Localized corrosion of the Mg alloys with inhibitor-containing coatings: SVET and SIET studies

Accepted Manuscript Title: Localized corrosion of the Mg alloys with inhibitor-containing coatings: SVET and SIET studies Author: A.S. Gnedenkov S.L. Sinebryukhov D.V. Mashtalyar S.V. Gnedenkov PII: DOI: Reference:

S0010-938X(15)30114-1 http://dx.doi.org/doi:10.1016/j.corsci.2015.10.015 CS 6511

To appear in: Received date: Revised date: Accepted date:

27-5-2015 26-8-2015 13-10-2015

Please cite this article as: A.S.Gnedenkov, S.L.Sinebryukhov, D.V.Mashtalyar, S.V.Gnedenkov, Localized corrosion of the Mg alloys with inhibitorcontaining coatings: SVET and SIET studies, Corrosion Science http://dx.doi.org/10.1016/j.corsci.2015.10.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Localized corrosion of the Mg alloys with inhibitor-containing coatings: SVET and SIET studies A.S. Gnedenkov [email protected]S.L. Sinebryukhov, D.V. Mashtalyar, S.V. Gnedenkov Institute of Chemistry of FEB RAS, 159 Pr. 100-letiya Vladivostoka, Vladivostok, 690022, Russia 

Corresponding author: Tel: +8 4232215284; fax: +8 4232312590;

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Highlights 

The way of self-healing coating formation on the Mg alloy has been suggested.



SVET and SIET were used for determining the kinetics of the self-healing process.



The self-healing mechanism has been studied and described in this work.

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Inhibitors ability to prevent the corrosion process in the defect zone was studied.



Self-healing composite coating avert intensive destruction of the magnesium alloy.

Abstract The way of self-healing coating formation at the surface of magnesium alloys on the base of plasma electrolytic oxidation method (PEO) has been suggested. Scanning Vibrating Electrode Technique (SVET) and Scanning Ion-Selective Electrode Technique (SIET) were used for determining the kinetics and mechanism of the self-healing process. The treatment of the PEOlayer by the solution containing 8-hydroxyquinoline enables one to decrease the current density of the composite coating in 30 times (3.2 μA cm-2) in the corrosion-active environment in comparison with the base PEO-coating (100 μA cm-2) and avert the intensive destruction of the material.

Keywords: A. Alloy; A. Magnesium; C. Oxidation; C. Oxide coatings; C. Passive films.

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1. Introduction Magnesium and its alloys possess many outstanding properties, such as low density, high strength, great damping capability, excellent fluidity for casting, good heat conductivity, low heat capacity, non-magnetic, and non-toxicity. These properties make magnesium and its alloys attractive to many industries [1–5]. Particularly in the automotive and aerospace industries, where the strength/weight ratio is a critical parameter, magnesium alloys have been regarded as a promising alternative to aluminum alloys [1, 6–8]. There is also a perspective of using Mg alloys in the different fields of medicine [9–16]. Currently, however, further expansion of magnesium alloy application in the automotive, aerospace, and medical industries are still unrealistic because of the poor corrosion resistance of the existing Mg alloys [17–19]. Before effective solutions to the corrosion problems of Mg alloys become available, a further expansion of their applications appears to be unlikely. To date, a large number of studies have been carried out to address corrosion issues and to improve the corrosion performance of Mg alloys [20–31]. The published results have clearly suggested that the corrosion of Mg is quite special in terms of its electrochemical behavior. The plasma electrolytic oxidation (PEO) method is widely used in both scientific and engineering fields for production of the oxide multifunctional coatings at the surface of the valve metals (owing to unipolar conductivity i.e. diode properties of the oxide of these metals) [32– 38]. During the PEO-process, the oxidation of the metal or alloy occurs using the high values of the applied electrode potential difference in comparison with conventional anodizing. The plasma discharges occurred on the electrode surface in such conditions at the critical values of the electric field strength (up to 1–10 MV cm-1). The temperature and pressure inside the discharge channel achieve up to 10000 K and 100 MPa, respectively. During the PEO-process an intensive ion and electric transfer is realized. This process promotes the electrochemical and plasmachemical synthesis of the anode material with electrolyte components [39, 40]. After the attenuation of the plasma channel is stopped, the sharp cooling of the discharge zone down to 3

