Electrolyte for anodising light metal or alloy to oxide ceramic coating - contg. ammonium di:hydrogen phosphate and ammonium acetate in aq. soln.

摘要

Electrolyte (I) consisting of an aq. soln. of NH4H2PO4 (II) and ammonium acetate (III) is used for producing oxide ceramic surface coatings, which adhere well and are resistant to thermal shock, on light metals or their alloys by plasma chemical anodic oxidn.. Pref. (I) contains 0.8-1.2 mole/l (II) and 0.8-1.2 mol/l (III). Anodic oxidn. is carried out at a current density of 0.01-0.1 A/cm2 of an impulse current with a frequency of 200-1000 Hz and at 250-320 V. USE/ADVANTAGE - (I) is stable for long periods and free from fluorides, which avoids disposal problems. It is suitable for bonding with polymers and is unchanged when subjected to alternating thermal shocks. The anodised metal or alloy can be bonded with polymers, e.g. a power input site in composites, and as substrate for paints or adhesives. In an example, a degreased and pickled sheet of AlMglSilMn was made the anode in an aq. soln. of 0.9 mole/l = 100 g/l (II) and 1.9 mole/l = 70 g/l (III) and subjected to plasma chemical anodic oxidn. at 0.05 A/cm2, 500 Hz and 300 V. A grey-white oxide ceramic coating was obtd.. This was resistant to thermal shock. When exposed to 5 cycles of 30 min. at 450 deg. C and chilling in a water bath, the surface coating did not flake off and no microcracks forme