GB2174719A - Solutions and processes for the treatment of metal surfaces - Google Patents

Abstract

A solution for conditioning a metal surface before phosphate conversion coating processes comprises metallic titanium, pyrophosphate and water soluble anionic organic compound.

Description

SPECIFICATION Solutions and Processes for the Treatment of Metal Surfaces It is well known to apply a conditioning solution to a clean metal surface before forming a phosphate conversion coating on the surface, especially before forming a zinc phosphate conversion coating by the application of an appropriate zinc phosphate coating solution. The conditioning solution serves to accelerate the subsequent chemical conversion reaction and to cause the phosphate coating to be of reduced weight, more homogeneous and of finer crystal structure. Thus it contributes to the formation of a dense coating of relatively low weight and that will give good corrosion resistance and adhesion of a subsequently applied paint film.

The known conditioning solutions, for instance as described in U.S. 2,874,081, 2,322,349 and 2,310,239 generally have as main constituents titanium, pyrophosphate, orthophosphate and sodium. The titanium may be colloidal metallic titanium and so the solutions are not true solutions.

When freshly prepared these solutions do give satisfactory conditioning for the subsequent application of a zinc phosphate coating. However the performance deteriorates when the solution is aged for more than, say, two days after initial preparation. Thus the crystal structure of the zinc phosphate becomes coarser and the coating weight may increase by 25 to 50% or more or in some instance a non-uniform coating or even yellow rust is formed instead of the desired conversion coating. The ageing can occur either upon leaving the solution to stand, unused, after initial make-up and before use or by subjecting it to occasional or repeated use.

Attempts to restore an aged solution to its initial effectiveness by regeneration, for instance by discharging part of the solution and replacing it with fresh solution, is not very satisfactory. It is difficult to restore the solution to its desired effectiveness and the process involves discharge and therefore wastage of useful solution components.

An aqueous solution according to the invention suitable for conditioning a clean metal surface before forming a phosphate conversion coating comprises at least 3 ppm metallic titanium, 60 to 360 ppm pyrophosphate ion, at least 150 ppm tota! phosphate, and 2 to 300 ppm water soluble anionic organic compound and has a pH of from 8 to 9.5.

A process according to the invention comprises preparing this solution and then conditioning a clean metal surface by application of the solution and then forming a phosphate conversion coating, preferably a zinc phosphate conversion coating, on the surface by application of an appropriate phosphating solution. A particular advantage of the solution is that satisfactory results can be obtained even though the conditioning solution is applied to the surface a prolonged time, generally at least two days, after preparation of the solution.

The performance of the conditioning solution is maintained for longer periods than is the performance of conventional conditioning solutions. For instance the increase in coating weight over a prolonged period of more than two days, for instance ten days, is often below 15% compared to values of 25 to 50% or more with conventional conditioning solutions. Similarly the crystal structure of the subsequently applied phosphate coating remains dense and fine even when the conditioning solution is aged.

Another advantage of the invention is that the solutions can be made up using relatively low quality water. Prior to the invention it has been normal to use high quality water for the make-up of the conditioning solution in an attempt at maintaining solution performance during ageing, but in the invention satisfactory performance can be obtained upon ageing even when lower quality water is used.

Although we refer to the aqueous conditioning composition as being an aqueous solution, and to the components as being water soluble, this does not necessitate that the solutions should be true solutions.

Thus the titanium is normally present as colloidal metal. Naturally however each of the components should be in the form of true or colloidal solutions such that they achieve the desired effect.

If the amount of titanium is below 3 ppm the crystal structure of the subsequently applied phosphate coating will generally be inadequately dense. Preferably the amount of titanium is below 100 ppm since although a dense crystalline coating structure is obtainable at higher amounts their use tends to be uneconomic.

The amount of pyrophosphate must be at least 60 ppm since lower amounts tend to result in the subsequently applied phosphate coating having an inadequately dense crystal structure. Lower amounts also lead to greater deterioration in performance of the solution upon ageing. If the amount of pyrophosphate is above 360 ppm it becomes difficult to form a zinc phosphate coating and, instead, the coating will tend to be an iron phosphate coating. By maintaining the amount of pyrophosphate between 60 and 360 ppm it is possible to obtain a subseqeunt phosphate coating having a low coating weight but a dense crystal structure and it is possible to suppress coagulation of the colloidal titanium from the conditioning solution.

The amount of total phosphate is calculated by measuring as PO4 all phosphate-type ions such as HPO4, PO4, and P07. If the amount is below 150 ppm there is a tendency for the colloidai titanium to coagulate and for the performance properties to deteriorate upon ageing. Amounts above 3,000 ppm can be used but are uneconomic.

