Problems and Issues Hard Anodizing of 7075 Aluminum

A. I thought I'd update our progress in the hope it may help someone else.

We have tested multiple process adjustments (especially extending rinse times and double dipping etc) without much success until recently. The parts are typically dyed for 20 minutes, but when we reduced the dye time to 15 minutes, the spots worsened. However, extending the dye time to 30 minutes has nearly eliminated the issue.

Before we noticed the dye time improvements, we noticed that larger runs had less of a problem, a run of the same material at 250 amps, vs the same material but double the surface area at 500 amps seemed to reduce the spotting.

We are trying to understand why increasing the dye time has had such a significant impact. One possibility we considered is that the gold spots could be caused by trapped acid within the pores, and the extended dye time may be allowing it to fully bleed out, enabling better dye absorption.

Cheers

A. Thanks Darren

Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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A. Keith

Contrary to what you may have heard, 7075 alloy is not that difficult to hardcoat.

A. To hardcoat 7075, try running the bath colder (we run at 14 °F). Also, make sure you have adequate agitation in the tank. After ramp, your voltage should be approx. 21V (not to exceed 30V).

A. Keith. Parts I worked with in industry the anodizer said about.001" per side. Most of what I worked on had to be post finished or polished. The anodizing was like flat paint. It was lapped or finished as required. In effect a very expensive process due to final fit at assembly.

What little I had done on racers was bout the same. It was a very hard surface but worked well. At least the coater or anodizing company was consistent. You could machine parts undersize to allow for it and use as is if possible. There are other durable coatings now that are just about as wear resistant and hard. Ace pistons have this and it takes some bad things happening in the motors to see scuffing now days.

A. Done all the time! One customer of ours routinely hardcoats 1/2 mil over size and post anodize hones parts to size. Cylinders are 7075 and since they're for aerospace industry, quality is paramount. They don't peel or crack!

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A. 200 g/L is OK, would be harder at 165.3 g/L. Dissolved aluminum at 8 g/L is OK, but set up a range and decant and add to hold within that range, I generally recommend 8 to 12 gm/L. Start by looking at the voltmeter at 8 volts, 5 minutes later raise to 16 volts, 5 minutes later quit looking at the volt meter and look at the ammeter and raise to 24 Amps per square foot(2.58 amps/dm2 to you).You MUST know the working exposed surface area or use a 1 sq ft panel of 7075 thru a separate 50 amp ammeter and set 24 amps on it. Set the rectifier on automatic current and let the voltage raise itself to hold the current constant. 0.002 should take about 50 to 60 volts and 75 minutes. After that the rate of thickness will fall off as the voltage goes higher and higher, I am not prepared to predict the top amperage and time for the 0.004 (100 microns).

A. In response, can the voltage be ignored by using the constant current setting on the rectifier. Setting the amps per sq dm at 2,5 from the beginning and let the rectifier control the voltage.

Thanks,

A. I too would like to understand the reasoning behind "8V and then 16V", before anodizing at the required current density. These voltages are meaningless without specifying the current densities they represent. I'm guessing that the intent is to anodize at the beginning of the process at a lower current density, if so, what is the purpose of this? Current limiting (to prevent burning) isn't the answer, a properly designed constant current rectifier will provide ideal current limiting; it needs no help from a forced lower voltage, or from the reduction in electrolyte conductivity caused by dissolved aluminum.

Ed. note: it is our sad duty to advise of the passing of Paul Yursis in August 2005.
Here is a brief obituary by Mike Caswell.

A. It is rather easy to hard coat 7075 we use a bath at 46 °F; we use 35 ASF 45 min and get 2 mil we do use an additive and lots of air agitation.

A. Your tank is running too hot! Try at 36 degrees and high agitation. Make sure you have tight contact on racks. Also use aluminum racks not titanium racks. Sometimes you have a better finish and better contact.

A. Apparently 7055 is not as commonly anodized as 7075 :-)

We have more than a dozen threads on this site about hard anodizing of 7075, but I'm not sure if they're highly relevant or totally irrelevant to your issues with 7055. What is your situation?

Regards,

? Hi Abdullahi. What prior research or experience points to this as a possible route to improved corrosion resistance? Thanks.

Regards,

?? No idea on why without more information.

Was the part dyed?

Sealing medium?

Coating thickness?

Environmental conditions (lighting conditions inside?, outdoor exposure?, temperatures?, etc.) part was exposed to after anodizing, and for how long?

A. Hello Ms. Balcazar,
Photos of the patches showing their shape, orientation relative to sharp edges or rack points, smeariness of margins, etc. would help.
When are the marks visible? What is the first process step that you can see them?
Also without any information whatsoever on your operating conditions and process steps, any answer the community might have will be no more than a guess.
Please provide some basic information to help us help you.

A. Hi cousin Nadeem. Apologies, but more words please !

Starting totally from scratch with what facilities, processing lines, accessory equipment, knowledge, chemicals, testing, etc. are required is the subject matter of the many books listed on this page. I am quite certain that we don't have to start from nothing, and that your question is far more specific than that, but there but there is no way for me to know where to start until you introduce yourself and your situation. Thanks!

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free pdf is currently available from academia.edu

However, on the off chance that you truly know nothing about the subject, please read our "Introduction to Anodizing", search this site for "hard anodize 7075", and view the Anodizing chapter of the Metal Finishing Guidebook -->

Luck & Regards,