Corrosion resistance of organic coating systems on aluminium alloy 7075-T6 with silicate and tungstate modified zirconium conversion layers
Journal
Surface and Coatings Technology
Journal Volume
531
Start Page
133583
ISSN
02578972
Date Issued
2026-07-01
Author(s)
Abstract
High-strength aluminium alloys, such as 7075-T6, rely heavily on organic coating systems for corrosion protection in aerospace applications. To ensure the long-term reliability of these systems, a robust conversion coating is essential to stabilize the metal/polymer interface and enhance overall protective performance. While traditional hexavalent chromium treatments offer superior protection, their toxicity has led to strict regulations. Trivalent chromium processes have been developed as alternatives; however, concerns persist regarding the potential oxidation of trivalent species into the hazardous hexavalent form of chromium. Consequently, zirconium-based conversion coatings (ZrCC) have emerged as a promising, environmentally benign alternative. Despite their advantages, the performance of ZrCCs is often compromised by coating defects and discontinuities, which lead to premature under-film corrosion. This study addresses this limitation by investigating the efficacy of inorganic post-treatments, specifically silicate and tungstate modification, in chemically modifying and passivating these defects. Scanning Electron Microscopy (SEM) and Auger Electron Spectroscopy (AES) analyses revealed distinct modification mechanisms within the conversion layers, where tungstate species appeared to primarily adsorb at the surface, whereas silicate treatment conferred deeper chemical integration associated with a significant reduction in fluorine content and modification of the coating structure. Electrochemical Impedance Spectroscopy (EIS) demonstrated that the silicate post-treatment yielded the highest low-frequency impedance (∼105 Ω·cm2) and retarded cathodic reactions. Furthermore, accelerated corrosion testing, including neutral salt spray test (SST) and ISO 12944-9 cyclic aging test, confirmed that both post-treatments improved the resistance to corrosion creepage and blistering under organic topcoats (acrylic and polyurethane) compared to the system applied on the untreated substrate. As a result, such inorganic post-treatment modifications demonstrate potential for improving the protective reliability of ZrCC-based coating systems.
Subjects
AA7075-T6 aluminium alloy
Corrosion resistance
Inorganic post-treatment
Zirconium conversion coating
Publisher
Elsevier B.V.
Type
journal article