GB2182679A

GB2182679A - Phosphate coating of metals

Info

Publication number: GB2182679A
Application number: GB08627039A
Authority: GB
Inventors: Ronald William Catton; Terence John Lyons
Original assignee: Pyrene Chemical Services Ltd
Current assignee: Pyrene Chemical Services Ltd
Priority date: 1985-11-12
Filing date: 1986-11-12
Publication date: 1987-05-20
Also published as: FR2593522A1; GB2182679B; IT8622259A1; IT8622259A0; IT1198070B; DE3635896C2; DE3635896A1; GB8527833D0; GB8627039D0; FR2593522B1

Abstract

A metal surface comprising iron is contacted with an aqueous phosphating solution comprising nickel ions and having a pH in the range 4.0 to 5.5, to form a coating comprising iron phosphate and nickel phosphate. An enamel frit is subsequently applied to the coated surface and is fired. The phosphating solution may contain an accelerator such as a fluoride in an amount up to 0.5 g/l and the nickel ions are preferably present in an amount 0.1-2.0 g/l and the phosphate ions in an amount 1-12 g/l. The enamel may be concentrated and is applied by conventional methods. A particular solid composition for use in forming the phosphating solution when dissolved in water comprises a mixture of an alkali metal or ammonium dihydrogen phosphate and a water-soluble inorganic nickel salt and optionally an alkali, a surfactant and/or an accelerator.

Description

SPECIFICATION Phosphate Coating of Metals Traditionally metal surfaces to be provided with a layer of enamel, for instance for use on cookers and saucepans and the like, must be subjected to a complicated sequence of preparation steps before application of the enamel. The substrate used generally is formed of high grade steel with a special low-carbon surface, which is expensive to produce. The sequence of pretreatment steps often includes at least 16 steps consisting of a cleaning sequence of 5 steps, a derusting sequence of 4 steps (where there were any signs of rust on the surface), and a sequence of preparation for an formation of a protective coating on the surface, followed by rinsing and neutralisation, before deposition onto the surface of the enamel frit. The coating is often provided by dipping the surface in a solution of nickel sulphate.

In GB 755559 it is proposed to form a phosphate coating on a metal surface, usually a heavy metal phosphate, and to convert at least part of the phosphate to oxide by heating the coated surface.

The oxide coated surface was then enamelled.

However, the process did not achieve very good results and was uneconomic to run. For these reasons the process was never commercialised.

In GB 1498490 it is proposed to form a nickel phosphate coating on steel as a pretreatment for various coatings such as paint and enamel coatings.

In the process disclosed it is always necessary to activate the surface of the steel before contacting the surface with nickel phosphate. The nickel phosphating soiutions are highiy acidic and deposit a high weight of nickel phosphate coating onto the metal surface. The adhesion of the subsequent enamel coating is not entirely satisfactory.

None of the processes in the prior art have produced substrate surfaces having satisfactory properties for receiving enamel coatings. All the pretreatment process have required many steps and are thus uneconomic. It has usually been necessary to use at least two coatings of enamel onto the coated surfaces to attain satisfactory adhesion to the substrate.

In the present invention a process for enamelling a metal surface comprising iron comprises treating the surface with an aqueous phosphating solution containing nickel ions and having a pH in the range 4.0 to 5.5 to form a coating comprising iron phosphate and nickel phosphate, and applying enamel frit to the coated surface and firing the frit.

The weight of the phosphate coating is preferably in the range 0.15 to 0.6 glum2, preferably in the range 0.2 to 0.3 g/m2.

The aqueous phosphating solution preferably comprises nickel ions in an amount of 0.1 to 2.0 gull, suitably in the range 0.3 to 1.0 g/l. Amounts above 2 g/l are unnecessary and may not dissolve into the solution. Phosphate ions are present in the solution in an amount in the range 1 to 12 g/l, preferably in the range 3 to 6 g/l. The phosphate is generally added as an alkali metal- or ammonium-dihydrogen phosphate, for example sodium potassium or ammonium dihydrogen phosphate. The alkali metal or ammonium is generally present in the phosphating solution in an amount in the range 0.3 to 3.0 g/l preferably in the range 0.5 to 2.0 g/l.

The total acid present in the phosphating solution is generally in the range 2 to 15 points, suitably in the range4to 10 points. The pH ofthesolution is suitably in the range 4.2 to 5.0.

The phosphating solution may contain a conventional accelerator, which may be organic or inorganic. Organic accelerators are for instance water-soluble aromatic nitro compounds such as sodium nitrobenzene sulphonate. Inorganic accelerators are, for instance, nitrate or nitrite.

Fluoride may be present generally in an amount of up to 0.5 g/l, suitably up to 0.2 g/l.

The phosphating step may be by dipping or by spraying. Dipping is usually carried out at a temperature in the range 25 to 60"C, suitably for a period of 1 to 5 mins. Spraying is generally carried out at a slightly lower temperature, for example in the range 20 to 500C, for a similar period of time, i.e.

1 to 5 mins.

The phosphating step may be preceded by a cleaning step, for example a conventional alkaline cleaning step. The cleaning step is generally followed buy a rinse which is followed by the phosphating step. This sequence of steps is particularly suitable for a dip-coating method.

Preferably the phosphating solution contains a surfactant to aid contact of the solution with the metal surface.

