The purpose of the study was to evaluate resistance to electrochemical corrosion of magnesium alloy WE43 after plastic working. Potentiodynamic and immersion corrosion tests were performed in solutions with concentration of 0,01 – 2 M NaCl. The tests of microstructure of magnesium alloy after its exposure in NaCl solutions were performed with application of scanning microscope. The results of measurements of alloy surface topography were given, which were obtained by means of optical profile measurement gauge. The results of carried out tests prove explicitly that corrosion properties of extruded magnesium alloy WE43 deteriorate with the increase of molar concentration of NaCl solution. It was proved that irrespective of the concentration of the solution, WE43 alloy features pitting corrosion. It indicates the necessity to apply protective layers on the parts made of the tested alloy

E. Hadasik

J. Przondziono

J. Szala

J. Wieczorek

W. Walke

English

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243METALURGIJA 52 (2013) 2, 243-246J. PRZONDZIONO, W. WALKE, E. HADASIK, J. SZALA, J.WIECZOREKCORROSION RESISTANCE TESTS OF MAGNESIUM ALLOY WE43 AFTER EXTRUSIONReceived – Prispjelo: 2012-06-04Accepted – Prihvaćeno: 2012-10-11Preliminary Note – Prethodno priopćenje ISSN 0543-5846METABK 52(2) 243-246 (2013)UDC – UDK 669.721:620.193=111J. Przondziono, E. Hadasik, J.Szala, J. Wieczorek, Faculty of Materials Science and Metallurgy, The Silesian University of Technology, Kato-wice, Poland, W. Walke, Faculty of Biomedical Engineering, The Sile-sian University of Technology, Gliwice, PolandThe purpose of the study was t o evaluate resistance to electrochemical corrosion of magnesium alloy WE43 after plastic working. Potentiodynamic and immersion corrosion tests were performed in solutions with concentration of 0,01 – 2 M NaCl. The tests of microstructure of magnesium alloy after its exposure in NaCl solutions were performed with application of scanning micr oscope. The r esults of measur ements of allo y sur face t opography w ere g iven, which were obtained by means of optical profi le measurement gauge. The results of carried out tests prove explic-itly that corrosion properties of extruded magnesium alloy WE43 deteriorate with the increase of molar concentra-tion of NaCl solution. It was proved that irrespective of the concentration of the solution, WE43 alloy features pitting corrosion. It indicates the necessity to apply protective layers on the parts made of the tested alloy.Key w ords: mag nesium allo y WE43 af ter ex trusion, elec trochemical c orrosion, pot entiodynamic and immersion tests, SEM, surface topographyINTRODUCTIONMagnesium alloys, depending on chemical compo-sition and formability, can undergo hot forming by means of the following methods: rolling, open die forg-ing and stamping, extrusion forging and sheet press forming. Extrusion is one of basic methods of shaping magnesium alloys, which enables to produce long pro-fi les with uniform cross-section. Concurrent extrusion and backward extrusion are both applied. Apart from matrix preparation and greasing, main parameters de-termining the process of extrusion include: temperature, degree of plastic forming and rate of strain. Extrusion forging of magnesium alloys is most often realised in temperature range 320 – 450 °C at the rate from 1 to 25 m/min. Recently, intensive development of hydro-static extrusion has been observed, which enables to perform the process in lower temperatures and obtain better grain size reduction of magnesium alloys [1−4].Application of magnesium alloys in aeronautic and automotive industries is to a great extent limited due to low corrosion resistance which results from insuffi cient protection of oxide layer that is created on the surface in oxidizing atmosphere or hydroxides layer in aqueous solutions [5−11].The purpose of this study was evaluation of resist-ance to electrochemical corrosion of magnesium alloy WE43 after extrusion. Corrosion tests were performed in NaCl solutions featuring various concentration of chloride ions from 0,01 – 2 M NaCl. Potentiodynamic tests enabled to register anodic polarisation curves. Im-mersion tests were performed that lasted 1 – 5 days. Hi-tachi scanning electron microscope with fi eld emission FE SEM S-4 200 enabled to make analysis of chemical composition in micro-areas. Surface topography of al-loy WE43 was measured by means of optical.MATERIAL AND METHODSAs stock material for the tests, samples of magnesi-um alloy WE43 after hot extrusion were used. Chemical