Sol-gel coatings based on GPTMS-TMOS precursor, including as dopants L-Cysteine and ZrO2 in different concentrations, were applied on the surface of AZ61 magnesium alloy. Their corrosion resistance was studied in 0.6M NaCl solution, by immersion up to 14 days. XRD spectra revealed Mg(OH)2 as the main corrosion product on the coated surface, while on the untreated AZ61 in addition there were several compounds of Zn with chloride. The localized corrosion attack on the untreated AZ61 was expressed by cracks and caverns, while on the dip-coated surface the corrosion was mostly through pitting. Two non-destructive electrochemical methods were employed, contrasting the electrochemical behavior of coated AZ61 with that of uncoated alloy. The tendency in the changes of the corrosion potential at open circuit correlated positively with SEM-EDS and XRD analysis. The EIS diagrams were fitted to equivalent-circuit model and the obtained corrosion resistance Rcorr (Rs + Rct) values strongly decreased over time. The ZrO2 and L-Cysteine effect is influenced by the pH changes of the solution, Zeta potential surface charge, chemisorption and desorption processes, internal stress in the sol-gel precursor, as well as the change in its structure, after the encapsulation of both dopants.
Magnesium (Mg) and its alloys have attracted the attention of the scientists as biodegradable medical materials for temporary implants in the human body (Zhang
The corrosion of Mg in aqueous solutions is faster in the presence of chloride ions and this leads to a rapid loss of mechanical strength of metal after its implantation. During the corrosion process, evolution of the hydrogen gas occurs (Eq. (4)), which may accumulate near the place where the implant was introduced, potentially damaging the surrounding tissues; likewise, bringing the local pH to alkaline values, because of the release of OH- ions (Liu
A physical barrier between the surface of the metal and the physiological body fluids could be created by coating the surface with a polymeric layer, which would also provide a mechanically strong bond with the bone (Zheludkevich
The characteristics of sol-gel coatings make them suitable for reducing the corrosion degradation rate of the metal and their use is a recent but promising approach. Sol-gel coatings have demonstrated good chemical stability and control of oxidation (corrosion). Furthermore, the sol-gel technique is respectful towards the environment, without the need for specific pretreatment of the metal surface and the formed coatings provide a protective resistance for the metal, being nontoxic (Wang and Bierwagen,
Sol-gel hybrid coatings are more popular than the inorganic oxide layers formed on the metal surface in terms of protection against corrosion, for two main reasons (Wang and Bierwagen,
Hybrid sol-gel coatings (Atik and Zarzycki,
Other studies report the incorporation of hydroxyapatite particles (HAp) in the matrix of a hybrid sol-gel, resulting in more effective barrier properties and better protection against corrosion of Ti6Al4V alloy surface, covered by a suitable film thickness (Barranco
The purpose of this work was the development and characterization of sol-gel hybrid coatings based on GPTMS+TMOS as precursors and doped with nanoparticles of ZrO2 and L-Cys nanoparticles, considered as inhibitor for protection of the commercial magnesium alloy AZ61surface against the corrosion. For the choice of the appropriate concentration for the composition of GPTMS + TMOS + L-Cys and GPTMS + TMOS + ZrO2, the yield of GPTMS + TMOS coatings with the addition of ZrO2 and L-Cyst in different concentrations was evaluated after each coating exposure for 14 days in 0.6M NaCl solution. Later, the selected hybrid sol-gel coating was loaded with both nanoparticles to obtain GPTMS + TMOS + L-Cys + ZrO2 coating. The research was carried out with different electrochemical and surface analysis methods, such as measurement of the open circuit potential value (OCP), Electrochemical Impedance Spectroscopy (EIS), Scanning Electron Microscopy (SEM) and X ray Diffraction (XRD).
