GB2147008A - Surface treatment - Google Patents

Abstract

A method for the in-situ treatment of a surface eg the anodisation of part of an aluminium or aluminium alloy component 1 to prepare it for adhesive bonding, in which method layers of gauze 2 or the like impregnated with a surface treatment liquid or gel, are applied to the component surface together with a mesh electrode 3 if required, and in which any trapped air or gaseous products of the treatment is drawn off through a gas-permeable sheet 4 forming part of a vacuum bag. <IMAGE>

Description

SPECIFICATION Surface treatment This invention relates to the surface treatment of components, for example the anodisation of aluminium and aluminium alloys prior to an adhesive bonding process or the like.

It may be required to treat the surface or a portion of the surface of a component using a liquid or gel treatment agent without immersing the component in say a tank containing the agent. Such a surface treatment may be intended say to adapt a surface to be more amenable to adhesive bonding, for example when making an in-situ patch repair on an item such as an aircraft wing. An example of a surface treatment which may improve the bonding of adhesives to the surface of an aluminium or aluminium alloy component is anodising. Thus, it is known to anodise an aircraft surface portion, prior to some further action thereon such as adhesively bonding a patch thereto, by constructing on that surface portion a so-called anodising cell.An acid electrolyte, for example a phosphoric acid gel is first applied to the surface, which surface forms the anode of the cell, followed by layers of glasscloth, gauze or felt impregnated thoroughly with more of the electrolyte, and then a stainless steel mesh cathode. A low voltage d.c. source is then connected between the surface and the cathode so that a current flows and anodisation of the surface takes place. During this process, oxygen is liberated at the surface.

Airtrapped in the layers of gel-impregnated gauze and the liberated oxygen may cause irregularities in the eventual anodicfilm, particularly if the process is being done in-situ in relation to the lower surface of a component, for example a portion of the undersurface of an aircraft wing since then the gas will tend to form voids at that surface. This invention provides a surface treatment method wherein the incidence of voids and irregularities is reduced by permitting the unwanted gases to be drawn off.

According to one aspect of the present invention, there is provided a method of treating the surface of a component wherein a surface treatment liquid or gel is positioned between said surface and a gaspermeable sheet, and a reduced pressure is maintained over the sheet to draw off via the sheet air and gaseous products of the treatment from between said surface and said sheet.

Preferably, a container including a non-porous, flexible sheet having a outlet for connection to pressure reducing means is sealed to said surface to maintain said reduced pressure.

Advantageously, the treatment liquid or gel may be applied to said surface by means of a porous matrix or wad of material impregnated with said liquid or gel. The matrix may consist of a plurality of layers of porous material, each layer being impregnated with said liquid or gel.

Conveniently, said method may be adapted to allow electrolytic treatment of said surface by placing an electrically-conductive member adjacent to and in contact with said liquid or gel. Naturally, said member may form the cathode of the electrolytic cell and the surface of the component to be treated, the anode.

According to another aspect of the invention, there is provided a method of anodising the surface of an aluminium or aluminium alloy component, the method including forming an anodising cell by applying a porous matrix impregnated with electrolyte to said surface and positioning against the matrix an electrically-conductive member for forming the cathode of the cell, and the method further including positioning a container over said cell and in sealing engagement with said surface, the container containing a mass of porous material and that side of the container which is nearest said cell comprising a flexible gas-permeable sheet, and maintaining a reduced pressure within the container so as to draw off from the cell, via said sheet, air and gaseous products of the anodising process.

According to yet another aspect of the invention, there is provided apparatus for use in the method described above comprising a container including a non-porous flexible sheet having an outlet for connection to pressure reducing means and a peripheral region for being sealed to said surface, the container containing a mass of porous material and that side of the container which is nearest said cell comprising a flexible gas-permeable sheet, a wad of electrolyteimpregnated material for being placed against said surface, and an electrically-conductive member for contacting a surface of said wad remote from said surface of the component.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing, the single figure of which is a diagrammatic sectional elevation of part of a component of which a surface portion is to be anodised, and an anodising cell and vacuum container arrangement made up on said surface portion.

The figure shows part of an aluminium or aluminium alloy component 1, a surface portion la of which is to be anodised by making up thereon an anodising cell and then allowing current to flow through the cell. To form the anodising cell, the surface 1a is liberally coated with phosphoric acid gel and then a porous matrix 2 comprising several layers of gauze, felt or glass-fibre cloth is applied to the surface. The layers making up the matrix are laid in turn, each one, when it has been laid, being thoroughly impregnated with more of the acid gel. A stainless steel mesh 3 is then placed over the gauze (and, for preferance, more of the gel is applied over the mesh) to complete the anodising cell - the surface 1 being the anode, the phosphoric acid gel the electrolyte and the mesh the cathode.

