Abdel Salam Hamdy. I. Introduction

Transcription

1 International Review of Chemical Engineering (I.RE.CH.E.), Vol. 2, N. 2, March 2010 VIRTUAL FORUM The Role of Nanotechnology in Designing High Performance Nano-Ceramic Coatings Abdel Salam Hamdy Abstract This paper discusses our recent research achievements in designing high performance environmentally-friendly coatings technologies for improving the corrosion resistance of the aluminum and magnesium alloys used in automotive and aerospace industries. Aluminum alloys are used widely in such industries due to the relatively light weight and high strength. However, the presence of alloying elements increases the inhomogeneous structure and hence, enhances the localized corrosion. Magnesium alloys have many attractive mechanical properties such as stiffness to weight ratio, ease of castability and ease of machinability. These mechanical properties make them competitive materials to aluminum alloys in a lot of applications. Moreover, the weight saving property of magnesium alloys is one of the most attractive issues from environmental view point especially in automobiles industry due to fuel consumption saving and consequently carbon dioxide emission reduction. However, magnesium has a very high chemical reactivity with environment especially in presence of chloride ions and oxygen. This extensively limits the widespread of magnesium in many applications. Chromate has been reported as the most efficient widespread conversion coatings for the corrosion protection of many metallic substrates. However, chromium is ranked among the top toxic substances. In this paper, newly developed environmentally acceptable surface treatments based on silica, ceria, molybdate, vanadia or stannate prepared either by chemical conversion method or by sol-gel method were proposed as alternatives to toxic chromate-based systems. Copyright 2010 Praise Worthy Prize S.r.l. - All rights reserved. Keywords: Chemical Conversion Coatings, Sol Gel Coatings, Surface Modification, Corrosion, Stannate, Silica, Ceria, Molybdate, Electrochemical Impedance Spectroscopy I. Introduction Aluminum and its alloys are used widely in aerospace, automotive, architectural, lithographic and packaging applications. The demand for environmental protection and energy saving have been promoting researchers to develop light-weight materials for automotive and aerospace materials. Magnesium alloys are considered to be an excellent material for reducing vehicle weight, lowering fuel consumption and, thereby, reducing CO 2 emissions. Several automobile manufacturers (Ford, General Motors, Chrysler, Volkswagen, Opel, and Fiat) have co-operated to develop new magnesium alloys for manufacturing less energy-consuming cars and hence, less polluted environment. According to the International Magnesium Association's website, world production of magnesium exceeds 429,000 tonnes per annum and the figure is increasing annually as the lightweight properties of magnesium alloys are used increasingly in the automotive industry as a means of reducing weight, increasing fuel efficiency and reducing greenhouse gas emissions. Magnesium alloys have a variety of excellent properties, including a high strength-to-weight ratio, low density, dimensional stability and castability. Therefore, they found widespread applications in many industrial sectors. However, magnesium alloys remain very susceptible to corrosion despite their excellent mechanical properties. Chromate has been reported as the most efficient widespread conversion coatings for the corrosion protection of many metallic substrates. However, the waste containing hexavalent chromate has many limitations due to the environmental consideration and health hazards. Corrosion and prevention of light alloys such as aluminum and magnesium is an important part of our research program and worldwide as well. This article discusses the recent research achievements in collaboration with several international institutions and relevant research work in this area in the world. The aim of this article is to deepen the current understanding of corrosion and protection of aluminum and magnesium and their alloys and to provide a base for future research work in this field. It will also report the recent development in designing eco-friendly conversion coatings based on stannate conversion coatings for magnesium alloys as alternatives to toxic chromate. Copyright 2010 Praise Worthy Prize S.r.l. - All rights reserved 256

