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The Techniques of the Surface Treatment of Aluminum Alloys(3)

1.The first stage of anodic oxidation

This is a stage where nonporous layer goes into the formation called ab. When it comes to electrify, the voltage will surge within a short period(from several to tens of seconds) and reach critical voltage.( the maximum voltage) demonstrates that the surface of anode has formed a continuous and nonporous film layer. The nonporous resistance is rather high, which stops the film from going to increase in thickness. The thickness of it is proportional to voltage, while grows in inverse proportion to the rate of oxide film dissolves in the electrolyte. Its thickness if between 0.01 and 0.1 micron.

2.The second stage of anodic oxidation

This is a stage where porous layer enters into the formation called cd. The place of the thinnest film will be firstly dissolved to holes where electrolyte can via to reach the fresh surface of aluminum. With that, the electrochemical reaction is able to keep going and then the resistance decreases, followed by the declining of voltage(the highest margin ranges from 10% to 15%) . And porous layer appears on the film.

The porous layer increases in thickness and becomes cd stage. At this moment, the voltage goes up steadily and slowly, when nonporous layer is dissolved into porous one continuously. As a result, the new nonporous layer is growing which leads to the constant thickening of porous one. When the rate between formation and dissolution reaches the dynamic balance, the thickness of film stops increasing and the reaction should halt ,too.

The techniques of anodic oxidation of oxide films of aluminum alloy 

1.The common techniques of anodic oxidation

They contain vitriolic anodic oxidation、chromic acid anodic oxidation、oxalic anodic oxidation and phosphoric acid anodic oxidation. The most common used is the first one.

2.Vitriolic anodic oxidation

At present, the most widely used technique around the world is anodic oxidation. Compared with other methods, it is superior in the cost of production、the characteristics of oxide film and properties. It possesses the merits with low cost, well apparent film, great properties in corrosion and friction resistance, easy tinting. It uses dilute sulfuric acid as electrolyte to anodize the products. Meanwhile the thickness of film is up to 5um—20um and its absorbability is good. And the film is colorless and transparent. This technique is simple and easy to operate.

3.Chromic acid anodic oxidation

The film which comes from chromic acid anodic oxidation is a bit thin, only 2-5um; it can stay at the original accuracy and surface roughness. The porosity is too low to be dyed; the film is soft and has weak corrosion resistance yet has good elasticity. It is corrosion preventative, but aluminum has low solubility in chrome, which results in liquid left in pinholes and cracks has little corrosion towards parts. And that is suitable for isostructural elements like casting. So this technique is used frequently in military. At the meantime, this technique helps check the quality of parts because brown electrolyte will outflow in the crack obviously.

4.Oxalic anodic oxidation

Oxalic anodic has little corrosion to the oxide film of aluminum, so the porosity of oxide film is small but the corrosion resistance and electrical insulation of film are better vitriol film. However, the cost of oxalic oxidation is three to five times higher than vitriol. Meanwhile oxalic can be reacted both in cathode and anode, which leads to its weak electrolyte stability. The color and lustre of oxalic oxide film will change with the conditions of technique, causing color differences of products so that this technique is limited in certain degree. But oxalic is universally taken as vitriolic oxidation additives.

5.Phosphoric acid anodic oxidation

Oxide film has a greater rate of dissolution in electrolyte than sulfuric acid; thus, the oxide film is thin ( only 3um) and has big aperture. Because of the excellent water resistance of phosphoric film, it can prohibit adhesive from aging due to hydration, which makes the combination of cement become better. Therefore, it is mainly used in the surface pretreatment of printer’s metal and the pretreatment of aluminous workpiece adhesive.

Hard anodizing of aluminum alloys

1.The characteristics of hard oxide film

Compared with conventional oxide film, hard anodizing of aluminum alloy has the following characteristics: it is thicker( usually >25um)and harder( >350 HV) , as well as it has better corrosion resistance、lower porosity and higher punch through voltage. While its surface flatness is poorer.

