coatings
Article A Zinc-Rich Coating Fabricated on a Magnesium Alloy by Oxide Reduction
Dongzhu Lu 1,2,3,* , Yanliang Huang 1,2,3 , Jizhou Duan 1,2,3 and Baorong Hou 1,2,3
1 CAS Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Science, Qingdao 266071, China; [email protected] (Y.H.); [email protected] (J.D.); [email protected] (B.H.) 2 Open Studio for Marine Corrosion and Protection, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China 3 Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China * Correspondence: [email protected]
Received: 1 April 2019; Accepted: 24 April 2019; Published: 25 April 2019
Abstract: The corrosion resistance of magnesium alloys could be enhanced by covering metallic coatings on the surface. The zinc-rich coating is one of these metallic coatings. To fabricate a zinc-rich coating on magnesium alloys, the substrate should be pretreated carefully, and a protective atmosphere is usually required. In this research, a zinc-rich coating was successfully fabricated on the AZ91D magnesium alloy in air by a diffusion alloying method, with zinc oxide as the zinc source. At the same time, the pretreatment of the magnesium alloy matrix was greatly simplified. The as-diffusion-alloyed zinc-rich intermetallic layer was investigated, utilizing SEM, EDS, and XRD, respectively. It is inferred that zinc oxide was reduced into Zn atoms by the active Mg atoms, and the Mg atoms were coming from the magnesium alloy matrix. Then the Zn atoms passed through the oxide film and formed an intermetallic layer on the magnesium alloy surface. Thus, taking advantage of the activity of Mg atoms, magnesium alloys could be surface alloyed with oxides.
Keywords: magnesium alloy; zinc oxide; diffusion; coating; surface treatment
1. Introduction Magnesium alloys are lightweight alloys, and could be utilized in various fields [1–3]. The corrosion resistance of magnesium alloys is poor [4–6], which limits further utilization of magnesium alloys. Coatings are fabricated on the surface to enhance the corrosion resistance of magnesium alloys [7–19]. Among which, the zinc coating is an effective one. It is reported that the weight loss of pure Zn is about half of the weight loss of pure Mg in the same duration after exposed to humid air [20]. Adding a little magnesium into pure zinc would reduce the weight loss further [20]. Moreover, zinc-rich coatings are frequently utilized in the electroplating method to fabricate protective coatings on magnesium alloys [17,21]. Magnesium alloys are active, and they are prone to react with oxygen and water to form MgO and Mg(OH)2 on the surface, which would lead to a weak adhesion of as-electrodeposited coatings [17,21]. Thus, before electroplating, magnesium alloys should be pretreated. Zinc immersion is one of the general processes to pretreat magnesium alloys before plating. Before the zinc immersion process, several pretreatment processes, including surface cleaning, alkaline cleaning, acid pickle, and acid activation, are commonly enlisted to remove soil, debris, oil, grease, oxides, etc. [17]. Then the magnesium alloy is immersed in special solutions for the zinc film to form [21].
