Phosphating solution for metal substrates

This invention relates to conversion coating of metal substrates which may comprise mixed metals such as steel, zinc or zinc based alloys or zinc aluminium alloy coated steel, aluminium or aluminium surfaces to provide a corrosion resistant surface which is subsequently coated with a paint coating. The process of the invention comprises contacting the metal surface with an aqueous composition comprising silica, phosphoric acid and a divalent metal ion, the composition being substantially free of chromium and of any oxy metal anions which the metal has a valency of at least 5. The metal surface is subsequently cured by drying, or preferably by heating immediately after contact with the composition with no intermediate rinsing stage. A paint coating is then applied. The process provides a painted surface, the coating providing an effective anti-corrosion coating and paint having good adhesion.

This invention relates to conversion coating of metal substrates which may 
comprise steel, zinc or zinc based alloys or zinc aluminium alloy coated 
steel, aluminium or aluminium alloy surfaces to provide a corrosion 
resistant surface which can subsequently be coated with a paint coating. 
In particular the invention relates to a dry-in-place metal coating process 
which is free of environmentally undesirable chromium and which will 
provide good results on the metal surfaces described above. 
It is standard practice to form protective corrosion resistant coatings on 
the types of metal surfaces described above. As well as improving the 
corrosion resistance of the painted surface the coating should also have 
good mechanical properties, i.e. good paint adhesion to the metal surface 
and a degree of flexibility. 
The conventional treatment of metals uses chromium based solution either 
for producing a conversion coating or for a final passivation rinse. A 
typical treatment sequence comprises cleaning (optionally with mechanical 
cleaning means or electro cleaning); rinsing; application of a chromate 
chemical composition or a phosphate coating composition comprising other 
metal ions in a conversion coating stage; rinsing, and; final passivation 
(chromium containing) or rinsing, which is followed by oven drying and 
paint application. Generally, the conversion coating solution is applied 
either by spray or immersion and subsequent rinsing steps are required. 
These processes are inconvenient because the series of process steps means 
that the treatment time is relatively long and in addition, because of the 
rinsing steps, significant problems of waste water and sludge disposal 
then arise. 
In recent years, "dry-in-place" or "no rinse" processes have been developed 
in which a pre-treatment solution is applied to the metal surface and is 
subsequently cured by heat or radiation. Any paint coating is then 
directly applied over the resulting layer with no intermediate rinsing 
step. This type of process has considerable advantages as the number of 
steps in the process sequence is reduced and therefore less time is 
required to treat the metal surfaces. In addition large amounts of rinse 
water are not required and therefore the problem of their disposal does 
not arise. 
The process speed becomes particularly important in on-line processes for 
example in coil-coating processes, where a continuous strip of sheet metal 
is uncoiled at the line entrance and recoiled at the exit, having been 
painted or otherwise treated. Line speeds can be up to 200 m/min. Thus, 
treatment times must be very short and a reduction in the number of 
process steps enables an desirable reduction of the line length. In 
addition, in such processes any coating composition imbalance, even if 
very promptly corrected could lead to huge losses and scrap, and therefore 
the coating compositions are preferably relatively simple and easily 
maintained. 
However, conventional processes still include chromium metal ions which are 
undesirable from an environmental point of view. One widely used chromium 
containing dry-in-place treatment is described in GB 1234181. 
EP-A-0478028 relates to providing crystalline zinc phosphate conversion 
coatings on metals. An initial activation step is used which tends to 
adversely affect the subsequent phosphating step. Silicate is therefore 
added to precipitate out any titanium ions which can then be removed from 
the phosphating composition. In this type of process, the phosphated 
surface is rinsed prior to paint coating and so the process described is 
not a dry-in-place conversion coating method. 
Favilla J. R. --"No Rinse Treatment for Aluminium" Product Finishing, 1990 
(11): 45-55 describes no-rinse processes. Generally the systems discussed 
are chromate containing however on page 50 non-chromium no-rinse 
treatments are described which are said to incorporate transition metals 
other than chromium. 
