PREPARATION OF Mg-Al LDH COATING ON AZ31 MAGNESIUM ALLOY UNDER MILD CONDITIONS

May 17 - 19, 2023, B no, Czech Republic, EU PREPARATION OF MG-AL LDH COATING ON AZ31 MAGNESIUM ALLOY UNDER MILD CONDITIONS Pa lína ŠOMANOVÁ, Ma in BUCHTÍK, Ja omí WASSERBAUER, Leoš DOSKOČIL B no Uni e si y o Technology, Facul y o Chemis y, B no, Czech Republic, EU, [email p o ec ed] h ps://doi.o g/10.37904/me al.2023.4712 Abs ac Magnesium and i s alloys a e in e es ing ma e ials bu hey ha e low co osion esis ance. Magnesium- aluminium double laye ed hyd oxide (Mg-Al LDH) coa ings we e p epa ed on AZ31 alloy by hyd o he mal me hod a 50 °C, 70 °C and 90 °C a di e en eac ion imes (3 h, 6 h and 24 h). The coa ings we e cha ac e ized by scanning elec on mic oscope (SEM) wi h ene gy dispe si e X- ay spec oscope (EDS), X- ay di ac ion (XRD) and po en iodynamic pola iza ion measu emen s in 0.15 M NaCl. The op imum condi ions o he p epa a ion o he coa ing included a empe a u e o 90 °C and a eac ion ime o 24 h, as he coa ing was composed o Mg-Al LDH and exhibi ed he highes co osion esis ance. Keywo ds: Mg-Al LDH coa ings, AZ31 alloy, co osion, mild condi ions 1. INTRODUCTION Magnesium and i s alloys ha e good mechanical p ope ies and low densi y, hanks o hese p ope ies i can be used in many in e es ing applica ions ( anspo indus y, medicine e c.). The disad an age o magnesium ma e ials is hei low co osion esis ance [1]. Fo his eason, a e magnesium alloys su ace ea ed by a ious me hods o inc ease hei co osion esis ance (e. g. supe hyd ophobic coa ings, High Veloci y Oxygen Fuel (HVOF) coa ing, Ni-P coa ing) [2]. One o he p omising and cu en ly popula me hod is he p epa a ion o a double laye ed hyd oxide (LDH) coa ing. Special a en ion is paid o he Mg-Al LDH ype coa ing. This coa ing has many in e es ing p ope ies and has he po en ial o inc ease he co osion esis ance o magnesium alloy [3-5]. The p inciple o educing he co osion a e o he subs a e lies in he mechanical ba ie and he abili y o hese laye s o ap co osi e anions (e.g. Cl-) in hei s uc u e. Fo his eason, he con ac be ween he co osi e en i onmen and he subs a e su ace is educed [2]. The s uc u e o LDH is made up o la laye s o oc ahed al clus e s [Mg(OH)6] ha a e connec ed by hyd ogen bonds. This s uc u e is o med om he magnesium ca ions Mg2+, which a e oc ahed ally coo dina ed by six hyd oxyl anions OH-. The basic s uc u al o mula o LDH is [MII1- xMIIIx(OH)2]x+[An-x/n·yH2O]x-, whe e MII ep esen s he di alen me al, MIII he i alen me al and An- he n-po en in e cala ed anion. In his s uc u e, he posi i e cha ge o he magnesium ca ions is compensa ed by he nega i e cha ge o he hyd oxyl anion. [5,6] The p epa a ion o LDH coa ings is usually ca ied ou by he hyd o he mal me hod, which equi es highe empe a u es. Howe e , his ou e is no a ac i e o indus ial applica ions. Recen ly, se e al pape s ha e been published on he p epa a ion o LDH coa ing unde mild condi ions. Shulha e al. [3] in es iga ed he e ec o chela ing agen s on he p epa a ion o Mg-Al LDH coa ing on AZ91 alloy. The au ho s showed ha LDH syn hesis was achie ed unde mild eac ion condi ions (95°C, 6h, pH 10) and acili a ed in he p esence o chela ing agen s such as NTA and EDTA. Dou e al. [6] in es iga ed he e ec o ini ial Al3+ ion concen a ion on he p epa a ion o Mg-Al LDH coa ings on AZ31 magnesium alloy. Thei esul s sugges ha di e en Al3+ concen a