An exploratory study of the possibility of using certain inorganic fused salts as electrolytes for the deposition of aluminum

Abstract

In order that further steps could be taken in the attempt to defeat corrosion, it was desirable to find a method for electrodepositing aluminum on other metal surfaces.

To meet this need, an investigation was conducted to attempt to find an electrolyte that would be suitable from the field of molten inorganic aluminum and alkali salts.

The principal work by other investigators on the electrodeposition of aluminum has been with aqueous solutions which have been proved unusable and with fused alkali-aluminum halide mixtures that have produced crystalline, poorly adherent aluminum coatings.

In the present investigations, seven inorganic, fused salt systems were studied. These systems were chloride, fluoride, cyanide, sulfide, thiocyanate, formate and fluoborate. No electrolyte was found that gave a compact, adherent aluminum deposit.

Investigation of a fused electrolyte containing 66 mol per cent aluminum chloride, 20 mol per cent sodium chloride and 14 mol per cent potassium chloride at 160 °C produced poorly adherent, dendritic aluminum deposits on a copper cathode at a current density of 0.833 ampere per square decimeter. This coating formed a surface alloy between aluminum and copper when electroplated pieces of copper were heat treated in an electric furnace at 550 °C for 45 minutes and at 1000 °C for one minute. The piece treated at 1000 °C seemed to have deeper penetration of the aluminum.

Ratios of 45 to 55, 42 to 58 and 40 to 60 mol per cent aluminum fluoride to potassium fluoride failed to produce a composition that would have a low viscosity at a temperature below the melting point of aluminum, 660 °C.

A fused bath of 70 weight per cent potassium sulfide, K₂S₈, and 30 weight per cent aluminum sulfide, Al₂S₇, did not yield aluminum after electrolysis at current densities of 2.5 and 5.0 amperes per square decimeter for three hours at a temperature of 300 °C and using platinum electrodes. A plot of cell potential versus current for the sulfide system indicated oxidation and reduction of the electrolyte with ultimate passivity of the anode.

Systems of cyanide, fluoborate and formate proved to be impractical because of the difficulty in handling or obtaining the chemicals involved.

Aluminum thiocyanate showed promise as an electrolyte but limitation of time prevented experimental work on this system.