electrolyte temperature is realized. This effect has an influence on the physicochemical properties of the formed surface layers [41]. PEO is a technique that operates at potentials above the breakdown voltage of an oxide film growing on the surface of a passivated metal anode and is characterized by multiple arcs moving rapidly over the treated surface. Complex compounds can be synthesised inside the high voltage breakthrough channels formed across the growing oxide layer. These compounds are composed of oxides of both the substrate material and electrolyte-borne modifying elements (e.g. silicon). Plasma thermochemical interactions in the multiple surface discharges result in a coating growing in both directions from the substrate surface. Although local temperatures are instantaneously extremely high, normally the bulk substrate temperature is below 100°C. At a particular combination of electrolyte composition and current regime the discharge modifies the microstructure and phase composition of the substrate from metal alloy to complex ceramic oxide. As a result, thick wear-resistant layers with excellent adhesion can be achieved on metal and alloy components, and the production cost is competitive with that of conventional anodizing processes [41]. Plasma phenomena considerably change the basic electrode processes because of both the enhancement of by-product physical and chemical processes and the instigation of new processes on the electrode surface. Thus, thermal and diffusion processes, new plasma chemical reactions and macro-particle transportation (i.e. cataphoretic effects) become possible during electrolysis. These processes are utilized in the various applications of plasma electrolysis, which include plasma-enhanced heat treatment and melting, welding, cleaning, etching and polishing, diffusion depletion and deposition. The layers are formed as a result of the modification of the basic electrode processes, principally by plasma-enhanced chemical reactions and diffusion processes on the electrode surfaces. This is primarily anode oxidation, in the case of PEO, and solution electrolysis and cation reduction [41].

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Variation of the electrolyte composition and mode of the coating formation enable one to regulate the chemical composition and, therefore, purposefully to change the surface layers properties [42–46]. The PEO-method is widely used all over the world for fabrication of coatings that improve the surface properties of various metals and alloys, and PEO-process is studied by various scientific groups [47–53]. There are a lot of scientific results concerning different aspects of PEO-process and because of its complicacy, we could not mention each side of this method. Currently, formation of the coatings with self-healing properties is one of the best ways of the materials protection against corrosion and mechanical influence. As a rule, the healing mechanism of the protective properties of such coatings is based on the inhibitive action of the substance in the coating composition. This inhibitor is activated during the changing of the environmental parameters at the beginning of the corrosion process. Formation of the self-healing coating connected with solving of the range of difficult tasks. Such tasks include the selection of the appropriate inhibitor for specific metal or alloy, finding the way of inhibitor location in the coating composition, and developing of the inhibitor activation mechanism. In this case, PEO-coating due to its convolute morphology and porous structure can be used as a container for corrosion inhibitors. However, there are no publications concerned with the formation of the self-healing coating by means of PEO-method. In this work, the new way of the self-healing coating formation is described. A detailed description of the electrochemical properties of such coating obtained at the surface of the Mg alloys by means of PEO-method with subsequent filling of this layer with inhibitor is also provided. According to the results of the previous studies (electrochemical impedance spectroscopy, potentiodynamic polarization and volumetric hydrogen measurements), the treatment of the base PEO-layer with 8-hydroxyquinoline significantly increased the protective characteristics of the coating at the surface of the magnesium alloy. The inhibitor-containing coating exhibits the enhanced protective properties (RP = 0.42 MΩ cm2; IC = 86 nA cm-2, where RP is polarization resistance, IC 5

