Consolidated Melting Gel Coatings on AZ31 Magnesium Alloy with

Subscriber access provided by The University of British Columbia Library

Applications of Polymer, Composite, and Coating Materials

Consolidated Melting-Gel Coatings on AZ31 Magnesium Alloy with Excellent Corrosion Resistance in NaCl solutions – An Interface study Mario Aparicio, Jadra Mosa, Gabriela Rodriguez, Jennifer Guzman, Quentin Picard, Lisa Klein, and Andrei Jitianu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20199 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 4, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Consolidated Melting-Gel Coatings on AZ31 Magnesium Alloy with Excellent Corrosion Resistance in NaCl solutions – An Interface study

Mario Aparicio1,*, Jadra Mosa1, Gabriela Rodriguez2, Jennifer Guzman2, Quentin Picard2, Lisa C. Klein3 Andrei Jitianu 2,4*

1

Instituto de Cerámica y Vidrio, Consejo Superior de Investigaciones Científicas (CSIC), Kelsen

5 (Campus de Cantoblanco), 28049 Madrid, Spain [email protected]; [email protected] 2

Department of Chemistry, Lehman College, CUNY, Davis Hall, 250 Bedford Park Boulevard

West Bronx, New York 10468, USA [email protected]; [email protected]; [email protected]; [email protected]; 3

Department of Materials Science and Engineering, Rutgers University, 607 Taylor Road,

Piscataway, NJ 08854, USA. [email protected] 4

Ph.D. Program in Chemistry and Biochemistry, The Graduate Center of the City University of

New York, 365 Fifth Avenue, New York, NY 10016, USA

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 42

ABSTRACT

Magnesium alloys, with a density two-thirds that of aluminum, are very attractive for industry. However, these alloys are extremely susceptible to corrosion in the presence of aggressive electrolytes such as NaCl solutions. Here, we designed hybrid coatings obtained by the consolidation of organically modified polysilsesquioxanes “melting gels” for the corrosion protection of AZ31 Magnesium Alloy in NaCl solutions. The main focus was to study the interaction between coatings and substrate, and the influence of the coating thickness on the final properties. Micro-scratch tests, adhesion by tape tests, confocal Raman microscopy, SEM-EDS and ToF-SIMS indicate good adhesion of coatings based on the interaction of melting gels and substrate. Those measurements indicate the presence of the Si-O-Mg bonds between the substrate and coatings. Electrochemical results show very low current densities (10-13 A cm-2) without any breakdown potential, and impedance values of 1010 Ohm cm2.

KEYWORDS Melting gels, Hybrid glasses, sol-gel, Hybrids Coatings, AZ31 Magnesium Alloy, Corrosion protection, NaCl solution

1.

INTRODUCTION With increasing demand for light-weighting in order to reduce fuel consumption and CO2

emissions, magnesium alloys, with a density two-thirds that of aluminum, are attractive for the transportation industry. This consideration of lightweight materials has been traditionally driven


2

Page 3 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


by the aerospace industry1,2. In addition to high strength to weight ratio and damage tolerance3, magnesium alloys benefit from high thermal conductivity, good dimensional stability, electromagnetic shielding behavior, high damping characteristic, good machinability, and easy recycling. However, these advantages are balanced by some major disadvantages: poor wear resistance, poor creep resistance and high chemical reactivity. One of the main drawbacks of magnesium and its alloys, which limits their use on a large industrial scale, is the fact that they have high surface reactivity. The main issue which hinders the use of magnesium and its alloys in the airspace industry, is their low stability at high temperature which can be associated with ignition and flammability3. There is a sustained effort to develop new ignition resistant and nonflammable magnesium alloys. It is well known, that magnesium is a thermodynamically active metal4 which is prone to corrosion in the humid environment due to the low standard potential which is -2.37eV5. The corrosion rate of the ultra-high-purity magnesium in the presence of 3.5% NaCl is ~0.25mm/year6. To reduce the corrosion rate, there is either a thermodynamic or a kinetic approach. Thermodynamically, it is possible to achieve this goal by alloying magnesium with inert noble metals7,8. The downside of this approach is the fact that the solubility of noble metals in magnesium is very low which is due to differences in crystal structure and electronegativity.

From the kinetic point of view, the higher stability of

magnesium can be obtained by passivation. The passivation is obtained by alloying magnesium with metals such as aluminum, zinc, manganese, silicon, copper, chromium, titanium, nickel and zirconium6,9. The common element present in the AZ magnesium type alloys is aluminum. The presence of aluminum increases the corrosion resistance, because in the humid environment oxides, hydroxides and magnesium carbonates6,10,11 are form on the surface of magnesium alloys. These species can partially improve the corrosion resistance. However, at present, there are no


3


Page 4 of 42

inhibitors that form passivating layers, which fully protect magnesium against corrosion12, due to fact that magnesium and its alloys have a high affinity for species like O2, OH-, or H2. Magnesium and its alloys are susceptible to corrosion in the presence of aggressive electrolytes such as NaCl solutions13-16. Some other downsides, of using magnesium and its alloys, are low elastic modulus, limited cold workability and shrinkage upon solidification17. A critical requirement in recent years has been enhanced corrosion behavior. Magnesium stability toward corrosion can be improved by using corrosion inhibitors12, phosphate coatings18, or perfluorinated polysiloxane coatings with graphene oxide19, sol-gel coatings etc.” Advances in mechanical properties and corrosion resistance have led to renewed interest in magnesium alloys20. Sol–gel technology is one possible approach for applying anti-corrosive coatings, being environmentally-compliant and compatible with organic paints. A key advantage of sol–gel films is their covalent bonding and strong adhesion to the substrate, which gives them good barrier properties that prevent access of the environment to the alloy surface21 24. Moreover, high homogeneity and chemical purity of the coating are desirable for application such as a protective coatings against corrosion. In the past, some organic and hybrid organic-inorganic sol–gel coatings were developed for the corrosion protection of AZ-group Mg alloys, but high temperature treatments required for coating densification were incompatible with the maintaining the microstructure and properties of the alloy. In these cases, the presence of residual defects, such as cracks and porosity, limited their performance during long exposure times to aggressive electrolytes. These defects resulted from high volumetric shrinkage of the sol-gel coating during the heat treatment, when compounds such as water and solvents were removed25-31.


4

Page 5 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Combinations of silica alkoxides with other oxide precursors have been used for the preparation of sol-gel coatings on magnesium alloy AZ31 in order to improve the adhesion of the coatings and anti-corrosion properties by increasing the network crosslinking and pH stability32,33. For example, the modification of the silica network by incorporation of different compounds, such as phosphonate functionalities, produced an improvement of the corrosion resistance of magnesium alloys due to the increased adhesion to the substrate34,35.

Other

approaches focused on the combination of anodized layers, with sol-gel coatings intended to seal pores in the anodized layer, and improve the corrosion resistance by eliminating diffusion paths for corrosive species36.

Another approach is the combination of sol-gel coatings directly

deposited on the metal substrate with an outer polymer coating37,38. Modified hybrid sol-gel coatings formed by organo-silanes with long alkyl chains39-41 and incorporation of nanoparticles in the sol-gel coatings42,43 are other strategies to increase the corrosion protection of AZ31 Magnesium alloys by increasing the network crosslinking and by improving the water repellency. Our work is based on “melting gel” coatings prepared by sol-gel processing using organically modified silicon alkoxides. The objectives are to obtain defect-free coatings and to eliminate the shrinkage of the sol-gel coating during the consolidation treatment. Originally, melting gels were reported by Matsuda et al. for a poly(benzylsilsesquioxane) system44. More recently,

melting

gels

were

prepared

using

phenyltriethoxysilane

(PhTES)

and

diphenyldiethoxysilane (DPhDES) or methyltriethoxysilane (MTES) and dimethyldiethoxysilane (DMDES)45-48. These syntheses were conducted using two steps of catalysis. The so called melting gels are in reality organically modified polysilsesquioxanes with a low softening point4648.

The defining property of these hybrid organic–inorganic melting gels is that they are rigid at


5


Page 6 of 42

room temperature, become fluid at a temperature T1 (~110 °C), and can be re-softened over and over again. However, after consolidation at a temperature T2 (T2 > T1) (135-170 °C), the gels are consolidated and transformed into hybrid glasses. The consolidation temperature T2 corresponds to the cross-linking of the silica chains into three dimensional networks.

