Aluminium discoloration

[neat2]

When I clean Al plates (AA5754) with trichloroethylene (TCE), I experience discoloration, which I do not understand how it forms. I would like to know the mechanism of how the discoloration forms and how can I avoid it. For your information, I am using the right grade of TCE as recommended by the supplier and I have been using it for long time.

 

[TVP]

Perhaps the TCE is reacting with something on the surface of the aluminum plate. Do you know what coatings, lubricants, etc. are on the plate?