The commercial rolled AZ61B, supplied by Magnesium Elecktron (UK), was employed as test material and its provided certified composition (wt.%) is: 6.2 Al; 0.74 Zn; 0.23 Mn; 0.04 Si; 0.004 Fe; 0.0013 Ca and the balance Mg. The square samples (2 x 3 x 0.3 cm) were cut and abraded with 350, 500, 1200 and 2000 grit SiC paper and mirror polished with 3-μm and 1-μm diamond paste. The 0.6 M NaCl solution was prepared from analytical grade reagent (Sigma-Aldrich, St. Louis, MO, USA) and ultrapure deionised water (18.2 MΩ·cm).
The sol-gel precursor was composed by GPTMS ((3-glycidyloxypropyl) trimethoxysilane) and the inorganic TMOS (tetramethyl orthosilicate), in a proportion of 4:1 by weight, both added to ethanol-water solution and acetic acid (HAc) as catalyst. The pH of the obtained sol-gel was maintained at a constant value of 4.0.
Nanoparticles of L-Cys (L-cysteine) were incorporated into the sol-gel precursor in three molar concentrations: 0.3, 0.6 and 0.9%, each amount of these added to 20 ml of the sol-gel precursor and mixed for 24 h with a magnetic agitator. The ZrO2 obtained from zirconium (IV) tert-butoxide (ZTB) was also evaluated for three molar concentrations: 0.3, 0.6 and 0.9%, each amount of this was also added to 20 ml of the sol-gel precursor and mixed for 24 h with a magnetic agitator. In the alcohol-water solution of the sol-gel precursor, ZTB undergoes very rapid hydrolysis, so that flocs and zirconia precipitates tend to form. The addition of a small amount of acetyl acetone (HacAc) and deionised water allows the Zr4+ ions to remain coordinated (Barranco
The experiments in this study were carried out with the mixture of the sol-gel precursor and inhibitors of ZrO2 and L-Cys, chosen in those concentrations that showed better protection resistances for AZ61, according to the Nyquist and Bode (EIS) diagrams.
The hybrid sol-gels were deposited on the metal samples, using an elevator, in such a way that the speed of immersion and ascent of the sample could be controlled. The speed was 3 mm·s−1, without keeping the samples submerged between the two events. Once out of the solution, the samples were suspended vertically inside a convection oven (Heraeus) for 3 hours, curing at 80 °C. The appearance of the coating was examined with the naked eye with respect to color, transparency, uniformity, homogeneity or any present defect.
The experimental setup consisted of a three-electrode cell (inside a Faraday cage) as follows: a saturated Ag/AgCl/KCl reference electrode, a platinum spiral as a counter electrode and AZ61 samples as working electrode. The cell was connected to Metrohm / EcoChemie Autolab PGSTAT302N potentiostat/galvanostat, equipped with a FRA32M frequency response analyzer module and a PARSTAT 4000 potentiostat. The AZ61 samples (working electrodes) were exposed to 100 ml of 0.6M NaCl solution at temperature of 21 ± 1 °C, for different periods of time (0 h, 1 h, 3, 5, 7 and 14 days) and before the EIS measurements the open circuit potential (Eoc) was recorded. The EIS measurements were conducted at OCP conditions, applying 10 mV sinusoidal signal amplitude with frequencies ranging from 10−5 to 10−2 Hz, taking 10 points per decade. The surface exposed to the electrolyte was 1.0 cm2 delimited by a circular ring of rubber. All tests were repeated at least 3 times to verify the repeatability of the results. The Elchem Analyst™ Gamry’s dedicated data-analysis program was used to build an equivalent-circuit model and then to fit that model to experimental data.
