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
or similar: Apticote Hard Anodised Composite Coatings:
“PolyLube:”
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
also
Alloy & Design Considerations given at
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
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:
Additional on-line anodizing references:
Some simple FAQs at
The Anodizing & Light Metals forum at finishing.com: (you can also search their Archives)
Forum at the Aluminum Anodizers Council:
Articles from the Aluminum Clinic in Products Finishing magazine:
A scientific study of the anodize mechanism & structure:
International Hard Anodizers Association. A rather small group of high quality anodizers & researchers interested in improving hard anodize coatings. Interesting symposium abstracts: Check their List of Members to see whether any are nearby:
Apticote Hard Anodising. Description of hard anodize. Mentions wear resistance; also option for improved fatigue & corrosion resistance.
P.S. Electroless nickel looks nice and is very low wear, too.
