Surface and Coatings Technology 103 104 (1998) 29 35 Enhanced corrosion protection of magnesium oxide coatings on magnesium deposited by ion beam-assisted evaporation F. Stippich a,*, E. Vera a, G.K. Wolf a, G. Berg b, Chr. Friedrich b a Physikalisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 500, 69120 Heidelberg, Germany b Institut für Werkstoffkunde, TH Darmstadt, Grafenstraße 2, 64283 Darmstadt, Germany Abstract In contrast to other light materials such as aluminium or titanium, the potential of magnesium to form stable, corrosionresistant films is limited to alkaline solutions. In neutral, acidic or saline solutions the protective hydroxide or carbonate film breaks through immediately and heavy corrosion takes place. The aim of this work was to deposit protective magnesium oxide layers, by ion beam-assisted deposition ( IBAD), which do not grow under natural circumstances. These coatings are different from coatings obtained by anodic oxidation, because of their partial crystallinity, good adhesion and very smooth, non-porous character. The MgO coatings were prepared with IBAD on Mg (hp), AZ91 magnesium alloy and AlMgSi0.5 substrates by the evaporation of high-purity MgO under bombardment with Ar+ ions with energies up to 15 kev. The Ar+ to MgO arrival ratio was chosen in order to obtain dense coatings. The coating thickness was 1 mm. The electrochemical and corrosion behaviour were determined by potentiodynamic controlled current potential measurements under pitting corrosion conditions and a standard salt-spray test. The crystallinity was determined by X-ray diffraction measurements and the surface quality by atomic force microscopy. The results prove that by IBAD, magnesium oxide coatings with or without the addition of other elements (Sn, Nd) of high hardness and low porosity and with good corrosion protection properties can be formed. The degree of crystallinity and the texture have a strong influence on the quality of the coatings and depend on the ion energy. 1998 Elsevier Science S.A. Keywords: Corrosion; IBAD; Magnesium; Magnesium oxide 1. Introduction deposited by ion beam-assisted deposition ( IBAD) [2 4] on magnesium, AlMgSi0.5 and AZ91. The influence Magnesium and its alloys have become more and of the Ar+ ion energy and angle of ion incidence during more important for automobile and aircraft components deposition was examined up to 15 kev with regard to [1]. Because of magnesium s insufficient corrosion resiscoatings. the crystallinity and corrosion protection power of the tance in neutral and acidic environments and its high Optimizations were made by codeposition of abrasion rate, alloys such as AlMgSi0.5 or AZ91 (9% 10% neodymium or tin with MgO. The idea was to Al, 1% Zn) are used for most technical applications modify the oxide layer and the cathodic reaction during instead of magnesium. Even these have disadvantages aqueous corrosion, respectively. in comparison with other light materials such as titanium The surface quality was determined by X-ray diffrac- or aluminium, but magnesium and its alloys have the tion ( XRD), atomic force microscopy (AFM) and advantages of low price, high strength-to-weight ratio microhardness measurements. The electrochemical and and good recycling properties. Ceramic coatings are corrosion behaviour was measured by potentiodynami- good candidates to protect magnesium and its alloys. cally controlled current potential measurements under They are inert in many environments and their isolating conditions of pitting corrosion. Application-orientated character suppresses galvanic corrosion that might arise examinations were made using a standard salt-spray test during technical applications. Furthermore, they may under conditions comparable to DIN 50 021. also reduce abrasion. Therefore, 1-mm-thick ceramic MgO coatings were 2. Experimental * Corresponding author. Tel: +49 6223 56 8371; e-mail: hr2@ix.urz.uni-heidelberg.de Magnesium, AZ91 (9% Al, 1% Zn), AlMgSi0.5 and 111 Si wafers were used as substrates. All samples 0257-8972/98/$19.00 1998 Elsevier Science S.A. All rights reserved. PII S0257-8972(98)00365-X
