Hard Anodize Problem. 2XXX Aluminum Burning

A. Sir,

2024 is a high copper alloy & cannot be hardcoated in conventional hardcoat systems. Here in our facility we use 2 different hardcoat systems, Martin HC & Imperv-X HC. Martin will process most alloys BUT NOT high copper ones. The Imperv-X system is specifically designed for high copper alloys and we hardcoat them daily with no problems.

A. Richard,

Your information is not complete which makes an answer difficult or at least required an assumption to be made which is not good.

Al 1.0 oz/gal or g/l or ? Sulfuric Acid ounce by weight or fluid ounce?

Normal hard anodize is 32 °F. Are you using a proprietary or an additive? Straight DC or superimposed AC or pulse?

The high copper content in your alloy makes the anodizing more difficult, but you should be able to get 1 mil.

If your final voltage is 16, you are hard pressed to call it type III anodize, and 1 mil will be very hard to get.

Include all of the process variables and it will be easier for people that are more knowledgeable than me to help you.

Jim

A. Your processing parameters are more appropriate for an anodize than a hardcoat. I doubt that this coating would meet the minimum wear requirements for a Type III hardcoating per MIL-A-8625. This could be contributing to your difficulties in achieving the desired thickness. Typical hardcoat anodizes utilize higher voltages and current densities with lower solution temperatures. The trade-off for these harder, thicker coatings is an increased tendency for "burning", particularly in the 2XXX series alloys. Special procedures and process controls can help minimize this phenomenon. Also, proper design and placement of auxiliary cathodes will help with coating uniformity.

Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.

Wadsworth, Ohio

Hi Mehdi.

By far the most complete reference to every type of anodizing is The Surface Treatment & Finishing of Aluminium and Its Alloys. It's a large, expensive, 2-volume book ... but hopefully there's a copy in a public or university library. It covers everything you can imagine =>

Regards,

A. 2024 and especially heat treated 2024 requires higher free acid, try 220 gm/L, then maybe 230 gm/L, let us know.
Always ramp 15 minutes up to a pre-determined 24 amps per square foot amperage then switch over to current control and let the voltage climb as the resistant thickness builds.

A. Hi, Ali.

In my very limited experience this is usually caused by poor contact. Thus the item initially gets little or no anodizing thickness, then when the voltage goes high, it breaks through whatever resistance was on the contact surfaces and exposes thinly anodized or non-anodized parts to 45-50 V and large currents, which immediately causes burning.

Regards,

A. How are you measuring hardness? When Reynolds published their first work on the glycolic/glycerin additive they got a average of 320 and a max of 365 Knoop on 2024-T3 at 1.8 mils and avg. 347 and max of 376 Knoop at 2.3 mils. So 300 is attainable. Be reminded that the Knoop diamond comes in on the cross section -- if close to the bottom of the thickness where the pore is small it gets a higher thickness, but when it comes in near the top of the inverse taper pore, it crunches and shows a lower thickness.
Now for ways to get a smaller pore (read harder): lower temperature, lower free acid, higher current density, and 4%/vol of a 50/50 glycolic/glycerin additive. Note also that all other allows get harder than the 2000's.