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Improved corrosion resistance of thermally sprayed coating via surface grinding and electroplating techniques
Surface & Coatings Technology 201 (2006) 737 – 743 www.elsevier.com/locate/surfcoat
Improved corrosion resistance of thermally sprayed coating via surface grinding and electroplating techniques Panadda Niranatlumpong ⁎, Hathaipat Koiprasert National Metal and Materials Technology Center, National Science and Technology Development Agency, 114 Thailand Science Park, Paholyothin Rd., Klong 1, Klong Luang, Pathumthani, Thailand Received 11 November 2005; accepted in revised form 16 December 2005 Available online 20 January 2006
Abstract Electroplating of Ni is one of the popular and well-studied surface engineering techniques used for corrosion protection in many industries. The equipment setup is cost-effective, simple and readily available to industries. This work explored the novel application of such electroplating technique to seal the porous surface of thermally sprayed coatings, with an aim to enhance the coating's ability to resist chemical corrosions further. The investigation employed a potentiostat as a testing method to study the corrosion behaviors of thermally sprayed samples, both plated and non-plated with Ni. The results can be summarized that the Ni plating of a few microns thick can successfully seal the arc-sprayed coatings of Hastelloy C-276. The plating layers provide a good corrosion protection for the underlying coatings. The corrosion behavior can be improved further by smoothening of the coating surface prior to the plating process. These encouraging findings will lead to the development of a sealing technique for the thermally sprayed coatings in order to enhance the corrosion resistance of the coatings and extend the limits of the coatings to more severe applications. © 2005 Elsevier B.V. All rights reserved. Keywords: Thermal sprayed coating; Coating sealants; Electroplating; Potentiostat
1. Introduction Corrosion is one of the major problems in chemical and many other industries [1]. Some parts damaged by corrosion and corrosive wear in these industries are currently being repaired and restored to their original dimensions using thermal spraying techniques. By rapidly depositing a coating with a thickness of several hundred microns or more to the surface, thermal spraying can provide a necessary volume of material with good adhesion strength to build up the damaged surface. The coating applications include chemical tank liner, anticorrosion/erosion coating for pipeline and corrosion resistant coatings for various parts and components in contact with a corrosive environment [2–5]. These thermal spraying techniques, including electric arc spraying and combustion flame spraying, are largely employed when thick coatings (greater than 0.5 mm. thick) are needed. Thermal spraying is also being used in upgrading the component materials by applying a high⁎ Corresponding author. E-mail address: [email protected] (P. Niranatlumpong). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.12.026
grade corrosion-resistant coating, such as Ni– or Co–base alloys, on steel or stainless steel substrate to improve the anticorrosion property of the components. The thermal spraying technique used in this work is electric arc spraying. Arc-spraying employs a coating metal in the form of two wires, which are electrically charged in an opposing manner. The wires are fed separately with an intersection at the wire ends, positioned at the front of the spray gun. An arc occurs at the intersection, where the wire metal is melted. The molten metal on the wire tips is atomized and propelled onto a substrate by means of compressed gas. As the molten droplets impact onto the substrate, they rapidly solidify to form a coating [6]. Arc-spraying can produce very thick coatings in a short period of time and, therefore, is widely used in component refurbishment where a thick surface build-up is required. Porosity, however, is a major drawback for the use of these arc-sprayed coatings as a corrosion prevention method [3,7]. Coatings produced via arc-spraying techniques often contain moderate to high porosity, ranging from 0.2% to 15%. These pores, although not large in size, are commonly of interconnected nature. Thus, molecular transport of the corrosive
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P. Niranatlumpong, H. Koiprasert / Surface & Coatings Technology 201 (2006) 737–743
Table 1 Sample preparation details Sample group
Substrate Hastelloy coating
Surface grinding
1 (ref.) 2 3 4 5
SS 304 SS 304 SS 304 SS 304 SS 304
✓
✓ ✓ ✓ ✓
✓ ✓
Ni Surface roughness plating (μm Ra)
✓ ✓
0.1 11.4 0.6 10.8 0.4
agent through these pores is likely. This can accelerate the corrosion failure of the coating via larger surface contact and/or crevice mechanism. In severe cases, the corrosive agents may have access to the coating/substrate interfaces and cause corrosion of the underlying components leading to part failures. Techniques for sealing of the porosity, such as chemical vapor deposition, laser glazing, phosphate treatment followed by heat treatment and detonation gun-sprayed coating [8–10], have been previously reported. These techniques require expensive equipment setup that may not be justifiable for repairing of many industrial parts. This project was, therefore, conducted with the objective of developing a low-cost technique for surface sealing of the arc-sprayed coating used in surface build-ups to improve the corrosion resistance. Electroplating was employed in this work to deposit a thin metallic layer to evenly cover up the coating surface and block any surface pores. Different surface conditions were investigated in comparison to a stainless steel 304 used as a reference material. Electrochemical corrosion of the coating system was studied using a potentiostat technique. 2. Experimental procedure The substrate and reference material used in this work was stainless steel 304 (nominal composition of Fe–18Cr– 8Ni–2Mn, in wt.%). The material was cut from a stainless steel bar to specimens of dimension 3 × 20 × 20 mm. The samples were prepared according to their groups as presented in Table 1. The samples to be coated (sample groups 2–5) were blasted on all surfaces with Al2O3 grit of no. 24 mesh size to increase the substrate surface roughness to approximately 8 μm Ra to promote a good coating adhesion. An electric arc-spraying system (Tafa 9000) was then utilized to produce coatings of roughly 500 μm thickness, using Hastelloy C-276 wire (nominal composition of Ni–2.5Co–15.5Cr–16Mo–4W– 5.5Fe, in wt.%) as a coating material. The spray parameters used in this experiment, as presented in Table 2, were optimized