The process of

consolidation of the melting gels and the formation of the hybrid glasses at the temperature T2 is taking place in two steps. First, by curing these gels to temperature T2 the reactivity of retained alkoxy and/or hydroxyl groups is increasing. Secondly, the alkoxy and/or hydroxyl groups bonded to the silsesquioxane molecular chains goes through a series of polycondensation reactions. These polycondensations determine the cross-linking of the polysilsesquioxanes to the point of forming an irreversible hybrid glass network49,50. The process of softening-becoming rigid-re-softening can be repeated many times, as long as the melting gels are not cured at the temperature of consolidation T2 (135-170 °C). This property can be used to great effect in processing after preparation of the coatings49-52. Our previous work in the field of corrosion protection of metal substrates using consolidated melting gel coatings was focused on the preparation and characterization of thick (more than 0.5 mm) and thin (up to 10 µm) hybrid glasses coatings on stainless steel53,54. These showed excellent behavior against corrosion in NaCl solutions. This behavior led us to conduct this study on the anticorrosive properties provided by these coatings when applied to more reactive metallic substrates like magnesium alloys. In addition, in this work we analyzed, for the first time, the response to corrosion of a system with two thin layers. Our aim is to control the thickness of the coating within the range practical for industrial use (up to 100 μm) to substantially improve the corrosion resistance of magnesium alloys.

Moreover, we have

investigated the interface between the two layers and its influence on the ability to limit


6

Page 7 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


corrosion. Using electrochemical testing, we have quantified the corrosion resistance provided by these coatings and the effect of thickness. This gives us a better understanding of the interaction between the two coatings and the interaction between the coating and the magnesium substrate.

2. MATERIALS AND METHODS Melting gel preparation in the methyltriethoxysilane - dimethyldiethoxysilane system was reported previously49,50,53-55 . However, a brief description of the melting gels preparation used in this study is presented here.

The precursors for melting gel synthesis were,

methyltriethoxysilane (MTES) (Sigma-Aldrich, Milwaukee, WI), a mono-substituted alkoxide and dimethyldiethoxysilane (DMDES) (Fluka Chemicals, Milwaukee, WI) a di-substituted alkoxide. For this synthesis, hydrochloric acid (37.4%) (Fisher Scientific, Atlanta, GA) and ammonia (30%) (Sigma-Aldrich, Milwaukee, WI) were used as catalysts. Anhydrous ethanol (Sigma-Aldrich, Milwaukee, WI) was employed as solvent. All reagents were used without further purification. The molar composition of the melting gel used in this study is 75% MTES 25% DMDES. The synthesis of the MTES-DMDMS melting gels had three steps. Initially, the first solution was prepared by mixing water with hydrochloric acid and then half of the alcohol was added. A second solution containing the other half of the ethanol and MTES was also prepared. This second solution was added dropwise to the first one under continuous stirring (~500 rpm).

The solution obtained for pre-hydrolysis was continuously stirred at room

temperature for 3 hours in a sealed beaker.

The molar ratios of the reagents

MTES:EtOH:H2O:HCl were 1:4:3:0.01. In the second step of the synthesis, the di-substituted


7


Page 8 of 42

alkoxide (DMDES) was diluted with ethanol using a molar ratio of DMDES:EtOH = 1:4. This solution was added dropwise into the MTES:EtOH:H2O:HCl initial mixture. This reaction mixture was kept under constant magnetic stirring (~500rpm) in a sealed beaker at room temperature for two additional hours, while the reagents underwent hydrolysis and condensation polymerization reactions. Finally, in the third step, the second catalyst, ammonia, was added dropwise by using a micropipette to the reaction mixture.

The molar ratio of (MTES +

DMDES):NH3 was 1:0.01. After this, the solution was stirred for one more hour in the sealed beaker and then for 48 hours at room temperature in an open beaker until gelation occurred. The beaker was kept open in order to allow the evaporation of the ethanol used as solvent and also of the ethanol generated during the hydrolysis – polycondensation reactions. The gel was thermally treated at 70oC for 17 hours. Ammonium chloride was formed as a by-product during gelation. It is not soluble in the organically modified polysilsesquioxanes. Dry acetone is a good solvent to decrease the viscosity of the polysilsesquioxanes, being chemically inert toward these and easy to remove via evaporation. In addition, the ammonium chloride is not soluble in the acetone. Taking advantage of the fact that acetone will lower the viscosity of the organically modified polysilsesquioxanes and will not dissolve the NH4Cl, the dry acetone was used to facilitate the filtration of the viscous polysilsesquioxanes.

For this 10 ml of dry acetone

(Spectranal, Sigma-Aldrich) were added into the organically modified polysilsesquioxanes and homogenized by stirring at 500rpm. After 30 minutes of homogenization the solution was filtered under vacuum. The supernatant was saved and transferred to another beaker and stirred with a speed of ~600rpm for ~6 hours until all acetone was evaporated. After the acetone purification treatment, the solution was stirred in an open beaker until it gelled once again. The MTES-DMDES based gel was thermally treated at 70°C for 24 hours. The purpose of this


8

Page 9 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


thermal treatment was to remove all traces of acetone and ethanol. This was followed by another thermal treatment at 110°C for removal of unreacted water. At this point, a transparent colorless melting gel was obtained. In its concentrated form, the melting gel was used for the preparation of the thick coating. However, for the preparation of thin coatings, the melting gel was diluted with absolute ethanol, using an ethanol/gel ratio of 1:1 (volume %). The complete dissolving of the melting gels in ethanol took ~36 h under a continuous shaking. In this study the substrates wereAZ31B magnesium alloy with the size of 3.0 cm x 2.5 cm were purchased from Magnesium Elektron North America Inc. (Madison, IL). The chemical composition of the AZ31B magnesium alloy is presented in Table 1

Table 1. Composition of the AZ31 magnesium alloy substrate Element

Mg

Composition Balance

Al

Zn

Mn

Cu

Ca

Fe

Ni

2.5-3.5

0.7-1.3

0.2-1

0.050

0.040

0.0050

0.005

(wt%)

Before coating, the substrates were manually polished using 30 microns 400 grit, 10 microns 800grit, 5 microns 1200 grit, 5 microns and 1 micron 3M™ (St. Paul MN) premium SiC abrasive discs until a mirror like surface was obtained on both faces. The substrates were handwashed with deionized (DI) water, and then rinsed with absolute ethanol and dried. Three different coatings based on the 75% MTES - 25% DMDES melting gel were prepared: a single thick-layer coating, single thin-layer coating and a coating consist two-thin-layers. For the preparation of the single-thick-layer coating, the AZ31B metal substrate was preheated at the


9


Page 10 of 42

temperature of consolidation of the hybrid glass (135°C) for 30 minutes before the deposition by pouring or dip coating. The consolidation temperature T2 was optimized for thick layers (~1mm) and was reported in previous studies49,50.

Briefly the consolidation temperature T2 was

established empirically by going through heating and cooling cycles until a temperature was reached after which the gels could not be softened51. After the deposition, the coating was thermally treated for 17 hours at 135°C. After this thermal treatment the melting gel was fully consolidated and transformed into a hybrid glass protective coating.

The single thin-layer

coating using the diluted melting gel was obtained at room temperature using a MTI Desktop Dip Coater (Richmond, CA, USA) with adjustable speed.

The support was immersed by

vertically dipping in the solution for 1 min and withdrawn with a speed of 10 cm/min. The coating was dried at room temperature for 1 h and then dried for 17 h at 135°C. The second layer was applied, after the cooling of the substrate + coating at room temperature, in the same way as the first layer and the coating consisting of two-thin-layers were obtained. Field Emission Scanning Electron Microscopy (FE-SEM HITACHI S-4700) combined with chemical analysis by Energy Dispersive X-ray Spectroscopy (EDX, NORAN system six) were used to evaluate the coating integrity (pores, cracks, delamination) and their chemical composition.

The thicknesses of the samples were studied on the cross-section of coated

samples. For this the coated metal substrate was cut with a diamond cutting machine, then the sample is placed in a mold and epoxy resin is added. After unmolding the sample, a slice was cut to obtain a fresh surface. Later, this was polished, and finally gold was deposited by sputtering before putting it into the electron microscope. The Time of Flight Secondary Ion Mass Spectrometer (ToF-SIMS) was used to evaluate the element distribution depth profile of the single-thin-layer coating and metal-substrate interface (TOF-SIMS5 – ION TOF equipment).


