We are anodizing some parts made of 7075 alloy. I have read your article regarding the issue and wonder if anything new has cropped up since that was written. We seem to have this problem off and on with parts that happen to be polished and hard anodized to 0.0008-inch thick and then are dyed black. We do not see the blistering problem until we remove the parts from the dye tank at which time we see spots that appear as raised material that breaks away when the material is probed. We typically process these parts at 40°F with an additive. The sulfuric concentration is 165 g/liter The second question has to do with the texture of the anodic coating. It appears to be a bit rough. It seems like the anodize is increasing the surface roughness more than I would normally expect. A.F.
I have anodized plenty of 7075 and had never seen the blistering condition until someone sent me a sample of it a few years ago. If the problem is metallurgical there is nothing that you can do in the anodizing process to prevent it. You could try anodizing at a slightly higher temperature, say, 50°F or even 60°F. I don’t think the hardness will suffer appreciably as long as you anodize at least 36 asf. High current density and cold temperatures can cause more roughening of the surface. There are a couple of other possible conditions that may be present with your parts, however.
You said the parts are polished. Sometimes when parts are polished some of the surface metal can actually be “folded over” forming small “pockets” that can open up in the finishing process. It doesn’t sound like this applies to your situation. Another possibility is that the parts are undergoing electrolysis or electrolytic corrosion in the black dye tank. This can be caused by too much titanium rack surface relative to the aluminum parts surface on the rack. A battery effect can be set up between the dissimilar metals and cause current to flow. Make sure the rack is electrically isolated from the tank if the tank is made of metal. This same condition can be caused by a contaminated dye bath.
A.F. writes back:
Interesting that you would mention the higher temperature anodizing bath. I ran a part at 48°F and the part came out much smoother and did not show the blistering problem after anodize. When I dyed the part, however, the problem did surface. I think both issues were taking place. We have had trouble with this hard anodizing bath since the additive was changed. I think the new additive just needs to run at a higher temperature on these alloys in order to avoid the problem mentioned. Our chemical supplier has also mentioned that the use of a magnesium anode in the dye tank could help prevent electrolytic corrosion on the parts. From what I have seen the higher temperature has done a world of good. Now I’m trying another dye color to verify that it isn’t the black dye. The black tank is our largest and oldest dye tank. I will say that this material (7075) is extremely sensitive to anodizing tank temperature and it gets very rough when processed at the lower temperatures allowed by this spec.
I once had a similar problem with the anodic coating popping off when anodized in a cold hard coat tank with an additive. I’m glad the higher temperature helped. By the way, the original Alcoa (Martin) hardcoat spec calls for anodizing at 50°F at 36 asf in a bath of 12% sulfuric and 1% oxalic acid. (The oxalic is the “additive” in that process). Even straight 15% sulfuric works well at these conditions.
As far as the mag anode in the dye tank goes, it does work most of the time, but it only “covers up” the real problem. The real problem is either excessive bath contamination and/or stray current electrolysis.