Type 2-1/2 & Room Temperature Hardcoat Anodizing

A. If your hard coat additive is the popular 2% Glycolic Acid with 2% Glycerin, YES, titration procedure is available, but cannot post it here. Your additive supplier should furnish a way to control the additive.

A. Hi Jane Jane.
Typically you'd send your anodizing additive supplier a specimen on a schedule determined by your bath volume/ throughput. Also type 2-1/2 around here (fishing reels, yes our nadcap shop has some fun) is more like 45-55 °F. Room temperature (70 °F) is straight TyII per spec requirements. So look at your process as much as your additives. I can't recall offhand what my anti-burn additive lists as the active but it smells like dry erase markers and costs about $850 us for a 5 gallon pail... there's some vocs in there ... I assume you run heavy air sparging or other aggressive mixing in the tank?

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A. The 'Sanford Process' is a proprietary hardcoat process. Please contact the licensor for information because we don't discuss proprietaries/brands here ( huh? why?). Apologies.

adv.
Luke Engineering has been hardcoating high silicon/copper aluminum die-castings for over twenty years with the unique process LUKONDC. Hardcoating thicknesses of up to 75 µm are possible, however, in production the "normal" thickness range is 20 to 50 µm.
We will be glad to hardcoat prototypes for evaluation, free of charge.

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"Chromating - Anodizing - Hardcoating"
by Robert Probert

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A. Hi, Joe.

I visited that supplier so long ago that I barely remember it, but to my knowledge the process is "Type 2-1/2" anodizing -- a simulated hard anodizing that does not require the electrolyte to be 28° F, but about halfway from there to ambient temperature. We don't discuss brands here ( huh? why?), but as you can see from the previous postings, there are additional brands of proprietary alternatives.

All proprietary processes will lack some degree of transparency or they wouldn't be proprietary of course, but if you want a transparent process you can operate at 28° F. and do real type 3 anodizing, or you can refer to Wernick, Pinner & Sheasby or Probert's "Aluminum How-To" for generic info on glycolic acid, glycerine, and similar additives. Good luck!

Regards,

A. I'm not sure whether you have the direction of the temperature change wrong, Garry, or I'm just misunderstanding you. In anodizing, what happens is that the electricity forms the coating, and the sulfuric acid simultaneously dissolves some of it. The hotter the acid, the faster the dissolution--which is why hard coat anodizing is done at much lower temperatures than conventional anodizing, so that the equilibrium point is at a thicker coating.

Purportedly, additives allow hard coat anodizing to be done in the 50 °F range you mention, rather than the usual 28-32 °F. While there are additives that work, the industry has also seen some snake oil, so I'd be slow to even swear that they all work, let alone try to explain how they work. I believe that most are based on oxalic and/or glycolic acid, and an expired patent search with those terms may tell you more. Hopefully, a supplier of these additives will reference a study on the subject; I will cheerfully read it because I haven't stumbled upon any explanations myself yet.

Readers may also be interested in letter 41283, "Is oxalic anodizing equal to hardcoat anodizing?".

Regards,

A. Garry,

Several companies sell additives to achieve "hard coat" equivalents. Some or most or all of them can duplicate some of the characteristics of hard coat but not all of them.

I have not heard anyone give a good reason for how or why they work.

They allow you to run a higher voltage and current density at a given temperature without burning.

The folks at Henkel in Atlanta can probably give you some of the information you are looking for.

Metalast in Nevada appear to come closer to true hard coat and at a very high speed. Their process is proprietary and you must buy in and pay royalties. It is unaffordable unless you are going to do a lot of hard coat or thick coat and then it is fast enough to be a profitable purchase.

They will not tell you what their additive is and have said on one occasion that they do not truly know why it works as well as it does.

Jim

A. Many commercial anodizers are lowering their bath temperature and calling it hardcoat. Except in special circumstances, very few traditional cold tank (25-40 °F) hardcoaters are raising their bath temperatures. This hot tank (50-65 °F) hardcoating is more appropriately referred to as pseudo or Type II 1/2 hardcoating.

All things being equal, higher bath temperatures result in softer, more porous anodic coatings. This larger pore structure can allow for greater terminal thicknesses. Other anodizer controlled variables such as; bath additives, acid concentration, dissolved aluminum, current density, electrical waveform, cathode design, agitation, and racking also have a significant effect on hardcoat quality. Bath additives, typically organic acids, raise the solution conductivity without increasing the acid attack on the anodic coating.

This can help mitigate problems from other less than ideal process parameters and opens the window for acceptable coatings under a wider range of conditions. However, acceptable is not necessarily the same as equivalent.

