P.A. Jeeva, S. Karthikeyan, S. Narayanan
Keywords: trivalent chromium coatings, corrosion resistance, taber abrasion, xps
Abstract:
An attempt has been made in this work to develop coatings with improved corrosion and abrasion resistance properties based on trivalent chromium which is eco-friendly in nature were tried besides their optical properties. Taber abrasion measurements confirmed the improvement of wear resistance of trivalent black coated surfaces. X-ray photoelectron spectra (XPS) was used to understand the surface morphologies of coatings being responsible for improvement in mechanical properties of automobile components. The enhancement of corrosion resistance is also validated through salt spray analysis.
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ISSN 1466-8858 Volume 16, Preprint 72 submitted 30 December 2013 Corrosion and abrasion resistance characteristics of trivalent black chromium electrodeposition P.A. Jeeva1, S. Karthikeyan2*, S. Narayanan1 1School of Mechanical &Building Sciences, VIT University, Vellore-632 014, India. 2*Surface Engineering Research lab, Centre for Nanobiotechnology, VIT University, Vellore-632 014, India. *Corresponding author (skarthikeyanphd@yahoo.co.in) Abstract An attempt has been made in this work to develop coatings with improved corrosion and abrasion resistance properties based on trivalent chromium which is eco-friendly in nature were tried besides their optical properties. Taber abrasion measurements confirmed the improvement of wear resistance of trivalent black coated surfaces. X-ray photoelectron spectra (XPS) was used to understand the surface morphologies of coatings being responsible for improvement in mechanical properties of automobile components. The enhancement of corrosion resistance is also validated through salt spray analysis. Keywords trivalent chromium coatings, corrosion resistance, taber abrasion, xps analysis, salt spray © 2013 University of Manchester and the authors. This is a preprint1of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 72 submitted 30 December 2013 Introduction The conventional black coatings involve hexavalent chromium which is highly toxic and banned by Environmental Protection Agency [EPA]. The alternate black coating is trivalent chromium which has to be meticulously formulated to improve hardness, abrasion resistance and corrosion resistance of the coatings. The development of corrosion resistant black coatings deserves much attention due to their high rate of deposition and inexpensive chemicals usage in industrial process. In general, the black coatings would exhibit better optical properties than mechanical properties. Hexavalent black chromium which is highly toxic was obtained by electrodeposition [Surviliene et.al. [1]] as well as by chromation conversion coatings [Nikolava et.al. [2]]. According to Aguilar et.al.[3], the external layers of black chromium coatings were mainly composed of Cr2O3. Anandhan[4] and his co-investigators found that beyond 4000C, the black chromium coatings were found unstable. A trivalent chromium formulation was proposed by Zeinab Abdel Hamid [5] using hexafluorosilicic acid and the resultant coatings has shown superior optical properties i.e. absorption coefficient of 0.97. The above author has used cobalt metal as additive to improve black colour and the optical properties. Bayati et.al.[6] are viewed that the addition of fluride with hexafluorosilicic acid were suitable materials for black chromium coating having absorption coefficient of 0.96. However, the uses of black chromium coatings on improvement of mechanical properties on metallic objects have not been exposed. This lead to an identification of appropriate black chromium coatings to be used for enhancing the wear resistance and corrosion resistances in order to extend the life time of machinery’s. © 2013 University of Manchester and the authors. This is a preprint2of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 72 submitted 30 December 2013 The performance of coatings is to be evaluated by taber abrasion resistant test, corrosion resistant measurement by electrochemical methods. The surface morphology of the coatings is the predominant properties of the coating which will be assessed by XPS. The absorption coefficient of the coatings is to be evaluated by UV-Visible spectrometer. Salt spray analysis is to be carried out to follow up the corrosion and get an idea about the performance of black chromium coatings. Experimental procedure For XPS and wear studies, mild steel specimens of 99.52% purity of size 2 x 5 x 0.2 cm3 and 100 x 100 x 4 mm3 were polished with fine grit paper and degreased with trichloro ethylene. They were rinsed in double distilled water. The composition of the mild steel used in the present study is given below: Carbon = 0.16%; Manganese = 0.3%; Silicon , Sulphur, Phosphorus = Nil; Aluminium = 0.02% and Iron = 99.52%. The optimized bath used in the present study had the following compositions. Cr metal (Trivalent) = 54.45 g/l= 270 g CrCl3 Co metal = 6.75 g/l= 20 g CoCl2 NaH2PO4 = 6 g/l NaF = 21 g/l pH = 4.6 Current density= 200-450 mA/cm2 Plating time = 7 min. © 2013 University of Manchester and the authors. This