Aluminum finishes: Anodize vs. Alodine/Iridite Properties

A. Greetings Sergio, First of all, thank you for posting a question with your needs laid out in an orderly fashion that allows us all to clearly give you a concise, direct, and appropriate answer (note to finishing.com: We should archive this as the "perfect question")

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 :-)

1. Anodize is, for the most part equipment wise going to be more expensive hands down because of the additional steps necessary in an anodize setup that are not needed in a chromate conversion coating line. Such as the seal tank and the rinses associated with it. Physically an anodize line is larger (for the most part, this is not ALWAYS the case, but generally speaking, it applies). Maintenance wise I would have to say that anodizing, and its associated chemicals, and waste treatment costs are actually lower, than chemical film because of a few things, Sulfuric Acid is much much cheaper than any chemical film product you will find. Sulfuric Acid runs us around 70-80 dollars for a 15 gallon drum. This is a very small quantity, and the larger quantities you purchase the larger your savings. Chemical film, even on the cheap side will run you 5.00 + per lb, and you can run through 150 lbs of chemical film very quickly, whereas a sulfuric acid bath has a longer bath life, at least in my experience.

2. Environmental regulations: Chromate Conversion Coatings have hexavalent chromium in them (the product I use has approx. 15-17% of hex. chrome in it). This is regulated, BUT because of the nonexistence of electrification, or in most cases a heated bath, they are exempt from most of the regulations that would say affect a hard chrome plater with the same concentration of hex. chrome in his process (because it is both electrified and heated). Anodizing is regulated, but not as heavily as other plating processes. Sulfuric Acid is nasty stuff by anyone's account, and it is nothing to be toyed with. Aluminum is not an EPA regulated "heavy" metal, therefore most generalized waste treatment systems designed to handle an effluent flow from an anodizing line would be sufficient and you take the separate things and you do with them as you need to by your regulations (sludge, liquid waste, etc.)

3. Training - Hire an outside consultant to advise you on this project if you have no one available with prior anodizing/chromating experience. You might also be able to hire the right person that has all this previous knowledge, and pay them enough to train the other workers and staff in question, and potentially save yourself a lot of money. But your best bet is still to try a consultant first, even if it is on a limited basis to say work alongside with a person with previous knowledge and let them coordinate the project. In most cases it shouldn't take but a few weeks to get a person familiar with either of these setups, to get them capable of operating a line and understanding how it works and how to "Think like a plater" (thank you Dr. Kushner for that quote) may take months and potentially even years. It's easy to do, it's hard to master is my thought on the whole thing. I can teach anyone how to run either of these lines, are they capable then of being able to trouble shoot a process and figure out why something went wrong? NO, they need more than just working on the line to know all this.
If you'd like, Kushner has a wonderful "Light Metals" based topic of education that you can order from their website (they are listed on finishing.com in one of the sections). Other than this source, the AESF (www.nasf.org) has one also, but I frankly enjoy reading the material from the Dr. as it is easy to comprehend, it goes at a pace that anyone can keep up with, it's clear, concise and to the point, but still being thorough enough to satisfy everyone's needs.

4.) Time/process differentials: These two cannot be directly compared in my opinion because they are both so different in nature as a comparison would essentially be pointless, but I will list the details for you nonetheless.
Chromate: Time to process - a Full blown chromate process can take from 15-20 minutes per run to 30-45 or more (all dependent upon the steps you decide to implement to best fit YOUR needs).
Anodize: Same case here an anodize can run you from 45 minutes to an hour to much more than that, also dependent upon your needs. In my experience most jobs will require 1.5-2 hours of time in the various tanks + loading/unloading.

5.) Quality - It's not something you can purchase, it's something more than that. It's based on a non-dollar amount commitment to the customer. You can improve quality through the proper equipment, but an improperly trained operator can mess up a job better than any piece of equipment can. Control your tank parameters as much as possible without spending a fortune, temperature, concentration of acids and chromates, concentration of potentially contaminating materials (alkaline cleaner drag in to acid tanks will kill them very very quickly as it neutralizes your acids, thereby having to replenish more often.) The best thing you can do to improve your quality process-wise is this: Counter flow rinses. The cleaner your parts go into your process tanks, the less problems you have, the more consistent your finish, the happier your customer, the longer they maintain business with you, the list goes on and on. Of all things I have seen that can be implemented in EVERY plating shop of any kind, this is the single most beneficial thing you can do (this is strictly my opinion mind you, take all of this with a grain of salt).

