Producing a Thick Anodic Coating on 6061-T6

Q. Our customer has requested .004-inch (100µ, 4 mil) coating thickness on 6061-T6. We normally anodize this part to .002-inch thickness. Our process is 28 V, bath temperature 32°F, for 1 hr. We are not sure of exactly what the best procedure is to hardcoat anodize to this coating thickness. If I raise the voltage, is there a better chance of achieving .004-inch thickness? I do not want to ruin the part. D.K.

A. Type III (hardcoat) anodizing is much more successful if it is processed by current density instead of voltage. If you set a particular voltage, the current (read current density) will almost immediately start to drop off as the voltage is steady and the resistance increases due to anodic coating buildup. Getting 4 mils under these circumstances would be difficult because the current would most likely trail off to zero, or near zero, and then it becomes a matter of never having enough amperage to get where you want to go. Here is what I suggest:

Determine the total surface area that gets anodized on each load or rack. If you don’t have that information, you will have to measure the part and calculate the area as closely as possible. Plus or minus 10% is acceptable. You could also ask the customer for that information. Usually, if there is a CAD drawing of the part, the computer can tell you what the area of the part is.

Once you know the area of the total load, including the rack if it is aluminum, determine what current density (cd) you want to use.

“Normal” cd for 6061 alloy in a hardcoat bath is usually 36 asf (amps per sq ft).
At this cd and a bath concentration of 165–220 g/L you will be able to get 2.5–3.0 mils in 60 min. Make sure the dissolved aluminum concentration in the bath is between 5 g/L and 15 g/L
Depending on the size and shape of the part and how it’s racked, you may be able to run at a higher cd than 36 asf. This depends on how good your cooling/temperature control is. Plus or minus 2°F from the control set point is the standard for anodizing. I use a bath temperature of 45o+ 2oF.

You must have good, solid contact between the parts and the rack in order to run at cd of 36 asf or higher.

Before turning the rectifier on, put the load in the tank and turn the voltage knob all the way to the right. This puts the rectifier in amperage control (mode).

Now you can put the load in the tank and turn on the rectifier (make sure the amperage knob is turned all the way to the LEFT before turning the rectifier on).

Using the amperage knob, use a 5-min ramp period to turn the amperage to the desired level. If the rectifier has an automatic ramping feature, use that to do the ramp-up.

If you have, for example, 10 ft2 on the load (parts + aluminum rack) and you run at 36 asf, the amperage will be 360. If you have 12 ft2 on the load and you run at 36 asf, the amperage will be 432. You get the picture.
For 4 mils using 36 asf, it should take about 75–90 min to get the coating.

Note that amperage will stay the same throughout the duration of the run and voltage will continue to climb higher as the anodic coating thickness builds.

Shooting for 4 mils, you may exceed the voltage limit of your rectifier. My guess is that it might take something close to 75 volts at the peak to maintain steady current throughout the entire run. If the voltage reaches the limit of the rectifier before the end of the run, you might as well stop the load and tell the customer that you can’t reach 4 mils with your rectifier setup.

If you can run the load at 40 asf you should be able to reach 4 mils in about 72–75 min.

If you can run at 48 asf you should get close to 4.0 mils in about 60 min.

Use the Rule of 720 to calculate the time of the run when the desired coating thickness is known.

Here is the Rule of 720 and its metric equivalent, 312:

Rule of 720:
Min to anodize = mils (of coating desired) × 720 / Amps/ft²
Rule of 312:
Min to anodize = microns (of coating desired) × 3.12 / Amps/ dm²

Of course, these equations can be manipulated to give the coating thickness if you know how long you want to anodize, or the current density required to achieve a certain coating thickness in a given amount of time.