K. Mohanam, G. Venkatachalam, S. Karthikeyan, P.A. Jeeva
Keywords: Electroless cobalt plating, tensile, corrosion, SEM
Abstract:
The electroless cobalt coatings on mild steel have been studied by weight gain method, corrosion studies by potentiodynamic polarization measurements and hardness by Vickers microhardness tester. The influence of surface roughness has greatly contributed for corrosion and tensile strength of the coatings. SEM and XRD measurements were performed to study the surface morphology of cobalt coatings on mild steel.
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ISSN 1466-8858 Volume 17, Preprint 19 submitted 31 March 2014 The influence of Cobalt coating on Corrosion and functional properties of Mild Steel in 3.5% NaCl K. Mohanam1, G. Venkatachalam1, S. Karthikeyan2, P.A. Jeeva1 1School of Mechanical and 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 The electroless cobalt coatings on mild steel have been studied by weight gain method, corrosion studies by potentiodynamic polarization measurements and hardness by Vickers microhardness tester. The influence of surface roughness has greatly contributed for corrosion and tensile strength of the coatings. SEM and XRD measurements were performed to study the surface morphology of cobalt coatings on mild steel. Keywords: Electroless cobalt plating, tensile, corrosion, SEM Introduction The electroless cobalt coatings are widely used plating process in industries because of their improved corrosion resistance and magnetic properties[1-5]. In particular, defence applications have drawn much attention on cobalt coatings to assess the signals from military armaments owing to its magnetic properties[6-10]. Besides the above, corrosion resistance behaviour is an important parameter for defence materials. Electroless nickel coatings have been generally used as defence coatings. However, the presence of phosphorus in EN induces non-magnetic character and hence EMI shielding is more for EN coatings[11-19]. In recent years, the combined actions of magnetic as well as corrosion resistance behaviour of cobalt have attracted the researchers. The research in this direction is still very scarce and hence the present study. 1 © 2014 University of Manchester and the authors. This is a preprint of 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 17, Preprint 19 submitted 31 March 2014 Experimental Procedure Weight gain method The Mild steel specimen was prepared for the following dimensions: 30 mm x10.60 mm x1.60 mm. They were mechanically polished with fine grit paper, washed with distill water. The bath for electroless plating was prepared as per the chemical composition given in table1. Prior to plating specimen, specimen was immersed in Hydro chloric acid to remove impurities like carbon. Then the process is carried out to protect the surface to be with cobalt coated. Figure 1 shows the schematic diagram of experimental setup. The initial mass of mild steel was weighed using electronic weighing machine. Then the mild steel were subjected to electroless cobalt plating(1 μm). The rate of deposition was calculated using the following formula: Where, W – Weight of the deposit (g) D – density of the deposit (g/cm3) T – plating duration (h) A – Surface area of the specimen (cm2) Micro hardness measurements Micro hardness measurements for mild steel on cobalt (20 x 50 x 2 mm3) were measured using Vicker‟s harness tester as per ASTM with a load of 100 g. A diamond shaped indentation was created on each coated mild steel at eight different locations and the mean of hardness was calculated from the diagonal of indentation on Vicker‟s scale using the formula. V.H.N = (1854 x load) / d2 where d = diagonal of the indenter Corrosion resistance measurements The potentiodynamic polarization and impedance studies were performed on mild steel on cobalt coated area of 10 mm2 exposed surface (test electrode) in 3.5% NaCl, 40 mm2 of 2 © 2014 University of Manchester and the authors. This is a preprint of 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 17, Preprint 19 submitted 31 March 2014 platinum electrode (counter electrode) and saturated calomel as reference electrode in three electrode cell assembly[19]. Potentiodynamic polarization method A constant quantity of 250 ml of 3.5% NaCl (sea water) solution was taken in a 250 ml beaker. The test electrode, reference electrode and the counter electrode were positioned in the electro chemical workstation (Sinsil Model 604E, USA) and the readings were recorded by shifting the potential ± 300 mV from OCP at a scan rate 10 mV.second-1 for the mild steel. The corrosion rate factors like Ecorr and Icorr, were recorded. The reduction in Ecorr and Icorr values indicated that the coatings are having good corrosion resistance than those reported earlier. Tensile strength measurements This measurement was carried out using tensometer as per ASTM for the tensile test. Scanning electron microscopic studies (SEM) The micro structural images of cobalt coated mild steel were studied using SEM analyzer. The coated test