Aluminum Anodizing vs. Powder Coating vs. Kynar vs. Clear Coating

A. According to Southern Aluminum Finishing, which offers both, powder coated aluminum seems like a better bet in a salt air environment from a corrosion resistance point of view. However, if these are public railings likely to become worn or deliberately scratched with coins, knives, etc., I think that changes the equation because anodized coatings are wear resistant and difficult to remove.

P.S. : These steel pipe railings in Asbury Park are more than ready for replacement!

A. You may want to consider "Hard" Anodizing. This is different from standard anodizing. Unfortunately you can only do it in natural or black, but since black is what you are wanting this may work. Standard anodizing offers no real wear protection, but it is more of a finish. The hard process has similar wear characteristics as hard chrome, just a little better. The 1.2 ~ 2 mils thickness is usually enough for most applications, but can go on as much as 9 mils. At 9 mils the material will turn from black to almost a grey / whitish color. I hope this may help.

A. Hi, Jennifer.

Both are widely available because neither is manifestly superior to the other for this application. But anodizing is rather limited in terms of truly lightfast colors, and white anodizing isn't available. If you like one of the bronze or champagne architectural anodizing colors, anodizing should prove outstanding, but they often don't suit people's taste for average residences.

Regards,

A. A very quick and non-destructive way would be to measure the coating thickness with an eddy current gauge. Even the best quality (Class 1 Architectural) anodizing is only required to be .0007" thick; whereas paint or powder would generally be about 3x that thick.

A. Hi John. Anodizing generally will not crack except at sharp corners; this cracking is not so much because of the vulnerability of that area to abuse, but because the coating builds perpendicular to the walls. Look at the millions of successfully hard anodized pots and pans that don't chip or flake despite the abuse.

Is it possible that your parts have sharp corners which don't yet, but could, have a radius? Powder coating is subject to no or very little coating on sharp outside corners. Good luck.

A. Although my first choice for treating pure aluminum might be anodizing, fancy aluminum wheels are pressure cast and have lots of tramp elements in them which would turn the anodizing greyish instead of bright, so I'd rule that out. A clear powder coat is okay, or polishing the aluminum and clear coating it. They should be reasonably bright but won't rival chrome plating. This last option would be the brightest, by far, but is difficult and very expensive and must be done perfectly, and the wheels are then quite delicate.

A. I don't think your rims would be harmed at that temp. 6061 T6 aluminum is heat treated for 18 hours at a far greater temperature, somewhere around 500 °C.

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A. 10 years with no fade is a tough spec for outdoor furniture. If you go the anodizing route, I think that you would be better off with a 2-step (architectural anodizing) as it does not use dye, but literally plates a metallic ion in the anodize. For a dye operation, the most fade resistant dye available (they all have a numerical rating) would be mandatory as well as a very tightly controlled seal step for the maximum fade resistance.

Powder coat can be good, BUT, this also requires the selection of a properly formulated paint that combines both durability, UV resistance and weatherability.

Then there is the possibility of an unsealed, clear, type II anodize with a powder coat on top.

A. 20 YEAR NON FADING BLACK - USE KYNAR 500 COATING ON MILL FINISH ALUMINUM.

A. Kynar filled the bill until you said cost effective.
With the optimum material formulation, powder coat will probably be a good option. Realize that changing color will shut down your paint line for upwards of half a day unless you go the high tech way up front.

A. Sodium Chloride will gradually dissolve aluminum oxide, hydrated aluminum oxide, dichromate occluded hydrated aluminum oxide, and aluminum. Use Kynar.

A. Hi David,

What Robert says makes sense, doesn't it.

My only query is why Kynar? Why not PVC, which would be far cheaper and easier to touch up... I'm not au fait with paints, only with solid plastics.

But Kynar may give a better bond to the substrate, I just don't know.

A. Jim,

If you want to go with powder, the pretreatment and paint formulation is going to make the difference as to the cost and weatherability. Find a company that powder coats this type of/or similar equipment and look at a job that they did that's been in the field for a couple of years. If you can't find anybody that does it successfully with powder, I would take that as my answer, stick with the anodizer.

A. Sheldon's answer sounds good. I'd add that the fading of anodizing tends to depend on what color you want. If you want a vibrant color from the rainbow, the anodizer will need an organic dye to achieve it, and such dyes do fade in strong sunlight. But if you want an architectural bronze/champagne color, these can be achieved with stable inorganic metals that are very resistant to fading. And natural non-dyed anodized aluminum doesn't fade.

