镁合金的表面处理（Surface treatment of magnesium alloy）

1、镁合金的表面处理（Surface treatment of magnesium alloy）Surface treatment of magnesium alloyLiu Zuming LebanonAbstract: the corrosion mechanism and corrosion protection of magnesium alloys are reviewed. The status, progress and problems are reviewed. The surface characteristics, surface treatment principle, p

2、rocess, application examples and future development direction of die casting magnesium alloy products are mainly introduced.Key words: magnesium alloy corrosion and protection surface treatment0 PrefaceWith light and Recyclable for magnesium alloy application characteristics, has become the ideal ma

3、terial for modern industrial products, especially the application of a large number of auto parts, telecommunications products to the development of light and thin and short and small direction of demand, makes the development of this new material is extremely optimistic about the prospects.With the

4、 development of modern technology, has been plagued by magnesium alloy industry related technical problems such as die-casting, recycling problem has been resolved, after the major process of magnesium alloy products - surface treatment technology has made progress, which makes the application of ma

5、gnesium alloy is constantly expanding, consumption has increased rapidly, the magnesium industry is the geometric speed growth.Proper surface treatment can make the product protective and decorative, and can give some special functions. For active magnesium, it is important to obtain high decorative

6、 and functional properties by surface technology, and improving its corrosion resistance is the most important aspect at this stage.Corrosion and protection of 1 magnesium alloyProperties of 1.1 magnesiumMagnesium element symbol Mg, atomic number 12, the electronic structure of 2-8-2, a negative sta

7、ndard electrode potential (-2.36V), a volatile electronic and oxidized, resulting in magnesium and magnesium alloy corrosion resistance is very poor, in the corrosive medium is prone to severe corrosion. The oxide film formed by magnesium alloy is loose and porous. The density coefficient of MgO and

8、 Mg2+ film is about 0.8 (10.2 magnesium alloy shows good ability to resist stress corrosion; in fluoride solution, magnesium alloy has good anti stress the performance of stress corrosion.(four) intergranular corrosion: magnesium alloys are generally insensitive to intergranular corrosion, and corro

9、sion can not develop along grain boundaries.(five) filiform corrosion: under the protective organic coating or anodic oxidation film, the corrosion of the battery on the surface of magnesium alloy leads to filiform corrosion.Corrosion Comparison between 1.4 magnesium alloy and other materialsIn 1980

10、s, the development of magnesium alloy materials has made great progress. It is found that the impurity elements such as Fe, Ni, Cu and the ratio of Fe to Mn have a great influence on the corrosion resistance of magnesium alloys. Therefore, a new generation of high purity die casting magnesium alloy

11、AZ91D has been developed. The corrosion resistance of high purity magnesium alloy was increased by 100 times, which is beyond the other competing materials, without the protection of the state can meet most of the requirements of automotive applications, and even exposed in the winter road condition

12、. The technical breakthrough is very important for the development of die casting magnesium alloy, and it is also an important reason that magnesium alloy can be widely applied. In terms of salt spray corrosion resistance, most of the die casting magnesium alloy brands have exceeded the aluminum all

13、oy A380 (as shown in Figure two).Corrosion protection technology of 1.5 magnesium alloyIn view of the corrosion mechanism of magnesium alloy, the research work of corrosion protection technology of magnesium alloy is mainly focused on the following aspects1.5.1 part design (as shown in Figure three)

14、A good protective measure should start with a good design. This is a systematic project. From the point of view of corrosion protection, the following factors should be considered in the design of a good magnesium alloy part:(1) water or other liquids can not be accumulated together to form a galvan

15、ic cell environment.(two) the corner should be designed as round arc, and sharp edge angle will reduce the protective effect.1.5.2 material selection (as shown in Figure three):In the product design, the choice of the raw materials used in the adjacent parts is also very important. It is necessary t

16、o ensure that the structural function is not affected, and the potential difference between the two materials is small, so as to ensure that no serious corrosion occurs.Another problem to note is the connection between the components and the material of the connecting medium and the anti corrosion t

