Steel alloys corrosion is ubiquitous and is conventionally protected by anticorrosion chromate coatings. However, the process suffers from the release of carcinogenic hexavalent chromium ions that needs to be replaced by an ecofriendly alternative. In this context, the need for the development of satisfactory ecofriendly chromium-free coating with superior corrosion performance is highly desirable. In the present study, we synthesized fully dispersible nanocrystalline Beta zeolite seeds and coated on steel alloys followed by steaming. The samples were characterized by XRD, FE-SEM, and DLS analyses. The anticorrosion behavior of the synthesized nanoparticle coatings on steel alloys was investigated by electrochemical measurements (DC polarization) and electrochemical impedance spectroscopy (EIS) in NaCl and acid and alkaline media under identical experimental conditions. The present study demonstrated that the nanozeolite coating can be a potential alternative for toxic and carcinogenic chromate coating.
Metal corrosion is inevitable and can be controlled and managed. There are direct and indirect expenses due to corrosion control. Every year billions of dollars are used for corrosion control. Therefore, corrosion control is very important to protect the environment and economy. Current method of steel corrosion control method uses chromate coating and is unacceptable due to their carcinogenic nature. Hence, a chromium-free technology with superior corrosion performance is highly desirable. In this context, there is obviously a need for more environmentally friendly corrosion-resistant coating materials such as polymers and inorganic materials. It is evident from the literature that the electrochemical deposition of conductive polymer coating suffers from low thermal stability and adhesion [
Zeolites are crystalline aluminosilicate materials, whose rigid structures define channels and cavities of strictly regular dimensions called micropores. Driven by a wide choice of applications and a multitude of challenges nanosized zeolites presented to materials scientists, nanocrystalline zeolites can be used in a variety of new and existing applications as in catalysts, ion exchangers, antibacterial materials, sensors, optical devices, anticorrosion, and so forth. There has been a rapid progress on the development of zeolite thin film for anticorrosion application. The method of preparation of zeolite significantly affects the structure and hence the anticorrosion. The anticorrosion efficiency is independent of the thickness of the coatings. It is well documented in the literature that for better performance the zeolite coating should be of less intracrystalline pores and no intercrystalline pores [
So far the anticorrosion studies are focused on hydrothermally synthesized pure-silica MFI (silicalite-1) system and anticorrosion ability zeolite AEL coating on aluminum alloy by ionothermal method is reported [
The present study aims at preparing Beta zeolite thin film on carbon steel by dry gel conversion method and to explore the anticorrosion protection of nanocrystalline zeolite coated carbon steel in aqueous solution.
Tetraethoxysilane (TEOS), aluminium isopropoxide (AlP), tetraethylammonium hydroxide (TEAOH, 10% aq), and Tween-80 were commercial samples from Merck and were used without further purification. Carbon steel samples were kindly provided by SABIC and were cut into 2 × 2 × 0.1 cm pieces and polished using sand paper. Prior to zeolite coating, it was cleaned with acetone and deionized (DI) water. It was rinsed three times with DI water and dried in air at room temperature.
In order to synthesize nanocrystalline Beta zeolite, a hydrolysed clear sol of TEAOH-AlP-TEOS-H2O was prepared at room temperature. The molar composition of the final solution was 1SiO2/4EtOH/0.27TEAOH/0.07AlP/20H2O. The clear sol was concentrated in a rotary evaporator at 80°C as described in the literature [
The nanoprecursor sol was deposited on the substrate by dip-coating (5-minute immersion). The coated substrate was dried at RT for 24 h followed by steaming at 180°C for 6 h.
The corrosion studies were carried out using a direct current (DC) polarizer equipped with a saturated calomel electrode (SCE) as a reference electrode and a platinum wire as an auxiliary electrode. The working electrode was either a coating-free or zeolite coated carbon steel substrate immersed in 50 cm2 in the solution. The corrosion behavior was studied using an aqueous solution of either a 0.1 M NaCl, 0.1 M H2SO4, or 0.1 M HCl at room temperature. The potentiodynamic polarization scans began at −150 mV versus SCE with a stabilization time of 15 minutes and continuously in the anodic direction at 10 mV s−1.
Powder X-ray diffraction patterns were recorded on a Regaku 2000 diffractometer using Cu-K
The particle sizes of the zeolite were determined by DLS measurement. A few drops of the zeolite sample were diluted with water and evaluated by DLS analysis. The results are shown in Table
The DLS result of nanocrystalline Beta synthesis.
Sample number | Sample name | Size (nm) |
---|---|---|
1 | NPs | 5.1 |
2 | Aged NPs | 20 |
3 | NZs | 45.0 |
The XRD analysis results of sample are shown in Figure
X-ray diffraction patterns of the nanocrystalline Beta zeolite.
The interplanar spacings calculated from the XRD patterns compare well with the reported values for zeolite Beta. Due to the small size of the crystals, however, the XRD peaks are broadened.
The TEM micrographs of the sample prepared by dry-gel conversion method are presented in Figure
TEM images of the nanocrystalline Beta zeolite.
FE-SEM images of nanocrystalline Beta zeolite thin film. (a) Before steaming; (b) after steaming.
The corrosion behavior of nanocrystalline Beta zeolite films was studied under kinetic control conditions. The previous anticorrosion study on zeolite coated metal substrate shows that the corrosion resistance of the zeolite coating is independent of the thickness of the coatings and the corrosion resistance is good for the zeolite coating with minimal intracrystal porosity [
Corrosion parameters derived from the polarization curves of bare and coated alloy plates in NaCl, H2SO4, and NaOH solutions.
Sample ID |
|
|
Corrosion rate (mm/year) |
---|---|---|---|
3 wt% NaCl solution | |||
TC1010 | −543.51 | −3.52 | 0.0034593 |
BZTC1010 | −660.2 | −3.54 | 0.0032988 |
0.1 M H2SO4 solution | |||
TC1010 | −517.61 | −0.82 | 1.7257 |
BZTC1010 | −498.62 | −1.41 | 0.4526569 |
0.1 M NaOH solution | |||
TC1010 | −297.33 | −3.28 | 0.0059535 |
BZTC1010 | −244.39 | −3.83 | 0.0016986 |
Comparison of the anticorrosion zeolite coatings produced by the in situ crystallization and the dry-gel-conversion processes in (a) 3% NaCl solution; (b) 0.1 M H2SO4; and (c) 0.1 M NaOH solution.
In the present study we have prepared nanocrystalline Beta zeolite coated carbon steel by dry-gel conversion method. The zeolite coated carbon steel plate was characterized by DLS, XRD, and SEM. The result showed that the Beta zeolite nanoparticles are crystalline and are about 15 nm. The corrosion resistance of zeolite coated film in the solution understudied is sulphuric acid > hydrochloric acid > sodium chloride. The method can be extended to larger substrates and can be a potential alternative for toxic and carcinogenic chromate coating. The present method of preparing anticorrosion zeolite coating could be able to protect the steel alloy samples from corrosion so that environment and economy will be managed.
The authors have declared that there is no conflict of interests.
The authors would like to thank Department of Chemistry, King Abulaziz University (Girls Campus), for providing the required facilities.