New development of anodizing process of magnesium alloys

Vol.14 Special 2.Trans. Nonferrous Met. Soc. ChinaOct.2004Article ID: 1003 - 6326(2004)S2 -0120 -06New development of anodizing process of magnesium alloys"BAI Li-qun, LI Di .(School of Materials Science and Engineering,Bejing University of Aeronautics and Astronautics, Beiing 100083, China)Abstract; Magnesium lly, a kind of eninenfriendly material with promising and exellent properties,isagood choice for a number of ppcatins. The research and development of anodizing on magnesium alysys and isaplietion stuation are reviewed, and the anodizing development trend on moagnesium alys is sunmarized.Key words: magnesium; magnesium alloy; anodizingCLC rumber: TG 176Document code: A1 INTRODUCTIONexploit their excellent properties for better serv-ice[1.2.4.5.8].Magnesium alloy is one of the lightest and eas-The corrosion resistance of Mg alloys can beiest machining structural metallic materials. Buresolved in three aspects: ①Strictly limiting thethe mechanical properties of pure Mg are poor. ltsimpurity content (Fe, Cu, Ni etc). For example,strength and the specific rigidity are similar tothe corrosion resistance of high purity Mg alloythose of Al alloys after forming alloys with Zn, AlAZ91HP in saltfog test is about 100 times thanand rare earth elements. The specific strength ofthat of AZ91C, exceeds that of die casting Al alloyMg alloys is much higher than that of Al alloys. InA380, and is much better than that of low carbonaddition, Mg alloys, as a kind of green material insteel;②Adopting flash solidification treatment,21-century, not only have low density, big energylaser treatment and ion implantation etc;③Trea-decay cofficient, good electric and thermal con-ting the surface of Mg alloy. According to require-ductivity properties, good vibration damping andment of corrosion resistance, chemical conversion, .electromagnetic shielding properties, but also areanodizing, organic coating, electroplating, electro-pollution free and easy recovery. Also our countryless plating， thermal spraying and some otherabounds in Mg resources. So the Mg alloys have .methods can be selected. For industrial produc-been used widely in many fields, especially in thosetion, chemical conversion and anodizing are com-where mass reduction is critical or particular tech-mon, effective, simple and economical wayslh4.5.8l.nical requirement are needed, such as automobile,But chemical conversion coating is very thin (3 - 5communication, aerospace, military， nuclear ener-μm) and harmful to environment and human beinggy industries. In recent years, the annul output ofbecause chromate is used in practice for many ca-Mg alloys increased by 20%，showing extensiveses. Anodizing of Mg alloys is one of the mainapplication prospectl-l.ways for improving the properties of anodized filmUnfortunately, however， there were somewith very good application effect and promisingproblems hindering. the widespread application ofmarket competition.Mg alloys in past a few years. Mg alloys are easyto burn during being smelt or machined because2 ANODIZING OF Mg ALLOYSthey are usually very active for negative electrodepotential(E"=-2. 34 V). In addition, the corro-The origina! anodizing process could date backsion resistance of Mg alloys is poor because theyat least to 1920s. Anodizing is an electrolytichave very high chemical and electrochemical activi-process for producing a thick, stable oxide protec-ty and their oxide films are porous. Poor corrosiontion film on metals or alloys. Al, Mg, Ti, Ta, V,resistance has become one of the most importantZr and their alloys can all be processed through an-challenges against the application of Mg alloys. Ef-odizing to improve their properties. The study oIfective surface protection is urgently needed to im-anodizing of Mg alloys started before 1951. But theprove the corrosion resistance of Mg alloys and toappearance of HAE and DOW17 processes in suc-①Foundatlon item; Project supported by Scientifie Research Foundation of Chi中国煤化工，Aeronautics ScienceFoundation CommitteeYHCNMHGRecived date: 2004 -05 - 10; Acepted date: 2004 -07 - 15.Corepondence; LI Di, Female, Professor; Tel: + 86-10- 82317113; E mail: dil9966@ vip. sina. comVol. 