Durability: Hrrrm, depends on what you want out of your durability. If you're using the process as a pre-finish for a painting process later down the line, its gotta be chromate, a cheaper finish that is so well made for this type of application, that it should be renamed to pre-paint Alodining :-). If by durability you mean longevity of service, an anodize will beat out a chromate 100 out of 100 times. Chromate is a surface build-up process, its measured in coating weight, whereas anodize is measured in MILS, or Microns (1 MIL = 25.4 microns). Anodize is scratch resistant, chromate is not. Anodize can be dyed any color, chromate cannot. Chromate is cheaper to process, Anodize is not. Once again, determine your application needs first, and you can then determine your optimum finish; it's not something you can do in reverse, you have to know your application needs before you can determine the finish to use.

As far as companies to help you with this: If you need chemicals, equipment, consulting, education, measuring equipment, testing equipment. Finishing.com is your best choice for listings on ALL of these things, first and foremost. After that, I would suggest reviewing the Thomas Regional guides of an area/location close to you (these are only for the US, none available for Mexico that I know of). After that, ask those of us around here. I can suggest people that I have dealt with and continue to deal with on a daily basis, those that have provided me services that I could not easily do without, and they've provided them in a timely manner, with quality service and quality backed products. But most of them are regional to me, and not applicable to the region you are in. Some might be willing to deal with you if you're willing to "pay their way" so to speak, but for the most part you'd be better off trying to find someone relatively close to you as they could provide value added services that those far a way cannot, at least in a timely manner that is.

Good Luck to you Sergio, and to your company.

If you have any questions that I haven't already exhausted here with answers, then please, feel free to reply with your questions. Thank you and have a wonderful day

A. Iridite coating is achieved by chemical reaction while anodizing requires electro-chemical process. Coating durability is better with anodized aluminum. This is why salt spray test duration is longer for anodized sample than for Iridited ones.

A. Iridite (aka chromate conversion coating, Alodine, etc.) are not abrasion resistant, they are super thin coatings of hundreds of thousandths, if not millionths...

They are corrosion resistant, but not abrasion resistant. They stand up to 336 hours min by military standards and generally speaking on the right alloys can reach and exceed 500 hours.

Industry wide chromate conversion coatings are a less expensive process, they are primarily used for paint base applications and applications where electrical conductivity is required, but you need better corrosion protection over a base aluminum.

A. I can't quite understand your 2nd paragraph Terence--there may be words missing or a typo--but:

Chromate conversion coating does not offer as much corrosion resistance as sulfuric acid anodizing. It may be enough, particularly when painted, but it's less. Chromic acid anodizing is a 3rd alternative, widely used in aerospace components because it offers corrosion resistance with less impact on fatigue life because it is thinner, and with no possibility of entrapment of corrosive sulfuric acid. Good luck.

Q. Thank you for responding to my question regarding "Alodine in lieu of anodizing". I would like to re-word some of my questions and hopefully you can provide some input.

Q1. The Mil-A-8625 spec suggest alum. alloys with over 5% copper should not be chromic acid anodized. This is also stated in the metals hdbk. Why is this, keeping in mind that H2SO4 is actually more aggressive than the CrO3? Is there something unique about the compatibility of copper precipitates and chromic acid?

Q2. Is it your opinion that skipping the de-oxidation treatment (typically nitric-hydrofluoric acid rinse) and relying strictly on alkaline rinse cycles to clean type 2219 alum. prior to anodize will end up with an acceptable coating? Do you have any experience with painted Alodine?, for example how much beyond the 168 hr salt fog would expect with paint (in this case, an aerospace grade used for hydraulic actuators). I appreciate any input you may have.

Best regards,

A. Alodine is a trade name for one brand of chemical used for MIL-C-5541 chromate conversion coating, Steve. You are right, your colleagues are wrong. If you check mil-specs or similar industry standards, the salt spray resistance and life of a chromated finish are perhaps half that of a standard anodized finish

From a career in metal finishing and a stint as a manufacturing technology consultant for one of the best-known management consulting firms I've come up with this maxim: "Cost reduction programs always fail". I'll call it "Mooney's Law" if no one else claims it :-)

Cost reduction programs must start with the premise that your quality is at least good enough and maybe too good; since this is never true, it's not surprisingly that programs founded on that faulty premise aren't successful.

So find a way in which the anodizing is limiting your quality, and ought to be eliminated -- for example, find a reason why the surface should be conductive, or why masking introduces quality problems -- and then you'll have a sound basis for a project which will both improve the product and save money :-)

A. Alodine is a Henkel Surface Technology trade name for a Mil-C-5541 chromate conversion coating. This is sometimes used as a paint base and sometimes as a final finish. It is conductive, reasonably corrosion resistant (but not as corrosion resistant as anodized finishes), and less expensive than anodized finishes because it is easier to apply because it is not an electrolytic process.

Anodized (also called anodic) coatings are very different from Alodine. There are several different types of anodic / anodized coatings. In brief, chromic acid anodizing is used for thin coatings where loss of fatigue strength would be a major issue (such as in aircraft components); but most anodized coatings are done in sulfuric acid. Anodized surfaces exhibit high electrical resistance. A Mil spec for anodized coatings is MIL-A-86025^⁸⁶²⁵.

