CHANGES IN BARRIER PROPERTIES OF PROTECTIVE COMPOSITE COATINGS ON ALUMINUM ALLOY DURING CLIMATIC TESTING Текст научной статьи по специальности «Химические науки»

i l St. Petersburg Polytechnic University Journal. Physics and Mathematics. 2022 Vol. 15, No. 3.1 Научно-технические ведомости СПбГПУ. Физико-математические науки. 15 (3.1) 2022

Conference materials

UDC 539.232+620.193.75

DOI: https://doi.org/10.18721/JPM.153.143

Changes in barrier properties of protective composite coatings on aluminum alloy during climatic testing

I. E. Vyaliy 1 e, V. S. Egorkin \ N. V. Izotov \ U. V. Kharchenko \ A. N. Minaev 2, S. L. Sinebryukhov 1, S.V. Gnedenkov 1

1 Institute of Chemistry FEB RAS, Vladivostok, Russia

2 Far Eastern Federal University, Vladivostok, Russia

H vyaly@ich.dvo.ru

Abstract: The paper reveals the changes in corrosion properties of the samples with composite coating subjected to atmospheric corrosion test for 6 months. Composite coating (CC) was formed by treatment of oxide layer obtained by plasma electrolytic oxidation (PEO) with suspension of superdispersed polytetrafluoroethylene microparticles (SPTFE) in polyvinylidenefluoride (PVDF). The corrosion current density was compared for the composite coatings, PEO-coated and untreated samples. It is shown that corrosion current density for the sample with CC tested for 6 months (2.9'10-11 A-cm-2) is more than 3 orders of magnitude lower in comparison with the sample with PEO-layer (3.5-10-8 A-cm-2) and almost 6 orders of magnitude less than for uncoated aluminium alloy (1.7-10-5 A-cm-2).

Keywords: aluminum, protective coating, plasma electrolytic oxidation, composite coating, electrochemistry

Funding: This study was funded by RFBR, grant number 19-29-13020.

Citation: Vyaliy I. E., Egorkin V. S., Izotov N. V., Kharchenko U. V., Minaev A. N., Sinebryukhov S. L., Gnedenkov S. V., Changes in barrier properties of protective composite coatings on aluminum alloy during climatic testing, St. Petersburg State Polytechnical University Journal. Physics and Mathematics. 15 (3.1) (2022) 253-258. DOI: https://doi.org/10.18721/ JPM.153.143

This is an open access article under the CC BY-NC 4.0 license (https://creativecommons. org/licenses/by-nc/4.0/)

Материалы конференции УДК 539.232+620.193.75 DOI: https://doi.org/10.18721/JPM.153.143

Изменение барьерных свойств защитных композиционных покрытий на алюминиевом сплаве при климатических испытаниях

И. Е. Вялый 1 и, В. С. Егоркин \ Н. В. Изотов \ У. В. Харченко А. Н. Минаев 2, С. Л. Синебрюхов 1, С. В. Гнеденков 1

1 Институт химии ДВО РАН, г. Владивосток, Россия 2 Дальневосточный Федеральный Университет, г. Владивосток, Россия

н vyaly@ich.dvo.ru

Аннотация. В работе представлены данные, характеризующие динамику изменения коррозионных свойств образцов с композиционным покрытием, подвергнутых атмосферной коррозии в течение 6 мес. Композиционное покрытие (КП) формировали обработкой оксидного слоя, полученного плазменно-электролитическим оксидированием (ПЭО), суспензией микрочастиц ультрадисперсного политетрафторэтилена (УПТФЭ)

© Vyaliy I. E., Egorkin V. S., Izotov N. V., Kharchenko U. V., Minaev A. N., Sinebryukhov S. L., Gnedenkov S. V., 2022.

Published by Peter the Great St. Petersburg Polytechnic University.

^St. Petersburg Polytechnic University Journal. Physics and Mathematics. 2022 Vol. 15, No. 3.1

в поливинилиденфториде (ПВДФ). Проведено сопоставление полученных значений тока коррозии после 3 и 6 месяцев испытаний с соответствующими значениями для необработанного образца и образца с ПЭО-покрытием. Установлено, что барьерные свойства образца с КП более чем на 3 порядка выше по сравнению с образцом с ПЭО-слоем и почти на 6 порядков выше, чем у сплава без покрытия.

