Fabrication and Characterization of a Hexavalent Chromium‐Free, Room Temperature Dry‐in‐Place Conversion Coating With Enhanced Corrosion Performance

ABSTRACT To mitigate the environmental impact of traditional chromate conversion coatings, this study presents a novel dry‐in‐place conversion coating (DCC) process that operates at room temperature without water rinsing. The coating was formed on AA3003 aluminum substrates by direct application of a conversion solution containing Cr(H₂PO₄) 3 and fluorometallates as main film‐forming agents. The chelator 1‐hydroxyethylidene‐1,1‐diphosphonic acid (HEDP) was added to improve both solution stability and coating corrosion resistance. The performance of the coatings was systematically evaluated through electrochemical measurements, copper sulfate spot testing, and neutral salt spray testing (NSS). At the same time, adhesion was assessed under both dry and wet conditions. Coating morphology and elemental distribution were analyzed using scanning electron microscopy (SEM) and energy‐dispersive X‐ray spectroscopy (EDS). Chemical states within the coating were further examined by X‐ray photoelectron spectroscopy (XPS). Results showed that a HEDP concentration of 0.6 g·L −1 yielded optimal corrosion resistance, with no visible corrosion products after 720 h of NSS exposure. The coating also maintained strong paint adhesion in both dry and wet states. EDS and XPS analyses confirmed that the coating consisted mainly of C, O, F, P, Zr, Ti, and Cr, with chromium present exclusively in the trivalent state (Cr 3+ ) and no hexavalent chromium detected, meeting environmental regulations.