Painting aluminum: preperation details
Posted: Thu May 11, 2017 1:18 pm
I'm writing, again, about painting aluminum because I've recently been involved with a friend's airplane paint job. While a plane is mostly 2024 cladded sheet and not 50 or 60 series aluminum alloys the paint prep. process is similar and these points can help make the job easier IF (huge consideration) you're planning to use the traditional chromic acid (alodine) conversion of aluminum oxide to aluminum chromate as the primer bond layer to the underlying metal.
Let's review. #1 if any aluminum alloy is left in the air for a few seconds it forms a 'self-healing' or depth limited few molecule thick layer by chemically combining aluminum with oxygen. The result is a layer or aluminum oxide that 'seals' the underlying metal. The word seal here is used to mean that the chemical reaction to oxygen is stopped because the layer of oxidized metal does not allow further depth of reaction down into the aluminum. This attribute of the Miracle Metal is why is has commercial value- if it reacted like iron/steel/alloys of iron then; Aluminum would dissolve very quickly into a pure shape of oxide with no metal remaining.
Steel slowly continues to oxidize; aluminum does not. Aluminum oxide forms in a few seconds and stops- adding a small percentage to that layer over time (many years) but remaining very thin layer. This layer melts at temperatures much higher than the underlying metal- so it needs to be taken into account when welding (various methods) but for consideration of gluing, painting, or coating aluminum this oxide film is not a very good surface to 'stick too'.
#2 Paint sticks to aluminum by one of two methods or a combination of the two methods. The first method is mechanical- where the surface of the metal is roughed up- this term rough is only taken to mean at a microscopic level. The roughness needed to bond paint is still pretty much smooth to the touch compared to a concrete block - say. If the surface is blasted by high speed particles (blasting w/ different media) the surface will become rough - this process is called creating an anchor pattern.
Think of the Velcro product as the surface of the metal that has been blasted- when that surface has been visually magnified many times so the anchor pattern can be seen. While that is not a perfect example- the idea being illustrated it the fact that even if the paint will not 'grab' chemically to the metals' surface- if the paint were to flood all the little tiny valleys and dry on the ridges too; that paint film with have a grip on the metal surface- which we refer to as mechanical adhesion.
There is another method of 'grabbing' the metals' surface and that it to chemically treat the surface with a very large crystalline (viewed in a microscope) film that gives an anchor pattern for the paint's primer layer. This method is to chemically convert the aluminum oxide to aluminum chromate and that is done by a few steps we've discussed before but will review here, as well.
The chemical conversion method chemically bonds to the aluminum without aluminum oxide- and the paint sticks well to the chromate converted surfaces- so this method is an alternative to blasting.
The ultimate method would be to blast, (blast media is not being discussed or explored) then follow all the steps to convert the surface to aluminum chromate and then primer and paint. However, it may be obvious this method, while the most effective paint preparation is also the most expensive- by far.
#3 the steps to chemically convert aluminum surfaces from aluminum oxide to aluminum chromate are; A) wash and clean with soap and water so all contaminants are removed from the metal. B) using an acid, cover the metal and allow the acid to 'etch' or chemically strip (combine with) the aluminum oxide layer; C) after the acid acts on the surfaces for a few minutes rinse continually with tap water, diluting the acid to neutral ph, flushing any remaining acid off the metal, cleaning the metal with a film of water and MOSTest important SEALing the air away from the aluminum...D) the coating the water covered aluminum with a chromic acid solution (hexavalent chromium in a mild acid) to allow the chrome to react with the aluminum BEFOre the oxygen can.
The result of this simple but potentially messy wetting, rinsing and re-coating process is a layer of aluminum chromate bonded to the aluminum.
This film will hold most paints well and is chemically bonded to the underlying metal so the "paint sticks" to the metal by both a mechanical adhesion from the primer film layer to the chromate and under that chromate a chemical bond to the metal's formerly oxygen linked -now chromium linked- molecules.
I'll expand on this a bit, and hopefully Chaps, our resident painting expert, will add points of clarification and other facts that pertain to this process.
Cheers,
Kevin Morin
Kenai, AK
Let's review. #1 if any aluminum alloy is left in the air for a few seconds it forms a 'self-healing' or depth limited few molecule thick layer by chemically combining aluminum with oxygen. The result is a layer or aluminum oxide that 'seals' the underlying metal. The word seal here is used to mean that the chemical reaction to oxygen is stopped because the layer of oxidized metal does not allow further depth of reaction down into the aluminum. This attribute of the Miracle Metal is why is has commercial value- if it reacted like iron/steel/alloys of iron then; Aluminum would dissolve very quickly into a pure shape of oxide with no metal remaining.
Steel slowly continues to oxidize; aluminum does not. Aluminum oxide forms in a few seconds and stops- adding a small percentage to that layer over time (many years) but remaining very thin layer. This layer melts at temperatures much higher than the underlying metal- so it needs to be taken into account when welding (various methods) but for consideration of gluing, painting, or coating aluminum this oxide film is not a very good surface to 'stick too'.
#2 Paint sticks to aluminum by one of two methods or a combination of the two methods. The first method is mechanical- where the surface of the metal is roughed up- this term rough is only taken to mean at a microscopic level. The roughness needed to bond paint is still pretty much smooth to the touch compared to a concrete block - say. If the surface is blasted by high speed particles (blasting w/ different media) the surface will become rough - this process is called creating an anchor pattern.
Think of the Velcro product as the surface of the metal that has been blasted- when that surface has been visually magnified many times so the anchor pattern can be seen. While that is not a perfect example- the idea being illustrated it the fact that even if the paint will not 'grab' chemically to the metals' surface- if the paint were to flood all the little tiny valleys and dry on the ridges too; that paint film with have a grip on the metal surface- which we refer to as mechanical adhesion.
There is another method of 'grabbing' the metals' surface and that it to chemically treat the surface with a very large crystalline (viewed in a microscope) film that gives an anchor pattern for the paint's primer layer. This method is to chemically convert the aluminum oxide to aluminum chromate and that is done by a few steps we've discussed before but will review here, as well.
The chemical conversion method chemically bonds to the aluminum without aluminum oxide- and the paint sticks well to the chromate converted surfaces- so this method is an alternative to blasting.
The ultimate method would be to blast, (blast media is not being discussed or explored) then follow all the steps to convert the surface to aluminum chromate and then primer and paint. However, it may be obvious this method, while the most effective paint preparation is also the most expensive- by far.
#3 the steps to chemically convert aluminum surfaces from aluminum oxide to aluminum chromate are; A) wash and clean with soap and water so all contaminants are removed from the metal. B) using an acid, cover the metal and allow the acid to 'etch' or chemically strip (combine with) the aluminum oxide layer; C) after the acid acts on the surfaces for a few minutes rinse continually with tap water, diluting the acid to neutral ph, flushing any remaining acid off the metal, cleaning the metal with a film of water and MOSTest important SEALing the air away from the aluminum...D) the coating the water covered aluminum with a chromic acid solution (hexavalent chromium in a mild acid) to allow the chrome to react with the aluminum BEFOre the oxygen can.
The result of this simple but potentially messy wetting, rinsing and re-coating process is a layer of aluminum chromate bonded to the aluminum.
This film will hold most paints well and is chemically bonded to the underlying metal so the "paint sticks" to the metal by both a mechanical adhesion from the primer film layer to the chromate and under that chromate a chemical bond to the metal's formerly oxygen linked -now chromium linked- molecules.
I'll expand on this a bit, and hopefully Chaps, our resident painting expert, will add points of clarification and other facts that pertain to this process.
Cheers,
Kevin Morin
Kenai, AK