14 Special 2New development of anodizing process of magnesium alloyshad promoted the commercial application ofposition is an alkaline aqueous solution includinganodizing to surface treatment for Mg alloyst' 9.hydroxylamine，phosphate anions and nonionicAnodizing produces a special electrochemicalsurfactants. Mg alloy AZ91D was anodized in thisconversion film on Mg alloys. In contrast to chem-electrolyte for 10 min with a current density from 2ical conversion, anodizing can produce a relativelyto 4 A/dm'. The corrosion area of the anodizedthick, hard and porous film. This porous film canfilm is less than 1% of the whole area after 5%be sealed by adsorption paint and olefin to improveNaCI salt fog spray for 330 h. The current densitycorrosion resistancelo0. Compared with Al alloys,at any given anodization potential can be chosenanodizing voltage and current density of Mg alloyslow enough so as to be outside the sparking regimeare higher. So the anodizing of Mg alloys is often(generally less than about 4 A/dm2) or high eaccompanied by sparkl". Anodizing is always di-nough to be within the sparking regime (generallyvided into common anoding and micro arc oxida-greater than about 4 A/ dm?). The phosphate con-tion. Through semicentennial development, largecentration is preferable between about 0. 1 mol/Lnumbers of new anodizing processes appear. In theand 0. 4 mol/L in electrolyte. When a high phos-following parts, the new process of common anodi-phate content solution is used, a relatively thickzing and micro arc oxidation will be respectively in-and harder anodized film is attained and it is neces-troduced.sary to control the solution temperature by coolingduring the period of anodization. But if a low phos-2.1 Common anodizingphate content solution is used, a relatively thin andThe electrolyte solution of anodizing on Mgvery smooth anodized fim will be attained and it isalloys can be acidic or alkaline. The most repre-unnecessary to control the solution temperature.sentative process is Dow17 and HAE in early time.The solution temperature does not influence theBut the electrolyte solutions of phosphate-series,film.fluoride series and environment -friendly-series are2. 1.2 Fluoride-seriesgradually developed since 1990s because of the re-Anodizing of magnesium was studied in solu-quirement on environment protection and the con-tions containing 3 mol/L KOH + 0. 6 mol/L KFtrol against cancer causing agents.+ 0. 21 mol/L Na PO,, with and without addition2.1. 1 Phosphate seriesof0.4 mol/L and 1. 1 mol/L of aluminate (Kha-A novel aqueous solution comprising dissol-selev O)[18]. Anodic films formed in a bath contai-ving silicate, carboxylate and alkali bhydroxide forning 1.1 mol/L aluminate at 15 mA/cm2 are com-anodizing of Mg and Mg alloys was invented byposed of a single oxide phase MgAl2O, spinel.Masashi in 198812]. The anodized film formed inMizuno"linvented an anodizing method.the anodizing solution has a superior properties,Through this method, an oxide film, which is ex-especially corrosion resistance, abrasion resistancecellent in resistance to corrosion and wear, can beand ornamental properties, as compared with anyformed on the surface of a product made of Mg orformer anodized film. A further improvement mayits alloy. This anodizing solution contains 5 - 250be obtained by adding one or more compounds, se-g/I of a silicate, 25 - 360 g/L of an alkaline com-lected from the group of borate, fluoride and phos-pound, 15- 100 g/L of a carboxylic acid and 1 - 50phate, to the foregoing anodizing solution.g/L of a fluorine compound. The product made ofA new anodizing solution of Mg alloy was in-Mg or its alloy is treated with this anodizing solu-vented by Macculloch and Rossfl11. The electrolytetion for 1 - 60 min with the solution controlled tois characterized by possessing pH of 7 or higher≤60 C and the current density to 0.2 - 5 A/dm'.and includes or comprises in water: at least oneFormation behavior of anodic oxide films orvolatile amine or ammonia and phosphoric acid or aMg alloy in fluoride electrolytes was investigatedwater soluble phosphate salt thereof. The aminewith attention to the effects of anodizing voltagecan be methylarmine, ethylamine, diethylene trand