30 F. Stippich et al. / Surface and Coatings Technology 103 104 (1998) 29 35 3. Results and discussion 3.1. Morphology and structure The morphology and structure of deposited MgO layers strongly depend on the ion energy. Under pure evaporation conditions without any assisting ion beam, the ceramic deposited on Si wafers showed a strong 111 texture (Fig. 1). The 111 orientation may have been caused by the 111 Si wafer orientation. Low 200 and 220 XRD patterns indicate regions of misorientation. The 111 orientation for evaporated- only coatings changed to a 220 orientation at 3 kev. Under consideration of the relative peak sensitivities, the signal intensity at 3 kev decreased to one-tenth of were metallographically polished up to 1-mm diamond paste followed by a 0.1-mm alumina suspension. Afterwards, they were cleaned in a supersonic bath with isopropanol. Prior to coating, the samples were sputteretched with a 10 kev argon ion beam with a fluence of 1016 cm 2 in the vacuum chamber. The coatings were prepared by evaporation of MgO and simultaneous argon ion bombardment with energies up to 15 kev. The ion angle of incidence was kept at 12 or 45, the argon to MgO arrival ratio (I/A) was #0.2 and the deposition rate was #0.3 nm s 1. Samples containing nominally 10% Sn or Nd were made by coevaporation of MgO and the corresponding metal using two independent evaporators. The evaporation materials were of 99.99% purity and the sample temperature was below 100 C during the coating process. The electrochemical current-density potential measurements were performed in 0.05 N NaCl solution at ph 7. All potentials are reported with respect to a saturated calomel electrode (SCE). All samples were measured in a single ramp with a potential sweep rate of 0.05 mv s 1 [ 5]. The corrosion behaviour was classified by the corrosion current i under open circuit corr potential E and the potential difference between the oc open circuit potential and the potential at which the protective coatings break through (E E ). The higher d oc this difference, the better the protective behaviour. Fig. 1. XRD pattern for MgO coatings deposited with IBAD with Ar+ ions up to 15 kev on silicon wafer. Fig. 2. AFM surface graphs as a function of the IBAD ion energy.
F. Stippich et al. / Surface and Coatings Technology 103 104 (1998) 29 35 31 Fig. 3. XRD pattern for MgO coatings deposited with IBAD at 7.5 kev and an angle of ion incidence of 45 on silicon wafer. Fig. 4. Current-density potential measurements in 0.05 N NaCl solution and a scan rate of 0.05 mv s 1 for MgO-coated Mg substrates for different ion energies. that with the evaporated-only coating. This indicates a high content of X-ray amorphous MgO. Above 5 kev the structure changed to a strong 200 texture and the X-ray amorphous content decreased. The signal intensity at 5.5 kev was one-third of that with the evaporatedonly MgO and at 10 kev the signal intensity was comparable to the 0 kev sample. At 15 kev a small 111 pattern could be found again and the intensity of the XRD pattern was one-fifth of that with the evaporated- only sample. AFM pictures showed that 200 orientated crystals (5 10 kev ) had columnar structures. Furthermore, increasing ion energies seemed to cause larger crystallites, as seen by AFM ( Fig. 2). Fig. 3 shows the XRD pattern for samples prepared under an angle of ion incidence of 45. Pure MgO coatings again showed a strong 200 texture. The addition of 10% Nd lowered the 200 signal and no other pattern appeared. With 10% Sn very broad 220
32 F. Stippich et al. / Surface and Coatings Technology 103 104 (1998) 29 35 Table 1 Corrosion currents and the width of the passive regions (E d E oc ) for different MgO coatings on Mg (hp), AZ91 and AlMgSi0.5 substrates Substrate Coating i corr E d E oc i ( 550 mv) (ma cm 2) (mv ) (macm 2) Mg 40 0 MgO 0 kev 0.1 200 3 kev 15 170 5.5 kev 0.3 90 10 kev 40 0 15 kev 2 85 30 10 Nd,MgO 7.5 kev, 45 0.03 70 Sn,MgO 2.5 50 AZ91 30 0 MgO 0.01 50 Nd,MgO 7.5 kev, 45 <4 0 Sn,MgO <2 15 AlMgSi0.5 0.05 30 500 MgO 0.006 875 2 Nd,MgO 7.5 kev, 45 0.02 950 0.4 Sn,MgO 0.2 780 4000 Fig. 5. Current-density potential measurements in 0.05 N NaCl solution and a scan rate of 0.05 mv s 1 for MgO-coated Mg substrates deposited with an angle of ion incidence of 45 and different alien elements at 7.5 kev. and 200 patterns appeared with low intensities. 