GB2313382A

GB2313382A - Metal coating

Info

Publication number: GB2313382A
Application number: GB9610783A
Authority: GB
Inventors: Jean-Marc Jailloux
Original assignee: Societe Civile des Brevets Henri Vidal
Current assignee: Societe Civile des Brevets Henri Vidal
Priority date: 1996-05-23
Filing date: 1996-05-23
Publication date: 1997-11-26
Also published as: ZA974532B; GB9610783D0; WO1997044502A1; AU2911397A

Abstract

A method of depositing a protective metal coating on a hot formed metal object 2, which may be a rolled steel strip, is characterised in that the object is protected from the atmosphere by means of a tunnel 3 of inert gas as it leaves the hot-forming stage. The object is then coated with droplets of molten metal by spray guns 7 to form a priming coating before it cools down. Subsequently, a further coating is applied. The priming coating is less than 30mm thick.

Description

Metal Coating Apparatus and Method This invention relates to the protection of metal objects, in particular those formed of ferrous metals, from corrosion by the application of a thin coating of another metal.

There are many applications where such protection is of great importance, for example in the field of civil engineering, where metal mechanical earth stabilising members must be capable of retaining their integrity for many years in corrosive environments.

One process of this type which has been known for many years is hot-dip galvanising. An iron or steel object is dipped into a bath of molten zinc which adheres to the object, thereby providing a protective coating. Since the use of pure zinc could result in the formation of a brittle alloy, small quantities of aluminium or magnesium may be added to the zinc.

More recently, improved processes have been developed in which the coating is provided by spraying a molten metal (in the form of droplets) onto the object which is to be protected. One successful process involves spraying an alloy of 85% zinc and 15 aluminium onto a steel object which has previously been carefully shot-peened. Shot peening is important in order to ensure that the surface is clean, oxide free and has the necessary rugosity for good adhesion of the coating.

The spraying is carried out using a gun in which alloy wire is fed to a torch or electric arc where it is melted before being sprayed using compressed air as a propellant. Since the coating has a porous structure, a minimum of 40 Zm (microns) is required to ensure that the object is adequately covered. However, it is generally recommended that a coating of about 100 ym be applied. International Standard ISO 2063 defines the characteristic properties of this process.

FR-A-2 575 765 describes an example of this type of process in which a metal object is shot peened and then heated to 100 C before being coated with molten zinc.

This process may be applied in a continuous manner.

Thus, a finished work piece may move along a production line from a first stage where it is shot-peened to a spraying booth where the coating is applied. The booth may contain, eg. 2, 4 or 6 spray-guns, depending on the thickness of the coating, the perimeter of the section to be coated and the speed of the line (which is typically 10 to 20 m/minute (0.17 to 0.33 m/s)).

Another method of protecting a metal object is described in WO 94/19640. Here, a cast iron pipe having a skin of oxides is taken directly from a heat treatment furnace and sprayed with a zinc-aluminium alloy to a density of at least 200g/m2 which is equivalent to 50-60 ym in thickness. A second coat of organic or mineral binder, such as bitumen, coal pitch or epoxy resin is then applied to seal the pores in the metal coating.

According to one aspect of the present invention, there is provided a method of depositing a protective metal coating on a hot-formed metal object, the coating being deposited on the object after the hot-forming of the object is completed, wherein a priming coating is deposited before the metal object cools down from the hot-forming process.

The term "priming coating means a coating that is applied to the object to provide a base onto which a further metal coating may be directly applied and to which it will form a firm bond. In addition, the priming coating will preferably provide a degree of corrosion resistance.

The invention may be applied to a metal object produced by a variety of hot forming processes such as forging, hot rolling etc. The object may be a discrete item or be produced in an endless fashion (eg steel strip).

By means of the invention, the coating is deposited on the metal whilst it is still hot from the hot-forming process. For example, in the case of a steel object, the temperature may typically be 800 to 900"C. In these conditions, suitable metal coatings, for example zinc, aluminium, or the zinc-aluminium alloy referred to above, will react with or diffuse into the hot metal object or otherwise form a strong bond thereto, even if the object is completely smooth. Thus, there is no need to ensure that the surface of the object is roughened, eg. by shot peening, before applying the coating.

If desired, the priming coating may be comparatively thick but this is not necessary, or even preferred. Indeed, where a high speed continuous production process is involved, applying thick coatings may cause difficulties. For example, in the prior art spraying process, to achieve coatings of a desired 100 sjm thickness, spray guns are directed at objects moving along a production line at about 10 to 20 m/min (0.17 to 0.33 m/s). In contrast, a typical rolling mill may produce steel strips at a rate of 2 to 5 m/s. Thus, if objects are to be spray-coated to 100 ym thickness as they leave a rolling mill, many (perhaps 10) spray guns will be needed unless the line is to be slowed down considerably. The use of so many spray guns would, of course, be complex and expensive.

