Letter 685

Microarc Oxidation

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Hello:
I am a student .Recently, I heard of a method to protect metals and the name of the method is microarc oxidation.Now I can not find any information about it.If there is any help available, it will be gratefully appreciated.
Thanks.

He Fei -
material dep of tsinghua univ

La société allemande exploite un procédé (kemical plasm coating) qui est peut être l'équivalent de ce que vous cherchez. Un procédé russe est aussi très proche. cela s'applique sur aluminium mais aussi sur magnésium

Bonne année

F GARNIER
- France

the microarc oxidation is a Russian technology called also plasma electrolytic oxidation (p.e.o.)It is a very revolutionary method to produce hard and corrosion resistant coatings on Al, Mg, Ti and other light metals.

aldo bordiga
- soncino-cr-italy

Hi Guys,

Micro-arc oxidation technology needs comprehensive evaluation. It is hard to find reported data and documented materials with confidence addressing the process performance (such as production efficiency, reproducibility, cost, etc.) and the properties of resultant coatings (such as corrosion resistance, wear resistance, hardness, smoothness, uniformity, adhesion, dyability, etc.). It is greatly appreciated if anyone can provide some information helpful to find out convincing data which are supporting any micro-arc oxidation process's success.

Ling

L. Hao

The microarc oxidation is still an expensive coating compared to traditional treatments on aluminium.The short life of the electrolyte and the great amount of power are limits.It cannot satisfy a big market but in some case I have found that the qualities of this treatment are very interesting. In the textile field there are many pieces that could take advantage of MAO coating instead of the plasma spray ceramic coatings or anodizing. About the reproducibility it requires proper position of the pieces in a well formulated electrolyte.

aldo bordiga
- soncino-cr-italy

Hello,

Let me introduce the technology of micro-arc oxidation.

MICRO-ARC OXIDATION

SUBJECT: Technology and equipment for wear-resistant, corrosion-resistant, electroinsulative and decorative coatings by the method of micro-arc oxidation on the parts made of aluminium and titanium alloys.

The method of micro-arc oxidation (MAO method) is very effective for protective and decorative coatings on aluminium profiles and other constructive elements, what is especially important for vehicles and constructions (aircrafts, boats, window frames, siding of buildings, roofs, etc.).

The method of micro-arc oxidation possesses high output, possibility of automatization the process and variation of coating properties within a wide range. The method provides for a deposit of oxide films with unique properties on the parts of complicate configuration and large dimensions. The process is environmentally safe.

MAO method uses the effect of forming micro-arc plasma discharges on the surface of the unit treated in water electrolytic solution; because of influence of these discharges on the articles, a multi-function protective coating forms.

Alkaline solutions of low concentrations (2-3%) are used in electrolytes, what ensure the environmentally safe process.

The method is useful for treating the passivating class metals, such as Aluminium, Titanium, Zirconium, etc., as well as their alloys.

The unique of the oxide coating made by MAO method is explained by the possibility of forming the different micro-structure and phase composition coating, also the fused amorphous oxide films.

The articles of Aluminium alloy with coating :

• have the increased corrosion resistance, also in the humid tropical climate (the test of samples in the 5% sea fog within 2000 hours did not show any trace of corrosion);
• have the increased wear resistance (wear speed of MAO-coating is lower by more than an order of magnitude as compared to those of mullite articles tested at the same conditions, microhardness of wear resistant coating is 15000 - 20000 MPa instead of 4000-6000 MPa on the anodised coatings;
• the breakdown voltage on the electroinsulating MAO coatings amounts to 5000V (5 times as high as under anodising).

General information about the technology proposed.

For application of the coating, the conventional bathes are used, which are normally used for galvanic process of anodising, but the quantity of them are considerably reduced (2 instead of 8-10); low concentration alkaline environment friendly solutions are used as electrolytes.

The process is carried out under higher voltage (more than 150- 200 V) than those by anodising one. Depending on the application of the coating, the power consumption varies between 0,80-27,0 kWatt×hour/dm2.

The dimensions of the bath depend on the dimensions of the parts to be coated; correlation of area of the part surface to the area of internal surface of the bath should be 1:10.