for the coating material to obtain uniform, high-density and good adhesion coatings. Sample groups 3 and 5 were ground using wet SiC paper after the coating process to achieve a coating surface roughness of less than 0.8 μm Ra, and still leaving a coating thickness of about 400 μm on the substrate. The average surface roughness of each group of samples is included in Table 1. Specified sample groups were then electroplated with Ni using a Watt's bath solution (300 g/lNiSO4·6H2O, 60 g/l NiCl2·6H2O, 45 g/l H3BO3). The samples were electroplated at 55 °C using optimized parameters, at 0.1 A and 2.5 V for 10 min, to achieve a uniform and continuous coverage of the Ni layer. The thickness of the Ni layer on the smoothened coatings was approximately 20 μm, while on the non-smoothened coatings the Ni layers were slightly thinner, at an average of about 16 μm thick, and more uneven. The as-plated samples were cross-sectioned to study the sealability of the Ni plating using a scanning electron microscope. The corrosion resistance of the samples was investigated in an electrochemical polarization experiment using a potentiostat technique. The NaOH solution used in this test was prepared from NaOH flakes of analytical reagent grade with a formula weight of 40. The NaOH flakes were dissolved in deionized water at 80 g/l to achieve a solution of 2 molar concentration and of pH 12. The potentiostat equipment was set up as shown in Fig. 1 [11]. The test sample was set up as the working electrode. Current was passed through the electrochemical cell. The potential between the working and the reference electrodes was measured and controlled to increase in fixed intervals while the current flow was measured. The results will be presented in the form of the polarization curve, which is the plot of the electrode potential versus the log of the current density. The values of Ecorr (electrode potential when corrosion begins; indicates the susceptibility to corrosion of the test sample) and Icorr (current density when corrosion begins; can be referred to as the severity of corrosion) can then be determined using Tafel extrapolation method and used to explain the corrosion behavior of the coating systems [12]. For each group of samples, at least 3 specimens were tested using the potentiostat technique in order to calculate the average Ecorr and Icorr of the test samples.
Table 2 Arc spraying parameters Arc voltage Current Air pressure Spray distance Traverse speed
31 V 150 A 4.14 bar 152 mm 432 mm/s
Fig. 1. Experimental setup of the potentiostat [11].
P. Niranatlumpong, H. Koiprasert / Surface & Coatings Technology 201 (2006) 737–743
3. Results and discussions As the samples underwent the electrochemical polarization test in 2 M NaOH solution, chemical reactions took place on the sample surfaces as follows: O2 þ 2H2 O þ 2Fe↔2Fe2þ þ 4OH−
ð1Þ
3O2 þ 6H2 O þ 4Cr↔4Cr3þ þ 12OH−
ð2Þ
O2 þ 2H2 O þ 2Ni↔2Ni2þ þ 4OH−
ð3Þ
In the case of the stainless steel reference samples and the Hastelloy-coated samples without the Ni electroplating, the corrosion will take place according to Eqs. (1), (2) and (3). Fe (OH)2, Cr(OH)3, Ni(OH)2 and possibly other oxides and spinels can then form on the surface as a passive layer [13–16]. When the samples were plated with Ni, however, the corrosion reaction changed to Eq. (3), with the passive layer of Ni(OH)2 [17–19]. The polished stainless steel substrate, tested using the electrochemical polarization technique, showed a formation of a passive film, see Fig. 2. As the electrode potential increases, the protective film undergoes chemical breakdown and the corrosion rate is accelerated. Similar behavior is found in testing of the polished Hastelloy sample. Metal dissolution of Hastelloy begins at a higher electrode potential than that of the stainless steel, and at the same potential the corrosion generally takes place at a lower rate (lower Icorr). At a higher potential, a protective film forms on the surface, thus reducing the corrosion rate. The film then undergoes chemical breakdown, followed by the formation of the second oxide. At still higher electrode potential, together with the breakdown of the second oxide film, the corrosion rate of Hastelloy was recorded as increasing rapidly. 3.1. Effect of Ni-electroplating on the corrosion behavior When Hastelloy is produced as a coating using an arcspraying technique, it was found that the corrosion properties
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deteriorated. The corrosion takes place as early as the electrode potential value of − 0.48 eV, in comparison to − 0.35 eV for the bulk Hastelloy sample. The distinct formation of the protective oxide film was also not evidenced. There may be a thin protective scale formed at around − 0.25 eV since the rate of increase of the current density drops slightly. However, the scale cannot provide a long-term protection, causing the acceleration in the current density at a higher potential. This is because the Hastelloy coatings contained a large amount of intersplat porosity. An intersplat pore can behave as a deep pit or crevice. Within the pit, oxidation took place resulting in the release of Fe2+, Ni2+ and Cr3+ species. These ions can then react with OH− to form protective hydrous oxide products. However, since the solution within the pit cannot flow as freely, there was not enough circulation and the solution in the pit became oxygen-starved. Thus, formation of the protective oxide scale cannot occur as readily as on the outer coating surface. Since the continuity of the oxide scale is interrupted, corrosion can persist locally within the pit. Furthermore, the cathodic nature of the protected outer coating surface may cause a galvanic cell with the internal surface of the crevice and markedly accelerate the corrosion, resulting in a high current density value, as shown in Fig. 3. Fig. 3 shows the effect on the corrosion behavior of the electroplating of Ni on the as-sprayed Hastelloy coatings. The corrosion is now taking place on the Ni layer instead of the more resistant Hastelloy coating. The polarization curve sees a decrease in the Ecorr value, indicating that corrosion can take place easier. However, as the electrode potential increases, the Ni layer can then form a continuous protective oxide film, causing the increase in the corrosion rate (the current density) to slow down at roughly the same potential as the as-sprayed coating. This is because the Ni-plating had closed-off small surface pores, reducing the geometrical effect that influences the pitting-type corrosion, see Fig. 4. If the surface pores are diminished, the continuous oxide film can be formed more easily and maintained on the surface. The protective film, however, cannot provide an effective protection, possibly due to the premature breakdown of the oxide film. The breakdown occurred as a result of localized corrosion in the concave region
Fig. 2. The potentiostat plots between the stainless steel substrate and bulk Hastelloy.
