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Evaluation of the electrochemical behavior of HVOF-sprayed alloy coatings—II
Surface & Coatings Technology 192 (2005) 278 – 283 www.elsevier.com/locate/surfcoat
Evaluation of the electrochemical behavior of HVOF-sprayed alloy coatings—II Devicharan Chidambarama,*, Clive R. Claytona, Mitchell R. Dorfmanb a
Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275, United States b Sulzer-Metco (US), Inc., Westbury, NY 11590-0201, United States Received 19 November 2003; accepted in revised form 2 August 2004 Available online 25 September 2004
Abstract The electrochemical behavior of four different high velocity oxy fuel (HVOF) sprayed coatings has been studied. These chromium alloy coatings were intended at replacing stainless steel alloy coatings. Stainless steel coatings formed by thermal spray processes provide poor corrosion resistance. Corrosion behavior of the coatings was studied in 0.1 M HNO3 solution using gravimetric analysis, open circuit potential (OCP) measurements and potentiodynamic polarization. These results have been compared with those obtained from bulk AISI 316 stainless steel that acted as control. As per this study, sprayed nickel–chromium alloy coatings containing molybdenum appear to exhibit corrosion resistance comparable to bulk AISI 316. D 2004 Elsevier B.V. All rights reserved. Keywords: Corrosion; Immersion test; Thermal spray; HVOF; Chromium alloy
1. Introduction Thermal sprayed coatings are used for various applications including corrosion protection [1]. However, conventional methods of spraying result in highly porous coatings, thereby limiting the use in corrosive environments. This problem has been circumvented largely with the advent of high velocity oxy fuel spray (HVOF) process. Improvements in the process conditions include increase in velocity and use of inert gas shrouding. The inert, high kinetic energy process has led to the development of thick, highdensity coatings containing lower amounts of oxides. The improved process results in coatings with very low porosity. In the past decade, increased interest in surface engineering of systems using advanced thermal spray processes for applications in aggressive aqueous corrosive environments, has been observed [2,3]. With the advent of the HVOF spray process, thermal sprayed coatings (TSC), which had limited * Corresponding author. Tel.: +1 631 632 8513; fax: +1 631 632 8052. E-mail address: [email protected] (D. Chidambaram). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.072
usefulness as corrosion protection coatings due to the presence of interconnected porosity in the structure of the coatings, have now gained popularity and are being studied extensively for their marked corrosion resistance. Internal combustion jet generated by the latest HVOF guns result in gas velocities that exceed 2100 m/s (7000 ft/s) and consequent particle velocities of around 400 to 800 m/s (1320 to 2600 ft/s) as compared to a velocity of ~1360 m/s (4500 ft/ s) generated in older guns. This higher kinetic-energy process gives rise to coatings with greater densities, increased thickness, smoother surfaces and lower oxide levels. A description of these systems could be found elsewhere [4]. Siitonen et al. have studied the porosity and electrochemical behavior of thermal sprayed Ni alloy coatings [5] and stainless steel coatings [6]. Normand et al. [7,8] studies on the corrosion behavior of thermally sprayed Inconel coatings in acidic medium showed that corrosion resistance increased with homogeneity in microstructure. Harvey et al. [9,10] have evaluated the corrosion resistance of nickel alloy coatings (Hastelloy, 625, stainless steel) formed by Diamond Jet and HVOF spraying. Their results indicated that corrosion resistance increases with a decrease in oxide
D. Chidambaram et al. / Surface & Coatings Technology 192 (2005) 278–283
content of the coatings. Zhang et al. [11] have studied the effect of spraying parameters on the microstructure and resultant properties of Inconel 625 alloy coatings formed through HVOF spraying. Stainless steel coatings have been produced by jet vapor deposition [12,13], arc deposition and HVOF spraying [14,15]. The electrochemical behavior of thermal sprayed stainless steel coatings have also been studied in aqueous media [12,13,16–18]. HVOF spraying of self-fluxing nickel alloys, that were extensively studied by Knotek et al. [19,20], have also been undertaken [21]. Recently, Kawakita et al. [22] have shown that HVOF spraying of Hastelloy can be optimized to result in dense zero porosity coatings with only 0.2% oxide content. Simard and Arsenault [23] have studied the corrosion behavior of HVOF sprayed SS316 in simulated marine environment. Aalamialeagha et al. [24] have evaluated the corrosion behavior of Ni–20Cr HVOF sprayed coatings and correlated it to the microstructure. There are inherent problems in spraying stainless steel coatings, which has also been shown by our earlier work [25]. These studies were conducted to identify alternative coatings with corrosion characteristics similar or better than bulk SS316. In this study, we report the electrochemical behavior of four different HVOF sprayed Ni–Cr–Mo based alloy coatings. Details about the coatings examined in this work can be found in part I of this study [25]. The electrochemical behavior of these coatings have been compared against that of wrought SS316 in dilute nitric acid solution. Besides providing acidic conditions for testing, nitric acid is a preferred medium for evaluation of these coatings as stainless steels are extensively used in nuclear fuel processing plants, wherein they are routinely exposed to nitric acid [26–28].