It has been found that only a very thin priming coating is required in order to provide a good base for further coatings and to protect objects from oxidation and surface contamination for a significant period of time in a normal air atmosphere. Thus, preferably the priming coating is just sufficiently thick to prevent the object from corroding in a normal air atmosphere.

Such a coating is preferably less than 30 ym or 20 ym, but it is particularly preferred for thinner coatings to be used since this allows for faster line speeds and/or fewer spray guns. Thus, ideally the coating is only a few microns thick, for example less than 10 ym, or preferably 2-5 ijm. Subsequently a further coating (or a number of further coatings) may be applied over the priming coating to provide the desired thickness of coating which will generally be around 100 ssm thick.

The priming and further coating(s) need not be of the same nature or composition. The priming coating should be formed of a material that will form a secure bond directly to the metal object and preferably also have good corrosion resistance. In particular, as it is applied to the metal object when the object is hot, it should form a satisfactory bond in such conditions. The further coating should provide a good bond to the priming coating, but it will generally not be necessary for it to be applied to the object when the latter is hot. One example of a preferred combination of coatings is to use a thin priming coating of pure aluminium covered with a thicker second layer of zinc-aluminium alloy.

The second coating can be applied some time after the priming coating (eg. several days later) and, in the meantime, the object may be stored in a normal air atmosphere. If necessary, the further coating could be applied at a remote site. The second coating may be applied by means of the prior art spray deposition method, but without shot peening, using the known line speed. The priming coating provides a suitably rough surface to which the second coating can key and the two layers become totally homogenous. The result is a coating that provides protection equivalent to a coating formed as a single layer.

Although, in some applications, the formation of a degree of oxide on the object before the priming coating is applied can be tolerated, this should be prevented in certain applications. Thus in a preferred method the metal object is substantially free of metal oxide when the priming coating is deposited. Preferably, the exposure of the object to oxygen is restricted after it is hot-formed. As a result of this, and the fact that there is no need to roughen the surface of the object, the previously important shot-peening stage can be eliminated, even in critical applications (ie. when the metal object is to be subjected to a very severe environment). This greatly simplifies the process and provides significant cost savings.

This is, in itself, believed to be an inventive method and therefore, viewed from a further aspect, the invention provides a method of depositing a protective metal coating on a hot-formed metal object, the coating being deposited on the object after the hot-forming of the object is completed, wherein, between the hotforming and the deposition of the coating, the exposure of the object to oxygen is restricted.

Preferably the coating is a priming coating as discussed previously.

One way to restrict the exposure of the metal object to oxygen and to ensure that deposition occurs at a high temperature is to deposit the coating just as the object passes from the final stage of a hot-forming process. However, in some applications it may be difficult to deposit the coating immediately the object leaves the hot-forming process. Even if the coating is applied only a few seconds later, with some metals this could result in the formation of a layer of oxide (eg.

mill scale) which would decrease the adhesion of the coating. Preferably, therefore, there is provided means to allow the metal object to pass from hot-forming apparatus to a remote deposition stage without its being exposed to oxygen to a significant degree. This may be achieved by surrounding the object with an inert gas, such as nitrogen.

Thus, there may be provided a tunnel through which the object passes, the tunnel being filled with inert gas from a supply which preferably maintains the inside of the tunnel at above atmospheric pressure. In this way, the entrance to and exit from the tunnel may be left open to permit the free passage of the objects which are to be coated whilst preventing the ingress of air. This facilitates the use of continuous production techniques.

The entrance and exit to the tunnel may be simple openings, but preferably, the tunnel has tugres at the entrance and exit in order to limit the loss of inert gas.

Although the method of invention may be applied to many different coating techniques, it is particularly applicable to a modified version of the known continuous process referred to above. Thus, preferably, spray guns are used to coat the metal object with droplets of molten metal.

The method can be applied to many kinds of metal objects, in particular those where corrosion resistance is critical, for example in the field of civil engineering where the objects are exposed to corrosive environments. The invention is particularly applicable to the production of objects that will be buried in soil, such as pipework or mechanical earth stabilising members. Since the latter may be produced in the form of strips using a rolling mill, it will be seen that the preferred forms of the present invention described above can be readily applied to their production.

It will be appreciated that the invention also extends to apparatus for carrying out the method. Thus, according to another aspect of the invention, there is provided apparatus for depositing a metal coating on a hot-formed metal object, the apparatus being arranged such that exposure of the metal object to air between the hot-forming and coating stages is restricted.