is corrosion current density) against influence of the corrosion active chloride-containing media (3 wt. % NaCl). The corrosion current density for self-healing coating was 3 orders of magnitude lower than this parameter value for MA8 alloy (1.5–2.5 wt. % Mn; 0.15–0.35 wt. % Ce; balance – Mg) without coating (IC = 53 μA cm-2). These properties were determined in a corrosive environment using a 3 wt. % NaCl solution and prevented the significant destruction of the material. These data have a good correlation with results obtained by various scientific groups dealing with corrosion inhibitors like 8- hydroxyquinoline [54–59]. MA8 magnesium alloy (chemical system Mg-Mn-Ce) has small quantity of the alloying elements, which make it rather inert to the corrosion media in comparison with some others types of the magnesium alloys. Manganese, presented in the alloy, increases it corrosion stability and durability. Cerium, also increases the corrosion stability of the alloy, and increases the plasticity [60]. The choice of the magnesium alloy, presented in this work, has been caused by the following reasons: MA8 is used as a material for aviation industry in Russian Federation; MA8 is one of the magnesium alloys with small quantity of the alloying additives (as it was previously mentioned) and can be used as a base for comparison with other Mg alloys comprised of higher amount of alloying elements. Besides, as it was previously established [61], MA8 magnesium alloy has high protective properties in the corrosion environment in comparison with other Mg alloys. The perspective method of the kinetics and mechanism establishment of the self-healing process is using of the localized scanning electrochemical methods of the surface investigation [61–71] such as Scanning Vibrating Electrode Technique (SVET) and Scanning Ion-Selective Electrode Technique (SIET) etc. SVET was originally devised for detecting the extra-cellular current near living cells in the 1970s [72]. It was firstly developed to study localized corrosion processes by Isaacs in the 1980s [73]. The electrochemical process of corrosion contains an ionic current flow in the electrolyte 6

balanced by the electron flow through the metal. The ionic current flow causes a potential gradient to exist in the solution at the electrochemically active site. SVET was designed to detect the potential gradient via a movable vibrating microelectrode. SVET has enjoyed wide acceptance as a powerful electrochemical technique for evaluation of corrosion inhibitor, detection of corrosion activity and quantification of corrosion defects in coatings. SVET has been used in the research of various types of corrosion, such as pitting [74], cut-edge corrosion [75], galvanic corrosion [73], microbiologically influenced corrosion [76], weld corrosion, and stress corrosion cracking [77]. In the case of corrosion of a coated metal, SVET is able to give detailed insights into the electrochemical interactions between a coating and its substrate at a defect, which has been provided valuable information on the corrosion mechanism by a coating, including the generation and development of defects, and the influence of pigments/ inhibitors on corrosion of substrate at a defect [61, 63, 78]. SIET works as a micro-potentiometric tool allowing measurements of specific ions at a quasiconstant micro-distance over an active surface in solution. Potentiometric measurements are conducted in a two electrode galvanic cell under zero current conditions. A potentiometric cell is composed of a reference electrode and an ion-selective microelectrode. A SIET device contains the following major parts: an ion-selective microelectrode mounted on a 3D computerized stepper-motors system, used to position and move the microelectrode over the sample. The sample in turn is placed on a movable holder where a reference electrode is also mounted. A video camera equipped with a long-distance lens providing magnification up to 400 times is located over the sample. The potential difference measured in the potentiometric cell is amplified and digitalized. Before ion-selective microelectrodes became interesting for corrosion scientists, the glass-capillary microelectrode with liquid membrane was introduced by life scientists [79, 80].

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2. Experimental 2.1. Samples In the present work, MA8 magnesium alloy (1.5–2.5 wt. % Mn; 0.15–0.35 wt. % Ce; balance – Mg) was use as a sample for investigation. All samples were mechanically ground for surface standardization with silicon carbide (SiC) papers with the decreasing of the grain size of the abrasive material down to 15 μm and further polished with aluminum oxide paper with the grain size down to 3 μm. After the polishing, the samples were washed with deionized water and dried on the air. Formation of the self healing coatings has been carried out in two steps. On the first stage, the PEO-coating was formed on the surface of the magnesium alloy in the base silicate-fluoride electrolyte (Na2SiO3∙5H2O, 1.5 g l-1 and NaF, 5 g l-1) in the bipolar polarization mode. During the anodic component of the process, the applied potential difference values increased from 30 to 300 V at a rate of 0.45 V s-1. The cathodic component was in potentiostatic mode fixed at a value of 30 V. The duration ratio of the anodic and cathodic pulses was equal to 1, so the duty cycles were 50%. The polarization frequency was 300 Hz. The root-mean square values of the applied potential difference were used in the formation process in this work. During the oxidation process, the temperature of the electrolyte (25 °C) was controlled by means of chiller Smart H150-3000 («LabTech Group» (UK)). The electrical parameters were controlled using an automated control system connected to a PC with appropriate software. The duration of the PEOprocess was equal to 600 s. A conventional reversible thyristor rectifier was used as a power supply [23]. The second stage consisted in the treatment of the PEO-coated samples with 8hydroxyquinoline (8-HQ), C9H7NO, solution. The 8-HQ solution was prepared by dissolution of 8-HQ and sodium hydroxide, NaOH, in deionized water at 90 °C under intensive stirring. It was experimentally established that the maximum concentration of 8-HQ (about 3 g l-1) dissolved at pH = 12.0–12.5. The specimens were immersed in the 8-HQ solution during 120 min at room