GB 2041987 relates to chromate free solutions used for coating surfaces of 
aluminium or aluminium alloy for dry-in-place processes. The specification 
relates specifically to aluminium surfaces and the compositions do include 
transition metal additives which are oxy metal anions, also 
disadvantageous from an environmental point of view. 
Chromate-free solutions are described in GB 2201157 in which silica 
products for treating surfaces are described. The application describes 
both no-rinse processes and systems which do involve a subsequent rinsing 
step. For the no-rinse treatment, a phosphating composition is described 
which comprises an aqueous dispersion of silica and an acidic trivalent 
metal compound. Oxy metal anions are specifically excluded from the 
composition and preferably also there is no divalent metal present. 
Various compositions comprising silica and phosphoric acid are known for 
treating metal surfaces but not as pretreatment for permanent paint 
coatings. FR-A-2272192 relates to treating surfaces so that they will 
support lubricants for cold forming of steel and JP-A-54130449 describes 
the formation of an insulating film on an electrical steel sheet for use 
in magnetic cores, transformers and electric motors. 
In order to provide an anti-corrosion coating on metal surfaces; which is 
suitable to be used as a base for subsequent paint coating, particular 
properties are required. In particular, the coating must provide good 
anti-corrosion properties which must not be adversely affected by the 
subsequent paint coat. In addition, the surface must be suitable to 
provide good adhesion for the paint coating. 
The present invention aims to provide an effective dry-in-place 
chromium-free treatment. 
Furthermore, the main components of the composition can be used across a 
range of steel, zinc or zinc alloys, aluminium or aluminium alloy metal 
substrates. Thus, metal processing of each of these metal surfaces can be 
effected with a minimum of composition changes. 
In accordance with the present invention, a method is provided for coating 
a metal surface comprising in a first step contacting the metal surface 
with an aqueous composition comprising silica, phosphoric acid and a 
divalent metal ion, the composition being substantially free of chromium 
and of any oxy metal anions in which the metal has a valency of at least 
5, and subsequently curing the coating metal substrate with no 
intermediate rinsing stage and in a second stage applying a second coating 
layer which is curable to form a fixed layer. 
Preferably in the process of the present invention, the aqueous composition 
is also substantially free of trivalent metal ions. 
Although silicon containing ions such as fluorosilicates are a well known 
component of conversion coating compositions, the use of silica is less 
well known and has an entirely different effect in a metal coating 
composition. In the present dry-in-place process, it is essential that 
after application of the composition to the pre-cleaned metal surface, the 
composition is cured. Curing is effected by drying, preferably be passing 
through an oven. Most preferably the metal should reach a PMT (peak metal 
temperature) of approximately 70 to 140 preferably 80.degree. to 
120.degree. C. This ensures the reaction is completed between the 
phosphating liquid remaining on the surface of the metal and the metal 
surface itself, resulting in a coating which provides an effective 
anticorrosion surface for the subsequent application of a paint coating. 
Thus, the silica from the composition forms part of the coating. 
In contrast in conversion coating processes which are not dry-in-place, 
although generally the metals are dried prior to application of paint 
coatings this is not essential and for example, if a water-based paint 
coating is applied, application can be carried out prior to drying. 
In the present invention, the expression "substantially free of" is 
intended to mean that none of these components have been added to the 
composition and the presence of any of these components is no more than by 
their incidental inclusion in any of the other components. Preferably they 
are present in amounts below 100 ppm most preferably below 50 ppm, most 
preferably below 30 ppm. 
In particular the compositions used in the process should be substantially 
free of chromium ions. 
The presence of oxymetal anions in which the metal has a valency of 5 or 
more are specifically excluded from the compositions claimed. They are not 
only environmentally undesirable but in addition have been found to be 
detrimental to the corrosion resistance of the conversion coatings formed 
from the compositions used in the claimed coating process. 
The quantities of the components in the composition can vary but are 
preferably chosen to suit the particular metal which is prevalent in the 
surface being treated and therefore depends upon whether the metal surface 
being treated is mainly steel, galvanised or aluminium (or aluminium/zinc 
alloy). 