ion has an impac on he co osion esis ance and s uc u e o he p epa ed LDH coa ing. May 17 - 19, 2023, B no, Czech Republic, EU The aim o his wo k was o p epa e Mg-Al LDH coa ing on AZ31 alloy unde mild condi ions wi h emphasis on he e ec o empe a u e and ime. The p epa ed coa ings we e cha ac e ized by po en iodynamic pola iza ion in 0.15 M NaCl. 1. MATERIAL AND METHODS 1.1 Ma e ial Magnesium alloy AZ31 was used in his expe imen . The elemen al composi ion o AZ31 alloy was de e mined by Glow-Discha ge Op ical Emission Spec oscopy (GDOES; Spec uma GDS 750, Spec uma Analy ik GmbH, Ho , Ge many). The elemen al composi ion o AZ31 in w .% consis o 3.60 Al, 1.34 Zn, 0.28 Mn, 0.03 Si, 0.002 Fe, 0.01 Sn and Mg. Dimensions o he AZ31 samples we e 20 mm × 20 mm × 5 mm. These samples we e g ound wi h 800 and 1200 g i SiC pape , insed by deionized wa e and isop opanol, and d ied wi h ho ai . 1.2 Coa ing p epa a ion The solu ion o Mg-Al LDH coa ing was p epa ed by dissol ing 0.1 M EDTA in 50 ml o deionized wa e . The pH o his solu ion was adjus ed by 2 M NaOH solu ion a he alue o 7 – 7.4. A e ha 0.25 M NaNO3 and 0.05 M Al(NO3)3 we e added o he solu ion and he pH alue was adjus ed by NaOH solu ion a 10 ± 0.1. The g ounded samples we e pu in o beake s wi h 50 ml o his solu ion and co e ed by aluminium oil. The beake s we e pu in o p ehea ed o en. The condi ions we e used: (i) 50 °C, 3 h, 6 h, 24 h, (ii) 70 °C, 3 h, 6 h, 24 h and (iii) 90 °C, 3 h, 6 h, 24 h. A e he eac ion he samples we e insed by deionized wa e , isop opanol and d ied wi h ho ai . 1.3 Cha ac e iza ion SEM – EDS Analysis The su ace mo phology and elemen al composi ion o he Mg-Al LDH coa ing was cha ac e ized using scanning elec on mic oscope (SEM, ZEISS, EVO LS-10) wi h an ene gy dispe si e X- ay spec oscope (EDS, Ox o d Ins umen s plc, Abingdon, UK). PDP Measu emen s Po en iodynamic pola iza ion (PDP) measu emen s we e pe o med using a Bio-Logic VSP-300 po en ios a (BioLogic, Seyssine -Pa ise , F ance). The measu ed samples we e used as wo king elec ode. The exposed a ea o he measu ed samples was 1 cm2. The sa u a ed calomel elec ode (SCE) was used as he e e ence elec ode and P mesh as he coun e -elec ode. A 0.15 M NaCl solu ion wi hou pH adjus men was used as a co osi e medium in he co osion cell wi h a olume o 200 ml. Open ci cui po en ial (OCP) we e s abilized du ing 60 min o exposu e. A e wa ds, PDP measu emen s we e pe o med in he po en ial ange om −150 mV o 500 mV s. OCP a a scan a e o 1 mV/s. XRD Analysis The phase composi ion o Mg-Al LDH coa ing was analysed by X- ay di ac ion (XRD; Panaly ical, Mal e n, UK). The X- ay sou ce was Cu Kα1 adia ion (λ =1.540598 Å), which was ope a ed a a ol age o 40 kV and a cu en o 30 mA wi hin he ange o 5° and 90° 2θ a a scanning a e o 0.01313° 2θ ime pe s ep o 46 s. 2. RESULTS AND DISCUSSION Visual obse a ion o he samples showed ha he coa ing changed om da k g ey o ligh g ey wi h inc easing empe a u e and ime, Figu e 1. May 17 - 19, 2023, B no, Czech Republic, EU Figu e 1 Colou ansi ion o he Mg-Al LDH coa ing wi h inc easing ime and empe a u e. Mg-Al LDH p epa ed o 3 h a 50 °C (le ), Mg-Al LDH p epa ed o 24 h a 90 °C ( igh ) Figu e 2 SEM images o AZ31 a e su ace ea men a 50 °C o 3 h (A) and 24 h (D), a 70 °C o 3 h (B) and 24 h (E), a 90 °C o 3 h (C) and 24 h (F) May 17 - 19, 2023, B no, Czech Republic, EU SEM analysis showed ha he mo phology o he coa ings changed wi h inc easing empe a u e and ime, Figu e 2. The su ace o he sample p epa ed a 50 °C o 3 h consis ed o a ne wo k s uc u e o Mg(OH)2 and islands o Mg(OH)2, Figu e 2A. EDS analysis also e ealed he p esence o alumina a ound 8 a .