The Surface Modification of Creep - Resistant

THE SURFACE MODIFICATION OF CREEP-RESISTANT MAGNESIUM ALLOYS BY PEO A. Kossenko, A. Lugovskoy, M. Zinigrad Ariel University, Ariel, Israel

Abstract Oxide ceramic coatings have been produced on Al-Ca-Sr magnesium alloys by plasma electrolytic oxidation (PEO) using various concentration of sodium silicate in weakly alkaline electrolyte. Potential-time responses were very varied for PEO in different electrolytes. A coating is formed from tree sublayers (outer, inner and transition), which differ significantly in structure, porosity, and composition. The main compounds of coating’s outer layer are periclase and forsterite in the dilute electrolytes and forsterite and various modifications of silicon oxide in the high concentrated electrolyte. The greatest hardness (874.7 HV10) of an oxide ceramic layer on the magnesium alloys was obtained in the outer sublayer of the coating; the inner oxide layer has a smaller hardness (236.1-458.6 HV10). Coatings derived using all electrolytes provided good corrosion resistance.

Introduction PEO is an electrolytic process (similar to anodizing), but the use of non-toxic weakly alkaline electrolyte and high voltage promotes the formation of millions of microscopic discharges with a very short lifetime. They melted and modified a growing oxide layer, change the structure, making it harder and denser. The most popular are basic silicate electrolytes that have a lower alkali content (0.5-1.0 g/L of alkali) for the oxidation of aluminum alloys and a higher basicity (up to 10 g/L of alkali) for the oxidation of magnesium alloys [1-4]. MRI153M and MRI230D are novel aluminum- and strontium- containing magnesium alloys, which were developed in the Magnesium Institute in Beer-Sheva and are produced by Dead Sea Magnesium Ltd. MRI153M is a die-cast, elevated-temperature, beryllium-free alloy with good castability, corrosion resistance, and mechanical properties at room temperature. It was patented by Dead Sea Magnesium and Volkswagen [5]. It remains creep-resistant at 130-150 °C under stresses equal to 50- 80 MPa. This alloy was developed for gearbox housings, valve covers,

94 intake manifolds, oil pans, oil pumps, and other parts. MRI230D is an elevated-temperature alloy developed for power-train applications, such as engine blocks, bedplates, and high-performance gearbox housings operating at 190 to 200°C (recently patented by Dead Sea Magnesium). AZ91D is a corrosion-stable hard magnesium alloy, which was initially developed for sand and metal casting, but is currently widely used for casting under pressure of chassis and motor parts, reducer bodies, casings, seats, and so forth [6]. AZ91D was used as base alloy (для сравнения). In spite of the fact that these alloys are classified as corrosion-stable, their corrosion resistance and wear resistance are still relatively low. In the present study, PEO of an creep-resistants Al-Ca-Sr magnesium alloys has been carried out in a wide range of concentration of silicate electrolytes with a 50 Hz pulsed bipolar regime. The microstructures, hardness, phase composition and corrosion properties of the coatings were studied. It is shown that three-layered dense coatings are formed in electrolyte with all concentration of silicate.

Experimental Sample preparation The composition of the magnesium alloys employed in this work is shown in Table 1.

Table 1. Chemical composition of the magnesium alloys (wt. %)

Element Rare Mg Mn Zn Al Ca Sr Sn Cu Be Alloy Earth Az91D bal. 0.30 0.70 9.0 NA NA NA NA 0.025 0.0008 MRI153 bal. 0.25 0.70 8.0 0.85 0.16 0.14 0.004 NA NA MRI230 bal. 0.30 0.08 6.5 2.3 NA 0.4 1 NA NA

The samples were cutting from plate obtained from Dead Sea Magnesium Ltd. into the size 22040 mm. Prior to the PEO process, the samples were mechanically polished with #600 and #1000 abrasive SiC papers, rinsed in ultrasonic bass with distilled water, and cleaned in acetone. The homemade machine equipment with stirring and cooling systems was used to perform the PEO process with an applied current density of 10A/dm2. An electrolyte with a silicate and low concentration

95 alkali (0.037 M NaOH) was used at the temperature 20-25 °C. To study the influence of the concentration of silicate ion on the hardness, structural change and corrosion behavior of the coating layers, four different conditions: 0.015, 0.030, 0.060 and 0.100M of Na2SiO3·5H2O were applied.