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Fig. 3. The potentiostat plots between the sample group 2, substrate coated with Hastelloy (Hastelloy C), and the sample group 4, substrate coated with Hastelloy and electroplated with Ni (Hastelloy C + Ni).
of the Ni-plating layer. This could also be due to the crevice corrosion behavior as in the previous sample (sample group 2), only less severe because there is more homogeneity in this sample. 3.2. Effect of coating surface roughness on the corrosion behavior When the coating surface was ground down to achieve the surface roughness of approximately 0.8 um Ra, the looser top layer of the coating was removed. At the same time, the depth of the pit was also reduced, see Fig. 5. This is because, in the arcspraying process, the coating achieves its density by deformation of the material droplets to the contour of the underlying surface upon impact. The density can then be increased further by the compressive force induced by the next impacting droplets, which assists in the splat compaction. The surface layer of the coating, however, was not further bombarded by the molten droplets, and therefore, tends to have larger intersplat
Fig. 4. Micrograph showing the sealing of the surface pore in the Hastelloy coating using the electroplating of Ni. The geometry of the opening of the pore can encourage pitting-type corrosion. This effect can be reduced when Niplating is applied on the surface of the coating.
pores. If the surface layer is removed, the new surface will be denser with pores smaller in size. The polarization result (Fig. 6) shows the Ecorr value displaying a slight increase but a significant improvement can be observed in the corrosion rate (the current density). This is because the smoothening helped to reduce the pitting effect of the coating surface. The shallower surface pit allows better circulation of the solution, which promotes the formation of the protective scale. Although the scale formation and breakdown cycle was not distinctively modeled in Fig. 6, the significant drop in the corrosion rate suggested its presence. 3.3. Combined effect of the Ni-electroplating and surface roughness of the coatings When the Hastelloy coating was ground down to achieve a smoother surface, followed by an electroplating of Ni, the potentiostat result shows an improvement in the corrosion behavior of the sample as evidence in Fig. 7. The Ecorr value of − 0.35 eV is relatively high and reflects the good resistance
Fig. 5. Micrograph showing the sealing of the surface pore in the Hastelloy coating after grinding using the electroplating of Ni (sample group 5). The size of the surface pore is reduced by the grinding.
P. Niranatlumpong, H. Koiprasert / Surface & Coatings Technology 201 (2006) 737–743
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Fig. 6. The potentiostat plots between the sample group 2, substrate coated with Hastelloy without surface grinding (rough), and the sample group 3, substrate coated with Hastelloy with surface grinding (smooth).
Fig. 7. The potentiostat plots between the sample group 3, substrate coated with Hastelloy with surface grinding (Hastelloy C), and the sample group 5, substrate coated with Hastelloy with surface grinding and topcoat with Ni electroplating (Hastelloy C + Ni).
Fig. 8. The potentiostat plots between the sample group 4, substrate coated with Hastelloy and electroplated with Ni without surface grinding (rough), and the sample group 5, substrate coated with Hastelloy with surface grinding and topcoat with Ni electroplating (smooth).
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Fig. 9. The potentiostat plots between the sample group 1, stainless steel 304 substrate (stainless), the sample group 3, substrate coated with Hastelloy with surface grinding (Hastelloy C smooth), and the sample group 5, substrate coated with Hastelloy with surface grinding and topcoat with Ni electroplating (Hastelloy C + Ni smooth).
to corrosion in NaOH solution of the coating system. The corrosion rate was also improved as indicated by the shifting of the curve to a lower current density. One other significant change is the well-apparent turns of the curve, which suggest the formation and breakdown cycle of the protective scale. The result also indicates the self-healing ability of the protective scale, seen in Fig. 7 as the sudden drop in the current density at around 0.4 eV. The improvement in the corrosion resistance came largely because of the smooth and near-defect-free surface of the Ni-plating layer, allowing less localized attack, see Fig. 5. As the corrosion becomes more uniform, the protective scale can also grow more uniformly and continuously. Also, with less localized attack occurring, the oxide scale can endure for longer and, therefore, provide an effective barrier against corrosion. Fig. 8 compares two samples, both electroplated with Ni, but with a different surface condition prior to plating. The plots show a dramatic improvement in the corrosion behavior when surface grinding was utilized. It was, thus, concluded that surface grinding to eliminate as much surface pores as possible is a necessary step in the coating fabrication. Fig. 9 presents the potentiostat results of the surface-ground coatings in comparison with the stainless steel substrate. It is
clear from the plots that the ground and Ni-plated coating (sample group 5) exhibits a higher corrosion resistance and a slower corrosion rate than the stainless steel 304 in NaOH solution. 3.4. Ecorr, Icorr and corrosion rate The average Icorr and Ecorr values were determined from the polarization curves using the Tafel extrapolation method as presented in Fig. 10. The Ecorr value represents the electrode potential when the corrosion state changes from cathodic to anodic nature, which indicates the susceptibility of the sample to corrosion attack. The Icorr value relates directly to the rate of corrosion. The graph shows that the Hastelloy coating with flat surface and with Ni-plating exhibits a preferable corrosion resistance in NaOH solution, i.e. High Ecorr and low Icorr. CR ¼
3:27 103 Icorr EW d
ð4Þ
The corrosion rate was determined using Eq. (4), where CR is the corrosion rate in mm/year, Icorr is the corrosion current density in μA/cm2, EW is the equivalent weight in grams and d
I corr (A) -1.50E-04
-1.00E-04
-0.35
-5.00E-05
Hastelloy coating, flat surface + Ni
-0.62
-0.7
5.00E-05
1.00E-04
Hastelloy coating, flat surface
-0.48
1.30E-04 3.40E-06 1.40E-04
Hastelloy coating
-0.45 -0.5
Stainless steel 304 (bulk)
-0.3
1.50E-04
5.80E-07
Hastelloy coating + Ni
-0.43
E corrosion I corrosion
0.00E+00
4.30E-07
-0.1
0.1
0.3
0.5
E corr (V) Fig. 10. Graph showing the average Ecorr and Icorr values for each set of samples tested.
