Estimating wear of hard anodized aluminum 3

[DReimer]

We are designing a cheap and cheerful linear guidance assembly using a formed sheet metal carriage supported using dirt-cheap skatewheel bearings within a hard anodized aluminum extrusion.

A vertical load of approximately 20 pounds will be supported by two 18mm (OD) bearings acting as wheels and rolling along the extrusion. There will be a moment load on the carriage that will be taken by the same type of bearing mounted within a delrin disk.

There is some concern about the wear of the aluminum with hardened steel bearings running on it. Our equipment often goes into food plants (not in the food processing area, though) and it is felt that visible metal wear particles would be offensive to many of our customers.

Is there a way to calculate an estimated material wear of the aluminum based on rolling-contact point loads? I've done some Googling without success. Any help is appreciated.

We will be doing physical tests of a prototype, but I want to move more quickly than that would allow.

Thanks,
Dean

 

[kenvlach]

Hi Dean,

The quick & easy answer is (probably) not. Hard anodize 'wear' is often due to excessive flexure of the aluminum substrate resulting in the anodize cracking and then crumbling. If the extrusion is stiff enough, smooth and kept clean, and if the all the anodizing steps are done properly, it should last a long time. I will look through my books & get back to you within 24 hrs.
Some questions first:

What is the alloy, e.g., 6061-T6?
Has the hard anodizing been done yet? [If not, I can make some recommendations re the processing.]
Will there be exposure to cleaning chemicals (hypochlorite bleach & alkaline detergents are common in food production plants)?
Will there be temperature variations, such as passing through ovens (altho not with Delrin)?
Do you want it dyed black or left natural or mind if dichromate sealed? (non-dyed 7075-T6 with dichromate seal gives a nice olive color).
Ken

 

[kenvlach]

Hi again,
It is better to prevent problems than to measure them, so I will make some recommendations on producing a high quality anodize coating at reasonable cost.
[The best requires lots of $ & time, e.g., Tufram® coating

http://www.magnaplate.com/coatings/tufram.html

or similar: Apticote Hard Anodised Composite Coatings:

http://www.poeton.co.uk/w1/a350.htm,

“PolyLube:”

http://www.poly-metal.com/polylube.shtml).

Anyway, most of my wear data is Taber test results for different anodizing conditions & alloys, not directly useful for you.

Typical hard anodize produced at 32^oF (& unsealed) has high abrasion resistance but also very high residual tensile stress at ambient or higher temperatures. Especially if processed through a final rinse of very hot water (to aid drying), the stress can exceed 75 ksi, and it can craze crack like crazy, resulting in negligible corrosion resistance along with poor fatigue behavior. [In thread307-134452, MikeHalloran describes a case of spontaneous corroding hard anodize.] Research has shown that even with a normal sealing in hot nickel acetate solution (as done with Type II anodize) does not yield corrosion resistance as good as that of the thinner Type II anodize.

I will first outline a procedure to minimize cracking and improve corrosion and fatigue resistance. This will require a cooperative anodizer, but hey, customer satisfaction should be their number one goal. I suspect some anodizers may even learn something. Explanations & details are then given.

1.) Design for Anodizing. Select a proper alloy, use sufficient radii of curvature, know the difference between anodize thickness and buildup.

2.) Proper pretreatment. Don't roughen: Soak clean as normal. Use the minimum time necessary in the hot caustic etch to remove the pre-existing oxide. Desmut as normal.

3.) Hard anodize at 50^oF (or even higher). This requires the use of a suitable solution (adding oxalic acid or proprietary additives to the typical 10 vol% sulfuric acid electrolyte.

4.) Cold water rinses until the part reaches ambient temperature.

5.) Dichromate seal while avoiding thermal shock and excessive temperature. Start with dichromate seal at ambient temperature. Insert part, then heat the solution to 155-160^oF. Maintain for 30 minutes, then shut off heat and allow solution and part to cool to 125^oF before removing. Lift part above solution and spray rinse with DI water before usual rinsing.

6.) Final Hot DI water rinse to be no more than 125^oF.

7.) Temperature of air drying cabinet to be no more than 160^oF.

8.) After receiving and inspecting the part, apply a PTFE-containing dry film lubricant such as Vydax.