Al content (Ono)[1s]. ln the range of forma-amine or ethanolamine.tion voltage between 2 V and 100 V, porous filmA method for anodizing of Mg and Mg alloyswas formed associated with high current density atusing electrolytic solution ( preferably derived pri-around 5 V. In the case of AZ91D, the criticalmarily from phosphoric acid), which contains am-voltage increased to 70 V and peculiar phenomenonmonia, amines or both of them， was inventedat 5 V was not observed, so that only barrier films(Ross, 2000)(4. The use of such an aqueous elec-were formed at a voltage lower than the criticaltrolytic solution can provide a more satisfactorybreaF was added in thefilm on Mg material with reduced recycle times.elec中国煤化工rmarkably inA composition and a method for making thiscrea|YHC N M H Gctively decreasedcormposition for anodizing Mg surface, invented bywith" Increasing AIU content. The passivationIlya and Kibbutz[15], has been disclosed, The com-effect of aluminum addition in the electrolytes is.122●Trans. Nonferrous Met. Soc. ChinaOct.2004more remarkable than that of the addition in mag-contains 20 - 300 g/L ydroxide, 5- 100 g/L bo-nesium substrates. The depth profiles of constitu-rate, 10- 200 g/L silicate and at least one of 10 -ent elements show that aluminum migrates into 0X-100 g/L tungstate and 20 - 30 g/L carbonate.ide film to reach near oxide/ substrate interface.Okahara[24] provided an anodizing method andMolar ratio of aluminum to magnesium increaseda composition for forming an anodized oxide film,with increasing voltage to attain0.42 at 80 V，andwhich has superior corrosion resistance withoutcrystalline MgAl2O, and MgO were found in theusing hazardous heavy metal salts. The composi-film.tion for anodizing Mg alloy includes silicate， AA suitable mixture without chromium is devel-compound, surface conditioner, base and water.oped for Mg alloys by treatment with the anodicThe new and environment friendly anodizingoxidation ( Wang)(19]. The aqueous solution isprocess for coloring of die casting Mg alloy was de-composed of NaOH, NaSiFg and organic additive.veloped (Lei)[25]. The anodized film thus formedThe morphology and component of the anodizedon Mg alloy is off-white, even, porous and slipper-fim have been analysed by XRD, SEM, XPS andy, and this film has favorable coloring effect to ob-EPMA respectively. The anodized film is made oftain manifold colors. The anodized films are com-MgO, MgAl2O, and 2Al2O3●2SiO2 ●H2O. Theposed of MgAl2O and MgO. The amount ofpotentodynamic polarization measurement showsMgAl2O increased and the amount of MgO de-that the film has outstanding protective ability increased with an increase in voltage.3.5% NaCl solution. Si and Al oxides make up theThe environment-friendly process of the ano-volume shrinkage of Mg oxides, so that the filmdized film for Mg alloys was studied (Qian)86,2)].has no crack， which provides a good conditionThe electrolyte solution consists of sodium salt,against pitting.hydroxide, borate and additive agent. The ano-2.1.3 Environment-friendly -seriesdized fim formed by this process on Mg alloys isTakahata and Kobayashilo, reported an aque-slivery gray. The corrosion resistance of anodized)us anodizing solution for anodizing article of Mgfilm is better than that of conventional film formedor Mg-base alloy. This solution contains, per literby DOW17 process. The anodized films consistvolume thereof, 20 - 300 g aluminate, 0.5-8 molmainly of MgO and Mgs B2O。and have porousof an alkali hydroxide per one mole of the alumi-structure. In addition, environment friendlynate, and at least one kind selected from the groupelectrolyte solution, containing aluminate, hydrox-consisting of 20 - 200 g of a boron compound, 2ide, borate and additive agent, for AZ91D die cast-50 mL of a phenol, 2-50g of a sulfate and5-70ging Mg alloy was studied. The slivery gray oxida-of an iodine compound. A process for coloring antion film was formed on the Mg alloy by thearticle of Mg or Mg-base alloys which comprisesprocess. The corrosion resistance and adhesion offorming an aluminum oxide-containing layer over afilm came near conventional film formed bysurface of said article and subsequently coloringDOW17 process. The anodized film was mainlysaid