%, bu he p esence o LDH was no de ec ed by XRD. These islands g ew wi h longe ime and subsequen ly co e ed he su ace o AZ31, Figu e 2D. Howe e , he su ace was no compac and con ained po es h ough which he co osi e en i onmen could pene a e o he magnesium alloy o AZ31. Samples p epa ed a 70 °C again exhibi ed a ne wo k s uc u e ha appea ed o be a honeycomb s uc u e a longe imes, Figu e 2B and 2E. The honeycomb s uc u e ob ained a 70 °C o 24 h had a la ge cell diame e and he cells we e discon inuously illed wi h sphe ical o ma ions, consis en wi h a chemical composi ion domina ed by Mg(OH)2. The honeycomb-like s uc u e was also obse ed a 90 °C a a ious empe a u es, bu a sho e eac ion imes he su ace o some cells was obse ed o be pa ially pe o a ed, Figu e 2C and 2F. This obse a ion sugges s ha he highe empe a u e acili a ed he g ow h o Mg(OH)2 inside he cells, leading o hei almos comple e closu e. The coa ing p epa ed a 90 °C o 24 h showed a closed su ace and was co e ed wi h e y ine Mg-Al LDH c ys als, Figu e 2F. This means ha he sample su ace was deg aded wi hou o ming a p o ec i e LDH coa ing, as can also be obse ed om he SEM analysis, Figu e 2A. The longe p epa a ion ime o he coa ing esul ed in ico alues simila o hose o un ea ed AZ31. These esul s sugges ha he su ace was co e ed wi h a laye o Mg(OH)2, as shown by SEM analysis (Figu e 2E), which p o ided pa ial p o ec ion o AZ31. The alue o ico o he samples p epa ed a empe a u e o 70 °C o 3 h and 6 h we e h ee imes highe han un ea ed AZ31. A wo old dec ease in he ico alue was obse ed wi h inc easing p epa a ion ime o he coa ing (24 h). This imp o emen is a ibu ed o he incipien o ma ion o he LDH coa ing, al hough i was no de ec ed by XRD analysis. This could be due o he low con en o LDH c ys als and/o hei nanoscale size. A a p epa a ion empe a u e o 90 °C and a p epa a ion ime o 3 h, he co osion esis ance o he sample dec eased ela i e o he AZ31 alloy. Longe p epa a ion imes o 6 h and 24 h imp o ed he ico alues by a ac o o 2 and 6 compa ed o un ea ed AZ31. Wi h espec o SEM and XRD analysis, he p esence o LDH coa ing was con i med o he samples p epa ed a 90 °C and 24 h. Thus, he inc eased co osion esis ance o he sample can be a ibu ed o he p esence o Mg-Al LDH coa ing on he AZ31 alloy. These esul s a e consis en wi h he expe imen s pe o med by Shulha e al. [5] on he AZ91 alloy. Table 1 Elec ochemical pa ame e s ob ained om po en iodynamic cu es o he co osi e en i onmen o a 0.15 M NaCl solu ion P epa a ion empe a u e (°C) P epa a ion ime (h) ico (µA/cm2) Eco (mV) Un ea ed AZ31 6.4 ± 0.6 -1.47 ± 0.02 50 3 18.6 ± 3.2 -1.44 ± 0.01 6 3.1 ± 2.1 -1.27 ± 0.03 24 7.7 ± 1.9 -1.41 ± 0.03 70 3 17.0 ± 2.8 -1.45 ± 0.02 6 21.6 ± 5.3 -1.46 ± 0.03 24 3.8 ± 0.9 -1.30 ± 0.02 90 3 19.4 ± 4.1 -1.47 ± 0.02 6 2.8 ± 2.2 -1.40 ± 0.01 24 1.1 ± 0.7 -1.34 ± 0.01 Po en iodynamic pola iza ion cu es we e measu ed in 0.15 M NaCl solu ion o un ea ed AZ31 alloy and o samples coa ed a 50 °C, 70 °C and 90 °C wi h eac ion imes o 3 h, 6 h and 24 h. The po enciodynamic May 17 - 19, 2023, B no, Czech Republic, EU cu es we e e alua ed and co osion pa ame e s, including