10

Page 11 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The analysis was performed over a 26 × 26 μm2 area using a pulsed 25 kV Bi+ ion source. Sputtering was achieved with a 2 kV oxygen beam, and data acquisition and post-processing analyses were carried out using Ion-Spec software. The Raman spectra of the consolidated hybrid glass coatings were recorded between 4000 and 250 cm-1 in order to analyze the interaction between the magnesium alloy substrate and the coating during the deposition and consolidation of the melting gel.

Raman spectra were recorded using a confocal Raman

microscopy integrated with Atomic Force Microscopy (AFM) on a CRM-Alpha 300 RA microscope (WITec, Ulm, Germany) equipped with Nd:YAG dye laser (maximum power output of 50 mW at 532 nm). Micro-scratch tests were used to study the adhesion of the thin coatings to the substrate. Micro-scratch tests were used to study the adhesion of the single and two-thinlayers coatings to the magnesium substrate. The “progressive load” mode with a normal load up to 250 mN was used to perform the scratch tests (Model APEX-1, CETR equipment). For this study, the test has a fixed duration of time of 240 seconds. During this period, the test starts with a load close to zero and gradually increases as the tip advances on the coating surface until reaching the maximum load (250 mN). In this way, it is possible to evaluate the load in which the coating begins to crack, when it begins to delaminate and if the coating pieces are more or less adhered to the metal substrate. The test is performed using a conical type diamond with 5 µm tip radius and a scratch of 3 mm length made in 240 s. Normal load applied (Fz), tangential force (Fx), apparent friction coefficient (Fx/Fz) and scratch depth (Z) vs. horizontal displacement (Y) were recorded. The coatings adhesion was also evaluated by the scrutiny of the residual scratch patterns using FE-SEM (HITACHI S-4700 field emission). Scratch tests for evaluating adhesion for the single-thin-layer and two-thin-layers coatings were accomplished using ASTM D3359. Indentations (Model APEX-1, CETR equipment) have been carried out for all the coatings using


11


Page 12 of 42

a Vickers Diamond indenter with a tip radius of 100 nm. The values provided are the average of nine measurements at loads ranging from 0 to 100 mN. Young´s Modulus and Hardness were determined from the indentation load-displacement curve. Electrochemical tests (direct and alternating current) were conducted at room temperature in NaCl 0.35 wt % solutions using an electrochemical unit (SP-200 from Bio-Logic SAS) and a three-electrode cell. A platinum mesh of 7.0 cm2 area was used as counter electrode, while a saturated calomel electrode (SCE) was used as reference electrode, and the coated samples was employed as working electrode with 0.785 cm2 of exposed area. The electrochemical tests were done at least twice for each sample investigated in this study. Bare AZ31B magnesium alloy was used as a blank. Potentiodynamic polarization measurements were performed at 0.167 mV s-1 after three hours of immersion. Electrochemical impedance spectroscopy (EIS) as a function of the immersion time in the electrolyte was performed sweeping frequencies from 20,000 to 10-2 Hz, modulating 0.050 and 0.015 V (rms) around the open circuit potential for thick and thin coatings, respectively. The Bio-Logic software was used to fit the impedance measurements to equivalent circuits.

3. RESULTS The consolidated melting gel coatings are transparent and uniform without visual evidence of defects. Figure 1 shows the cross-section SEM micrographs of the single-thick-layer coating on the metal substrate, showing a uniform and crack-free layer with a thickness of around 1100 μm.

The thickness of the metal substrate surface affected by the interaction

between the AZ31B magnesium alloy and the hybrid glass coatings is about 7.0 μm. For


12

Page 13 of 42

comparison, the thickness of the coating modified by this interaction is around 8.7 μm. EDS analysis allows the identification of the elemental composition of the coating and the substrate. There is a clearly interdiffusion zone with the presence of Si and Mg in both the coating and the substrate, with a higher Mg in the interdiffusion zone of the metal surface.

a)

b) Metal

Coating

Coating

Resin

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Metal 2

1

Interface

2

1

Figure 1. Cross-section Scanning Electron Microscopy (SEM) micrographs of the metal substrate coated with the single-thick-layer: a) general view at low magnification, and b) detail of the metal-coating interface, including energy dispersive X-ray spectroscopy (EDX) measurements at different positions.

Regarding the samples protected with thin coatings, Figure 2 shows a study of the crosssection of the single-thin-layer coating on the metal substrate.


13


a)

b)

Resin

Si

Coating

Si

Intensity (a.u.)

Mg Counts (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 42

+ +

Mg

Al

Metal 0

200

400

600

800

1000

+

1200

1400

Sputtering time (seconds)

Figure 2. Cross-section analysis of the metal coated with single-thin-layer: a) SEM micrograph including energy dispersive X-ray spectroscopy (EDX) along the metal-coating interface, and b) ToF-SIMS (Time of Flight Secondary Ions Mass Spectrometer) depth profiles.

The SEM image displayed in Figure 2a shows again the interaction of the metal substrate with the hybrid glass produced during the preparation and consolidation of the coating. In this case, the thickness of the affected surface of the magnesium substrate during the interaction is about 8.3 μm, following the same behavior observed with the thick coating. The total thickness of the coating, delimited by the resin on the left and the metal substrate on the right, is around 11.5 μm. EDS analysis, with a spatial resolution between 1 and 3 µm3, across the interface has been performed, and the Si and Mg profiles have been included as an insert in Figure 2a56-58. The results confirm the presence of a considerable amount of Si in the affected area of the alloy surface and the diffusion of Mg into the coating. The cross-section SEM image including EDS analysis on selected points of the metal substrate protected with the coating consisting of two-thin-layers is shown in Figure 3.


14

1

Coating

3

1

Resin

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

2

4

Metal

Page 15 of 42

3

4

Figure 3. Cross-section analysis of the metal coated with two-thin-layers: SEM micrograph including energy dispersive X-ray spectroscopy (EDX) measurements at different positions. The image clearly shows the presence of the two thin layers deposited on the metallic substrate. The fracture observed in the area near the interface between the metal and the coating is due to the process of cutting and polishing. The EDX analysis in points 2 and 3 show again the interaction between the surface of the magnesium alloy and the coatings, produced during the preparation of the inner layer. The analysis in point 1 shows only the presence of Si signal, clearly delimiting the location of the second layer. The presence of a magnesium signal in the inner layer and a Si signal in the metal-coating interface are new indications of the interaction between coating and metal substrate. Although, in this case, it is more difficult to give a precise value of the thicknesses, it can be estimated that the thickness of the inner layer is approximately 14 μm, while the outer one is around 42 μm. The Raman spectra of the single-thin-layer coating on the metal substrate in a plane XY and XZ are shown in Figure 4.


15


b)

c)

a)

Si-O-Si 8 δSiO---Mg-O member ring δSi-O-Si---MgO

Red Blue

Intensity (a.u.)

Si-O-Mg Si-O-Si

Resin

ʋ sSi-O-C ʋsSi-O-Si ρCH3

δasCH 3 δsCH3

ʋSi-C

δSi-O-C

Metal

Coating

SiO2 --Mg 1600

1400

1200

1000

800

600

400

200

-1

Raman shift (cm )

d)

e)

f)

Red Blue Blue

Red

Si-O-Mg

2 µm

δSiO---Mg-O

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 42

1600

1400

1200

1000

800

600 -1

Raman shift (cm )

400 1600

SiO2 --Mg

200 1400

1200

1000

800

600

400

200

-1

Raman shift (cm )

Figure 4. Confocal Micro-Raman (CRM) coupled with AFM analysis of the single-thin-layer coating: (a) AFM confocal microscopy image of cross-section along the metal-coating interface showing the area XY for analysis; (b) x-y Raman image corresponding to this area (integration time of 0.05 s). The colors in the spectra correspond to different areas in the Raman image using a filter for 1120 cm-1; (c) main Raman spectra associated with the two different colors from b; (d) AFM confocal microscopy image of surface coating showing the line XZ for analysis; (e) x-z Raman depth scan image corresponding to this line (integration time of 0.05 s). The colors in the spectra correspond to different areas in the Raman image using a filter for 1120 cm-1; (f) main Raman spectra associated with the two different colors from (e).


