GB932923A - Coating metallic sheet or strip material with powdered annealing separator substances - Google Patents

Abstract

932,923. Coating articles. ARMCO STEEL CORPORATION. June 26, 1961, No. 23107/61. Class 117. [Also in Group XV] An adherent coating of an annealing separator in dry form and devoid of combined moisture is formed on the surface of metallic sheet or strip stock by creating between the said surface and at least one electrode spaced therefrom an electrostatic field by applying to said electrode an electrical potential sufficiently positive with respect to the potential of said sheet or strip surface to produce a positive corona about the said electrode without producing a negative corona at the said surface, and introducing into the space between the said electrode and said surface an annealing separator in finely-divided form entrained in a dry, non-reactive gas, the position of introduction being closer to the said surface than to the said electrode and outside the area of the said corona. Suitable annealing separators include magnesium oxide, calcium oxide, alumina and silica. The metal sheet may be of iron, steel or silica-iron. Dried air, hydrogen, gases rich in hydrogen and argon are specified as the entraining gas. It is advantageous to isolate the separator laden gas from the electrodes by a blanket of entraining gas free from the separator substance. As shown (Fig. 2), the finely-divided separator substance is fed from a hopper 1 by means of an agitator 3 and screen 2 into delivery means 4 and entrained in a moving gas stream by means of a Venturi feed system 6. The separator laden gas passes through a feed channel 7; a gas blanket may be provided by means of an additional conduit 8. The strip to be coated, 9, is unrolled from roller 10 and is tensioned by rollers 12 and 13 outside the insulating housing 14 surrounding the electrodes, all the rollers being earthed. Rod-like electrodes 15, 16, 17 and 18 extending transversely across the strip and less in length than the width of the strip are connected to the positive side of a highvoltage supply 19. Following the rod electrodes there is a plate electrode 20 preferably of brass or other metal, connected to its own voltage supply so that the potential applied to it is about half that applied to the rods. This electrode ensures deposition of any remaining entrained separator substance and promotes even distribution of the deposited separator.