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temperature. After that, the samples were dried at 140 °C for 20 min. 8-HQ was presented as a sorbate state on the surface of the PEO-coating according to the result of Raman spectroscopy. The surface morphology and thickness of the produced coating were studied by scanning electronic microscope (SEM) EVO 40 (Carl Zeiss, Germany). PEO-coating cross-section was prepared by the metallographic method of the sample processing. 2.2. Electrochemical measurements In this work, in order to investigate the corrosion activity of the magnesium samples with coatings and to study the capability of the 8-HQ to inhibit the corrosion process, the SVET/SIET system (Applicable Electronics, USA) was used. 2.2.1. SVET measurements For SVET measurements, the probe was an insulated Pt–Ir wire with a Pt black deposited on the spherical tip of 10 μm diameter. The probe was located 100 μm above the surface and vibrated in the perpendicular direction to the surface (Z axis) with 20 μm amplitude. The frequency of probe vibration was 398 Hz. 2.2.2. SIET measurements The H+-selective microelectrode for SIET measurements was prepared from single-barrelled glass capillary with an outer diameter of 1.5 mm. For pulling the capillary with conic tip by means of thermal treatment, the P-97 Flaming/Brown Micropipette Puller (Sutter Instruments Company) device was used. The diameter of the apex of the conic tip of the glass capillary was equal to 1 μm. The capillaries were then silanized by injecting 200 μl of N,Ndimethyltrimethylsilylamine in a glass preparation chamber at 220 °C. The membrane for H+selective microelectrode was composed of 0.5 wt.% polyvinylchloride, 9.9 wt.% hydrogen ionophore I – tridodecylamine, 88.9 wt.% 2-nitrophenyloctyl ether and 0.7 wt.% potassium tetrakis ( 4-chlorophenyl) borate. The inner reference solution contained a buffer made of 0.01 M KH2PO4 in 0.1 M KCl.

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The liquid membranes were embedded in the glass tip using an optical microscope with two 3D micromanipulators. The column length of the membrane of cocktail (in the pH-microelectrodes), was about 25–30 μm. A silver chlorinated wire was inserted into the internal solution as the inner reference electrode. Selective microelectrode was located 50 μm above the monitored surface for SIET measurements. The H+-selective microelectrode has been calibrated using buffer solutions according to the Nernst equation. The Nernst slope was 58 mV. A silver-chloride Ag/AgCl/0.1 M KCl, 0.01 M KH2PO4 electrode was used as an external reference electrode. The microelectrodes were mounted on the SVET/SIET system. Scanning of the area under study occurred by means of program software ASET-2 (ScienceWares). A preamplifier of 1 PΩ input impedance was used to measure the potential. The magnesium samples of a size of 2 cm × 1 cm × 0.2 cm with coatings were fixed at the surface of the polymeric tablet. The investigated area of the magnesium samples with different protective coatings did not exceed 16 mm2 (after isolation with wax). All the samples were studied in 0.05 M NaCl solution. The development of the corrosion process at the surface of the MA8 alloy with base PEOcoating was monitored by means of SVET/SIET methods during 48 hours of the samples exposure to the corrosion-active media. The development of the corrosion process at the surface of the composite inhibitor-containing coating due to its better protective properties was carried out over 168 hours. 2.3. Formation of the artificial defect It is well known that pores and different defects presented in the coatings or developed during the service life create pathways for corrosive species initiating corrosion processes on the metal surface [37, 63, 71, 80–82]. Therefore, the investigation of the protective properties of the coating with artificial defect in the presence of the corrosion inhibitor is the best way to understand and study deeply self-healing process of surface layers. Active corrosion protection provided by the inhibitor-containing coatings has been tested in the place with intentionally 10

created artificial defects. Defects were made to induce a local corrosion activity and monitor its progress during immersion in NaCl corrosive solution [81]. In order to accelerate the corrosion process, the artificial defect at the surface of the base PEOcoating and self-healing coating was made by means of scratch-test method using Revetest-RST (CSM Instruments, Switzerland) (Fig. 1) just before starting the experiment. The Rockwell diamond indenter with a diameter of the conic tip of 200 μm and with the angle of the top 120° was used for making a scratch. The constant rate and the load were 1 mm min-1 and 8 N, respectively. The length of the artificial scratch at the surface of the coating was equal to 2 mm. The depth of the defect estimated by profilometer was about 20 μm and the width was 250 μm.

