World's #1 finishing resource since 1989
No login needed: Chime right in

topic 58771

Spotting on aluminum parts with chromate finish

Current question and answers:

March 1, 2021

Q. Hello,

While looking on the internet I came across a thread from a few years ago, it's a problem we are experiencing today so I thought I would reach out? I am looking for someone that can verify what foreign substance is on some product we produced and had Chromate Treated (MIL-DTL-5541F Type II, Class 3 Clear).

I have attached a picture of the defective part:

  

We have several parts that exhibit this condition, it is not a common issue but relegated to a point in time which pointed me back to the supplier (not a very good supplier). I am sure we have solved the issue by switching suppliers to a (NAPCAP) supplier with much better process controls in place. My question is this:

The parts in question are used in a medical application, our customer would not be concerned about the visual but has asked me questions such as "What are the stains/black spots?, Can they migrate? and What could have caused this?"

Do you have a way to test parts in this condition? I ultimately need to convince the customer that all is OK and that what we are seeing is a visual surface issue and not anything we are introducing (for example) into the air? My initial comment to them was that I believe this could be a reaction to Machine Coolant that perhaps was left on the parts too long after manufacture but I am really not sure. At the end of the day I need to understand what is on the part (scientifically) and to determine if it?s under the coating or on top of the coating?

Can you help, if so what is the cost to test ?

Lou Mautone
Elk Grove Village, Illinois
^- Reply to this post -^

---

---

---

Previous closely related Q&A's starting in:

span class="date">July 8, 2008

Q. We make a 6061 aluminum part that is clear chromated.

The customer has complained that the chromating is turning dark under the washers used for mounting (steel, yellow zinc)

What would cause this and how do we prevent it.

Mike Wilson
sheet metal fabrication - Kernersville, North Carolina

---

July 11, 2008 A. Use cadmium plated fasteners instead of zinc. If they have a cow about that, have them try stainless steel, 18/8 and not 400 series. James Watts - Navarre, Florida July 11, 2008 A. It is simple galvanic corrosion. "Clear" chromate is the most overrated and mis-used title in our industry. Use heavy yellow chromate on the zinc plated washers and heavy yellow chromate on the aluminum surface. Eventually with temp and humidity cycling, that too, will fail. Robert H Probert Robert H Probert Technical Services Garner, North Carolina

---

---

To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)

---

---

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 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 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

---

---

---

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.

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

---

---

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

---

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 Waste Water 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 H Probert Robert H Probert Technical Services Garner, North Carolina

---

---

Clear Zinc Chromate has 'burn mark'

September 11, 2020

Q. Hi all, I am a quality engineer working in Singapore.
I encountered an issue where goods received after plating had a "black line with red-ish at inner".

What will be the most possible for such defect occur?
Visually the defect look like a "burn marks", however my plating contractor mention this is due to porosity trapping chemical and the defect forms while time passed. Any idea?

Gray Lee
- Kallang, Singapore