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topic 58771

Spotting on aluminum parts with chromate finish

A discussion started in 2012 but continuing through 2018

black spots on chromated part 1
Single top light

black spots on chromated part 2
Top light & LED side light. Note some areas appear white where before it was gold. Also some of the black seems textured.

August 6, 2012

Q. I'm looking for some help narrowing down what it is and the source of black spots on some chromated 7075 Al parts. I am the quality manager of the machine shop working with the plating shop to eliminate this issue.

Extruded 7075 T651 is milled then tumbled with ceramic media. From there it is sent to a vendor for "chemical conversion coating class 3 per MIL-C-5541" Most parts seem fine, but we find some with small black spots, or lines that generally follow the mill marks. In a few bad cases it looks like there is some tiny clear or white crystals forming in the area of the spot.

Photos thru a 20x microscope and handheld camera. Roughly 0.1" x 0.1" view
Dale Puch
Quality Manager - Orlando, Florida, USA

August 7, 2012

A. 1. I do not know how hot is T651, but any heat on 7000's moves the zinc out of alloy into the grain boundaries as molecular zinc (which forms white compounds with chemicals like sulfates, acetates, and others)

2. You mention "tumbled", well the media must be designated "aluminum only", and if any steel has been run (even on the night shift) with the same media, you will pound iron particles into the soft aluminum - which turn orange and brown.

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

August 8, 2012

Q. We only tumble 6000 and 7000 series aluminum in that tumbler and media.

Is there a good way to confirm this is caused by the zinc? You indicate this is a material issue. Do you mean the material may be out of spec, or just problematic?

Assuming zinc is the reason, what steps in the chem film process are most likely to be causing or able to control this. Especially since 90% of the parts seem fine. I can not change the material specification, but is there something I can request of the extruder for future material orders?

Dale Puch [returning]
- Orlando, Florida, USA

August 9, 2012

A. Since it is only occurring on 7075, then that points to the possibility that the zinc has moved -- not necessarily because the metal is inferior, but naturally because of having been heated (has it?). What kind of deoxidizer? Let's look at a piece with a 10X loop after the deox and see if we can notice a raise or a pit.

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

August 25, 2012

Q. No heating of the part other than 120 °F or so during parts of the chromate.
Lab test results for several locations on the same part

"The results of SEM-EDS analyses indicate that contamination during the conversion coating process is a likely cause of the spots. Analysis has not provided any evidence of coating rupture; therefore it is possible that deposits of Zn and Cu have later undergone reactions with the environment. The levels of Zn, Cu and O detected in certain areas suggest that both zinc and copper oxides may be present."

Table 1 -Semi-Quantitative Chemical Composition in Wt. %
Location O(%) Cr(%) Fe(%) Ni(%) Al(%) Zn(%) Cl(%) Cu(%)
Scraped off Dark Material 39.4 16.4 1.7 0.4 35.5 4.3 -- 2.3
Darker Spot 13.6 4.2 -- -- 77.4 4.8 -- --
Lighter Spot 10.6 1.2 -- -- 81.4 4.2 0.6 2.0
Scraped Area 5.5 1.0 0.2 0.1 86.4 4.9 0.2 1.5
Clean Area 5.9 2.4 -- -- 85.4 4.8 -- 1.4

Dale Puch [returning]
- Orlando, Florida, USA

August 28, 2012

A. Keep bringing us more data. I have two more thoughts. (1) Ask the chromater to increase the alkaline etch to remove the work hardened streaks where the metal pulled thru the extrusion die, and (2) be sure he is using a "mixed acid deoxidizer based on Ferric Sulfate" and also sulfuric acid, fluoboric acid, and nitric acid (this may help remove the zinc and copper from the surface before you get to the chromate. See if his deox is air agitated because ferric changes to ferrous unless air agitated, and ferrous does NOT deoxidize. Note that we chromate "aluminum" only and the aluminum chromate formulation makes copper black and makes zinc a dark dull gray. You see all that zinc and copper in your analysis, 7075 is usually over 5%/wt zinc. Earlier I asked how hot temper 651 was because if it was heated at the mill, then the zinc walked ("migrated") out of alloy and into the grain boundaries as molecular metal. Laying on top it reacts badly with the "aluminum" chromate formula.

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

October 15, 2012

Q. I have an electronic product manufactured from Aluminium, and chromated after that. When the product is delivered in a European country from India, small black spots on them were observed which has led to severe customer dissatisfaction.

black spots on chromated part 3

Please help me to understand what could be the black spots in the aluminium extrusions which are having chromated.

Arun Kumar
- Kochi, Kerala, India

White powder in alodine body

February 9, 2017

Q. Dear All,
We have an issue of white powder in Alodine process on aluminum body at inner surface, holes and edges. I have tried many ways like altering conc. of degreasing and activation, increase in time and flow of water washing, extra water washing after degreasing but still not found good result. Requesting for suggestion.

Raghavendra C H
Plating engineer - India

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White spots - chem film?

November 14, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Does anyone know what would cause white blurry spots on class 1A gold chem film? Could it be porosity in the material (aluminum casting)

Karie Foster
- Santa Ana, California, USA

simultaneous February 12, 2018

A. Hi Karie,
Casting alloys are harder to get an even-colored Chem Film on because of Silicon content. I would look 'upstream' in your process; specifically, to your deox tank. 7xxx especially will foul the deox with Silicates, that can then re-adhere to the part surface and render even-colored Chem Film coating difficult. Keep your total silicates in the deox below 30 ppm, and be sure to spot-check parts coming out with a clean kimwipe or white cotton glove and be certain there is no smut left. Focus on the surfaces where you notice spotting most often. I'll bet you see them in locations that have similar geometry.

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

February 12, 2018

A. "Silicon" on the surface will give a gray spot. There is no "silicate" in aluminum alloys 7000s but there is about 5% zinc which makes white salts.

Come back, tell us your chemical process line. I suspect you are going from alkali to acid and leaving salts in the pores which pop out when you hit warmer temperatures; and most salts of aluminum ARE white.

Try cleaning in a phosphoric/detergent acid type product (all cleaning companies have such a formulation). Stay out of the alkali etch because it increases the "silicon" exposure and decreases the aluminum exposure.

Consider using a real cast deox because a wrought metal deox will not work on high silicon casting. The book Aluminum How-To goes into more detail about pre-treating castings.

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

February 13, 2018

A. To clarify, if I may, since my wording was not that great; silicon only in its pure form is an alloying constituent of AL 7xxx. I of course acknowledge that silicates as such do not come from the metal. I start calling it that when it becomes "gross crud in solution/suspension that is interacting with other gross crud in same and messing up my nice tanks". Not too scientific. That's simply a general term for many Si-and-O-bearing, rather insoluble, substances and once a deox tank with plenty of oxygen has built up a ton of, well, gross crud in it you've got all the ingredients... settleable solids in a strongly acidic deox tank do happen when it's dirty enough. Free Si still in solution is the problem for later processing.
Please do take Mr. Probert's advice to heart. He's got more experience teaching this stuff than just about anyone!

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

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