The role of anions in the formation and corrosion resistance of the plasma electrolytic oxidation coatings

Abstract

Abstract:Combination of KOH with each of Na 2 SiO 3 , Na 3 PO 4 and NaAlO 2 , formed three different coating solutions to produce plasma electrolytic oxidation (PEO) coatings on the surface of AM50 magnesium alloy. The surface morphology, cross section, chemical composition, corrosion resistance and structure of each of the coatings was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS). The results showed that different anions, i.e., SiO 3 2-, PO 4 3-and AlO 2 -, influence the coating characteristics such as thickness, chemical composition and coating structure. The results showed that thicknesses of the Si-, P-and Al-coatings are 8, 4 and 1 µm, respectively. Moreover beside MgO existing in structure of all three coatings, specific phases namely Mg 2 SiO 4 , Mg 3 (PO 4 ) 2 and MgAl 2 O 4 were formed in the structure of the Si-P-, Al-coating, respectively. It was revealed that usage of SiO 3 2-instead of PO 4 3-or AlO 2 -led to formation of a coating layer with better corrosion protection properties. The better performance of the Si-coating compared to P-or Al-coatings is considered to be due to the fact that the thickness, the number of open pores and the resistance of the barrier layer are formed under such an optimum conditions which result in a higher corrosion resistance.

The effect of current mode and discharge type on the corrosion resistance of plasma electrolytic oxidation (PEO) coated magnesium alloy AJ62

“…3 show some evidence of intermetallics being retained at, or near, the coating/substrate interface. A similar interface appearance has been seen in a PEO-coated AM50 magnesium alloy [15]. This irregularity in the coating/substrate interface has a beneficial effect in improving the adhesion between the coating and substrate.…”

“…The model chosen for the fitting was a commonly used model for PEO [15,16,41] and other ceramic coatings [42,43]. The choice of the circuit was a balance between a reasonable fitting of the experimental values and a good description of the electrochemical system by keeping the number of circuit elements at a minimum.…”

“…The sub-processes include both mass transfer and charge transfer and can be in series or parallel with each other [43]. More complex equivalent circuits have been proposed by Ghasemi et al [15] for PEO-coatings produced with KOH/Na 3 PO 4 and KOH/ NaAlO 2 electrolytes. In this model, a more general Constant Phase Element (CPE) was used instead of a capacitive element, which reflects the distributed surface reactivity, surface roughness, electrode porosity, and current and potential distributions associated with electrode S1 S0…”

“…The PEO method can be used to form a thick, hard and adherent ceramic coating on the surface of Mg alloys as well as the valve metals (Al and Ti) and their alloys [14]. Coating surface morphology, structure and corrosion resistance are affected by many parameters including electrolyte composition [15][16][17][18], substrate material [19], and the electrical parameters, mainly current mode and current density [9,20]. Different current modes have been utilized in the PEO treatment including, DC, AC, unipolar and bipolar current modes [21][22][23].…”

Desarrollo de la industria de aleaciones de magnesio en Colombia - Una oportunidad

“…Por ejemplo, se están implementando técnicas de conversión sobre la superficie del Mg a través de anodizados que tienen como objetivo generar capas protectoras de óxidos y otros compuestos a partir del sustrato, de electrolitos y parámetros eléctricos preseleccionados. La técnica mencionada anteriormente es conocida como oxidación electrolítica por plasma (PEO por sus iniciales en inglés), la cual hace del Mg un mejor material de ingeniería debido al incremento de su resistencia a la corrosión y desgaste, además de hacerlo más biocompatible [34][35][36][37][38][39][40][41][42][43].…”