The SEM-EDX Phillips and XL-30 ESEM JEOL JSM-7600F scanning microscopes were used to characterize the morphological and elemental changes occurring on the surface of AZ61 coated with sol-gel samples, after their immersion in 0.6M NaCl. Their surfaces were treated previously with a solution composed of CrO3, AgNO3, Ba(NO3)2 and water, according to the ISO
Values of Eoc of sol-gel coatings applied on AZ61 surface after exposure to 0.6M NaCl at 21 °C for 14 days
AZ61 | Sol-Gel (precursors) | +0.3% L-Cys | +0.6% L-Cys | +0.9% L-Cys | +0.3% ZrO2 | +0.6% ZrO2 | +0.9% ZrO2 | |
---|---|---|---|---|---|---|---|---|
14 days | −1.58± 0.002 | −1.56 ± 0.002 | 1.52 ± 0.002 | 1.55 ± 0.002 | −1.55 ± 0.002 | −1.56 ± 0.002 | −1.54 ± 0.002 | −1.55 ± 0.002 |
In the search for a sol-gel hybrid coating that delays the corrosion process of the AZ61 Mg alloy, the Nyquist and Bode diagrams of the coatings (
Values of Rcorr (Rs + Rct) of AZ61 surfaces, untreated and dip-coated with different sol-gels hybrid systems, after 14 days of exposure to 0.6M NaCl (21 °C)
Rcorr kΩ·cm−2 | AZ61 untreated | Sol-Gel precursor | +0.3% L-Cys | +0.6% L-Cys | +0.9% L-Cys | +0.3% ZrO2 | +0.6% ZrO2 | +0.9% ZrO2 |
---|---|---|---|---|---|---|---|---|
0 h | 4.26 | 54.10 | 33.81 | 45.29 | 14.77 | 15.00 | 75.10 | 11.28 |
14 days | 1.61 | 2.38 | 5.73 | 1.39 | 3.28 | 6.05 | 6.37 | 7.45 |
(a) Nyquist diagrams of AZ61 surfaces, untreated and dip-coated with different sol-gel hybrid systems, after zero time of exposure to 0.6M NaCl (21 °C); (b) zoom of Nyquist arcs.
Bode diagrams of AZ61 surfaces, untreated and dip-coated with different sol-gels hybrid systems, after exposure to 0.6M NaCl: (a) at zero time and (b) after 14 days.
The electrochemical behavior of the newly formed sol-gel hybrid coating was monitored by Nyquist and Bode EIS diagrams, during its exposure to 0.6M NaCl for 14 days.
Nyquist (a) and Bode (b) diagrams of AZ61 surface and surface coated with hybrid sol-gel system (GPTMS-TMOS + 0.3% L-Cys + 0.6% ZrO2), during its exposure to 0.6M NaCl (21 °C) for 14 days.
Values of EIS equivalent-circuit (Fig.4) components presenting untreated AZ61 surface and dip-coated with hybrid sol-gel system (GPTMS-TMOS precursor + 0.3% L-Cys + 0.6% ZrO2), after 14 days of exposure to 0.6M NaCl (21 °C)
Sample | Rs (Ω) | Rct (kΩ) | R3 (kΩ) | L1 (H)x103 | CPE1 (S*s^n)x10-5 | n | Chi x10-3 |
---|---|---|---|---|---|---|---|
AZ61 Untreated | 72.24 | 2.78 | 4.59 | 1.84 | 2.98 | 0.81 | 7.87 |
Sol-Gel hybrid | 27.9 | 3.98 | 6.03 | 8.37 | 1.08 | 0.85 | 3.69 |
Equivalent-circuit model based on Bode EIS diagrams for untreated AZ61 surface and dip-coated with hybrid sol-gel system, after 14 days of exposure to 0.6M NaCl (21°C).
SEM-EDS and XRD techniques provided an additional information for the changes that have occurred on the untreated AZ61 surface and that of dip-coated with the hybrid system (GPTMS-TMOS+0.3%L-Cys+0.6% ZrO2), after their exposure for 14 days to 0.6M NaCl.
a) Untreated AZ61 surface and b) dip-coated with hybrid sol-gel system, after 14 days of exposure to 0.6M NaCl (21 °C); the corrosion layer and coating have been removed.
