Electrolytic deposition of amorphous and crystalline zinc-calcium phosphates

Amorphous Zn-Ca phosphates and crystalline Zn-3(PO4)(2) . 4H(2)O conversion layers on cathode substrates we re prepared by electro lysis of mixtures o f acid ic solutions saturated with meta I phosphates. The solutions contain ed tricalcium phosphate (Ca-3(PO4)(2)) and/or zinc phosphate dihydrate (Zn- 3(PO4)(2) . 2H(2)O). The depositions was carried out with constant or pulsa ting cathode current densities in the range 20-70 mAcm(-2) at 20-70 degrees C. The deposition of the uniform crystalline Zn-3(PO4)(2) . 4H(2)O was per formed at a pulsating cathode current density of 70 mA cm(-2) at 70 degrees C for periods up to 10 min. Amorphous deposits of Zn-Ca phosphates contain ing 20 wt% H2O with variable Zn-to-Ca ratios were deposited at a constant c athodic current of 30 mAcm(-2) at 20 degrees C for 3 min. Surface areas of the amorphous deposits were of the order of 28 m(2) g(-1). X-ray diffractio n, differential thermal analysis and thermogravimetry were used to investig ate phase formation and transitions at increasing temperatures. The amorpho us Zn-Ca phosphate deposit was after calcination at 900 degrees C transform ed to crystalline phosphates containing the beta-Ca-3(PO4)(2) or Ca3-xZnx(P O4)(2) and alpha-CaZn2(PO4) phases. (C) 1998 Kluwer Academic Publishers.