Hardcoating Mixed Tempers

Question:

We are having a problem with a difference in coating buildup on mixed heats of the identical parts in our hardcoat anodizing process. We run many small parts for a customer, which should be of 6061-T6 and could have been mixed with 6061-T6511. Identical parts were measured for coating thickness to find that there is only 0.0002-inch total difference in size (done with an Etalon 0.00005-inch indicating micrometer). After hardcoating the pieces to 0.002-inch thick coating, we found not only a difference in looks (light and dark coloration), but found the darker color was up to 0.0005-inch smaller in size than the lighter colored parts. Could these be the T6511 with a slower coating build-up rate? These different colored parts were scattered on the same rack and had good firm contact points. I can’t seem to find any substantial documentation (other than they have the same chemical analysis) to give the customer to back up my theory and would be glad to hear any comments on this. B.R.

Answer:

Your theory is correct. Even slight differences in heat treating can make a difference in the rate of coating buildup and in the final coating thickness on a part. It is difficult to predict which parts will have more coating or less coating (i.e., T-6 vs. T-6511). You can test that yourself by anodizing, in the same batch, two parts of known, but different, temper. Note the resulting color and anodic coating thicknesses.

In so-called architectural and commercial anodizing processes where parts often get a heavy caustic etch, the differences in temper can be seen when the parts come out of the caustic tank. The etching residue, or smut, on the parts will be different colors (usually shades of gray, brown or black) for differences in temper. Temper differences are exhibited as different alloying elements “in solution” that precipitate near the part surface during heat treat. Sometimes these differences in temper are large enough to influence the way the parts respond to the anodizing process and can result in differences in coating thickness, color and rate of coating buildup.

It is usually assumed that the darker part will have greater coating thickness. An example of this would be when two parts of identical heat treat characteristics come out of the anodizing process as different colored parts. The reason for this may be that one part had a loose rack contact and did not get as much current as the other part. Or one of the parts may be shielded on the rack by other parts causing a difference in coating thickness. However, when the darker parts are found to have either the same coating, or less coating, it is a good bet that the parts are of differing temper.

Of course, parts of differing temper but of the same alloy will analyze chemically the same. It is best to keep track of each batch of parts by both alloy and temper and don’t ever mix parts if they are different in either alloy ortemper. This, obviously, requires careful control throughout the production cycle.