17、reatment processIts also a matter of great concern.Figure three corrosion protection methods for magnesium alloysThe electrochemical activity order of some commonly used materials (at room temperature, seawater) is listed below. The closer to Mg, the better the compatibility with magnesium:magnesium

18、magnesium alloyAluminum alloys 5000, 30000, 1100, 6000cadmiumAluminum alloy 2000steel13% chromium plated stainless steel (reactive)18-8 not plated steel (active)aluminumzincAdmiralty brassNickel (active)Al Cu alloyCopper, copperNickel (inert)Nickel copper alloytitanium18-8 stainless steel (inert)gol

19、d1.5.3 metallurgical controlMetallurgical factors include alloy elements, impurity elements, phase composition and microstructure.Different elements have different effects on the corrosion resistance of magnesium alloys. According to the alloy elements commonly used in magnesium alloys, they are gen

20、erally divided into three categories, as shown in table 1. Effect of alloy elements on corrosion rate of magnesium alloy,Usually, the content of the third kinds of elements is reduced to below the critical value, and the method of increasing purity is adopted to increase the corrosion resistance.1 A

21、l, Mn, Na, Si, Zr and other elements which have little influence on corrosion rate2, the corrosion rate of Zn, Cd, Ca, Ag and other elements slightly larger3, the corrosion rate of Ni, Fe, Cu, Co and other elements greatly increasedTable 1 Effect of different elements on corrosion resistance of magn

22、esium alloyPhase composition has a great influence on the corrosion resistance of magnesium alloy. The processing mechanism of alloy is different, the phase composition and content are different, and the influence mechanism is different. For example, rapid solidification can improve the phase compos

23、ition and microstructure of the material, make the matrix more homogeneous, reduce defects and inhibit local corrosion.On the other hand, it is to develop new alloy system with high corrosion resistance to adapt to the influence of different corrosive environments.1.5.4 surface treatmentBefore the s

24、ignificant progress of metallurgical control and new alloy development, corrosion control of magnesium alloys by various surface treatment technologies is undoubtedly the most important research topic in the industry.The surface treatment technology and process of magnesium alloy are described brief

25、ly.Surface treatment of 2 magnesium alloyRequirements for main surface properties of 2.1 magnesium productsUsually used to replace the aluminum magnesium alloy, iron or plastic, one of the principle of surface treatment is to maintain the superior performance of the material itself, but more importa

26、nt is to the performance of some difference from other materials up to achieve the performance requirements of products, such as corrosion resistance. Some of the major surface properties of magnesium alloy products are listed below.Performance requirements of 2.1.1 automobiles, motorcycles and tool

27、s(1) general mechanical properties: hardness, strength, impact resistance, fatigue resistance and so on.(two) protection: corrosion resistance, chemical resistance, weather resistance, pollution prevention.(three) decorative: tinting, gloss, etc.(four) workability: such as weldability, adhesion and

28、so onPerformance requirements of 2.1.2 3C products(1) general mechanical properties: hardness, strength, impact resistance, fatigue resistance and so on.(two) protection: corrosion resistance, chemical resistance, weather resistance, pollution prevention.(three) thermal conductivity(four) conductivi

29、ty(five) electromagnetic shielding property(six) workability: such as weldability, adhesion and so on(seven) decorative: tinting, gloss, etc.Most of the product parts will not put forward higher requirements for all the performance, usually only for the part of the performance of their products put

30、forward high requirements, in order to reduce the technical threshold, reduce the comprehensive cost of products.Surface analysis of 2.2 die casting magnesium alloy productsTo develop a perfect surface treatment technology and process, must fully grasp the influence of physical and chemical properti

31、es of the raw material surface, analyze the influence of surface condition of magnesium alloy die casting process for processing.Effect of pristine loose film on 2.2.1 surfaceIn the ordinary atmosphere environment, the surface of magnesium alloy products will form Mg (OH) 2 and MgCO3 based non dynam

32、ic coating, which is not good for corrosion protection, not suitable for long-term protection, should be removed before treatment.Influence of 2.2.2 release agent adhesionIn the most practical die casting method at present, the mold release agent is adhered to the surface of the raw material, and it

