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Evaluation of the electrochemical behavior of HVOF-sprayed alloy coatings
Surface and Coatings Technology 176 (2004) 307–317
Evaluation of the electrochemical behavior of HVOF-sprayed alloy coatings 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, USA b Sulzer-Metco (US), Inc., Westbury, NY 11590-0201, USA Received 7 October 2002; accepted in revised form 27 May 2003
Abstract High-velocity oxy-fuel (HVOF) spraying is a versatile technique that can yield high-density coatings with porosities less than 1% by optimization of the process variables. Oxidation of the coating material can be greatly reduced by inert gas shrouding during the spraying process. This work evaluates the electrochemical behavior of HVOF-sprayed coatings aimed to have better or similar corrosion behavior as stainless steel AISI 316 in acidic medium. A total of eight different chromium-containing coatings with varying proportions of alloying elements such as Ni, Mo, Si, Fe, Co, W, B and C have been studied in hydrochloric acid medium. Porosity measurements, gravimetric analysis, open-circuit potential measurements, potentiodynamic polarization and optical microscopy have been performed. The electrochemical behavior of each coating has been compared with that of bulk AISI 316. The results indicate that HVOF-sprayed AISI 316 coating offers a lower corrosion resistance compared to bulk AISI 316. High nickel- and chromium-containing coatings appear to offer a corrosion resistance comparable to bulk AISI 316. However, results indicate that other alloying elements like molybdenum might be essential for obtaining higher corrosion protection. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Corrosion; Immersion test; Thermal spray; HVOF; Chromium alloy
1. Introduction In the past decade, increased interests in surface engineering of systems using advanced thermal-spray processes for applications in aggressive aqueous corrosive environments have been observed w1,2x. With the advent of the high-velocity oxy-fuel (HVOF) spray process, thermal-sprayed coatings (TSCs), which had limited 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 corrosion-resistant properties. TSCs are widely used for providing wear resistance. However, many of these coatings have to function in an aggressive environment and hence, their corrosion behavior becomes important. For reclaiming a wide range of petrochemical-process components such as *Corresponding author. Tel.: q1-631-632-9272; fax: q1-631-6328052. E-mail address: [email protected] (C.R. Clayton).
storage vessels, heat exchangers, pipe end fittings and valves, which are subjected to severe erosive wear and corrosive conditions, Amoco Oil Company routinely employs the HVOF process to apply AISI 316L and Hastalloy C-276 coatings w1x. Neville and Hodgkiess have studied various TSCs for aqueous corrosion and wear properties w3x. The influence and role of galvanic interactions in the corrosion behavior of cermet coatings such as WC–Co–Cr and WC– Ni–Cr, metallic coatings such as Inconel 625 (Cr–Mo– Si–Ni) and Ni–Cr–C–B coatings on mild steel substrates have been studied in simulated seawater w4,5x. Siitonen et al. have investigated the corrosion behavior of various coatings w6–8x. In one of the studies w6x corrosion resistance and connected porosity of alumina and metallic nickel-base alloy were studied. Another study w8x by the same authors have also shown that the corrosion resistance of these coatings cannot be improved by any post treatments, however, shrouding of the plasma jet with an inert gas seemed to reduce the amount of oxide in the coating. Stainless steels have
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972Ž03.00809-0