Coatings 2019, 9, 278; doi:10.3390/coatings9040278 www.mdpi.com/journal/coatings Coatings 2018, 8, x FOR PEER REVIEW 2 of 8 Coatings 2019, 9, 278 2 of 8 grease, oxides, etc. [17]. Then the magnesium alloy is immersed in special solutions for the zinc film to form [21]. The zinc-rich coating could also be fabricated by a thermalthermal evaporationevaporation methodmethod [[18].18]. In this method, a vacuum environment is required, and the temperature for the evaporation of the original Zn powder is 600 °C.◦C. Similarly, the magnesium sp specimenecimen could could be be obtained obtained by by thermal thermal diffusion, diffusion, which buries the magnesium alloy specimen into th thee pure pure Zn Zn powder, powder, and keeps keeps the the diffusion diffusion system system at an elevatedelevated temperaturetemperature for for the the zinc-rich zinc-rich coating coatin tog grow.to grow. In this In this method, meth theod, di theffusion diffusion system system could couldbe buried be buried in compact in compact carbon carbon powder powder to further to further prevent prevent oxidation oxidation [19]. [19]. Thus, it is crucial to clean the surface of a magnesium alloy matrix to prohibit the formation of the magnesium oxide oxide film film in in the zinc immersion method, method, and and it it is is also also crucial crucial to to keep a a relatively inertinert atmosphere, atmosphere, to prohibit the oxidation of pure metals in the the thermal thermal evaporation evaporation method and the the thermal diffusion diffusion method. It It seems seems as as though though oxides oxides are are always always unfavorable, unfavorable, and and should should be be avoided. avoided. In this this research, research, a amethod method based based on onthe thediffusion diffusion alloying alloying process, process, but utilizing but utilizing zinc oxide zinc as oxide the diffusionas the di ffsourceusion sourceinstead insteadof pure ofZn pure powder Zn powderto fabricate to fabricatezinc-rich zinc-richcoatings on coatings magnesium on magnesium alloys, is raised.alloys, isThe raised. pretreatment The pretreatment process of process the magnesium of the magnesium alloy matrix alloy is matrix simplified is simplified effectively eff ectivelyby this method.by this method. Meanwhile, Meanwhile, the environment the environment requirement, requirement, such as a suchvacuum as a or vacuum inert atmosphere, or inert atmosphere, becomes unnecessary.becomes unnecessary.
2. Experimental Experimental Procedures Procedures An as-cast AZ91D magnesium alloy with with the composition of of 9.1 9.1 wt.% wt.% Al, Al, 0.52 0.52 wt.% wt.% Zn, Zn, 0.26 0.26 wt.% wt.% Mn and Mg as balance, balance, was utilized as the matrix. matrix. Samples Samples were were cut cut into into specimens specimens with with a a dimension dimension of 15 mm 10 mm 3 mm from the AZ91D magnesium alloy sheets with an electrical discharge of 15 mm × 10 mm × 3 mm from the AZ91D magnesium alloy sheets with an electrical discharge cutting machine, and and then polished with with SiC papers to 1000 grit, rinsed with alcohol, and naturally dried up. The didiffusionffusion sourcesource isis a a mixture mixture of of 1 1 g g NH NH4Cl4Cl powder powder and and 10 10 g ZnOg ZnO powder. powder. Photographs Photographs of the of theNH NH4Cl4 powderCl powder and and the ZnOthe ZnO powder powder are shownare shown in Figure in Figure1. The 1. magnesium The magnesium alloy specimensalloy specimens were wereinserted inserted in the in di theffusion diffusion source. source. The diTheffusion diffus sourceion source and theandmagnesium the magnesium alloy alloy specimens specimens were were put putinto into a 30 mLa 30 crucible mL crucible for the for following the following heat treatment. heat treatment. A schemetic A schemetic diagram diagram of the di ffofusion the diffusion system is systemshown inis shown Figure2 in. Figure 2.
Figure 1. Photographs of the NH 4ClCl powder powder ( (aa)) and and the the ZnO ZnO powder powder ( (bb).).
After the furnace temperature reached 430 °C and was kept stable for minutes, the filled crucible was put into the furnace as quickly as possible, and kept at 430 °C for 2 h for the surface alloying process on the magnesium alloy specimen to proceed. The whole surface alloying process was carried out in the air; neither vacuum nor inert gases were utilized. After the surface alloying process, specimens were taken out from the furnace and cooled down naturally in the air. Coatings 2019, 9, 278 3 of 8 Coatings 2018, 8, x FOR PEER REVIEW 3 of 8
Figure 2.2. SchematicSchematic diagram diagram to to show show the the diff usiondiffusion system, system, ZnO wasZnO the was only the Zn only donor Zn in donor the di ffinusion the source,diffusion and source, the di andffusion the alloyingdiffusion process alloying was process carried was out carried directly out in directly the air. in the air.