0.7
P. Niranatlumpong, H. Koiprasert / Surface & Coatings Technology 201 (2006) 737–743 Table 3 Corrosion rates of the test samples
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Acknowledgements
Sample groups
Corrosion rate (mm/year)
1. Stainless steel 304 2. Hastelloy coating 3. Hastelloy coating with flat surface 4. Hastelloy coating + Ni 5. Hastelloy coating with flat surface + Ni
0.006 1.78 0.043 1.79 0.008
is the metal or alloy density in g/cm3 [20]. The calculated values were summarized in Table 3. The result shows that the rates of corrosion of sample group 1 (stainless steel reference sample) and sample group 5 (sample coated with Hastelloy with flat surface and an outer layer of plated Ni) are very similar, with the corrosion rate slightly higher in sample group 5. The rate of corrosion of sample group 5 is significantly lower than those of the Hastelloy coating samples without flattened surfaces or Ni plating (sample groups 2–4). This indicates that the plating of Ni can considerably slow down the corrosion process of the thermal-sprayed coating in NaOH solution. 4. Conclusions Conventional Ni plating can successfully be used to seal the surface pores of an arc-sprayed coating. At the Ni-plating thickness of about 15–20 μm, the surface roughness of the plating is affected by the roughness of the underlying coating. Thus, some defects are still present on the outer surface. The surface pores on the non-plated coating and the high surface roughness of the plated and the non-plated coatings encourage corrosion attack. Grinding of the coating surface can remove the large surface pores to an extent, but some small pores are still present, which hinder the ability of the coating to form a protective scale. Electroplating of Ni on the ground coating can seal-off the pores and provide a smooth finish to the sample, which improves the ability to form a protective scale and, in turn, enhances the corrosion resistance of the Hastelloy coating. It is, therefore, promising that the double-layer coating system, consisting of electric arc-sprayed Hastelloy with surface grinding and Ni electroplating as a top coat, can be used to refurbish worn and damaged parts, where the thermal-sprayed coating is used to build up the surface volume required and Ni plating to improve its corrosion resistance.
The authors would like to thank the members of the FAMD unit at the National Metal and Materials Technology Center for the use of the electrochemical polarization equipment and Assistant Prof. Siriluck Nivitchanyong for consultation. References [1] K.J. McNaughton, and the staff of Chemical Engineering, Materials Engineering II, Controlling Corrosion in Process Equipment, McGrawHill Publications Company, New York, 1980. [2] T. Sundararajan, S. Kuroda, T. Itagaki, F. Abe, Proceedings of the 2003 ITSC, Florida, 2003, p. 495. [3] P. Siitonen, T. Konos, P.O. Kettunen, Proceedings of the 7th NTSC, Massachusetts, 1994, p. 105. [4] X. Huang, R. Wang, L. Xu, H. Luo, X. Yin, Proceedings of the 15th ITSC, France, 1998, p. 1013. [5] D. Toma, W. Brandl, G. Marginean, Surf. Coat. Technol. 138 (2001) 149. [6] R.C. Tucker Jr., ASM Handbook Volume 5 Surface Engineering: Thermal Spray Coatings, ASM International, Ohio, 1994, p. 497. [7] T. Tajiri, Z. Zeng, Proceedings of the 1st ITSC, Montreal, 2000, p. 697. [8] R. Rajendran, V.S. Raja, R. Sivakumar, R.S. Srinivasa, Surf. Coat. Technol. 73 (3) (1995) 198. [9] H.L. Tsai, P.C. Tsai, Surf. Coat. Technol. 71 (1) (1995) 53. [10] S. Ahmaniemi, P. Vuoristo, T. Mantyla, Surf. Coat. Technol. 151–152 (2002) 412. [11] J.R. Davis, Davis & Associates (Eds.), Corrosion Understanding the Basics, ASM International, Ohio, 2000, p. 453. [12] M.G. Fontana, Corrosion Engineering, International Edition, McGraw-Hill Book Company, 1987, p. 499. [13] D. Marijan, M. Vukovic, P. Pervan, M. Milun, Croat. Chem. Acta 72 (4) (1999) 737. [14] M. Pourbaix, Lectures on Electrochemical Corrosion, Plenum Press, New York, 1973. [15] M. Vukovic, Corros. Sci. 37 (1) (1995) 111. [16] N. Krstajic, M. Popovic, B. Grgur, M. Vojnovic, D. Sepa, J. Electroanal. Chem. 512 (2001) 16. [17] A. Seyeux, V. Maurice, L.H. Klein, P. Marcus, J. Solid State Electrochem. 9 (5) (2005) 337. [18] C. Silverman, Corrosion 37 (9) (1981) 546. [19] R.J. Smith, R.E. Hummel, J.R. Ambrose, Corros. Sci. 27 (8) (1987) 815. [20] J.R. Scully, D.W. Taylor, 9th edition, Corrosion: Metals Handbook, vol. 13, ASM International, Ohio, 1987, p. 212.
