Investigation of Inhibition Mechanism of Chromate-Free Coatings

Abstract

Aluminium alloys used in aerospace industry such as Al–Cu–Li are susceptible to corrosion when exposed in aggressive environments. The known solution to this problem is based chromate materials that are incorporated into coating system as a leachable pigment, whose release of hexavalent and trivalent species protect the alloy at coating damage areas such as scratches or cracks. However, chromates have been heavily regulated and restricted due to their carcinogenic nature. The development of chromate-free viable alternatives began in 1970s where some potentials candidates were investigated. They were found to significantly reduce corrosion rate of aluminium alloy in corrosive environments through the formation of protective layers. However, their versatility and self-healing capabilities against corrosion are not comparable with the chromate materials. In this work, the potential and effectiveness of phosphate base and synthetic carboxylic candidates were evaluated, and some selected leachable pigments were incorporated into organic coatings at different volume concentrations (PVC) for the protection of Al–Cu–Li alloy exposed in aggressive environments. The quantitative leaching measurement of inhibitive species were conducted using an inductively coupled plasma optical emission spectroscopy (ICP-OES). These pigments were able to leach out from the primer into the test solutions, whose rate increased with immersion time but decrease with increasing pH of the bulk solution. Furthermore, the leached inhibitive species formed a protective film in the scribed surfaces. The protective film formation and inhibition mechanism were proposed based on the evidence obtained. Electrochemical impedance spectroscopy (EIS) measurements were conducted to determine the barrier properties of the loaded and blank primers as a function of PVC and immersion time. In each case, the EIS study has confirmed the inhibitive properties of inhibitor-loaded primers by significantly increasing the impedance modulus of the coated alloy substrate. Morphological studies were conducted on the unexposed and exposed coating samples using various characterisation techniques for the purpose of gaining new insights about the transport paths of the inhibitive species within the primers. Microscopy revealed cavities as evidence of dissolution and release of large clusters of inhibitive species from the inhibitor-loaded coating samples. The cavity networks provided excellent transport path for both inward migration of aggressive solution and the release of dissolved inhibitive species.

Date of Award	5 Oct 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stuart Lyon (Co Supervisor) & Xiaorong Zhou (Main Supervisor)