Effect on dimensions of anodizing/hardcoating and stripping & re-anodizing

A. General rule of thumb, 50% in and 50% out. Although, I don't believe this is an exact science. I'm sure James will correct me if I'm wrong (I meant that as a compliment to your knowledge, James). Your diameter should be .2508, if its an outside diameter.

A. Todd,

Alloy type does make a difference in the buildup to penetration ratio. Type III Sulfuric anodize (hardcoat) --.8 mils typically is 50% penetration and 50% buildup per surface which means that your dia. would be .2508. Type II sulfuric anodize the ratio varies more, i.e. 6061 will give you approx. 80% penetration and 20% buildup or .2532^^.2503 diameter.

Aluminum 2000, 5000, 7000 series all vary as to the ratio. One method I use to determine what % is buildup, I measure the parts before and after, using my thickness gage to I measure the amount of anodize I applied, take the before and after size divide it by two and subtract it from my thickness per side and that tells me what my penetration and buildup is, this method may seem obvious but when I have to hold a tight tolerance, I don't assume and it's helpful on repeat orders. In short there are a lot of variables that can effect buildup and penetration, including the different alloys.

I'm sure that if I too am not correct, my friend James, will set me straight. Hope this helps.

A. I really do not know. I always assumed that I got less effective build up out of alloys with higher amounts of acid soluble metals like copper and magnesium. I also assumed that the ratio changed slightly. Never investigated actual numbers as most customers wanted a given final dimension. I agree with Lamar that keeping good records is the best way to have it come out right the next time.

A. One more thing that may need to be addressed in this discussion is the amount of material one removes during the etch process prior to anodizing. Some anodizers may remove as much as .001" prior to anodizing.

A. Hi, Muhammad.

It is possible to strip hardcoat anodizing in chromic-phosphoric acid without attacking the substrate--see letters 2776 and 3397. But that does not mean there will be no dimension change. Rather, even if you don't consume any aluminum in etching, when one anodizes .002" thick, roughly .001" of the aluminum substrate was consumed in building the anodized film (per the discussion above); so when you strip the part it will be .001" smaller than it was before you anodized it.

Good luck.

A. Your frame can be chromate conversion coated for corrosion resistance without affecting dimensions, but that's not really as good as anodizing in some ways. You should probably allow for the growth in anodizing, which is about 50 percent of the specified thickness of the anodizing, because the other half is "penetration". Typical sulfuric acid anodizing is probably about 0.0002"-0.0005" thick (even slightly thicker if you want to color it black because the more saturated the color, the more dye you need to absorb into the anodizing pores, so the thicker you need the anodize layer to hold it).

Anodizing can have a large effect on fatigue strength, which is also a good topic for engineering students to study.

A. No, it's not possible, Kenneth. The anodizing is not a finish that is added on top of aluminum like paint or electroplating. Rather, it is a conversion process whereby aluminum from the item combines with oxygen to form aluminum oxide. If you strip that aluminum oxide, even with zero attack on bare aluminum, the part will be smaller than when you started because some of the aluminum was consumed in generating the aluminum oxide.

For rough purposes, half of the anodizing thickness is consumed aluminum. So if you have a plate that is 1" thick and you want to anodize it on both sides to .002" anodizing thickness, it will be 1.002" thick after you anodize it and .998" thick if you strip the anodizing with no material loss. Good luck.

A. Hi Edward. In the real world it probably won't work; in the theoretical world it is not impossible.

Say the initial thickness requirement was for .001" of anodizing on a 1" radius (2" diameter) cylinder. In rough numbers, .0005" of aluminum would be consumed in the building of that film thickness, so the radius after anodizing the first time would be 1" minus .0005" plus .001", equals 1.005". If you strip this with a chromic phosphoric acid mix so you lose lose no raw aluminum, it will be .9995" radius after stripping. If you now apply .002" of anodizing on the second try, you'll consume .001" of aluminum in the process. So the radius will be .9995 minus .001" plus .002", equals 1.005". Theoretically possible :-)

A. Hi, Steve. In practice it's almost always true, due to dimensional considerations. Hard anodizing is considered to be about .002" thick. In building a .002" hard anodizing layer, you will have consumed about .001" of aluminum from the substrate. When you strip the old anodizing, even if you lose no raw aluminum, the substrate will thus be .001" inches smaller than it started. You would have to anodize .004" thick to make up the difference and that thickness almost surely won't be practical.

Regards,

A. Hi Leo. As you see, we've appended your inquiry to a thread which discusses the effect of anodizing (and stripping and re-anodizing) on the dimensions of parts. It does sound like these parts may have been stripped a few times by the anodizer, upsetting the dimensions. We probably don't have enough info to solve the problem, but we do have enough to help you get your arms around it. And here are some other things to consider:

- 2024 is one of the most difficult alloys to successfully anodize because only the aluminum content can be anodized, whereas 2024 has a lot of other garbage in it, including over 4% copper. The job would be easier in an alloy like 6061 if that suited your needs.

- Anodizing IDs can be very difficult, especially when they are deep. Holes of 0.257" dia. in pins of 2" long, comprise very deep . . . and I doubt that such parts can be successfully anodized in run-of-the-mill fashion; rather, they probably require custom jigging with solution propelled through the holes, and perhaps with a center cathode.

Again, I don't have the facts, but if I were asked to guess the most probable case, the anodizer priced the job for routine processing, tried that a few times and failed a few times.

Regards,

Thank you. Everything you say makes sense. And here I thought I was helping the jobshop by specifying an easier to machine aluminum. I've asked that he re-run the job with 6061. I really don't care if the inside of the .257 hole gets plated. I was concerned about the outside.

A. You should be fine with that if the anodizing is done as specified.

Q. Jeffrey
Thanks for the response. Is controlling the thickness of hardcoat on threads any more difficult if you are doing it to a large amount of identical parts at one time as apposed to just one part.

Geoffrey

A. Threads should NEVER be anodized. On ANY sharp edge the coating penetrates half in half out, that leaves solid aluminum oxide on the edge, which readily breaks off when you screw in the opposing thread. Threads should be masked.

A. Hi Dominic,
Since it is a color anodized part, it would have been 'clear anodized' prior to the dye stage, probably to about 15-20 microns thick (I am just estimating here ... you have to check it with a coating meter).
Best way to go about it would be, to measure all the dimensions as they are now, and then measure them again after stripping the anodized coating. The difference will give you an idea as to how much you have to coat back - remember, anodizing builds up half penetration and half growth.
Yes, you can hardcoat anodize back to your required size, easily even up to 80 - 100 microns on a 6061 alloy.
Threaded holes may have to be retapped, as strict control is not possible.