The colloidal titanium is present as anionic dispersed colloidal particles and these colloidal particles have a tendency to coagulate on ageing, leading to a deterioration in the performance properties of the solution. The pyrophosphate inhibits coagulation but the inhibition is inadequate over prolonged ageing periods, as a result of which the coating weight of a subsequently applied phosphate conversion coating solution may increase by 25 to 50%. The use of excess pyrophosphate, to prevent the coagulation of the colloidal titanium, causes reaction with the metal surface (especially when it is of steel) in such a way as to suppress the desired subsequent phosphate conversion reaction. In the invention the desired effects are achieved without the disadvantages as a result of including water soluble anionic organic compound in the solution.Thus titanium coagulation is inhibited without deactivating the metal surface against the subsequent phosphate conversion reaction.

The water soluble anionic organic compounds are preferably water soluble anionic salts of compounds containing a plurality of carboxylic or sulphonic acid groups. The anionic compounds cause electrical repulsion between the colloidally dispersed titanium particles and so help maintain them in colloidal solution. The anionic compounds can thus be considered to be acting as dispersing agents and when polymeric anionic compounds are used their molecular weight should be such that the compounds will serve as dispersing agents.

The preferred anionic compounds are water soluble polycarboxylic acid and polysulphonic acid compounds. Suitable polycarboxylic compounds can be homopolymers or copolymers of an acrylic monomer, for instance acrylic acid, with acrylic acid ester, acrylamide, acrylonitrile or unsaturated dicarboxylate. Other suitable polycarboxylic compounds are copolymers of maleic acid with isobutylene or styrene. Suitable sulphonic acid compounds are the formaldehyde condensates of naphthalene sulphonic acid.

The anionic organic compounds are generally present as water soluble salts for instance of an alkali metal, ammonium or amine such as alkylamine. The preferred compounds are present as sodium or ammonium salts. The amount of anionic compound must be at least 2 ppm as lower amounts will give inadequate dispersing effect of the colloidal titanium and inadequate suppression of ageing properties. If the amount of anionic compound is above 300 ppm it is liable to interfere with the formation of the subsequent phosphate conversion coating.

The pH of the solution should be between 8 and 9.5. If it is below 8 it is close to the isoelectric point of the colloidal titanium, thereby increasing the risk of coagulation of the titanium, with the risk of increased coating weight and coarser phosphate crystal structure. If the pH is above 9.5 the solution will be too strongly alkaline to give the desired conditioning effect.

Generally the pH is from 8.3 to 9. Generally the amount of titanium is from 5 to 50 ppm. Generally the amount of pyrophosphate is from 100 to 360 ppm. Generally the amount of phosphate is from 300 to 1500 ppm. Generally the amount of anionic compound is from 2 to 100 ppm.

The metal surface that is treated in the invention is generally of steel and will normally be cleaned by a conventional alkaline cleaner before application of the conditioning solution. The solution may be used fresh but it is an advantage of the invention that it can be used a considerable time, usually at least two days, typically five to fifteen days, after initial make up without it being necessary to drain away active solution and replace it with fresh solution. Application of the conditioning solution can be in conventional manner and at conventional temperatures, for instance by immersion at room temperature.

The phosphate coating is preferably a zinc phosphate coating and may be formed by application of an appropriate zinc phosphate coating solution. This is preferably formulated in a known manner such that the coating weight is low, typically 1 to 3 g/m2. The phosphate coating may be rinsed, post-treated, dried and painted in conventional manner.

The following are some examples.

Solutions 1 to 8 Solutions 1 to 8 according to the invention, and comparative solutions A, B and C, were formulated as shown in Table 1, wherein the amounts are ppm of active compound. The polycarboxylate was 40% active and was SN Dispersant 5020 manufactured by SAN Nopco KK. The sulphonate dispersantwas 90% active and was Labellin FW-N manufactured by Daiichi Kogyo Seiyaku KK.

The water used for preparing the solutions was tap water having a specific conductivity of 150 lis/cm.

TABLE 1 Solution 1 2 3 4 5 6 7 8 A B C Titanium 10 10 10 20 10 10 10 20 10 10 10 Pyrophosphate 166 166 166 332 166 166 166 332 166 166 166 Totai P04 595 595 595 1190 595 595 595 .1190 595 595 595 Polycarboxylate 3 30 150 30 - - - - - 500 Sulphonate - - - - 6 60 300 60 - - 700 pH 8.5 8.5 8.6 8.8 8.5 8.5 8.5 8.8 8.5 8.7 8.5 EXAMPLE 9 Sample steel for testing, JIS-G-3141, SPCC, was cleaned in an alkaline degreasing solution Fine Cleaner L4422 (manufactured by Nihon Parkerizing Co.Ltd.) having pH value about 10.2 and total alkalinity 16+1 point (titrated by using 10 ml sample, indicator; bromphenol blue, titration liquid; 0.1 N H2SO4). The cleaning was at 40+10C for 180 seconds. The steel was then water rinsed and immersed in one of the conditioning solutions shown in Table 1 at room temperature for 30 seconds.