2 II. Experimental II.1. Materials Different specimens of AZ91D and AA6061, in the form of 60mm 30mm taken from sheet of 3 mm thick, were abraded to 800 finish with SiC grit papers, degreased in acetone, washed with distilled water, and dried in dry air. II.2. Coatings II.2.1. Sol Gel Technology The specimens were subjected to surface modification either by etching or oxide thickening before applying the sol gel coatings. After surface pretreatment, the specimens were dipped into the silica, ceria or molybdate sol for 10 minutes, dried at 110 C for 30 minutes and heat treated for 30 minutes. II.2.2. Chemical Conversion Coatings The magnesium specimens were treated in a diluted solution of stannate salt with different concentrations for about 15 min after some surface modification. II.3. Testing II.3.1. Electrochemical Impedance Spectroscopy (EIS) EIS technique was used to evaluate the electrochemical behavior of the coated samples in 3.5% NaCl solution open to air and at room temperature for up to 30 days. A three-electrode set-up was used with impedance spectra being recorded at the corrosion potential Ecorr. A saturated calomel electrode (SCE) was used as the reference electrode. It was coupled capacitively to a Platinum wire to reduce the phase shift at high frequencies. EIS was performed between 0.01 Hz and 65 khz frequency range using a frequency response analyzer (Autolab PGSTAT30, Eco-Chemie, The Netherlands). The amplitude of the sinusoidal voltage signal was 10mV. II.3.2. Cyclic Voltammetry Measurements Cyclic voltammetry measurements of the samples previously immersed for seven days in 3.5% NaCl solution were made at a scan rate of 0.07 mv/s using Autolab PGSTAT30, Eco-Chemie, The Netherlands. The potential was recorded starting from a cathodic potential (-100 mv) and be allowed to sweep to anodic potential direction till it reaches the pitting potential. At that potential, a sudden shift in the current to the active direction will be observed. At the pitting potential the sample will be enforced to sweep again in the cathodic direction. The exposed surface area was 4 cm 2. All curves were normalized to 1 cm 2. II.3.3. Surface Characterization SEM and EDS were used to examine the surface morphology of the coated samples before and after the immersion in 3.5% NaCl solution. SEM images of the samples that immersed in 3.5%NaCl for seven days, washed with deionized water and then dried were obtained using a digital scanning electron microscope Model JEOL JSM 5410, Oxford Instruments, Japan. Microprobe analysis was performed using energy dispersive spectrometry, EDS, Model 6587, Pentafet Link, Oxford microanalysis group, UK. Also, digital camera photos were taken to investigate the types of corrosion produced on the substrate surfaces after seven days of immersion in 3.5% NaCl solution. III. Results and Discussion III.1. Sol Gel Nanocoatings The most common problems associated with applying the sol gel technique for metals coatings are; the poor adhesion performance of the coatings formed and the absence of the coating systems that based on environmentally acceptable salts. Our recent research showed that the coating based on environmentally acceptable salts such as silica, ceria, vanadia and molybdate can be tailored via sol-gel route to produce a functionally gradient coating that can provide covalent bonding for strong coating adhesion and act as a barrier to limit the water/aggressive ions transport to attack the materials surface [14]-[27]. The strategy we used is based on modifying the metal substrate surface followed by simple immersion in the sol of corresponding metals to form gel. After, we heat such gel at elevated temperature to form inorganic oxides. Such process was successfully applied into aluminum and its alloys. To achieve the highest corrosion