layer with an anodized aluminum coloring dyecomposed of MgAl2O. The film was irregularis also disclosed.porous structure, whose aperture was great largerThe invention relates to an electrocoatingthan anodized film of Al alloy.method for fabricating Mg oxide insulating film on2.1.4 Amineseries and othersMg alloy surface, wherein the power supply provi-Formation behavior and structure of newly es-ding alternating polarity is utilized in electrolytetablished anodized films grown on magnesium andcomposedof5-250g/LNaOHand1-40g/LAZ alloys were studied in organic electrolyte con-Naz SO, or alternatively Cu(OH)2 and Al(OH)staining 1 mol ●L 1 triethylamine/ ethylene glycolmay be used instead of Na2 sO, (Akira)[21].solution with varying water content ( Asoh andA new anodizing process， investigated byOno)28]. The growth behavior and appearance ofZHANGl9,22,23], based on an environment friendlyanodized films were significantly affected by theelectrolyte. solution that contains none of chro-water content. In the region between 10% andmate, phosphate or fluoride can enhance the corro-40% of water content, compact barrier-type filmssion protection of magnesium alloy significantly.were obtained regardless of substrate purity. TheBy comparison with the films produced by two a-surface appearance of obtained films was transpar-vailable classic processes , the new film can provideent and enamel-like, which was different frommore effective corrosion protection to substrate.those formed usually in aqueous solutions. In addi-The new anodized film, having a number of porestion,中国煤化工order of resist-that are relatively uniform in size as well as distri-ancelectrolyte contai-bution on a microstructure scale and do not tranve-ningYHCNMHG0%，thepprrse the entire film, forms a relatively intact andances were yellowish white and whitish gray, recompact barrier layer. The electrolyte solutionspectively. The composition of anodized films de-Vol. 14 Special 2New development of anodizing process of magnesium alloystermined by FT-IR and GD OES analysis showedformed.the incorporation of organic substance (carbon, ni-The development of anodized filns at a con-trogen, oxygen) into the film. The incorporationstant current density of 10 mA●cm一2 has beenbehavior of organic species was also changed bystudied for Mg-W alloys, containing 0. 4% andwater content and the amount of incorporated or-1. 0%W (in mole fraction), in 3 mol. L-1 ammo-ganic species increased with decreasing water con-nium hydroxide/0. 05 mol●L ' ammonium phos-tent in the electrolyte.phate electrolyte at 293 K (Bonilla)a3]. During an-The invention is an Mg alloy ultrasonic anodi-odizing to about 50 V, a relatively smooth film de-zing method (Guo, et al)29]. After alkali-washingvelops. Cross- sectional atomic force microscopyand acid- washing, the Mg alloy workpiece is placedsuggests that the film may be finely porous. Within the electrobath. It adds 260 - 460 g/L stabilizingan increase in voltage, the film transforms gradual-agent to existing electrolyte and during the oxidi-ly to a coarse, porous morphology due to dielectriczing course, exerts ultrasonic field (80 - 300 kHz)breakdown. Significant regions of both types ofused to produce 0H-' and H2O2, and utilizes ul-film morphology coexist during anodizing to 15(trasonic to raise reacting speed and provide atom OV. Magnesium, phosphorus, oxygen, hydrogenfor the oxidizing reaction. The stabilizing agentand nitrogen species are presented throughout thecan be glycol, glycerine and carbamide etc. Thefilm thickness at the resolution of the measure-aperture of anodized film was decreased. Andments. The molar ratio of 0 to Mg is in the rangegrowth rate and smoothness of the anodized filmfrom 1.7 to 2.1, reducing with an increase in volt-were increased, so corrosion resistance was also in-age for films formed up to 220 V. The molar ratiocreased.of P to 0 increases with increase of voltage, start-ing at about 0. 