AfterThen the residual furnace temperaturepowder on the reached magnesium 430 ◦C allo andy wasspecimens kept stable was cleaned for minutes, up, and the the filled as-surface crucible wasalloyed put intospecimens the furnace were as washed quickly with as possible, running and wate keptr atand 430 alcohol◦C for 2successively, h for the surface and alloying then naturally process ondried the up. magnesium Cross-sections alloy specimen of the as-surface to proceed. alloyed The wholemagnesium surface alloy alloying specimens process were was carriedobtained out with in the a air;saw. neither Then the vacuum cross-sections nor inert were gases polished were utilized. and et Afterched thesuccessively surface alloying for microstructure process, specimens observation. were takenThe polished out from cross-sections the furnace andwere cooled etched down until naturallythe as-surface in the alloyed air. coating showed up clearly on the specimenThen surface, the residual and then powder washed on the with magnesium alcohol and alloy naturally specimens dried was up. cleaned Microstructure up, and the photographs as-surface alloyedand composition specimens analysis were washed results of with the runningas-diffusion-alloyed water and alcohol specimens successively, were obtained and thenby a naturallyscanning driedelectronic up. microscope Cross-sections (S-3400N, of the as-surfaceHitachi, Tokyo, alloyed Japan). magnesium Phase identification alloy specimens was werecarried obtained out utilizing with aan saw. X-ray Then diffractometer the cross-sections (Ultima were IV, polished Rigaku, and Tokyo, etched Japan). successively The Gibbs for microstructure free energies observation. of relative Thechemical polished reactions cross-sections were calculated were etched by a untilsoftware the as-surface(HSC 6.0). alloyed coating showed up clearly on the specimen surface, and then washed with alcohol and naturally dried up. Microstructure photographs and3. Results composition analysis results of the as-diffusion-alloyed specimens were obtained by a scanning electronic microscope (S-3400N, Hitachi, Tokyo, Japan). Phase identification was carried out utilizing After being diffusion alloyed at 430 °C for 2 h with 10 g ZnO and 1 g NH4Cl as the diffusion ansource, X-ray a dicoatingffractometer was fabricated (Ultima IV, on Rigaku, the AZ91D Tokyo, magnes Japan).ium The alloy Gibbs specimen free energies as shown of relative in Figure chemical 3a. It reactionscould be werefigured calculated out that by the a software thickness (HSC of the 6.0). as-diffusion alloyed coating can be hundreds of micrometers. From a magnified photograph of the coating shown in Figure 3b, it could be observed 3. Results that the coating contains multiple phases. Some phases have a network shape morphology, while otherAfter phases being are di embeddedffusion alloyed in the at 430network.◦C for 2Dendri h withtes 10 gcould ZnO be and observed 1 g NH4Cl clearly. as the diTheffusion size source,of the adendrites coating wasembedded fabricated in onthe the networks AZ91D magnesiumis estimated alloy to be specimen tens of asmicrometers shown in Figure to two3a. hundred It could bemicrometers. figured out that the thickness of the as-diffusion alloyed coating can be hundreds of micrometers. From a magnified photograph of the coating shown in Figure3b, it could be observed that the coating contains multiple phases. Some phases have a network shape morphology, while other phases are embedded in the network. Dendrites could be observed clearly. The size of the dendrites embedded in the networks is estimated to be tens of micrometers to two hundred micrometers.
Figure 3. A Cross-section of the as-diffusion-alloyed intermetallic layer on an AZ91D magnesium alloy specimen with zinc oxide as the only Zn donor in the diffusion source (a) and partially enlarged zone (b) which is marked with a rectangle in (a). Coatings 2018, 8, x FOR PEER REVIEW 3 of 8
Figure 2. Schematic diagram to show the diffusion system, ZnO was the only Zn donor in the diffusion source, and the diffusion alloying process was carried out directly in the air.
