Micro –arc anodic oxidation is a well- known method for the production of ceramic layers and ceramic oxides, having very good properties in heat wear resistance and corrosion, along with dielectric, thermal insulation, interface adhesion properties.

Micro arc oxidation mechanism

The curve representing the time-dependence of the voltage was divided into three regions.

Dependence chart:

Under certain circumstances, micro-arc oxide films can form.
OA- anodic oxidation;
DS- spark discharge
DMA- micro-arc discharge
DA- disruptive discharge

Micro-arc oxidation process  = Complex mechanism which can be divided into several stages developing simultaneously or successively.

blue – Chemical interaction of the basic material and the layer formed with the electrolyte

red – Electro-chemical processes taking place until the discharge ignition, in the places where discharge does not take place at that time.

grey -Actual micro-arc oxidation.

Micro arc oxidation technologial parameters and used equipment

Micro-arc oxidation process

Electrolyte
Support material
Treatment regime and duration =  Composition, structure and properties of the layers obtained

Electrolytes

In preparing the electrolyte, attention must be paid to order of mixing or dissolving the parts, because it affects not only the voltage and ignition of electrical micro -discharge, but it often determines the quality of layers and the lifetime of the electrolyte.
The electrolytes used for micro- arc oxidation are often subject to hydrolysis, due to the content of salts of the weak acids. The factors that favor this harmful phenomenon are: increase of the electrolyte temperature, in particular over 40 ° -50 ° C , diluting of the electrolyte , especially in hot water and absence of alkalinization.

With increasing the electrolyte temperature, oxidation speed increases and the breakdown voltage decreases, but in this case it may worsen the characteristics of the layers, for example: disruptive voltage decreases and porosity increases.

They can be classified by:

– type of electric current (continuous, alternate or their overlapping); polarity of voltage applied (anode, cathode, anode – cathode, cycling regimes of different polarity to the presence of a break without electricity);

electrical discharge character (spark, micro-arc, arc electrophoresis);
process control degree (manual, semiautomatic, automatic);

MICRO-ARC TREATMENT INSTALLATION

Micro- arc oxidation installation is composed of:

– two stainless steel tanks connected by a recirculation circuit of the electrolyte by means of a pump; the first tank has a 8liter-capacity and it is intended for the micro-arc treatment; the second tank has a 50 liter-capacity and the electrolyte circulates through it;
a cooling system made of stainless steel pipe;
an elecvtrical hood having the role of exhausting gases resulted in the micro-arc oxidation process;
technological voltage source;
sample lifting and lowering device.

Preparation of electrolytes for micro- arc oxidation

An aqueous sodium aluminate solution has been prepared, having the following composition: sodium aluminate, NaAlO2  0,1mol/l sodium hydrate, NaOH  0,05 mol/l.
The aqueous sodium aluminate solution was prepared by two methods:
– sodium aluminate was prepared starting from aluminum sulphate and the sodium hydrate was purchased;
– both sodium aluminate and sodium hydrate were purchased from a specialized firm.

Micro- arc oxidation was carried out using a technological source of direct current, which provides voltages ranging between[0-600]V and amperages ranging between[ 0,30] A. The power unit provides a constant voltage with an accuracy of ± 5 % of the desired value.
The samples were made ​​of pickeld carbon steel sheet OL37 , 1mm thickness. The surface area of the micro -arc oxidation of the sample is 4 cm² on each side . The rod is protected with a varnish , not to be subject to oxidation.

Electrolysis bath was made of stainless steel. It has the shape of a parallelepiped with dimensions of 20cm x 20cm x 25cm, in the front is executed a window that allows the observation of micro -arc oxidation .
Bath walls serve as the cathode. Cathode surface area (400cm²) is about 100 times larger than the anode surface area.
Before micro- arc oxidation process, no prior treatment of samples has been performed.

Experimental results

Samples subject to micro- arc oxidation process are presented in the table below:

The corrosion resistance of the oxide film was evaluated by the salt spray test , according to European norms ME D17 1058 / – J.
Salt spray tests consisted of placing the samples in a stove brand Erichsen 606/1000 no. 08145 / 5, with operating temperature adjusted at 35 ° C +- 2 ° C. In this environment, the samples were sprayed with a sodium chloride NaCl with a concentration of 5%- 0.5 % .

Salt spray test results:

Descărcări electrice pe suprafaţa probei în condiţiile de lucru selectate

A. Electric discharges at the sample surface at the beginning of the micro-arc oxidation process
B. Electric discharges at the sample surface during the micro-arc oxidation process
C. Moving of micro- arc discharge to disruptive discharge – end of micro- arc oxidation process.

Measuring the layer thickness by optical microscopy method

Conclusions

By help of the micro- arc oxidation installation we have demonstrated the possibility of using the micro -arc oxidation technique for obtaining aluminum oxide layers on the surface of the iron which does not possess rectification properties.
The layers obtained are porous and do not provide a sufficient corrosion protection. There is the possibility of increasing the corrosion resistance of the oxide layers, by adding in the used aqueous solution of the electrolyte, of an amount of a polyethylene fine powder (0.1-1.0 ) % by mass, the dispersion [ 40 ]. The layers obtained have a higher resistance to corrosion than those obtained without the addition of polyethylene and the electrolyte does not warm up significantly, allowing the micro -arc oxidation at almost constant temperatures of (20 ± 5 ) °C .