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topic 44778

Stains in Hard Anodize of Type 6061 Aluminum

A discussion started in 2007 but continuing through 2018


Q. We are having a cosmetic issue with some parts we produce for a Medical Customer. We are having an external supplier perform a Hard Anodize with a PTFE coating per AMS AMS2482 [link by ed. to spec at TechStreet]. The raw material is 6061-T6511.

Essentially what happens is that on one face (and only one face) of the component we begin to see stains below the anodize (which is dyed black). These stains only appear when the parts are held at certain angles under direct light, but have resulted in rejections from our customer claiming the parts are stained when in fact it appears the anodic layer is not compromised in any way.

I visited the supplier for an audit to try and determine what condition was causing these stains. During processing they keep the parts entirely vertical so there appears to be no justifiable reason for the stains to appear on only one side of the component. While I was unable to get an accurate photo of the condition we are seeing in the anodized state, when we had the anodize stripped you can actually see the stains quite well.

You may view a picture of the stripped component:


I am wondering if anyone else has encountered this situation before and if they have a solid fix to prevent this issue in the future. I appreciate any help you can offer.

Jonathan Severino
Consumer (Quality Engineer) - Piscataway, New Jersey, USA


A. Is there any reason for these "stained" area to have been work hardened more than other areas? Is it on the same place on each part? The anodic pore structure builds 90 degrees perpendicular to the (mini) surface. If that surface is roughened, then the pores build at various random slanted angles and reflect a different light. If that surface is work hardened, then the pores build with a random pattern which looks like a "stain"

Have the parts been tumbled, blasted,or sanded, any of which leaves a uniform surface to the naked eye, but imbeds hard trash into the soft (Kleenex-like) aluminum. Also, non-uniform manual blasting looks good to the naked eye, but looks awful after being magnified by the hard coat anodic build-up.

Come back with some more info, pictures if possible, surely some of us can crack this one.

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. Jonathan

The symptoms you describe are characteristic of extrusions, although they usually appear as an "apple-core phenomenon" -- all the way through. If so, cut back on the etch time; and if permissible, dressing after anodize with a buff can help hide the flaws. Else manufacture from a wrought alloy.

Willie Alexander
- Colorado Springs, Colorado


A. Before I attribute this to material defect, I'd like to ask few question:
1. It happens only on one side, as you said, does it happen on the same orientation? let say one part's surface A racked facing to you got this mark, another part's surface B (instead of A)facing to you got this mark too?
2. Have you checked condition of surface of the part after pretreatment but before anodizing? any abnormality?
a. if abnormal, you need adjust process (especially etching time, deox time) and method of mechanical polish.
b. if nothing wrong, then most likely it is material fault as I am sure you would find (more easier to find also) the mark after hard anodize but before black dye.

John Hu
- Singapore


Q. The parts go through a brush finishing operation before going to anodize. That being said, the process is performed to all sides of the item and there is no reason for it to occur consistently on only one side.

Some responders have suggested that this might be attributable to improper quenching of the aluminum altering the crystalline structure. Is anyone aware of any screening that can be done before we go through all of the machine time and get all the way to anodize? My guess is there is no economical test that can be performed on a lot to lot basis.

Jonathan Severino [returning]
- Piscataway, New Jersey

June 18, 2013

Q. We have started getting some staining on our solid parts we anodize. We only anodize to esthetics. Occasionally we get this stain and haven't been able to really determine what it is. All of our anodizing tanks are circulated from one mother tank. It is isolated to one anodizing tank so we don't think it's any other pre-treatment issue.

Sulfuric 200 g/l
Aluminum 12 g/l
WM75LA 1%

staining on anodized parts

Thanks in advance

Scott Frazier, Anodizing Manager
pots & pans & trays - Spokane Valley, Washington, USA

June 20, 2013

A. I will guess that the rack hangs slightly different (3-axis) or the solution flow is not the same or the temperature varies in this tank more than the others.

James Watts
- Navarre, Florida

December 30, 2013

Q. We hard anodize some parts for a customer of ours. They are turned 6061-T6 aluminum with a tight bore through the center and some set screw holes thru to the tight bore. The set screw hole and the bore both get plugged prior to hard anodizing.

The past couple of runs we have been getting the parts back from the plater with a white crust that is building up around the plugged holes. Is this caused by the plater not washing the parts? If not what is causing this and how can we fix the problem so we don't have to wash every part by hand. Thanks.