Then the residual powder on the magnesium alloy specimens was cleaned up, and the as-surface alloyed specimens were washed with running water and alcohol successively, and then naturally dried up. Cross-sections of the as-surface alloyed magnesium alloy specimens were obtained with a saw. Then the cross-sections were polished and etched successively for microstructure observation. The polished cross-sections were etched until the as-surface alloyed coating showed up clearly on the specimen surface, and then washed with alcohol and naturally dried up. Microstructure photographs and composition analysis results of the as-diffusion-alloyed specimens were obtained by a scanning electronic microscope (S-3400N, Hitachi, Tokyo, Japan). Phase identification was carried out utilizing an X-ray diffractometer (Ultima IV, Rigaku, Tokyo, Japan). The Gibbs free energies of relative chemical reactions were calculated by a software (HSC 6.0).
3. Results
After being diffusion alloyed at 430 °C for 2 h with 10 g ZnO and 1 g NH4Cl as the diffusion source, a coating was fabricated on the AZ91D magnesium alloy specimen as shown in Figure 3a. It could be figured out that the thickness of the as-diffusion alloyed coating can be hundreds of micrometers. From a magnified photograph of the coating shown in Figure 3b, it could be observed that the coating contains multiple phases. Some phases have a network shape morphology, while other phases are embedded in the network. Dendrites could be observed clearly. The size of the Coatingsdendrites2019 ,embedded9, 278 in the networks is estimated to be tens of micrometers to two hundred4 of 8 micrometers.
FigureFigure 3.3. AA Cross-sectionCross-section of of the the as-di as-diffusion-alloyedffusion-alloyed intermetallic intermetallic layer layer on an on AZ91D an AZ91D magnesium magnesium alloy specimenalloy specimen with zincwith oxide zinc oxide as the as only the Znonly donor Zn donor in the in di theffusion diffusion source source (a) and (a) partially and partially enlarged enlarged zone Coatings(zoneb )2018 which (,b 8), whichx is FOR marked PEERis marked withREVIEW awith rectangle a rectangle in (a). in (a). 4 of 8
From the microstructure observation of the coating, it could be inferred that the formation of the coating is is not a simple solid diffusion diffusion process, li liquidquid phase should also be involved in the surface alloying process, process, since since a a pure solid diffusion diffusion process is driven by the concentration gradient which commonly results results in in a a layer layer structure, structure, while while solidification solidification of of a amolten molten phase phase is isprone prone to toresult result in ina networka network shape shape eutectic eutectic multiple-phase-microstructur multiple-phase-microstructuree [10]. [10 ].The The existence existence of of a aliquid liquid phase phase in in the diffusiondiffusion alloying process is reasonable [22], [22], beca becauseuse the eutectic temperature in the Mg–Zn binary system could even be 341 °C,◦C, according to the Mg–Zn binary phase diagram [[20].20]. From the EDS resultsresults shownshown inin FigureFigure4 ,4, it it could could be be figured figured out out that that there there are are mainly mainly Mg, Mg, Zn, Zn, Al, Al,and and O elementsO elements in thein the coating. coating. Mg Mg is is the the dominant dominant element element in in thethe AZ91DAZ91D magnesium alloy alloy matrix. matrix. Compared to the original content which was 0.52 wt wt.%.% in in the the AZ91D AZ91D magnesium magnesium matrix, matrix, the the relative relative content of ZnZn increasedincreased significantlysignificantly in in the the magnesium magnesium alloy alloy surface, surface, which which could could be attributedbe attributed to theto thecontinuous continuous input input of Zn of atoms Zn atoms from from the di theffusion diffusio source.n source. There There was 9.1 was wt.% 9.1 Al wt.% in the Al originalin the original AZ91D AZ91Dmagnesium magnesium specimen, specimen, hence Al hence could Al also could be foundalso be in found the coating. in the coating. The existence The existence of the O of element the O elementis inferred is inferred to be related to be torelated the oxidation to the oxidation of active of metallic active metallic atoms on atoms the cross-section on the cross-section of the AZ91D of the AZ91Dmagnesium magnesium alloy specimen. alloy specimen.
