It is a RAM module from a MacBook freshly obtained from one of Apple distributors; so not touch by any customers yet. Nevertheless, the notebook is working fine. This was identified because a customer wanted to get additional RAM installed in his machine.
François, another reader, provides us additional information:
We have identified the same phenomenon in 3 black MacBooks. RAM modules were covered by such "corrosion" or rust. After analyzing it closer, it turns out to be...thermal paste! It is possible to wipe it away from RAM modules. Then, once opened, we could notice spots of thermal paste over the motherboard at several location.So nothing bad, but Apple should maybe look a bit pay more attention to the way manufacturers are assembling MacBooks.