16

Page 17 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4a shows an Atomic Force Microscopy (AFM) confocal microscopy image of the cross-section along the metal-coating interface showing the area for analysis (delimited by a red line). Figure 4b shows the Raman depth-profiling (x-y direction) image of the area selected in Figure 4a (acquisition time of 0.05 s). Blue areas in Figure 4b correspond to the peak from the 1120 cm-1 band, assigned to SiO-Mg bonds related to interaction between the consolidated melting gel and metal substrate. The red areas are related to the 1415 cm-1 band (asymmetric CH3 deformation band, characteristic of the hybrid glass coating). Figure 4c shows the main Raman spectra of imaging (Figure 4b) associated with both colors: red, hybrid coating; and blue, hybrid glass – magnesium alloy interaction. Blue spectra show additional bands that also appear in the red spectra because of the interaction between the magnesium substrate and the coating. Most of these peaks are related to the silica backbone of the hybrid glass: bands at 1415 cm-1, 1266 cm-1 and 742 cm-1 are assigned to methyl groups bonded to silica network, and correspond to asymmetric and symmetric CH3 deformation and bend CH3 Raman active modes, respectively. The Si-O-Si ring breathing vibrations are present at 466 cm-1 for the 8-member ring structure while the less intense vibration is found at 588 cm-1 and is assigned to the 6-member ring structure. At 705 cm1

appears a stretching vibration band of Si-C bonds. The main bands of the Si-O-Si stretching

were observed at 790 cm-1 that overlaps with stretching vibration of Si-O-C bonds, and at 1090 cm-1 (ʋas Si-O-Si) that overlaps with Si-O-Mg stretching vibration at 1120 cm-1 59,60. The isolated peak at 864 cm-1 is assigned to deformation of Si-O-C bonds. Several bands can be assigned also to the interaction between the consolidated melting gel and magnesium substrate such as 708 cm1,

associated with the modes of MgO4 in the Si–O–Mg linkages61. The weak peaks placed at 570

and 410 cm-1 are most likely caused by bending vibrations of the SiO4 tetrahedra coupled with


17


Page 18 of 42

Mg-O stretching modes60. The band found around 200 cm-1 can be assigned to the coupling action of the Si-O-Si bending vibration and Mg-O62. Figure 4d displays an AFM confocal microscopy image of the surface coating showing the line XZ (green) for analysis. Figure 4e shows depth profile Raman shift images XZ corresponding to this line (integration time of 0.05 s). The colors in the spectra correspond to different areas in the Raman image using a filter for 1120 cm-1. Figure 4f shows the main Raman spectra associated with both colors. Red spectra correspond to the hybrid glass coating and present analogous bands to that in the red spectra of Figure 4c. However, for the blue spectra in this XZ profile study, specific bands associated with the interaction between the consolidated melting gel coating and magnesium alloy substrate can be observed, demonstrating again that a thick interface is formed by reaction between the metal substrate and the coating, in agreement with SEM and ToF-SIMS results. The assignments for this interface layer are analogous to the XY Raman analysis. Figure 5 shows the results of the micro-scratch tests (normal load applied (Fz), tangential force (Fx), apparent friction coefficient, Fx/Fz (COF) and scratch depth (Z) vs. horizontal displacement (Y)) and SEM micrographs of the scratch patterns for single-thin-layer (Figure 5a) and coating consisting of two-thin-layers (Figure 5b). The test performed on both coatings shows certain similarities but also important differences. The cracking of the outer layer without detachment is observed at Fz values lower than 50 mN with both types of coatings. The SEM images of the substrate protected with the single-thin-layer coating show the spallation of the coating from the metal surface at Fz values around 75 mN, whereas it is necessary to reach 200 mN to observe the magnesium alloy in the case of using the coating consisting of two-thin-layers. This behavior can be related to the higher thickness of the outer layer of the two-thin-layers coating, and also with a good adherence between layers. The middle


18

Page 19 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SEM image (higher magnification) of the coating consisting of two-thin-layers corresponds to Fz values of around 150 mN, and it is possible to see where the indenter has detached the outer layer and has only cracked the inner one without removing it.

a)

b)

500 µm

50 µm

50 µm

500 µm

50 µm

50 µm

100 µm

100 µm

Figure 5. Micro-scratch tests of coated magnesium substrates, plotting normal load (Fz), tangential force (Fx), apparent friction coefficient, Fx/Fz (COF) and depth (Z) vs. horizontal displacement (Y): a) Single-thin-layer coating, and b) Coating consisting of two-thin-layers.

The adhesion of the coatings was assessed by scratching the coated surface following ASTM D3359 standard test on the single-thin-layer and two-thin-layers coatings. Both coatings show a good adhesion, “small flakes of the coating are detached at intersections; less than 5 % of the area is affected (Classification 4B)”. The average hardness values obtained for thick and thin coatings are 0.035 and 0.196 GPa, respectively. No difference is observed between the coatings of one and two thin layers, indicating that the influence of the substrate is negligible. The thick


19


coating is softer because of the manufacturing process and its very high thickness. The values of the Young´s Modulus are in agreement with the hardness: 1.46 and 1.88 GPa, for thick and thin coatings, respectively. The corrosion susceptibility of the coated substrates was studied in comparison with the bare AZ31B magnesium alloy. The polarization curves and impedance measurements have been recorded in 0.35 wt % NaCl solution. Figure 6 shows the potentiodynamic polarization curves of the protected metal substrates and the bare alloy after three hours of immersion. In this case, the magnesium alloy shows initially a slight protection as a result of the presence of oxides on the surface. The bare metal substrate shows active dissolution, while the coated material shows excellent corrosion protection effectiveness, especially with the thick and coating consisting of two-thin-layers. 0.0

One thin layer -1.0

E vs SCE / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 42

Two thin layer Mg Bare

-2.0

-3.0

-4.0

One thick layer -5.0

-6.0 1.E-17

1.E-15

1.E-13

1.E-11

j

1.E-09

1.E-07

1.E-05

1.E-03

/ A cm-2

Figure 6. Potentiodynamic polarization curves of the coated substrates in comparison with the bare substrate after three hours of immersion in 0.35 wt. % NaCl solution.


20

Page 21 of 42

While the substrate protected with single-thin-layer provides a significant reduction of the corrosion process, showing a low current density more than five orders of magnitude lower than that of the bare substrate, the single-thick-layer and the coating consisting of two-thin-layers present extremely low current densities (around 10-4 nA cm-2) without any breakdown potential (potential at which passivity breaks and current density increases in a monotonic way with potential). During polarization, the parameters of these coatings remain unchanged. This is strong evidence for a very stable superficial film. The impedance results confirm the behavior interpreted from the polarization curves. Figure 7 displays Bode plots for the AZ31B magnesium substrate protected with the three types of coatings after different immersion times (up to two months). These are plotted in comparison to the bare AZ31B magnesium alloy after one hour in 0.35 wt% NaCl solution. Two time constants are observed in the spectra of the bare metal substrate. The relaxation process at medium frequencies (30 Hz) is associated with the charge transfer process on the alloy surface. The time constant at low frequencies (< 0.1 Hz) can be attributed to ion diffusion through the corrosion product layer63. 14

Mg BARE (1 hour) One-thin-layer (4 hours) Two-thin-layers (4 hours)

12

One-thick-layer (1 day) One-thin-layer (72 hours) Two-thin-layers (65 days)

20

One-thick-layer (74 days) One-thin-layer (9.5 days)

0 -20

10

Phase (deg)

log (|Z| / ohm cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8

6

-40 -60 -80

4

-100

2

-120

-2

-1

0

1

2

3

4

-2

log (f / Hz)

-1

0

1

2

3

4

log (f / Hz)

Figure 7. Electrochemical Impedance Spectroscopy (EIS) measurements of the metal substrate


21


Page 22 of 42

with three coatings after different immersion times in 0.35 wt.% NaCl solution. The measurement of the bare substrate after one hour of immersion is included for comparison.