Improved corrosion resistance of thermally sprayed coating via surface grinding and electroplating techniques Panadda Niranatlumpong ⁎, Hathaipat Koiprasert National Metal and Materials Technology Center, National Science and Technology Development Agency, 114 Thailand Science Park, Paholyothin Rd., Klong 1, Klong Luang, Pathumthani, Thailand Received 11 November 2005; accepted in revised form 16 December 2005 Available online 20 January 2006
Abstract Electroplating of Ni is one of the popular and well-studied surface engineering techniques used for corrosion protection in many industries. The equipment setup is cost-effective, simple and readily available to industries. This work explored the novel application of such electroplating technique to seal the porous surface of thermally sprayed coatings, with an aim to enhance the coating's ability to resist chemical corrosions further. The investigation employed a potentiostat as a testing method to study the corrosion behaviors of thermally sprayed samples, both plated and non-plated with Ni. The results can be summarized that the Ni plating of a few microns thick can successfully seal the arc-sprayed coatings of Hastelloy C-276. The plating layers provide a good corrosion protection for the underlying coatings. The corrosion behavior can be improved further by smoothening of the coating surface prior to the plating process. These encouraging findings will lead to the development of a sealing technique for the thermally sprayed coatings in order to enhance the corrosion resistance of the coatings and extend the limits of the coatings to more severe applications. © 2005 Elsevier B.V. All rights reserved. Keywords: Thermal sprayed coating; Coating sealants; Electroplating; Potentiostat
1. Introduction Corrosion is one of the major problems in chemical and many other industries [1]. Some parts damaged by corrosion and corrosive wear in these industries are currently being repaired and restored to their original dimensions using thermal spraying techniques. By rapidly depositing a coating with a thickness of several hundred microns or more to the surface, thermal spraying can provide a necessary volume of material with good adhesion strength to build up the damaged surface. The coating applications include chemical tank liner, anticorrosion/erosion coating for pipeline and corrosion resistant coatings for various parts and components in contact with a corrosive environment [2–5]. These thermal spraying techniques, including electric arc spraying and combustion flame spraying, are largely employed when thick coatings (greater than 0.5 mm. thick) are needed. Thermal spraying is also being used in upgrading the component materials by applying a high⁎ Corresponding author. E-mail address: [email protected] (P. Niranatlumpong). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.12.026
grade corrosion-resistant coating, such as Ni– or Co–base alloys, on steel or stainless steel substrate to improve the anticorrosion property of the components. The thermal spraying technique used in this work is electric arc spraying. Arc-spraying employs a coating metal in the form of two wires, which are electrically charged in an opposing manner. The wires are fed separately with an intersection at the wire ends, positioned at the front of the spray gun. An arc occurs at the intersection, where the wire metal is melted. The molten metal on the wire tips is atomized and propelled onto a substrate by means of compressed gas. As the molten droplets impact onto the substrate, they rapidly solidify to form a coating [6]. Arc-spraying can produce very thick coatings in a short period of time and, therefore, is widely used in component refurbishment where a thick surface build-up is required. Porosity, however, is a major drawback for the use of these arc-sprayed coatings as a corrosion prevention method [3,7]. Coatings produced via arc-spraying techniques often contain moderate to high porosity, ranging from 0.2% to 15%. These pores, although not large in size, are commonly of interconnected nature. Thus, molecular transport of the corrosive
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P. Niranatlumpong, H. Koiprasert / Surface & Coatings Technology 201 (2006) 737–743
Table 1 Sample preparation details Sample group
Substrate Hastelloy coating
Surface grinding
1 (ref.) 2 3 4 5
SS 304 SS 304 SS 304 SS 304 SS 304
✓
✓ ✓ ✓ ✓
✓ ✓
Ni Surface roughness plating (μm Ra)
✓ ✓
0.1 11.4 0.6 10.8 0.4
agent through these pores is likely. This can accelerate the corrosion failure of the coating via larger surface contact and/or crevice mechanism. In severe cases, the corrosive agents may have access to the coating/substrate interfaces and cause corrosion of the underlying components leading to part failures. Techniques for sealing of the porosity, such as chemical vapor deposition, laser glazing, phosphate treatment followed by heat treatment and detonation gun-sprayed coating [8–10], have been previously reported. These techniques require expensive equipment setup that may not be justifiable for repairing of many industrial parts. This project was, therefore, conducted with the objective of developing a low-cost technique for surface sealing of the arc-sprayed coating used in surface build-ups to improve the corrosion resistance. Electroplating was employed in this work to deposit a thin metallic layer to evenly cover up the coating surface and block any surface pores. Different surface conditions were investigated in comparison to a stainless steel 304 used as a reference material. Electrochemical corrosion of the coating system was studied using a potentiostat technique. 2. Experimental procedure The substrate and reference material used in this work was stainless steel 304 (nominal composition of Fe–18Cr– 8Ni–2Mn, in wt.%). The material was cut from a stainless steel bar to specimens of dimension 3 × 20 × 20 mm. The samples were prepared according to their groups as presented in Table 1. The samples to be coated (sample groups 2–5) were blasted on all surfaces with Al2O3 grit of no. 24 mesh size to increase the substrate surface roughness to approximately 8 μm Ra to promote a good coating adhesion. An electric arc-spraying system (Tafa 9000) was then utilized to produce coatings of roughly 500 μm thickness, using Hastelloy C-276 wire (nominal composition of Ni–2.5Co–15.5Cr–16Mo–4W– 5.5Fe, in wt.%) as a coating material. The spray parameters used in this experiment, as presented in Table 2, were optimized