Alternatively, the phosphating step may be combined with a cleaning step, for example by including surface active agents in the phosphating solution in amounts such that the solution removes dirt and grease from the metal surfaces. In a process in which cleaning and phosphating are combined it is found to be useful to use two successive phosphating steps, using solutions containing the same active ingredients. This is because dirt removed from the surface by the cleaning agents in the first combined cleaning phosphating step contaminates the coating solution and thus the coating itself. The second coating step provides a coating with a relatively uncontaminated surface to receive the enamel frit. Suitably a batch of clean phosphating solution is used in the second phosphating step for a period of time and is then used in the first phosphating step.Batches of solution contaminated by use in the first step are discarded after a period of time. In this way the surface of the coating is substantially free from contamination. The combined cleaning and phosphating process is particularly preferred for spray coating methods.

The phosphating step is suitably followed by one or more rinsing steps, for example water rinses, prior to deposit of the enamel frit onto the phosphate coated surface.

The chemicals for providing the phosphating solution, may be supplied as a concentrated aqueous solution which can be diluted in line to give a phosphating solution of the desired concentration.

Alternatively, the chemicals may be provided as a mixture of water soluble powders, for example a mixture comprising an alkali metal or ammonium dihydrogen phosphate and a water soluble inorganic salt of nickel. It may be necessary to include a solid alkali such as ammonium carbonate to give a solution of an appropriate pH. The solid mixture may also include surface active agent, for example a solid surface active agent, or a liquid surface active agent in an amount at which the solid mixture remains dry and handlable, for example up to 5% by weight. The concentrate or solid mixture usually contains all the necessary desired ingredients for the final coating solution, for example it will generally contain an accelerator.

The enamel coating may be a conventional enamel coating, e.g. one comprising two or more coating layers or it may be an enamel requiring only a single coating layer, such as a pickle-free enamel.

The enamel is coated onto the surface by conventional methods usually being applied as a powder and then fired to cure the coating. An example of a suitable frit is Tube Investment Enamellers 2378.

By the process of the invention it is possible to form a coating comprising iron phosphate and nickel phosphate of a lowerweightthan in GB 1498490, which requires smaller amounts of nickel starting materials and is therefore a more efficient process. Lighter coatings are also preferable to heavy coatings since they promote greater adhesion between the metal substrate and the enamel coating. The process allows lower quality steel to be used as the substrate, thereby reducing the cost.

The process of the invention gives far stronger adhesion between the substrate and the enamel than conventional pretreatment processes as evidenced by the Dropping Weight test for enamelled substrates.

The following example illustrates the invention.

EXAMPLE A phosphating solution contained nickel ions in an amount of 0.6 g/l, phosphate in an amount of 2 g/ I, fluoride ions in an amount of 0.1 g/l, ammonium ions in an amount of 1.0 g/l, and surfactant in an amount of 0.9 girl. The total acid in the solution was 5 points and the solution had a pH of 4.6. A very low carbon steel Virkosaal plate was pretreated by subjecting itto a conventional alkali cleaner followed by-a water rinse. The plate was then dipped in the phosphating solution at a temperature of 50 C for a period of 5 min. The phosphating step was followed by two rinsing steps. The weight of the resultant phosphate coating was 0.25 g/m2.

The treated surface was coated with an enamel frit (Tube Investment Enamellers 2378/1362) and fired at a temperature of 830+ 1 00C to form the enamel coating. The enamel substrate was subjected to a dropping weight test, in which a 500 g weight was dropped 1 m onto a 25 mm diameter ball on a 25 mm anvil.

The tested substrate was inspected according to the publication "Visual Classification of Adhesion of Vitreous Enamel to Steel" published by the Institute of Vitreous Enamellers and was classified as excellent.

Claims

1. A process for enamelling a metal surface which comprises iron, in which the surface is treated with an aqueous phosphating solution containing nickel ions and having a pH in the range 4.0 to 5.5 to form a coating comprising iron phosphate and nickel phosphate, an enamel frit is applied to the phosphate coated surface and the frit is fired.

2. A process according to Ciaim 1 in which the weight of the phosphate coating is in the range 0.15 to 0.6 g/m2.

3. A process according to any preceding claim in which the phosphating solution contains an accelerator.

4. A process according to Claim 3 in which the accelerator comprises fluoride.

5. A process according to any preceding claim in which the phosphating solution contains nickel ions in an amount in the range 0.1 to 2.0 g/l, phosphate ions in an amount in the range 1 to 12 g/l and fluoride ions in an amount up to 0.5 girl.

6. A process according to any preceding claim in which the phosphating solution has a total acid present in the range 2 to 15 points and/or a pH in the range 4.2 to 5.0.

7. A process according to any preceding claim in which the phosphating solution contains a su rfactant.

8. A process according to any preceding claim in which the phosphating step is preceded by an alkaline cleaning step.

9. A process according to any preceding claim in which the metal surface is contacted with a phosphating solution, as defined, in two successive steps.

10. A process according to any preceding claim in which the surface is dipped in the phosphating solution at a temperature in the range 25 to 600C preferably for a period in the range 1 to 5 minutes.

11. A process according to any preceding claim in which the metal surface is sprayed with the phosphating solution at a temperature in the range 20 to 50"C preferably for a period in the range 1 to 5 minutes.

12. A concentrated aqueous solution capable of being diluted to form a phosphating solution as defined in any of Claims 1 to 7.

13. A solid composition comprising a mixture of an alkali metal or ammonium dihydrogen phosphate and a water-soluble inorganic nickel salt and optionally an alkali, a surfactant and/or an accelerator, the solid mixture being capable of being dissolved in water to form a phosphating solution as defined in any of Claims 1 to 7.