Die casting Aluminum chroming is quite complex and difficult electroplating process.It very hard to reach 100% yield of good quality,which is caused sometimes by die casting process,polish etc.However,there are several points we need to take into consideration.
It is very important for chroming parts to have proper hanging fixture.Especially,hanging fixture has to be big enough for big size chroming parts in order to ensure good conductivity.It is necessary to avoid to use copper hanging fixture because copper will be dissolved in degrease solvents and aluminum parts will be stained and have pitting or blister on surface.
After degrease totally,plating supplier need to control concentration,temperature,time for alkali etch.Al2O4 will be dissolved,which will cause over corrosion and discontinue etch.Therefore,it is very important for rinse after alkali etch.
Acid etch can remove all oxide and leave free aluminum substrate since all of element except for aluminum do not react with alkali.Water is strictly prohibited during acid etch otherwise the quality of chroming will be affected.Also,it is necessary to control time,concentration and time.25℃ is better for acid etch.Concentration will be 50% phosphoric acid,25% nitric acid,0-5% hydrofluoric acid and 20-25% desmut.Time will be within 0.5-1 minute for die casting aluminum.
Aluminum die casting parts have to pre-plating before final chroming.Double zincate is popular method for pre-plating.
Temperature need to be 15-25℃ during zincate.The deposit of zinc on surface will not be uniform if temperature is too high.
Concentration of zincate need to be strictly controlled.NaOH to Zn is 10-12:1 normally.If proportion of NaOH is too high,surface will have pitting after chroming.If proportion is too low,surface will have blister.
After double zincate,nickle-copper-chroming will be apply on aluminum die casting parts.