Comparative Study on Filiform Corrosion of Automotive Hot Dip Galvanized and Zn‐Al‐Mg Coated Steels Under Acetic Acid Accelerated Test

ABSTRACT Acidic residues trapped in automotive structural crevices, where incomplete paint coverage often occurs due to electrodeposition electric‑field shielding, can initiate filiform corrosion (FFC). This study develops an accelerated test using 0.5 M acetic acid (50°C, 95% RH) to evaluate the FFC susceptibility of hot dip galvanized (HDG) and Zn‐Al‐Mg (ZnAlMg) coated steels with four surface treatments (phosphating or Zr‐based conversion, with or without topcoat). Electrochemical and microscopic characterizations show that ZnAlMg exhibits superior early general corrosion resistance when an organic coating is applied. However, ZnAlMg is more susceptible to FFC (filament length: 1.26–1.99 mm) than HDG (0.22–1.05 mm), which is attributed to the preferential dissolution of its Mg‐containing phases and the subsequent galvanic coupling with exposed steel substrates. The full topcoat system fails to suppress FFC of ZnAlMg. Despite its higher general corrosion resistance, ZnAlMg requires rigorous selection of pretreatment processes for service in acidic environments.