactured by BASF Farben und Lacke.) 
______________________________________ 
Method of application B 
30 days 60 days 
CRS.sup.1) 
Z.sup.2) 
ZE.sup.3) 
CRS Z ZE 
______________________________________ 
Surface acc. 
m0/g0 m0/g0 m0/g0 m0/g0 m0/g0 m0/g0 
to DIN 
53209.sup.4) 
Cutting acc. 
0.3 0.7 0.7 0.5 1.0 1.1 
to DIN 53167 
in mm 
Chipping K 2 K 4 K 2 K 3 K 7 K 2 
behavior 
acc. to 
VW standard 
______________________________________ 
______________________________________ 
Method of application C 
30 days 60 days 
CRS.sup.1) 
Z.sup.2) 
ZE.sup.3) 
CRS Z ZE 
______________________________________ 
Surface acc. 
m0/g0 m0/g0 m0/g0 m0/g0 m0/g0 m0/g0 
to DIN 
53209.sup.4) 
Cutting acc. 
0.3 0.4 0.5 0.5 0.6 0.5 
to DIN 53l67 
in mm 
Chipping K 2 K 3 K 1 K 3 K 6 K 2 
behavior 
acc. to 
VW standard 
______________________________________ 
.sup.1) CRS = coldrolled steel St 1405 
.sup.2) Z = hotgalvanized steel, 10 .mu.m coating on both sides 
.sup.3) ZE = electrolytically galvanized steel, 7.5 .mu.m coating on both 
sides 
.sup.4) Ratio of degree of blistering to blister size. 
In determination of the degree of blistering of paints in accordance with 
DIN53 209, the blistering which occurs in paints is defined by indication 
of the degree of blistering. According to this standard, the degree of 
blistering is a measure of the blistering which has occurred in a paint 
according to the frequency of the blisters per unit area and the size of 
the blisters. The degree of blistering is expressed by a code letter and a 
code number for the frequency of the blisters per unit area and by a code 
letter and a code number for the size of the blisters. The code letter and 
the code number m0 mean no blisters while m5 defines a certain frequency 
of the blisters per unit area in accordance with the degree of blistering 
patterns of DIN 53 209. 
The size of the blisters is denoted by the code letter g and a code number 
in the range from 0 to 5. The code letter and code number g0 mean "no 
blisters" while g5 corresponds to the size of the blisters in accordance 
with the degree of blistering patterns of DIN 53 209. 
By comparing the paint with the degree of blistering patterns, it is 
possible to determine the degree of blistering for which the pattern is 
most similar to the appearance of the paint. 
According to DIN 53 167, the salt spray test according to this standard is 
used to determine the behavior of lacquers, paints and similar coatings 
under the effect of sprayed sodium chloride solution. If the coating has 
any weak spots, pores or damage, the sodium chloride solution creeps 
beneath the coating from those defects, resulting in a reduction or loss 
of adhesion and in corrosion of the metallic substrate. 
The salt spray test is used to reveal such defects so that any creepage can 
be detected. 
In the context of this standard, creepage is understood to be the 
penetration of sodium chloride solution from a point of intentional damage 
(scratch) or from existing weak spots (for example pores, edges) to the 
interface between the coating and the substrate or into the interface 
between individual coatings. The width of the zone with reduced or lost 
adhesion is a measure of the resistance of the coating on the particular 
substrate to the effect of sprayed sodium chloride solution. 
The VW standard P-VW 1210 is an alternating test consisting of a 
combination of various standard tests. Thus, in the present case, a 30/60 
day test cycle consists of salt spray testing for 4 h in accordance with 
DIN 50 021, standing for 4 h at room temperature, and testing for 16 h in 
a constant condensed moisture atmosphere in accordance with DIN 50 017. 
At the beginning of the test, the test specimen is bombarded with a certain 
quantity of steel shot having a certain particle size distribution. After 
the test period, a characteristic value is assigned to the degree of 
corrosion. 
In accordance with code numbers of 1 to 10, the code number 1 stands for no 
visible corrosion while the code number 10 means that virtually the entire 
surface is corroded.