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been arc-deposited and HVOF-sprayed earlier w9,10x. The electrochemical behavior of wrought stainless steels has been compared with their thermal-sprayed counterparts w11,12x. Corrosion resistance of jet-vapor deposited stainless steel coatings has been studied in hydrochloric acid medium w13,14x. Ni superalloy (Anval 625 and Anval 718) coatings have been HVOF-sprayed and studied w15x. HVOFsprayed Ni–W–Cr–B–Si coatings have been studied for various properties including corrosion resistance in NaCl solution w16,17x. Self-fluxing nickel-based alloys that were extensively studied by Knotek et al. w18–21x have been applied by HVOF spraying w22x. Corrosion behavior of the Ni-based Inconel 625 alloy coatings sprayed using HVOF method has been studied w23x. Metallic coatings with alloying elements similar to ones studied here have been characterized w24,25x and their performance evaluated w26,27x. Effects of heat treatment on HVOF-sprayed Ni–Cr–W–Mo–B coatings have been studied in sulfuric acid w28x. The newest HVOF guns generate an internal combustion jet with gas velocities that can exceed 2100 mys (7000 feetys), whereas the older systems could only approach 1360 mys (4500 feetys). The high gas velocities are capable of producing particle velocities of approximately 400–800 mys (1320–2600 feetys), which are significantly higher than low-velocity combustion processes w29x. However, combustion temperatures are similar to those of low-velocity processes; combustion fuels include propylene, propane, natural gas, hydrogen, acetylene and kerosene. The major advantage of this higher kinetic-energy process is that coatings with greater density are possible. Other benefits include increased thickness capability, smoother surface finishes, lower oxide levels and less effect of the environment (reduced decarbonization, oxidation and loss of key elements by vaporization) during the spray process. HVOF processes are suitable not only for applying tungsten carbide–cobalt and nickel chromium–carbide systems, but also for depositing wear- and corrosion-resistant alloys such as Inconel (NiCrFe), Triballoy (CoMoCr) and Hastalloy (NiCrMo) materials. HVOF-sprayed MCrAlY coatings are also replacing some low-pressure plasma-sprayed coatings for high temperature oxidationyhot corrosion and TBC bond-coat applications for repair and restoration of existing components. Low melting-point ceramics such as alumina and alumina–titania are also applied via some HVOF processes for abrasive wear and dielectric applications. HVOF systems operate by injecting powder into a stream of burning gases. The material melts or softens, and is propelled against the substrate. A characteristic of most HVOF guns is the multiple shock-diamond pattern that is visible in the flame. Upon exiting the nozzle, compressed flame gases undergo free expansion,
thereby generating a supersonic jet. The high gas velocity results in high sound levels of over 120 dB, which implies that proper soundproofing and ear protection are needed. The benefits of HVOF technology are many. Applications that had shown only marginal benefits from TSCs years ago, are now achieving success through HVOF technology. One such application is in the replacement of hard chrome plating. Environmental restrictions and the cost of waste disposal of chromium have made HVOF technology more attractive. Today, HVOF materials are being applied to hydraulic rods, landing gears and the internal diameter of large bore cylinders as hard chrome replacements. Boeing has approved HVOF spraying of carbide materials on the landing gears of 737 and 757 commercial airliners. In this work, we evaluate the electrochemical behavior of eight different HVOF-sprayed Cr-based coatings with varying proportions of alloying elements such as Ni, Mo, Si, Fe, Co, W, B and C. The coatings have been subjected to porosity measurements, gravimetric analysis, open-circuit potential (OCP) measurements, potentiodynamic polarization and optical microscopy and these results have been compared with that of bulk AISI 316. 2. Experimental 2.1. Materials used The materials used in this research consist of eight different TSCs on mild steel substrate and stainless steel AISI 316 as a control. The compositions of the coatings and their designations are given in Table 1. The two major elements 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. It should be noted that of the coatings tested were— AISI 316 type (FeCr01), Hastalloy C-22 type (NiCr03), Inconel 718 type (NiCr01), high chromium self-fluxing alloy and cobalt-based alloy coatings. The coatings were obtained using a Sulzer-Metco Diamond Jet䉸 2600 Hybrid HVOF system. The DJ2600 hardware was used to spray coatings using hydrogen as the fuel gas. Fuel gases are mixed in a proprietary siphon system in the front portion of the gun. Mixed gases are ejected from the nozzle and are ignited external to the gun. Ignited gases form a circular flame configuration, which surrounds the axially injected powder. The combustion gases, along with the heated powder, are accelerated further through the converging–diverging nozzle to hypersonic velocities. In contrast to most other guns, that use water as the only cooling medium, in this process air is used to cool the combustion portion of the air cap, while water is used to cool the rest of the gun. This greatly reduces the amount of water-
