Different Mg/SiC ML samples were fabricated on Si wafer substrates varying the structural parameters and/or the capping layer material and its thickness. They were characterized in the EUV range, at 121.6 nm, and in the visible spectral range a few weeks after deposition. Their performance was verified years after deposition, a period during which the samples were initially stored in a sealed plastic box (3.5 years) and then in boxes exposed to regular atmosphere (five additional months). After four years, samples show a large surface modification, probably due to Mg corrosion, which affects their reflectance in the visible spectral range and at 121.6 nm. Nevertheless, the performance in the EUV is not related to the degraded surface characteristics, since some samples still perform well, despite the observed alteration. The degree of degradation is likely to be related to the structural parameters of the multilayer and the characteristics of the capping layer. In particular, all the samples with a Si top layer underwent a complete degradation, samples with a SiC top layer deposited over the last SiC layer exhibit unaltered performances, and samples with modified ratios show partial degradation, which can be associated with the thickness of Mg in the bilayers. A detailed AFM analysis shows that the physical process responsible for degradation is present in all samples at different evolution stages, and thus further reflectance reductions have to be expected in those samples which are degrading more slowly. The processes occurring on the surface are compatible with a large interdiffusion at the interfaces and, at the top surface, oxidation of Mg itself. This fact has been confirmed by XPS analysis, which revealed the presence of Mg in the top layers. A potential solution to slow down this process might consist in increasing the SiC top layer thickness for protection, or adding a barrier layer between the SiC and the first two Mg layers.