EDS elemental analysis of several zones on the AZ61 untreated surface (
On the dip-coated AZ61 surface with the hybrid sol-gel system (GPTMS-TMOS precursor + 0.3% L-Cys + 0.6% ZrO2), “white” particles and two distinctive morphologies are observed, after the removal of corrosion layer and coating (
According to reports, it is considered that the corrosion products formed on Mg-alloys surfaces consist of three layers, varying in the content of O and Mg, which concentrations increase in the internal sublayer, as a part of corroded Mg-Al-Zn alloy surface (Afrin
XRD spectra of corrosion products formed on AZ61 surface: (a) untreated and b) dip-coated (GPTMS-TMOS precursor + 0.3% L-Cys + 0.6% ZrO2), after exposure for 14 days to 0.6M NaCl.
The values of Rcorr (Rs + Rct) of AZ61 surfaces, dip-coated with different sol-gel hybrid systems (
The Zeta potentials of ZrO2 have been measured in 1mM NaCl (Liu
It has also reported that ZrO2 presents high adsorption capacity (Liu
On the other hand, it was reported that the molecule of L-Cysteine is stable in pH between 3 and 7, having a neutral charge (zero) (Zhang
It was further reported (Sunwoo
It could be concluded, after the revision of above results, reported in the literature, that there are several characteristics of ZrO2 and L-Cyst, which should be taken into account before introducing as dopants in GPTMS-TMOS precursor, because their behavior is influenced by the pH changes of the solution at the interface of AZ61-sol-gel-electrolyte, including: Zeta potential (surface charge); chemisorption and desorption processes; internal stress in the sol-gel precursor (GPTMS-TMOS), as well as the change in its structure, after the introduction of particles of ZrO2 and L-cys and their encapsulation. Based on the results presented in our study, we cannot recommend their use as dopants in GPTMS-TMOS precursor.
Sol-gel coatings based on GPTMS-TMOS precursor, including as dopants L-Cysteine and ZrO2 in different concentrations, were applied on the surface of AZ61 magnesium alloy. The corrosion resistance of the formed dip-coated hybrid systems was studied in 0.6M NaCl solution, by immersion for up 14 days. XRD spectra revealed that the main corrosion product on the coated surface was Mg(OH)2 brucite, while on the untreated AZ61 surface, in addition to brucite, there were several compounds of Zn with chloride, in the presence of NaCl. The corrosion attack on the untreated AZ61 surface is well localised, expressed by cracks and caverns, while on the dip-coated surface the corrosion attack mostly occurs through pitting with a different diameter size.
Two non-destructive electrochemical methods were employed, contrasting the electrochemical behaviour of the dip-coated AZ61 with that of uncoated alloy. The tendency in the changes of the corrosion potential at open circuit correlated positively with SEM-EDS and XRD analysis of the surface. The EIS diagrams provided additional information and the results obtained were fitted to an equivalent-circuit model. Based on the Bode diagram of EIS, the corrosion resistance Rcorr (Rs + Rct) values were calulated. They showed that after exposure for 14 days in 0.6m NaCl the values of Rcorr significantly decreased.
A possible explanation of the results reported in this study is that the behavior of ZrO2 and L-Cysteine, used as dopants in GPTMS-TMOS precursor, is influenced by the pH changes of the solution at the interface of AZ61-sol-gel coating-electrolyte, including: Zeta potential (surface charge) of the dopants; chemisorption and desorption processes; internal stress in the sol-gel precursor, as well as the change in its structure, after introduction of particles of ZrO2 and L-cys and their encapsulation.
L. Hernández gratefully thanks CONACYT for his scholarship as Ph.D. student at CINVESTAV-IPN and for the research stay at CENIM/CSIC (the National Center for metallurgical Research of Madrid, Spain). The authors acknowledge LANNBIO-CINVESTAV for permitting the use of their facilities, as well to D. Aguilar-Treviño, D. Huerta-Quintanilla, and G. Espinoza-Gurriz for their technical assistance.