3. Results and discussion In order to observe the significant suppression of corrosion activity in micro-scale defects made in inhibitor-containing coatings loaded with 8-HQ in comparison with base PEO-coating the localized electrochemical measurements were performed. Since SVET and SIET techniques provide detailed information about the type of corrosion process, realized at the heterostructure/electrolyte interface and also shows the self-healing effect in the provoked defects on the surface of the coating with corrosion inhibitors [83–86], these methods were used in this work in order to study the peculiarities (kinetics and mechanism of the self-healing process) and the protective properties of the formed inhibitor-containing coating on the surface of the Mg alloy. Measurement of localized currents can provide information on kinetics of electrochemical processes in coatings defects as well useful insights on the self-healing properties. Fig. 2 depicts SEM images of the surface and cross-section of the inhibitor-containing coating. The thickness of the coating was determined by SEM analysis of the cross-section. The average thickness of the composite coating has not changed as compared to the base PEO-coating and was equal to 16 μm. In general, the surface of the coating was uniform and no relevant defects 11

were detected by SEM analysis, at the same time such coating obtained by PEO-method has a porous structure which could be used as a container for inhibitor. In order to carry out the comparative analysis, the artificial defect area of the base PEO-coating without inhibitor was investigated first. The development of the corrosion process began immediately after the contact of the defect zone with the aggressive environment (0.05 M NaCl). The local anodic area (zone with higher values of the current density, red-orange area in color version and dark grey in black-and-white version) has been registered by SVET method on the left side of the defect zone after two hours of the sample exposure (Fig. 3). The artificial defect was made at the surface of the magnesium alloy with coating treated by 8HQ in the same condition mentioned above (for the base PEO-coating). The values of the depth, length, and width of the artificial defect were also the same as for the defect on the surface of the base PEO-layer. The beginning of the corrosion process for the specimen with inhibitor-containing coating was registered on the right side of the defect zone by SVET method after 4 hours of the sample exposure to 0.05 M NaCl solution (Fig. 4). There was an anodic area with higher values of the current density (red-orange area in color version and dark grey in black-and-white version). The difference of the current density values between cathodic (blue area in color version and pale grey in black-and-white) and anodic regions (between zones with lower and higher values of the current density, respectively), which has been established by SVET method, was 100 μA cm-2 for the sample with base PEO-coating (Fig. 3) after 2 hours of the sample exposure to the aggressive media. For the composite inhibitor-containing coating, the corrosion process (formation of the anodic zone) was activated only after 4 hours of the exposure (Fig. 4). The difference of the current density values between cathodic (blue and green areas in color version and pale grey and grey in black-and-white) and anodic areas was only 3.2 μA cm-2 (Fig. 4). Therefore, the sample with base PEO-coating was being destroyed in the defect zone more than

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30 times faster than that of the sample with composite inhibitor-containing coating at the initial stage of the corrosion (first 4 hours of sample exposure to 0.05 M sodium chloride solution). The microgalvanic couple formation in the defect zone of the base PEO-coating has been established by SIET method after 2 hours of the sample exposure to the corrosion-active media. The left side of the defect serves as a local anode (area with lower values of the pH, blue area in color version and pale grey in black-and-white), the right side as a local cathode (area with higher values of the pH, red area in color version and dark grey in black-and-white) (Fig. 5). The cathodic reaction was the hydrogen reduction (1), which was followed by the local alkalization of the solution up to pH = 12.6 (Fig. 5), according to the SIET-data by means of the H+-selective microelectrode. 2H2O + 2e– → H2 + 2OH– (in an alkaline or neutral solution)

(1)

Oxidation of the magnesium (2) occurred on the anodic part of the sample. Mg = Mg2+ + 2e–

(2)

The decreasing of the pH values in the local anodic zone down to below 8 (Fig. 5) caused by the reaction of the magnesium dissolution and hydrolysis (3): Mg + 2H2O → Mg(OH)2 + 2H+ + 2e–

