-- The Home Page of the Finishing Industry
A website for Serious Education, promoting Aloha,
& the most FUN smiley you can have in metal finishing.
    no popups, no spam
on this site
current topics
topic 3985

Aluminum discoloration, pale yellow to dark brown



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


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


Aluminum oxides may appear in different colors depending on hydration states and other variables. Look in a "CRC Handbook of Chemistry & Physics" [link is to info about book at Amazon] 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


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 rinse in water. Does any one have any idea what causes this discoloration

Robert Tan
- Kuala Lampur, Malaysia

May 6, 2011

I found the same gold brown coloring on aluminum surface. The material was tested for 24 hrs in Demineralised water of 40 degrees 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 From the Netherlands

O Wille
- Petten, Netherlands

This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site

ADD a Comment to THIS thread START a NEW threadView CURRENT TOPICS

Disclaimer: It's not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices

©1995-2018, Inc., Pine Beach, NJ   -   About   -  Privacy Policy
How Google uses data when you visit this site.