composition of the alloy is presented in Table 1.Table 1  Chemical composition of magnesium alloy WE43 / % massY Nd RE Zr Mn Cu Ni Zn Mg4,0 2,0 3,4 0,4 0,15 0,03 0,005 0,2 restMeasurements were made in 0,01; 0,2; 0,6; 1 and 2 M NaCl solution. Solution temperature during the test amounted to 21 ± 1 °C.Resistance to electrochemical corrosion was evalu-ated on the ground of registered anodic polarisation curves. System VoltaLab®PGP 201 by Radiometer was used for potentiodynamic test. Saturated calomel elec-trode (NEK) of KP-113 type served as reference elec-trode, whereas platinum electrode of PtP−201 type was used as auxiliary electrode. The tests began with deter-mination of opening potential EOCP. Later, anodic po-larisation curves were registered, beginning with meas-urement of potential with the value E = EOCP − 100 mV. Potential changed in the anodic direction at the rate of 1 244  METALURGIJA 52 (2013) 2, 243-246J. PRZONDZIONO et al.: CORROSION RESISTANCE TESTS of MAGNESIUM ALLOY WE43 AFTER EXTRUSIONmV/s. When anodic current density reached 10 mA/cm2, polarisation direction was reversed and return curve was registered. Opening potential EOCP of tested samples stabilised after 30 min.On the ground of registered anodic polarisation curves, typical values describing resistance to electro-chemical corrosion were determined, i.e.: corrosion po-tential, corrosion current density and corrosion rate. Stern method was applied to determine polarisation re-sistance.Immersion tests were performed in ambient tempera-ture in 0,01 – 2 M NaCl solution during 1 – 5 days. After suitable preparation of samples surface, they were weighted and mass m0 was determined. After immersion of the alloy in NaCl solutions during 1 – 5 days, the sam-ples were taken out and corrosion products were removed in reagent containing 200 g/l CrO3 and 10 g/l AgNO3. Next, they were fl ushed with distilled water, degreased with acetone, dried and weighted again, which enabled to determine mass m1. Performed tests enabled to determine corrosion rate. Qualitative and quantitative analysis of chemical composition in micro-areas was made by means of electron scanning microscope with fi eld emission FE SEM S-4 200 Hitachi aided by spectrometer EDS Voy-ager 3 500 Noran Instruments. The study also presents measurement results of geometrical features of AZ31 al-loy surface after corrosion tests. The tests were performed by means of optical profi le measurement gauge Micro’prof. (CWL 3 000) made by FRT.RESULTSPotentiodynamic tests performed in NaCl solutions with various molar concentrations enabled to determine corrosion properties of magnesium alloy WE43 after hot extrusion. Corrosion resistance test results (mean values of measurements) have been compared in Table 2. An-odic polarisation curves are presented in Figure 1.It was discovered that with the increase of chloride ion concentration, corrosion characteristics of magne-sium alloy WE43 deteriorates. Decrease of polarisation resistance and increase of corrosion current density and corrosion rate can be seen. Polarisation resistance of the alloy tested in 0,01 M NaCl is Rp = 9,23 Ωcm2, whereas in 2 M NaCl – Rp = 1,66 Ωcm2. Corrosion current den-sity increases from Icoor = 0,0028 A/cm2 (0,01 M NaCl) to Icoor = 0,016 A/cm2 (2 M NaCl). Corrosion rate deter-mined in 0,01 M NaCl is Corr. = 0,64 mm/year and in 2 M NaCl – Corr. = 3,66 mm/year.Corrosion rate in the immersion test was determined on the ground of the equation (1): StmmV 10 −=  (1)where:  V – corrosion rate, mg/(cm2day); S – surface area (cm2), t – exposure time (day).Table 3 presents immersion test results.Immersion test results for the tested alloy confi rmed its greater predisposition for electrochemical corrosion with the increase of solution molar concentration.Tests made by means of scanning microscope proved that after 1 day of immersion test in 0,01 M NaCl solu-tion, WE43 alloy corrosion is of selective and non-uni-form character. Oblong and round pits can be seen on the surface of the alloy (Figure 2). Corrosion progresses along with time of immersion in NaCl solution, and the number of pits increases (Figure 3). The analysis proved that WE43 alloy shows the structure of a solid solution with precipitation of phases containing neodymium and yttrium (e.g. Mg41Nd5, Mg24Y5, Mg14Nd2Y). It was proved that phases are placed both inside grains and on their boundaries.With the increase of NaCl solution concentration, corrosion intensifi es. Pits substantially enlarge their size (Figure 4).Table 2  Results of