2. Experimental Four different coatings sprayed using HVOF process on mild steel substrate were used. Stainless steel AISI 316 acted as control. The compositions of the coatings and their designations are given in Table 1. The two major elements Table 1 Chemical composition of the powder used for thermal spraying Sample no.
Material designation
Starting material
Composition (wt.%)
1
FeNi01
Fe-based super austenitic steel
2
CoCr01
Co–Cr–Mo–Ni–C
3
CrNi01
4
NiCr02
High chromium self-fluxing alloy Ni–Mo–Cr–Fe–W
5
wrought AISI 316
20 Ni; 19 Cr; 15 Mo; 6 Si; 3 W; 3 Cu; 2 B; 0.3 C; Fe-Balance 29Cr; 8.5Mo; 3Ni; 3Fe; 2C; 1.5Si, Co-Balance 39Ni; 3Mo; 1Si; 1B; Cr-Balance 16Cr; 15Mo; 6Fe; 3W; Ni-Balance 17Cr; 12Ni; 2.5Mo; 1Si; 0.08 C; Fe-Balance
Cr–Ni–Mo
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Table 2 Spray parameters used to obtain the coatings Carrier gas
Nitrogen
Pressure (MPa/psi) FMR Feed rate Spray distance
0.75/110 55 40 g/min 230 mm
Gases
Oxygen
Hydrogen
Nitrogen
Pressure (MPa/psi) FMR Flow (scfh)
1.16/170 22 327
0.96/140 62 1450
0.75/110 66 1250
in the alloy have been used for material designation, followed by a number if there are two or more alloys with the same two major alloying elements. The coatings were obtained using a Sulzer-Metco Diamond JetR 2600 Hybrid High Velocity Oxy-Fuel (HVOF) system. Spray parameters employed during the coating process are listed in Table 2. High purity powders (N99%) were used in the process to obtain 400–500 Am thick coatings on 2575 mm mild steel substrates, as well as 2-mm-thick free standing coatings. More information on the hardware and specific details about the process conditions can be obtained elsewhere [25]. The coupons were lapped using a 150-grit diamond wheel. These coupons were used for potentiodynamic and open circuit potential studies. Freestanding coatings were obtained for use in porosity measurements and gravimetric studies. The tests were conducted in 0.1 M nitric acid (pH~1.0) supplied by Fisher scientific. All samples were rinsed with deionized water and degreased with isopropanol prior to experimental use. Deionized water was used throughout this study. Thick free-standing coatings measuring ~2.5 cm2.5 cm2 mm in dimension, obtained by delamination from the surface, were used for gravimetric studies carried out in quiescent 0.1 M HNO3 (250 ml) in glass beakers. The immersion tests were carried out for a period of 8 days (~200 h). A corrosion flat cell, exposing 1-cm2 area of the electrode to the electrolyte, was used for all open circuit potential (OCP) measurements and potentiodynamic polarization. A platinum-coated niobium mesh was used as the counter electrode. All potentials were measured with respect to a saturated calomel electrode (SCE), which served as the reference electrode. The deaeration of the electrolyte achieved by vigorously bubbling high purity nitrogen was started 30 min prior to the experiment and continued until the completion of the experiment. Vigorous bubbling of the gas stirred the solution and hence the process was not under diffusion control. OCP was monitored for 500 s. Then the sample was potentiodynamically polarized at a scan rate of 1 mV/s from ~250 mV cathodic of OCP until either ~100 mA current or damage of the coating was achieved. Electrochemical measurements were performed using PC4/FAS1
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Table 3 Porosities of the coatings as measured by oil-impregnation method
Table 4 Gravimetric analyses experiment results
Sample no.
Coating
Density of coating (g/cm3)
Porosity of coating (%)
Sample no.
Specimen
1 2 3 4
FeNi01 CoCr01 CrNi01 NiCr02
7.2 7.6 7.0 8.6
3.2 2.3 3.6 0.8
Corrosion rate in 0.1 M HNO3 in Ampy (mpy)
1 2 3 4 Control
FeNi01 CoCr01 CrNi01 NiCr02 SS316
32.26 (1.27) 144.78 (5.70) 43.18 (1.70) 13.97 (0.55) 5.84 (0.23)
potentiostat supplied by Gamry Instruments. The DC 105 corrosion measurement software, also supplied by Gamry Instruments, was used to analyze the data and obtain the potentiodynamic polarization parameters. The corrosion current was calculated using the Stern–Geary equation [29]. If the potential (E), current (I), corrosion current (I corr) and corrosion potential (E corr) are known, then using the anodic tafel slope (b a=slope in the tafel region of the anodic polarization curve) and the cathodic tafel slope (b c=slope in the tafel region of the cathodic polarization curve), we can find the polarization resistance (R p) as follows [29]: Polarization resistance: Rp ¼ jdE=dIapp jEEcorr f0
in Table 3. Oil-impregnation method, which is based on Archimedes principle, is the standard industrial practice for determination of porosity of coatings. They range from 0.8% to 3.5% porosity. The low porosities indicate the spray parameters used to be optimum. The inverse correlation between the porosities and density is illustrated in Fig. 1. The porosities compare favorably to values obtained for HVOF sprayed coatings: 1.1% to 4.2% for stainless steel by Kawakita et al. [30], 1.6% austenitic steel by Zhao et al. [31], ~1.0% for SS316L by Zhao and Lugscheider [32].
