Hard Anodizing Defects: stains, flow marks, color problems

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.

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.

A. Before I attribute this to material defect, I'd like to ask few questions:
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.

A. Dean

You should consider the coating defective if it does not meet established acceptance criteria.

Is the coating failing in some manner?

A. Seems to me that black would mean black, so I would assume that if the term "black" is used, it would be implied that the coating would be dyed, as opposed to "natural".

A. Because of it's naturally dark color, many people mistakenly assume that hardcoat anodize is dyed. Because of this confusion and to insure that they are getting hardcoat and not clear commercial anodize, often "black" hardcoat is specified. We always clarify this with our customer. Natural hardcoat on 6061 will appear dark grey to black (Class 1). To get a deep black, a secondary dye step is required (Class 2).

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.

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!
Ramajayam

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.

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!

A. Check for the thickness of the coating in the affected area, we suspect that the thickness may be low due to either (1) air (gas) gap, or (2) air agitation hitting that spot and displacing anodizing solution. Then, how do you test the sealing, is it ASTM B136?

A. We should separate source of problem;
1-during anodizing
2-during service

1-If your sealing bath or inorganic gold bath's pH balance out of limit and if you add ammonia ⇦this on eBay or Amazon [affil links] to these bath during a bar inside of bath you may see this appearance. My advice: check the pH values often and not to add any chemical if there is a bar inside of bath ,you can add chemicals to bath empty times.Also these additions should be diluted.
2-anodizing layer is robust between pH 4 and pH 8. If you use some chemicals which out of these values like high alkaline detergent derivatives for washing, cleaning you will see the similar appearance as other contributors mention. Also, oily chemicals causes similar appearance.
Good luck

A. Seems the second part made good contact and the first part broke contact and partially chewed away. Tell us the anodic coating thickness on each part.

A. Hi Josh,
Yup that's a defect that is impossible to have come solely from the anodize process... it just makes it more obvious as the caustic etch prep bares the sins of the mill. I always strongly encourage our customers, and anyone working with extrusions, to remove the outer mill layer mechanically prior to sending for anodize, whether through blasting, machining, or even sanding to the desired finish with alox grit. It might take some experimentation tp determine how deep the surface defect goes. Granted, some stock has defects deeper than a quick once-over will remove, but it looks like you could get serious improvement from a little extra prep.

A. I am guessing that these are castings. If so, this is likely porosity which is exposed by the etching and/or hardcoat process.