Fabrication of Vanadium–Zirconium Conversion Coatings on 6061 Aluminum Alloy and Performance Modulation by Additives

Aluminum alloys used in harsh environments often suffer from inadequate protection due to the limited compactness and stability of existing chromate-free conversion coatings. This study designs and optimizes a corrosion-resistant vanadium-based conversion coating on 6061 aluminum alloy and investigates the influence of additives on its structure and performance. The effects of solution pH (2–4), reaction temperature (35–75 °C), and immersion time (10–30 min) on coating corrosion resistance were examined. The optimal parameters were determined as pH = 3, 55 °C, and 25 min, yielding a compact coating with excellent corrosion resistance (icorr = 0.335 μA·cm−2, Ecorr = −0.596 V, |Z| = 48.7 kΩ·cm2). To further enhance performance, polyvinyl alcohol (PVA), chitosan (CS), and a combination of sodium hexametaphosphate and cerium nitrate (SHMP + Ce(NO3)3) were introduced into the conversion solution. Characterization by SEM, AFM, and contact angle measurements showed that SHMP + Ce(NO3)3 significantly improved coating uniformity and compactness (Rq = 131 nm, Ra = 107 nm), resulting in superior corrosion resistance (icorr = 0.055 μA·cm−2, |Z| = 67.9 kΩ·cm2). The coating exhibited strong adhesion (grade 5B) and no visible corrosion after 72 h of neutral salt spray exposure, demonstrating excellent protective capability. In contrast, PVA produced porous coatings with reduced resistance, while CS provided only limited improvement.