When the prevalent metal in the metal surface for coating is a galvanised 
metal the silica content is generally at least 0.01, preferably at least 
0.05, and most preferably at least 0.1 moles/liter Generally the silica 
content is no greater than 1.0, preferably no greater than 0.5, and most 
preferably no greater 0.2 moles/liter. The total phosphate content in the 
composition is generally at least 0.02, preferably at least 0.05 and most 
preferably at least 0.1 moles/liter. Usually it is no greater than 0.5, 
preferably no greater than 0.2 and most preferably no greater than 0.1 
moles/liter. The free phosphoric acid content is generally at least 0.02, 
preferably at least 0.03 and most preferably at least 0.05 moles/liter. 
Generally it will be no greater than 0.5, preferably up to 0.25 and most 
preferably up to 0.1 moles/liter. The amount of divalent metal ion in the 
composition is generally at least 0.001, preferably at least 0.01, and 
most preferably at least 0.025 moles/liter in the aqueous composition. 
Generally it will be no greater than 0.5, preferably no greater than 0.2 
and most preferably no greater than 0.05 moles/liter (based on metal ion 
content). 
For galvanised surfaces the preferred molar ratios of the components range 
from 1:1 to 1:0.7 for silica:total phosphate ion; around 1:3 to 1:5 for 
metal ions:total phosphate ion; and 1:2.5 to 1:7 for the metal 
ions:silica. 
When the metal surface to be treated is predominantly aluminium or 
zinc/aluminium alloy, the optimum composition will generally comprise from 
at least 0.01, preferably at least 0.05 and most preferably at least 0.1 
moles/liter silica. Generally the aqueous composition will include no 
greater than 1.0, preferably no greater than 0.5 and most preferably no 
greater 0.2 moles/liter. 
For the total phosphate ion content, generally this will be at least 0.02, 
preferably at least 0.1 and most preferably at least 0.2 moles/liter. 
Generally the total phosphate content in the aqueous composition will be 
no greater than 2.0, preferably no greater than 0.5 and most preferably no 
greater than 0.25 moles/liter. The free phosphoric acid content is 
generally at least 0.02 preferably at least 0.03 and most preferably at 
least 0.04 moles/liter. Generally the free phosphoric acid content will be 
no greater than 0.5, preferably no greater than 0.2 and most preferably no 
greater than 0.1 moles/liter. 
The metal ion content in the composition will generally be at least 0.001, 
preferably at least 0.03 and most preferably at least 0.05 moles/liter. 
Generally it will be present in an amount no greater than 0.5, preferably 
no greater than 0.2 and most preferably no greater than 0.1 moles/liter. 
Thus in a composition for treating metal surfaces which are predominantly 
of aluminium or zinc/aluminium alloy, the preferred molar ratio of 
silica:total phosphate ion is in the range of about 1:0.8 to 1:1.5; the 
preferred ratio of divalent metal ions:total phosphate ion is in the range 
1:2.5 to 1:5 and the preferred ratio of divalent metal ions:silica is in 
the range of about 1:2.5 to 1:3. 
In particular, for use on aluminium surfaces, it is preferable that the 
aqueous composition should also include an activator preferably fluoride 
ions. Generally fluoride ions will be provided in the form of hydrogen 
fluoride. Fluoride may be present in amounts up to 0.5 moles/liter, 
preferably in amounts from 0.01 to 0.1 moles/liter and most preferably 
from 0.02 to 0.03 moles/liter. The addition of hydrogen fluoride to the 
composition may also contribute to the free acidity in the solution. Since 
hydrogen fluoride has a tendency to react with silica to produce fluoro 
silicate, when the composition includes hydrogen fluoride the composition 
is preferably prepared and stored in a two-pack form in which the contents 
of the two packs are mixed shortly prior to use. Thus, in the two-pack 
system the silica will be present in one of the packs and the hydrogen 
fluoride will be kept separate in the second pack. Preferably the first 
pack will comprise phosphoric acid, metal ion and hydrogen fluoride and 
the second pack will comprise silica. Preferably both are in the form of 
an aqueous based composition. 