ð1Þ
Corrosion current as given by Stern–Geary equation [29]: ð2Þ Icorr ¼ 1= 2:303 Rp ½ba bc =ðba þ bc Þ The terms dcurrentT and dcurrent densityT have been used interchangeably as the sample area was 1 cm2. Reproducibility of data was ensured by conducting quadruplicate tests. Details of the experimental procedures for all above measurements can be obtained from the first portion of this study [25].
3. Results and discussion Coatings were observed to be dull and rough when viewed using an optical microscope. Representative digimicrographs of the cross sections of coating were shown in the first part of this study [25]. The porosities and the densities of the four coatings as determined by oil impregnation tests are provided
4. Gravimetric analysis The average corrosion rates, as calculated from gravimetric tests conducted on duplicate samples, are provided in Table 4. The corrosion rates of the coatings range from as low as 13.97 Ampy (0.55 mpy) for NiCr02, to as high as 144.78 Ampy (5.70 mpy) for CoCr01. AISI 316 control sample has a corrosion rate of 5.84 Ampy (0.23 mpy). Therefore, NiCr02, which has the lowest corrosion rate amongst coatings evaluated in this study, is on par with bulk SS316. While CoCr01 has a corrosion rate over 20 times that of bulk SS316, the other two coatings (FeNi01 and CrNi01) with corrosion rates ~25–50 Ampy (1–2 mpy) offer good corrosion resistance.
5. Open circuit potential measurements and potentiodynamic polarization Table 5 lists the OCP after 500 s of exposure to the electrolyte and the results from potentiodynamic polarization. The OCP of the coatings vary between 230 to +127 mV vs. SCE. The listed OCP values (after 500 s) were all active Table 5 Open circuit potential measurements and potentiodynamic polarization results
Fig. 1. Graph showing the relationship between density and porosity of the coatings. In general, density is observed to be inversely proportional to porosity, except for CoMo01.
Sample no.
Specimen
OCP vs. SCE (mV)
E corr vs. SCE (mV)
I corr (A/cm2)
Rp (V cm2)
1 2 3 4 Control
FeNi01 CoCr01 CrNi01 NiCr02 SS316
127 230 40 68 30
3.5 235 131 75 8.5
4.34105 1.46105 2.20106 8.98107 9.18107
1.24103 2.48103 2.17104 3.18104 5.20104
D. Chidambaram et al. / Surface & Coatings Technology 192 (2005) 278–283
Fig. 2. Potentiodynamic polarization curves for the super austenitic steel and cobalt-based coatings in 0.1 M HNO3 acid solution.
compared to the potential observed on exposure (0 s) to electrolyte. Formation of a passive film could have been difficult in a deaerated electrolyte leading to the active behavior. Bulk SS316 exhibited an OCP of 30 mV vs. SCE and hence falls in the range of the OCP values of the coatings. The corrosion potential, E corr, is seen to fall in the range 235 to 3.5 mV vs. SCE. The Eobserved corr in nitric acid were all noble compared to those observed in hydrochloric acid and reported in part I of this study [25,33]. Polarization of metals and alloys in solutions containing nitrates, has been shown to result in the formation of surface nitrides [34–37]. We could speculate the formation of passive nitride films on polarizing the metallic coatings in the nitrate medium to be the reason for the observed ennoblement of corrosion potential. AISI 316 has a corrosion potential of 8.5 mV vs. SCE. Overall, the corrosion current density varies from 43.4 to 0.9 AA/cm2. Super austenitic SS alloy coating, FeNi01, exhibited the highest corrosion current density. CoCr01 followed FeNi01 with I corr of ~15 AA/cm2. Both alloys, NiCr02 and CrNi01, containing high amounts of Cr–Mo– Ni are observed to have corrosion currents similar to that of bulk SS316. In fact, NiCr02 has a slightly lower corrosion current than the control. The low corrosion current densities exhibited by these alloys, are likely attributed to the protective nature of chromium, molybdenum and nickel. It has been shown earlier that molybdenum and nitrogen lead to an increase in the corrosion resistance of steels [38]. All the systems studied here were found to be under anodic control as determined by their Tafel slopes. Given the experimental conditions (deaerated), this result is not surprising as inclusion of oxygen might have brought some of the systems under diffusion control. Polarization resistance (R p) values, calculated using the Stern–Geary equation [29], varied from 1.24103 V cm2 (FeNi01) to 3.18104 V cm2 (NiCr02). The polarization resistance value is considered to be directly proportional to the corrosion resistance of the material. SS316 exhibited an R p of 5.2104 V cm2, which is slightly higher than those