Ключевые слова: алюминий, защитное покрытие, плазменное электролитическое оксидирование, композиционное покрытие, электрохимия

Финансирование: Исследование выполнено при поддержке гранта РФФИ № 19-29-13020.

Ссылка при цитировании: Вялый И. Е., Егоркин В. С., Изотов Н. В., Харченко У. В., Минаев А. Н., Синебрюхов С. Л., Гнеденков С. В. Изменение барьерных свойств защитных композиционных покрытий на алюминиевом сплаве при климатических испытаниях // Научно-технические ведомости СПбГПУ. Физико-математические науки. 2022. Т. 15. № 3.1. С. 253-258. DOI: https://doi.org/10.18721/JPM.153.143

Статья открытого доступа, распространяемая по лицензии CC BY-NC 4.0 (https:// creativecommons.org/licenses/by-nc/4.0/)

Introduction

Protection of aluminum and its alloys, which are prone to corrosion in halide-containing media, is an important scientific and sought after practical task [1]. The performed studies established a high level of protection provided by a composite coating (CC), where heterogeneous oxide layer is used as a matrix for the polymer layer on AMg3 aluminum alloy [2, 3]. However, the conditions of exploitation and electrochemical express-testing may vary significantly [4]. Therefore, it is necessary to verify the obtained parameters under appropriate exploitation conditions. For this purpose, as the most suitable for testing and evaluating the influence of these factors on the anti-corrosion properties of composite coatings the estimation of protective properties assessed via electrochemical study of samples subjected to atmospheric corrosion test for 3 and 6 months was performed.

Materials and Methods

Plasma electrolytic oxidation of AMg3 aluminum alloy samples with size of 30 * 30 * 2 (mm)3 was carried out in bipolar mode for 15 minutes. The voltage during the anodic period was increased from 30 to 540 V at a rate of 3.4 V/s. During the cathodic period current density was maintained at 0.12 Acm-2. The duty cycle was equal to 1. The silicate electrolyte (20 g/L Na2SiO35H2O, 10 g/L Na2B407 10H20, 2 g/L NaF and 2 g/L KOH) and polarization mode led to formation of the layer with a complex morphology [1].

To form CC, superdispersed polythetrafluoroetylene (SPTFE) particles were added to the 6% polyvinylidenefluoride (PVDF) solution in N-methyl-2-pyrrolidone [1]. Then the samples were dip-coated and dried at 65 °C for 3 h.

To study the dynamics of changes in electrochemical properties during the atmospheric corrosion, the samples were installed at the Marine Corrosion Test Station of the Institute of Chemistry of FEB RAS, located on Russkiy Island, Rynda Bay [5]. This water area has a salinity of seawater close to the average oceanic (34%o) and classifies according to ISO 9223:2012 as category C3, i.e. a marine environment with medium atmospheric corrosivity. Samples with size of 30*30*2 (mm)3 were exposed at an angle of 45 to the horizon on racks located about 20 m away from the coastline. The witness samples were taken after 3 and 6 months of field testing.

The morphology of the composite coatings was investigated by scanning electron microscopy (SEM) using Carl Zeiss EVO 40 microscope with an acceleration voltage of 20 kV.

Electrochemical properties were investigated using ModuLab XM ECS (Solartron analytical, Farnborough, UK). Measurements were carried out in a three-electrode cell in a 3 wt.% NaCl. Platinum mesh was used as a counter electrode, and saturated calomel electrode was used as a reference electrode. The exposed area of samples was equal to 1 cm2. The samples were kept

© Вялый И. Е., Егоркин В. С., Изотов Н. В., Харченко У. В., Минаев А. Н., Синебрюхов С. Л., Гнеденков С. В., 2022. Издатель: Санкт-Петербургский политехнический университет Петра Великого.

in a solution for 60 minutes prior to electrochemical tests to achieve a steady state. To record the impedance spectrum, a sinusoidal signal with an amplitude of 10 mV (rms) was used. The experiments were carried out in the frequency range from 0.1 MHz to 0.01 Hz at a logarithmic sweep of 10 points per decade. The potentiodynamic polarization measurements were carried out at a scan rate of 1 mV s-1 in the range from Ec -0.25 V to Ec +2 V, where Ec is the corrosion potential. The values of polarization resistance Rp were determined in separate experiments from the linear potential current density plot in range of E ± 20 mV as the R = AE/Aj.