Coating characterization The surface and cross-section morphologies of the PEO coatings were examined by scanning electron microscopy (SEM) in a JEOL JSM- 6510LV instrument with energy-dispersive X-ray spectroscopy (EDS, or EDX) – NORAN SYSTE 7. The Proza Phi-Rho-Z matrix correction algorithm was used for the quantitative analysis. Phase compositions of coatings were determined with a PANalytical X’Pert Pro X-ray diffractometer (Cu-Kα radiation) in Grazing incidence mode, using a scan with a grazing angle Ω of 3, a step size in θ of 0.02 and a range of 2θ from 20 to 90. Depth of penetration (x, cm) [7, 8] in an X-Ray analysis was calculated. The corrosion behaviours of specimens were investigated by potentiodynamic polarization tests in 3.5 wt. % NaCl solution, using an Autolab PGSTAT12 potentiostat/galvanostat with the General Purpose Electrochemical System (GPES) version 4.9 software. A three-electrode cell, with a stainless steel counter electrode and a saturated calomel reference electrode, was employed. The polarization resistance of a sample was determinate at a scan rate of 1 mV/s, from 250 mV below the OCP [9]. A Buehler Micromet 2100 microhardness tester was used to monitor the microhardness of the oxide layer on the cross section. The microhardness was determined according to ASTM E384, C1327, and B578 as the mean of five measurements for each sublayer under a load equal to 10-100 g. Result and discussion Potential – time responding during PEO Plots of the dependence of the formation voltage of the oxide layer on the oxidation time are presented in Fig. 1. The oxidation process was impeded at low concentrations of silicate in the electrolytes, especially for alloy AZ91D. In the electrolytes with 0.015 and 0.030 M

96 Na2SiO2·5H2O the process stopped at 20 and 50 minutes, respectively (Fig. 1a). An electric arc was ignited at one place on the sample. The oxidized layer was then destroyed, and a depression was burned out on the surface.

Fig. 1. Potential-time responses for PEO in different electrolytes for several magnesium alloys: a) AZ91D, b) MRI 153, c) MRI 230. The plots of the variation of the voltage with the process time that were obtained during the oxidation of different alloys in electrolytes with the same silicate concentration are identical. The voltage for formation of the oxide layer did not exceed 300 V, while the voltage for oxidation of the aluminum alloys was 300-330 V for similar silicate concentrations [10]. One exception is the oxidation of alloy MRI230 in the electrolytes with 0015 and 0.030 M Na2SiO2·5H2O (the voltage at 60 min during the process was 320 V and 315 V, respectively). During

97 oxidation of the magnesium alloys, the first stage of the process (the transition to a steady-state regime) has a greater duration: 3 min, as opposed to 1 min for an aluminum alloy [11]. Phase composition (XRD) Fig. 2 presents data on the phase composition of the oxide ceramic layers obtained. The quantitative analysis results presented were obtained as a result of a Rietveld refinement using the X'Pert HighScore Plus software package, version 2.2e, from PANalytical B.V. The goodness of fit (GOF 1.56-3.48) indicates the fairly high reliability of the treatment performed. The depth of the phase analysis performed (x), which amounted to 11.7-26.2 μm, so the data presented reflect the phase composition of the outer layer of the coating.

Fig. 2. XRD-based phase composition of the coatings formed during 60 min

with various silicate concentrations (M Na2SiO2·5H2O) in the electrolyte on various Mg alloys: AZ91D, MRI153, MRI230. The principal phases comprising the oxide ceramic layer on the all silicate concentration in the electrolyte are periclase and forsterite. As is seen, in the electrolyte with 0.100 M Na2SiO2·5H2O the oxide layer is 97-99% forsterite. Alloy MRI230 has a greater forsterite content in the coating over the entire range of silicate concentrations in the electrolyte.

Chemical composition and structure of layers on magnesium alloys On the typical photomicrographs of a cross section of an oxide ceramic layer (Fig. 3) the arrow shows the line for performing the EDS

98 Fig. 3. Cross section (left) and line scan (right) of PEO coating of MRI153 magnesium alloy for different electrolyte (from top to down: 0.015; 0.030; 0.060; 0.100M Na2SiO2·5H2O)

99 line scan (in the direction from the base alloy through the oxide ceramic layer to the plastic vessel into which the sample was poured during its preparation. The coatings obtained upon oxidation of the magnesium alloys have some the features. There is a clearly expressed dense transition layer with a thickness of about a micron between the base metal and the inner layer of the oxide. In this layer (according to the EDS data in fig. 3) there is an abrupt drop in the content of magnesium and aluminum, and there is an increase in the oxygen content, which does not vary further in the direction from the transition layer to the surface. The oxygen concentration in the coating is maintained at the 53% level for the electrolytes with a silicate concentration equal to 0.015 - 0.100 M

Na2SiO2·5H2O. A zone with an oxygen content equal to 8 - 12% was discovered between the base metal and the transition layer. On the SEM photomicrographs there are no structural changes that define this sublayer (Fig. 3). The diffusion of oxygen into the bulk of the base metal occurs under the action of the elevated temperature and pressure of the microplasma discharge. When the aluminum alloys were oxidized in similar electrolyte, this zone was not detected by the analytical methods employed. Discharge channels that begin on the outer surface of the transition layer and pass through both principal sublayers are clearly noticeable in the coating.