When the prevalent metal surface for coating is steel, the composition will 
generally contain at least 0.05, preferably at least 0.1 and most 
preferably at least 0.25 moles/liter silica, generally being no greater 
than 1.0, preferably no greater than 0.5 and most preferably a maximum of 
0.35 moles/liter in the aqueous composition. The amount of total phosphate 
ion will generally be at least 0.05, preferably at least 0.1 and most 
preferably at least 0.5 moles/liter. Generally no greater than 2.0, 
preferably no greater than 1.0 and most preferably no greater than 0.5 
moles/liter of the aqueous composition. 
The free phosphoric acid content is generally at least 0.05 and preferably 
at least 0.1 moles/liter, generally being no greater than 0.5, preferably 
no greater than 0.3 and most preferably no greater than 0.2 moles/liter. 
The metal ions will generally be present in an amount of at least 0.001, 
preferably at least 0.05 and most preferably at least 0.1 moles/liter, 
generally no greater than 1.0, preferably no greater than 0.5 and most 
preferably no greater than 0.2 moles/liter of the aqueous composition. 
Thus, for treatment of predominantly steel surfaces, the preferred molar 
ratios for silica:total phosphate ion is in the range of about 1:1 to 1:2; 
for divalent metal ions:phosphate ion is preferably in the range of about 
1:2.5 to 1:5; and for divalent metal ions:silica the molar ratio is 
preferably in the range of about 1:1 to 1:3. 
Any divalent metal ion may be used as the divalent metal ion for use in the 
composition: divalent transition metal ions such as Mn, Co, Fe, Ni, Zn or 
alkaline earth divalent metal ions such as Mg, Ca, Sr, or Ba. Preferably 
the divalent metal ion is other than nickel for environmental reasons. 
Preferably calcium, zinc or magnesium ions are used to provide the 
divalent metal ion, most preferably calcium. They are generally added to 
the aqueous composition in the form of a non-interfering oxide, hydroxide 
or salt such as a carbonate. They may however, be added in the form of a 
phosphate salt, when an additional source of acid, such as hydrogen 
fluoride is present in the composition, thereby contributing to the 
phosphoric acid content in the composition. Alternatively the metal itself 
may be added to acidic composition to dissolve. 
The silica particles are high surface area particles which are dispersed in 
solution to form a homogeneous, that is colloidal, dispersion (which can 
be clear or hazy) or capable of becoming dispersed in solution. The use of 
fume or precipitated silica, is preferred, especially those commercially 
available in the form of relatively high solids content viscous 
dispersions and in the form of silica sold under the trade names Aerosil 
(trade mark of Degussa). Mixtures of different forms of silica may be used 
if desired. 
The phosphoric acid is generally added to the aqueous composition in the 
form of an aqueous solution for example a 50% active or higher aqueous 
solution. When an additional source of acid is present in the composition, 
phosphate ions may be added, for example, as divalent metal phosphate, 
thereby contributing phosphoric acid to the composition. 
In particular in the compositions for treatment of galvanised metals or 
steel metal surfaces, preferably the composition also includes boric acid. 
When the prevalent metal surface for coating is galvanised, boric acid is 
generally present in an amount of at least 0.02, and most preferably at 
least 0.075 moles/liter. Generally the boric acid will be in an amount no 
greater than 0.5, preferably no greater than 0.2 and most preferably no 
greater than 0.1 in the aqueous composition. For steel surfaces, a 
slightly higher concentration of boric acid is preferred. Preferably this 
will be at least 0.05 and most preferably at least 0.1 moles/liter. 
Generally the amount will be no greater than 0.7, preferably no greater 
0.4 and most preferably no greater than 0.2 moles/liter. 
The aqueous compositions are prepared by the incorporation of the necessary 
ingredients into deionised water. The order of addition may be in any 
convenient way but is generally by firstly preparing a concentrate of the 
composition. 
The present invention also includes a concentrate for a solution for metal 
coating comprising silica, phosphoric acid and a divalent metal ion, the 
concentrate being substantially free of chromium, any oxy metal anions in 
which the metal has a valency of at least 5 and any trivalent metal ions. 