C p

Results and Discussion

Analysis of the SEM-images in Fig. 1 indicates that PEO-layer formed in the established oxidation mode and electrolyte, have a complex morphology. Application of PVDF solution with SPTFE microparticles atop the PEO-layer leads to a shielding of microdefects and pores and covers surface with uniformly distributed particles. PVDF solution embeds microparticles and provides adhesion to the PEO-coating.

Moreover, the microparticles form large agglomerates, which probably better cover the porous part of the PEO-layer and increase its anti-corrosion parameters [1].

SEM image of the CC surface after 6 months of the atmospheric exposure is presented in Fig. 2. Composite coating has high strength and resistance to temperature changes, as well as resistance to ultraviolet radiation and atmospheric precipitation. The SPTFE particles sealed the pores and defects, preventing pitting corrosion. At the same time, there are no defects in the PEO-coating, which, during 6 months of atmospheric corrosion, remained completely covered with a PVDF/SPTFE-layer.

In the investigated range of potentials, the potentiodynamic curve for an uncoated aluminum alloy is the typical for this material. After the cathodic part of the curve (Fig. 3), a breakdown of the natural oxide-hydroxide film occurs with a corresponding sharp increase in the current density during the development of the corrosion process. The potentiodynamic curves obtained for the coated samples are located in the zone of significantly lower currents compared to the curve for the uncoated alloy and demonstrate a significant inhibition of the corrosion process.

Fig. 1. SEM-images of the PEO and composite coatings formed on the AMg3 aluminum alloy

Fig. 2. SEM-image of the composite coating after 6 months of the atmospheric corrosion test

Fig. 3. Evolution of the electrochemical properties for uncoated, PEO-coated and composite polymer contained AMg3 aluminum alloy during atmospheric exposure

Electrochemical studies of the samples found that the formed protective layers significantly reduce the corrosion current density.

Thus, for a sample with a composite coating, the corrosion current density is equal to 7.5T0-12 A cm-2, which is more than 4 and 5 orders of magnitude lower than this parameter for the PEO-coated (/ = 8.410-8 A cm-2) and for the uncoated AMg3 aluminum alloy (/ = 1.110-6 Acm-2), respectively.

The composite coating has the highest barrier properties among the studied layers due to its multimodal textured surface (Fig. 1). According to the results of [6], the area of direct contact of such coatings with an aggressive medium, calculated using the Cassie-Baxter equation, is no more than 3% of the total area of the sample immersed in a chloride-containing media. The stable state of the entrapped air in the irregularities of the multimodal surface, which prevents the formation of a wetting film between the liquid and the developed surface of the coating, provides high anticorrosion properties [6]. Therefore, a sample with a composite coating, having a multilevel relief formed by agglomerates of SPTFE microparticles, has a higher value of polarization resistance (R = 3.91010 fi cm2) compared to samples without a coating (R = 1.9105 fi cm2) and with a pEo-coating (R = 2.4-104 fi cm2).

The results of electrochemical studies of samples after atmospheric corrosion are presented in Table 1 and Fig. 3. For sample with PEO-coating, the corrosion current density during 6 months exposition is reduced by 2.5 times due to the sealing of its pores with corrosion products. Similarly, the polarization resistance increases for it (in 3 times). Also, the corrosion current density for the sample with CC tested for 6 months (/ = 2.910-11 Acm-2) is 3 orders of magnitude lower in comparison with the sample with PEO-layer (/ = 3.510-8 Acm-2) and almost 6 orders of

Table 1

Electrochemical parameters calculated for the samples without coating, with PEO-layer and composite coating before and after atmospheric corrosion test for 0, 3 and 6 months

Samples B , mV •a5 Pc, mV E, Vvs. SCE c7 I, A-cm 2 R , fi'cm2 p'

