Abstract
The mechanism of iron corrosion protection by thin siloxane films was clarified. Quartz crystal microbalance technique (QCM) was applied to control the vapour phase deposition of alkoxysilanes and the formation of thin siloxane films. It was shown that the addition of water vapour increased the thickness of the grafted siloxane films. Crystal-like films spontaneously grow to 10–16 monolayers at 100% RH of Ar flow due to the catalytic effect of the surface. X-ray photoelectron (XPS) and Auger spectroscopies analysed the thin siloxane films and Scanning Kelvin Probe (SKP) showed the formation of iron-siloxane bonds passivating the iron surface. The films showed high hydrophobicity and corrosion inhibition in humid air contaminated by sulphur dioxide. Thick films were less ordered, hydrophilic and accelerated the corrosion of iron. For corrosion protection, the presence of oxygen in the atmosphere is extremely important. In a wet Ar atmosphere, contaminated by sulphur dioxide, the surfaces are not stable and quickly corroded. Oxygen adsorption stabilizes the surface oxide film that correspondingly preserves the anchoring iron-siloxane bonds and enables corrosion protection by the coating.
Highlights
Corrosion protection of metallic construction by polymeric coatings is widely applied.It was supposed that polymeric coatings protect the substrate by acting as a barrier toward water, oxygen and ions
Monitoring of the frequency of Fe electrodes in the Quartz crystal microbalance technique (QCM) cell after letting the vapours of butyltrimethoxysilane (BTMS) in the flow of Ar is shown in the figure
A vapour of pure BTMS was added to Ar flow (0% RH) that decrease the frequency by 20 Hz (Figure 3a)
Summary
It was supposed that polymeric coatings protect the substrate by acting as a barrier toward water, oxygen and ions. Oxygen and water quickly saturate the coating and these species could not be rate-determining factors of corrosion protection by advanced coatings [1,2]. The wet adhesion of the polymer to the substrate and the stability of chemical bonds at the interface can be important factors determining the overall coating lifetime [7,8]. The strength of the contact of the epoxy polymer and Al/Al2 O3 is very weak and can be determined by water diffusion to the interface [8]. It is believed that the formation of the ‘interphase’ intermediated the nature of the inorganic substrate and the organic polymer determines the final adhesion stability of the metal-polymer joint [9]
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