Alodine and Anodize Defined

by Herman Dierks and Gil Alexander

Fri, 13 Jan 1995

I am building my rudder now and I am quite amazed about all of the messages about priming. I am confused about the anodizing process. Why do you do it and how is it done?. What materials do you use?

The topic here is preventing corrosion. See the June 92 issue of RVator for a brief discussion of the different types of corrosion.

First, there are two different processes and you don't want to get confused. They are alodine and anodizing. (Note, see the June 92 issue of The RVator for a discussion of the different types of corrosion).

ALODINE. This is the process that you will use most with painting so lets discuss first. This is the process where you first acid etch the AL and then apply the Alodine conversion coating. Alodine is a strong oxidizer and does a chemical conversion of the surface (oxidizes it) and is the best way to prep it for painting. The paint is suppose to bond to the alodine'd surface better. For interior parts it probably would not have to be done but most of us do it as it is not much more work if you are going to etch the parts. I would alodine the non-alcad parts for sure just to be safe. This process just requires you buy the quart(s) or gal of Alodine from any of the AC supply houses. DuPont calls their process 225S cleaner and 226S conversion coating, but it is the same thing. You make sure it is rinsed off and you do not let it dry on the surface as it will leave a film. You just want it to convert the surface and rinse it off and wipe dry.

ANODIZE. This is an electro-chemical process that typically requires extensive equipment to do it right. If you buy the Phlogiston prebuilt spars, they come anodized and are the nice gold color. This is a very good corrosion prevention process but it also hardens the surface of the material. Van wrote an article in the Dec 1991 issue of "The RVator" that addressed the issue of Anodized spars and the affect on fatigue strength. The issue is what does anodizing do to the service life of the spars? His conclusion was that anodizing offers more than double the corrosion resistance of priming. However, it also reduces the fatigue life by up to 50%. The normal fatigue life was computed to be 24,000 hours on regular aerobatic use so the anodized life would be 12,000 hours. That would be 80 yrs at 150 hrs/yr. Also, these are conservative numbers. If you get corrosion in a spar it also lowers the service life (and maybe yours also) so anodizing them is a good tradeoff.

If you are building a RV, I don't think you need to worry about anodizing, esp. if you buy the pre-built wing spars as it is already done. If you build your own spars, then you may want to do this if you have access to a commercial shop that can do this properly. Because this involves the use of strong acids, it needs to be done properly. See prior messages on this over the last few weeks. Doing the simpler alodine and epoxy primer at home can give superior results at minimum cost and avoide the fatigue strength issue. See note below on epoxy primers.

Due to the cost of this anodize process (for small lot sizes) and due to the surface hardening, it is not something you need to do for the other parts of your RV. We have 50 yr old Cessnas flying around that are not even primed inside so I don't think we need to get too carried away here. If you live along a salt water coast, then the alodine and priming discussed above should cover you.

My origional note had some words on expoxy primers being porus and maybe not protecting as well as I thought they would and that zinc chromate may still be a fast but effective way to prime the interior. Gil Alexander responded with the following excellent discussion of epoxy primers and I am including it here as it is a logical place for it. I think the message is that not all primers are created equal and you need to read the label (and know what to look for).

Gil Says:

This is a good reason to use a MIL Spec 23377 type epoxy primer. The spec calls for a minimum of 52% of it's pigment to be Strontium Chromate - the approved replacement for Zinc Chromate. These primers are usually a yellow/green 'sort-of-ugly' color (the spec calls the raw pigment "deep yellow").

Aluminum corrosion resistance is required to be better than 1000 hours in a 5% solution salt spray test. Water resistance is required to be 4 days immersion im 120 degree water (try this with your spray can paint!!), as well as 24 hours in 250 degree lubricating oil!! These primers get much harder than zinc chromate, and are also resistant to MEK solutions. The "speciality" epoxy primers in previous postings are commercial (Boeing, MacDoug) equivalents of this MIL Spec, and are usually refered to as "Fluid Resistant Epoxy Primer".

I'm not sure if the porosity comments above apply to this class of product.

If you want to buy some, it's probably at your auto paint store as PPG Industries DP-70/DP-701 primer.

For steel, I prefer cadmium plating, and you can still paint over the plating if you want.

protect that aluminum ... Gil Alexander, RV-6A, #20701

Gil Alexander
Herman Dierks

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