Hardcoat Anodize Thickness Problem

Q. I have a question about hard anodizing. During the anodizing cycle, how can we check to see if the coating thickness is reaching the thickness requirement stated on the drawing? The thickness requirement is 0.0020 – 0.0025 inches. It is not easy to match this thickness when using the current density method and controlling the cycle time. Should we power off and pick the part out of the tank to measure the thickness, and if the desired thickness has not been reached put the parts back in the tank to anodize longer? We are anodizing 6061 T651 alloy. Bath conditions are 220 – 240 g/L sulfuric acid; temperature: -3 – -5°C; dissolved aluminum of 2 – 30 g/L; current density is 2.5 asd. When we process the parts using the above conditions to get the desired thickness, the anodizing time is 80 min (15 min ramp-up plus 65 min at current density). C.H.

A. I don’t understand why you are having trouble getting the required coating thickness when you say you are running by current density. If you know the total surface area of the load and the required coating thickness, then you can use the “Rule of 720” to calculate the length of time it will take under “standard” conditions to achieve the desired coating thickness.

The Rule of 720 works quite well, but the time to achieve the desired coating thickness can be affected by the alloy. It is very straightforward for 6063, 5005 and 5052 alloys. If you are anodizing 6061, it may take a few minutes longer depending on the current density used. Series 2000 alloys can take up to 50 percent longer in some cases. You say the current density you are using is 2.5 asd (23 asf). This is lower than optimum for 6061 alloy. Perhaps that is why you are taking 80 min to anodize to the proper thickness. If you use 3.9 asd (36 asf) you should be able to get .002-inch (2 mils) coating in about 45 – 50 min, including a 5-min ramp period. Using this current density might burn the parts if they are thin gauge and have sharp edges. In that case you will want to find the highest current density at which you can anodize without burning. Set up the anodizing conditions on the first load, no matter what the alloy is. Adjust the conditions, if necessary, to run the remaining loads in order to get the desired thickness. It is very important to maintain tight control of the bath conditions and anodizing parameters. I do not have an issue with your bath conditions, but I recommend using 15 g/L as the maximum dissolved aluminum concentration instead of 30 g/L.

It is not a good practice to stop the load part way through the anodizing cycle. This results in a separate layer of anodic coating each time you stop and start again. This may not provide the coating integrity that is achieved by having only one layer of aluminum oxide.

Here are the Rule of 720 and the Rule of 312 (for metric) in case you do not have that information:

Rule of 720: Minutes to anodize = mils (of coating desired) × 720
                                            Amps per square foot

Rule of 312: Minutes to anodize = microns (of coating desired) × 3.12
                                            Amps per square decimeter

Note: 25 microns = 0.001” (1 mil)

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.