03 for voltage below 50 V and reac-2.2 Micro- arc oxidationhing 0.29 at 330 V.Micro-are oxidation ( MAO) is also calledAccording to Jjiangh), a surface treating tech-plasma anodizing or plasma electrolytic oxidationnology for improving the quality of ceramic layer(PEO). In 1932，two German scientists Gunters-on Mg alloy and making its surface benefit the postchulze and Betz studied this process. But Markovtreatment includes pre -treating Mg alloy, puttingis called the“father" of the process because he suc-it in phosphate- or silicate series alkali electrolyte,cessfully transferred micro arc oxidation of Al fromand micro-arc oxidizing the surface of Mg ally tolaboratory experiments to industrial production inform a ceramic layer8. The phosphate-series con-1970s30. MAO gradually attracted scientists' in-sist of 10 - 60 g/L sodium hexametahposphate andterest in the late of 1970s, and became research hot2 - 20 g/L sodium borate. The silicate series con-at home and broad in 1980s.sist of 5- 60 g/L sodium silicate, 2 - 20 ml/L thi-The process of MAO is to immerse the work-ethanolamine and 2 - 20 g/L potassium hydroxide.piece in an electrolyte as anode and provide cath-The pH value of two series was controlled at 8 - 12ode, then to pass a current through the electro-and stabilizing agent is needed during micro-arc ox-lyte. The plasma is discharged by an external pow-idization. Its advantages are simple process, higher source in an electrolyte near the surface of theproductivity and less environment pollution. Theworkpiece. The oxygen plasma causes partialcorrosion resistance and abrasion resistance of ce-short- term surface melting. Ultimately the ceramicramics films were remarkably increased. .oxide film which bonds base material in metallur-A patent by Huang[20 invented that the com-gy，is formed under the effects of thermochemis-posite ceramic films consist of two layers, the in-try，plasma chermistry and electrochermistry-1o].ner layer film of silicon-containing composite Mg-The porosity of the film remarkably decreases. SoAl oxide and the outer layer film of MgAI silico-the corrosion resistance and abrasion resistance in-oxide of 5 - 70 μm thickness each!8J. The compos-crease more largely than those of the common ano-ite ceramic film is formed through surface treat-dizing.ment，degreasing, neutralizing， water washing,An aqueous solution, containing an alkali mealmicro-arc anodizing, water washing, drying andsilicate, an alkali metal hydroxide and a fluoride,sealing treatment or painting. The composite ce-for coating Mg and its alloys has been disclosed inramic film has no holes and cracks， so it showsa patent by Kozak31,x2]. In the process, an electri-high corrosion resistance, high abrasion resistancecal potential between 150 V and 400 V is appliednd smooth surface.between the Mg article serving as the anode andthe cathode immersed in the bath until visibleme中国煤化Issfully electrode-spark is discharged on the surface of the metal.pcCNMHGAZ91D Mg speci-The potential difference is maintained for a few mi-mer!This process wasnutes until the coating with desired thickness isnamed double electrode microarc electrodepositionTrans. Nonferrous Met. Soc. ChinaOet.2004(DEME) by YuanGang I.. The rare earth elementhighly susceptible to corrosion. This has limitedCe was detected in films when CeF, ●H2O wastheir wide application. The simplest way to avoidadded to the electrolyte. By immersing the samplescorrosion is to treat the surface. Anodizing is onewith and without DEME treatment in 3. 5% NaClof the effective ways of surface treatment. Com-solution, it was found that the corrosion rate ofpared to Al alloy, anodizing process of Mg alloys isspecimens treated using the DEME process waspremature and the theory needs further research.much lower than that of untreated specimens. InDeveloping environment-friendly electrolyte solu-order to reduce their porosity, samples withtion is one of development trend of anodizing. An-DEME films were further painted with three dif-odizing treatment is developing from unitary wayferent kinds Ccoatings: sol-gel, polysilicone andto complex way. In addition, power source is fromepoxy lacquer. Samples coated with DEME flmAC/DC to PC or AC/DC along with PC.and epoxy coating could sustain a 336 h neutral saltspray test.REFERENCES3 ANODIZED FILM STRUCTURE OF Mg AL- [1] JIAO Shurqiang, KUANG Ya-fei, CHEN Jin-hu, etal. Corrosion behavior and anodization of Mg and itsLOYSalloys [J]. 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