Ross Braastad
- Ham Lake, Minnesota, USA

January 9, 2014

A. Hi Ross,
Most of the staining or salt build-up I have experienced in finishing operations generally is due to residual acidity or alkalinity or hard water rinsing mainly because of poor post rinsing.You may introduce a de-mineralized water rinse preferably hot before drying besides the usual counter-flow or cascade rinses.This may help avoiding corrosion or water mark after processing.
Good luck!

Subramanian Ramajayam
Subramanian Ramajayam
Bangalore, Karnataka, India

First of two simultaneous responses -- January 9, 2014

A. Obviously the plugs are leaking, then, as Rama said above, alkali bleeds in, then acids bleed in, then salts bleed out.

robert probert
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

Second of two simultaneous responses -- January 10, 2014





Cloudy stained hardcoat anodize

November 2, 2018

Q. I recently picked up a batch (72) parts from my local metal finisher. I specified:
Hard Coat / Black w/Teflon .002 (do not exceed .2672" on shaft / .2665" min.) (.2672/.2665) Rack with 4-40 threaded hole in stem 'only'
The raw material is 6061 and I hand deburr and polish these parts to soften the edges more. As a result of the polishing, the part has a nice clean shine. They were then cleaned in the ultrasonic cleaner, rinsed in clean water and hand dried before placing them in the parts box.

When I picked them up at the plant, I did a quick inspection. I found the parts to have cloudy staining and some even had finger prints on them. I brought this up immediately. They told me that was the PTFE. They tried to remove it and failed. And told me the only way was to buff it off. I don't think it was and feel there was something wrong in the process. I am a one-man shop and these parts are serious loss due to cosmetics.







What could have happened here? Is there any resolution?

Bill Milashius
Owner. - WHITE BEAR LAKE, Minnesota USA
  ^- Privately contact this inquirer -^

November 5, 2018

A. The water dip PTFE is a very expensive product and requires extreme care. Obviously floating soil has messed up the clarity of the coating, probably because of too many parts drag-in along impurities. What can you do? You need to contact the supplier of the PTFE for advice on how to recover, I cannot post his name here. I tell my clients to filter the top of the tank frequently with cheese cloth to keep the solution surface clear.

robert probert
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

November 7, 2018

Q. I have contacted the manufacturer of the PTFE. They said it was water soluble and should come off with hot soapy water. That didn't work. Neither did soaking the part in MEK for an hour. I did put it in the ultrasonic and it pulled the dye out, but still had the staining. The maker said if a piece of tape would stick to the part, their product was not on this part. I did this and the tape stuck.

Could this be a result of poor contact with the part while anodizing" Or a gradual loss of contact with the part over the process time?

Bill Milashius [returning]
- White Bear Lake, Minnesota

November 7, 2018

A. Hi Bill- Oh man what a mess, that is a huge bummer after putting so much time into your parts. Couple things to chime in here. There's more than one issue.

The fingerprints- If you were wearing gloves while handling and buffing your parts at all steps, I'd look at the plater's racking staff not wearing gloves- a big no-no on cosmetic parts, even more so when it's a highly polished finish. Usually it's not as big a deal on parts to be hardcoated since most of the time they will be caustic etched before processing, and lose enough thickness to get through fingerprint residue- chlorides and oils on the surface, neither of which you want going into an ano tank on parts - but you buffed and then ultrasonic cleaned yours before shipping to the plater, so they should have a clean surface. Based on your description of your prep steps, if you did not handle those parts without gloves at any time, IMVHO it is on the plater to remedy this.

Is a total strip and replate an option considering your dimensional requirements?

Also, were parts handled by any operator between hardcoat and PTFE dip, and how do they guarantee that the Sulfuric acid is completely removed from parts before immersion in the teflon tank? Water based PTFE suspensions are sensitive to pH and operate ideally at pH 9-10.

Gonna have to agree with Robert here that waterborne PTFE tanks are a very delicate thing. They attract soils and greases like moths to a porch light! I skim ours by hand regularly, let the skimmed off material settle out in a bucket, and literally throw away the top layer. It's just too twitchy to take any risk of oil contamination with that stuff, and it doesn't take much to cause problems! Those little bubble patterns around one of the features in your pics make me very suspicious of grease.

It might not be a bad idea to send out a couple test or scrap pieces- Same material, same deburr and buff process, same cleaning steps, same handling protocols- and ensure that the plater is in fact capable of running this process correctly before committing to a whole load of good parts. Even a few 6061 test panels polished to the same standards and sent out in pristine condition will tell you SOMETHING ...

Good luck!

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

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