3.2. Corrosion The crystalline content decreased in the order MgO>Nd,MgO>Sn,MgO. The electrochemically determined corrosion protection The hardness values were independent of the structure behaviour of ceramic IBAD MgO coatings depends and morphology. Considering the experimental error, strongly on the substrate. With regard to their corrosion no differences between the ion beam-assisted coatings resistance the pure substrates can be grouped as follows: could be found, and they had a universal hardness of Mg<AZ91<AlMgSi0.5. None of the MgO coatings in 7200 MPa. The evaporated 111 MgO had a lower this work protected all three substrates equally well. hardness value of 4600 MPa. The best protection for Mg (hp) was given by evapo-
F. Stippich et al. / Surface and Coatings Technology 103 104 (1998) 29 35 33 Fig. 6. Current-density potential measurements in 0.05 N NaCl solution and a scan rate of 0.05 mv s 1 for MgO-coated AZ91 substrates deposited with an angle of ion incidence of 45 and different alien elements at 7.5 kev. Fig. 7. Current-density potential measurements in 0.05 N NaCl solution and a scan rate of 0.05 mv s 1 for MgO-coated AlMgSi0.5 substrates deposited with an angle of ion incidence of 45 and different alien elements at 7.5 kev. rated-only MgO coatings which showed a corrosion deposited with an angle of ion incidence of 45 showed current of #0.1 ma cm 2 and a passive area of about no improvement compared with 12 ( Fig. 5). However, 200 mv ( Fig. 4). Samples with a high amorphous alien elements such as Nd and Sn reduced the cathodic content which were deposited with energies between 3 current density and the corrosion current. The coating and 5.5 kev also had good corrosion properties with 10% Nd showed a very low corrosion current of (Table 1). The passive region was of 170 mv width for #0.03 macm 2. the 3 kev sample, and the 5.5 kev sample showed a low Coatings with strong 200 texture, which grew best corrosion current of #0.3 macm 2. MgO coatings at 10 kev, gave only poor results on magnesium (hp)
34 F. Stippich et al. / Surface and Coatings Technology 103 104 (1998) 29 35 only MgO coating and the Nd,MgO coating were less effective than expected. On magnesium (hp) the best result was obtained with pure MgO coatings deposited under the assistance of 3 and 5.5 kev under Argon ion bombardment. Both samples showed only small corroded areas of approxi- mately 0.01 or 0.1% of the surface area. On the Sn-added sample 50% of the surface area was lightly corroded. Heavy corrosion where more than 90% of the surface was destroyed was observed on all other samples. The 15 kev sample is not shown but the protection behaviour was analogous to the 10 kev sample. On AZ91 all three coatings were suitable. No corrosion products could be seen by eye. The dark areas on the coated samples are painted areas and not corrosion products. Microscopic examination was necessary to obtain differences between the coatings. The protection behaviour can be grouped as Sn,MgO>MgO>Nd,MgO. The results suggest that the adhesion and stress are much more important factors in the case of salt-spray testing than in the electrochemi- cal short-term evaluations. substrate. The reason for this behaviour can be found in the cathodic part of the current-density potential curves (Fig. 4). The current density in this region is proportional to the porosity of the coatings. Because of the extremely high corrosion rate of pure magnesium, dense coatings are the most powerful protection layers and the adhesion and the stress play a less important role. Samples with different orientations or a highly amorphous content have fewer pores than the columnar growing 200 texture. On AZ91 the quality of the coating can be correlated not only to the porosity (Fig. 6). Coatings that con- tained alien elements such as Sn or Nd reduced the cathodic current density but showed no passive region in the anodic part, while the 100 MgO layer was a good candidate for protective layers. The corrosion current was 1/3000th of the uncoated substrate and