A different behavior is observed for the magnesium alloy protected with the single-thinlayer coating. The impedance at low frequency decreases slowly from the beginning of the immersion in NaCl solution, reaching an impedance modulus value of 1.9∙107 Ohm cm2 after 9.5 days of immersion. Although this value is substantially lower than that obtained with the singlethick-layer coating, it is important to note that it remains four orders of magnitude higher than the bare substrate. The impedance spectra of the single-thin-layer coated substrate after four hours of immersion present one time constant attributable to the consolidated melting gel coating properties. The increase of the immersion time up to 72 hours begins an impedance reduction and the presence of a new time constant at around 10-1 Hz associated with the charge transfer process at the metal/coating interface. In this curve, it is also possible to observe a third time constant at 100 Hz attributed to Mg oxides/hydroxides on the alloy surface. A further increase of the immersion time (9.5 days) leads to an additional decreasing of the impedance modulus and a more defined time constant associated with the corrosion process. Now, the frequency of this time constant is around 30 Hz, similar of that shown by the bare substrate. This shifting to higher frequencies prevents easy identification of the time constant associated with the oxide layer on the metal surface. The impedance results of the substrate protected by the coating consisting of two-thinlayers are quite interesting. The value after two months of immersion was 2.0∙1010 Ohm cm2, only one order of magnitude lower than that achieved with the single-thick-layer coating. As for


22

Page 23 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the thick coating, it is only possible to observe one time constant ascribed to the properties of the hybrid glass coating. The phase angle is also close to -90°, an indication of a capacitive behavior in the frequency domain under study. After two months of immersion, a decrease of the phase angle is observed at frequencies below 0.1 Hz, probably linked with the slow electrolyte permeation in the coating.

4. DISCUSSION The SEM analysis shows that even in the thick coating, defects are not observed throughout the cross-section (Figure 1a). Figure 1b displays a detail of the interface between the AZ31B magnesium alloy substrate and the hybrid glass coating. In this image, it is possible to observe how the deposition and consolidation of the coating has initiated an interaction between the metallic substrate and the hybrid glass. As a consequence of this interaction, the surface of the metal and the part of the coating in contact with the substrate has been altered. The chemical interaction between the magnesium substrate and the hybrid glass coating, observed in EDS, appears to facilitate the adhesion of the coating to the substrate and, accordingly improve the corrosion resistance provided by the single-thick-layer coating. In the profile of concentrations (Figure 2a insert), the decrease of Si towards both surfaces is clearly observed. On the left of the Si profile, there is a drop in concentration due to the separation of the resin with the coating. In the right area of this profile there is a gradual reduction of Si as a consequence of the diffusion of this element. In the case of the Mg concentration profile, a drop in the metal surface is observed due to the interaction with the coating and diffusion of Mg. In our previous work, the metal substrate was stainless steel, and there was little interaction between the metal substrate and


23


Page 24 of 42

coatings53,54. In the present study, the interaction is evident and produces a strong bonding of the coating to the magnesium alloy. ToF-SIMS depth profiles of the single-thin-layer coating on the metal substrate (Figure 2b) support the EDS results, showing an elemental concentration gradient in the interface area. Unlike our previous work with stainless steel, a second thin layer was proposed to improve the corrosion resistance of a metal as reactive as magnesium alloy used53,54. The analysis of the cross section of the coating consisting of two-thin-layers (Figure 3) indicated that while the thickness of the inner thin layer is similar to the value obtained with the single layer coating, the thickness of the outer thin layer is somewhat higher, probably due to easy selfadherence.

The values obtained in this study per layer (11.5 μm) are higher than those

previously obtained on stainless steel (about 5 μm). The greater thickness appears to be due to the rapid chemical interaction between the melting gel and the magnesium alloy during application. Structural characterization of the hybrid glasses was previously performed using Raman, 13C

and

29Si

NMR spectroscopy and Small Angle X-ray Scattering (SAXS)49,55. Quantitative

29Si

NMR of the hybrid glasses samples showed characteristic signals for D1, D2, T2, and T3

species. Moreover, using a two-dimensional

29Si

NMR more extended structural motifs within

the framework were identified, which revealed the existence of two distinct types of T3 Si environments (the dominant type of Si sites in both materials) that correspond to Si atoms located in regions with different extents of framework condensation. The SAXS spectra revealed the presence of a homogeneous glassy phase. There was no evidence for separate well-defined phases formed by mono substituted species (CH3-SiO3) or the di-substituted species ((CH3)2SiO2) that are obtained by hydrolysis and polycondensation of the MTES and DMDES.


24

Page 25 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Moreover, the SAXS data confirmed the absence of any nano- or micro-pores. In this study using the Raman spectroscopy we were able to identify the presence of an interface between the AZ31B magnesium alloy and the melting gels. At this interface the Si–O–Mg linkage was detected. The production of porous Mg oxides/hydroxides during the corrosion process of magnesium alloys13 leads to the detachment of coatings reducing the corrosion protection, so the development of well-bonded coatings is essential for the corrosion protection of these alloys. During the micro-scratch tests for both types of layers (Figure 5), the coating pieces peeled off by the indenter are small, occurring very close to the path. Even coating sections remain on the same position after the test. This behavior indicates good adhesion, a necessity for improved corrosion resistance. One of the most interesting points of the potentiodynamic polarization measurements (Figure 6) is the significant difference in the electrochemical behavior between thin-coatings with one and two layers.

Although the single-thin-layer coating provides a significant

improvement of the corrosion resistance, it appears that the coating is permeable to the electrolyte and the current density rapidly increases with the potential. However, the addition of the second thin-layer appears to have a significant effect in improving corrosion resistance, with a substantial reduction of the current density and elimination of the breakdown potential. In this way, the behavior of the coating consisting of two-thin-layers is similar to that offered by the thick coating. Taking into account the significant difference in thicknesses between the singlethick-layer coating (1100 µm) and the coating consisting of two-thin-layers (56 µm), the anticorrosive properties offered by the two thin layers is remarkable. This behavior can be explained assuming a combination of the properties offered by each thin layer: the inner thin


25


Page 26 of 42

layer allows a good adhesion to the magnesium substrate, and consequently a reduced thickness; and the second thin layer provides an excellent barrier against the permeation of the electrolyte based on a high thickness, and a good adhesion to the inner layer. Considering the results of the impedance measurements, all the coated substrates present a very good corrosion resistance. The behavior of the metal protected with the single-thick-layer coating does not show any significant change during two months of immersion in the NaCl solution. The impedance value at low frequency (10-2 Hz) was 4.0∙1011 Ohm cm2 after two months of immersion, around eight orders of magnitude higher than that of the bare substrate. The spectra show only one time constant associated with the properties of the hybrid glass coating. The phase angle, close to -90°, reveals a highly capacitive behavior in the frequency domain under study associated with a quasi-ideal coating without signals of corrosion attack after two months of immersion. In comparison with our previous results on stainless steel protected with a thick coating, the impedance values and shape of the curves are similar. This adds confidence to the argument for magnesium alloys to replace stainless steel with melting gels53. While a single thin layer coating is not as good as the results with stainless steel substrate, this is not expected due to the more reactive behavior of the magnesium alloy54. Nevertheless, the electrochemical performance of the magnesium alloy substrate protected by the coating consisting of two-thin-layers is close to that observed with the single-thick-layer coating as in the case of polarization curves. The impedance modulus value shows only a very small change during immersion for up to two months, indicating a highly stable coating. In the case of uncoated magnesium alloy, an induction loop can be observed, showing angle phases greater than 0° around 0.01 Hz. In the case of the coated magnesium alloy, we think the protection


26

Page 27 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


provided by these coatings is excellent and the corrosion negligible under these test conditions. Consequently, the induction loop is not observed. Table 2 compares the impedance modulus at 0.01 Hz of the AZ31 Mg alloy protected by the coatings shown in this work with that from other coatings prepared on the same substrate according to recent literature results. Other parameters, such as thickness, NaCl concentration and immersion time have also been included in the table due to their importance in the electrochemical performance. The best results (higher impedance values) are provided by the sol-gel method in comparison with others processing techniques, probably due to the lower amount of residual defects produced by using the sol-gel process. As shown in Table 2, the hybrid glass coatings described in this work present improved corrosion protection after two months of immersion, especially for the coating consisting of two-thin-layers that combine high impedances and low thickness. Table 2. Comparison of the impedance values at 0.01 Hz of AZ31 Magnesium Alloy protected with different coating in NaCl solutions. Thickness (µm)

NaCl (wt. %)

Immersion

│Z│

(days)

(ohm cm2)