The invention also includes a two-pack concentrate in which at least one 
of the components of the composition is in a first pack and at least one 
other component is in a second pack. In particular the invention comprises 
a two-pack concentrate in which the first pack comprises at least hydrogen 
fluoride and the second pack at least silica. The concentrate should 
include each of these components in a concentration so that dilution with 
water will produce the required end-use composition including the specific 
amounts of each component, as described above. Preferably, the end-use 
composition will comprise dilutions of a one or two-pack concentrate. 
The compositions can be prepared by the addition of the components in any 
convenient order. It is generally convenient to mix the silica with at 
least a portion of the water prior to mixing with the remaining component 
of the composition. The remaining components can be added subsequently, 
optionally also dissolved in a portion of the water. 
The invention is particularly aimed at providing a fast and efficient 
treatment for a coil coating metal conversion process. 
For coil coating, the coating is generally applied by roll coating or 
reverse roll coating, or by passing the uncoiled metal sheet through a 
bath of the composition so that application is by immersion. However, any 
other standard application form can be used such as by spraying or 
conventional spray/dip treatment. Usually contact is at ambient 
temperature. 
After application of the coating composition, the amount of teh composition 
may be controlled by passing the coated metal through Squeegee rolls 
optionally having an engraved surface. The amount of composition remaining 
on the metal surfaces should preferably be sufficient to produce a coating 
weight of from 0.25-5 g/m.sup.2 after curing. 
The metal to be treated is generally first cleaned and rinsed in a 
conventional manner. No activation step is necessary and the cleaned metal 
is then contacted with the coating composition. 
The curing step comprises drying the coating. Generally curing is by 
heating and this may be carried out by conventional means, for example by 
passing the coil or coated article through an oven or exposing to IR 
radiation. Preferably the PMT reached on curing is from at least 
60.degree. C., preferably at least 70.degree. C. most preferably at least 
80.degree. C., generally no greater than 140.degree. C., preferably no 
greater than 120.degree. C. and most preferably no greater than 
100.degree. C. Once the coated metal surface has been cured, the 
secondcaating layer, e.g. paint coating, can be immediately applied. 
The present invention is a suitable anti-corrosion base coat for any 
curable second coating, in particular paint coatings. Suitable coating 
layers are for examples acrylate resins, polyester resins, silicon 
modified polyester resins, polyvinyl chloride based mixed polymers and 
fluorocarbon resins, in particular polyvinylidene fluoride or paints 
containing these materials. Paint coatings are generally applied in two 
layers: a primer layer, followed by a topcoat. Other suitable second 
coating materials are for example organic materials such as dispersions of 
resin powder in a plasticising medium, for example organosols such as 
polyvinylchloride plastisols. A primer coating may be applied prior to 
such a plastisol. 
Alternatively the second coating may comprise for example an adhesive which 
can be applied as a liquid to which a smooth or textured laminate based on 
polymeric substances such as softened PVC or polytetrafluoroethylene can 
be adhered. Curing of the second coating layer generally comprises drying 
and optionally this is with the aid of heat or radiation. The second 
coating layer can be applied by any conventional method, for example for 
paint, generally by spraying, brushing or rolling. 
The following examples illustrate the invention.

EXAMPLE 1 
An aqueous concentrate was formed by mixing 1.01/moles phosphoric acid with 
water and subsequently adding and mixing 0.27 moles of calcium hydroxide. 
Separately, 0.58 moles of boric acid was mixed with water. The two aqueous 
solutions were mixed together and 1.25 moles Aerosil 200(trade mark) and 
deionised water were added to a total volume of 1000 ml. The solution was 
mixed in a Silverson (trade mark) mixer. 
The resultant concentrate was a white acidic, viscous liquid. 
A working solution was then prepared comprising 15% of this concentrate, 
the remaining 85% comprising deionised water. 
The composition was applied to the surface of hot dip galvanised and 
electro galvanised metal plates. Prior to application of the coating 
composition, the metal plates had been treated by cleaning and 
demineralised rinsing. Application of the coating composition to the 
plates was by uniform wetting of the surface by Sheen spinner. The sheen 
spinner is a simple horizontal rotating plate (usually up to 1000 r.p.m.). 