is a preprint3of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 72 submitted 30 December 2013 Evaluation of black trivalent Cr coatings through different techniques Taber abrasion resistance measurement The abrasion resistances of the black chromium coated specimens of size 100 x 100 x 4 mm3 were measured as per ASTM D-4060 through Taber abraser both in as plated and annealed conditions. The abrading wheels were allowed to rotate on the coatings at a load of 100 g. Before the start of the experiment, the specimens were accurately weighed. Then, the wheels were allowed to rotate against the deposit for 1000 cycles with the above load. After that the specimens were removed and weighed again. The experiment was repeated for another 1000 cycles on the specimens. The average weight loss was taken as the Taber wear index or Abrasion resistance. Taber wear index = Average weight loss (in mg) for 1000 cycles Optical properties of black Cr coatings Solar absorptance of the black Cr coatings were measured with UV visible spectra in the region of wave length from 200 to 400 nm and IR visible were obtained from 750 to 1000 nm regions and low absorptance in the visible region(>400-700 nm) were measured. The absorptance values of the black coatings layers were calculated using Kirchhoff’s law, which is given below. A+R+T = 1 where A is absorptance, T is transmittance, and R is reflectance of surface. In the opaque surfaces, their transmittance is zero, it can be expressed as: A+R = 1; so, A = 1 – R. © 2013 University of Manchester and the authors. This is a preprint4of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 72 submitted 30 December 2013 Corrosion resistance measurements Salt spray analysis for corrosion resistance of black coated samples As per ASTM B-117, the black Cr coated steel panels were tested in SF 850 salt spray cabinet in 3.5% NaCl (sea water medium) to evaluate the corrosion resistance of the coatings. Based on the appearance of formation of red rust spots on the coated samples used under annealed conditions, the corrosion degree of the samples was evaluated. Surface Characterization (XPS or ESCA analysis) The black Cr coated samples of size 10 x 10 mm2 under annealed conditions were used to carry out the surface characterization using a physical electronics PHI 5600 ESCA system (Precision = ±0.2 eV; Vacuum pressure = 10-9 Torr) with Al Kα monochromatic source was used to obtain oxidation states of species along with chemical composition of surfaces. Results and Discussions Taber abrasion resistance The results of abrasion resistance of the coatings measured by Taber abraser as per ASTM D-4060 are presented in Table 1. It has been found that the trivalent black chromium coatings improved the abrasion resistance both as plated as well as annealed conditions. This is due to the presence of intermetallic phases by Cr-Co contents. The increased values of abrasion resistance after annealing at 300oC may be due to the precipitation hardening of chromium particles. The results are in good agreement with those reported earlier for electroless deposition of Ni-P-Chromium oxide composite coatings. © 2013 University of Manchester and the authors. This is a preprint5of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 72 submitted 30 December 2013 Measurement of optical properties Absorption and emission values for the black Cr3+ coatings obtained in this study are presented in Figures 1 and 2. It can be visualized from the figure that the absorption values change from 0.92 to 0.81. These results are in good correlation with those reported earlier for black Cr coatings [7-9]. The better absorption and emittance for coatings obtained at 7th minute is due to highly regular shape, improved micro hardness and less dendrite structure of chromium layers on metal surface. Also, the incorporation of cobalt into the lattice plane of chromate film may enhance the optical properties of the coatings. In addition, the current density (0.4 A.cm-2) played significant role on the optical properties of the coatings [10]. It was observed from Figure 2 that there is no appreciable change in absorptance for black chromium coatings after annealing at 300oC. Hence, these coatings may be used for solar energy applications. © 2013 University of Manchester and the authors. This is a preprint6of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 72 submitted 30 December 2013 Corrosion resistance studies XPS analysis of Cr black coatings Figure 3 shows the XPS analysis of the annealed surface of electrodeposited trivalent chromium black coatings clearly showed the presence of the both chromium and cobalt peaks. The spectrum of the ejected photoelectron displays peaks at the kinetic energy from which binding energy could be inferred. Since the core binding energies are signatures of the elements present, the photoelectron spectra could be used to establish elemental identities. In the case of chromium, the 2p region showed a doublet 2p3/2-2p. The peaks for black chromium correspond to the Cr 2p3/2 core levels. The measured binding energy for Cr2p3/2 is very close to binding energies of Cr(OH)3 which as the binding energy of 577.3 eV corresponding to Cr(III). The