Before starting the anodisation, a vacuum container or bag is made up over the anodising cell. To do this, strips of double-size adhesive tape 7 are applied to the surface la all around the perimeter of the anodising cell. The electrical connecting lead 3a from the mesh cathode 3 is brought out of the cell over the tape 7 and then more double-sized adhesive tape 7a is laid over this connecting lead. A sheet 4 able to pass gas but not the acid gel is then laid over the cell and into engagement with the tape 7 and 7a all round the cell. The sheet 4 may comprise a so-called microporous film, for example a piece of the commerically available material 'Celguard 4510' which is polypropylene film about 50-75 micrometers in thickness and having numerous microscopic holes or slits, perhaps 0.4 micrometres across, formed therein.

A series of gas-permeable breather layers 5 each made of non-woven felt material such as 'Tiger Vac NW5' or of glasscloth or sackcloth are placed over the film. Further double-sized adhesive tape 7b is applied to the film around the series of breather layers and finally a sheet 6 of non-porous material is laid over the breather layers and stuck to the tape 7b.

The sheet 6 has a gas outlet tube 6a formed therein and this tube is connected to a suction pump (not shown) to enable a partial vacuum to be maintained between the sheets 4 and 6 and hence also between the sheet 4 and surface la.

To operate the anodising ceil, an electrical voltage source (not shown) of perhaps 1 to 4 volts d.c. is connected between the surface la and the mesh cathode 3. During the anodising treatment, oxygen is generated at the surface la. This is drawn off through the sheet 4 and layers 5, along with any air that may have been trapped in the gauze 2, by the suction pump. Meanwhile, the vacuum container defined by the sheets 4 and 6 and the breather layers 5 inside the container are pushed by the pressure of the air outside the container into engagement with the mesh electrode 3 and the gel impregnated layers 2 so helping to maintain the layers 2 and the gel in close engagement with the surface la.The microporous sheet 4 thus, while allowing the relatively easy passage of gas, prevents the electrolyte gel from being drawn away from surface la toward the outlet to the pump. The action of the ambient air pressure also maintains the vacuum container and anodising cell in position on surface la, i.e. so the method and arrangement shown can also be used 'upside down' so to speak, i.e. where the undersurface of an article such as an aircraft wing is to be anodised in situ, the anodising cell and vacuum container can be made up on that undersurface and then kept in place by the pressure of the ambient air pushing the container and the cell components upwards against the surface.However, the pressure within the container should not be made so low that the force exerted on the layers 2 by the sheet 4 is sufficient to cause over-consolidation of the layers 2 and hence obstruct the free passage of the liberated oxygen. A pressure of around 250 Torr or so has been found preferable.

Instead of making up the anodising cell and vacuum container in-situ as described above, either or both or these items could be assembied perhaps at a site where the facilities and/or conditions are more appropriate to such assembly, and then taken and applied as a 'package' to the aircraft wing or like article to be treated.

As will be appreciated, various parameters of the anodisation process, for example the voltage, current density, acid gel concentration and viscosity, anodising time and the initial cleaning and subsequent rinsing of the anodised surface, may alp need to be controlled to give the desired anodising effect.

Generally, these parameters may be set the same as they are for the known anodisation process using an anodising cell.

Possibly, one or more of these parameters may be advantageously varied from the usual setting when the method and arrangement shown in the drawing is used - such optimum settings can easily be obtained by experiment.

Claims (8)

1. A method of treating the surface of a component wherein a surface treatment liquid or gel is positioned between said surface and a gaspermeable sheet, and a reduced pressure is maintained over said sheet to draw off via the sheet air and gaseous products of the treatment from between said surface and said sheet.

2. A method of treating the surface of a component according to claim 1 wherein a container including a non-porous, flexible sheet having an outlet for connection pressure reducing means, is sealed to said surface to maintain said reduced pressure.

3. A method of treating the surface of a component according to claim 1 or 2, wherein a porous matrix is impregnated with said liquid or gel and said matrix is placed against said surface.

4. A method of treating the surface of a component according to claim 3, wherein said matrix comprises a plurality of layers of porous material impregnated with said liquid or gel.

5. A method of treating the surface of a component according to any one of claims 1 to 4, and adapted to allow electrolytic treatment of said surface, wherein an electrically-conductive member is placed adjacent to and in contact with said liquid or gel.

6. A method of anodising the surface of an aluminium or aluminium alloy component, the method including forming an anodising cell by applying a porous matrix impregnated with electrolyte to said surface and positioning against the matrix an electrically-conductive member for forming the cathode of the cell, and the method further including positioning a container over said cell and in sealing engagement with said surface, the container containing a mass of porous material and that side of the container which is nearest said cell comprising a flexible gas-permeable sheet, and maintaining a reduced pressure within the container so as to draw off from the cell, via said sheet, air and gaseous products of the anodising process.

7. Apparatus for use in the method of claims 5 or 6, comprising a container including a non-porous flexible sheet having an outlet for connection to pressure reducing means and a peripheral region for being sealed to said surface, the container containing a mass of porous material and that side of the container which is nearest said cell comprising a flexible gas-permeable sheet, a wad of electrolyteimpregnated material for being placed against said surface, and an electrically-conductive member for contacting a surface of said wad remote from said surface of the component.

8. A method of anodising the surface of a component substantially as hereinbefore described with reference to the accompanying drawing.