Ted, I just wanted to note that in your reply to this letter you mistyped the Mil spec for anodizing as Mil-A-86025, when it is MIL-A-8625 / MIL-PRF-8625 [on DLA]. Lee Gearhart

oops Thanks for the correction, Lee. If you didn't tell me, it could have become the start of a bad habit :-)

A. Since you have used the chem film treatment before, you must have had the biocompatibility info for this. I suggest you refer to MSDS for Aluminum and Aluminum Oxide, the main substance in anodic coating. From there, you might be able to make biocompatibility comparison.

? ! Hi. I can't bring myself to believe that a chromate conversion coating process, which leaves hexavalent chrome on the surface of the part, is biocompatible.

Regards,

Q. Ron,

I have a very similar need for biocompatibility data on anodized aluminum. Have you had any luck?

Q. Ron, have you got any answer regarding biocompatibility of Anodized Aluminum?

"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. If you check the actual specification for the chem-film process (the "Alodine" you are looking for is a trade name of one of the many chemicals you are allowed to use - the specification is MIL-DTL-5541 [on DLA]) you'll find that there is a class 3 that specifies a conductivity test - that would give you some of your numbers. Class 1A is less conductive, but still useable. For more specific numbers you can refer to any of the reference books noted through the links on the homepage - especially "The Surface Treatment and Finishing of Aluminum and Its Alloys" by Wernick Pinner and Sheasby (I think the most current version is just by Pinner and Sheasby)..

Anodize itself (specification MIL-A-8625 / MIL-PRF-8625 [on DLA]) is non-conductive... so if you are looking for a conductive coating you pretty much can't use anodize. It will - at some high level of voltage - eventually breakdown, but that will also destroy your coating and can basically be disregarded.

With regard to heat conductivity - I'm not sure... But, it basically doesn't matter. Heat conductivity is rarely an issue for any Room Temperature conditions, and anything over 140 °F will cause the chem-film (Alodine) to break down and lose all corrosion resistance (and adhesion - I believe).

So - which one are you looking for? Heat conductivity or electrical conductivity? If the first and not the second, and chem-film and anodize are your only choices - anodize is the only way to go. If the second but not the first, chem-film. If you need both, you need to look at different technologies - probably nickel plate being the first among them.

Good luck!

A. There is electrical resistance information for Class 3 chem film (Alodine® is a brand, with several varieties) in MIL-DTL-81706 [on DLA] MIL-DTL-81706B CHEMICAL CONVERSION MATERIALS FOR COATING ALUMINUM AND ALUMINUM ALLOYS (02-MAY-2006). The resistance of Class 1A coatings is higher but not specified. I'm not aware of thermal conductivity data for chem film. The conductivity probably decreases upon heating above ~100 °C due to dehydration.

Anodize is primarily aluminum oxide, a pretty good insulator. The behavior varies with anodizing conditions, thickness, aluminum alloy and humidity (if unsealed). The breakdown voltage is typically about 30 Volts per micron of anodize thickness, but significantly lower on Cu- and Si-containing Al alloys. Thermal conductivities are <10% that of bulk alumina. However, anodize has a high thermal emissivity, so aluminum heat sinks are typically given ~8 microns of anodize.

For much more information on anodic coatings, see Chapter 12 of The Surface Treatment and Finishing of Aluminum and Its Alloys.

A. Hi, Bob

Anodizing would help. "Alodine" might contain hexavalent chromium, so should not be used in water supply. Stainless steel would probably be better, or the mesh filter could perhaps be made in one step by electroforming it out of nickel. Good luck.

Regards,

A. Hi Stephanie. We combined a number of threads on this subject and then appended your inquiry to it, so this dialog should answer your questions ... but get back to us if it needs fine tuning for your situation.

Regards,

Hi Madhu. We appended your inquiry to a thread where Ken V (who is a great researcher) says he can't find any data on it, and Jim G. warns of the likely unsuitability of chromate conversion coatings for temperatures over 140 °F. Are you still satisfied with the wording of your postings after these two relevant comments? Good luck.

Regards,

Yes.

Regards,

A. Hi Homer. Your inquiry is a bit abstract, but I guess I should assume that you are a purchaser of this service, not an operator of the processing line :-)

Your vendor has probably switched from hexavalent chromating to RoHS-compatible trivalent chromate conversion coating of aluminum. Although both clear and yellow chromating were common in the old days of hexavalent chromating, with the yellow coloration being intrinsic to hexavalent chromate, the MIL-DTL-5541 approved trivalent coatings are essentially colorless.

Regards,

A. The gray color could be from imbedded iron or other metal when the tumble media had been used on metal other than aluminum.

Other causes could be heat treatment, work hardening, or out-of-control chromate solution.