The metal test plate was fastened to the rotating plate, brushed with the 
solution and rotated for a short time (usually 30 seconds to 1 min). In 
this way the liquid composition was evenly spread on the shole surface 
with consistent coating weight. 
After application of the coating composition the coating was cured by oven 
drying at a temperature of 100.degree. C. for 1 minute. The coating 
weights obtained were 0.15-0.3 g/m.sup.2. 
A paint coating was then directly applied over the resulting layer. The 
paint coatings used were: 
a) A black polyester powder paint (produced by Croda) which was applied 
electrostatically and stoved at 210.degree. C. with a thickness of 50 
.mu.m. 
b) A chrome epoxy primer, followed by a white PVF (polyvinylidenefluoride) 
top-coat (produced by Beckers) were applied by bar coating in which a 
precision machined bar which is spirally engraved is rolled on the flat 
test panel, previously wet by the paint to be applied. The amount of paint 
remaining on the surface (so the paint thickness after curing) depends on 
the depth and width of the engraved spiral. Paint application was followed 
by stoving at a peak metal temperature (pmt) of 220.degree. C. to give a 
primer coating of from 7 to 10 .mu.m thick and a top coat of from 25 to 30 
.mu.m. 
c) A stove enamel (produced by Trimite) was used to test protective 
coatings on aluminium. The enamel was applied by Sheen spinner and stoved 
at 170.degree. C. to give a dry film thickness of 25-30 .mu.m. 
The coated, painted metal plates were then tested for corrosion and 
mechanical properties using the following tests: 
Corrosion Tests 
Corrosion resistance was evaluated by salt spray tests. Steel and 
galvanised metal plates were tested according to ASTM B117 and aluminium 
using acetic salt spray tests, according to ASTM B287. 
Mechanical Tests 
Test A 
Mechanical properties were evaluated by cross cut adhesion tests in which 
the painted surface was engraved, by a sharp knife, with ten parellel 
cuts, 1.5 mm apart from each other. A further ten cuts were made, 
perpendicular to the former, so that a network of small squares, having 
1.5 mm sides, resulted. Erichson indentated then followed, to see whether 
the coating and/or paint flaked from the metal plate and was continued to 
a depth at which adhesion losses begin to apear. Adhesion loss was 
detected by taping. 
Test B 
Reverse impact tests were carried out on each plate to British Standard 
3900-E, in which a specified weight was allowed to fall from a 
preetermined height, onto the reverse side of a painted panel under 
testing. Paint adhesion was then checked by taping on the top of the 
resulting conical deformation. 
Test C 
T-bend tests were carried out on the powder coated paint surfaces by 
bending each plate by 180.degree. C., so as to fold it back onto itself, 
tightly. This was followed by placing tape over each plate and evaluating 
the conversion coating/paint coating loss. For the paint coated plates 
according to b, T-bend stripline tests were carried out followed by taping 
and evaluation. 
Scores out of 10 were given to each of the plates for each test 
performance. In each of these tests 10 signifies good results with no 
cracking or adhesion loss and 1 signifies considerable adhesion loss. 
EXAMPLE 2 
A two-pack concentrate composition was prepared: the first pack by mixing 
1.31 moles phosphoric acid in water with 0.45 moles zinc carbonate 
followed by 0.16 moles hydrogen fluoride. The composition was made up to 
1000 ml with deionised water. The second pack was prepared by adding 
de-ionised water to 1.25 moles or Aerosil 380 and mixing using a Silverson 
mixer. Water was added to a total volume of 1000 ml. The first pack formed 
a clear acidic solution and the second pack formed a white thixotropic 
neutral composition. 
Immediately prior to use, the two-pack concentrate was mixed and a working 
composition was prepared comprising 15% of each concentrate, the remaining 
70% comprising deionised water. The coating weights obtained were 0.15 to 
0.3 g/m.sup.3. 
A selection of the tests set on in Example 1 were carried out on metal 
plate samples comprising cold rolled steel, aluminium, Galfan (trade mark) 
(95% zinc, 5% aluminium), Zalutite (trade mark) (45% zinc, 55% aluminium), 
hot dip galvanised and electrogalvanised metals. 
