EP0074211B1

EP0074211B1 - Coated metal substrate and method of coating a metal substrate

Info

Publication number: EP0074211B1
Application number: EP82304451A
Authority: EP
Inventors: Cyril Dawes; John David Smith
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1981-09-05
Filing date: 1982-08-24
Publication date: 1987-11-04
Also published as: AU552307B2; HU186571B; BR8205184A; AR228794A1; NZ201811A; YU199782A; EP0074211A1; ES8307909A1; AU8796882A; SU1364242A3; IN159201B; PL238136A1; DE3277585D1; ZA826202B; ES515496A0; JPS5852474A

Description

This invention relates to a method of coating a non-alloy steel substrate with a corrosion resistant coating.
It is well known that most steels are prone to environmental attack and become badly corroded in a relatively short period of time, particularly when exposed to a moist environment containing salt. In order to reduce the corrosion of steel, it is also well known to coat the steel with a corrosion resistant coating such as paint composition containing a polymer. However, if such a paint composition is applied directly to the surface of the steel substrate, an effective adhesion between the paint composition and the steel is not usually obtained. In particular, even when the steel substrate has been chemically cleaned before application of the paint composition thereto, the paint peels away from the surface of the substrate relatively easily, particularly when subjected to changes in ambient temperature and humidity. Once the paint composition peels away from the steel substrate, the latter is immediately prone to corrosion through oxidation.
In order to improve the adhesion between the steel substrate and the paint composition, it is known to effect pretreatment of the steel substrate by a process known as phosphating. In a typical phosphating treatment, the steel substrate is first cleaned for 2 to 5 minutes using an alkaline cleaner maintained at 50 to 70°C, then the cleaner is removed by rinsing the cleaned substrate in two successive rinsing operations with water at ambient temperature, each operation being of a duration of half to one minute. Then, the cleaned and rinsed steel substrate is sprayed with a zinc phosphate solution maintained at 40 to 70°C, the spraying operation taking about one and a half minutes. Alternatively, the steel substrate can be immersed in a zinc phosphate solution for about 5 minutes. Following this, the substrate is rinsed with water at ambient temperature and again rinsed with water at ambient temperature which is often de-ionized. As an alternative to this second rinse, the substrate may be subjected to a chromate rinse. A further rinse with deionised water at ambient temperature may be effected and will be effected if the above mentioned chromate rinse has been performed. Finally, the component is dried in an oven and is then ready for painting with a paint composition.
With this conventional phosphate pretreatment process, there are a number of disadvantages. The phosphating solution requires close chemical control to maintain consistent results. Control of effluent from the treatment plant is essential since excessive pollution of the site drainage system with zinc ions is not permitted. Additionally, it is likely that legislation will be enacted in the near future to restrict the phosphate ion concentration in site effluent discharge. High levels of plant maintenance are also required to maintain consistent results. The number of clean water rinsing operations required in the process makes itvulnerable to rising water costs. The process requires a high capital investment. The phosphating process is usually sited in a flow line arrangement organised for specific products and this greatly reduces or often eliminates any flexibility of product throughput. Lastly, the phosphating process is sensitive to the cleaniness of the components to be processed therefore requires close control at the cleaning stage.
An object of the present invention is to provide a method of coating a non-alloy steel substrate in which the above disadvantages can be obviated or mitigated and in which the corrosion resistance of the coated substrate can be materially improved.