1100

0.35

74

1011.6

56

0.35

65

1010.3

11.5

0.35

9.5

107.3

Hybrid sol-gel Si + TiO2-CeO2 NP

1.8

0.30

2.0

103.0

26

Hybrid sol-gel Si

2.0

Hanks

6

105.8

30

Hybrid sol-gel Si-Ti-Zr

5.0

0.03

14

105.7

31

Hybrid sol-gel Si-Zr + HQ

3.0

0.03

14

106.0

32

Hybrid sol-gel Si-P

0.7

Hanks

0.05

103.7

34

Coating composition

Hybrid sol-gel Si (melting gel)


Reference Present work

27


Page 28 of 42

Hybrid sol-gel Si + polymer

12

0.30

28

108.9

38

Hybrid sol-gel Si + lanthanides

---

0.03

7

106.5

40

Hybrid sol-gel Si + polymer + SiO2 NP

53

3.50

27

106.1

41

Hybrid sol-gel Si + SiO2 NP

4.0

0.60

1.25

104.5

42

Zeolitic imidazolate + SiO2 NP

---

3.50

0.125

104.7

43

Ca myristic coating by EL

33

3.50

8

105.2

64

Polyether imide / poly-dopamine

29

3.50

35

104.0

65

Polypyrrole by EL

50

3.50

1

104.0

66

Magnesium phosphate

35

1.00

0.02

103.5

18

Mg-based coating + SiC NP by PEO

12

3.50

0.04

104.2

67

Hybrid sol-gel Si + polymer

14

3.50

30

109.6

68

Hybrid sol-gel Si

2.2

3.60

0.04

105.0

69

Mg-based oxides by PEO

12

3.50

0.02

105.7

70

Hybrid: organic-inorganic coating. NP: nanoparticles. HQ: 8-Hydroxyquinoline. EL: electrodeposition. PEO: Plasma electrolytic oxidation Only the coatings prepared by Lamaka et al. show similar results considering the impedance values and without taking into account other parameters, such as thickness or immersion

time68.

The

main

ingredients

of

their

coating

composition

are

aminopropyltriethoxysilane, poly(bisphenol A-co-epichlorohydrin)glycidyl end-capped and diethylenetriamine, with only a 3.0 wt% of the silane in the final solution. Instead, their coating has a high organic content, which is quite different from our silane-only coatings. The different chemical composition changes all of the properties, including thermal and chemical stability, mechanical properties and adhesion.


28

Page 29 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Equivalent circuits and fitting parameters corresponding to the EIS spectra. CPEcoat

Rs

CPEcoat

Rs

a)

CPEox

b)

Rcoat

CPEdl

Rcoat Rox

Rct CPEcoat

Rs

CPEox

Rs

CPEdl

CPEdl

d)

c)

Rox

Rcoat

Rct

Rct

Rcoat 2

(Ωcm )

Rox

CPEcoat Fcm-2sa-1

2

(Ωcm )

Bare Mg, 1hour (d) Thick, 1day (a)

Fcm-2sa-1

a 3.7 10

2.3 1012

2.0 10-11

0.96

12

-11

0.95

Thick, 74days (a)

2.0 10

2-thin, 4 hours (a)

5.1 1011

2.8 10-10

0.97

2-thin, 65 days (a)

3.2 1010

3.2 10-10

0.97

1-thin, 4 hours (a)

1.8 10

8

-10

0.92

1-thin, 72 hours (b)

3.4 105

3.3 10-10

0.95

1-thin, 9.5 days (c)

5

-10

3.4 10

2.1 10

7.7 10 3.6 10

3

3.5 107

Rct

CPEox

(Ωcm2)

0.86

1.7 10

5.2 10-8

0.97

0.98

CPEdl Fcm-2sa-1

a

-5

1.6 10

L

a

-3

0.75

2.3 107

6.5 10-9

0.69

7

-8

0.72

2.3 10

3

1.2 10

1.8 10

A detailed analysis of the EIS measurements using equivalent circuits was performed and is presented in Table3. The EIS spectra for samples with single-thick-layer, coating consisting of two-thin-layers and single-thin-layer (only after four hours of immersion) can be modelled with one time constant reflecting the resistance of the coating to the electrolyte permeation through pores or defects (Rcoat) and capacitive response of the coating (CPEcoat). A constant phase element (CPE) was applied because the slopes of the log |Z| - log freq. curves were not 1 as a consequence of small changes in the coating thickness related with the roughness of the metal.


29


Page 30 of 42

The spectra for the sample with single-thin-layer after 72 hours of immersion can be modelled with two additional time constants: CPEox and Rox related to the magnesium oxide/hydroxide on the surface of the metal substrate and, CPEdl and Rct attributed to the double-layer on the metal substrate in contact with the electrolyte and the resistance associated with the charge transfer process. In the case of the substrate protected with the single-thin-layer coating after 9.5 days of immersion, the shifting of the time constant related to the corrosion process at higher frequencies covers the third time constant (Mg oxide/hydroxide). The fitting errors were less than 10%. The Rcoat values (Table 3) show again the differences among the coatings. The samples with singlethick-layer and the two-thin-layers give very high values, pointing to excellent corrosion resistance. The increase of the immersion time in NaCl solution up to two months gives a slightly lower value in the case of the sample with single thick-layer and a reduction of more than one order of magnitude in the case of the sample with two-thin-layers. This difference can be attributed to the significant difference in thickness between the two coatings. In the case of the substrate protected with single-thin-layer, the value of Rcoat after 4 hours of immersion is smaller by more than three orders of magnitude, in comparison to that with coating consisting of two-thin-layers, under the same conditions. This major difference illustrates the improvement brought about by the second outer layer, with its higher thickness and good adherence with the inner layer. The extended immersion time for the sample with single-thin-layer leads to a reduction of the Rct value by approximately three orders of magnitude due to the intake of NaCl solution. The electrolyte permeation through the coating initiates the corrosion process, in the sample with single-thin-layer after 72 hours of immersion. In spite of this, the high values of Rct can be associated with a slow corrosion process at this stage. The Ccoat values are very low for all coatings, a signal of good barrier properties against electrolyte penetration. Not surprisingly,


30

Page 31 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the value of Ccoat for single-thick-layer of about 1 mm is an order of magnitude lower than that obtained with thin layers.

5. CONCLUSIONS Uniform and crack-free hybrid glass coatings with thickness in two different ranges: 1 mm and 10-60 µm were obtained on AZ31 magnesium alloy by consolidation of melting gels based on methyltriethoxysilane (MTES) and dimethyldiethoxysilane (DMDES). Micro-scratch tests, Confocal Raman-AFM microscopy, FE-SEM-EDS and ToF-SIMS analysis indicate a good adhesion of thin coatings. Interactions between melting gels and substrates result in Si-O-Mg bonds and interdiffusion during the preparation and consolidation of the coatings. The coating consisting of two-thin-layers (56 µm) provide the best results in terms of corrosion resistance and reduced thickness. Electrochemical results show extremely low current densities (around 10-13 A cm-2) without any breakdown potential, and impedance values of 1010 Ohm cm2 and one time constant associated with the coating. The two-thin-layer approach gives a coating where the inner thin layer produces good adhesion to the substrate and the outer layer provides a barrier to the permeation of the electrolyte.

AUTHOR INFORMATION Corresponding Authors *Phone: +1 718 9606770. Fax: +1 718 9608750. E-mail: [email protected]


31


Page 32 of 42

*Phone: +34 91 7355840. Fax: +34 91 7355843. E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work is supported by NSF – DMR Award #1313544, Materials World Network, SusChEM: Hybrid Sol–Gel Route to Chromate-free Anticorrosive Coating and by Ministerio de Economía y Competitividad, Spain (PCIN-2013-030). The authors thank Miguel Gómez, Desiree Ruiz and David Soriano for support in coatings characterization.

REFERENCES (1) Duffy L., Magnesium Alloys: The Light choice for Aerospace, Materials World 1996, 4, 127130. (2) Gray J.E., Luan B., Protective coatings on magnesium and its alloys — a critical review, J. Alloys Compd. 2002, 336, 88-113. (3) Czerwinski F., Controlling the ignition and flammability of magnesium for aerospace application, Corros. Sci. 2014, 86, 1-16.