Designing for hard anodize (hardcoat):
1.) Alloy selection-- see the brief Reference Guide at

http://www.anodizing.org/reference_guide.html,

also
Alloy & Design Considerations given at

http://www.mastermetal.com/white.htm

Comments: Of the extrusion alloys suitable for hard anodizing, 5xxx have best corrosion resistance but not much strength strength, 6061-, 6063-T6, etc. have good corrosion resistance and strength, and 7075-T6 has best strength but poorer corrosion resistance. Forget 2024, etc. -- softest hardcoat & worst corrosion resistance.

2.) Radius of curvature – avoid sharp inside corners and edges; these can cause spalling. For a 0.002” coating, at least 1/16” radius is necessary. See Table III in MIL-A-8625F ANODIC COATINGS FOR ALUMINUM AND ALUMINUM ALLOYS, available at

http://assist1.daps.dla.mil/quicksearch/

3.) Buildup – see Para 6.in MIL-A-8625F. About 0.001” of aluminum is consumed in forming a 0.002” coating, so the buildup is only 0.001” (per surface). However, a threaded hole get buildup on the face of each thread, so either oversize it or have the plugged during processing.

4.) Sealing. Hardcoat is (non-dyed and) unsealed unless otherwise specified. Sealing improves corrosion resistance but softens the hardcoat by about 30%. Most anodizers use a nickel acetate sealing solution at about 170^oF. A dichromate seal gives better corrosion resistance and also improves fatigue resistance. I did some research with dichromate sealing in 1997 for a US Navy contractor & found not exceeding 160^oF avoided crazing. Metalast has published abrasion and salt spray test results for different hardcoat seals which revealed surprisingly poor salt spray test results for nickel acetate sealing, which was attributed to crazing of the hardcoat –
”Performance Results for Sealed Type III Anodic Oxides” by Tami Westre et al. of Metalast* (2001).

From a following study, “Sealing: Enhance Anodic Coatings’ Performance” by Ling Hao, Ph.D of Metalast* (2001):

p. 8 “Only dichromate sealing and silicate sealing, among the traditional sealing processes, appear to be able to meet the requirements for the applications of Type III anodized aluminum. Dichromate sealing is especially capable of enhancing paint adhesion and minimizing the loss in fatigue strength of anodized aluminum, apart from improving the corrosion resistance.”

p. 11-12 “The crazing of anodic coatings may happen when the anodic coatings produced at a low temperature and a high current density are rapidly transferred to a sealing solution operated at high temperatures via a cold rinse..... the crazing of anodic coatings in sealing is mainly caused by the tensile stress built up in the anodic coating, stemming from the considerable difference in thermal expansion coefficient between anodic coating and substrate aluminum....Crazing is substantially reduced as sealing temperature is lowered. A short warm rinse between a cold rinse and a high temperature sealing treatment can minimize the occurrence of crazing in a subsequent sealing process.”

*Metalast has a good on-line anodizing library (some articles pertain to advantages of their proprietary anodizing additive & process). Registration required:

http://www.metalast.com/documents/doc_library.html#Reports

Additional on-line anodizing references:
Some simple FAQs at

http://www.anodizing.org/faqs.html

The Anodizing & Light Metals forum at finishing.com:

http://www.finishing.com/Letters/index.html#2a

(you can also search their Archives)

Forum at the Aluminum Anodizers Council:

http://www.anodizing.org/

http://wwwboard/wwwboard.html

Articles from the Aluminum Clinic in Products Finishing magazine:

http://www.pfonline.com/dp/clinics/index.cfm?cat=AF06&zone=ALUMFIN

A scientific study of the anodize mechanism & structure:

http://www.nucotec.de/pdfs/AnodicOxideFilm.pdf

International Hard Anodizers Association. A rather small group of high quality anodizers & researchers interested in improving hard anodize coatings. Interesting symposium abstracts:

http://www.ihanodizing.com/program.pdf

Check their List of Members to see whether any are nearby:

http://www.ihanodizing.com/

Apticote Hard Anodising. Description of hard anodize. Mentions wear resistance; also option for improved fatigue & corrosion resistance.

http://www.poeton.co.uk/w1/a300.htm

P.S. Electroless nickel looks nice and is very low wear, too.

 

[DReimer]

Wow. Thanks for all the info. It will take some time for me to digest it all!

We will have the prototype version ready to test next week. In the event we are concerned with cracking or wear, we have actually found a urethane coated bearing that look like it would work exceptionally well in the application. It would add about $15 to the assembly, but would make it quieter and smoother as well.

