Anodizing Fundamentals

Question:

I have just started my enterprise and couldn’t find any information on anodizing using a high start-up amperage versus a more slow and even increase of amperage within the first seconds of having the load in the tank. I am working with a voltage of 15 V right now and cannot imagine that the same quality of coating will develop when having the full amperage within the first few seconds before the resistance builds up compared to a slow increase in order to give the material time to build up resistance. H.R.

Answer:

You do not say what type of anodizing you are intending to do, so I will assume you are doing Type II.  You are essentially correct in your assessment of the conditions required upon start-up of a load of material to be anodized. On the other hand, it is usually not as critical as you might think. It is important to permit a relatively slow and even increase in voltage during the starting period. This is also called the ramping period. It is during this phase that the very thin barrier layer is formed and the initial growth of the porous anodic film starts to develop. Generally, the ramping period only needs to be from 30 sec–5 min in length in order for a uniform anodic layer to form over all surfaces of the parts. The uniformity of the initial build-up determines the degree of uniformity of the coating over the finished part.

The “slope” of the ramp is determined by the type of finish to be produced, and also by the size, shape and alloy of the parts being anodized. There is no rigid set of rules governing the ramping period. I can give you some examples, though:

? Long, thin-walled extrusions need a relatively slow ramp period so that the current flows uniformly through the part from end to end. If high current is applied in only a few seconds (essentially no ramp) the current may tend to concentrate at the ends of the parts at first and you could end up with more coating near the ends than in the middle. This would be of special concern if the parts were to be colored either by organic dying or by electrolytic deposition after anodizing.

• In contrast, a “short, heavy” part may not be so sensitive to uniform current distribution because it is a better conductor of electric current and, therefore, the ramp time could be relatively shorter.
• Many small, thin sheet metal parts on a load may require more ramp time, especially if they are to be colored.
• Loads of densely racked extrusions would require more ramp time than a more loosely racked, or “open” load of parts.
• When I’m working with hardcoaters in their anodizing plants we often use a standard 5-min ramp for hard, thick coatings. It helps ensure a more uniform final coating.
• High-copper alloys may require a more gentle ramping period.

I hope you get the idea. It’s largely a matter of common sense and experience.
Generally speaking, Type II is done in a bath of 15–20% by weight (not by volume) sulfuric acid and temperature of 68–80°F. The goal is usually to build an anodic coating thickness of anywhere from 0.0001 inch (1/10 mil, or 2.5 µm) up to perhaps 0.001 inch (one mil, or 25 µm).

In order to do this, it is best to anodize by current density, not by voltage. Voltage can be used as a gauge, but current density is preferred in order to reach a specific coating thickness in a given amount of time. 

Type II anodizing is usually run at a current density of 10–24 asf, or in metric terms 1.0–2.6 A/dm2. It is pretty obvious, then, that if the total part surface area is, say, 10 ft2 (0.9 m2 or 90 dm2 ), and you want to anodize at 12 asf (1.3 A/dm2), the total load will run at about 120 A. When parts are anodized at this current density under the bath conditions stated above, you will build very close to 1.0 mil (25 µm) of coating thickness in 60 min.

The “ramp time” for a load like this should, in most cases, be no more than 30 sec–1 min, but it could be longer without affecting the overall run of the load.  During the ramp time the amperage is raised as smoothly as possible to (in this case) 120 A and left to run for as long as you wish to build the coating required, but usually no more than 60 min. 

By and large, these relationships are linear, so if you want to build 1 mil (25 µm) in 30 min you would run the current density at 24 asf (2.6 A/dm2). In other words, twice the current density. for half the time, etc. 

You will also need to control the amount of dissolved aluminum in the bath at 5–15 g/L. Use the “Rule of 720”, or “Rule of 312”, given on the previous page, to calculate the anodizing time needed. 

That is the bare minimum you need to get started. But there is so much more.