Corrosion Resistance of Aluminum Alloys Treated by Micro Arc Oxidation

CORROSION RESISTANCE OF ALUMINUM ALLOYS TREATED BY MICRO ARC OXIDATION IN DIFFERENT ELECTROLYTES

Michael Remennika,b, Alexey Kossenkoa, Alex Lugovskoya, Michael Zinigrada a Faculty of Natural Science, Ariel University Center of Samaria, Science Park, Ariel, 40700,Israel b Department of Chemistry, Bar-Ilan University, Ramat-Gan, 52900, Israel

Abstract. Corrosion resistance of aluminum oxide layer, obtained on aluminum parts and aluminum coated steel, by Micro Arc Oxidation (MAO) technology, in different electrolytes, was examined. KOH, H3PO4 and Na2SiO3 addition to Na2SiO3 based electrolytes was tested. Additionally, the efficiency of MAO technology as an anti- corrosion treatment for aluminum alloys examination was performed. Corrosion rate of MAO treated aluminum parts was measured by linear sweep voltammetry method, in 3.5% NaCl aqueous solution. Obtained results presented in this paper.

INTRODUCTION

Corrosion is one of the most destructive processes that occurs on metals, and causes huge physical and esthetical damage on them. Corrosion protection of aluminum (Al) alloys is an important industrial problem. Traditionally, it can be solved with hexavalent chromium (Cr6+) coatings. However, these coatings are hazardous, and have to be replaced. One of the most promising alternatives to Cr6+ corrosion protection technology is the protection with aluminum oxide layer obtained by Micro Arc Technology (MAO). It's important to note that anodizing is one of the alternative technologies for aluminum protection, but in one hand the obtained oxide coating has much poorer mechanical properties, in comparison to those of oxide coating obtained by MAO technology, in the other hand electrolytes used for anodizing are less "green" and therefore it makes them a problematic waste. MAO technology is a relatively new and effective method of producing thick, hard and well-adhered ceramic coatings with excellent wear resistant properties on light alloys-especially on aluminum and titanium [1]. This technology is based on the anodic polarization of processing materials in an aqueous electrolyte solution under plasma discharge conditions on the surface of the anode. These conditions arise after exceeding the critical values of the polarization potential. As a result, on the anode surface, oxide-based layers with special properties different from those of conventional anodic films are formed. The process consists of numerous simultaneous discharges over the whole surface of the anode [2]. According to Ref. [3], the current of a single discharge is 10−3–10−2 A, the life time is 10–20 ms, the electrical power is 0.2–1.0 W, and the quantity of discharges is 105 per cm2. Micro Arc Oxidation of metals is a complex process combining concurrent partial processes of oxide film formation, dissolution and dielectric breakdown. The probability of dominance for any of these partial processes in the overall process depends on the nature of both the metal and the electrolyte, as well as on the current regime employed. The ultimate stage of the MAO treatment is a quasi-stationary state of persistent anodic micro discharges, which exhibit a progressive change in characteristics during the electrolysis. At high discharge temperatures and pressures (reaching about 2 × 103 to 3 × 103 ◦C and ~102 MPa, respectively), solid products of

59 electrolysis and adsorbed gel layers are deposited on the metal surface in the form of high-temperature oxide phases or glassy ceramic coatings. Under those extreme conditions, which occurs during MAO process, aluminum oxide can crystallize to different crystallinic forms of the oxide under different conditions, like  -Al2O3,  - Al2O3,  -Al2O3 and others. Depending on the purpose of the oxidation, the MAO treatment is typically carried out for between 5 and 180 min at current densities of 500–2000Am−2 and voltages of up to 800V [4-5]. After MAO process the wear resistance, corrosion resistance, mechanical strength and electrical insulation of the metals and their alloys can be greatly enhanced. Lots of studies [6-8] have indicated that the properties of coatings produced by MAO mainly depend on the nature of the substrate metal, the type of power source, the applied current density, the compositions and concentration of the electrolyte, etc. MAO processes are commonly carried out in weak alkaline electrolytes containing anions such as silicate, phosphate, aluminate, etc., which is environmental friendly. However, the influence of silicate and phosphate electrolyte systems on the microstructure and composition of MAO coatings has rarely been considered [9]. With the introduction of hard ceramic coatings by means of micro arc oxidation, the wear resistance, corrosion resistance, mechanical strength, and electrical insulation of the metals and their alloys can be effectively increased [10].