feeble peak at binding energy value 62.8 eV also showed the presence of Cr3+ state. The core peak for cobalt appeared at Co2p3/2 with binding energy values 785 eV. The O1s XPS spectrum of the black coatings indicates one chemical states of oxygen at a binding energy of 531 eV. The bonding in the coatings was composed of F1s and P2p at binding energy values of 313 eV and 295 eV respectively indicating that chromium is attached with oxygen as Cr(OH)3 which occupied the top layer of black coatings. The trace appearances of flurine atom and phosphorus atomic peaks have come from the additives. i.e. sodium fluride and sodium dihydrogen phosphate. The binding energy value of O KLL at 1008 eV which is in close proximity to Co2p3/2 peaks at binding energy 785 eV confirming that cobalt is existing as Co3O4 in the © 2013 University of Manchester and the authors. This is a preprint7of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 72 submitted 30 December 2013 inner layer of the coatings evidence from their higher binding energy values compared with Cr(OH)3.[11-12] Analysis of salt spray test The results of salt spray test are shown in table 2. The progress of corrosion on black coated steel samples using trivalent chromium coatings as well as the uncoated steel is shown. For steel samples, it was noticed that 30% red rust formed on uncoated sample at 30 minutes stay in salt spray chamber. 2% rust area formation on trivalent black chrome after 960 hours stay in salt spray chamber. Therefore, it can be concluded that corrosion resistance of black trivalent chromium coatings in salt spray is 32 times higher than uncoated steel specimens. Conclusions 1. A suitable eco-friendly bath has been formulated based on trivalent chromium, cobalt and sodium dihydrogen phosphate. 2. Trivalent black coated surfaces enhanced the abrasion resistance both in the as plated as well as annealed condition. 3. The precipitation hardening of Cr particles resulted intermetallic phase formation by Cr-Co contents. 4. The existence of Cr3+ in the coatings was evidenced from binding energy values by XPS analysis. 5. The trivalent chrome coated surfaces could with stand up to 960 hours salt spray resistance which is 32 times higher than mild steel. © 2013 University of Manchester and the authors. This is a preprint8of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 72 submitted 30 December 2013 References [1]. S. Surviliene, L. Orlovskaja, S. Biallozor, Surf. Coat. Technol. 122 (1999) 235. [2]. M. Nikolova, O. Harizanov, P. Steftchev, I. Kristev, S. Rashkov, Surf. Coat. Technol.34(1988)501. [3]. M. Aguilar, E.Barrera, M. Palomar-Pardavé, L. Huerta, S. Muhl, J. Non Cristal. Sol. 329 (2003) 31. [4]. C. Anandan, V.K. William Grips, K.S. Rajam, V. Jayaram, Parthasarathi Bera, Appl. Surf. Sci. 191 (2002) 254. [5]. Z. Abdel Hamid, Surf. Coat. Technol. 203(2009) 3442. [6]. M. R. Bayati, M. H. Shariat, K. Janghorban, Renewable Enery. 30 (2005) 2163. [7] F.J. Monteiro, F. Oliviera, R. Reis, O. Paiva, Plat. Surf. Fin. (1992) 46 [8] M. Koltun, G. Gukhman, A. Gavrilina, Sol. Energy Mater. Sol. Cells 33 (1994) 41. [9] R.E. Peterson, J.W.Ramsey, J. Vaccum Sci. Technol. 12 (1975) 174. [10] J.H. Lin, R.E. Peterson, J. Vaccum Sci. Technol. 12 (1975)174. [11]. G.D. Wilcox, Journal of Corrosion Science and Engineering, 6, 2003. [12]. L.U. Ogbuji, Journal of Corrosion Science and Engineering, 6, 2003. © 2013 University of Manchester and the authors. This is a preprint9of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 72 submitted 30 December 2013 Table 1 Abrasion resistance for trivalent black chromium coatings S.No. Nature of the system Taber wear index (load 1000g) for 1000 cycles (in grams) As plated Annealed at 300oC 1. Trivalent chromium 0.024 0.018 coatings Table 2 Results of salt spray analysis as per ASTM B-117 Time Uncoated Appearance of black (hr) Steel Trivalent Cr coatings 0 White Black 0.5 30% red rust area Black 1 100% red rust area Black 36 100% red rust area Black 120 100% red rust area Black 240 100% red rust area Black 480 100% red rust area Black 960 100% red rust area 2% red rust 1100 100% red rust area 5% red rust 1200 100% red rust area 30% red rust area 10of a paper that has been submitted for publication in the Journal of © 2013 University of Manchester and the authors. This is a preprint Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 72 submitted 30 December 2013 Legends for figure 1. Absorption results for trivalent black chromium coatings (as plated) 2. Absorption results for trivalent black chromium coatings (annealed) 3. XPS analysis of trivalent black chromium coatings 14 Reflectance 12 10 8 6 4 2 0 0 200 400 600 800 1000 Wave length (nm) 1200 Figure 1 Reflectance 14 12 10 8 6 4 2 0 0 200 400 600 800 1000 Wave length (nm) 1200 Figure 2 11of a paper that has been submitted for publication in the Journal of © 2013 University of Manchester and the authors. This is a preprint Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 72 submitted 30 December 2013 Figure 3 12of a paper that has been submitted for publication in the Journal of © 2013 University of Manchester and the authors. This is a preprint Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work.