Hardness

The greatest hardness (874.7 HV10) of an oxide ceramic layer on the magnesium alloys was obtained in the outer sublayer of the coating in the electrolyte with a silicate concentration equal to 0.060 M

Na2SiO2·5H2O. The inner oxide layer has a smaller hardness (236.1- 458.6 HV10). The data from the hardness measurements are presented in Table 2. The measurements performed are in complete agreement with the results of the quantitative phase analysis of the coating (see the preceding section). The compound forsterite, which is the principal coating-forming phase, has the greatest hardness.

100 Table 2. Dependence of the hardness of the oxide ceramic coatings on −2 magnesium alloy (HV10, kgs·mm ) on the silicate content in the oxidation electrolyte (M Na2SiO2·5H2O)

AlloyM AZ91D MRI153 MRI230 outer inner outer inner outer inner layer layer layer layer layer layer Na2SiO2·5H2O 0.015 475.3 255.3 546.4 326.5 703.1 236.1 0.030 746.5 354.4 690.3 307.5 809.9 378.6 0.060 804.6 375.3 874.7 356.6 868.0 329.6 0.100 683.5 362.8 700.9 366.9 873.0 327.8

Electrochemical corrosion of magnesium alloys samples The corrosion characteristics of untreated magnesium alloys and samples with an oxide ceramic coating are shown in Table 3.

Table 3. Corrosion characteristics of magnesium alloys for different concentrations of Na2SiO2·5H2O in the electrolyte

Alloy M Corrosion current density, Corrosion potential, V μA·cm−2

Na2SiO2·5H2O AZ91D MRI153 MRI230 AZ91D MRI153 MRI230 Base alloy −1.472 −1.148 −1.421 950 244 102 0.015 −1.453 −1.468 −1.497 11.9 41.3 9.99 0.030 −1.490 −1.438 −1.447 12.6 28.7 30.7 0.060 −1.456 −1.483 −1.435 25.9 33.9 45.6 0.100 −1.337 −1.483 −1.485 168 9.07 20.9 Alloy M Polarization resist., kOhm Corrosion rate, mm·year−1 AZ91D MRI153 MRI230 AZ91D MRI153 MRI230 Na2SiO2·5H2O Base alloy 4.09 16 38.55 21.8 5.59 2.32 0.015 17.97 7.216 2.422 0.272 0.944 0.229 0.030 0.399 2.397 1.841 0.876 0.657 0.702 0.060 0.7548 3.737 3.085 0.594 0.775 0.604 0.100 7.674 5.691 4.167 3.84 0.208 0.478

The measured corrosion potential of the oxidized magnesium alloys is below the corrosion potential of the untreated alloys in several

101 cases (for example, the corrosion potential of alloy MRI153 is 1.148 V, and the corrosion potential of the oxidized alloy in an electrolyte with

0.060 M Na2SiO2·5H2O is 1.483 V). However, the corrosion current density of the oxidized alloys in the electrolytes with all of the silicate indices and concentrations used is 1–2 orders lower. As a result, the smallest corrosion rate for all the alloys was obtained in the electrolyte −1 with 0.060 M Na2SiO2·5H2O (0.594–0.775 mm·year ).

Conclusions The oxide layer on the magnesium alloys was obtained using PEO in an electrolyte containing 0.015-0.100 M Na2SiO2·5H2O and low concentration alkali (0.037 M NaOH). The following alloys, which differ in corrosion resistance, properties, and areas of application, were subjected to oxidation: AZ91D, MRI153, and MRI230. Stable characteristics of the coatings were obtained for all the alloys at all silicate concentrations. The thickness of the coatings was 30 and 50 μm, respectively. The hardness of the inner layer was 325-375 HV10, and that of the outer layer was 700-879 HV10. According to the X-ray powder diffraction analysis data, the outer layer of the coating obtained in electrolytes with the concentrations just indicated consists mainly of magnesium silicate (forsterite), which has a high hardness (7 Mohs). The most corrosion-resistant coatings on all the alloys were obtained in an electrolyte containing 0.060 M Na2SiO2·5H2O. The corrosion rate was 0.59-0.78 mm·year−1, which is 40 times smaller than the corrosion rate of the untreated alloys.

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