electrochemical corrosion resistance tests of magnesium alloy WE43 (mean measurement values)NaCl concentra-tion /M Ecorr /mV Icorr /A/cm2 Rp /Ωcm2 Corr. /mm/year0,01 -1 565 0,0028 9,23 0,640,20 -1 751 0,0071 3,64 1,630,60 -1 780 0,011 2,20 2,511,00 -1 792 0,013 1,98 2,972,00 -1 687 0,016 1,66 3,66Figure 1 Anodic polarisation curves of WE43 alloy245METALURGIJA 52 (2013) 2, 243-246J. PRZONDZIONO et al.: CORROSION RESISTANCE TESTS of MAGNESIUM ALLOY WE43 AFTER EXTRUSIONGenerally speaking, increase of exposure time is re-sponsible for greater alloy surface subject to corrosion (Figure 5).Tests in 2 M NaCl solution proved that few non-corroded spots can be observed on the surface of the alloy. Corrosion becomes more uniform. Intermetallic phases rich in Nd and Y, cube-shaped, are more and more visible deep in the pits. (Figure 6).The tests enable to conclude that plastic working process is responsible for structure orientation, and thus - properties anisotropy. Depending on the direction of the orientation, corrosion of the respective grains pro-ceeds more intensively in some crystallographic planes (Figure 7).Tests performed with optical gauge proved that cor-rosion of WE43 alloy subject to exposure in 0,01 M NaCl is uniform. After 1-day test depth of pits does not exceed 6 µm (Figure 8), whereas after 5-day tests in 2 M NaCl solution, depth of pits can even exceed 15 µm (Figure 9).Figure 2  Pitting corrosion of WE43 (0,01 M NaCl, 1 day)Figure 3  Pitting corrosion of WE43 (0,01 M NaCl, 3 days)Figure 4 P its on the surface of the alloy (1 M NaCl, 1 day)Table 3 Immersion test resultsNaCl concen-tration MCorrosion rate / mg/(cm2day)1 day 3 day 5 day0,01 0,29 0,39 0,760,2 0,41 1,27 1,430,6 0,94 1,87 2,011 1,42 2,48 2,692 1,61 2,66 2,94Figure 5 Alloy surface after exposure in 1 M NaCl for 3 daysFigure 6  Deep pit with visible intermetallic phases (2 M NaCl, 3 days)Figure 7  Crystallographic grains oriented corrosion (2 M NaCl, 5 days)Figure 8  Roughness (a) and waviness (b) of WE43 alloy (0,01 M NaCl, 1 day)a)b)246  METALURGIJA 52 (2013) 2, 243-246J. PRZONDZIONO et al.: CORROSION RESISTANCE TESTS of MAGNESIUM ALLOY WE43 AFTER EXTRUSIONCONCLUSIONSApplication of magnesium alloy WE43 after plastic working is to a great extent dependent on resistance to electrochemical corrosion, especially in chloride solu-tions. Results of performed tests explicitly prove dete-rioration of alloy corrosion properties with the increase of molar concentration of NaCl solution. Potentiody-namic tests performer in solution with concentration of 0,01 – 2 M NaCl proved that increase of chloride ion concentration is responsible for decrease of corrosion potential and polarisation resistance, as well as increase of corrosion current density and alloy corrosion rate. Deterioration of corrosion properties with the increase of NaCl concentration was also confi rmed in immersion test as well as during surface topography test.Anodic dissolution of the surface of the alloy and a large number of pits were observed. With the increase of solution concentration, the pits feature increasingly bigger surface and depth. The surface that was corroded to the greatest extent was the surface of WE43 alloy after corrosion tests in 2 M NaCl solution. WE43 alloy corrosion is of inter-crystallic character.Following the tests in 2 M NaCl solution, areas with oriented microstructure, connected with plastic strain, can be seen. Due to anisotropy of features in the ex-truded alloy WE43, it can be expected that there is a crystallographic direction in which corrosion will pro-ceed with greater intensity.Potential for application of extruded magnesium al-loy WE43 in aeronautic and automotive industries is related to the necessity of protective layers application.a)b)Figure 9  Roughness (a) and waviness (b) of WE43 alloy (2 M NaCl, 5 days)AcknowledgementsFinancial support of Structural Funds in the Opera-tional Programme–Innovative Economy (IE OP) fi -nanced from the European Regional Development Fund - Project “Modern material technologies in aerospace industry”, No POIG.0101.02-00-015/08 is gratefully acknowledged.REFERENCES[1] A. Kiełbus, D. Kuc, T. Rzychoń: Magnesium alloys – mi-crostructure, properties and application. Monograph. Mo-dern metallic materials - presence and future. Department of Materials Engineering and Metallurgy, Katowice 2009.[2] K. Bryła, J. Dutkiewicz, P. 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Corrosion resistance tests of magnesium alloy WE43 after extrusion

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