electrodes were prepared for size 10 x 10 mm2 and placed firmly on crucible to be examined for SEM images. The SEM images were portrayed by using GEMINI SUPRA 55 model with FESEM and with the magnification range of 25kX. Results and Discussions Stress-strain behaviour studies The Figure 2 shows result of Co coating affects the stress strain behaviour of Mild steel. Till the elastic limit, there is no major difference in the value of yield stress. But a significant difference is visible in the stress strain curve after yield stress. This is due to the high ductility of Co metals. The differences in the stress stain behaviours are insignificant from elastic limit to plastic instability. There is minor change in the values of ultimate stress and failure stress if the mild steel is coated with cobalt. Uncoated mild steel exhibits low percentage of elongation then the coated one. Micro hardness measurements The hardness of the electroless deposited cobalt coatings determined by Vicker‟s hardness tester is presented in Table 2. The hardness values increased to 2 or 3 times by Co coating 3 © 2014 University of Manchester and the authors. This is a preprint of 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 17, Preprint 19 submitted 31 March 2014 due to the precipitation of inter metallic phases (Co3P2) appeared in XRD data. The increased hardness is claimed for precipitation hardening mechanism through the formation of Co3P2. Corrosion resistance studies Potentiodynamic polarization studies The corrosion resistance experiments were performed for hard coatings on Mild steel has been done by employing current-voltage measurements using polarization studies. The coated surface was exposed to sea water medium for 5 hours and OCP was noted. The shift of Potential to more positive direction by coating indicated the corrosion resistance of the surface. Also Icorr has reduced by coatings shown in figure 3. The results are given in table 3. Scanning Electron Microscopic studies The formation of layered structures indicates the Co3P2 phase on mild steel. The aggregation of Co-P is clearly evident that through the back ground of cotton like layers. The presence of micro cracks are indicating that cobalt is a soft metal which has deposited on steel are shown in figure 4. The figure 5 indicates corrosion of mild steel in sea water medium through the severe attack of sea water mild steel particularly at the corners than the rest of the metal. The presence of deep pits, dendrites and scratches confirmed the attack of Cl- on mild steel. X-Ray Diffraction Analysis X-ray diffraction patterns, as shown in Figure 6, trace the properties of crystalline and amorphous peaks on coated hardened and coated without hardened. These patterns gradually change from sharp to very broad peaks with increasing phosphorus content as the structural composition slowly changes from crystalline to amorphous. Spectra show sharp peaks around 14 , 17 , 25 , 45 , 65 , 78 corresponding to nickel and very few diffuse diffraction peaks at (110), (220), (100), (111) planes of varying phases of cobalt phosphate. Surface roughness Mild steel is made of fine refined surface oriented atomic species. The incorporation of Co decreased the roughness which is evident from Ra values are given in table 4 Cobalt coated on mild steel and table 5 mild steel. 4 © 2014 University of Manchester and the authors. This is a preprint of 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 17, Preprint 19 submitted 31 March 2014 Conclusion An engineering coating based on Co and Mild steel was successfully developed. There is a little influence in values of field stress and ultimate tensile stress due to cobalt coating on mild steel. XRD studies revealed that the formation Co3P phase. The coating exhibited higher hardness than Mild steel both plated as well as heat treated condition at 4000C. The enhancement of hardness due to the precipitation hardening effect by forming Co3P Phase. The Co3P coatings offered better corrosion resistance in sea water medium than Mild steel alloy this was confirmed by potentiodynamic polarization studies. Cobalt phosphorus coating have significantly reduced the tensile stress of Co alloy validating that the incorporation of Co3P in Iron matrix. References [1] I H M, Aly., M M, Younan and M T, Nageeb (2003) “Autocatalytic (Electroless) Deposition of Ternary Nickel-Cobalt-Phosphorus Alloy”, metal finishing, Pp 37-42 [2] W J, Cheonga., Ben L, Luana and David W, Shoesmitha.,(2004) “The effects of stabilizers on the bath stability of electroless Ni deposition and the Deposit”, Applied Surface Science vol 229, Pp 282–300 [3] S L, Chenga., T L, Hsua., T Leea., S W, Leeb., J C, Huc and L T, Chenc.,(2013) “Characterization and kinetic investigation of electroless deposition of pure cobalt thin films on silicon substrates”, Applied Surface Science vol 264, Pp 732– 736 [4] Y,Sverdlov and Y Shacham-Diamand.,(2003) “E lectroless deposition of Co(W) thin films”, Microelectronic Engineering vol 70, Pp 512–518 [5] Bala S, Haran., Branko N,Popov and Ralph E, White.,(1998) “Studies on Electroless Cobalt Coatings for Microencapsulation of Hydrogen Storage Alloys”, Journal of the Electrochemical Society, Vol. 145, No. 9, Pp 3000-3007 [6] W H, Safranek.,(1974) “The Properties of Electrodeposited Metals and Alloys”, American Elsevier Publishing Co. First Edition. 