33、 will also be involved in the interior of the raw material, which leads to the uneven surface chemical properties of the product, which brings difficulties to the processing work.Effect of 2.2.3 alloy surface segregation effectsIn the molding process, some alloy elements will be enriched in the surf

34、ace segregation, such as the AZ91 series of Al, Zn in the concentration of surface segregation caused by die casting, surface chemical properties and matrix internal inconsistencies, which leads to increased difficulty of processing.Influence of uneven forming of 2.2.4A release agent and the concent

35、ration of residual segregation of alloy composition is not uniform, everything is different, for the same part, each part is completely different; the two refers to the molding process, the crystal structure of each part is not uniform. Such as the gate and overflow part of a dense fine, a rough por

36、ous,Some parts of the dense part and rough part mixed together, these will affect the treatment process and treatment effect.Surface treatment technology of 2.3 magnesium alloyDifferent products and their application environment, as well as the different parts of the same product, the surface treatm

37、ent processes and methods are different.Here is a brief introduction to the various treatment processes.Fig. four schematic diagram of surface treatment technology for magnesium alloy2.3.1 chemical conversion coatingChemical conversion coating is mainly aimed at the pretreatment of coating:(1) chrom

38、ium inorganic salt conversion coating. The chromate composite film prepared by chemical treatment of chromic acid and dichromate as the main component has good corrosion resistance, strong adhesion and automatic repair ability. At present, the related process is relatively mature, but its production

39、 process is unfavorable to environmental protection, and has gradually been replaced by other non chromium processing technology.(two) non chromium inorganic salt conversion coating. At present, phosphate, manganese salt series, coating corrosion resistance close to chromium film and other performan

40、ce indicators are to meet the requirements. As a result of environmental protection requirements, it has gradually become the mainstream technology used in the current research and production.(three) non chromium organic conversion coatings. The earliest research and development in Japan, some domes

41、tic foreign enterprises have begun to introduce into production, mainly for both high corrosion resistance and good conductive properties of 3C products.(four) chemical oxidation film. The conversion film obtained by various chemical oxidation methods is mainly applied to the pretreatment of low dem

42、anding products and temporary protective treatment between processes. Domestic aerospace, military enterprises in 60s as early as the relatively mature processing technology.(five) comparison of chemical conversion membranes. The properties of several commonly used chemical conversion coatings are s

43、hown in table two.Anodic oxide film on 2.3.2An electrochemical conversion film, which is thicker than the chemical conversion film, has high strength, high hardness, good wear and corrosion resistance, and has a porous film, which can be used as a good substrate before coating. After closed treatmen

44、t, the corrosion resistance of the workpiece is better, and can be used as the final coating directly.2.3.3 micro arc oxidationAlso known as the ceramic surface, is the development of anodic oxidation technology, its principle is to produce plasma micro arc spark in the anode region, the workpiece s

45、urface (anode) local temperature to 2000 DEG C, so that the anodic oxide cladding on the surface of the workpiece, and the metal substrate and produce metallurgical bonding, forming a layer of dense a ceramic oxide film closely (Figure five, figure six), the wear resistance and corrosion resistance,

46、 strength, hardness, adhesion of the anodic oxide film is much better than ordinary.Fig. five surface observation of oxide film after micro arc treatment six SEM observation of specimen cross section2.3.4 electroplatingThe domestic aerospace industry as early as 80s has frequency of cloth of magnesi

47、um and magnesium alloy plating process specification, gold-plated mature technology widely used in aerospace and military industry, but in the folk at present has not developed mature plating process. In the early development of magnesium alloy industry in Europe, Japan and Taiwan, there is a small

48、batch of trial production.Electroless 2.3.5 platingThe direct electroless nickel plating process is mature and has been used in actual production. Direct electroless nickel plating technology of magnesium alloy, compared with the traditional methods, much simpler, shorter process, coating adhesion, with metal luster, hardness of 500-680HV, has good conductivity (impedance 50-100 Mu Omega.Cm), the excellent performance of EMI. In addition, the treatment