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of NiCr02 and CrNi01. Thus, these two alloy coatings can be considered to exhibit a corrosion resistance comparable to that of bulk SS316. Figs. 2 and 3 provide the anodic polarization plots for the control and four coatings. A minimum of 600-mV passivation was observed for all systems before transpassive breakdown. The corrosion rates from gravimetric studies and corrosion currents from polarization studies are plotted in Fig. 4. Except for CoCr1, it can be seen that these two curves have fairly similar trends for the most part. The super austenitic steel based alloy coatings exhibited relatively poor corrosion resistance in nitric acid, with a corrosion rate ~32 Ampy (1.26 mpy) in gravimetric test, corrosion current 43.4 AA/cm2 and exhibited a pseudopassivity with current density increasing at a slow rate with potential. However, the OCP of 127 mV vs. SCE was found to be the most noble of all systems evaluated in this study. The corrosion current is almost two orders of magnitude higher than that of the bulk SS316, thus agreeing with similar work performed on these materials in other media [33]. The high corrosion current could be due to the presence of high amount of iron. The polarization plot for cobalt based alloy coating, CoCr01, is illustrated in Fig. 2. While CoCr01 performed the worst in gravimetric tests, it fared better than FeNi01 in accelerated tests. It exhibited a corrosion rate of 144.78 Ampy (5.7 mpy) in immersion studies and a corrosion current of 14.6 AA/cm2 in polarization studies. While the coating exhibited passivity in the region 100 to 900 mV vs. SCE, it also had the highest current density during the passive state. CoCr01 also has the most active potential at 235 mV vs. SCE. The absence of localized pits after gravimetric experiments indicated the coatings had undergone uniform corrosion. However, qualitative analysis showed the preferential leaching or dissolution of cobalt [33]. Once again, this confirmed the homogeneity of the coating as shown in the earlier part of this study. The polarization behavior of the high chromium selffluxing alloy, CrNi01, is shown in Fig. 3. CrNi01 exhibited a
Fig. 3. Potentiodynamic polarization plots for the two high nickel containing alloy coatings and bulk AISI 316 (control) in 0.1 M HNO3 acid solution.
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Fig. 4. Graph of corrosion rate (in mpy) from gravimetric studies and corrosion current (in A/cm2) from polarization studies.
corrosion rate of 43.18 Ampy (1.7 mpy) in immersion studies and an I corr of 2.2 AA/cm2, passivity from 50 to 950 mV vs. SCE. Its electrochemical behavior was second only to NiCr02. The nickel-based alloys, NiCr02, exhibited a highly corrosion resistant behavior. Corrosion rate was determined to be 12.7 Ampy (0.5 mpy) in immersion studies and a corrosion current of 0.9 AA/cm2 was observed in polarization tests. Passivity was exhibited over a wide range of potential from 50 to 750 mV vs. SCE. The measured passive current of 10 AA/cm2 was also the lowest observed in this study. The corrosion current density and the passive current were both lower than that of the control. The OCP of 30 mV vs. SCE was slightly more active compared to SS316. The polarization resistances of both CrNi02 and NiCr01 coatings are similar to that of the control and are provided in Table 5. One of the reasons for high corrosion resistance of the NiCr02 as compared to CrNi01 could be the five-fold increase in the molybdenum content in NiCr02. Determination of the exact nature of the surface for understanding the lower corrosion rate requires extensive surface analyses and is work for the future. Evaluation of the control (SS316) shows a corrosion rate of 5.86 Ampy (0.231 mpy) in gravimetric tests and a corrosion current of 0.92 AA/cm2 in polarization test. SS316 exhibited passivity in the range 250 to 1400 mV vs. SCE. The OCP was measured to be 30 mV vs. SCE. Earlier we had shown that although nickel–chromium based alloys exhibited excellent corrosion resistances, nickel and chromium alone were not sufficient for synthesizing a coating to replace SS316 and other alloying elements, such as molybdenum, were needed [25]. This study clearly shows that the high Mo containing Ni alloy coating, NiCr02, has electrochemical behavior comparable to that of wrought SS316. The accelerated tests showed corrosion current lower than SS316, while the long-term tests showed a comparable corrosion rate.
6. Conclusions Four different HVOF sprayed alloy coatings were studied. The coatings had low porosity and the density was found to be inversely proportional to the porosity. Of the four coatings studied, the iron-based (super austenitic steel) and the cobalt-based coatings exhibited poor corrosion resistance in comparison to the high nickel–chromium based alloy coatings. The nickel-based high molybdenum containing alloy coating, NiCr02, offered superior corrosion resistance as compared to SS316 (control). The chromiumbased alloy, CrNi01, exhibits an electrochemical behavior second only to NiCr02. Most notably, the NiCr02 (Ni–Mo– Cr–Fe–W) coating on a steel substrate, containing 16% Cr, 15% Mo, 6% Fe, 3% W and 60% Ni, exhibited a corrosion resistance better than SS316 coating in 0.1 M nitric acid and could potentially be used as a replacement for stainless steel coatings in nitric acid medium for use in this medium.
Acknowledgements This work was supported in part by the Strategic Partnership for Industrial Resurgence (SPIR), a program of the New York state, under the contract number 431N107A. We thank Mr. Israel Aguilar of Sulzer-Metco (U.S.) for assistance in preparation of the samples and determination of porosity of the coatings.
