Rechargeable aqueous zinc batteries offer low cost, safety, and good cycling capacity, largely due to the water-compatible Zn-metal anode. However, Zn anodes corrode in aqueous electrolytes. While such corrosion is well known, distinguishing the relative importance of corrosion contributions toward anode capacity loss remains less understood. Here, by systematically cycling Zn anodes with controlled loading and under different aging conditions, we successfully decouple and quantify the aging-induced contributions toward anode degradation in mildly acidic aqueous electrolytes. While some losses occur due to the irreversible consumption of Zn into corrosion by-products, we demonstrate that the bigger contributor to this efficiency loss (over 80%) is the physical screening effect of evolved gases, preventing the reversible dissolution of deposited Zn. Understanding the crucial role of evolved gas during cell aging, and how it can accumulate and effectively passivate large sections of the battery anode, will have important implications in the development of rechargeable aqueous zinc batteries.