We have explored the effect of the presence of the sulfide layer on the corrosion performance of sulfidized brass-coated steel cords extracted from tires and squalene formulations. The corrosion performance of sulfidized brass cords extracted from various rubber formulations was tested in 3% NaCl electrolyte solution using DC polarization techniques. We found that the copper sulfide layer made the sulfidized brass cords passive and protected them from corroding in severe corroding environment (3% NaCl). The degree of protection offered by the sulfidized brass cords was found to vary depending on the formulation in which they were sulfidized. Our results suggest that the type of sulfide layer formed on the sulfidized brass cords is different in different compounds, depending on the individual compound formulation ingredients. Hence, the type of protection that the sulfide layer offers depends on the formulation. Generally, a higher loading of sulfur in the rubber formulation led to formation of a poor copper sulfide adhesion layer which performed worst under aging and corrosion testing environments. The cords in such cases were found to show corrosion cracking after corrosion testing. Resins along with normal sulfur loading did not show this effect. However, resins along with higher loading of sulfur were found to further deteriorate the corrosion performance of the copper sulfide adhesion layer. We are proposing that the copper sulfide is modified in terms of its crystal structure, which makes it protective in some cases and crack in other cases. Also, a correlation was found between the level of iron migration and the corresponding corrosion performance of the sulfidized tire cord using SIMS analysis on various sulfidized cords. The migration of iron was more pronounced in cases where high sulfur was used, which ultimately resulted in poor corrosion performance. A new mechanism for adhesion degradation of rubber to brass-plated steel cords is proposed based on these corrosion studies. It is proposed that the aging of the cord, which results in loss of rubber-to-brass bonding, is due to iron dissolution from the sulfidized tire cord. It is further discussed how this overall approach of testing the corrosion protection properties of the copper sulfide adhesion layer as a function of formulation ingredients might be useful to formulate newer formulations for better and durable adhesion performance in rubber-to-brass bonding in tires.