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Aluminum discoloration, pale yellow to dark brown

Q. Hello,

I'm seeing discoloration (from pale yellow to severe dark brown) on the Aluminum surface in one our component. I made an EDX analysis, and all I found out is Oxygen (high level) with Aluminum of Course and a little trace of Carbon. These came after dipping the part on an Ultrasonic Rinsing tank for 45 degrees C at 3.5 hours.

Does anybody know what causes this discoloration? I thought aluminum oxides appear white (if its oxidation).


Avelino Buenafe
- Shenzhen, People's Republic of China

A. Hi Avelino,

The detecting limits of EDX on elements range from 0.5 to 1%. This means that it is difficult for you to find out the trace contamination that causes the surface discoloration of your aluminum parts if the contaminate contents on the surface are below the detection limits of EDX. SIMS may provide you with sufficient information.

Good luck.

Ling Hao
- Grand Rapids, Michigan

A. Aluminum oxides may appear in different colors depending on hydration states and other variables. Look in a CRC Handbook of Chemistry & Physics [adv: on Amazon & AbeBooks & eBay affil links] if available for aluminum compounds and their colors.

The colors that you see may also have as much to do with the surface finish as the color of the material on the surface. A microscopically rough surface will appear dark next to a polished surface due to light scattering.

I will agree with Ling in principle, but not on details. EDS detection limits on homogeneous samples can be better than 0.1%, although not so good for elements at the light end of the detection range. Current detectors can detect chlorine (a common culprit for aluminum discoloration) at about 0.1%. Your problem with detection and one that Ling has addressed previously may be that a thin film of contamination would not be easily detected by EDS.

As Ling suggests, SIMS is a good technique for detecting thin films and low concentrations. The problem with SIMS in this case may be that the detection limits are too good and the technique cannot be quantified. SIMS would detect 2 ppm of chloride and you couldn't tell for sure whether it was 2 ppm, which is probably not an issue here, or 2000 ppm, which would be significant. I would suggest x-ray photoelectron spectroscopy (ESCA or XPS) as an alternative for further study. The technique will analyze thin films (20 A) and detection limits are good enough (about 0.1%). In addition, XPS will give you information about the compounds present, e.g. whether the oxygen is aluminum oxide, aluminum hydroxide, etc.

Good Luck.

larry hanke
Larry Hanke
Minneapolis, Minnesota

Q. I would like to know what causes such discoloration of Al surfaces. Currently my company is running a product, upon ultrasonic cleaning in a alkaline solution, it discolors when rinsed in water. Does anyone have any idea what causes this discoloration?

Robert Tan
- Kuala Lampur, Malaysia

I found the same gold brown coloring on aluminum surface. The material was tested for 24 hrs in Demineralised water of 40 °C.
Only flat plate areas were brown, while other parts in the same assembly stayed gray aluminum.
pH of the water was approx. 7 and the electrical guidance parameter is 0,9 micro-Siemens, so the water is found pretty clean ?
Anyone who can explain the brown coloring ?
Will the coloring process be cumulative?

Onno Wille
- Petten, Netherlands
May 6, 2011

Steam turns aluminum yellow

Q. Hi, I would like to ask about aluminum memory substrate before nickel plating. Why the aluminum substrate surface become yellow when we expose the aluminum surface to steam?

Muhammad Nafis Sabidi
Uekatsu Industry (M) Sdn Bhd - Sungai Petani
October 3, 2019

A. In my opinion, aluminum forms with steam, a growth of the oxide layer. This very thin oxide layer can appear colored from light to dark depending on its thickness. It is not a natural color, but an interference coloring. The incident light is reflected in color by the non-colored oxide layer through the interference.

J. Beerli
- Hallau, Switzerland
October 8, 2019

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