• Topic 55760 "Aluminum Anodizing flakes off when heated to 125 °C"

A. Hi Dave. 130 °C will certainly affect anodized aluminum.
We added your question to one of at least 4 major threads we have on the subject of heat resistance of anodized aluminum and you can review them for numerous perspectives.

But one major effect will be that if the anodizing is dyed the color will be at risk at that temperature and will require at least temperature rated dyes or two-step anodizing where the 'dyes' are actually metallic salts rather than organic dyes.

A second major effect is that the anodize will certainly crack at that temperature because the aluminum substrate expands 5X as fast as the anodized layer. But whether that cracking will result in unacceptable flaking & spalling gets complicated, and that's where you'll want to read some of these experiences.

Luck & Regards,

A. Hi David, Sheasby and Pinner say that the anodic coating begins to crack above 70 °C.

Thanks for your response to my question.

A. If you mention thermal resistance of anodize layer:

"The Properties of Electrodeposited Metals and Alloys"
by William Safranek

on eBay or

AbeBooks

or Amazon

(affil links)

A. Can you give us a little more data on the application?

"The Properties of Electrodeposited Metals and Alloys" by Safranek notes that the coefficient of linear expansion of the anodic coating is about one fifth that of pure aluminum and that 'This, obviously, is the reason for the susceptibility of coatings to crazing when significant temperature changes are encountered'.

Maybe one person's 'crazing' is another person's 'dustiness'?

Perhaps - the parts are an assembly of steel and anodized aluminum to which a plastic component is molded. Part temperature reaches the values indicated, and inspectors have objected to 'dustiness' of the anodized coating. It is probable that the appearance is a result of the heating (particularly since it is not evident prior to the molding operation), but may also be 'flash' or dust from the molding/flash removal process, as yet unanalyzed.   Thanks for your prompt reply.

"The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys"
by Wernick, Pinner
& Sheasby

(note: this book is two volumes)

on eBay or

AbeBooks

or Amazon

(affil links)

A. As you can see, we found a related letter and added your inquiry to it, Jim. Anodized coatings, whether thick or thin, are brittle. They crack under mechanical deformation; and they crack from temperature changes because the coefficient of expansion is so much lower than that of aluminum.  

I don't know for a fact that you can't go to 300 °C without cracking (and the stove-topable hard coated frying pans must approach that temperature) but I would be surprised if you could. Further, I'd be very surprised if the coefficient of expansion of any aluminum alloy differed significantly from any other, so I don't think the alloy matters a lot for this particular consideration. If the part is susceptible to fatigue failure, note that these cracks act as stress risers and reduce the fatigue strength of the component. "The Surface Treatment & Finishing of Aluminum and its Alloys" can probably offer you the detailed info you want.

A. Maybe I'm missing the scope of the question here. By normal atmospheric environment, do you mean exterior based usage common to a fresh water environment? A salt water environment, a controlled temp., internal lab environment?

I guess my outlook would that in normal temp ranges of let's say a max of 100 °F. Just to be safe. In a fresh water atmosphere, your largest concern will not really be the atmosphere, but the light exposure to the part. Unless you're using an electrolytic or similar type process, organic dyes will fade over a given period of time from tests I have seen. These ranged anywhere from little loss over 1 year, to major loss over a few months depending on color compounds. Gold dyes and pigments (such as Gold FAO [ferric ammonium oxalate] from U.S. Specialty). Has what I would consider to be incredibly good light fastness, and yet I'd only recommend it for partial, moderate exterior uses because of the fact its not intended to be an exterior based dye. I guess there's more to the question than you detailed, and more information would need to be presented to further give you any kind of approximate time frame that was even remotely worth using.

Before I finish this up for you. I'll pass on this bit of information I just read about weather fastness. This is from the book The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick, R. Pinner, P.G. Sheasby the single most informative resource I have ever seen related to aluminum and anodizing.

Two gentlemen by the name of Speiser and Schenkel tested Sandoz (now Clariant) dyes for weather-fastness. It was determined based upon their study that Gold 4N (comparable to the Gold I have mentioned above from U.S. Specialty) lost approximately 37% of its color base over a 6 year period of time. How does that relate to your conditions, its unknown. But it does state they placed samples at 13 different sites around the world to average out the exposures and understand which environments were most accepting and the harshest to the dyes. For comparison, a Black MLW dye only lost 4% of its color over that same 6 year time frame. According to this Sandoz then rated the dyes on a % basis being 10% perfect, 20% very good, 30% good and 40% moderately good. Anything over 40% was deemed unacceptable and therefore was not incorporated into their Sanodal line of exterior use products.

Hope that at least puts you on the right path, if your environment is a controlled, interior lab environment or interior setting I doubt these facts would relate very much to you, but it helps to at least give you something to go on.

A. Alright, so you're more concerned about the actual anodizing layer breaking down more than the dye itself fading away. Are you using the gold for more or less I.D. purposes then?

Anodized aluminum, specifically 6061-T6 in this instance, hardcoat anodized can withstand roughly 2100 °F temps for a VERY short period of time (I'm talking seconds). 400 °C works out to be roughly what? 700 °F-800 °F (just roughing it there, didn't do the conversion). I wouldn't expect the material to last an overly large period of time under those conditions as you're really starting to get into the upper temp. range of what the base material can handle in and of itself. I had a customer that was using parts as "ironing bases" for the textile industry. He came to the conclusion that using the blue dye he was using for I.D. purposes was simply a waste of money because it came down to the fact the dye would wear off quickly under those extreme temps. We opted instead for a hardcoat anodize for coloration and wear and apparently that worked sufficiently for his uses.

I'd suggest that you'd get a somewhat longer lifespan from a hardcoat finish than you would from a standard anodize. If for no other reason than the fact that the thicker coatings would protect the base material from the heat better and thereby not allow the structure to break down so quickly under those conditions from the inside out.

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

Also available in Spanish

You'll love this book. Finishing.com has sold almost a thousand copies without a single return request :-)

A. The easy answer is don't powder coat it, Celine. There is no reason that an anodized surface should not offer the look and corrosion resistance you want without further coatings. But you can spray an air-dry lacquer, or a two-component automotive clearcoat on it if you wish. Powder coating is an application method rather than a distinct type of coating; generally, the same coating is achievable with other application methods.

Although the block would return to original dimensions when it cools, the anodized coating could be cracked by the heat expansion involved in the curing cycle for powder coating.

A. Hi Max. The dullness could be from deterioration of the organic dyes, or it could be heat-related crazing/cracking.

Everbrite [a finishing.com supporting advertiser] has made the point elsewhere on this site that you can simply wet the surface with water as a good guide. If it's shiny when wet, a clear coat will probably help a lot. If water doesn't make it shine, neither will a clear coating. It's hard to say whether the surface is mechanically polishable though -- sounds unlikely but maybe not impossible -- it's probably more practical to strip and re-anodize though. Good luck.

Regards,