The present invention refers to a sensor unit (10, 10′) for an electronic module (1) which is small and lightweight comprising at least one exposed component (5, 5 a, 5b) containing a chemical element and/or a chemical compound at its surface which is adapted to undergo a color change observed in visible light due to presence of a corrosive, the exposed component preferably containing elementary copper. The invention further refers to a respective electronic module (1) and a procedure to calculate the level of corrosive exposure of such an electronic module (1).