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cooling required, and increases the overall thermal efficiency of the gun. The equipment was specifically designed to allow high particle temperatures and velocities. Table 2 lists the spray parameters used to obtain various coatings. Nitrogen was used as a cooling gas instead of air to minimize the oxidation of the powder particles during the spray process. The powders used were (99% or more) pure. The coatings were sprayed on to mild steel coupons of dimension 25=75 mm2 (1 inch=3 inch) to obtain 400–500 mm thick coatings. The coupons were then cut into sections of 25=37 mm2 and lapped using a 150grit diamond wheel. These coupons were used for potentiodynamic and OCP studies. Freestanding coatings were obtained for use in porosity measurements and gravimetric studies. The tests were conducted in 0.1 M hydrochloric acid supplied by Fisher scientific. The hydrochloric acid was diluted to 0.1 M using de-ionized water. The pH of the acid was approximately 0.7, as measured using a Agy AgCl Sure-Flow䉸 electrode (model 9165BN) interfaced with a bench top pHyISE meter (model 410A), both supplied by Orion Research Inc. 2.2. Porosity measurements The porosities of the coatings were determined by the oil-impregnation method, which is the standard practice of measuring porosity in industry. This technique is based upon the Archimedes principle. The weight of a dry freestanding coating is noted (Cdry). The coating is then impregnated with oil. The weight of the coating is noted again (Coil). The coating is then immersed in water and weighed (Cwater). With these measured values the porosity (connected to surfaceyinterface) of the coating can be determined by a simple calculation procedure, assuming that the density of oil (roil) and the density of water (rwater) is known, as given below: Weight of oilsCoilICdry Volume of oils(CoilyCdry)yroilsVo Volume of coatings(CoilyCwater)yrwatersVc Density of coatingsCdry yVc Porosity (%)s(Vo yVc)=100
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Table 2 Spray parameters used to obtain the coatings Carrier gas Pressure (MPaypsi) FMR Feed rate Spray distance
Nitrogen 1.03y110 55 40 gymin 230 mm
Gases
Oxygen
Hydrogen
Nitrogen
Pressure (MPaypsi) FMR Flow (scfh)
1.16y170 22 327
0.96y140 62 1450
0.75y110 66 1250
2.3. Optical microscopy Coatings with the substrate were mechanically polished across the cross-section. Samples were prepared as recommended by ASTM E 1920–97, ‘Standard Guide for Metallographic Preparation of Thermal-Sprayed Coatings’. These were then analyzed metallographically under an optical microscope. The optical microscope supplied by MEIJI was interfaced with a Nikon digital camera (model DXM 1200) capable of a resolution of 3840=3072 pixels (12-million pixel resolution). All images (digimicrographs) were taken with maximum resolution and at high sensitivity. 2.4. Electrochemical studies 2.4.1. Gravimetric studies Thick freestanding coatings (f2 mm in thickness), which were delaminated from the surface, were used for gravimetric studies. These ‘as-sprayed’ samples were degreased using isopropanol and dried. The samples were then weighed using an analytical balance (model AB104) with a readability of 0.1 mg supplied by Mettler Toledo Inc. Coatings of dimensions 2.5=2.5 cm2 were then immersed in 250 ml of the 0.1 M HCl in glass beakers. These electrolytes were open to atmosphere. The changes in the appearance of the solution and the coating were monitored every day for 8 days (f200 h). The volume of the solution was kept constant by adding de-ionized water to account for the loss of water through evaporation. At the end of the period the
Table 1 Chemical composition of the powder used for thermal spraying S. No.