EXAMPLE 3 
An aqueous concentrate composition was prepared by dissolving 0.55 moles 
calcium hydroxide and 1.46 moles phosphoric acid in deionised water, 0.44 
moles boric acid were added followed by 0.91 moles Aerosil 200 and 
deionised water to a total volume of 1000 ml, the composition being mixed 
in a Silverson mixer. The resultant composition was a white acidic 
thixotropic liquid. A working composition was prepared comprising 33% 
concentrate, the remainder being de-ionised water. The coating weights 
obtained were 0.6 to 0.8 g/m.sup.2. 
Corrosion and mechanical property tests were carried out as described in 
Example 1 on cold rolled steel metal plates. 
Comparative Example A 
A chromium containing metal treatment concentrate was prepared comprising 
1.05 moles (105 parts by weight chromic acid), 16 parts by weight wheat 
starch, 1.67 moles (100 parts by weight) Aeorsil 350 (trade mark), 0.05 
moles (5.53 parts) zinc carbonate and deionised water to a total 1000 
parts. The pH of the composition was 3.0 and chromium VI:chromium III 
ratio was 0.55. A working composition was prepared comprising 25% 
concentrate, the remainder comprising deionised water. The coating weights 
obtained were 0.25-0.3 g/m.sup.2 on cold rolled steel metal plates, 0.3 to 
0.35 g/m.sup.3 on galvanised plates and 0.4 to 0.45 g/m.sup.2 on aluminium 
metal plates. Comparative tests for corrosion and mechanical properties 
were carried out as described in Example 1 for cold rolled steel, hot dip 
galvanised, electro galvanised and aluminium metal substrates. 
Results 
The results of the corrosion resistance tests are given in tables 1, 2 and 
3. All of the corrosion resistance results indicate acceptable corrosion 
resistance properties. 
The results of the mechanical property tests for each coating composition, 
and metal plate with paint coat a) are given in Table 4 and with paint 
coat b) in Table 5. The three results recorded in each case are for Tests 
A, B and C respectively. 
As can be seen from the results in Tables 4 and 5 high mechanical 
performance is achieved using the compositions of the invention. The 
results also show that the composition of Example 1 is particularly 
preferred on galvanised surfaces, the composition of Example 2 is 
particularly preferred on aluminium surfaces and the composition of 
Example 3 is particularly preferred on steel surfaces. All of the results 
show performance approximately the same as using the conventional chromium 
based phosphating solution (comparative example A) but without the 
addition of this undesirable component. 
In each case, it has been found that an increase in the strength of the 
working solution results in an increase of corrosion protection but a 
decrease in the adhesion of the protective layer to the metal. Thus the 
concentrations described above show the preferred amounts to achieve a 
balance between corrosion protection and mechanical properties. 
TABLE 1 
______________________________________ 
SALT SPRAY RESISTANCE ASTM B117 
Average mm Paint Loss on Cross Hatch: 
Powder Coat Paint Coating 
Exposure 
Time Solution 
Metal Surface 
(hrs) Ex. 1 Ex. 2 
Ex. 3 Ex.4 
______________________________________ 
Cold Rolled Steel 
336 -- -- 2 1.5 
Electrogalvanised 
500 1 -- -- 1 
Hot Dip galvanised 
336 1 -- -- 1 
500 2 -- -- &lt;1 
Galfan* 500 -- 1 -- &lt;1 
Zalutite* 500 -- &lt;1 -- &lt;1 
______________________________________ 
TABLE 2 
______________________________________ 
SALT SPRAY RESISTANCE ASTM B117 
Average mm Paint Loss on cross Hatch: 
Primer - PVF Coating 
Solution 
Exposure Comp. 
Metal Surface 
Time (hrs) 
Ex. 1 Ex. 2 
Ex. 3 Ex. 