GB-A-2026045 discloses a method of treating a steel substrate prior to painting, in which method the substrate is gas soft nitrided with gas components generating no free carbon to form a nitride layer, after which the substrate is cooled by standing in air and then directly painted. GB-A-2026045 recommends the nitride layer to be mainly epsilon phase, although mixed epsilon and gamma' phases are not precluded as the nitride layer.
Haeterei Technische Mitteilungen, vol. 29 No. 1. March 1974, pages 42-49 (J. Wuenning "Neues Verfahren und Anlagen zum Nitrieren mit epsilon-Verbindungsschicht") discloses the fact that gamma' nitride can be expected to separate out of epsilon- and alpha-phases during cooling, and also discloses that the gamma' phase can be reduced or eliminated by regulating the concentration of ammonia and of the other components in the heat treatment atmosphere. Moreover this article teaches that the segregation of gamma' nitride phase depends on the cooling rate of the treated sample and discloses that, when . carbonitriding in gas, the cooling rate may be increased as required from furnace or lock-chamber cooling to quenching in oil, emulsion or water.
According to the present invention, there is provided a method of coating a non-alloy steel substrate comprising the steps of effecting on the substrate a pre-treatment operation which includes heat treating the substrate so as to produce an epsilon iron nitride surface layer thereon, and subsequently coating the pre-treated substrate with a solid organic polymer coating composition, characterised in that after the heat treating step and before the coating step the heat treated substrate is quenched in oil, degreased and then heated to remove moisture from the micropores of the epsilon iron nitride surface layer.
The non-alloy steel is preferably one which contains up to 0.5 wt% carbon.
Preferably, the epsilon iron nitride surface layer is formed by heat treating the steel substrate at a temperature of 550 to 720°C for up to 4 hours in an atmosphere of ammonia, ammonia and endothermic gas, ammonia and exothermic gas or ammonia and nitrogen, with the optional inclusion of at least one of carbon dioxide, carbon monoxide, air and methane, followed by cooling. The terms "exothermic gas" and "endothermic gas" are well understood in the art. Carbon dioxide, carbon monoxide, air and exothermic gas are oxidizing gases. Carbon dioxide, methane and endothermic gas are carburizing gases. The heating to remove moisture from the micropores is effected to a temperature of e.g. 200°C.
The invention is applicable to any non-alloy steel substrates which are required to be rendered corrosion resistant, for example steel sheets, tubes, rods or other articles of manufacture produced by, for example, rolling, pressing, forging or extruding.
The polymer coating composition may be applied by means of a wet process, for example one employing a solvent to disperse the polymer coating composition, but may alternatively be effected by powder coating.
In addition to increasing the adhesion between the steel substrate and the organic polymer coating composition, the epsilon iron nitride layer produced as described above imparts other advantageous properties to the substrate. In particular, it is found that increased strength in components made from thin strip or sheet can be obtained. The epsilon iron nitride layer is very hard (approximately 1100HB) and has anti-seizure properties which can be exploited in certain applications merely by masking the required areas during the application of the polymer coating composition. The processing costs are lower than for phosphating and a further reduction may be obtained when only corrosion resistance is required by the use of higher temperatures and shorter times within the above ranges.
It is preferred to effect the heat treatment operation so that the epsilon iron nitride surface layer has a thickness of about 25 pm. Thicknesses greater than about 25 pm can lead to spalling or cracking of the surface layer. Typically, such a layer thickness of about 25 pm can be obtained by heat treatment at 660°C for 45 minutes. Such a layer thickness may also be produced by heat treatment at 570°C for 3 hours or at 610°C for 90 minutes. However, heat treatment temperatures and times may be employed to produce layer thicknesses of less than 25 pm, eg down to 15 µm. For example, heat treatment at 570°C for 2 hours can be employed to produce a layer thickness of 16-20 um.
The advantageous effects of the s-nitride pretreatment will become apparent from the following Test 1:

