Thermoelectric technology has witnessed a resurgence in recent years due to increasing demands for sustainable energy sources and efficient cooling systems. Recently, the introduction of Te-free thermoelectric modules using non-toxic, abundant materials including p-type MgAgSb and n-type Mg₃(Sb,Bi)₂ marked a significant breakthrough. Despite promising performance, questions persist regarding long-term robustness and stability, especially in harsh environments. In this study, a thorough exploration of thermoelectric modules is conducted, focusing on their performance degradation under various conditions. Through elemental mapping analysis, degradation mechanisms are identified within the modules during cycling in argon environments, where atomic migrations and the formation of complex oxides at contact regions are key factors. Furthermore, cycling tests in air reveal significant degradation, prompting the exploration of protective strategies. Surface coatings using atomic layer deposition (ALD) emerge as a promising solution, particularly by HfO₂, demonstrating superior protective effects. Furthermore, re-soldering effectively restores module performance is found, highlighting the importance of developing advanced soldering techniques to promote magnesium-based thermoelectric technology as a sustainable alternative to Bi₂Te₃. These findings emphasize the importance of exploring novel contact materials and demonstrate the potential of ALD as a universal approach to enhancing module reliability and robustness.