(3)

Increasing of the protective properties of the composite coating is provided by the chelate compound, magnesium hydroxyquinalinate Mg(8-HQ)2, formed as a result of reaction of the 8HQ with magnesium ions (2) according to the generalized reaction (4). Mg(8-HQ)2 protects PEO-coating from the direct corrosion influence of the environmental media. 8-HQ deposited at the surface of the PEO-coating is slightly soluble in the neutral media and it located in the PEOcoating pores in the initial condition. The chemical interaction of the 8-HQ with magnesium ions (4) begins after coating damage (occurrence of the microdefects) as a result of the corrosion process. Changing of the pH values in the defect anodic zones (3) and in cathodic zones (1) intensifies the 8-HQ solubility and makes its reaction with magnesium ions (4) possible.

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O Mg + 2

N

Mg

N

+ H2

(4)

N OH

O

It should be noted that since the pH value at the surface of the base PEO-coating in the defect zone reaches 12 (Fig. 5), the 8-HQ, adsorbed at the surface of the PEO-layer, will be easier dissolved and react with magnesium ions, because 8-HQ solubility increase in the alkaline media. The higher value of the pH in contrast to the undamaged part (cathodic area, blue, green and yellow area in color version and grey in black-and-white) of the composite coating has been registered by SIET method in the defect zone (anodic area, red area in color version and dark grey in black-and-white) after 4 hours of the exposure of the sample with inhibitor-containing layer to the sodium chloride solution (Fig. 6). This effect can be explained by the reaction of the magnesium ions obtained from the anodic reaction (2) with 8-HQ adsorbed at the coating surface by the reaction (4). The alkalization process connected with the free hydroxide ions excess obtained by the cathodic reaction (1). Fixation of the magnesium ions with 8-HQ according to reaction (4) creates the excesses of non-bonded OH– in the defect zone. Whereas, at the surface of base PEO-coatings (without 8-HQ), Mg ions fixate the hydroxide groups, and, therefore, cannot prevent a decrease to pH (3). A small value of the ΔpH = 0.6 between extremum points (anodic and cathodic areas) for the sample with composite inhibitor-containing coating (Fig. 6) in comparison with that for the base PEO-layer ΔpH = 7.2 (Fig. 5) at the initial stage of the sample exposure to the 0.05 M NaCl demonstrated that the inhibitor-containing coating lowered the intensity of the corrosion process. This effect could be explained by the correlation of the parallel reaction (1) and (2), due to the 8-HQ solution treatment the low rates of the magnesium oxidation (2) and hydroxide ion release (1) were observed. The formation of the hydroxide ions in the solution by the reaction (1) is decelerated; therefore, the alkalization of the solution stayed 14

negligible. Therefore, it was established by SVET/SIET methods the positive action of the inhibitor at the initial stage of the corrosion process. The main chemical reactions are (1)–(4). There is a cathodic reaction (1) of the hydrogen evolution, which is followed by alkalization process and anodic reaction of the magnesium dissolution (2). These are the base process occurring in the defect zone for the PEO-coating. Enhanced protective properties of the inhibitor-containing coating is a result of the reaction of the magnesium ions with 8-HQ (4) that lead to the formation of the magnesium hydroxyquinalinate Mg(8-HQ)2 in the defect zone (anodic areas). Mg(8-HQ)2 protects PEOcoating from the direct corrosion influence of the environmental media. These processes occurred on the surface of the coatings in the chloride-containing media and were confirmed by SVET and SIET data. Other chemical reactions could be also occur in such media on the surface of the defect zone, but according to the literature data [87] and the results of the previous studies [60] these reactions practically did not have a significant influence on the main corrosion process on the magnesium surface. The local anodic zone destruction of the base PEO-coating has been confirmed by optical microscopy (Fig. 7) (left side of the defect). It should be noted that SVET method stopped registering the changing of the electrochemical activity in the defect zone after 18 hours of exposure of the sample with base PEO-layer. This effect could be explained by decreasing of the corrosion process due to the formation of the corrosion products layer, notably the magnesium hydroxide. This result also could be caused by the general increasing of the depth of the defect due to the rapid alloy dissolution that led to the accuracy decreasing of the current density measurements as a result of the decreasing of the receptiveness. At the same time, the SIET method, being more responsive for detecting the changes in the electrochemical activity on the microscale level, continued to detect the changes of the pH values.