Cost of 1 m2 of corrosion resistant coating is $3.00.

Oleg Demin
South Ealing, London W5 4RD - United Kingdom

There is at least one company in the West doing micro-arc oxidation. They have ties to MIT. They claim that they have got the process time down to 15 minutes - apparently the original Soviet technique took hours, which is probably where the charges of energy-inefficiency originated as well.

Christopher E. Harris, PhD (graduand)
Private Researcher - New Zealand

Several companies in Europe and more recently Asia and the USA are using it for wear resistant and corrosion resistance on Al Mg and Ti alloys. There is a good deal of technical information available on the internet.

Robert Altham
Cambridge, England

Ed. note: Sorry if this page is growing cryptic, dear readers, but heavy editing has been necessary to preserve our policy that these public discussions remain technical and non-commercial. A lot of people are coming to this page trying to sell their MAO offerings. It's not fair to the readers, and it's not fair to the site's supporting advertisers who make this discussion site possible, to ask them to pay for the posting of testimonials to, and contact info for, their competitors. Ted Mooney, P.E.  finishing.com Brick, NJ

Dear Sir

I need more information about (mao) if it is possible.

Basheer Ahmed
- Iraq

Microarc Oxidation

Microarc oxidation (MAO) technology allows obtaining coatings with a unique complex of physical and mechanical properties on metals of valve group - aluminum,zinc,magnesium titanium and their alloys.
Compared to conventional anodising techniques the MAO process is able to produce ceramic coatings with higher performance, which particularly, have superior microhardness, abrasion resistance and corrosion resistance.

Advantages of the MAO process:
can be used to anodise alloys difficult to anodise with traditional anodising processes, e.g. high silicon content die castings or high copper content aluminum alloys
capability of producing high-thickness coatings (up to 300 microns)
pre- and post-treatments of the surface are not necessary
usage of environmentally friendly and inexpensive electrolytes
decorative attractiveness of colours of the coatings

The typical properties of the coatings are:
high hardness - up to 24000 MPa
low coefficient of friction f=0,005-0,01
high wear resistance ip=10-12
heat resistance (working temperatures - up to 800-1200 C)
piezoelectric properties
corrosion resistance in various mediums
dielectric durability 10-20 V/micron
heat conductivity factor 5-10 W/(m c)
surface finish Rz~1-40 microns (without post-processing) and Ra~0,04-0,08 (after polishing)
microstructure - 1-10 microns.

The MAO process exploits microarc discharges to form coatings on the metal surface. In fact, MAO is a surface modification process because the oxides layer grows into the part depth due to the oxidation of the metal. The microarc anodizing and oxidizing and the coating formation occur when high (up to 1000V) asymmetric pulsed voltage is applied between a metal part and an electrolyte bath. The process parameters (current and voltage modes, electrolyte composition and concentration) are optimised to achieve a desired combination of the coating properties.
For each material the process parameters are determined experimentally.

The following table shows the comparative characteristics of MAO technology and anodising:

Parameter
Anodizing MAO technology
Potential, V 10 - 30 100 - 300 and more
Current density, A/dm˛ up to 10 20 - 100
Hardness, kg/dm˛ up to 500 - 600 up to 2000 -250
Coating type amorphous crystalline
Electrolyte type acid alkaline
Microarc charges no yes

Equipment

The equipment for the MAO process includes a special power supply able to generate positive and negative pulses, an electrolytic bath, a control unit, an electrolyte conditioning system, an exhaust ventilation system. The main characteristics of the equipment are the current density (ranged from 80 to 160 A/dm2) and the productivity (m2 per hour).
The MAO equipment varies from small capacity units to large industrial plants. Below are given examples of the MAO production equipment . . .

John .Y . Wang
- SHAOXING, ZHEJIANG, CHINA

Just because any process doesn't seem to have any kind of limitations, you can not dismiss it. I have tested MAO alumina coatings for biological applications in my lab and they have shown very low wear and friction as compared to many other processes. They have their limitations, very high temperature and pressure are required to deposit these coatings, which makes these coatings not very industry friendly.

JUHI BAXI
- College Station, TX, US

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