resistance, the surface of the aluminum samples were modified using etching, oxide thickening or both together prior to dipping in a corresponding salt solution based on silica, ceria, vanadia or molybdate prepared by sol gel method. The optimum conditions under which such treatments can provide good corrosion protection to the aluminum substrate in NaCl solution were determined using EIS and polarization measurements. Experimental data showed that applying the sol gel coating directly to the aluminum surface without any surface modification resulted in formation of mud-like cracked coating of zero adhesion. The coating formed in that case can easily be removed with the tip of fingers. The specimens that passed with a picking step in a dilute solution of potassium hydroxide followed by an oxide thickening step in boiling distilled water prior to 257

3 immersion in sol gel showed the best corrosion resistance. Among the sol gel salts we have studied, molybdate sols showed the highest corrosion resistance even after 30 days of immersion in NaCl solution. Electrochemical impedance spectroscopy measurements allow the estimation of coating degradation and corrosion kinetic. The surface resistance of molybdate is 32X104 Ω.cm2 which is equal to the double of the resistance obtained from silica (16X104 Ω.cm2) and almost six times of that obtained from the as-polished samples (5.2X104 Ω.cm2). Ceria and vanadia treatments showed acceptable surface resistances of 11X104 Ω.cm2 and 14X104 Ω.cm2 respectively but still less than the resistance obtained form molybdate (Fig. 1). Fig. 1. Electrochemical Impedance Spectroscopy of sol gel coatings after 30 days of immersion in NaCl solution The EIS Bode plots of different surface treatments are compared in Fig. 1. Generally, the impedance spectra of bare Al alloy reveals one time constant around 10 Hz attributed to the charge transfer resistance of corrosion process (9, 22, 26). The presence of the surface treatments promotes the increasing of the low frequency impedance by almost 2-10 orders of magnitude compared to the bare alloy. After 30 days of immersion in NaCl solution, the spectrum of silica treatment shows the presence of two time constants. The time constant with the maximum at around 4x102 Hz in the phase angle plot was assigned to the presence of silica film, and the another one at low frequency, 0.2 Hz, may be attributed to the first signals of a charge transfer controlled process at the metal oxide/ silica coating interface. On the other hand, vanadia, ceria and molybdate sol gel coatings showed only one time constants. For vanadia and molybdate, the time constant was around 3 Hz. For ceria, the time constant was around 1 Hz. The presence of time constant at medium (or low) frequencies for long immersion times attributed to relaxation of mass transport in the solid phase due to the growth of the corrosion product layer (21, 22, 26). SEM-EDS (Fig. 2) revealed formation of compact Mo-rich aluminum oxide film uniformly distributed over the surface of aluminum substrate [14], [18], [23], [24]. Therefore, no pitting or crevice corrosion was observed after immersion in NaCl solution for molybdate treated samples. Conversely, silica and ceria treatments showed micro-cracked surface films [14], [18]-[20]. Although vanadia treated samples showed compact film distributed uniformly over the surface as well, few tiny pits were observed at the end of the experiments [14], [18], [21], [25]. The surface topography and appearance of the film formed due to molybdate treatment was much more compact and has smoother appearance than the coatings of other salts. Accordingly, it seems that the superior corrosion protection of molybdate treatment is due to conversion of molybdenum oxide from less corrosion resistant species such as Mo3d3 to more corrosion resistant oxides such as Mo 3p1 and Mo 3p3 as confirmed by XPS studies in previous work to form molybdate rich aluminum oxide layer [14], [18], [23], [24]. Silica Ceria Molybdat e Vanadia Figs. 2. SEM of different sol gel coatings after 30 days in NaCl solution 258

