I have a problem when machining aluminium in my machine shop. There are times when the machines aluminium surface turns yellowish (somtimes brown). I do not know what is the main cause of the problem. Is it due to the coolant used?Aluminum oxides may appear in different colors depending on hydration states and other variables. Look in a CRC Handbook of Chemistry Physics 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.Other related question