co osion cu en densi y (ico ) and co osion po en ial (Eco ), we e ob ained as shown in Table 1. These esul s showed ha he alue o Eco shi s o mo e posi i e alues wi h inc easing p epa a ion ime and empe a u e compa ed o uncoa ed AZ31. This indica es an imp o emen in co osion p ope ies om a he modynamic poin o iew. XRD analysis o he Mg-Al LDH coa ing p epa ed a 90 °C o 24 hou s is shown in Figu e 3. The esul s showed ha i was possible o p epa e Mg-Al LDH coa ing on AZ31 magnesium alloy unde mild condi ions. This ac is con i med by he peaks a 11.7° 2θ° and 23.3° 2θ°, which a e cha ac e is ic o LDH. The XRD spec a o he o he samples did no con ain his pai o peaks, which may indica e ha sui able condi ions o LDH coa ing o ma ion we e no achie ed. Howe e , i canno be excluded ha in some cases LDH c ys als may ha e been p esen , bu only in small amoun s and e y ine. Figu e 3 XRD spec um o AZ31 ea ed wi h Mg-Al LDH coa ing a 90 °C o 24 h 3. CONCLUSION The p epa a ion o he Mg-Al LDH coa ing was ca ied ou a 50, 70 and 90 °C and a di e en imes o 3, 6 and 24 h. Su ace analysis showed ha he mo phology a ied depending on he eac ion condi ions. Fine Mg- Al LDH c ys als we e demons a ed on he AZ31 magnesium alloy wi h an obse able honeycomb s uc u e when he eac ion condi ions we e a 90 °C and a 24 h. This condi ion p o ed o be op imal also om a co osion poin o iew, as he coa ing had he highes co osion esis ance han he o he samples including he ba e AZ31 alloy. ACKNOWLEDGEMENTS This wo k was suppo ed by Speci ic Uni e si y Resea ch a FCH BUT, P ojec N . FCH-S-23-8208, Minis y o Educa ion, You h and Spo s o he Czech Republic and In e nal p ojec o PhD s uden s FCH BUT-2022. REFERENCES [1] HORST, F ied ich E. and MORDIKE, Ba y L. Magneisum Technology: Me allu gy, Design Da a, Ap-plica ions Ge many: Sp inge , 2006. ISBN 10 3-540-20599-3. [2] DOSKOČIL, L., ŠOMANOVÁ, P., MÁSILKO, J., BUCHTÍK, M., HASOŇOVÁ, M., KALINA, L., WASSERBAUER, J. Cha ac e iza ion o P epa ed Supe hyd ophobic Su aces on AZ31 and AZ91 Alloys E ched wi h ZnCl2 and SnCl2. Coa ings. 2022, ol. 12, pp. 1414. A ailable om: h ps://doi.o g/10.3390/coa ings12101414. May 17 - 19, 2023, B no, Czech Republic, EU [3] SHULHA, T. N., SERDECHNOVA, M., LAMAKA, S. V., WIELAND, D. C. F., LAPKO, K. N., ZHELUDKEVICH, M. L. Chela ing agen -assis ed in si u LDH g ow h on he su ace o magnesium alloy. Scien i ic Repo s. 2018, ol. 8, pp. 16409. A ailable om: h ps://doi.o g/10.1038/s41598-018-34751-7 [4] DOU, Z., ZHANG, Y., SHULHA, T., CUI, R., SERDECHNOVA, M., TIAN, H., YAN, T., BLAWERT, C., LI, L., ZHELUDKEVICH, M. L., CHEN, F. Insigh in o chela ing agen s imula ed in-si u g ow h o MgAl-LDH ilms on magnesium alloy AZ31: The e ec o ini ial ca ionic concen a ions. Su ace and Coa ings Technology. 2022, ol. 439, pp. 128414. A ailable om: h ps://doi.o g/10.1016/j.su coa .2022.128414. [5] ZHANG, X., ZHANG, M., LI, R., FENG, X., PANG. X., RAO, J., CONG. D., YIN, C., ZHANG, Y., Ac i e Co osion P o ec ion o Mg–Al Laye ed Double Hyd oxide o Magnesium Alloys: A Sho Re iew. Coa ings. 2021, ol. 11, pp. 1316. A ailable om: h ps://doi.o g/10.3390/coa ings11111316. [6] GUO, L., WU, W., ZHOU, Y., ZHANG, F., ZENG, R., ZHENG, J. Laye ed double hyd oxide coa ings on magnesium alloys: A e iew. Jou nal o Ma e ials Science & Technology. 2018, ol. 34, pp. 1455-1466. A ailable om: h ps://doi.o g/10.1016/j.jms .2018.03.003.