4 Abdel Salam Hamdy with stannate without any surface modification (0.37X104 Ω.cm2) (33-41). Linear polarization, cyclic voltammetry, electrochemical impedance spectroscopy, SEM-EDS, and XRD analysis was performed to assess the performance of the stannate conversion coatings in NaCl solution. It was found that, stannate conversion coatings improve the pitting and crevice corrosion resistance due to the formation of tin oxide-rich magnesium hydroxide layer that act as a barrier to oxygen diffusion to the metal surface, and hence, shift the pitting potential to nobler one (Figs. 3 and 4). III.2. Chemical Conversion Coatings Advanced chrome-free chemical conversion coatings based on ceria, zirconia and stannate were designed to improve the corrosion performance of Mg alloys in chloride containing environments (28-41). In recent studies (33-41), the effect of stannate conversion coating on the corrosion behavior of AZ91D was investigated. Experimental data showed that the highest surface impedance and pitting corrosion resistance was obtained from the samples that subjected to alkaline etching followed by acidic etching prior to stannate coatings (0.4X104 Ω.cm2) and the samples that directly treated Figs. 3. SEM of the as-polished samples before and after corrosion in NaCl solution Figs. 4. SEM of the stannate coated samples before and after corrosion in NaCl solution (self-healing action to cover the pitting zones) Results showed that increasing the stannate concentration from g/l affects negatively the Copyright 2010 Praise Worthy Prize S.r.l. - All rights reserved 259 International Review of Chemical Engineering, Vol. 2, N. 2

5 Abdel Salam Hamdy corrosion resistances (33-41). The highest surface resistances obtained from the samples that coated with diluted stannate conversion coatings (25 g/l). The effect of surface modification prior to stannate coating was also studied. Results showed that modifying the magnesium surface prior to stannate coatings has a marked effect in improving the pitting and crevice corrosion resistance (38, 41). pitting corrosion (self-healing action) as shown in Figs. 6 [38]-[41]. (a) (a) (b) Figs. 6. Optical photos of the stannate coated samples before and after corrosion in NaCl solution (self-healing action to cover the pitting zones) IV. (b) Figs. 5. Optical photos of the as-polished samples before and after corrosion in NaCl solution Conclusion This article reviews our research activities for designing new eco-friendly surface treatments for aluminum and magnesium alloys in chloride containing environments. The effect of surface modification prior to applying sol gel coatings of salts like ceria, vanadia, silica and molybdate on aluminum alloys was found to have a marked role in the protection performance of aluminum alloys. The optimum conditions under which sol gel coatings can provide good corrosion resistance were determined. The surface modification of magnesium alloys prior to applying stannate conversion coatings was determined. A simple treatment step in a diluted stannate solution without any surface modification was found promising. The surface resistances obtained was five times higher than the surface resistance of as-polished samples. The optimum conditions for improving the corrosion protection of magnesium alloys, considering the economical aspects and industrial applicability, were determined. A simple treated process based on treating the magnesium surface directly with a diluted stannate solution was found promising to improve the localized corrosion resistance of magnesium after seven days of immersion in 3.5% NaCl solution. Results showed that the samples that directly treated with diluted stannate without any surface modification showed an improved surface resistance of about (5.4X103 Ω.cm2) which is five times higher than the surface resistance of as-polished samples (0.08X104 Ω.cm2). SEM-EDS showed that direct Mg surface treatment in stannate solution plays an important role in inhibiting the active surface sites, rejecting the chloride ions from the surface and forming uniformly distributed magnesium hydroxide layer enriched with tin oxide. Moreover, direct treatment of Mg in stannate solution has a strong ability to repair the surface defects and References [1] Copyright 2010 Praise Worthy Prize S.r.l. - All rights reserved 260 A.S. Hamdy, A.M. Beccaria and T. Temtchenko, Surface Coatings & Technology, 155, 2002, International Review of Chemical Engineering, Vol. 2, N. 2