2.The process characteristics of hard anodizing

There is no essential differences between hard anodizing and conventional oxidation in principles、equipment、techniques and testing. And hard oxidation tries to decline the solubility of oxide film, with the following major characteristics:

a.The temperature of bath solution is low (generally about 20 degrees, its horniness is under 5 degrees). Usually, it has harder oxide film due to it.

b.Low viscosity of bath solution(common sulfuric acid is 20%, its horniness is under 15 degrees)  has small rate of dissolution towards film.

c.The bath solution adds organic acid and sulfuric acid adds oxalic or tartaric acid.

d.Applied pressure and current is high (common current is 1.5A/dm2 and pressure is below 18V, with hard current is 2~5A/dm2 and pressure is above 25V. Its peak is 100V)

e.Applied pressure suits to increase the voltage progressively because of its high pressure and current. If it takes along time to settle, it will have large energy consumption. Meanwhile , hard oxidation usually uses impulsing power source or special wave form source.

Hard anodizing of cast aluminum alloys

Cast aluminum alloys often needs hard anodizing to level up its properties. It usually uses Al / Si Alloys and Al / Cu alloys because AI Alloys has great casting character and corrosion resistance which is to be used most, universally for structural parts and components , sometimes accompanied with copper and magnesium to improve its mechanical property and heat resistance. Al / Cu alloys is frequently used casting alloys for bearing large dynamic and static loads and less complex sand casting. Cast aluminum alloys needs to improve electrolyte and wave form source because it contains nonmetal elements. Electrolyte usually can add some metal—salt 、organic acid、sulfuric acid—oxalic—tartaric acid solution or sulfuric acid— dry oil solution. The forms f source are generally changed into DC superposition、aymmetrical current、pulse current, among which pulse current has the better effect. Before the oxidation of electric casting, we should remove burr of water caltrop in case of current concentration. 

Micro - ARC oxidation of aluminum alloy

1.The principle of MAO

  Micro-arc oxidation is also called micro-plasma surface ceramic-coating. It is a method that use arc discharge to increase and activate the reaction on the anode bases on white anodizing, and forms a high quality enhanced ceramic membrane on the surface of artifact, which made by aluminum, titanium, magnesium and its alloys. It makes metal on the surface of artifact interact with electrolyte solution and form micro-arc discharging by using micro-oxidation power to applied voltage on the artifact. With the factors of high temperature and electric field, etc. it forms a ceramic membrane on the metal surface to strengthen the surface of artifact.  

2.The features of MAO

   a. It greatly improves the surface hardness (HV>1200), which is higher than the hardness of high carbon steel, high alloy steel and high-speed tool steel after heat treatment.

   b. It has good wear resistance property. 

   c. It has good heat-resistance and corrosion resistance (CASS salt spray test>480h), which can  fundamentally address the drawbacks of aluminum, magnesium and titanium alloy materials in their applications. So this technology has a bright prospect.

   d. It has excellent electrical insulation property, which insulation resistance can up to 100MΩ.

   e. This technology is stable and reliable with simple equipment while its reaction can happen under normal temperature operation, which is convenient procedure and easy to preform.

   f. Ceramic membrane that forms in matrix in situ is firmly combined, compact and uniform. 

3.The applications of MAO

   Micro-arc oxidation is a new surface treatment technology of aluminum alloys. It combines the ceramics of alumina with the metallic of aluminum alloy to make aluminum alloys’ surface has better physical and chemical properties. But it is not extensively used in China for the reasons of technology and economy. However, oxide film can be used in lots of fields including aircraft engines and automobile engines, petrochemical industries, textile industry and electronics industry, etc. because of its special properties.   

4.The disadvantages of MAO

   Micro-arc oxidation can produce spark discharging and spark eroding, roughing surface of the product. It can cause waste because the rough layer need to be grinded off when using. Its energy consumption is high, which is five times higher than common oxidation.