[M_D]

Successful anodizing really depends on just a few basic factors, such as parts that have a clean surface and are free of oxide, a good electrolyte, and getting the right amount of current for the right time. But, the devil is in the details and it isn’t always as easy as it sounds.

We know that the electrolyte is critical, but yet there is some latitude for mix ratio, although certain contaminants are bad news.

The time and current can actually be varied quite a bit, and still have nice looking parts. A shorter anodize time will result in slower dying and/or a final lighter color. It should still be even though. Anodizing for longer periods can help speed dying and result in deeper colors, although that only goes so far and then there is no benefit to appearance.

You do need to be sure the connections are good, I would recommend doing a few test parts where the part connection is out of the acid. Try to make the connections as good as possible. If that doesn’t fix the problem then something else is the cause.

I don’t know what kind of parts you are doing, so it would be hard for me to recommend any specific cleaning cycles. What you need to end up with is a fully clean part, with as little oxide as possible. Sanding a test area would be a good step. If you find it works better, then you may need to use a more aggressive de-oxidizing step in the cleaning cycle. Oxide will cause unevenness and light depths of colors. I have found our machined parts anodize nicely without de-oxidizing at all. We can scrub them with bristle brush and dish soap until the water sheets, and that gives as good of results as anything. We don’t do them like that now because it is impractical except for limited volume.

When the same parts are tumbled to de-burr and even the surface that completely changes things. Then they need to be cleaned much more aggressively. We are using the SP cleaner, and have found when heated to 200º is cleans faster and more completely. For parts with very low mass that will heat up almost instantly, it will do a fair job of de-oxidizing. It will not satisfactorily de-oxidize larger parts, because their mass doesn’t allow them to reach the high temperature fast enough. Those parts require a lye etch or equivalent. A general rule I have found that works is this: if the parts develop a light coating of smut in the lye etch, they are probably de-oxidized well enough. If the lye is mixed too weak and it is cold (room temp), it can take forever to work. We use a strong enough solution and heat it to 140º or so, and it takes 1-3 minutes to get the smut layer that says it is de-oxidized. The lye will work faster when heated, but if it is heated too hot it can get weird, 120- 140º is more than sufficient.

The smut needs to be cleaned, Caswell’s de-smut works well for this. It also works faster when heated, but it isn’t required. When the smut from the etch process is gone, and the parts have a bright look they should anodize to a nice finish.

I am assuming you don’t handle the parts once the cleaning is started, and rinse real well between steps.

I’m curious, what color are you dying the parts? Also, what alloy is the aluminum and what kind of surface?

If you try some tests and can’t get it to come around, you might suspect the CD is out of line; maybe the meter you use is wrong. Also, if using distilled water to mix the electrolyte doesn't solve the problem, I think it is a good possibility the parts need to be cleaned better, whether just degreased and/or de-oxidized better I couldn't say with the information I have.