A. The above comments have said it pretty well. Let me add that the anodic coating consists of a hexagonal cellular structure and within each cell is a pore. The density of the coating is the size and number of the cells and pores within a given area. This can often be indirectly measured by using coating weights and the number of cycles on a Taber Abrasion tester, prior to breakthrough. The pore size and cellular structure are determined by electronic field effects. The pore size depends linearly upon the anodic voltage and upon the electrolyte pH. The number of cells is due to the current density, as is the rate of growth. As the anodic coating grows it is also dissolving. At a point when the rate of growth equals the rate of dissolution the part is dimensionally shrinking.

Hardcoat anodizing allows a higher current density to be applied without the parts burning (increases the rate of growth/cell density), and slows down the rate of dissolution (temperature = time in chemical reactions) thus allowing thicker coatings which are more dense.

Anodizing additives work by reducing the rate of dissolution and at the same time allowing higher current densities, thus still causing a dense/thick coating. Additives work by affecting the pH and conductivity of the anodizing electrolyte at the surface of the part.

Personally, I have seen 6-8 mil thick coatings per surface, produced on conveyor rollers, at a temperature of 68-72 °F using an electrolyte additive, and high current densities. This same tank chemistry also produced 60 °F coatings which passed Taber abrasion testing and coating weights per MIL-A-8625 / MIL-PRF-8625 [on DLA]. It might well be argued that those coatings were not as dense as cold temperature produced hardcoat, however I do not know of any studies comparing the density of cold produced hardcoat versus additive produced hard coat.

An excellent study of the field effects during anodizing which relates anodizing parameters to oxide growth is:

Theoretical Modeling of Porous Oxide Growth on Aluminum published 1992 in J. Phys. D: Appl. Phys. 25 pages 1258-1263.

A. There are a number of non-licensed hardcoat additives on the market. These additives, when added to the anodize tank will help retard the tendency toward burning. They will also allow a moderate increase in processing temperature. Care should be taken to follow the manufacturer's recommendations.

A. Anodizing additive packages are available from a number of metal finishing chemical suppliers as are proprietary anodizing "systems" which include bath chemistries. The literature details a number of beneficial organic additives which may be less expensive to purchase. There is no such thing as an industry standard, most anodizers have their own favorite.

Many refer to hot tank hardcoating as pseudo or Type II 1/2 hardcoating. It may meet the minimum requirements for hardcoating under certain conditions but may not be equivalent to traditional cold tank processing. In general, with all factors being equal, colder is harder.

A. I agree with Chris. There is no substitute for low temperature hardcoat anodizing. Certain additives can enhance the process, but CONTROL and AGITATION are equally, if not more important!

There is little "magic" in proper hardcoating. It is a matter of sticking to the basics.

A. Hello Ning. The anodizing reaction consists of two competing reactions: building up of the oxide through electricity, and destruction of the oxide by acid attack. So running this process at low temperatures, where the acid is less active, speeds up the process rather than slowing it down.

There are numerous additives and patented or trade secret processes for hard anodizing and "type 2-1/2" (higher temperature pseudo-hardcoat anodizing). Chris gave us the general hint that they increase conductivity without increasing acid attack. But the people who have developed these generally wish to license the technology (or sell the additive) rather than telling you what it is and how to make it. Contact suppliers like Clariant, Metalast, N>Tec, or U.S. Specialty Color, or do a Google search.

Although hard anodizing has a grayish cast compared to thinner anodizing, it can be dyed to various colors. Dyes need to be sealed, and this sealing process softens the hard anodized coating somewhat, so hardcoat is not usually dyed, but it is not truly rare for parts to be hard coated or type 2-1/2 anodized and then dyed. For example, some ammunition is hardcoated but must be dyed to distinguish live from dummy rounds.

Hi Ted,

Thanks for the detailed explanation.

A. The question isn't quite a valid one, Ronald; you wouldn't take a pile of parts and sort them, claiming these are hard, and those are conventional. Rather, each part meets certain specs or it doesn't. In broad generalities though, hard anodized parts would probably have a coating of 2 thousandths of an inch, and would be unsealed whereas conventional parts would have a thickness of well under a thousandth of an inch and would pass a seal test. But there are also "Type 2-1/2" coatings that may be halfway in between. Please try to describe the actual situation because we can probably quickly offer an answer to your particular situation but it can take a book to answer generalized questions covering all possible variations and their almost endless "ifs, ands & buts" :-)
Thanks.

Q. Dear Ted,

Thanks for your kind response. My professional knowledge in anodizing is pretty limited which explained the manner of the question posed. My current situation is that I need to verify that the plater is really doing hard matte anodizing on the parts instead of normal anodizing. I tried testing the first batches by running a round edge tool lightly across the surface and a line could be seen where the tool moves across. Am I right in concluding that this should not happen with hard matte anodizing? As such, I am seeking for a better/accurate methods to verify the parts.

A. The principal and easiest distinction between hard anodizing and conventional anodizing is probably the thickness; unless otherwise agreed, hardcoating must be .002" thick according to MIL-A-8625 / MIL-PRF-8625 [on DLA]. When you review that spec, you'll see that it also references a hardness test that hard anodizing is expected to pass.