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Evaluation of the electrochemical behavior of HVOF-sprayed alloy coatings—II Devicharan Chidambarama,*, Clive R. Claytona, Mitchell R. Dorfmanb a
Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275, United States b Sulzer-Metco (US), Inc., Westbury, NY 11590-0201, United States Received 19 November 2003; accepted in revised form 2 August 2004 Available online 25 September 2004
Abstract The electrochemical behavior of four different high velocity oxy fuel (HVOF) sprayed coatings has been studied. These chromium alloy coatings were intended at replacing stainless steel alloy coatings. Stainless steel coatings formed by thermal spray processes provide poor corrosion resistance. Corrosion behavior of the coatings was studied in 0.1 M HNO3 solution using gravimetric analysis, open circuit potential (OCP) measurements and potentiodynamic polarization. These results have been compared with those obtained from bulk AISI 316 stainless steel that acted as control. As per this study, sprayed nickel–chromium alloy coatings containing molybdenum appear to exhibit corrosion resistance comparable to bulk AISI 316. D 2004 Elsevier B.V. All rights reserved. Keywords: Corrosion; Immersion test; Thermal spray; HVOF; Chromium alloy
1. Introduction Thermal sprayed coatings are used for various applications including corrosion protection [1]. However, conventional methods of spraying result in highly porous coatings, thereby limiting the use in corrosive environments. This problem has been circumvented largely with the advent of high velocity oxy fuel spray (HVOF) process. Improvements in the process conditions include increase in velocity and use of inert gas shrouding. The inert, high kinetic energy process has led to the development of thick, highdensity coatings containing lower amounts of oxides. The improved process results in coatings with very low porosity. In the past decade, increased interest in surface engineering of systems using advanced thermal spray processes for applications in aggressive aqueous corrosive environments, has been observed [2,3]. With the advent of the HVOF spray process, thermal sprayed coatings (TSC), which had limited * Corresponding author. Tel.: +1 631 632 8513; fax: +1 631 632 8052. E-mail address: [email protected] (D. Chidambaram). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.072
usefulness as corrosion protection coatings due to the presence of interconnected porosity in the structure of the coatings, have now gained popularity and are being studied extensively for their marked corrosion resistance. Internal combustion jet generated by the latest HVOF guns result in gas velocities that exceed 2100 m/s (7000 ft/s) and consequent particle velocities of around 400 to 800 m/s (1320 to 2600 ft/s) as compared to a velocity of ~1360 m/s (4500 ft/ s) generated in older guns. This higher kinetic-energy process gives rise to coatings with greater densities, increased thickness, smoother surfaces and lower oxide levels. A description of these systems could be found elsewhere [4]. Siitonen et al. have studied the porosity and electrochemical behavior of thermal sprayed Ni alloy coatings [5] and stainless steel coatings [6]. Normand et al. [7,8] studies on the corrosion behavior of thermally sprayed Inconel coatings in acidic medium showed that corrosion resistance increased with homogeneity in microstructure. Harvey et al. [9,10] have evaluated the corrosion resistance of nickel alloy coatings (Hastelloy, 625, stainless steel) formed by Diamond Jet and HVOF spraying. Their results indicated that corrosion resistance increases with a decrease in oxide
D. Chidambaram et al. / Surface & Coatings Technology 192 (2005) 278–283
content of the coatings. Zhang et al. [11] have studied the effect of spraying parameters on the microstructure and resultant properties of Inconel 625 alloy coatings formed through HVOF spraying. Stainless steel coatings have been produced by jet vapor deposition [12,13], arc deposition and HVOF spraying [14,15]. The electrochemical behavior of thermal sprayed stainless steel coatings have also been studied in aqueous media [12,13,16–18]. HVOF spraying of self-fluxing nickel alloys, that were extensively studied by Knotek et al. [19,20], have also been undertaken [21]. Recently, Kawakita et al. [22] have shown that HVOF spraying of Hastelloy can be optimized to result in dense zero porosity coatings with only 0.2% oxide content. Simard and Arsenault [23] have studied the corrosion behavior of HVOF sprayed SS316 in simulated marine environment. Aalamialeagha et al. [24] have evaluated the corrosion behavior of Ni–20Cr HVOF sprayed coatings and correlated it to the microstructure. There are inherent problems in spraying stainless steel coatings, which has also been shown by our earlier work [25]. These studies were conducted to identify alternative coatings with corrosion characteristics similar or better than bulk SS316. In this study, we report the electrochemical behavior of four different HVOF sprayed Ni–Cr–Mo based alloy coatings. Details about the coatings examined in this work can be found in part I of this study [25]. The electrochemical behavior of these coatings have been compared against that of wrought SS316 in dilute nitric acid solution. Besides providing acidic conditions for testing, nitric acid is a preferred medium for evaluation of these coatings as stainless steels are extensively used in nuclear fuel processing plants, wherein they are routinely exposed to nitric acid [26–28].
