GB989807A - Formation of corrosion resistant boride coatings on metal compositions - Google Patents

Abstract

A corrosion resistant boride coating is formed on a metal composition of melting point at least 600 DEG C., in which over 50 mol. per cent of the composition is at least one of the metals with atomic numbers 23-29, 41-47 and 73-79, by the method comprising (1) forming an electric cell containing the metal composition and boron as electrodes joined together through an external circuit and a fused salt electrolyte composed of at least one alkali metal fluoride and from 0.5 to 50 mol. per cent of at least one alkali metal fluoborate, which may be prepared in situ by dissolving BF3 vapour in a fused bath of the alkali metal fluoride, the electrolyte being maintained at 600-800 DEG C. in the absence of oxygen such as by using a vacuum, (2) controlling the current flowing in the cell, e.g. increasing the resistance in the circuit for a slower deposition rate or applying a voltage of up to 0.5 volt for a faster deposition rate, during the formation of the boride coating so that the current density does not exceed 3-amp./sq. dm., and (3) interrupting the flow of electric current after the desired thickness of silicide coating is formed. Preferably a porous graphite or perforated tantalum basket containing pieces of boron is used instead of a single piece of boron for the anode. Examples of boride coated products disclosed are strips of iron or steel with coating of Fe2B and FeB, Ni with coating of Ni2B; other specified metal compositions are Co, Cr, Nb, Mo, W, Re, Cu, Ta, Pt, "Inconel" and "Hastelloy" (Registered Trade Marks), and iron base alloys.ALSO:A corrosion-resistant boride coating is formed on a metal composition of melting point at least 600 DEG C., in which at least 50 mol per cent of the composition is at least one of the metals with atomic numbers 23-29, 41-47 and 73-79, by the method comprising (1) forming an electric cell containing the metal composition and boron as electrodes joined together through an external circuit and a fused salt electrolyte composed of at least one alkali metal fluoride and from 0.5-50 mol per cent of at least one alkali metal fluoborate, which may be prepared in situ by dissolving BF3 in a fused bath of the alkali metal fluoride, the electrolyte being maintained at a temperature 600-800 DEG C. in the absence of oxygen such as by using a vacuum, (2) controlling the current flow in the cell, e.g. increasing the resistance in the circuit for a slower deposition rate or applying a voltage of up to 0.5 volt for a faster deposition rate, during the formation of the boride coating so that the current density does not exceed 3 amp/sq.dm., and (3) interrupting the flow of electric current after the desired thickness of boride coating is formed. Preferably a porous graphite or perforated tantalum basket containing pieces of boron is used instead of a single piece of boron for the anode. Examples of boride coated products disclosed are strips of iron or steel with coating of Fe2B and FeB, and Ni with coating of Ni2B; other specified metal compositions are Co, Cr, Nb, Mo, W, Re, Cu, Ta, Pt, "Inconel" and "Hastelloy" (Registered Trade Marks), and iron base alloys.