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According to comparison of the photos obtained by optical microscopy (Fig. 7 and Fig. 8), it has been established that the specimen with inhibitor-containing coating after 24 hours exposure to the corrosion-active media undergoes less corrosion destruction in contrast to the samples with base coating in the same condition. The destruction intensity increased from the left to the right side of the defect zone (Fig. 8) that was confirmed by SVET method (Fig. 4). SVET registered in that area (right side of the defect) the local anodic zone with higher value of the current density. The corrosion process on the surface of the base PEO-layer began to develop over the entire defect zone uniformly after 30 hours of the sample exposure, according to the SIET data (Fig. 9). The pH values in the entire defect zone was less than 8, indicated that all this area was the anodic one (blue area in color and pale grey in black-and-white). After 30 hours of the composite coated sample exposure, the current density value in corrosionactive zone of the defect increased up to 6 μA cm-2 (Fig. 10a). The corrosion process began to develop more intensively over the entire length of the defect (green, yellow, and red area in color and grey, and dark grey in black-and-white). Analysis of the pH values distribution at the surface of the sample under study show that the corrosion process inhibition by the 8-HQ action continued; therefore, higher pH value was only found in the defect zone (red area in color and dark grey in black-and-white). However, at this stage of the sample exposure, the value of the pH in the anodic area increased by only 0.77 (8.96 after 4 hours (Fig. 6), 9.73 after 30 hours of sample exposure (Fig. 10b)) as a result of the accumulation of the hydroxide ions. The ΔpH value (between anodic and cathode zone of the sample) was equal to 0.56, in contrast to the sample with base PEO-layer ΔpH = 2.7 (Fig. 9). This result demonstrated that a low rate of corrosion processes occurred at the surface of the composite inhibitor-containing layer (reaction 1 and 2), which reduced the corrosion destruction. It has been established from the analysis of the experimental SVET-data (Fig. 11) that in the local anodic area of the artificial defect (central zone of the figure) on the composite coating surface, the gradual intensification of the corrosion processes occurred (increasing of the current 16

density values with the increasing of the exposure time). However, the maximum current density value for the sample with self-healing coating after 7 days of exposure was just only 12 μA cm-2. At the same time, for the sample with base PEO-coating, the current density was equal to 100 μA cm-2 (Fig. 3) after just 2 hours of the specimen exposure. The maximum current density of the base PEO-coating was more than 8 times higher as compared to self-healing coating. The current density map obtained by SVET for the sample with inhibitor-containing coating method is in a good agreement with optical microscopy data (Fig. 12). These results demonstrate the development of the defect zone under the corrosion-active media influence (the increasing of the corrosion destruction of the defect). In the course of time, the partial destruction of the active zone occurred. However, this process passed with the lower rate for the sample with composite coating in comparison with the specimen with base PEO-layer (Fig. 7 and Fig. 8). The analysis of the dynamics of the pH values distribution at the surface of the composite coating during 7 days of the sample exposure (Fig. 13) showed that there was a positive effect of the deceleration of the corrosion process by inhibitor action after immersion in the corrosionactive media. The defect zone (central zone of the figure) was still more alkaline (red in color version and more dark grey in black-and-white) in comparison with undamaged part of the sample which indicated to the inhibitor protection of the anodic area against corrosion process. The ΔpH value at the surface was equal to 1.2 and demonstrated that the self-healing process continued in the artificial defect zone after 7 days of the exposure. For the sample with base PEO-coating, the ΔpH = 7.2 was already higher after 2 hours of the exposure to the corrosionactive solution (Fig. 5). This value is 6-time higher than one for the sample with self-healing coating. The samples with base PEO-coatings and with self-healing coatings were washed with deionized water and dried with air at the end of the experiment in order to remove the corrosion products formed in the defect zone. The depth of the artificial defect was estimated at the end of the SVET/SIET tests by profilometer. The depth of the defect zone was equal to 23 μm for the base 17

PEO-coating and 20 μm for the self-healing coating. It should be noted that the values of the depth for the both coatings were equal to 20 μm before the corrosion process beginning. Obtained results confirm the conclusion based on the SVET/SIET experimental results and indicate the significantly lower dissolution rate of the sample with defect on the self-healing inhibitor-containing coating in comparison with the sample with base PEO-layer. The depth of the defect increased by 3 μm for the sample with base PEO-coating after 2 days of the exposure, whereas, the depth practically was unchanged for the specimen with self-healing inhibitor-containing layer even after 7 days of the exposure. This result showed that the selfhealing reactions within the coating, inhibited the corrosion process reducing the sample destruction. Therefore, the corrosion rate was greatly reduced during the long exposure of the magnesium alloy with composite coating to the corrosive environment.