AMg3 17.5 154.8 -0.67 1.1-10-6 2.4-104

AMg3 after 3 months 7.6 207.9 -0.73 1.6-10-6 2.5-103

AMg3 after 6 months 11.2 179.3 -0.95 1.740-5 3.8-102

PEO 39.0 429.5 -0.87 8.4-10-8 1.9405

PEO after 3 months 86.3 334.3 -0.79 6.9-10-8 4.3-105

PEO after 6 months 78.3 422.1 -0.69 3.5-10-8 5.7-105

CC 488.8 243.2 -0.59 7.5-10-12 3.9-1010

CC after 3 months 279.3 416.4 -0.53 1.5-10-11 4.9-109

CC after 6 months 305.0 302.3 -0.48 2.9-10-11 2.3-109

magnitude less than for uncoated AMg3 aluminum alloy (/ = 1.7-10-5 Acm-2). The polarization resistance for this sample over the same period showed the value 2.3-109 fi-cm2, which is more than 3 orders of magnitude higher than one for sample with a PEO-layer (Rp = 5.7-105 fi-cm2) and almost 7 orders of magnitude greater than for uncoated AMg3 aluminum alloy (Rp = 3.8102 fi-cm2).

Comparison of the corrosion current values obtained as a result of field tests (Table 1) with the literature data showed that after six months of exposure, both PEO (Ic = 3.5-10-8 Acm-2) and composite coating (Ic = 2.9-10-11 Acm-2) have higher barrier properties compared to other PEO coatings (Ic = 3.9-10-8 Acm-2) [5], (Ic = 1.5-4.5-10-6 Acm-2) [7], as well as composite coatings (Ic = 7.610-10 Acm-2) [7]. The initial values of corrosion current density for Alodine 1200s and PreCoat A32 chromium (VI) containing conversion coatings (Ic = 8.0-9.5 10-7 Acm-2) in the work [8] an order of magnitude higher compared to the values registered for the PEO-layer (Ic = 8.4-10-8 Acm-2) and 5 orders of magnitude higher than for the composite coating (Ic = 7.5• 10-12 A-cm-2). Even after 6 months of exposure the CC shows higher level of barrier properties (Ic = 2.9-10-11 Acm-2) in comparison with PTFE-containing IFKhANAL-3 conversion coating (Ic = 1.0-2.0-10-6 Acm-2) [9]. It can also be noted that the corrosion current density for the presented composite coating is one order of magnitude lower than estimated for the superhydrophobic coating formed by treatment of laser-processed aluminum alloy with fluorosilane (Ic = 10-10-10-11 Acm-2) presented in [10].

Conclusion

It has been established that composite coating provides reliable protection even after a long-term exposure, the difference in currents is half an order of magnitude. This result is ensured by the high stability of both the polymeric materials and the PEO-coating used as the matrix. The increase in protective parameters for the PEO-coating is explained by the sealing of its pores with corrosion products.

Acknowledgments

Far Eastern Center for Electron Microscopy, Federal State Institution of Science A.V. Zhirmunsky National Scientific Center of Marine Biology of the Far Eastern Branch of the Russian Academy of Sciences (Vladivostok, Russia) for assistance in the study of coating morphology.

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^St. Petersburg Polytechnic University Journal. Physics and Mathematics. 2022 Vol. 15, No. 3.1 ^

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THE AUTHORS

VYALIY Igor E.

vyaly@ich.dvo.ru

ORCID: 0000-0003-3806-1709

MINAEV Aleksander N.

aminaev@mail.ru

ORCID: 0000-0002-8072-306X

EGORKIN Vladimir S.

egorkin@ich.dvo.ru ORCID: 0000-0001-5489-6832

SINEBRYUKHOV Sergey L. sls@ich.dvo.ru

ORCID: 0000-0002-0963-0557

IZOTOV Nikolai V.

nikolaj.izotov@mail.ru ORCID: 0000-0001-9504-1523

GNEDENKOV Sergey V.

svg21@hotmail.com

ORCID: 0000-0003-1576-8680

KHARCHENKO Ulyana V.

ulyana-kchar@mail.ru ORCID: 0000-0001-5166-5609

Received 23.05.2022. Approved after reviewing 24.07.2022. Accepted 24.07.2022.

© Peter the Great St. Petersburg Polytechnic University, 2022