2. Experimental Four different coatings sprayed using HVOF process on mild steel substrate were used. Stainless steel AISI 316 acted as control. The compositions of the coatings and their designations are given in Table 1. The two major elements Table 1 Chemical composition of the powder used for thermal spraying Sample no.
Material designation
Starting material
Composition (wt.%)
1
FeNi01
Fe-based super austenitic steel
2
CoCr01
Co–Cr–Mo–Ni–C
3
CrNi01
4
NiCr02
High chromium self-fluxing alloy Ni–Mo–Cr–Fe–W
5
wrought AISI 316
20 Ni; 19 Cr; 15 Mo; 6 Si; 3 W; 3 Cu; 2 B; 0.3 C; Fe-Balance 29Cr; 8.5Mo; 3Ni; 3Fe; 2C; 1.5Si, Co-Balance 39Ni; 3Mo; 1Si; 1B; Cr-Balance 16Cr; 15Mo; 6Fe; 3W; Ni-Balance 17Cr; 12Ni; 2.5Mo; 1Si; 0.08 C; Fe-Balance
Cr–Ni–Mo
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Table 2 Spray parameters used to obtain the coatings Carrier gas
Nitrogen
Pressure (MPa/psi) FMR Feed rate Spray distance
0.75/110 55 40 g/min 230 mm
Gases
Oxygen
Hydrogen
Nitrogen
Pressure (MPa/psi) FMR Flow (scfh)
1.16/170 22 327
0.96/140 62 1450
0.75/110 66 1250
in the alloy have been used for material designation, followed by a number if there are two or more alloys with the same two major alloying elements. The coatings were obtained using a Sulzer-Metco Diamond JetR 2600 Hybrid High Velocity Oxy-Fuel (HVOF) system. Spray parameters employed during the coating process are listed in Table 2. High purity powders (N99%) were used in the process to obtain 400–500 Am thick coatings on 2575 mm mild steel substrates, as well as 2-mm-thick free standing coatings. More information on the hardware and specific details about the process conditions can be obtained elsewhere [25]. The coupons were lapped using a 150-grit diamond wheel. These coupons were used for potentiodynamic and open circuit potential studies. Freestanding coatings were obtained for use in porosity measurements and gravimetric studies. The tests were conducted in 0.1 M nitric acid (pH~1.0) supplied by Fisher scientific. All samples were rinsed with deionized water and degreased with isopropanol prior to experimental use. Deionized water was used throughout this study. Thick free-standing coatings measuring ~2.5 cm2.5 cm2 mm in dimension, obtained by delamination from the surface, were used for gravimetric studies carried out in quiescent 0.1 M HNO3 (250 ml) in glass beakers. The immersion tests were carried out for a period of 8 days (~200 h). A corrosion flat cell, exposing 1-cm2 area of the electrode to the electrolyte, was used for all open circuit potential (OCP) measurements and potentiodynamic polarization. A platinum-coated niobium mesh was used as the counter electrode. All potentials were measured with respect to a saturated calomel electrode (SCE), which served as the reference electrode. The deaeration of the electrolyte achieved by vigorously bubbling high purity nitrogen was started 30 min prior to the experiment and continued until the completion of the experiment. Vigorous bubbling of the gas stirred the solution and hence the process was not under diffusion control. OCP was monitored for 500 s. Then the sample was potentiodynamically polarized at a scan rate of 1 mV/s from ~250 mV cathodic of OCP until either ~100 mA current or damage of the coating was achieved. Electrochemical measurements were performed using PC4/FAS1
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Table 3 Porosities of the coatings as measured by oil-impregnation method
Table 4 Gravimetric analyses experiment results
Sample no.
Coating
Density of coating (g/cm3)
Porosity of coating (%)
Sample no.
Specimen
1 2 3 4
FeNi01 CoCr01 CrNi01 NiCr02
7.2 7.6 7.0 8.6
3.2 2.3 3.6 0.8
Corrosion rate in 0.1 M HNO3 in Ampy (mpy)
1 2 3 4 Control
FeNi01 CoCr01 CrNi01 NiCr02 SS316
32.26 (1.27) 144.78 (5.70) 43.18 (1.70) 13.97 (0.55) 5.84 (0.23)
potentiostat supplied by Gamry Instruments. The DC 105 corrosion measurement software, also supplied by Gamry Instruments, was used to analyze the data and obtain the potentiodynamic polarization parameters. The corrosion current was calculated using the Stern–Geary equation [29]. If the potential (E), current (I), corrosion current (I corr) and corrosion potential (E corr) are known, then using the anodic tafel slope (b a=slope in the tafel region of the anodic polarization curve) and the cathodic tafel slope (b c=slope in the tafel region of the cathodic polarization curve), we can find the polarization resistance (R p) as follows [29]: Polarization resistance: Rp ¼ jdE=dIapp jEEcorr f0
in Table 3. Oil-impregnation method, which is based on Archimedes principle, is the standard industrial practice for determination of porosity of coatings. They range from 0.8% to 3.5% porosity. The low porosities indicate the spray parameters used to be optimum. The inverse correlation between the porosities and density is illustrated in Fig. 1. The porosities compare favorably to values obtained for HVOF sprayed coatings: 1.1% to 4.2% for stainless steel by Kawakita et al. [30], 1.6% austenitic steel by Zhao et al. [31], ~1.0% for SS316L by Zhao and Lugscheider [32].