Every finish has a limited life so, while powder coating may well be better in Fresno, we must compare actual example to actual example. We're always dissatisfied with an actual example that has seen some years of exposure when it's compared to a catalog idealization :-)

A. There are a handful of letters in the archives from people who had railings powdercoated for a coastal environment and later regretted their decision. In the responses it is usually suggested that the prep work wasn't correct and/or the wrong type of powder was applied. More than once the suggestion has been made that the most resilient finish for an environment near salt-water (which your yacht would certainly quality as) is anodize followed by powder-coating.

Check through the archived letters before you make any switches to your current process - if nothing else they will alert you to problem areas you will want to discuss with your vendor before processing.

Good luck!

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Ed. note:Threads about the option of sequentially BOTH anodizing and powder coating include: Letter 1245 "Anodizing as a pretreatment for powder painting aluminum extrusions", and Letter 41055 "Powder coating anodized aluminum parts". The search engine can find more.

A. Hi. If there are sharp corners, the anodize probably does chip. If edges are radiused, then it's far less likely to be a problem.

An anodized finish looks like a honeycomb; the dye (if applicable) is absorbed into the comb. After dyeing, the anodizing must be sealed, which completely closes off those openings. If not sealed properly, dirt can get into those holes and be almost impossible to scrub out. There are "seal quality tests" that you should run to ensure that the anodizing has been properly sealed. The most obvious initial test is that if the parts feel sticky, they're almost surely not fully sealed. Good luck.

Regards,

A. Hi Laura. For interior application, corrosion is unlikely to be an issue with either coating. And, unless the handles are used a lot more heavily than I can picture, I shouldn't think that general wear-through would be a major concern either.

An anodized surface is an aluminum oxide surface, and pretty scratchproof. It's hard to mark it with sandpaper when you're trying! So if you have a lot of concern over scratching, then the anodized surface is the winner. And if you want to retain the metallic look, anodizing wins again.

But if you want to disguise the metallic look, powder is better. Also powder is more versatile in being able to give you more and richer colors. The finish should also be less likely to fingerprint.

I think, in summary, for interior use it comes down to aesthetics more than engineering qualities.

A. Hi Ben.

I'm not sure that there is a clear call on that one, but I'd probably go with anodizing.

Regards,

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A. Hi Jack. You probably already realize that such parts are often made of stainless steel, and that high quality stainless steel is both better and more expensive than aluminum (no matter what you coat it with). Aluminum is no match for electropolished type 316 stainless steel.

But Sharkhide or Protectaclear from Everbrite [a finishing.com supporting advertiser], or even a 2-part automotive clearcoat will improve the aluminum quite a bit.

You are right that powder coating doesn't stick well to polished surfaces; but more precisely, metals need a proper pretreatment before powder coating and these pretreatments tend to not be shiny clear, but "toothy" or opaque, which fights against your desire to retain a shiny finish. A clear Alodine or Iridite chromate conversion coating before clear powder coating should help though. Good luck.

Regards,

A. Remember that powder coating has different properties than anodizing, including the fact that it's not as abrasion resistant. So you need to think about how and how often these tubes telescope.

The powder coating will be thicker. The anodizing is probably between .0002 and .0007" thick. Depending on what you specified, but half of that buildup is made good by aluminum consumed from the surface in the course of building the anodized film, so the "added" thickness would be half of that.

How thick the powder ought to be will depend on the service condition, etc., but it's probably, say, ten times as thick as the net gain from the anodizing.

A. Mark,

There is a significant difference between anodizing, and powder coating thickness. Anodizing, typically, can range from .0002"-.002" While your powder coating is typically .010+"

A. Hi Brad. If by a "corrosive environment" you mean acid or alkali or other chemicals, we'd have to know what environment you mean. But if you just mean severe service in a natural or salty environment like an oceanfront house or automotive exterior, and assuming that the metal has been properly pretreated, the general rule of thumb is probably that organic coatings like powder coating or epoxy will outperform anodizing. If wear or scratching are involved, anodizing will generally outlast organic coatings. Hardcoat anodizing, which I'll loosely define as anodizing of 0.002" thickness, is generally not considered decorative like thinner "Type 2" anodizing.

The reason so many different surface treatments are available is because each of them is better than the other alternatives under certain circumstances, and it is very difficult to say which is "best" without a really carefully described scenario. For example, powder or epoxy may not penetrate into recesses, crimped areas, etc.

Regards,