Novel green manufacture of metallic aluminum coatings on carbon steel by sol–gel method

Abstract

Industrial processes for fabricating hot-dipping aluminum coatings on carbon steels involve problems related to equipment complexity, environmental issues and high energy consumption. To address these problems, a novel method for manufacturing metallic aluminum coating on carbon steel Q235 at room temperature by sol–gel method was developed in this work. Both the single-layer coating (47 μm) and the double-layer coating (97 μm) specimens were prepared by spraying some aqueous silica sol slurries containing spherical and flaky micro metallic aluminum powders on the steel surface at room temperature and then drying them at 50 °C. When the two coating specimens were heated at 500 °C for 10 h, heated double-layer specimens were thus obtained. It was found that the double-layer and the heated double-layer specimens didn't rust at all after being soaked in aerated 3.5 wt% NaCl for 30 days. The shielding effect of the compact top coating was the main anticorrosion mechanism of the double-layer coating based on some electrochemical impedence spectroscopies and potentiodynamic polarization curves. Both coatings comprised only one metallic Al phase based on XRD. A very small quantity of Al₂O₃ phase appeared only after heating both coating specimens at 500 °C in the air for 10 h. In both cases the coatings didn't crack at all after being heated at 500 °C in the air for 15 h by SEM observation and the oxidation rates of the steel substrates under these conditions were reduced by over 72% owing to the presence of the coatings. The average adhesive strengths of the single-layer and double-layer Al coatings were 12.06 MPa and 11.23 MPa, respectively, which were much larger than the corresponding value (max 8 MPa) of an ordinary anti-rusting epoxy coating on Q235 steel. Compared to the conventional hot-dipping aluminum or aluminized process, this novel method eliminates all the high temperature processes and thus saves a lot of energy, eliminates the use of all hazardous fluorides or chlorides and explosive H₂, avoids the formation of the voids inside aluminized coating, reduces the hot-dipped Al coating defects, can be applied for the steel plates with over 0.8 mm thickness, and can be applied in situ to repair damaged Al or Zn coatings.