32

Page 33 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(4) Feliu Jr. S., Maffiotte C., Galvan J.C., Barranco V., Atmospheric corrosion of magnesium alloys AZ31 and AZ61 under continuous condensation conditions, Corros. Sci. 2011, 53, 18561872. (5) Yang W., Xu D., Wang J., Yao X., J. Chen, Microstructure and corrosion resistance of micro arc oxidation plus electrostatic powder spraying composite coating on magnesium alloy, Corros. Sci. 2018, 136, 174-179. (6) Cao F., Song G.L., Atrens A., Corrosion and passivation of magnesium alloys, Corros. Sci. 2016, 111, 835-845. (7) Perrault G., The potential-pH diagram of the magnesium-water system, J. Electroanal. Chem. Interfacial Electrochem. 1974, 51, 107–119. (8) Sadeghi A., Hasanpur E., Bahmani A., Shin K.S., Corrosion behavior of AZ31 magnesium alloy containing various levels of strontium, Corros. Sci. 2018, 141, 117-126. (9) Walton C.A., Martin H.J., Horstemeyer M.F., Whittington W.R., Horstemeyer C.J., Wang P.T., Corrosion stress relaxation and tensile strength effects in an extruded AZ31 magnesium alloy, Corros. Sci. 2014, 80, 503-510. (10) Calado L.M., Taryba M.G., Carmezin M.J., Montemor M.F., Self-healing ceria modified coating for corrosion protection of AZ31 magnesium alloy, Corros. Sci. 2018, 142 12-21. (11) Cui Z., Li X., Xiao K., Dong C., Atmospheric corrosion of field-exposed AZ31 magnesium in a tropical marine environment, Corros. Sci. 2013, 76, 243-256. (12) Lamaka S.V., Vaghefinazari B., Mei D., Petrauskas R.P., Hoche D., Zheludkevich M.L., Comprehensive screening of Mg corrosion inhibitors, Corros. Sci. 2017, 1287, 224-420.


33


Page 34 of 42

(13) Pardo A., Merino M.C., Coy A.E., Arrabal R., Viejo F., Matykina E., Corrosion behaviour of magnesium/aluminium alloys in 3.5 wt.% NaCl, Corros. Sci. 2008, 50, 823-834. (14) Singh I.B., Singh M., Das S., A comparative corrosion behavior of Mg, AZ31 and AZ91 alloys in 3.5% NaCl solution, J. Magnesium and Alloys 2015, 3, 142-148. (15) Samaniego A., Llorente I., Feliu Jr. S., Combined effect of composition and surface condition on corrosion behavior of magnesium alloys AZ31 and AZ61, Corros. Sci. 2013, 68, 66-71. (16) Aung N.N., Zhou W., Effect of grain size and twins on corrosion behaviour of AZ31B magnesium alloy, Corros. Sci., 2010, 52, 589-594. (17) Mordike B.L., Ebert T., Magnesium: Properties - applications - potential, Mat. Sci. Eng. 2001, A 302, 37-45. (18) Jayarj J., Raj S.A., Srinivasan A., Ananthakumar S., Pillai U.T.S., Dhaipule N.G.K., Mudali U. K., Composite magnesium phosphate coatings for improved corrosion resistance of magnesium AZ31 alloy, Corros. Sci. 2016, 113, 104-115. (19) Ikhe A.B., Kale A.B., Jeong J., Reece M.J., Choi S-H., Pyo M., Perfluorinated polysiloxane hybridized with graphene oxide for corrosion inhibition of AZ31 magnesium alloy, Corros. Sci. 2016, 109, 238-245 (20) Mathaudhu S., Magnesium technology: Magnesium alloys in U.S. military applications: Past, Current and future solutions, Magnesium Technology 2010, 30, 27-30. (21) Durán A., Castro Y., Aparicio M., Conde A., Damborenea J.J., Protection and surface modification of metals with sol–gel Coatings, Int. Mater. Rev. 2007, 52, 175-192.


34

Page 35 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Rosero-Navarro N.C., Pellice S.A., Durán A., Aparicio M., Effects of Ce-containing sol–gel coatings reinforced with SiO2 nanoparticles on the protection of AA2024, Corros. Sci. 2008, 50 1283-1291. (23) Montemor M.F., Ferreira M.G.S., Analytical and microscopic characterisation of modified bis-[triethoxysilylpropyl] tetrasulphide silane films on magnesium AZ31 substrates, Prog. Org. Coat. 2007, 60, 228-237. (24) Wang D., Bierwagen G.P., Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. 2009, 64, 327-338. (25) Rosero-Navarro N.C., Curioni M., Castro Y., Aparicio M., Thompson G.E., Durán A., Glass-like CexOy sol–gel coatings for corrosion protection of aluminum and magnesium alloys, Surf. Coat. Technol. 2011, 206, 257-264. (26) Zaharescu M., Predoana L., Barau A., Raps D., Gammel F., Rosero-Navarro N.C., Castro Y., Durán A., Aparicio M., SiO2 based hybrid inorganic–organic films doped with TiO2-CeO2 nanoparticles for corrosion protection of AA2024 and Mg-AZ31B alloys, Corros. Sci. 2009, 51, 1998-2005. (27) Rueda L.M., Hernández C.A., Viejo F., Coy A.E., Mosa J., Aparicio M., Diseño de recubrimientos multicapa barrera-biomimético base TEOS-GPTMS sobre la aleación de magnesio Elektron 21 de potencial aplicación en la fabricación de implantes ortopédicos, Revista de Metalurgia 2016, 52, Article number e075.


35


Page 36 of 42

(28) Correa P.S., Malfatti C.F., Azambuja D.S., Corrosion behavior study of AZ91 magnesium alloy coated with methyltriethoxysilane doped with cerium ions, Prog. Org. Coat. 2011, 72, 739747. (29) Hu R.G., Zhang S., Bu J.F., Lin C.J., Song G.L., Recent progress in corrosion protection of magnesium alloys by organic coatings, Prog. Org. Coat. 2012, 73, 129-141. (30) Zomorodian A., Brusciotti F., Fernandes A., Carmezim M.J., Moura e Silva T., Fernandes J.C.S., Montemor M.F., Anti-corrosion performance of a new silane coating for corrosion protection of AZ31 magnesium alloy in Hank's solution, Surf. Coat. Technol. 2012, 206, 43684375. (31) Lamaka S.V., Montemor M.F., Galio A.F., Zheludkevich M.L., Trindade C., Dick L.F.P., Ferreira M.G.S., Novel hybrid sol–gel coatings for corrosion protection of AZ31B magnesium alloy, Electrochim. Acta 2008, 53, 4773-4783. (32) Galio A.F., Lamaka S.V., Zheludkevich M.L., Dick L.F.P., Müller I.L., Ferreira M.G.S., Inhibitor-doped sol–gel coatings for corrosion protection of magnesium alloy AZ31, Surf. Coat. Technol. 2010, 204, 1479-1486. (33) Reyes Y., Durán A., Castro Y., Glass-like cerium sol-gel coatings on AZ31B magnesium alloy for controlling the biodegradation of temporary implants, Surf. Coat. Technol. 2016, 307, 574-582. (34) Khramov A.N., Balbyshev V.N., Kasten L.S., Mantz R.A., Sol–gel coatings with phosphonate functionalities for surface modification of magnesium alloys, Thin Solid Films 2006, 514, 174-181.


36

Page 37 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(35) Khramov A.N., Johnson J.A., Phosphonate-functionalized ORMOSIL coatings for magnesium alloys, Prog. Org. Coat. 2009, 65, 381-385. (36) Tan A.L.K., Soutar A.M., Annergren I.F., Liu Y.N., Multilayer sol–gel coatings for corrosion protection of magnesium, Surf. Coat. Technol. 2005, 198, 478- 482. (37) Barranco V., Carmona N., Galván J.C., Grobelny M., Kwiatkowski L., Villegas M.A., Electrochemical study of tailored sol–gel thin films as pre-treatment prior to organic coating for AZ91 magnesium alloy, Prog. Org. Coat. 2010, 68, 347-355. (38) Brusciotti F., Snihirova D.V., Xue H., Montemor M.F., Lamaka S.V., Ferreira M.G.S., Hybrid epoxy–silane coatings for improved corrosion protection of Mg alloy, Corros. Sci. 2013, 67, 82-90. (39) Zucchi F., Frignani A., Grassi V., Balbo A., Trabanelli G., Organo-silane coatings for AZ31 magnesium alloy corrosion protection, Mat. Chem. Phys. 2008, 110, 263-268. (40) Montemor M.F., Ferreira M.G.S., Electrochemical study of modified bis[triethoxysilylpropyl] tetrasulfide silane films applied on the AZ31 Mg alloy, Electrochim. Acta 2007, 52. 7486-7495. (41) Wang H., Akid R., Gobara M., Scratch-resistant anticorrosion sol–gel coating for the protection of AZ31 magnesium alloy via a low temperature sol–gel route, Corros. Sci. 2010, 52, 2565–2570. (42) Peres R.N., Cardoso E.S.F., Montemor M.F., de Melo H.G., Benedetti A.V., Suegama P.N., Influence of the addition of SiO2 nanoparticles to a hybrid coating applied on an AZ31 alloy for early corrosion protection, Surf. Coat. Technol. 2016, 303, 372-384.


