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The protective film formed in a defect by leaching of lithium oxalate from model organic coatings during neutral salt spray exposure has been investigated. A scribed area of about 1mm width was introduced on the coated AA2024-T3 aluminium alloy. The scribed area was examined before and after exposure in neutral salt spray environment for 4, 8, 24 and 168 h by scanning and transmission electron microscopies. Itwas found that the lithiumoxalatewas able to leach fromthe organic coating during neutral salt spray exposure and it promoted the formation of a film that provided effective corrosion protection to the alloy. The typical film morphology consists of three different layers, including a relatively compact layer near the alloy substrate, a porous middle layer and a columnar outer layer. Variation of the filmmorphology was also observed at different locations of the scribed alloy surface, which may be related to the difference of local concentration of lithium species. Electron energy loss spectroscopy detected lithium, aluminium and oxygen in the film. Although the film showed the varied morphologies in different regions of the scribed area, the alloy substrate was protected from corrosion when the film was formed. [ABSTRACT FROM AUTHOR]

The influence of a low concentration of strontium chromate on the corrosion inhibition of superpure aluminium and AA2014-T6 aluminium alloy in 0.6 M chloride solution has been investigated to simulate the leaching process of inhibitors from coatings. The potential-time and polarisation behaviour show influences on both the anodic and cathodic kinetics on the superpure aluminium surface. However, predominant cathodic inhibition was observed for the AA2014-T6 aluminium alloy. It was evident that chromium species were reduced at cathodic second phase particles, forming a thin passive film at the cathodic sites, which blocks the oxygen reduction reaction and, consequently, provides effective corrosion inhibition. [ABSTRACT FROM AUTHOR]

In this work, cerium 4-hydroxycinnamate (Ce(4OHCin) 3) and strontium zinc phosphosilicate (SZP 391) were separately incorporated as leachable anticorrosive pigments in model organic coatings for safeguarding the structural integrity of AA2198-T851 aluminium alloy under neutral salt spray (ASTM B-117) and immersion test conditions. The coated samples were artificially damaged with a scribe to reveal the underlying substrate. After 840 h of salt spray and immersion tests, the unexposed and exposed samples were characterized with scanning electron microscopy (SEM), interferometry, X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS). The SEM and interferometry revealed relatively rough surface morphology and porous networks due to the dissolution and release of large inhibitor clusters from the organic coatings. These networks provide pathways for transport of inhibitive species. The leached inhibitive species formed a precipitate of protective film in the scribed surfaces, whereas the XPS detected and provided information about the oxidation state of the protective species. EIS study has confirmed the inhibitive properties of the adsorbed film by significantly increasing the impedance modulus of the substrate. The protective films revealed a monolayer structure. • Inhibitor pigments SZP 391 and Ce(4OHCin) 3 incorporated in organic coatings leached and effectively inhibited the corrosion of Al–Cu–Li alloy. • The pigments SZP 391 and Ce(4OHCin) 3 may be new alternative benign inhibitor in place of the hazardous hexavalent chromium. • The SEM and interferometry revealed rough morphology and porous networks due to leaching of inhibitor from organic coatings. • Porous networks provide easy pathways for both inward migration of aggressive ions and outward movement of inhibitive species. • XPS data indicated that the inhibitive species detected within the damaged area oxidised from one state to another. [ABSTRACT FROM AUTHOR]