Silicate coatings for high-temperature corrosion protection of steel in oxidative environment: processes at surface and interface

Oxidation of steel in atmosphere of combustion products limits its application in industrial practice. Once steel is exposed to oxidation conditions above ∿ 570 ℃, a multilayer scale constantly forms consisting of FeO (wustite) layer next to steel surface, Fe₃O₄ (magnetite) and Fe₂O₃ (haematite) at the scale-gas interface. The aim of this work is to study the processes at surface and interface of silicate barrier coatings that prevent oxygen transport to steel surface and thus suppress it the progressive oxidation at elevated temperatures in contact with combustion products. The water-borne silicate composition was applied to Cold Rolled Steel (CRS) substrate. Almost complete curing of silicate coating by 650 ℃ was confirmed with FTIR and TG / DTG / DTA. SEM images show that application of silicate composition (2.0 mol. Si / L) to bare CRS substrate resulted in coating formation after subsequent curing at 650 ℃ for 0.5 h. The protective performance of the developed silicate coatings was tested in air at 650 ℃ by substrates weight gain monitoring. The increase of silicate composition concentration resulted in enhancement of protective performance of 1- layer coatings while the best result exhibited 3-layer coating. The increase of composition concentration (b - 0.125, c - 0.25, d - 0.5 and e - 1.0 mol. Si / L) on CRS substrate resulted in enhancement of protective performance of 1-layer coatings. The best result exhibited 3-layer coating applied from 2.0 mol. Si / L composition - f. The formation of silicate coating on steel surface was simulated by ab initio Hartree-Fock method (within restricted open-shell variant) with valence-only basis set SBKJC and respective effective core potential using the GAMESS (Version 6.4) program package. Within 25 - 700 ℃ temperature interval, the calculated Gibbs energy for the reaction of silicate species bonding to Fe₂O₃ cluster appears to be close to - 270 kJ / mol ensuring the formation of Fe₂O₃ / Silicate interlayer responsible for adhesion and protective performance of barrier silicate coating. (author)