What is the Correct Anodizing Specification?

Q. My company fabricates aluminum and steel pedestrian bridge railing among other bridge parts. We recently got an aluminum railing job that called for “Type I” anodizing per MIL-A-8625. There was no anodic coating thickness called out. We are not anodizers and we are at a loss as to how to write up a meaningful anodizing specification for this railing. We feel it is important to call out the right finish for this railing that will be installed on a bridge over salt water. We want the anodizer to know specifically what we want so that they have some measurable standards to go by. To we who are not knowledgeable about anodized finishes, this can be very confusing and frustrating. We feel it is important to produce a high quality product and the anodizing is an important part of this product. Perhaps you could give me an overview of the basics, what the different finishes are and how they should be specified. P.D.

A. You say your customer called out “8625 Type I.” It sounds like the customer is confused and most likely is not at all familiar with anodized finishes. I come across poor or incorrect finishing specifications on a regular basis. This sort of thing usually starts with a well-intentioned designer or engineer who is trying their best to interpret one of the anodizing specs and ends up calling out something entirely different than what is required for the application. I’ll do my best to give you an overview and make it as concise and explicit as possible.

MIL-A-8625, Type I is anodized in chromic acid. It is a VERY thin coating that is used mainly in the aerospace industry as a corrosion resistant base for paint. Many aircraft parts are then painted with a zinc chromate primer. Unless your customer also wants the railing painted, this is certainly the wrong finish to call out for bridge railing.

MIL-A-8625, Type II is what many might refer to as “regular anodizing”. Call it “clear anodizing,” if you will. The anodizing is performed in a bath of diluted sulfuric acid. Type II can also overlap, sometimes, the so-called “architectural” anodized finishes. (More about that below.) For a MIL-A-8625, Type II coating you would also call out what coating thickness you want.

MIL-A-8625, Type III anodic coatings are also referred to as ‘hardcoat” anodized finishes. I think you said that the anodizer you spoke with offered this as an alternative to Type I. Generally speaking, Type III anodizing is produced using higher current densities and higher voltage than Type II coatings. The Type III coatings can be harder than the Type II coatings and generally are. According to 8625, if no coating thickness is called out for Type III coatings it is assumed to be 2.0 mils. A 2.0 mil anodic finish in the case of this project would most likely not be cost effective. That’s the only reason why I wouldn’t specify it. Otherwise it’s an excellent coating. The specification makes it incumbent upon the customer (this would be you, or your customer in this case) to call out the coating thickness wanted. Often times determining the coating thickness is a collaboration between the customer and the anodizer. In the case of your railing, you could call out Type III with some minimum thickness such as 0.8 mils or 1.0 mils.

In my experience, aluminum bridge railing also falls within the purview of “architectural” and the anodic coatings for this product could easily be called out under the commonly used spec for architectural work: AAMA 611-98, Voluntary Specification for Anodized Architectural Aluminum
There are two types of architectural finishes: Class I and Class II. Class I is called a “high performance anodic finish used in exterior applications receiving periodic maintenance such as curtain wall. Minimum coating thickness is 0.7 mils.” Class II is referred to as a “commercial anodic coating...” primarily used on store front and other “less-than-monumental” (my quotes) commercial applications. The minimum coating thickness is 0.4 mil for Class II. The AAMA (American Architectural Manufacturers Association) spec also calls out various quality tests in Section 9.0. Any of these tests may be specified by the customer to be performed by the anodizer or by an outside testing lab. These include some important performance evaluation tests such as anodic coating thickness, coating weight and quality of seal. These tests are spelled out under various ASTM standard methods.

The Class I and Class II architectural finishes are not to be confused with the Class 1 and Class 2 finishes in MIL-A-8625. Under the 8625 specification, Class 1 simply means “non-dyed” and Class 2 means the anodic coating is dyed (colored) after the anodizing step and before sealing. Coloring can be done with organic or inorganic dyes.

My recommendation is that you call out MIL-A- 8625, Type II (since your customer is already referring to that spec) and you specify the coating thickness as well as the quality evaluation testing that should be done. I recommend a minimum anodic coating thickness of 0.8 mil (20µ) and that you ask the anodizer to give you certs that indicate that they have measured the coating thickness of every load (average of at least five readings on every load) to meet your minimum requirement. Otherwise you could get coating thicknesses that are inconsistent. Remember that 8625 states that the coating thickness should be called out by the customer. Note that the Type II anodize with minimum 0.8 mil thickness would also fall under the AAMA 611-98 spec as a Class 1 coating with the minimum thickness called out at 0.8 mils instead of 0.7 mils.

For the salt water application, I prefer a minimum 1.0 mil (25µ) coating, but that would certainly cost more. That would be your decision.

On anodic coatings such as this and for this application, the quality of seal is just as important as the coating thickness, so you could also call out the ASTM test for quality of seal. In this case the anodizer would test seal quality on a few coupons of the same alloy as the railings and furnish the certified test results to you as well. That specification is ASTM B 680-80, “Standard Method for Seal Quality of Anodic Coatings on Aluminum by Acid Dissolution.” It’s referred to as “The Acid Dissolution Test.” The anodizer may charge extra for doing a few acid dissolution tests, but it wouldn’t be that much. Or they might choose to have an outside lab do the testing. By the way, here are the other common test method standards for quality evaluation that I mentioned: ASTM B 137 for evaluation of coating weight, ASTM B 244 or ASTM B 487 for measurement of coating thickness.

I think you would be best served by calling out MIL-A-8625, Type II, Class 1 (non-dyed) with a minimum thickness of 0.8 mils. You would get the same exact coating if you called for AAMA 611-98, Class I with minimum thickness of 0.8 mils. The two different specs would be overlapping in this case. You can decide about asking for testing and certs.

You mentioned that there are also aluminum castings as part of this job. I would need to know what the cast alloy is in order to advise you on that.