5 © 2014 University of Manchester and the authors. This is a preprint of 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 [7] Volume 17, Preprint 19 submitted 31 March 2014 R C, Agarwala and Vijaya Agarwala.,(2003) “Electroless alloy/composite coatings: A review”, sadhana Vol. 28, (3&4), Pp. 475–493. [8] P, Louda., (2007) “ Applications of thin coatings in automotive industry”, Journal of Achievements in Materials and Manufacturing Engineering, Vol.24, No.1, Pp 51-56. [9] ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, Dec 2000, Pp 345-348 [10] Liyuan Qin, Jianshe Lian and Qing Jiang., (2010) “Enhanced ductility of high-strength electrodeposited nanocrystalline Ni–Co alloy with fine grain size”, Journal of Alloys and Compounds, Vol.504S, PpS439–S442 [11] G. Venkatachalam, S.Narayanan, C.Sathiya Narayanan and R.Abhishek: Journal of Manufacturing Engineering, Vol.6, (2011), p.1-4. [12] A, A, Karimpoor, U, Erb, K,T, Aust, and G. Palumbo, (2003) “High strength nanocrystalline cobalt with high tensile ductility”, Scripta Materialia, Vol. 49 Pp. 651–656 [13] Y.C. Zhou , Tonomori. T and Yoshida. A (2002) “Fracture characteristics of thermal barrier coatings after tensile and bending tests” Surface and Coatings Technology,Vol.157 Pp.118–127. [14] Stephane Haag, Michel Burgard, and Barbara Erns (2006) “Pure nickel coating on a mesoporous alumina membrane: Preparation by electroless plating and characterization” Surface & Coatings Technology, Vol. 201 Pp. 2166–2173 [15] Tim Hilditch , Dale Atwell , Mark Easton , Matthew Barnett “Performance of wrought aluminium and magnesium alloy tubes in three-point bending” Materials and Design, Vol. 30 Pp. 2316–2322 [16] Jaroslav Mackerle (2005) “Coatings and surface modification technologies: a finite element bibliography (1995–2005)” Modelling Simul. Mater. Sci. Eng. Vol.13 Pp.935–979 [17] Schenzel HG, AND Kreye H. (1990) “Improved corrosion resistance of electroless nickel phosphorus coatings”, Plat. Surf. Finish., Vol.77, Pp.50-54. [18] Kumar PS, and Nair PK.(1994) “X-ray Diffraction studies on the relative proportion and decomposition of smorphous phase in electroless Ni-B deposits” Nanostructured Mater.,Vol.4, Pp.183-189. 6 © 2014 University of Manchester and the authors. This is a preprint of 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 17, Preprint 19 submitted 31 March 2014 [19]. „Ethane-2- thioamido-4-amino-N-(5-methylisoxazol-3-yl)-benzene sulfonamide: A novel inhibitor for the corrosion of mild steel in 1N HCl‟, S. Karthikeyan, N. Arivazhagan, S. Narayanan, Journal of Corrosion Science &Engineering, Vol.16, 2013. Table 1 The bath used for metallization of mild steel had the following chemicals: Sl.No chemical composition weight 1. Cobalt Sulphate-0.12M 33g/L 2. Sodium acetate-0.6M 50g/L 3. Sodium Hypo phosphate-0.32M 28g/L 4. pH 4.8 5. Temperature 700C Table 2.Micro hardness measurement S.No Material Vickers Hardness Number Load:100g [V.H.N] 1 Mild steel 180 2 Cobalt coating 390 3 Heat treated surface 812 Table 3. Corrosion results Tafel slopes Nature of Ecorr (mv) Icorr (µA.cm-2) ba bc Uncoated -412mv 1.82X10-4 60 66 Coated -310mv 5.8x10-5 52 63 substrate 7 © 2014 University of Manchester and the authors. This is a preprint of 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 17, Preprint 19 submitted 31 March 2014 Table.4. Surface roughness result for cobalt coated mild steel Sl.No Ra(µm) 1 0.3737 2 1.1497 3 0.3322 Average 0.6185 Table.5. Surface roughness result for uncoated mild steel Sl.No Ra(µm) 1 1.6502 2 1.6570 3 1.4297 Average 1.5790 Legends for figure 1. Experimental apparatus of autocatalytic cobalt deposition. 2. Stress strain curve for steel with and without coated Experimental. 3. Cobalt coated Mild steel & Mild steel. 4. SEM image of Cobalt coated Mild steel. 5. SEM image of Uncoated Mild steel. 6. XRD Patterns of electroless Cobalt coated mildsteel with and without hardening. 8 © 2014 University of Manchester and the authors. This is a preprint of 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 17, Preprint 19 submitted 31 March 2014 Figure 1 Figure 2 9 © 2014 University of Manchester and the authors. This is a preprint of 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 17, Preprint 19 submitted 31 March 2014 Figure 3 Figure 4 10 © 2014 University of Manchester and the authors. This is a preprint of 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 17, Preprint 19 submitted 31 March 2014 Figure 5 Intensity (a.u) 6000 Co Without hardening 5000 4000 3000 2000 10 20 30 40 50 60 70 80 Intensity (a.u) 60000 50000 Co Hardened 40000 30000 20000 10000 0 10 20 30 40 50 60 70 80 Figure 6 11 © 2014 University of Manchester and the authors. This is a preprint of 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.