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topic 40535

Problems and Issues Hard Anodizing of 7075 Aluminum

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A discussion started in 1998 but continuing through 2018


Q. Does anyone have any advice on hardcoating 7075 alloy aluminum? We hardcoat 1" dia rings at 40 F, 18% sulfuric acid, with a proprietary hardcoat additive. Occasionally the parts will burn in the hardcoat tank ( applying .002 thick hardcoat dyed black. I suspect the alloy is the problem since we have little problem with other customers parts of other alloys. The parts are being racked on 6000 series fixtures. Have tried ramping up the current slowly at the beginning of the hardcoat cycle with some success. Would a rack made of the same alloy as the parts help? Any other ideas? Thank you.

Keith Rosenblum
plating shop - St. Paul, Minnesota

"Surface Treatment & Finishing of Aluminium and Its Alloys"
by Wernick, Pinner & Sheasby
from Abe Books
info on Amazon


A. Keith

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

Charlie Grubbs
- Alpharetta, Georgia


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).

Henry Wolff
- Delta, B.C., Canada

To minimize your searching efforts and to offer multiple viewpoints, we've combined some threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.

Hardcoating 7075 Aluminum Alloy


Q. I have a problem with coating hard anodizing on aluminum alloy 7075: the coating looks good when the anodizing is complete. The problem was when the coating needed to be machined down to blueprint size the coating started to peel off as if the coating didn't bond to the aluminum. I need to know if aluminum alloy 7075 can be hard anodized?

Albert Romero Jr
- Phoenix, Arizona, U.S.A


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!

milt stevenson jr.
Milt Stevenson, Jr.
Anoplate Corporation
supporting advertiser 
Syracuse, New York

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Q. I have been having trouble Hard Anodizing 7075 alum, I seem to look great while the parts are wet but after drying the parts look very dusty. The temperature in the anodize tank is 45 °F, volts 25 to 30, time 60 mins.
What's my problem ?

Richard Harrison
metal finishing - Lynn, Massachusetts



A. Hardcoat is done by AMPS per SQUARE FOOT, typically 36 a.s.f. The power must be ramped slowly and the voltage steadily increased until a set voltage, usually typical thickness is achieved at a set terminal voltage - 2 mils at 70 volts, etc. The ramp and characteristic voltage/thickness relationship is highly alloy dependent and 7075 is not one of the easier alloys to hardcoat - the dustiness sounds like early onset of burning. Lastly, 45 °F is rather high for hardcoat - try to get it down to 32 °F.

milt stevenson jr.
Milt Stevenson, Jr.
Anoplate Corporation
Syracuse, New York


A. Tell us more. Surface Prep, Free Acid, Dissolved Aluminum, racking, thickness of part, current density, ramp time to c.d...

Some 7075,depending on heat treatment at the mill or subsequent heat treatment, may have precipitated the zinc into the grain boundaries, then when you anodized, it pulled out (dissolved (unplated) like the anode in a zinc plating tank).

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide


A. It's relatively easy to hard anodize 7075. These seem like Type II conditions. Perhaps too high a temperature, insufficient agitation for parts cooling, improper current density (e.g., from excessive initial & inadequate final voltages)... Please describe the following:
Current density.
Voltage (initial voltage & ramp rate).
Thin sheet or any unusual geometry.
Anode-to-cathode area ratio.
Desired and actual anodize thickness.

Ken Vlach
- Goleta, California
contributor of the year honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.


A. It is powdery,right?
if possible, adjust to bath temperature: -3 °C; current density: 32A/ft^2

John Hu
- Singapore


A. Let's See ... If the coating is of a gold-ish color and the dusty like effect is of the alloy and is correct. Remember that the color of the hardcoat is due to the alloy materials.

Chris Snyder
plater - Charlotte, North Carolina


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.

Steve Tanner
- Garland, Texas

January 21, 2017

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.

James Kokiel
- Bristol Connecticut

Hard anodizing of 7055 aluminum alloy

August 13, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Does anyone have any experience in hard anodize on alloy 7055 ?



August 2014

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?


Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
We need "Aloha" now more than ever

Sodium Sulfonate Additive for increased corrosion resistance of anodized aluminum

August 29, 2015

Q. I am undertaking a research on anodization of 7075 aluminium alloy. Actually I want to use sulfuric acid anodization with organic acid additives. I don't know which of the sodium sulfonates to use as additive. Is it also possible to use more than one additive in the anodization? The research is basically to improve corrosion resistance of the alloy.

Abdullahi Mohammed
Student - Minna,Niger,Nigeria

August 2015

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


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
We need "Aloha" now more than ever

November 2, 2015

Q. I have anodized 7075 and it has gone green. Any ideas on why?

David Catchpole
- UK, London

November 8, 2015

?? 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?

Marc Green
Marc Green
anodizer - Idaho

September 9, 2016



October 30, 2018

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.

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

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