for the coating material to obtain uniform, high-density and good adhesion coatings. Sample groups 3 and 5 were ground using wet SiC paper after the coating process to achieve a coating surface roughness of less than 0.8 μm Ra, and still leaving a coating thickness of about 400 μm on the substrate. The average surface roughness of each group of samples is included in Table 1. Specified sample groups were then electroplated with Ni using a Watt's bath solution (300 g/lNiSO4·6H2O, 60 g/l NiCl2·6H2O, 45 g/l H3BO3). The samples were electroplated at 55 °C using optimized parameters, at 0.1 A and 2.5 V for 10 min, to achieve a uniform and continuous coverage of the Ni layer. The thickness of the Ni layer on the smoothened coatings was approximately 20 μm, while on the non-smoothened coatings the Ni layers were slightly thinner, at an average of about 16 μm thick, and more uneven. The as-plated samples were cross-sectioned to study the sealability of the Ni plating using a scanning electron microscope. The corrosion resistance of the samples was investigated in an electrochemical polarization experiment using a potentiostat technique. The NaOH solution used in this test was prepared from NaOH flakes of analytical reagent grade with a formula weight of 40. The NaOH flakes were dissolved in deionized water at 80 g/l to achieve a solution of 2 molar concentration and of pH 12. The potentiostat equipment was set up as shown in Fig. 1 [11]. The test sample was set up as the working electrode. Current was passed through the electrochemical cell. The potential between the working and the reference electrodes was measured and controlled to increase in fixed intervals while the current flow was measured. The results will be presented in the form of the polarization curve, which is the plot of the electrode potential versus the log of the current density. The values of Ecorr (electrode potential when corrosion begins; indicates the susceptibility to corrosion of the test sample) and Icorr (current density when corrosion begins; can be referred to as the severity of corrosion) can then be determined using Tafel extrapolation method and used to explain the corrosion behavior of the coating systems [12]. For each group of samples, at least 3 specimens were tested using the potentiostat technique in order to calculate the average Ecorr and Icorr of the test samples.
Table 2 Arc spraying parameters Arc voltage Current Air pressure Spray distance Traverse speed
31 V 150 A 4.14 bar 152 mm 432 mm/s
Fig. 1. Experimental setup of the potentiostat [11].
P. Niranatlumpong, H. Koiprasert / Surface & Coatings Technology 201 (2006) 737–743
3. Results and discussions As the samples underwent the electrochemical polarization test in 2 M NaOH solution, chemical reactions took place on the sample surfaces as follows: O2 þ 2H2 O þ 2Fe↔2Fe2þ þ 4OH−
ð1Þ
3O2 þ 6H2 O þ 4Cr↔4Cr3þ þ 12OH−
ð2Þ
O2 þ 2H2 O þ 2Ni↔2Ni2þ þ 4OH−
ð3Þ
In the case of the stainless steel reference samples and the Hastelloy-coated samples without the Ni electroplating, the corrosion will take place according to Eqs. (1), (2) and (3). Fe (OH)2, Cr(OH)3, Ni(OH)2 and possibly other oxides and spinels can then form on the surface as a passive layer [13–16]. When the samples were plated with Ni, however, the corrosion reaction changed to Eq. (3), with the passive layer of Ni(OH)2 [17–19]. The polished stainless steel substrate, tested using the electrochemical polarization technique, showed a formation of a passive film, see Fig. 2. As the electrode potential increases, the protective film undergoes chemical breakdown and the corrosion rate is accelerated. Similar behavior is found in testing of the polished Hastelloy sample. Metal dissolution of Hastelloy begins at a higher electrode potential than that of the stainless steel, and at the same potential the corrosion generally takes place at a lower rate (lower Icorr). At a higher potential, a protective film forms on the surface, thus reducing the corrosion rate. The film then undergoes chemical breakdown, followed by the formation of the second oxide. At still higher electrode potential, together with the breakdown of the second oxide film, the corrosion rate of Hastelloy was recorded as increasing rapidly. 3.1. Effect of Ni-electroplating on the corrosion behavior When Hastelloy is produced as a coating using an arcspraying technique, it was found that the corrosion properties
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deteriorated. The corrosion takes place as early as the electrode potential value of − 0.48 eV, in comparison to − 0.35 eV for the bulk Hastelloy sample. The distinct formation of the protective oxide film was also not evidenced. There may be a thin protective scale formed at around − 0.25 eV since the rate of increase of the current density drops slightly. However, the scale cannot provide a long-term protection, causing the acceleration in the current density at a higher potential. This is because the Hastelloy coatings contained a large amount of intersplat porosity. An intersplat pore can behave as a deep pit or crevice. Within the pit, oxidation took place resulting in the release of Fe2+, Ni2+ and Cr3+ species. These ions can then react with OH− to form protective hydrous oxide products. However, since the solution within the pit cannot flow as freely, there was not enough circulation and the solution in the pit became oxygen-starved. Thus, formation of the protective oxide scale cannot occur as readily as on the outer coating surface. Since the continuity of the oxide scale is interrupted, corrosion can persist locally within the pit. Furthermore, the cathodic nature of the protected outer coating surface may cause a galvanic cell with the internal surface of the crevice and markedly accelerate the corrosion, resulting in a high current density value, as shown in Fig. 3. Fig. 3 shows the effect on the corrosion behavior of the electroplating of Ni on the as-sprayed Hastelloy coatings. The corrosion is now taking place on the Ni layer instead of the more resistant Hastelloy coating. The polarization curve sees a decrease in the Ecorr value, indicating that corrosion can take place easier. However, as the electrode potential increases, the Ni layer can then form a continuous protective oxide film, causing the increase in the corrosion rate (the current density) to slow down at roughly the same potential as the as-sprayed coating. This is because the Ni-plating had closed-off small surface pores, reducing the geometrical effect that influences the pitting-type corrosion, see Fig. 4. If the surface pores are diminished, the continuous oxide film can be formed more easily and maintained on the surface. The protective film, however, cannot provide an effective protection, possibly due to the premature breakdown of the oxide film. The breakdown occurred as a result of localized corrosion in the concave region
Fig. 2. The potentiostat plots between the stainless steel substrate and bulk Hastelloy.