37


Page 38 of 42

(43) Wu C., Liu Q., Chen R., Liu J., Zhang H., Li R., Takahashi K., Liu P., Wang J., Fabrication of ZIF-8@SiO2 Micro/Nano Hierarchical Superhydrophobic Surface on AZ31 Magnesium Alloy with Impressive Corrosion Resistance and Abrasion Resistance, ACS Appl. Mater. Interfaces 2017, 9, 11106-11115. (44) Matsuda A., Sasaki T., Hasegawa K., Tatsumisago M., Minami T., Thermal softening behaviour and application to transparent thick films of poly (benzylsilsesquioxane) particle prepared by sol-gel process, J. Am. Ceram. Soc. 2001, 84, 775-780. (45) Masai H., Tokuda Y., Yoko T., Gel-melting method for preparation of organically modified siloxane low-melting glasses, J. Mater. Res. 2005, 20, 1234-1241. (46) Jitianu A., Amatucci G., Klein L.C., Phenyl-substituted siloxane hybrid gels that soften below 140°C, J. Am. Ceram. Soc. 2009, 92, 36-40. (47) Takahashi K., Tadanaga K., Matsuda A., Hayashi A., Tatsumisago M., Thermoplastic and thermosetting properties of polyphenylsilsesquioxane particles prepared by two step acid-base catalyzed sol–gel process, J. Sol-Gel Sci. Technol. 2007, 41, 217-222. (48) Klein L.C., Jitianu A., Organic-Inorganic Hybrid Melting Gels, J. Sol-Gel Sci. Technol. 2011, 59, 424-431. (49) Jitianu A., Doyle J., Amatucci G., Klein L.C., Methyl modified siloxane melting gels for hydrophobic films, J. Sol-Gel Sci. Technol. 2010, 53, 272-279. (50) Jitianu A., Lammers K., Arbuckle-Keil G.A., Klein L.C., Thermal analysis of organically modified siloxane melting gels, J. Therm. Anal. Calorim. 2012, 107, 1039-1045.


38

Page 39 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(51) Takahashi K., Tadanaga K., Matsuda A., Hayashi A., Tatsumisago M., Thermoplastic and thermosetting properties of polyphenylsilsesquioxane particles prepared by two step acid-base catalyzed sol–gel process, J. Sol-Gel Sci. Technol. 2007, 41, 217-222. (52) Jitianu A., Gambino L. and Klein L.C., Sol-Gel Packaging for Electrochemical Devices” in Sol-Gel Processing for Conventional and Alternative Energy”, Edited by Aparicio M., Jitianu A., Klein L.C., Springer, New York, 2012, 375-392 (53) Aparicio M., Jitianu A., Rodriguez G., Degnah A., Al-Marzoki K., Mosa J., Klein L.C., Corrosion protection of AISI 304 stainless steel with melting gel coatings, Electrochim. Acta 2016, 202, 325-332. (54) Aparicio M., Jitianu A., Rodriguez G., Al-Marzoki K., Jitianu M., Mosa J., Klein L.C., Thickness-properties synergy in organic–inorganic consolidated melting-gel coatings for protection of 304 stainless steel in NaCl solutions, Surf. Coat. Technol. 2017, 315, 426-435. (55) Jitianu A., Cadars S., Zhang F., Rodriguez G., Picard Q., Aparicio M., Mosa J., Klein L.C., 29Si

NMR and SAXS investigation of the hybrid organic-inorganic glasses obtained by

consolidation of the melting gels, Dalton Transaction 2017, 46, 3729-3741. (56) Ae T., Coy M., Nowell M., Optimizing spatial resolution for EDS analysis, EDAX Insight 2013, 11, 3. (57) Beaman D. R., Solosky L. F., Accuracy of quantitative electron probe microanalysis with Energy Dispersive Spectrometers, Analytical Chemistry 1972, 44, 1598-1610.


39


Page 40 of 42

(58) Hodoroaba V.D., Rades S., Salge T., Mielke J., Ortel E., Schmidt R., Characterisation of nanoparticles by means of high-resolution SEM/EDS in transmission mode, IOP Conf. Series: Materials Science and Engineering 2016, 109, 012006. (59) Kalampounias A.G., IR and Raman spectroscopic studies of sol–gel derived alkaline-earth silicate glasses, Bull. Mater. Sci. 2011, 34, 299-303. (60) Durben D.J., McMillan P.F., Wolf G.H., Raman study of the high-pressure behavior of forsterite (Mg2SiO4) crystal and glass, American Mineralogist 1993, 78, 1143-1148. (61) Tsai M.T., Preparation and crystallization of forsterite fibrous gels, J. Eur. Ceram. Soc. 2003, 23, 1283-1291. (62) Wang L., A. Lu, Z. Xiao, J. Ma, Y. Li, Modification of nano-fibriform silica by dimethyldichlorosilane, Applied Surface Science 2009, 255, 7542-7546. (63) Zanotto F., Grassi V., Frignani A., Zucchi F., Protection of the AZ31 magnesium alloy with cerium modified silane coatings, Mater. Chem. Phys. 2011, 129, 1-8. (64) Zhong Y., Hu J., Zhang Y., Tang S., The one-step electroposition of superhydrophobic surface on AZ31 magnesium alloy and its time-dependence corrosion resistance in NaCl solution, Appl. Surf. Sci. 2018, 427, 1193-1201. (65) Wang C., Shena J., Xie F., Duan B., Xie X., A versatile dopamine-induced intermediate layer for polyether imides (PEI) deposition on magnesium to render robust and high inhibition performance, Corros. Sci. 2017, 122, 32-40.


40

Page 41 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(66) Hatami M., Yeganeh M., Keyvani A., Saremi M., Naderi R., Electrochemical behavior of polypyrrole-coated AZ31 alloy modified by fluoride anions, J. Solid State Electrochem 2017, 21, 777-785. (67) Nasiri Vatan H., Ebrahimi-kahrizsangi R., Kasiri-asgarani M., Structural, tribological and electrochemical behavior of SiC nanocomposite oxide coatings fabricated by plasma electrolytic oxidation (PEO) on AZ31 magnesium alloy, J. Alloys Compd. 2016, 683, 241-255. (68) Lamaka S.V., Xue H.B., Meis N.N.A.H., Esteves A.C.C., Ferreira M.G.S., Fault-tolerant hybrid epoxy-silane coating for corrosion protection ofmagnesium alloy AZ31, Prog. Org. Coat. 2015, 80, 98-105. (69) Diaz L., García-Galván F.R., Llorente I., Jiménez-Morales A., Galván J.C. Feliu Jr S., Effect of heat treatment of magnesium alloy substrates on corrosion resistance of a hybrid organic–inorganic sol–gel film, RSC Adv. 2015, 5, 105735-105746. (70) Madhan Kumar A., Hwan Kwon S., Chul Jung H., Hee Park Y., Jeong Kim H., Seon Shin K., Fabrication and Electrochemical Corrosion Behavior of PEO Coatings on Strip-Cast AZ31Mg Alloy in 3.5% NaCl Solution, Ind. Eng. Chem. Res. 2014, 53 9703-9713.


41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Counts (a.u.)


Resin

Confocal Raman depth scan

Mg

Mg

Si Coating

Coating

Mg

Metal-Coating interaction Si-O-Mg bonds (1120 cm-1)


Page 42 of 42

Consolidated Melting Gel Coatings on AZ31 Magnesium Alloy with

Recommend Documents