EXPERIMENTAL DETAILS

Micro Arc Oxidation process

Rectangular examples of aluminum alloy were treated by MAO process. Each example was treated in different electrolyte composition; (1) first one in 1 gr/L KOH + 10 gr/L Na2SiO3, (2) second one in 10 gr/L Na3PO4 + 10 gr/L Na2SiO3, and the (3) third one in 20 gr/L Na2SiO3. All the processes lasted for 90 minutes, the current density was 20 A/dm2 and the temperature was 20-30 ºC.

Corrosion test

For examine MAO technology efficiency for corrosion resistance of aluminum, in generally, in addition to the MAO treated examples in different electrolytes, bare aluminum alloy example was tested. The anti-corrosion behavior of the coated aluminum alloy and the bare aluminum alloy was tested on an Autolab PG12 potentiometer, using an aqueous solution of 3.5% NaCl as the corrosive media and a three electrode corrosion cell as the test chamber, where Ag/AgCl electrode functioned as a reference electrode and a hoof like stainless steel as a counter. The linear sweep voltammetry tests were conducted in a polarization corrosion voltage from -350 to 350mV (relatively to OCP) at a sweeping rate of 1 mV/s. All of the coated Al alloy examples were immersed in the aqueous solution of 3.5% NaCl for one hour before performing linear sweep voltammetry test in order to reveal the effect of pre-immersion in the corrosive medium on the corrosion-resistance of the Micro Arc Oxidation coatings. Corrosion current was measured as a parameter that indicates the speed of the corrosion process, and therefore a corrosion rate in the units of mm corrosion in year.

RESULTS AND DISCUSSION

60 After 90 minutes of MAO treatment the obtained oxide thickness was measured. The thickness of the obtained oxides was 100-110µm in the sample number 1, 90- 100µm in sample number 2, and 130-140µm in sample number 3. Corrosion resistance test witch was done on the bare aluminum provided the next -5 -2 result: corrosion density icorr.= 1.307·10 A·cm , polarization resistance Rp= 3 -1 -1 4.162·10 Ohm, corrosion rate Corr.rate= 1.425·10 mm·year , corrosion potential Ecorr.= -0.858 V . In comparison to any aluminum sample treated by MAO process, in any electrolyte, it's unequivocal that corrosion resistance improved. Results of corrosion test of aluminum alloy parts protected by MAO technology, in different electrolytes are presented in table 1.

Table 1. Results of the corrosion tests.

Corr.rate -2 -1 Sample Electrolyte Ecorr. (V) icorr. (A·cm ) Rp (Ohm) (mm·year ) Na SiO 10 gr/L + 1 2 3 -1.336 6.14E-07 6.02E+05 6.70E-03 KOH 1 gr/L Na SiO 10 gr/L + 2 2 3 -0.903 5.05E-08 4.46E+06 5.51E-04 Na3PO4 10 gr/L

3 Na2SiO3 20 gr/L -1.218 1.36E-07 1.28E+06 1.48E-03

One can see that, in general, there's no doubt that MAO treated aluminum alloy has much higher corrosion resistance than bare aluminum alloy. MAO treatment improves corrosion resistance by more than three orders of magnitude.

Potentiodynamic curves of corrosion tests are presented in figure 1.

Fig. 1. Potentiodynamic curves of corrosion tests; 1 – MAO treated aluminum alloy in electrolyte with KOH addition, 2 - with Na3PO4 addition, 3 - with Na2SiO3 addition, 4 – bare aluminum.

After examining three electrolytes based on sodium silicate (Na2SiO3), with different additives (KOH, Na3PO4, and double concentration of Na2SiO3), it can be

61 concluded that addition of sodium phosphate helps obtaining less porous oxide layer, and by that causes better anti-corrosion protection. Though in sample 3 the oxide layer 20-30% thicker that in the other two electrolytes, was obtained, corrosion resistance test showed worst results, in comparison with the one with sodium phosphate addition. It can be explained by the fact that the obtained oxide contains more amorphous silicate complexes, and causes higher porousness.

SUMMARY Efficiency of the MAO technology for improving corrosion resistance of aluminum and aluminum alloys is enormous and unambiguous. Addition of Na3PO4 to the electrolyte improved corrosion resistance of the obtained coating. High concentration of Na2SiO3 in the electrolyte contributes for higher growth rate of the oxide, but makes the oxide layer more porous and therefore less protective. There are lots of possible additives for electrolyte that can be examined. The next stage of our research is improving anti-corrosion properties of MAO treated aluminum alloys, by addition of ceramic nano particles to the electrolyte.

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