## Question:

We’re new at anodizing and we would like you to explain how to calculate the time it takes to get a given coating thickness when you have more or less “standard” anodizing conditions. Is the same method used for working with both Type II (standard sulfuric acid anodizing) and Type III (hardcoat anodizing)? D.M.

## Answer:

This is such a fundamental of anodizing, and yet many folks don’t really understand how it works. And, very few people actually ask the question. A lot can be said about this subject, but I will try to keep it short and to the point.

There is a very simple calculation that can be done to tell the anodizer how long to anodize to achieve a certain coating thickness under a range of more or less standard anodizing conditions.

The English version of this formula is called “The Rule of 720”. The metric version I call “The Rule of 312”. Here are the formulas:

Rule of 720:

Minutes to anodize = (mils of coating desired × 720) / Amps per ft^{2}

Rule of 312:

Minutes to anodize = (microns of coating desired × 3.12) / Amps per dm^{2 }

These formulas apply to both Type II and Type III anodizing and are good for a fairly wide range of operating conditions. This means that in most cases, the formulas apply for bath conditions of:

12–20% by weight sulfuric acid; 40–80°F (4–27oC); and dissolved aluminum of 2–20 g/L.

This covers most of the operating conditions that you will find for these types of finishes. The alloys being anodized are also important to consider. Most aluminum alloys of the 5000, 6000 and 7000 series conform to these formulas. Alloys of the 2000 series do not necessarily conform because of their high copper content.

These formulas tell us that if we know the coating thickness required and the current density at which we want to anodize, then we can calculate the length of the anodizing cycle in minutes. Likewise, if we want to set the anodizing cycle time and we know the coating thickness required, then we can find the current density needed to get that thickness in the time allotted.

It becomes obvious that we need to run by current density, not voltage, in order to make use of these very helpful formulas. I know that there are still many anodizers who do not run by current density. They run by voltage, using an approximation of the anodizing conditions from past experience for the particular parts, or part types, that they are anodizing at the time.

Anodizing by voltage can result in longer cycle times and inaccurate coating thickness. In order to run by current density, the surface area of the parts (and the racks if they are aluminum), must be known. This can be a pain to calculate, but it’s worth it in the long run. Anodizing by current density vs voltage has been the subject of past articles in this column and several full-length articles by other authors over the years. I’ll keep preaching. I’m sure this won’t be the last article to talk about it.