Galvanized sheet metal is generally produced by immersing the sheet in a bath of molten zinc and providing a coating on both faces of the sheet. This method of manufacture has been fully developed and enables the production of sheet which is protected against corrosion for many years.
However, a new market has for some time provided outlets for sheet which is galvanized on one face and not on both. This market is provided by the automobile industry, in which the increasingly frequent salting of roads in winter causes an increased rate of corrosion of vehicle bodywork. Although in many cases in the building industry, sheet galvanized on both faces is tolerable, this is not the case in the automobile industry as it is not possible to provide a sufficiently smooth layer of paint on a layer of zinc. This layer of zinc is also disadvantageous with respect to the spot welding of the sheet.
Various solutions have already been proposed for coating the sheet on one of its faces alone. Franch Patent Application No. 2,344,640 thus relates to a method in which a bath of molten zinc is provided and the metal strip is passed close to the free surface of the zinc bath so that the surface tension and the wettability of the molten metal enable the formation of a meniscus on the free surface of the bath which contacts the face of the strip which is facing towards it.
In accordance with the method disclosed in the French Patent Application No. 2,348,278, the strip to be coated is passed through a bath of molten zinc, and the face of the sheet which is not to be coated is simultaneously contacted with a cylinder rotating about an axis parallel to the level of the bath, the contact between the face and the cylinder being maintained for the entire period of immersion of the sheet in the bath.
Japanese patent application No. 77-151,638 also proposes to mask the face which is not to be coated.
Belgian Pat. No. 859,420 relates to a method in which a thin jet of molten zinc is sprayed onto the face of the strip of sheet to be coated.
The first two techniques mentioned above do not enable the thickness and uniformity of the deposited layer to be controlled. In addition, in the second of these approaches, difficulties arise from the accumulation of zinc on the portions of the cylinder which project beyond the longitudinal edges of the strip of metal, as well as with respect to protecting the other non-coated face of the sheet.
The method which makes use of masking generally requires the subsequent removal of the coating designed to mask the non-galvanized face.
The method using a zinc jet is difficult to carry out, as it is very difficult to form a very thin jet which is perfectly laminar, especially if it is desired to produce a coating having a thickness in the range of some tens of microns.
Further proposals relating to the single face coating of substrates have been made.
French Pat. No. 1,153,715 relates to a device for the metallic coating of metal strip on one or both faces in which the molten metal is contained in a crucible having outlet lips pressed against the strip so that the width of the orifice formed between the two lips is small enough to cause any metal which is applied to the strip to be entrained from the application device as a result of the displacement of the strip, under the effect of the capillary forces resulting from the surface tension between the molten metal and the strip to be coated. In this way the displacement of the strip, which is wetted by the molten metal, causes the progressive removal of the metal from the crucible. A device of this type is disadvantageous for several reasons. For example the required contact between the distribution lips of the crucible and the strip to be coated causes the lips to become worn.
Moreover the fact that only the capillary forces resulting from the surface between the molten metal and the strip are used to pump the liquid from the crucible limits the coating speed. The possibilities for adjusting layer thickness with a device of this type are also restricted.
A further device based on the pumping action caused by capillary forces is disclosed in the U.S. Pat. No. 3,201,275 and in the German Auslegeschrift No. 1,080,373.
Lastly, in accordance with the U.S. Pat. No. 1,973,431, liquid metal is caused to flow through a passage whose cross-section corresponds to that of the layer to be deposited, the outlet end of this passage itself being located at a distance from the substrate which is exactly equal to the thickness of the layer to be deposited. This method is similar to extrusion methods and is very difficult to carry out unless considerable pressure is exerted on the metal. In addition, bearing in mind phenomena related to capillarity, this method is restricted to relatively large coating thicknesses.
Attention should also be given to U.S. Pat. No. 2,937,108 which describes a system for maintaining a molten mass of a metal adapted to coat a strip against a portion of this stretch whose speed of linear displacement is selected as a function of the thickness desired of the coating. This mass of metal is fed by at least one and possibly two rollers which may be immersed or may dip into a bath of the metal, or which may be in contact with a roller dipping into the metal bath. In either case, the molten metal is entrained by the surface of a roller into contact with the strip so that a meniscus is formed between a vertical bath of the strip and a portion of the surface of a roller adjacent this strip.
For a given geometry, especially for a given diameter of the coating roller, the mass of molten metal maintained against the strip is constant. Under these conditions the thickness of the deposit formed on the surface of the strip by molten metal cannot be varied except as a function of the speed of displacement.
This limitation in the adjustability constitutes a significant disadvantage from the point of view of industrial application of the technique since it is necessary either to operate at less than maximum strip speed (thereby decreasing productivity) or to modify the geometry by changing the parts (at considerable expense) for variation of the thickness. In fact, for reasonably thick coatings, the system of this patent must operate so slowly as to be impractical on a commercial scale.
U.S. Pat. No. 3,522,836 relates to the continuous production of wire from a molten metal which is displaced from a crucible through a horizontal passage which opens towards a cold drum.
A piston is displaced into the molten metal to cause the molten metal to flow through this passage and form a meniscus at the outlet of the distribution passage. As the cold drum rotates in contact with the meniscus, it draws from the latter a strand of the metal, which cools and solidifies on the surface to form the wire.
In this case as well, the possibilities for controlling the flow of the liquid metal are limited by the static equilibrium conditions established by the pressure exerted by the liquid, and the surface tension of the liquid capable of maintaining a stable meniscus. The possibilities of varying the flow are extremely limited and indeed care must be taken with any modification of the operating parameter to avoid the rupture of the meniscus and hence the disturbance of the static equilibrium. Under these conditions, moreover, it is practically impossible to coat a strip with a layer of a metal whose thickness can be varied independently of the speed of displacement of the metallic substrate.