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messydeer
Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA
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 Posted: Tue Jul 04, 2006 8:56 am Post subject: Alodine and Zinc Chromate, Part III |
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I'd like to share with you answers to questions I had about using Alumiprep, Alodine, and Zinc Chromate. Below is a response by Dr. Sheldon Dean, President of Dean Corrosion Technology, Inc. I had asked him about reusing Alumiprep and whether scotchbriting without an acid etch would work. Also if alodine could be skipped and just put on zinc chromate.
...The etch solution is basically an acid etch that is intended to remove a very thin layer of aluminum together with the oxide on the surface. The reason to do that is to allow any particles, dirt or grease that may be on the surface to be undercut and dispersed by the detergents in the etch. If your Scotch Brite abraded surface were really clean and free of particles then maybe it would not be necessary to use the etch cleaner, but it is sort of an insurance policy against the occasional lapse in obtaining a really clean starting surface. Regarding reuse of the etch solution, you can do that up to the point where the solution begins to lose potency. If the solution becomes dark in color, or does not brighten the aluminum after immersion it is spent and should be discarded.
The Alodine treatment produces a conversion coating on the aluminum that is much thicker than the normal air formed oxide. It does provide some corrosion protection because of its greater thickness, and it also does not hydrate when exposed to high humidity. One mechanism of paint failure involves the conversion of aluminum oxide to aluminum hydroxide from high moisture content environments. The aluminum hydroxide is much more voluminous than the oxide and that causes the paint to blister and spall off. Usually the conversion of oxide to hydroxide requires something like chloride to be present so if you had a really well washed surface (rinsed with chloride free deionized water) maybe the paint adhesion to the aluminum surface would be adequate without Alodine. Again, it is like an insurance policy to cover less than perfect operations.
The zinc chromate paint is inhibited so corrosion is not a major issue. However, it is vital to have the best paint adhesion you can obtain because corrosion is only one mechanism for loss of paint adhesion. Temperature swings and sun exposure are equally important. The coefficient of thermal expansion of paint is at least 10 times that of aluminum metal so every time the surface cycles from very cold to very hot the paint is being squeezed and stretched probably beyond its yield point. Sun exposure causes the organic vehicle in the paint to cross link and/or oxidize and that makes it chalk and become brittle. In most cases the corrosion of the aluminum is not a major problem so the use of paint is for cosmetic reasons. Atmospheric corrosion of aluminum alloys usually results in a mild surface pitting of the aluminum surface that gives it a dull grey appearance. There is little loss of strength that accompanies the corrosion. The exception to this observation is in cases where alloys like 2024 T6 or 7075 T6 heat treated to obtain maximum strength are used. These alloys are susceptible to exfoliation and stress corrosion cracking in marine atmospheres and that can cause major problems. In that case most folks believe that paint can help. However, there are tempers of these alloys that will resist these corrosion problems, and the use of alclad products also prevents corrosion damage.
Regarding the question of whether you can skip the etch and Alodine treatments the answer is that it depends on what you are trying to achieve. The Scotch Brite treatment will improve paint adhesion, and in many cases that would be enough. However, It is impossible for me to anticipate the degree of severity of the environments that your product may encounter in its use so it would be only a guess if I told you either that you needed them or not. The other key issue is how long do you expect these surfaces to perform before you would need to maintain them? The etch and Alodine treatments should give you several years of good performance before spalling of the paint occurred...
Think I'll continue to Scotchbrite, etch, alodine, and zinc chromate the interior.
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noelloveys(at)yahoo.ca
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| Posted: Tue Jul 04, 2006 7:06 pm Post subject: Alodine and Zinc Chromate, Part III |
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Very factual. Thank you.
Noel
[quote] --
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