Figure 4. ((aa)) The The morphology morphology of of a a cross-section cross-section of of the the intermetallic intermetallic layer layer on on a a surface-alloyed surface-alloyed AZ91D magnesium alloy specimen with zinc oxide as as the the only only Zn Zn donor donor in in the the diffusion diffusion source; ( b)) The The EDS EDS resultresult of the point located at the cross marked in (a).
Major elements inin thethe surface-alloyedsurface-alloyed coating coating on on the the AZ91D AZ91D magnesium magnesium alloy alloy specimen specimen could could be befigured figured out out from from the elementthe element maps maps shown shown in Figure in Fi5gure. Comparing 5. Comparing with which with inwhich the matrix, in the thematrix, relative the relativecontent content of Mg is of obviously Mg is obviously less in theless coating.in the coating. Whereas, Whereas, Zn is richerZn is richer in the in coating the coating compared compared with withwhich which in the in matrix, the matrix, since since the main the main source source of Zn of is theZn is di fftheusion diffusion source, source, which which was composed was composed of the of the ZnO powder and the NH4Cl powder outside the specimen. Besides, the Al element and the O element could also be observed in the coating.
Figure 5. Morphology of a cross-section of the intermetallic layer on an as-surface-alloyed AZ91D magnesium alloy specimen obtained with zinc oxide as the Zn donor and the corresponding Mg, Zn, O, Al element maps. Coatings 2018, 8, x FOR PEER REVIEW 4 of 8
From the microstructure observation of the coating, it could be inferred that the formation of the coating is not a simple solid diffusion process, liquid phase should also be involved in the surface alloying process, since a pure solid diffusion process is driven by the concentration gradient which commonly results in a layer structure, while solidification of a molten phase is prone to result in a network shape eutectic multiple-phase-microstructure [10]. The existence of a liquid phase in the diffusion alloying process is reasonable [22], because the eutectic temperature in the Mg–Zn binary system could even be 341 °C, according to the Mg–Zn binary phase diagram [20]. From the EDS results shown in Figure 4, it could be figured out that there are mainly Mg, Zn, Al, and O elements in the coating. Mg is the dominant element in the AZ91D magnesium alloy matrix. Compared to the original content which was 0.52 wt.% in the AZ91D magnesium matrix, the relative content of Zn increased significantly in the magnesium alloy surface, which could be attributed to the continuous input of Zn atoms from the diffusion source. There was 9.1 wt.% Al in the original AZ91D magnesium specimen, hence Al could also be found in the coating. The existence of the O element is inferred to be related to the oxidation of active metallic atoms on the cross-section of the AZ91D magnesium alloy specimen.
Figure 4. (a) The morphology of a cross-section of the intermetallic layer on a surface-alloyed AZ91D magnesium alloy specimen with zinc oxide as the only Zn donor in the diffusion source; (b) The EDS result of the point located at the cross marked in (a).
Major elements in the surface-alloyed coating on the AZ91D magnesium alloy specimen could
Coatingsbe figured2019, out9, 278 from the element maps shown in Figure 5. Comparing with which in the matrix,5 the of 8 relative content of Mg is obviously less in the coating. Whereas, Zn is richer in the coating compared with which in the matrix, since the main source of Zn is the diffusion source, which was composed ZnOof the powder ZnO powder and the and NH the4Cl NH powder4Cl powder outside outside the specimen. the specimen. Besides, Besides, the Al element the Al element and the Oand element the O couldelement also could be observed also be observed in the coating. in the coating.
