Low-temperature zinc phosphating accelerated by triazoles for microalloyed steel in automotive cataphoresis
Kacem Jaioua, M. Boudalia, Hajar Hajjaoui, Moussa Ouakki, A. Zaroual, Anton José Garcia, Abdelkbir Bellaouchou, Mohammed Cherkaoui, Hatem M.A. Amin
Abstract
• Triazole derivatives (BTA and ATA) were evaluated as accelerators for low-temperature zinc phosphating of S420MC steel. • BTA and ATA significantly enhanced corrosion resistance and coating adhesion. • Surface analysis revealed phase transition from phosphophyllite to hopeite with triazoles. • Triazole-modified phosphate coating resulted in denser and more protective layers. • The combined phosphating–passivation–triazole treatment outperforms conventional conversion coatings, offering a scalable solution for the automotive industry. This study demonstrates the high performance of novel triazole-based accelerators in low-temperature phosphate conversion treatments for cataphoresis applications in the automotive industry, with a focus on microalloyed steel S420MC. For the first time, 3-amino-1,2,4-triazole (ATA) and benzotriazole (BTA) were incorporated into the zinc phosphating coating and evaluated for their impact on corrosion resistance and paint adhesion. Electrochemical methods, including potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS), along with salt spray and cross-cut tests, were employed to assess corrosion performance. Surface characterization was conducted using SEM, EDX, and XRD. The results revealed that triazole additives significantly enhanced corrosion resistance compared to sole phosphating or passivating, with the highest performance at 0.3 g L⁻¹. Coatings with ATA and BTA achieved corrosion protection efficiencies of 98.3 % and 98.4 %, respectively, surpassing the 96.9 % achieved by standard passivation. This improvement is attributed to the coordination of triazoles with Zn²⁺ and Fe²⁺ ions and their electron-donating ability, which accelerates the phosphating reaction. XRD analysis revealed that triazole additives suppress the formation of phosphophyllite and promote hopeite, indicating a shift in the coating formation mechanism. Coatings also retained excellent paint adhesion after 500 h of salt spray testing, with sub-film corrosion propagation limited to ≤2 mm. These findings demonstrate that triazole derivatives are effective low-temperature accelerators for zinc phosphating, offering improved corrosion resistance and long-term durability in automotive coatings.