Material designation
Starting material
Composition (%)
1 2 3 4 5 6 7 8 9
CoCr01 CrNi01 NiCr01 CoMo01 FeCr01 NiCr03 CoCr02 CrNi02 Wrought AISI 316
Co–Cr–Mo–Ni–C High chromium self-fluxing alloy Inconel 718 type Co–Mo–T-800 type SS316 Hastalloy C-22 type Co–Cr–C–W–C Ni–50Cr Bulk material
29Cr; 8.5Mo; 3Ni; 3Fe; 2C; 1.5Si, Co-balance 39Ni; 3Mo; 1Si; 1B; Cr-balance 21Cr; 8Mo; 3Fe; 0.5C; Ni-balance 28Mo; 17Cr; 3Si; Co-balance 17Cr; 12Ni; 2.5Mo; 1Si; 0.1C; Fe-balance 21Cr; 14Mo; 6Fe; 3W; Ni-balance 25Cr; 10Ni; 7W; 0.5C; Co-balance 50Cr; 50Ni 17Cr; 12Ni; 2.5Mo; 1Si; 0.08C; Fe-balance
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coatings were removed and rinsed thoroughly and carefully with de-ionized water so as not to remove any layer formed on the surface of the coating. The coatings were then dried thoroughly and weighed, at constant intervals for a period of 24 h to check for the constancy of their weight. This was necessary as these coatings are porous and hence may have adsorbed water. Using the weight change in gravimetric testing (W), exposed area of the sample (A), density of coating (D) and time of gravimetric analysis or immersion (t), we calculate the corrosion rate using the following equation in ASTM G31: Corrosion rates
KW DAt
where K is a constant. To obtain the corrosion rate in mils per year (mpy), Ks3.45=106 with W in mg, D in gycm3, A in cm2 and t in h. A few coatings were observed using an optical microscope after exposure to the electrolytes for 200 h and compared. Some coatings on which pits had developed were also observed under the optical microscope. 2.5. Electrochemical cell and instrumentation A corrosion flat cell supplied by Princeton Applied Research (K0235) was used for all OCP measurements and potentiodynamic polarization. One-square-centimeter area of the electrode was exposed to the solution. A platinum-coated niobium-mesh was used as the counter electrode. All potentials were measured with respect to a saturated calomel electrode (SCE), which was used as the reference electrode. Electrochemical polarizations and measurements were performed using PC3 potentiostatygalvanostatyZRA and PC4yFAS1 model potentiostats supplied by Gamry Instruments Inc., which have maximum sensitivities up to 0.01 fA and down to 1 mV resolution. The DC corrosion techniques software, DC105, supplied by Gamry Inc. was used to control the potentiostats. The Gamry potentiostats and their controlling software use control loop algorithms to accurately measure and correct for uncompensated resistance thus eliminating the manipulation of the supporting electrolyte or the cell geometry. 2.6. OCP and potentiodynamic polarization The coatings bonded to the substrate were used for both the OCP and polarization experiments. The polished coatings were rinsed with de-ionized water and degreased using isopropanol. The electrolyte was deaerated for 30 min prior to the experiments by vigorously bubbling high purity nitrogen. The sample was exposed to the electrolyte and the OCP was monitored for 500 s. Then, the sample was potentiodynamically polarized
Table 3 Porosities of the coatings as measured by oil-impregnation method S. No.
Coating
Density of coating (gycm3)
Porosity of coating (%)
1 2 3 4 5 6 7 8
CoCr01 CrNi01 NiCr01 CoMo01 FeCr01 NiCr03 CoCr02 CrNi02
7.6059 6.9816 8.077 7.982 7.610 8.217 7.962 7.2006
2.263 3.536 0.452 2.906 2.098 1.023 1.279 2.62
at a scan rate of 1 mVys from approximately 250 mV below OCP to ;100 mA current or damage of the coating, whichever was earlier. The electrolyte was deaerated throughout the experiment by constant bubbling of high purity nitrogen. The vigorous bubbling of the gas stirred the solution and hence the process was not under diffusion control. All electrochemical tests were repeated to assure the reproducibility of the data. The potentiodynamic polarization parameters were obtained by analyzing the data using the DC105 corrosion measurement software. The corrosion current was calculated using the Stern–Geary equation. If the potential (E), current (I), corrosion current (Icorr) and corrosion potential (Ecorr) are known, then using the anodic tafel slope (basslope in the tafel region of the anodic polarization curve) and the cathodic tafel slope (bcs slope in the tafel region of the cathodic polarization curve), we can find the polarization resistance (Rp) as follows w30x: Polarization resistance: RpsZdEydIappZEyEcorr;0 Corrosion current (Stern–Geary equation w30x): z Icorrswy1yŽ2.303Rp.zw ~ybabcyŽbaqbc.~ x
|x
|
3. Results and discussion 3.1. Porosity measurements The porosities and the densities of the eight coatings as determined by impregnation tests are provided in Table 3. They range from 0.452 to 3.536%. These low values are a result of HVOF spraying, as mentioned earlier. NiCr01 (21Cr; 8Mo; 3Fe; 0.5C; Ni-bal) is the least porous at 0.452%. In general, we observe the density to be higher when the coating has a lower porosity and vice versa. This is illustrated in Fig. 1. However, there is a discrepancy in the case of CoMo01, which could be due to the variations in alloying elements and their molecular masses. The porosities ranged from
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Fig. 1. Graph showing the relationship between density and reciprocal of porosity of the coatings. In general, density is observed to be inversely proportional to porosity, except for CoMo01.