Figure 5.5. Morphology of a cross-section of the intermetallicintermetallic layer onon anan as-surface-alloyedas-surface-alloyed AZ91DAZ91D magnesium alloyalloy specimenspecimen obtained obtained with with zinc zinc oxide oxide as as the the Zn Zn donor donor and and the the corresponding corresponding Mg, Mg, Zn, Zn, O, CoatingsAlO, 2018 elementAl element, 8, x FOR maps. maps. PEER REVIEW 5 of 8
From the the XRD XRD pattern pattern shown shown in in Figure Figure 6,6 it, itcould could be befigured figured out out that that there there are Mg are0.97 MgZn0.970.03,Zn MgO,0.03, MgO,Mg32(Al, Mg Zn)32(Al,49, Mg Zn)7Zn49,3 Mg, AlMg7Zn34Zn, AlMg11, Al42ZnMg11 phases, Al2Mg in phases the as-diffusion-alloyed in the as-diffusion-alloyed coating. coating.MgO is inferred MgO is inferredto be formed to be in formed the duration in the durationwhen the whencross-sectio the cross-sectionn of the coating of the was coating exposed was in exposed the air, and in the other air, andphases other including phases includingMg0.97Zn0.03 Mg, Mg0.97Zn32(Al,0.03 ,Zn) Mg4932, (Al,Mg7 Zn)Zn349, AlMg, Mg7Zn4Zn311,, AlMg Al2Mg4Zn are11, Alinferred2Mg are to inferred be in the to beoriginal in the coating. original Comparing coating. Comparing with a binary with alloy a binary system, alloy the system, constituent the constituent of a ternary of alloy a ternary system alloy is systemmuch more is much complex. more complex.According According to the melting to the points, melting these points, phases these would phases solidify would one solidify by one one from by onethe melt from in the the melt cooling in the process cooling [16]. process [16].
Figure 6. The XRD pattern of the intermetallic layer fa fabricatedbricated by surface alloying on the AZ91D magnesium alloy with ZnO as the only Zn donor.
4. Discussion Since the Zn element in the coating is significa significantlyntly increased compared with which in the AZ91D magnesium matrix,matrix, there there should should be be a Zna Zn element element input input continuously continuously from from the ditheffusion diffusion source. source. As there As wasthere only was ZnO, only whichZnO, which contains contains Zn in theZn in di fftheusion diffusion source, source, ZnO is ZnO inferred is inferred to be reduced to be reduced into Zn into atoms, Zn andatoms, Zn and atoms Zn were atoms transferred were transferred and dissolved and dissolved into thesurface into the of surface the AZ91D of the magnesium AZ91D magnesium alloy specimen. alloy Relativespecimen. chemical Relative reactions chemical are reactions listed as are follows listed [as23 follows]. [23].
NHNH Cl4 Cl NH= NH g 3 (g HCl g ) + HCl (g) (1)(1)
2Mg + O2(g) = 2MgO (2) 2Mg O g 2MgO (2)
MgO HCl g Mg OH Cl (3)
Mg l Mg g (4)
Mg s Mg g (5)
Mg ZnO MgO Zn (6)
NH4Cl is unstable at the diffusion alloying temperature of 430 °C, and it would decompose into NH3 gas and HCl gas [23,24] as shown in Equation (1). HCl gas could promote the oxidation of Mg atoms at the AZ91D magnesium alloy surface as shown in Equations (2) and (3). The oxide film on the AZ91D magnesium alloy specimen surface could be compact or porous. Whereas even a compact oxide film formed initially, this compact film could be converted into a porous one with the help of gaseous HCl [24] as shown in Equation (3). A porous oxide film on the AZ91D magnesium alloy specimen is much more helpful for the mass transport between the AZ91D magnesium alloy matrix and the diffusion source. It is reported that in Al-rich magnesium alloys, the oxidation of Mg is more preferred than Al [4], and the content of Al could increase at some positions on the magnesium alloy surface, thus as a result, local melts could emerge. The existence of liquid Mg has the possibility of releasing gaseous Coatings 2019, 9, 278 6 of 8
MgO + HCl(g) = Mg(OH)Cl (3)
Mg(l) = Mg(g) (4)
Mg(s) = Mg(g) (5)
Mg + ZnO = MgO + Zn (6)