But a hint is: if the parts still look like aluminum, they almost surely aren't hard anodized; if they are black although not dyed, they probably were hard anodized; some alloys turn dark gray when hard anodized, but some alloys turn dark gray even from a conventional anodize and black from hard anodizing so it's more difficult to tell.

A. While I'm hesitant to offer the following, because as Ted said - it is very dependent upon your exact circumstance, there is a substantial weight and thickness difference between a MIL-A-8625 type II (conventional) and a MIL-A-8625 type III (hard) anodize coating.

Type II has to have at least 1000 mg/sq foot coating weight (approx. 4 micrometers under normal conditions) and pass certain tests for corrosion resistance. Type III has to be a minimum of .002 inches thick (50.8 micrometers) unless specified otherwise, with a coating weight of 4320 mg/sq foot per 0.001 inch.

So - if the parts were done to spec (rather than just "hard anodize"), and if they pass all other testing requirements, you could sort them by checking the anodize thickness.

What is your exact situation?

A. Hi Shaul. Yes there's a difference. Hard anodizing is thicker and therefore offers more electrical, wear, and corrosion resistance. This is reflected by the hard anodized coatings requiring higher voltage rectifiers to achieve them. But again, if you would please describe your actual situation, we can avoid these generalized answers which always have exceptions and which are often misleading. Good luck.

Regards,

A. Hi Shaul,

I delayed replying to see if you would describe, as suggested by Ted, the actual situation. It can be so much easier to offer advice if the real problem is known.

I am uneasy about measuring resistivity. Firstly, how are you going make repeatable measurements? It is quite easy to to measure the resistance of say, aluminium wire. But a very thin, high resistance anodic film on a highly conductive base is a very different matter. You would need to get a repeatable surface area of contact. In reality the surface undulations would make the intimate contact, required for getting a flow of current, highly variable. Secondly, moisture in anodic pores or surface defects could substantially reduce the apparent resistance.

Voltage breakdown, to measure film thickness, might be more helpful. ISO anodizing standards include such tests.

It does not follow that hard anodized films are thicker than convention ones though they usually are. It is perfectly possible to have 25 µm in both. This is a instance where knowledge of the actual situation would be so useful. (Sorry, I couldn't resist turning the knife).

Ultimately hard anodizing is defined by hardness and not thickness. An abrasion test with a suitable abrasive medium might separate the sheep from the goats.

Your comments please.

Harry

Hi Naveed. As you know, there's no "best" except in response to a specific list of requirements. If oxalic acid and sulfuric acid together were "best", everyone would be using them ... whereas I suspect that there are many more anodizers using straight sulfuric acid (Type II anodizing) than such mixes.

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 :-)

What alloy(s) do you plan to process, and what thickness of anodizing are you seeking? Thanks!

Glycolic acid is also sometimes used as a hardcoat additive, and Robert Probert's "Aluminum How-To" offers some info and formulations. You can perhaps also find more by searching the site for "glycolic anodize" and "oxalic anodize". Good luck.

A. Hi Alaattin. We combined a bunch of threads on that subject here, and then appended your inquiry to it. I think you'll find your answers on this page now.

Additives don't reduce required anodizing time -- the 720 rule still holds. They allow higher temperature pseudo hard anodizing, so refrigeration may be simpler, but they don't reduce energy requirements by much, because any heat the rectifier puts in, the cooling system still has to take out. Good luck!

Regards,

A. Hi Brian. What's the secret formula for Coca-Cola and Big Mac's special sauce? :-)

The formulas wouldn't be proprietary if they weren't trade secrets, but as an introduction you can look up "MAE (multipurpose anodizing electrolyte) Process" to learn that it (and probably they) involve glycolic acid and/or glycerin and/or oxalic acid. We appended your inquiry to a long thread with lots of info and reference sources.

Good luck.

Regards,

A. 35 microns is, to us, 0.0014 inch thick. After you pass about 0.0008 you need more voltage to overcome the resistance.
Depending on the alloy you can get 0.001" with about 24 volts (but not on the 2000 alloys). Come back and tell us your top available voltage. If you've got about 30 volts you can get 0.0014" without an additive and without cooling so low. The colder requires more voltage and since you do not need the small pore or abrasion resistance, you may stay up around 72 °F (22 °C). Oxalic, glycolic, and glycerin additives will not help you get thickness, those additives only make the pore smaller which you do not need. Again, all you need is enough voltage to overcome the resistance at 0.001" so it will go on to 0.0014".

A. Eric,

adv.
Contact me directly about suggested additives to produce up to 100 microns of hard anodized aluminum coating without going into deep cooling.

Also the newly developed technology of micro-arc aluminum anodizing produces up to 55 microns extremely hard coating.

Thank you for your replies! Our rectifier goes up to 24V/3000A so we took your advice and just went for it. Managed to get 34 µm first try and will make another run later this week.