GB1199335A - Improvements in Aluminiding - Google Patents

Abstract

1,199,335. Electrolytically aluminiding metals. GENERAL ELECTRIC CO. Oct. 25, 1967 [Nov. 10, 1966], No.48516/67. Heading C7B. A method of forming an aluminide coating on a metal composition, the latter having a melting point of greater than 600‹ C., at least 50 mole percent of said metal composition being at least one of the metals selected from the class consisting of metals whose atomic numbers are 23 to 29, 41 to 47 and 73 to 79 inclusive, which method comprises (1) forming an electric cell containing said metal composition as the cathode, joined through an external electrical circuit to an aluminium anode or an anode of an alloy having aluminium as its most electro-positive component and a fused salt electrolyte consisting of (a) one or more of the alkali metal fluorides or a mixture of one or more of the alkali metal fluorides with one or more of the fluorides of calcium, strontium and barium and, (b) from 0À01-5 mole percent of aluminium fluoride, said electrolyte being maintained at a temperature of at least 600‹ C. but below the melting point of said metal composition in the substantial absence of oxygen and (when said metal composition includes a metal of atomic number 23, 24, 41, 42, 73 or 74) in the absence of carbon contacting the electrolyte and (2) controlling the current flowing in said electric cell so that the current density of the cathode is in the range from 0À05 to 10 amperes/dm<SP>2</SP> during the formation of the aluminiding coating. The term "aluminide" designates any solid solution or alloy or intermetallic compound of Al and the base metal. The cell generates electric current. The deposition rate of the Al may be decreased by inserting some resistance in the circuit or may be increased by impressing a voltage, e.g. to obtain constant current densities during the reaction. Specified fluorides that may be used are those of Li, Na, K, Rb and Cs, e.g. a eutectic mixture of NaF and LiF or of KF and LiF. The process may be carried out under vacuum or an inert atmosphere, e.g. of forming gas (10% H 2 , 90% N 2 ), N 2 , Ar, He, Ne, Kr or Xe. The electrolyte may be purified, e.g. from carbon, by an initial electrolysis. The anode may be an Al-Ni alloy rod, an aluminided Ni strip or be liquid Al in a graphite basket which has been shielded by a woven (Registered Trade Mark) "Monel" screen to prevent carbon from contaminating the fused salt bath. The aluminide has a hard, wearresistant surface. The method is modified for forming an aluminide coating on a metal composition selected from the class consisting of Ti, Zr, Hf (atomic numbers 22, 40, 72) and alloys thereof wherein at least 50 mole per cent of the alloy is Ti, Zr or Hf. The electrolyte is treated to remove carbon and oxygen and a negative potential of 0À1 to 5À0 volts is applied to the cathode prior to and during immersion of the cathode in the electrolyte to complete the electrical circuit.