NH4Cl is unstable at the diffusion alloying temperature of 430 ◦C, and it would decompose into NH3 gas and HCl gas [23,24] as shown in Equation (1). HCl gas could promote the oxidation of Mg atoms at the AZ91D magnesium alloy surface as shown in Equations (2) and (3). The oxide film on the AZ91D magnesium alloy specimen surface could be compact or porous. Whereas even a compact oxide film formed initially, this compact film could be converted into a porous one with the help of Coatings 2018, 8, x FOR PEER REVIEW 6 of 8 gaseous HCl [24] as shown in Equation (3). A porous oxide film on the AZ91D magnesium alloy specimenMg further is [4], much as shown more helpful in Equation for the (4), mass and transport the sublimation between of the solid AZ91D Mg magnesiummay also happen alloy matrix[25] as andshown the in di Equationffusion source. (5). Liquid Mg, gaseous Mg and Mg atoms could react with ZnO to produce Zn atomsIt as is reportedshown in that Equation in Al-rich (6). magnesium alloys, the oxidation of Mg is more preferred than Al [4], and theMeanwhile, content of oxidation Al could increaseof Mg is atreported some positions to occur on at thethe magnesiuminterface between alloy surface, the MgO thus film as and a result, the localair in meltsa regular could oxidation emerge. process The existence of magnesium of liquid alloys Mg has[4,5]. the In possibilitythe diffusion of releasingalloying process gaseous here, Mg furtherthe MgO [4 ],film as showncontacted in Equation with the (4),mixed and powder the sublimation which contains of solid ZnO Mg may directly, also happenthus it is [25 possible] as shown for inthe Equation active Mg (5). atoms Liquid to pass Mg, through gaseous Mgthe MgO and Mg film atoms and react could with react ZnO with to ZnOproduce to produce Zn atoms Zn as atoms shown as shownin Figure in 7. Equation (6). Meanwhile,Zn atoms are oxidation transported of Mg into is the reported magnesium to occur alloy at the surface interface continuously between thethrough MgO filmvacancies and the in airthe inMgO a regular film, oxidationand a Zn-rich process layer of magnesium would form alloys on [4this,5]. magnesium In the diffusion alloy. alloying Since processthe eutectic here, thetemperature MgO film of contacted the Mg–Zn with binary the mixed system powder could which be 341 contains °C which ZnO directly,is below thus the itdiffusion is possible alloying for the activetemperature, Mg atoms the toZn-rich pass through layer would the MgO be molten film and in reactthe diffusion with ZnO alloying to produce process. Zn atoms In the as cooling shown inprocess, Figure the7. melt solidified gradually, and the coating which composed of various phases formed.
Figure 7. Schematic diagram of the in-situ re reductionduction of ZnO by the active Mg.
ZnThe atoms Gibbs are free transported energies of into relati theve magnesium chemical equations alloy surface are continuouslyshown in Figure through 8. It vacanciescould be found in the MgOthat most film, of and thea chemical Zn-rich layerequations would are form preferred, on this except magnesium Equations alloy. (4) Since and (5). the Although eutectic temperature pure liquid ofMg the and Mg–Zn pure solid binary Mg system both have could difficulty be 341 ◦becomingC which ispure below gaseous the diMgffusion in the alloying thermal temperature, calculations thewhich Zn-rich describe layer the would preference be molten of reactions in the di ffunderusion id alloyingeal conditions process. as In shown the cooling in Figure process, 8, the the Gibbs melt solidifiedenergies of gradually, Equations and (4) theand coating (5) may which change composed in a real sophisticated of various phases diffusion formed. alloying system. In fact, theseThe reactions Gibbs freeare energiesreported ofin relative some literatures chemical equations [25,26]. The are shownexistence in Figureof Al 8and. It couldZn both be foundin the thatintermetallic most of the melt chemical and in the equations magnesium are preferred, alloy matrix except could Equations affect the (4) evaporation and (5). Although of liquid pure Mg liquid and Mgthe sublimation and pure solid of Mgsolid both Mg. have It could diffi cultybe easier becoming for an pureMg atom gaseous in the Mg liquid in the or thermal solid phase calculations in the whichmagnesium describe alloy the surface preference to become of reactions an Mg underatom in ideal the gaseous conditions phase, as shown considering in Figure that8 ,the the melting Gibbs point of Mg could be reduced by adding Al and Zn into the Mg solid solution.