Conclusions A method to form a protective coating on magnesium alloys surface has been developed. This coating possesses self-healing properties that protect it from damage during exposure to a corrosion-active solution. The SVET method, which measures the difference of the current density values between cathodic and anodic regions, showed that treatment of the PEO-coating with 8-hydroxyquinoline solution (3 g l-1) lead to the decreasing of the current density values in 30 fold (from 100 μA cm-2 down to 3.2 μA cm-2) in the corrosion conditions (0.05 M NaCl) and prevented the intensive destruction of the material. The SIET method, which studies of the character of the local pH distribution at the sample surface, allowed for the analysis of the mechanism of the self-healing process. Inhibitive effect is based on the activation of the corrosion inhibitor (8-HQ) of the composite coating in the alkaline media.

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Acknowledgments This work was supported by the Grant of Russian Scientific Foundation (project №14-33-00009) and Russian Government (Federal Agency of Scientific Organizations).

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Figure Captions

Fig. 1. The photo of the artificial defect on the PEO-layer magnesium alloy MA8 surface. The investigated area is inside the contour.

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Fig. 2. SEM images of the surface (a) and cross-section (b) of the inhibitor-containing coating.

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Fig. 3. Current density distribution at the surface of the base PEO-layer with defect after 2 hours of the sample exposure to 0.05 M NaCl. The local anodic area (zone with higher values of the current density, red-orange area in color version and dark grey in black-and-white version) is on the left side of the defect zone.

30

Fig. 4. Current density distribution at the surface of the composite inhibitor-containing layer with defect after 4 hours of the sample exposure to 0.05 M NaCl. There was an anodic area with higher values of the current density (red-orange area in color version and dark grey in black-andwhite version) on the right side of the defect zone.

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Fig. 5. pH distribution at the surface of the base PEO-layer with defect after 2 hours of the sample exposure to 0.05 M NaCl. The formation of the microgalvanic couple.

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Fig. 6. pH distribution at the surface of the composite inhibitor-containing layer with defect after 4 hours of the sample exposure to 0.05 M NaCl. The anodic area with higher values of the pH.

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Fig. 7. The photo of the base PEO-coating with defect after 24 hours of the sample exposure to 0.05 M NaCl.

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Fig. 8. The photo of the composite inhibitor-containing coating with defect after 24 hours of the sample exposure to 0.05 M NaCl.

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Fig. 9. pH distribution at the surface of the base PEO-layer with defect after 30 hours of the sample exposure to 0.05 M NaCl. The entire defect zone was the anodic one (blue area in color and pale grey in black-and-white).

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Fig. 10. Current density (a) and pH (b) values distribution at the surface of the composite inhibitor-containing layer with defect after 30 hours of the sample exposure to 0.05 M NaCl. The corrosion process began to develop more intensively over the entire length of the defect.

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Fig. 11. SVET mapping of the current density distribution at the surface of the composite inhibitor-containing coating with defect upon exposure of the sample to 0.05 M NaCl (in days): a) 2, b) 3, c) 4, d) 5, e) 6, and f) 7. Increasing of the current density values with the increasing of the exposure time occurred in the local anodic area of the artificial defect (central zone of the figure).

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Fig. 12. Photos of the corrosion process distribution at the surface of the composite inhibitorcontaining coating with defect upon exposure of the sample to 0.05 M NaCl (in days): a) 2, b) 4, and c) 7.

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Fig. 13. SIET mapping of the pH distribution on the surface of the composite inhibitorcontaining coating with defect upon exposure of the sample to 0.05 M NaCl (in days): a) 2, b) 3, c) 4, d) 5, e) 6, and f) 7. The defect zone (central zone of the figure) was more alkaline (red in color version and more dark grey in black-and-white) in comparison with undamaged part of the sample.

44