6 [2] A.S. Hamdy, A.M. Beccaria and T. Temtchenko, Surface Coatings & Technology, 155, 2002, [3] A.S. Hamdy and A.M. Beccaria, Corrosion Prevention & Control, 48 (4) 2001, 143 [4] A.S. Hamdy, A Novel Coating System for the Corrosion Protection of Aluminum Alloys and Composites in Marine Environments Paper presented at the IIW International Congress on Welding and Applied Processes, CMRDI, Cairo, Egypt, Nov 29-Dec 1, [5] A.S. Hamdy, A.M. Beccaria and R. Spiniello, Corrosion Prevention & Control, 48 (3) 2001, 101. [6] A.S. Hamdy and A.M. Beccaria, Surface and Interface Analysis, 34, 2002, [7] A.S. Hamdy, Surface Coatings & Technology, 200 (12-13), 2006, [8] A.S. Hamdy, "The Effect of Surface Preparation Prior to Silica- Ceria Ceramic Coatings on the Corrosion Protection of AA6061 T6" Paper presented at the European Corrosion Congress EUROCORR 2005, Portugal. [9] A.S. Hamdy A.M. Beccaria and P. Traverso, J. Applied Electrochemistry, 35, 2005, [10] A.S. Hamdy and A.M. Beccaria, J. Applied Electrochemistry, 35, 2005, [11] A. S. Hamdy "Novel Eco-friendly Surface Coatings for Automotive and Aerospace Materials". (Invited presentation at the Arabian-European Symposium on Environmental Protection, Tunisia, September, 2005). [12] A.S. Hamdy "Novel Coating Systems for Corrosion Protection of Aluminum Alloys and Composites in Marine Environment", presented at Symposium Materials Processing Challenges for the Aerospace Industry, Materials Science & Technology Conference and Exhibition (MS&T 06), October 15-19, 2006, Cinergy Center, Cincinnati, OH, USA, PP [13] A.S. Hamdy, Fluoropolymer coatings for highly aggressive environments (PVDF Coatings), Invited Chapter in High performance organic coatings: selection, application and evaluation, Woodhead Publishing Limited, Abington Hall, Abington Cambridge, CB21 6AH, UK, in press. [14] A.S. Hamdy, A.K. Ismail, D.P. Butt and A.A. Ismail, "Comparative Studies on Anticorrosion Nano-Particle Silica-, Ceria- and Molybdate Based Thin Films Adsorbed on Aluminum Alloys". (Paper presented at the International Symposium on the Role of Adsorbed Films and Particulate Systems in Nano and Biotechnologies, University of Florida, Gainesville, Florida, USA August 24-26, 2005). [15] A.S. Hamdy, Surface Coatings & Technology, 200 (12-13), 2006, [16] A.S. Hamdy, A.A. Ismail, A.K. Ismail, and D.P. Butt, "Novel Silica-Based Ceramic Coatings Prepared by Sol-Gel Method for Aluminum Alloys". (Paper presented at the European Corrosion Congress EUROCORR 2005, Portugal). [17] A.S. Hamdy, J. Surface & Coatings Technology, Vol. 201/1-2 pp , [18] A.S. Hamdy, J. Materials Letters, Volume 60, Issues 21-22, September 2006, Pages [19] A.S. Hamdy, A.A. Ismail, A.K. Ismail, and D.P. Butt, "Novel Anti-corrosion Nano-Sized Vanadia-Based Thin Films Prepared by Sol-Gel Method for Aluminum Alloys, (Paper presented at 4th Japanese-Mediterranean Workshop on Applied Electromagnetic Engineering for Magnetic, Superconducting and Nano Materials, (JAPMED 4), Sheraton Cairo Hotel, Egypt, September 17-20, 2005). [20] A.S. Hamdy, A.K. Ismail, D.P. Butt and A.A. Ismail, "Sol-gel Prepared Anti-Corrosion Vanadia-Based Ceramic Coatings for 6061-T6 Aluminum Alloy ". (Paper presented at the International Symposium on the Role of Adsorbed Films and Particulate Systems in Nano and Biotechnologies, University of Florida, Gainesville, Florida, USA August 24-26, 2005). [21] A.S. Hamdy and D.P. Butt, J. Anti-Corrosion Methods and Materials, Vol. 53, Issue 4, P , [22] A.S. Hamdy and D.P. Butt, J. Materials Processing Technology, Vol. 181, No. 1-3, PP.76-80, [23] A.S. Hamdy, A.K. Ismail and D.P. Butt "Nano-Particle Molybdate Based Anti-corrosion Thin Films for Aluminum Alloys". (Paper presented at US-Egypt Workshop on "Synthesis, Characterization and Industrial Application of Nano-particles and Nano-structured Materials", Alexandria, Egypt, November [24] A.S. Hamdy, Progress in Organic