 

[unclesyd]

You might want to look at the Aluminum Anodizing offered by these people to help with friction factor. I have used this process for several components with excellent results.

http://www.tiodize.com/anodizing.html

I worked on the idea of an added lubricant on several other anodizing processes by coating after the anodizing. I used several different baked on lubricants with very good success. I found that a vacuum coating/impregnation process gave the best results.

kenvlach
Have you ever looked at this process?

http://www.microplasmic.com/

 

[kenvlach]

Dean,
With the PU-coated bearing, the hardcoat I as described should last ~forever.

Most hard anodize failures I have examined were due to poor initial design, improper processing or corrosion. Few were due to actual abrasion. Note that the food processing industry uses alkaline cleaners, sanitizing chemicals and frequent washdowns, so it must be considered a corrosive environment.

Dichromate sealing is recommended for improving the corrosion resistance of hard anodize. It also reduces the tensile residual stress within the hardcoat, which improves fatigue resistance & minimizes cracking at the cost of about a 30% decrease in hardness. The surface is more slick than unsealed hardcoat, so easier to clean and avoid stains. Some paints do not stick well, so testing is advisable. With epoxy primer and PU topcoat, it is used for long-term underwater listening devices by the US Navy.

unclesyd,
I am trying to keep the cost down. Some anodizers do the mid-temperature hard anodizing as their normal process; e.g., some of the coating baths in Paragraph 6.8 of MIL-A-8625F. Likewise, dichromate sealing is readily available (although mistakenly performed at higher T).

I like the micro-arc processes for oxidation coating Al, Mg & Ti. There is usually a porous outer layer which is ideal for baked-on PTFE coatings. Some well-known brand are Keronite®

http://www.keronite.com/

and KEPLA-COAT®

http://www.ahc-oberflaechentechnik.de/ahce/products/coating_service/kepla_coat.htm

[informative website].

 

[BohdanL]

Hi, Ken

first thanks for the nice information in your answer about anodizing. Could you please look at my question? I am working as researcher at university. One of my subject is effect of hard anodizing on fatigue. Here the residual stresses can be crusial. Knowing that after hard anodizing tensile residual stresses are developing in the coating i am still trying to find reliable references to confirm this. Could you please help me, if you have any references were the stresses were measured, or may be advice suitable method to measure

thanks in advance

Bohdan

 

[kenvlach]

Hi BohdanL,

Welcome to Eng-Tips.

I don't have much time today, but can give you a start:

1) Nearly all of the Al fatigue work involves aircraft alloys such as 2024 & 7075 and newer versions, so search aluminum & aerospace sites & NASA.

2) For fatigue applications, the Al alloy is shot-peened before anodizing. This minimizes progagation of cracks from the anodize to the metal.

3) The easiest way to measure stress in a coating is to coat one side only of a thin, flat test strip. The resulting curvature can be used to back-calculate the stress. See

A Practical Guide to Understanding, Measuring and Controlling Stress in Electroformed Metals at

http://www.finishing.com/Library/stein/strspapr.html

These people sell a sample holder and have some more info:

http://www.specialtytest.com/

Ken

 

[REMDESIGN]

We design and build pipe line inspection tools. Our parts are made of Al.7075 & Hard Anodized. Do you know about any coating/treatment for making parts H2S and NACE BRINE compatible?
Please let me know ASAP,

Tank you,
Lev

 

[REMDESIGN]

We design and build pipe line inspection tools. Our parts are made of Al.7075 & Hard Anodized. Do you know about any coating/treatment for making parts H2S and NACE BRINE compatible?

Please let me know ASAP,

Tank you,

Lev

 

[kenvlach]

REMDESIGN,
Is this at ~ambient temperatures?

I don't have experience regarding Al use in pipelines.

For marine applications (both harbor & ocean bottom), we used the following: Al 7075-T6 forging, hard anodized with (lowered temp. as above) dichromate seal, 2 coats of epoxy primer (MIL-PRF-23377; MIL-PRF-85582 is similar), and a polyurethane topcoat (I think it was MIL-PRF-85285).
Probably Ameron or others sell marine grade paints for less than the aerospace paints mentioned, but many primers have difficulty sticking to sealed anodize. Test the adhesion.

The MIL specs. & associated QPL's can be obtained at ASSIST:

http://assist.daps.dla.mil/quicksearch/