Steel rebars with oxide scale, which is usually rich of defects, are widely used in industry. On the surface of the oxide scale, it is difficult to prepare a conversion film with desirable corrosion resistance because of the acidity of conventional conversion solutions. In this work, a sol–gel solution (pH = 11) was employed to retard the diffusion of cerium ions avoiding the formation of cerium salt precipitates at room temperature. After treatments, the diffusion of cerium ions would be accelerated and a protective conversion film would form to repair natural defects on oxide scale of steel rebars at a higher temperature. This strategy can not only avoid the peeling of the oxide scale by the acid solution, but also prevent a large amount of non-uniform precipitation of the film-forming Ce 3+ ions. The conversion film@oxide scale was characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization curve (PPC) etc. It is found that the corrosion resistance of the prepared film@oxide scale was enhanced greatly when the concentration of added cerium ion (III) was 0.5 M, with pH adjusted to 11. After 30 days of immersion, R ct of surface treated rebar is more 180 times that of blank sample. The excellent corrosion resistance is closely related to the double-layer structure of the prepared sol-gel film, especially an about 2 nm inner layer of cerium oxide.
Jiao Li,Tianshu Li,Jodie A. Yuwono,Guozhe Meng,Zhiyuan Feng
Purpose This study aims to develop a sol-gel-based coating to provide a long-lasting corrosion protection on AZ31 Mg alloy. Silane-based sol-gel coatings have been successfully applied to Mg alloys for corrosion protection. However, the micro or nano defects formed during condensation and solidification will cause the coating failure during a long-lasting immersion in a saline solution. More durable corrosion-protective sol-gel coatings are needed. Design/methodology/approach A sol-gel-based coating was modified on AZ31 Mg alloy by levodopa (DOPA). The long-lasting corrosion protection mechanism was studied by multiple electrochemical testing methods and surface characterization techniques. Findings Long-term testing by electrochemical impedance spectroscopy in aqueous 0.1 M NaCl indicated that the modified DOPA@sol-gel coating exhibited significant corrosion protection performance (>14 days). In comparison, the DOPA-free sol-gel coating failed only after three days of testing. The improved corrosion protection is attributed to the self-polymerized DOPA filling to the micro or nano defects in the glassy cross-linked networks of the sol-gel coating, which greatly improves the compactness of the coating. Originality/value The method of this study is simple and easy to process, the raw materials are green and the protective effect is excellent, which is of significance for the study of magnesium alloy corrosion protection.
Epoxy resin is widely used in the field of anticorrosive coatings for its outstanding properties. However, environmentally friendly two-component waterborne epoxy coatings often have micropores in its films, making their barrier properties against corrosive media unsatisfactory. α-zirconium phosphate (α-ZrP), known as a kind of flexibly designable two-dimensional nanoparticle , has outstanding mechanical and barrier properties. However, the common problem of incorporating such inorganic fillers into epoxy resins is the poor interface compatibility. In this work, α-ZrP was firstly exfoliated by Tris-(hydroxymethyl)-aminomethane (Tris) aqueous solution, then acidified by diluted hydrochloric acid (HCl) and finally covalently functionalized by 3-aminopropyltriethoxy silane (KH-550). Subsequently, waterborne epoxy composite coatings (KWEPc) with K-ZrP as functional filler were prepared and applied on Q235 carbon steel substrates by blade-coating. With KH-550 modifying, K-ZrP showed an improved dispersion in waterborne epoxy coatings (WEPc). The structure and morphology of prepared K-ZrP was characterized and confirmed by FTIR spectra, XPS and EDS-mapping. The results showed that K-ZrP is evenly modified with amino siloxane. Electrochemical tests revealed that the corrosion rate of KWEPc was one order of magnitude lower than that of WEPc. KH-550 act as a “bridge” among inorganic filler, epoxy resin matrix and Q235 carbon steel substrates.
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