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topic 5993

E Coating Zinc Based Alloys


We are attempting to produce an "e coat" black finish onto Acuzinc castings. The inherent microporosity in the castings (which we are unable to prevent) is causing the coating to bubble when gas is released.The e-coat process is baked at 200 deg C .Is there a way of eliminating the bubble or is there an alternative solution?

Ian Clarke
- Sheffield, England


From experience with a similar type problem , try....

if outgas is from base material.....

prebake parts 5 degrees hotter x .5 hrs longer than coating requires prior to processing

if outgas is from microporosity absorbing? coating....

try a real slow heat up , ramped if you will....

also....allow parts to ambient cure for a longer duration before bake.

I ran across something like this years back , can't place it , but experimentation along these lines got us through it.

Ron Landrette
plating equipment supplier - Bristol, Connecticut


Hi, Ian,

based my experience, you should select an e-coat with comparative low curing temperature in avoiding the gas releasing. Furthermore, the pretreatment like pre-baking, emulsifier de-water treatment, etc. can be your best choice before your electrophoretic deposition.

Thanks and best regards,

Hunter Y. H. Yuan
- Zhongshan, GD, China


My company has an E-Cote process that normally bakes at 320 F (160 C). It can be dyed nearly any color, and we have black dye. One may use a lower curing temperature but the finish is not quite as hard. We use 100 C for silver so it does not discolor while baking.

Terry Collins
Gold Touch Inc.
supporting advertiser 
Cleveland, Ohio

gold touch

October 12, 2010

The most common reason for this type of defect is surface discontinuities on the zinc substrate. Most commonly due to surface cold shuts. Cleaning fluids can get trapped in these surface defects and remain trapped until the curing operation, during which the gasses escape and leave a blistering like defect on the coating surface. As mentioned before, a "pre-bake" operation will help cure this because it gasses out the trapped fluids before the coating occurs. The other solution is to eliminate the defect - which can be done by improving die temperature and/or cavity fill time in the die casting process.

If the blisters are large mounds from the substrate, it is most likely gas porosity that is expanding in volume during the curing operation. The only way to fix this problem is by eliminating the porosity in the process or lower the curing temp/duration....

Ryan Winter
- maybrook, New York USA

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