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Fig. 3. The potentiostat plots between the sample group 2, substrate coated with Hastelloy (Hastelloy C), and the sample group 4, substrate coated with Hastelloy and electroplated with Ni (Hastelloy C + Ni).
of the Ni-plating layer. This could also be due to the crevice corrosion behavior as in the previous sample (sample group 2), only less severe because there is more homogeneity in this sample. 3.2. Effect of coating surface roughness on the corrosion behavior When the coating surface was ground down to achieve the surface roughness of approximately 0.8 um Ra, the looser top layer of the coating was removed. At the same time, the depth of the pit was also reduced, see Fig. 5. This is because, in the arcspraying process, the coating achieves its density by deformation of the material droplets to the contour of the underlying surface upon impact. The density can then be increased further by the compressive force induced by the next impacting droplets, which assists in the splat compaction. The surface layer of the coating, however, was not further bombarded by the molten droplets, and therefore, tends to have larger intersplat
Fig. 4. Micrograph showing the sealing of the surface pore in the Hastelloy coating using the electroplating of Ni. The geometry of the opening of the pore can encourage pitting-type corrosion. This effect can be reduced when Niplating is applied on the surface of the coating.
pores. If the surface layer is removed, the new surface will be denser with pores smaller in size. The polarization result (Fig. 6) shows the Ecorr value displaying a slight increase but a significant improvement can be observed in the corrosion rate (the current density). This is because the smoothening helped to reduce the pitting effect of the coating surface. The shallower surface pit allows better circulation of the solution, which promotes the formation of the protective scale. Although the scale formation and breakdown cycle was not distinctively modeled in Fig. 6, the significant drop in the corrosion rate suggested its presence. 3.3. Combined effect of the Ni-electroplating and surface roughness of the coatings When the Hastelloy coating was ground down to achieve a smoother surface, followed by an electroplating of Ni, the potentiostat result shows an improvement in the corrosion behavior of the sample as evidence in Fig. 7. The Ecorr value of − 0.35 eV is relatively high and reflects the good resistance
Fig. 5. Micrograph showing the sealing of the surface pore in the Hastelloy coating after grinding using the electroplating of Ni (sample group 5). The size of the surface pore is reduced by the grinding.
P. Niranatlumpong, H. Koiprasert / Surface & Coatings Technology 201 (2006) 737–743
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Fig. 6. The potentiostat plots between the sample group 2, substrate coated with Hastelloy without surface grinding (rough), and the sample group 3, substrate coated with Hastelloy with surface grinding (smooth).
Fig. 7. The potentiostat plots between the sample group 3, substrate coated with Hastelloy with surface grinding (Hastelloy C), and the sample group 5, substrate coated with Hastelloy with surface grinding and topcoat with Ni electroplating (Hastelloy C + Ni).
Fig. 8. The potentiostat plots between the sample group 4, substrate coated with Hastelloy and electroplated with Ni without surface grinding (rough), and the sample group 5, substrate coated with Hastelloy with surface grinding and topcoat with Ni electroplating (smooth).
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Fig. 9. The potentiostat plots between the sample group 1, stainless steel 304 substrate (stainless), the sample group 3, substrate coated with Hastelloy with surface grinding (Hastelloy C smooth), and the sample group 5, substrate coated with Hastelloy with surface grinding and topcoat with Ni electroplating (Hastelloy C + Ni smooth).
to corrosion in NaOH solution of the coating system. The corrosion rate was also improved as indicated by the shifting of the curve to a lower current density. One other significant change is the well-apparent turns of the curve, which suggest the formation and breakdown cycle of the protective scale. The result also indicates the self-healing ability of the protective scale, seen in Fig. 7 as the sudden drop in the current density at around 0.4 eV. The improvement in the corrosion resistance came largely because of the smooth and near-defect-free surface of the Ni-plating layer, allowing less localized attack, see Fig. 5. As the corrosion becomes more uniform, the protective scale can also grow more uniformly and continuously. Also, with less localized attack occurring, the oxide scale can endure for longer and, therefore, provide an effective barrier against corrosion. Fig. 8 compares two samples, both electroplated with Ni, but with a different surface condition prior to plating. The plots show a dramatic improvement in the corrosion behavior when surface grinding was utilized. It was, thus, concluded that surface grinding to eliminate as much surface pores as possible is a necessary step in the coating fabrication. Fig. 9 presents the potentiostat results of the surface-ground coatings in comparison with the stainless steel substrate. It is
clear from the plots that the ground and Ni-plated coating (sample group 5) exhibits a higher corrosion resistance and a slower corrosion rate than the stainless steel 304 in NaOH solution. 3.4. Ecorr, Icorr and corrosion rate The average Icorr and Ecorr values were determined from the polarization curves using the Tafel extrapolation method as presented in Fig. 10. The Ecorr value represents the electrode potential when the corrosion state changes from cathodic to anodic nature, which indicates the susceptibility of the sample to corrosion attack. The Icorr value relates directly to the rate of corrosion. The graph shows that the Hastelloy coating with flat surface and with Ni-plating exhibits a preferable corrosion resistance in NaOH solution, i.e. High Ecorr and low Icorr. CR ¼
3:27 103 Icorr EW d
ð4Þ
The corrosion rate was determined using Eq. (4), where CR is the corrosion rate in mm/year, Icorr is the corrosion current density in μA/cm2, EW is the equivalent weight in grams and d
I corr (A) -1.50E-04
-1.00E-04
-0.35
-5.00E-05
Hastelloy coating, flat surface + Ni
-0.62
-0.7
5.00E-05
1.00E-04
Hastelloy coating, flat surface
-0.48
1.30E-04 3.40E-06 1.40E-04
Hastelloy coating
-0.45 -0.5
Stainless steel 304 (bulk)
-0.3
1.50E-04
5.80E-07
Hastelloy coating + Ni
-0.43
E corrosion I corrosion
0.00E+00
4.30E-07
-0.1
0.1
0.3
0.5
E corr (V) Fig. 10. Graph showing the average Ecorr and Icorr values for each set of samples tested.