______________________________________ 
Cold Rolled Steel 
1000 -- -- 3 2.5 
Electrogalvanised 
1000 3.5 -- -- 3 
Hot Dip 1000 1 -- -- 1.5 
Galvanised 1000 -- 1 -- 1 
Galfan* 1000 -- 1.5 -- 1.5 
Zalutite* 
______________________________________ 
*Galfan and Zalutite are trade marks 
TABLE 3 
______________________________________ 
ACETIC SALT SPRAY RESISTANCE ASTM B287 
Average mm Paint Loss on Cross Hatch 
Exposure Time 
Solution 
Metal Surface hr Ex. 2 Comp. Ex A 
______________________________________ 
Aluminium (stove enamel) 
1000 1 &lt;1 
Aluminium (primer/top 
2000 &lt;1 &lt;1 
coat) 
______________________________________ 
TABLE 4 
______________________________________ 
MECHANICAL TESTS 
Results for Powder Coat Paint Coating 
Solution 
Metal Surface 
Ex. 1 Ex. 2 Ex. 3 Comp. Ex. A 
______________________________________ 
Aluminium -- 8 9 10 -- 6 7 9 
Cold Rolled Steel 
8 4 5 -- 10 10 9 
10 9 10 
Electrogalvanised 
10 9 10 9 3 4 -- 10 7 10 
Hot Dip Galvanised 
10 9 10 9 4 9 -- 10 8 9 
Galfan 10 7 10 9 5 9 -- 10 9 9 
Zalutite 10 10 9 10 8 9 -- 10 8 9 
______________________________________ 
TABLE 5 
______________________________________ 
MECHANICAL TESTS 
Results for Primer and Top-Coat Paint Coating 
Solution 
Metal Surface 
Ex. 1 Ex. 2 Ex. 3 
Comp. Ex. A 
______________________________________ 
Aluminium -- 10 9 8 -- 10 7 9 
Cold Rolled Steel 
10 7 10 -- 9 4 9 
7 3 7 
Electrogalvanised 
10 6 9 10 3 6 -- 9 6 10 
Hot Dip Galvanised 
8 7 9 9 2 7 -- 10 6 10 
Galfan 10 8 7 10 4 9 -- 10 4 9 
Zalutite -- 9 5 8 -- 6 6 8 
______________________________________ 
Comparative Examples B and C 
The adverse effect of oxymetal anions in the coating composition is 
illustrated by the following comparative example. 
Compositions B and C were prepared in deionised water as set out in table 6 
below. 
TABLE 6 
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Solution 
B C 
Component mole/l mole/l mole/l 
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Silica (Aerosil 200) 
0.16 0.16 
Phosphoric acid (100%) 
0.81 0.81 
Boric Acid 0.16 0.16 
Zinc ions (added as ZnCO.sub.3) 
0.078 0.08 
Molybdic acid 0.06 -- 
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The compositions were each applied to cold rolled steel plates which had 
been brushed and cleaned with an alkali cleaner to a complete water-break 
free surface. Each composition was applied by the sheen spinner disc then 
stoved at 120.degree. C. p.m.t. (peak metal temperature). A black 
polyester powder coating was applied electrostatically and stoved at 
210.degree. C. to a paint thickness of 50 .mu.m. 
Tests were carried out for corrosion (2 plates) and mechanical properties 
(1 plate) as described in example 1, using salt spray tests and mechanical 
tests A, B and C. 
The results given in Table 7 show salt spray results as average mm of 
corrosion creepage from the cross-hatch, and maximum and minimum values 
are given in brackets, after 240 hours exposure to the salt spray test. 
The mechanical test results are in points out of 10 with high results 
indicating good properties. 
TABLE 7 
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Solution 
Test B C 
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Salt Spray Test 
Plate 1 Total Loss 
21/2 (11/2-4) 
Plate 2 7 (6-11) 2 (11/2-3) 
Mechanical Properties 
Test A 9 8 
Test B 10 6 
Test C 9 5 
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As shown, although the addition of molybdate ions produces a benefit 
relating to the mechanical properties of the paint, it also results in a 
detrimental effect on the corrosion resistance. The concentrations used 
above are slightly higher than those of the working compositions of the 
present invention which are exemplified below. As also explained below in 
the present invention, increasing concentration has been found to increase 
corrosion protection and decrease mechanical properties which indicates 
that the corrosion protection obtained at the preferred concentrations of 
the invention would be so poor as to be unsuitable for practical use.