Improved metal coating baths

A metal is provided with an inorganic coating of a salt or oxide by immersion in an acid coating solution which contains a surface-active agent and has floating on its surface a layer of inert liquid. The inert liquid may be paraffin wax, liquid paraffin, petroleum jelly, an animal or vegetable oil such as lanolin, tallow or castor oil, bitumen, a low molecular weight synthetic resin or a low melting point natural resin. The layer may contain a surface-active agent, perferably the one used in the coating solution, and a dye to give an estimate of the layer thickness and to give warning that the solution is hot since the layer reduces fuming. Specifically disclosed are solutions based on (1) phosphates of iron, manganese, zinc, solium, potassium, ammonium and magnesium, which solutions may also contain accelerators such as nitrates, nitrites, chlorates, bromates and nitroguanidine; (2) oxalates: in an example the solution also contains manganese, nitrate and sodium thiosulphate; and (3) chromates: passivating baths for aluminium and magnesium, respectively, contain (a) chromium trioxide and phosphoric acid, and (b) sodium dichromate, magnesium sulphate and manganese sulphate. The production of sulphide coatings is also referred to. Concentrates of the acid coating material and the inert liquid may be made up for subsequent dilution. The resulting coatings are stated to improve corrosion resistance and paint adhesion and to serve as lubricant or lubricant-carrying layers in drawnig processes. In one Example (X) aluminium panels are degreased by immersion in a 3% metasilicate solution containing a surface-active agent and covered with a layer of oil.