the passive region had a width of #50 mv. On AlMgSi0.5 all coatings were suitable ( Fig. 7). As on AZ91, the MgO coating showed the lowest corrosion current ( Table 1). The largest passive area with 950 mv was obtained for the Nd-added sample. The samples containing Sn had the smallest passive region and the highest corrosion current. The current densities at 550 mv, where the substrate had already undergone SCE anodic corrosion, showed the same tendency ( Table 1). Preliminary results of the salt-spray tests for Mg (hp) and AZ91 substrates after a timespan of 7 h are given in Fig. 8. In contrast to the electrochemical measure- ments this application-orientated test showed better results than the electrochemical examinations for Sn-added coatings on both substrates. The evaporated- Fig. 8. Comparison of selected samples after 7 h in salt-spray tests. The dark areas of the coated AZ91 substrates are painted areas and not corrosion products. 4. Conclusions In general, good corrosion properties are expected by amorphous coatings because of their low porosity [6, 7]. The present study shows that even crystalline magnesium oxide coatings are good candidates for corrosion-resistant coatings on magnesium and magnesium alloys if columnar structures are avoided. The high crystallinity and the strong texture of these coatings make a very useful substrate-coating system which can be controlled by the ion beam parameters and additional deposition of other metals, probably leading to mixed oxides. Regarding the high corrosion rate of pure magnesium, protective coatings must primarily have low porosity. Amorphous coatings serve this demand and show good protection properties on Mg (hp). The addition of Nd and Sn enhances the amorphous fraction and influences the electrochemical reactions. The role of the rare earth elements in the corrosion mechanism of magnesium is still not clear but they are already known to reduce the corrosion rate of magnesium [8]. Tin is known to act as a promotor. This lowers the cathodic reaction and the corrosion rate decreases under open circuit potential. On AlMgSi0.5 MgO and Nd,MgO coatings have a very low corrosion current. The width of the passive region is also very important [9]. These coatings only failed at higher anodic polarizations than the substrate. At anodic polarizations where the substrate showed heavy corrosion they were still able to give protection. On AZ91 substrates the pure MgO coatings and Sn,MgO had very good corrosion protection properties. Compared with other coatings, the IBAD MgO, Nd,MgO and Sn,MgO layers showed good properties on corroding substrates. Long-term salt-spray tests on
F. Stippich et al. / Surface and Coatings Technology 103 104 (1998) 29 35 35 larger substrates and thicker coatings are necessary to [2] G.K. Wolf, K. Zucholl, M. Barth, W. Ensinger, Nucl. Instrum. clarify further the quality of the coatings for practical Meth. Phys. Res. B 21 (1987) 570. [3] R. Emmerich, B. Enders, W. Ensinger, in: T.S. Sudarshan, J.F. applications. Braza (Eds.), Surface Modification Technologies VI, The Minerals, The electrochemical results suggest the following Metals and Materials Society, Warrendale, PA, 1993, p. 811. ranking of corrosion protection: MgO #Sn, [4] H. Wituschek, G.K. Wolf, Surf. Coat. Technol. 60 (1993) 556. (3 6 kev) MgO<Nd, MgO. A preliminary salt-spray test [5] F. Stippich, Diploma thesis, Heidelberg (1994). gave slightly different results, i.e. Nd, MgO<Sn, [6] T.P. Hoar, J. Electrochem. Soc. 17 22C (1970) 117. [7] A.G. Revesz, J. Kruger, in: R.P. Frankenthal, J. Kruger (Eds.), MgO<MgO. (3 6 kev) Passivity of Metals, Electrochemical Society, Princeton, NJ, 1978, pp. 137 155. [8] O. Lunder, M. Videm, K. Nisancioglu, Corrosion Resistant Magnesium Alloys, SAE Technical Paper 1995, International Congress References and Exposition, February 1995. [9] B. Enders, S. Krauß, G.K. Wolf, Corrosion properties of aluminum [1] F. Hehmann, Produktvielfalt und Verbrauchsinnovationen: Schlüsbased alloys deposited by ion beam assisted deposition, Surf. Coat. sel für eine nachhaltige Expansion auf dem Magnesium-Markt, Technol. 65 (1994) 203. Metallwirtsch. Metallmarkt 3 (1993) 288.