Test 1

A number of test plates of non-alloy steel having a carbon content of 0.10% were subjected to a phosphate pretreatment or a pretreatment according to the present invention and were then coated with one of three organic coating systems. The resultant samples were subjected to a salt-spray test in accordance with ASTM Standard B117-64 in which 5 plus or minus 1 parts by weight of sodium chloride are dissolved in 95 parts by weight of distilled water, the pH of the solution being adjusted so that, when atomised at 35°C (95°F) the collected solution will have a pH in the range of 6.5 to 7.2 and the temperature in the exposure zone of the salt-spray chamber is maintained at about 35°C (95 plus 2 minus 3°F)
After removal from the salt-spray test, the performance of the samples was evaluated by mesauring the degree of creep of corrosion from two diagonal lines scratched through the organic coating in the form of a cross. The creep was checked by applying 710 tape manufactured by the 3M's Company over the diagonal lines and then removing it by pulling back rapidly at 180 degrees to the sample surface. Pass or failure judgements were made using a permissible creep value of 2 mm on either side of the diagonal lines. The results are illustrated in the Table below:-
In the column headed "ε-Nitride Pretreatment", the steel samples were pretreated by heating them for two hours at 570°C in the case of samples (A)1 to 3, (A)5 and (B)1 and for 45 minutes at 660°C in the case of sample (A)4, in an atmosphere of 50% ammonia and 50% endothermic gas followed by slow cooling under a protective atmosphere of the same composition. The resultant steel samples had an epsilon iron nitride surface layer of a thickness of 16-20 pm.
It will be seen from the above Table that e-nitride pretreatment of the steel samples produces a substantial improvement in corrosion resistance under salt spray conditions. Therefore the e-nitride pretreatment is considered to be particularly suitable for steel articles for under bonnet motor vehicle applications. It will be appreciated that salting of roads during the winter time makes it necessary for exposed parts of the vehicle which are formed of steel to be rendered resistant to salt spray corrosion as effectively as possible.
Some details of the invention will now be described in the following Test 2, in which unless otherwise stated the percentages are by volume:

Test 2

A yoke body for a small electric motor was manufactured from non-alloy steel according to British Standard BS 1449 CS3 (C content=0.₁₀ wt%). The body was then nitrocarburised for 2 hours at 570°C in an atmosphere of 50% ammonia and 50% endothermic gas mixture (40% nitrogen, 40% hydrogen and 20% carbon monoxide) followed by slow cooling under the protection of the same atmosphere, to produce an epsilon iron nitride surface layer having a thickness of 16-20 µm on the body.
After this, an epoxy/phenolic/urea polymer coating formulation sold by International Paints Ltd under the code No. 0830X3020 was applied to the nitrocarburised body and stoved to produce a polymer coating having a thickness of 12-15 µm.
The resultant yoke body had a salt-spray resistance of more than 400 hours as measured according to the above-described Salt-Spray Test.
The Applicants have found that the epsilon iron nitride layer itself has an inherent resistance to corrosion by humidity and this property is particularly useful in cases where the organic coating becomes chipped in service or where it is desired to leave part of the surface of the epsilon iron nitride layer uncoated with the organic layer.

Claims

1. A method of coating a non-alloy steel substrate comprising the steps of effecting on the substrate a pre-treatment operation which includes heat treating the substrate so as to produce an epsilon iron nitride surface layer thereon, and subsequently coating the pre-treated substrate with a solid organic polymer coating composition, characterised in that after the heat treating step and before the coating step the heat treated substrate is quenched in oil, degreased and then heated to remove moisture from the micropores of the epsilon iron nitride surface layer.

2. A method as claimed in claim 1, wherein said heat treatment step is effected by heating, the steel substrate at a temperature of 550 to 720°C for up to 4 hours in an atmosphere of ammonia, ammonia and endothermic gas, ammonia and exothermic gas or ammonia and nitrogen, with the optional addition of at least one of carbon dioxide, carbon monoxide, air and methane.

3. A method as claimed in claim 1 or 2, wherein the coating composition is applied by means of a wet process.

4. A method as claimed in claim 1 or 2, wherein the coating composition is applied by powder coating.

5. A method as claimed in claim 1 or 2, wherein the heat treatment operation is effected so as to produce a surface layer having a thickness of not more than about 25 um.

6. A method as claimed in claim 5, wherein the heat treatment operation is effected so as to produce a thickness of about 15 to 25 um.

7. A method as claimed in claim 5, wherein the heat treatment operation is effected so as to produce a thickness of about 25 µm.