Structures engineer
AFulmer said:
I could surely be mistaken but I always understood that pure aluminum will quickly oxidize ("rust"?) when exposed to air but the aluminum oxide created on the surface is what is extremely corrosion resistant. It is only when we break that "layer" of aluminum oxide (drilling, scratches, etc.) that we expose the underlying metal to corrosion potential.
Some good replies, but I think we need to define, alodine, anodized and primers a little.
Here is an article related to alodine and anodize and RV spars:
http://www.matronics.com/rv-list/hovan/tips/AlAnDef.html
As was mentioned most of the "sheet" on our planes are clad. PURE aluminum is very soft and PURE means it does not have alloying. Any PURE metal tends to resist corrosion. Alloys will corrode. Aluminum like 2024 has other elements like copper, zinc and magnesium in it as well as Aluminum (90%-94.7%). As you know "entropy" means things tend to go back to their natural state, where pure metals tend not decay or corrode. So the PURE aluminum clad is a great layer of corrosion resistant material.
Really Apples and Oranges, Anodize and Prime.
Also GREEN as a standard color is long gone. Chromate is also all but gone as the standard for primers. You can get primer in any color, grey, yellow, white to name a few. Most "chromate" primer is gone for toxic EPA reasons. It did make a good primer because it was some what sacrificial. It worked great but there are other epoxies that are superior which seal the material and chemical bond making corrosion unlikely as long as the layer of primer is not compromised (scratched off).
You would not "HARD" anodize large parts and sheet. The normal etch, alodine and prime is all you need.
As far as anodizing causing cracks and fatigue reduction, one of the biggest problems is Hydrogen Embrittlement. If the process is not done properly you can cause this condition, and it will indeed cause fatigue problems and actually lead to corrosion.
I can see alodine clad parts as the only process, but cost and time get in the way. Why? The clad is good enough. That is the argument for no prime of the clad parts. As was mentioned the extruded angles are not clad and need to be fully protected with primer, not just alodine.
Alodine is a chemical conversion that "stains" the metal with a gold tint typically but can be clear or other colors. It leaves a "film" but it is not durable on its own. It does not make the surface hard. This is a passivisation process, making the material "passive" in relation to another material. It basically is a layer to resist oxidization. It makes a very "tacky" surface for primer to attach to. It is usually a step prior to priming, not as a final finish on its own. It could be used on its own, but it is not really meant to be a final finish.
Hard Anodize is a electrochemical process (electrical current is used) and adds material (not really but the part grows). The surface is hard and because it is porous it can be colored. Material is not added, aluminum oxide is "grown out" of the surface during anodizing, and then becomes aluminum hydrate that is extremely hard. Parts will become dimensionally larger after the process, so "material is added". The real advantage is looks and hard wear surface, since aluminum is soft. This is great for consumer products and "dress up items", but practically on aerospace structure its use is limited. Its nice looking but not use this widely on primary aircraft structure.
Van does anodize the spars. What type of anodize not sure there is type I, II and III. Type III is the hard kind, which is not used I believe. The type II is the thinner coating. If I had a choice I would rather have the aluminum, Scotch-Bite, etched, alodine and epoxy primed, but that is the way it comes. My RV-6 I built the spars, but I think all RV's including my RV-7 come pre-built.
It does not mean it will necessarily have lower fatigue life if the process was done properly. The stress levels and cycles are low, so it is not an issue IMHO. According to the 1992 article referenced above Van says the fatigue life is 1/2 when anodized, but still acceptable.
There are fasteners on large aircraft made of titanium and aluminum collars. The aluminum collars are anodize bright pretty colors. The hard coating (type III) gives it durability against the tools used during installation. You can imagine aluminum nuts installed with a steel tool may be subject to abuse, so the hard layer is goodness here.
G
CJ