Figure 8. Gibbs free energies of relative chemical equations. Coatings 2018, 8, x FOR PEER REVIEW 6 of 8
Mg further [4], as shown in Equation (4), and the sublimation of solid Mg may also happen [25] as shown in Equation (5). Liquid Mg, gaseous Mg and Mg atoms could react with ZnO to produce Zn atoms as shown in Equation (6). Meanwhile, oxidation of Mg is reported to occur at the interface between the MgO film and the air in a regular oxidation process of magnesium alloys [4,5]. In the diffusion alloying process here, the MgO film contacted with the mixed powder which contains ZnO directly, thus it is possible for the active Mg atoms to pass through the MgO film and react with ZnO to produce Zn atoms as shown in Figure 7. Zn atoms are transported into the magnesium alloy surface continuously through vacancies in the MgO film, and a Zn-rich layer would form on this magnesium alloy. Since the eutectic temperature of the Mg–Zn binary system could be 341 °C which is below the diffusion alloying temperature, the Zn-rich layer would be molten in the diffusion alloying process. In the cooling process, the melt solidified gradually, and the coating which composed of various phases formed.
Figure 7. Schematic diagram of the in-situ reduction of ZnO by the active Mg.
The Gibbs free energies of relative chemical equations are shown in Figure 8. It could be found Coatingsthat most2019 of, 9 ,the 278 chemical equations are preferred, except Equations (4) and (5). Although pure liquid7 of 8 Mg and pure solid Mg both have difficulty becoming pure gaseous Mg in the thermal calculations which describe the preference of reactions under ideal conditions as shown in Figure 8, the Gibbs energies of Equations (4) and (5) may change in a real sophisticated diffusion diffusion alloying system. In fact, fact, these reactions are reportedreported inin somesome literaturesliteratures [[2525,26].,26]. The The existence existence of of Al Al and Zn both in the intermetallic melt and in the magnesium alloy matrix could affect affect the evaporation of liquid Mg and the sublimation of solid Mg. It It could could be be easier easier for for an an Mg Mg atom in the liquid or solid phase in the magnesium alloy surface to become an Mg atom in the gaseous phase, considering that the melting point of Mg could be reduced byby addingadding AlAl andand ZnZn intointo thethe MgMg solidsolid solution.solution.
Figure 8. Gibbs free energies of relati relativeve chemical equations.
5. Conclusions To conclude, it is possible to fabricate a Zn-rich coating on magnesium alloys by the powder thermal diffusion alloying method with ZnO powder as the only Zn donor. Some ZnO powder could be reduced to produce Zn atoms by taking advantage of the intrinsic activity of Mg atoms in the magnesium alloy matrix. As the MgO film is full of vacancies, the as-reduced Zn atoms would diffuse into the magnesium alloy matrix across the porous MgO film. This method is an economic method to fabricate Zn-rich coatings on magnesium alloys. Since stable ZnO is utilized as the raw material, transport and storage of the diffusion source would be much easier.
Author Contributions: Conceptualization, D.L.; Methodology, D.L.; Software, D.L.; Validation, D.L.; Formal Analysis, D.L.; Investigation, D.L.; Resources, D.L.; Data Curation, D.L.; Writing–Original Draft Preparation, D.L.; Writing–Review & Editing, D.L.; Visualization, D.L.; Supervision, Y.H., J.D. and B.H.; Project Administration, D.L.; Funding Acquisition, D.L. Funding: This research was funded by the Key Research & Development Program of Shandong Province, China [No: 2018GSF117039] and the Applied Research Program of Qingdao, China [No: 17-1-1-22-jch]. Acknowledgments: The authors would like to thank all the anonymous reviewers for their kind help and useful advice. Conflicts of Interest: The authors declare no conflict of interest.
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