Electropolishing of Aluminum vs Bright-dip

A. Hi Rob. We moved your posting from where you had put it to this thread because of the great answer below from Harry Parkes where he explains why so few shops do electropolishing even when it can be fine for the purpose. You might also wish to see topic 1243 for 2nd and 3rd opinions and discussion of appropriate alloys, and consult ASM Metals Handbook Vol.5, which has great coverage on the interrelationship issues related to alloy composition, bright dipping, electropolishing, and anodizing.

Luck & Regards,

A. Hi,
I haven't been on this site for a couple of weeks and so didn't see your question until now. Are you still monitoring for replies?

 Ed. note: Welcome back Harry! We'll remind Paul that he has a reply, but please note that in addition to this forum running interactively, we're also trying to make it a permanent information resource. Hundreds of people read the postings for each one who posts, so anything you offer will be very helpful to many readers in any event :-)

A. Hi,

Comparing good chemical brightening with good electropolishing, electropolishing generally gives a brighter, smoother finish. However on alloys of less than 99.8% purity the loss of brightness with even modest anodic films of 3 µm, the benefit is slight when compared to the loss in brightness due to the anodic film. Even on 99.8-99.85 based alloys (car trim/cosmetic containers) the gain is relatively small. The exception here is when the alloy does not contain the 0.03-0.08% copper added to enhance chemical brightening (on phosphoric acid based baths). The main benefit is on super-pure aluminium (99.99) especially lighting reflectors where maximum reflectivity is required and brightness loss on anodising is small.

The choice of which to use is not simply down to current versus no current. The cost of electropolishing is considerably higher than chemical. This is partly due to higher chemical cost and electrical power. But, of great importance is the amount of work that can be processed and racking requirements.

Dealing with racking first. With chemical brightening the cross sectional area of racks and number of contact points is largely dependent on the anodising current (~ 12 asf).
The current density for electropolishing is 5 to 10 times greater. As heating is proportional to the square of the current the impact on burning at contact points and rack size is significant. The use of aluminium racks rather than titanium help here.

A very important feature of anodising is that the throwing power is exceptionally good. Parts can be tightly packed in a way that would be disastrous if used in electroplating. Current distribution in electropolishing is much closer to that of electroplating. Consequently parts have to be much more widely spaced: this means that the throughput of work is considerably lower than that of a plant of comparable size that is using chemical brightening.

All the electrical power use in electropolishing is dissipated as heat and cooling is needed on all but the most lightly used of plants.

One advantage of electropolishing is that it does not use nitric acid and so the emissions of nitric acid vapor and oxides of nitrogen are avoided. If you operate in a restricted zone consider using the services of a specialist anodiser rather than install uncompetitive plant.

A. Hi,
Ted has attached your email to a reply I gave in 2009. Dealing specifically with extrusions, I suggest that you consider using 6463 alloy which is designed for maximum brightness.
Harry

Dear Mr. Harry,
Thanks a lot for the reply. I will go for 6463 alloy.

Once again thanks for your reply and also a great thanks to Mr. Ted and finishing.com

Regards

A. Hi Nigel,

It's been a long time since I measured specular gloss and can't quote a value. Your choice of alloy looks good. This is the procedure I would try:

Mechanically polish the significant surface/s. The aim is to produce a compact, homogeneous layer of aluminium.

Chemically or electrolytically polish the aluminium to remove fine scratches and produce a bright, clean surface.

Anodize for the minimum time needed to get the required corrosion resistance: the anodizing stage will lower the total and specular reflectivity.

Harry