0.7
P. Niranatlumpong, H. Koiprasert / Surface & Coatings Technology 201 (2006) 737–743 Table 3 Corrosion rates of the test samples
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Acknowledgements
Sample groups
Corrosion rate (mm/year)
1. Stainless steel 304 2. Hastelloy coating 3. Hastelloy coating with flat surface 4. Hastelloy coating + Ni 5. Hastelloy coating with flat surface + Ni
0.006 1.78 0.043 1.79 0.008
is the metal or alloy density in g/cm3 [20]. The calculated values were summarized in Table 3. The result shows that the rates of corrosion of sample group 1 (stainless steel reference sample) and sample group 5 (sample coated with Hastelloy with flat surface and an outer layer of plated Ni) are very similar, with the corrosion rate slightly higher in sample group 5. The rate of corrosion of sample group 5 is significantly lower than those of the Hastelloy coating samples without flattened surfaces or Ni plating (sample groups 2–4). This indicates that the plating of Ni can considerably slow down the corrosion process of the thermal-sprayed coating in NaOH solution. 4. Conclusions Conventional Ni plating can successfully be used to seal the surface pores of an arc-sprayed coating. At the Ni-plating thickness of about 15–20 μm, the surface roughness of the plating is affected by the roughness of the underlying coating. Thus, some defects are still present on the outer surface. The surface pores on the non-plated coating and the high surface roughness of the plated and the non-plated coatings encourage corrosion attack. Grinding of the coating surface can remove the large surface pores to an extent, but some small pores are still present, which hinder the ability of the coating to form a protective scale. Electroplating of Ni on the ground coating can seal-off the pores and provide a smooth finish to the sample, which improves the ability to form a protective scale and, in turn, enhances the corrosion resistance of the Hastelloy coating. It is, therefore, promising that the double-layer coating system, consisting of electric arc-sprayed Hastelloy with surface grinding and Ni electroplating as a top coat, can be used to refurbish worn and damaged parts, where the thermal-sprayed coating is used to build up the surface volume required and Ni plating to improve its corrosion resistance.
The authors would like to thank the members of the FAMD unit at the National Metal and Materials Technology Center for the use of the electrochemical polarization equipment and Assistant Prof. Siriluck Nivitchanyong for consultation. References [1] K.J. McNaughton, and the staff of Chemical Engineering, Materials Engineering II, Controlling Corrosion in Process Equipment, McGrawHill Publications Company, New York, 1980. [2] T. Sundararajan, S. Kuroda, T. Itagaki, F. Abe, Proceedings of the 2003 ITSC, Florida, 2003, p. 495. [3] P. Siitonen, T. Konos, P.O. Kettunen, Proceedings of the 7th NTSC, Massachusetts, 1994, p. 105. [4] X. Huang, R. Wang, L. Xu, H. Luo, X. Yin, Proceedings of the 15th ITSC, France, 1998, p. 1013. [5] D. Toma, W. Brandl, G. Marginean, Surf. Coat. Technol. 138 (2001) 149. [6] R.C. Tucker Jr., ASM Handbook Volume 5 Surface Engineering: Thermal Spray Coatings, ASM International, Ohio, 1994, p. 497. [7] T. Tajiri, Z. Zeng, Proceedings of the 1st ITSC, Montreal, 2000, p. 697. [8] R. Rajendran, V.S. Raja, R. Sivakumar, R.S. Srinivasa, Surf. Coat. Technol. 73 (3) (1995) 198. [9] H.L. Tsai, P.C. Tsai, Surf. Coat. Technol. 71 (1) (1995) 53. [10] S. Ahmaniemi, P. Vuoristo, T. Mantyla, Surf. Coat. Technol. 151–152 (2002) 412. [11] J.R. Davis, Davis & Associates (Eds.), Corrosion Understanding the Basics, ASM International, Ohio, 2000, p. 453. [12] M.G. Fontana, Corrosion Engineering, International Edition, McGraw-Hill Book Company, 1987, p. 499. [13] D. Marijan, M. Vukovic, P. Pervan, M. Milun, Croat. Chem. Acta 72 (4) (1999) 737. [14] M. Pourbaix, Lectures on Electrochemical Corrosion, Plenum Press, New York, 1973. [15] M. Vukovic, Corros. Sci. 37 (1) (1995) 111. [16] N. Krstajic, M. Popovic, B. Grgur, M. Vojnovic, D. Sepa, J. Electroanal. Chem. 512 (2001) 16. [17] A. Seyeux, V. Maurice, L.H. Klein, P. Marcus, J. Solid State Electrochem. 9 (5) (2005) 337. [18] C. Silverman, Corrosion 37 (9) (1981) 546. [19] R.J. Smith, R.E. Hummel, J.R. Ambrose, Corros. Sci. 27 (8) (1987) 815. [20] J.R. Scully, D.W. Taylor, 9th edition, Corrosion: Metals Handbook, vol. 13, ASM International, Ohio, 1987, p. 212.