ð1Þ
Corrosion current as given by Stern–Geary equation [29]: ð2Þ Icorr ¼ 1= 2:303 Rp ½ba bc =ðba þ bc Þ The terms dcurrentT and dcurrent densityT have been used interchangeably as the sample area was 1 cm2. Reproducibility of data was ensured by conducting quadruplicate tests. Details of the experimental procedures for all above measurements can be obtained from the first portion of this study [25].
3. Results and discussion Coatings were observed to be dull and rough when viewed using an optical microscope. Representative digimicrographs of the cross sections of coating were shown in the first part of this study [25]. The porosities and the densities of the four coatings as determined by oil impregnation tests are provided
4. Gravimetric analysis The average corrosion rates, as calculated from gravimetric tests conducted on duplicate samples, are provided in Table 4. The corrosion rates of the coatings range from as low as 13.97 Ampy (0.55 mpy) for NiCr02, to as high as 144.78 Ampy (5.70 mpy) for CoCr01. AISI 316 control sample has a corrosion rate of 5.84 Ampy (0.23 mpy). Therefore, NiCr02, which has the lowest corrosion rate amongst coatings evaluated in this study, is on par with bulk SS316. While CoCr01 has a corrosion rate over 20 times that of bulk SS316, the other two coatings (FeNi01 and CrNi01) with corrosion rates ~25–50 Ampy (1–2 mpy) offer good corrosion resistance.
5. Open circuit potential measurements and potentiodynamic polarization Table 5 lists the OCP after 500 s of exposure to the electrolyte and the results from potentiodynamic polarization. The OCP of the coatings vary between 230 to +127 mV vs. SCE. The listed OCP values (after 500 s) were all active Table 5 Open circuit potential measurements and potentiodynamic polarization results
Fig. 1. Graph showing the relationship between density and porosity of the coatings. In general, density is observed to be inversely proportional to porosity, except for CoMo01.
Sample no.
Specimen
OCP vs. SCE (mV)
E corr vs. SCE (mV)
I corr (A/cm2)
Rp (V cm2)
1 2 3 4 Control
FeNi01 CoCr01 CrNi01 NiCr02 SS316
127 230 40 68 30
3.5 235 131 75 8.5
4.34105 1.46105 2.20106 8.98107 9.18107
1.24103 2.48103 2.17104 3.18104 5.20104
D. Chidambaram et al. / Surface & Coatings Technology 192 (2005) 278–283
Fig. 2. Potentiodynamic polarization curves for the super austenitic steel and cobalt-based coatings in 0.1 M HNO3 acid solution.
compared to the potential observed on exposure (0 s) to electrolyte. Formation of a passive film could have been difficult in a deaerated electrolyte leading to the active behavior. Bulk SS316 exhibited an OCP of 30 mV vs. SCE and hence falls in the range of the OCP values of the coatings. The corrosion potential, E corr, is seen to fall in the range 235 to 3.5 mV vs. SCE. The Eobserved corr in nitric acid were all noble compared to those observed in hydrochloric acid and reported in part I of this study [25,33]. Polarization of metals and alloys in solutions containing nitrates, has been shown to result in the formation of surface nitrides [34–37]. We could speculate the formation of passive nitride films on polarizing the metallic coatings in the nitrate medium to be the reason for the observed ennoblement of corrosion potential. AISI 316 has a corrosion potential of 8.5 mV vs. SCE. Overall, the corrosion current density varies from 43.4 to 0.9 AA/cm2. Super austenitic SS alloy coating, FeNi01, exhibited the highest corrosion current density. CoCr01 followed FeNi01 with I corr of ~15 AA/cm2. Both alloys, NiCr02 and CrNi01, containing high amounts of Cr–Mo– Ni are observed to have corrosion currents similar to that of bulk SS316. In fact, NiCr02 has a slightly lower corrosion current than the control. The low corrosion current densities exhibited by these alloys, are likely attributed to the protective nature of chromium, molybdenum and nickel. It has been shown earlier that molybdenum and nitrogen lead to an increase in the corrosion resistance of steels [38]. All the systems studied here were found to be under anodic control as determined by their Tafel slopes. Given the experimental conditions (deaerated), this result is not surprising as inclusion of oxygen might have brought some of the systems under diffusion control. Polarization resistance (R p) values, calculated using the Stern–Geary equation [29], varied from 1.24103 V cm2 (FeNi01) to 3.18104 V cm2 (NiCr02). The polarization resistance value is considered to be directly proportional to the corrosion resistance of the material. SS316 exhibited an R p of 5.2104 V cm2, which is slightly higher than those
281