Additionally, there is provided apparatus for depositing a metal coating on a hot-formed metal object, the apparatus being arranged such that only a coating sufficient to protect the metal object from oxidation in a normal air atmosphere is initially deposited onto the object before it cools down from the hot-forming process.

Preferably the coating is formed using spray-guns, as discussed above. These may be located to spray the object immediately after it has been hot-formed.

However, as discussed previously, the coating apparatus may be-located downstream and in such an arrangement means is preferably provided to prevent oxidation of the metal as it passes to the coating stage.

An embodiment of the invention will now be described, by way of example only, and with reference to the single figure.

Rollers 1 are the final stage of a conventional rolling mill, the rest of which is not shown. Steel strip 2 (which passes continuously through the entire apparatus) is formed into its final profile by the rollers, which it leaves at a speed of about 3 m/s. At this stage the steel is at a temperature of about 900"C.

Immediately the steel leaves the rollers 1 it enters tunnel 3 via entrance 4. The tunnel is supplied with a continuous stream of nitrogen gas via inlet 5.

This prevents oxidation or surface contamination of the strip. The nitrogen is supplied at a sufficient rate to maintain the inside of the tunnel at above atmospheric pressure, despite the losses of gas from entrance 4 and exit 6. The gas escaping from the entrance and exit does, in fact, provide some benefit as it tends to reduce the amount of oxygen in contact with the steel 2 in the regions just outside the tunnel.

Subsequently, the steel 2 leaves the tunnel via exit 5 where it is sprayed by two spray guns with a molten alloy comprising 85% zinc and 15% aluminium. The spray guns are each of a conventional type in which alloy wire 8 is melted by means of an oxy-acetylene flame and then sprayed using a compressed air as a propellant. At this stage, the steel is coated with a priming coating having a thickness of a few microns.

The steel is then allowed to cool, and may be cut to lengths or rolled for storage as desired. The steel is then stored in a dry, normal air atmosphere. Later, (perhaps after some weeks) the steel is fed through a conventional spray-coating apparatus (not shown) at a speed of about 10 to 20 m/minute. This results in a much thicker coating of about 100 ym, which bonds to the first coating, becoming homogenous therewith. There is no need to shot peen the steel prior to application of the second coating, because the priming coating ensures that the second coatig forms a good bond.

Claims

1. A method of depositing a protective metal coating on a hot-formed metal object, the coating being deposited on the object after the hot-forming of the object is completed, wherein a priming coating is deposited before the metal object cools down from the hot-forming process.

2. A method as claimed in claim 1, wherein the priming coating is just sufficient to protect the object from oxidation in a normal air atmosphere.

3. A method as claimed in claim 1 or 2, wherein the priming coating is less than 30 ym thick.

4. A method as claimed in claim 1, 2 or 3, further comprising the step of depositing a further protective metal coating on the metal object.

5. A method as claimed in any preceding claim, wherein the hot-forming process comprises hot rolling.

6. A method as claimed in any preceding claim, wherein the priming coating comprises zinc, aluminium or zincaluminum alloy.

7. A method as claimed in any preceding claim, wherein, between the hot-forming and the deposition of the initial coating, the exposure of the object to oxygen is restricted.

8. A method as claimed in any of claims 1 to 6, wherein the metal object is substantially free of metal oxide when the priming coating is deposited.

9. A method as claimed in claim 7 or 8, wherein the coating is deposited just as the object passes from a hot-forming process.

10. A method as claimed in claim 7 or 8, wherein there is provided a tunnel through which the object passes between the hot forming and the deposition stage, the tunnel being filled with inert gas.

11. A method as claimed in any preceding claim, wherein spray guns are used to coat the metal object with droplets of molten metal.

12. A method as claimed in any preceding claim, wherein the metal object is a mechanical earth stabilising member.

13. Apparatus for depositing a metal coating on a hotformed metal object, the apparatus being arranged such that only a coating sufficient to protect the metal object from oxidation in a normal air atmosphere is initially deposited onto the object before it cools down from the hot-forming process.

14. Apparatus as claimed in claim 13, wherein the coating is deposited by spray guns.

15. Apparatus as claimed in claim 13 or 14, wherein the apparatus is arranged such that exposure of the metal object to air between the hot-forming and coating stages is restricted.

16. A metal object formed using the method of any of claims 1 to 11 and/or the apparatus of claim 13, 14 or 15.

17. A metal object having a protective metal coating in the form of a priming coating less than 30 ;im thick.

18. A metal object as claimed in claim 16 or 17 in the form of a mechanical earth stabilising member.

19. A method substantially as hereinbefore described, with reference to the accompanying drawing.

20. An apparatus for depositing a metal coating substantially as hereinbefore described with reference to the accompanying drawing.

21. A metal object substantially as hereinbefore described with reference to the accompanying drawing.