Coatings, Volume 56, Issues 2-3, PP , [25] A.S. Hamdy, and D.P. Butt, "Novel Anti-Corrosion Nano-Sized Ceramic-Based Thin Films Prepared by Sol-Gel Method for Aluminum Alloys"", (Paper presented at Symposium Nanostructured Materials: Synthesis, Characterization and Applications, Materials Science & Technology Conference and Exhibition (MS&T 06), October 15-19, Cinergy Center, Cincinnati, OH, USA. [26] A.S. Hamdy, D.P. Butt and A. A. Ismail, J. Electrochimica Acta, 52 (2007) [27] Hamdy A. S., "A novel approach in designing high performance eco-friendly coatings for aerospace and automotive industries.", Invited distinguished lecturer at the Summer Program on "Advanced Materials and Structures -AMS'08", School of Engineering, Universitatea 'Politehnica" din Timisoara, Romania, May 28-30, [28] Hamdy A.S., and Farahat M., "Novel approaches in designing protective coatings for magnesium alloys", Electrochemical Society Symposium on Coatings and Corrosion Protection, October 4-9, 2009, Vienna. [29] Hamdy A.S., "Enhancing corrosion resistance of magnesium alloy AZ91D in 3.5% NaCl solution by cerate conversion coatings, J. Anti-Corrosion Methods and Materials, Vol. 53, No. 6 (2006) [30] Hamdy A.S., "Green cerate based surface treatment for improving the corrosion resistance of magnesium alloy AZ91D in marine environments", Symposium Green Engineering for Materials Processing, Materials Science & Tech. Conference and Exhibition (MS&T 06), Oct , 2006, Cinergy Center, Cincinnati, USA, PP [31] Hamdy A. S., J. Electrochemical and Solid-State Letters, Vol.10, No.3, C21-C25, [32] Hamdy A. S. and Butt D., "Eco-friendly conversion coatings for automotive and aerospace materials. Invited talk at the European Coatings Conference "New Concepts for Anti-Corrosive Coatings", June 2007, Berlin, Germany. [33] Marx B., Hamdy A.S., Butt D. P. and Thomsen D., Assessing the performance of stannate conversion coatings on Mg alloys, Symposium "Corrosion and Coatings Challenges in Industry", 88th Annual Meeting, Boise, ID, June 19, 2007, co-located with the 62 nd Annual Meeting of the American Chemical Society. [34] Thomsen D., Hamdy A.S., Marx B. and Butt D. P., Corrosion behavior of some newly developed Mg alloys in NaCl solution, Symposium "Corrosion and Coatings Challenges in Industry", 88 th Annual Meeting, Boise, ID, June 19, 2007, co-located with the 62 nd Annual Meeting of the American Chemical Society. [35] Hamdy A.S., Marx B., Butt D. P. and Thomsen D., Novel ecofriendly stannate-based conversion coatings for Mg alloys, Surface Treatments and Processing Session, Symposium: Automotive and Ground Vehicles: Materials and Processes for Vehicles, MS&T '07 Conference and Exhibition Sept , 2007, Cobo Hall, Michigan. [36] Marx B., Hamdy A.S., Butt D. P. and Thomsen D., Corrosion of newly developed magnesium alloys in chloride containing solutions, Automotive Symposium MS&T '07 Sept , 2007, Cobo Hall, Michigan. [37] Hamdy A.S., Marx B. and Butt D. P., A new approach in designing eco-friendly low cost chemical conversion coatings for magnesium alloys, Keynote Speaker at 6 th International Symposium on Surface Protective Coatings, Goa, India, Feb , [38] Hamdy A. S., J. Surface Coatings and Technology, 203 (2008), pp [39] Hamdy A.S., and Farahat M., "Zirconia based ceramic coatings for magnesium alloys", The European Corrosion Congress, paper 261

7 #7816, Session Coatings WP 14, EUROCORR 2009, 6-10 September 2009, Nice, France. [40] Hamdy A.S., Over 10-years of muti-international cooperation in designing high performance environmentally friendly coatings technologies for automotive and aerospace materials, Invited keynote speaker at conference Cooperation with Germany experience, new forms and perspectives, in memoriam of 150 years of Alexander von Humboldt, September 2009, Kishinev, Moldova, sponsored by the Alexander von Humboldt Foundation, Germany. [41] A. S. Hamdy (Editor): "High Performance Coatings for Automotive and Aerospace Industries, Nova Science Publishers, New York, USA, available online at cts_id=11740 Authors information Max Planck Institute of Colloids and Interfaces, Am Mühlenberg 1, Potsdam, Germany, Phone: (+49) , Fax: (+49)