What is the purpose of controlling the concentration of aluminum in the anodizing bath? I’ve heard several differing ranges of concentrations as being the optimum. What is your opinion about this? L.B.
This is a question basic to aluminum anodizing and, oddly enough, it’s not asked very often. Maybe it never occurs to most people or perhaps it seems like a “dumb” question. It certainly is not a “dumb” question, in fact, it’s a very important one to ask and have some knowledge about.
The best range of dissolved aluminum in the anodizing electrolyte actually depends on the sulfuric acid concentration of the bath. The lower the acid concentration, the higher the concentration of aluminum can be. European standards, I believe, range from 25 g/liter for a 7% (wgt) acid bath to around 12 g/liter aluminum for a 400 g/liter (~35% by wgt) acid bath. In the United States, the generally accepted, and used, range for both Type II and Type III anodizing is 5 g/liter to 15 g/liter aluminum. I have seen normal operating ranges from a very low 2–5 g/liter up to as high as 20 g/l dissolved aluminum in the bath. Generally speaking, a range of 5–15 g/liter is good for nearly every application. And 8–12 g/liter is super, if you want to control it that closely. The concentration of aluminum can be controlled by either decanting the bath periodically, and the “spent” acid can be used in the waste treatment plant for pH adjustment, or by the use of ion exchange of the anodizing electrolyte. I will save the details of the mechanics of this for another time.
How does the dissolved aluminum affect the bath? I won’t go into great detail here, but there are some simple points to remember about this. One, higher levels of dissolved aluminum lower the conductivity of the bath. In the range of 5–20 g/liter, the conductivity change is virtually unnoticeable. I have seen sulfuric acid electrolytes with a dissolved aluminum content of upwards of 80 g/liter where the conductivity of the bath is such that the anodizer could only get half the output of the rectifier at full capacity. At levels above roughly 25–30 g/liter, the aluminum sulfate wants to precipitate—precipitate on the work, on the tank at solution level, on the bus bar, on the ventilation hoods and in pumps and heat exchangers in the cooling system. It can also affect the dying of the parts adversely.
The presence of some dissolved aluminum acts to buffer the electrolyte against burning. That’s not to say that the presence of dissolved aluminum prevents burning. There are certainly lots of conditions that can cause burning of parts but no dissolved aluminum in the bath can lead to burning more easily than the presence, say, of a minimum level of two g/liter. That’s why it is a good idea not to start up a new bath with less than about two g/liter dissolved aluminum.