of NiCr02 and CrNi01. Thus, these two alloy coatings can be considered to exhibit a corrosion resistance comparable to that of bulk SS316. Figs. 2 and 3 provide the anodic polarization plots for the control and four coatings. A minimum of 600-mV passivation was observed for all systems before transpassive breakdown. The corrosion rates from gravimetric studies and corrosion currents from polarization studies are plotted in Fig. 4. Except for CoCr1, it can be seen that these two curves have fairly similar trends for the most part. The super austenitic steel based alloy coatings exhibited relatively poor corrosion resistance in nitric acid, with a corrosion rate ~32 Ampy (1.26 mpy) in gravimetric test, corrosion current 43.4 AA/cm2 and exhibited a pseudopassivity with current density increasing at a slow rate with potential. However, the OCP of 127 mV vs. SCE was found to be the most noble of all systems evaluated in this study. The corrosion current is almost two orders of magnitude higher than that of the bulk SS316, thus agreeing with similar work performed on these materials in other media [33]. The high corrosion current could be due to the presence of high amount of iron. The polarization plot for cobalt based alloy coating, CoCr01, is illustrated in Fig. 2. While CoCr01 performed the worst in gravimetric tests, it fared better than FeNi01 in accelerated tests. It exhibited a corrosion rate of 144.78 Ampy (5.7 mpy) in immersion studies and a corrosion current of 14.6 AA/cm2 in polarization studies. While the coating exhibited passivity in the region 100 to 900 mV vs. SCE, it also had the highest current density during the passive state. CoCr01 also has the most active potential at 235 mV vs. SCE. The absence of localized pits after gravimetric experiments indicated the coatings had undergone uniform corrosion. However, qualitative analysis showed the preferential leaching or dissolution of cobalt [33]. Once again, this confirmed the homogeneity of the coating as shown in the earlier part of this study. The polarization behavior of the high chromium selffluxing alloy, CrNi01, is shown in Fig. 3. CrNi01 exhibited a
Fig. 3. Potentiodynamic polarization plots for the two high nickel containing alloy coatings and bulk AISI 316 (control) in 0.1 M HNO3 acid solution.
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Fig. 4. Graph of corrosion rate (in mpy) from gravimetric studies and corrosion current (in A/cm2) from polarization studies.
corrosion rate of 43.18 Ampy (1.7 mpy) in immersion studies and an I corr of 2.2 AA/cm2, passivity from 50 to 950 mV vs. SCE. Its electrochemical behavior was second only to NiCr02. The nickel-based alloys, NiCr02, exhibited a highly corrosion resistant behavior. Corrosion rate was determined to be 12.7 Ampy (0.5 mpy) in immersion studies and a corrosion current of 0.9 AA/cm2 was observed in polarization tests. Passivity was exhibited over a wide range of potential from 50 to 750 mV vs. SCE. The measured passive current of 10 AA/cm2 was also the lowest observed in this study. The corrosion current density and the passive current were both lower than that of the control. The OCP of 30 mV vs. SCE was slightly more active compared to SS316. The polarization resistances of both CrNi02 and NiCr01 coatings are similar to that of the control and are provided in Table 5. One of the reasons for high corrosion resistance of the NiCr02 as compared to CrNi01 could be the five-fold increase in the molybdenum content in NiCr02. Determination of the exact nature of the surface for understanding the lower corrosion rate requires extensive surface analyses and is work for the future. Evaluation of the control (SS316) shows a corrosion rate of 5.86 Ampy (0.231 mpy) in gravimetric tests and a corrosion current of 0.92 AA/cm2 in polarization test. SS316 exhibited passivity in the range 250 to 1400 mV vs. SCE. The OCP was measured to be 30 mV vs. SCE. Earlier we had shown that although nickel–chromium based alloys exhibited excellent corrosion resistances, nickel and chromium alone were not sufficient for synthesizing a coating to replace SS316 and other alloying elements, such as molybdenum, were needed [25]. This study clearly shows that the high Mo containing Ni alloy coating, NiCr02, has electrochemical behavior comparable to that of wrought SS316. The accelerated tests showed corrosion current lower than SS316, while the long-term tests showed a comparable corrosion rate.
6. Conclusions Four different HVOF sprayed alloy coatings were studied. The coatings had low porosity and the density was found to be inversely proportional to the porosity. Of the four coatings studied, the iron-based (super austenitic steel) and the cobalt-based coatings exhibited poor corrosion resistance in comparison to the high nickel–chromium based alloy coatings. The nickel-based high molybdenum containing alloy coating, NiCr02, offered superior corrosion resistance as compared to SS316 (control). The chromiumbased alloy, CrNi01, exhibits an electrochemical behavior second only to NiCr02. Most notably, the NiCr02 (Ni–Mo– Cr–Fe–W) coating on a steel substrate, containing 16% Cr, 15% Mo, 6% Fe, 3% W and 60% Ni, exhibited a corrosion resistance better than SS316 coating in 0.1 M nitric acid and could potentially be used as a replacement for stainless steel coatings in nitric acid medium for use in this medium.
Acknowledgements This work was supported in part by the Strategic Partnership for Industrial Resurgence (SPIR), a program of the New York state, under the contract number 431N107A. We thank Mr. Israel Aguilar of Sulzer-Metco (U.S.) for assistance in preparation of the samples and determination of porosity of the coatings.
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