Replacing silver metallization with earth-abundant materials in Si solar cells is a critical step towards the sustainable growth of photovoltaics. In this work, we investigate fire-through processes using Al for the metallization of solar cells with p-type passivating contacts, with the goal to achieve a thin depth of contact, oppositely to what is used for standard back-surface field Al contacts. The interactions taking place during firing between the different elements in our contact stack (Al, SiNx:H, SiCx(p) and tunnel SiOx) are studied. We discuss how Al and SiNx react during firing by performing contact formation through different nitride layers, and show that increasing the N content of the silicon nitride can reduce Al penetration depth. Finally, we also investigate the mechanisms behind metal-induced passivation degradation and identify possible ways of mitigating them by employing an adapted SiOx/SiCx stack to allow for Ag-free metallization of tunnel oxide passivating contacts.