Azinc phosphate conversion coating was then applied by immersion at 43+10C for 120 seconds in a zinc phosphate solution Bonderite L3080 (manufactured by Nihon Parkerizing Co. Ltd.) having free acidity 0.8 to 1.0 point (valuetitratedfor 10 ml sample, indicator; bromphenol blue, titrating solution; 0.1 N NaOH) and total acidity 22 to 24 point (value titrated by using 10 ml sample, indicator; phenolphthalein titrating solution; 0.1 N NaOH) and containing nitrite accelerator in an amount of 2.5 to 3 point (saccarometer method). The steel was then rinsed in water and then in deionised water and dried at 100"C.

In each instance the coating was observed visually, the coating weight was determined as the difference before and after stripping the coated steel using 5% solution of chromic acid anhydride, and the size of the crystal grains was measured by a scanning type electron microscope (JSM-T100 Model manufactured by Nihon Denshi KK).

The process is conducted both using freshly made conditioning solutions and solutions that had been aged by standing for ten days.

The results are shown in Table 2.

TABLE 2 Solution 1 2 3 4 5 6 7 8 A B C Fresh coating wt. g/m2 2.10 2.19 2.25 2.12 2.18 2.16 2.24 2.15 2.14 - Crystal size um 2-3 2-3 2-3 2-3 2-3 2-3 2-3 2-3 2-3 - - Aged coating wt g/m2 2.32 2.26 2.36 2.23 2.46 2.37 2.53 2.42 3.03 - Weightincrease ratio % 10.5 3 5 5 13 10 13 12.5 42 - Crystal size um 2-4 2-3 2-3 2-.-3 2-4 2-4 -24 2-4 58 All the coatings obtained after using conditioning solutions 1 to 8 and comparative solution Awere dense and uniform but the phosphate conversion coatings following the use of conditioning solutions B and C were non-uniform and included yellow rust.

EXAMPLE 3 Conditioning solutions 1' to 8' and A', B' and C' were made up as in solutions 1 to 8 and A two C except that the dilution water was industrial water having a specific conductivity of 600 us/cm. The resultant conditioning solutions were tested in a phosphating process as in Example 2. The results are shown in Table 3.

TABLE 3 Solution 1' 2' 3' 4' 5' 6' 7' 8' A' B' C' Fresh coating wt g/m2 2.21 2.20 2.22 2.20 2.30 2.31 2.27 2.25 2.56 - Crystal size um 2-3 2-3 2-3 2-3 2-3 2-3 2-3 2-3 5 - - Aged coating wt g/m2 2.34 2.23 2.30 2.27 2.37 2.32 2.32 2.33 4.2 - Weight increase ratio % 5.9 1.4 3.6 3.1 3.0 0.5 2.2 3.5 64 - Crystal size um 2-4 2-3 2-3 2-3 2-3 2-3 2-3 2-3 W10 The coating obtained using solutions 1' to 8' and A' were dense and uniform while the coatings using solutions B' and C' were non-uniform and included yellow rust.

As can clearly be seen from Tables 2 and 3 the solutions according to the invention are satisfactory both when fresh and aged whilst the comparative solutions are unsatisfactory after ageing and comparative solutions B and C are unsatisfactory even when fresh.

Comparison of Tables 2 and 3 shows that performance is not adversely affected by using industrial water instead of tap water.

As a result of the invention the amount of conditioning solution and the frequency of renewal of the conditioning solution can be reduced. The process is of particular value in full dip treatments for car bodies.

Claims (13)

1. An aqueous solution suitable for conditioning a clean metal surface before formation of a phosphate conversion coating and comprising at least 3 ppm metallic titanium, 60 to 360 ppm pyrophosphate ion, at least 150 ppm total phosphate as PO4, and 2 to 300 ppm water soluble anionic organic compound and having a pH of 8 to 9.5.

2. A solution according to claim 1 in which the amount of titanium is below 100 ppm and the amount of total phosphate is below 3,000 ppm.

3. A solution according to either preceding claim in which the amount of titanium is 5 to 50 ppm.

4. A solution according to any preceding claim in which the amount of pyrophosphate is 100 to 360 ppm.

5. A solution according to any preceding claim in which the amount of phosphate is from 300 to 1500 ppm.

6. A solution according to any preceding claim in which the amount of anionic compound is from 2 to 100 ppm.

7. A solution according to any preceding claim in which the pl-l is from 8.3 to 9.

8. A solution according to any preceding claim in which the water soluble anionic organic compound is a dispersing agent selected from water soluble anionic homopolymers or copolymers containing recurring carboxylic acid groups and from formaldehyde condensates of naphthalene sulphonic acid.

9. A solution according to any preceding claim in which the anionic organic compound is in the form of a sodium orammonium salt.

10. A process in which a solution according to any preceding claim is prepared and a clean metal surface is conditioned by application of the solution and a zinc phosphate coating is then formed on the surface.

11. A process according to claim 10 in which the solution is applied to the surface more than two days after it was prepared.

12. A process according to claim 10 or claim 11 in which the weight of the zinc phosphate coating is from 1 to 3 g/m2.

13. A process according to any of claims 10 to 12 in which the metal surface is a car body and the zinc phosphate conversion coating is obtained by immersing the metal surface in a zinc phosphate conversion coating solution.