3.6 to 6.2% w16x and 2.68% w31x for NiWCrBSi coatings, ;2.0% for NiCrWMoB coatings w28x, ;0.7% for Ni–Cr alloy coating w32x and ;1.0% for stainless steel w10x all formed using HVOF spraying. The coatings thus having similar or lower porosities comparatively, confirm that the spray parameters used were optimum. Digimicrographs showed that the surface of all the coatings was dull and rough. Representative digimicrographs of the cross-sections of a coating are shown in Fig. 2. In general, these coatings had fewer voids than observed normally in coatings sprayed through alternative processes. There were relatively more voids towards the surface of the coating than at the coating–substrate interface. This is due to the repeated impingement experienced by the inner layers. Porosity determined from micrographs of these coatings indicated that these coatings had lower than 5% porosities, in agreement with the oil-impregnation method. 3.2. Gravimetric analysis The results of the gravimetric studies are provided in Table 4. These are the average corrosion rates as calculated from gravimetric tests conducted on duplicate specimens. Gravimetric measurements indicate extremely low corrosion rates with just three systems having a corrosion rate of more than 3 mpy. The corrosion rate ranges from 1.11 to 20.44 mpy, with the majority of the systems exhibiting a rate in the range of 1–2 mpy. AISI 316 control sample has a corrosion rate of 0.2 mpy.
These results are in agreement with other work done on similar system w33,34x. Therefore the least corroding coating is approximately five times more corroding than bulk AISI 316. Still, an absolute corrosion rate of approximately 1–2 mpy is considered to be extremely corrosion resistant. 3.3. OCP polarization
measurements
and
potentiodynamic
The results of the OCP and potentiodynamic polarization are provided in Table 5. The OCP of the systems can be seen to vary between y325 and y55 mV vs. SCE. These are the OCP after 500 s of exposure to the electrolyte. In all cases the potential after 500 s was more active than immediately after immersion. This is expected as the electrolyte was deaerated and thus the formation of a passive film is difficult. In comparison, bulk AISI 316 has an OCP of y125 mV, which falls in the range of the OCP values of the coatings. OCP values also agree with reported values w34x. The corrosion potential, Ec, is seen to fall in the range y335 to y164 mV. In all cases the corrosion potential of a coating was observed to be more noble in nitric acid compared to hydrochloric acid w35,36x. This might be due to formation of passive nitride films on the metallic electrodes when polarized in the nitrate medium. AISI 316 has a corrosion potential of y284 mV. CrNi01, CrNi02, NiCr01 and CoMo01 coatings all have a more noble corrosion potential than AISI 316.
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Fig. 2. Digimicrographs of the cross-section of the coatings (a) NiCr01 and (b) NiCr03. The coating surface is on the right side of the image (a) and the coating–substrate interface can be easily seen in both digimicrographs. The digimicrographs show that these HVOF-sprayed coatings have very low porosity in comparison with coatings formed using alternative spray processes. This gives rise to the high densities observed in these coatings. Table 4 Gravimetric analyses experiment results S. No.
Specimen
Corrosion rate in 0.1 M HCl (mpy)
1 2 3 4 5 6 7 8 Control
CoCr01 CrNi01 NiCr01 CoMo01 FeCr01 NiCr03 CoCr02 CrNi02 SS316
2.211 2.078 1.105 1.142 20.437 1.616 9.734 3.255 0.200
The corrosion current density varies from 44 to 0.27 mAycm2. The highest corrosion current density is observed for the HVOF-sprayed AISI 316 alloy, namely,
FeCr01. Both the alloys, NiCr01 and CrNi01 containing high amounts of Cr–Mo–Ni, are observed to have very low corrosion current densities and hence a high resistance to corrosion. In fact, NiCr01 (0.27 mAycm2) has a lower corrosion current than the control AISI 316 (0.53 mAycm2). Once again, this could be explained to be due to the protective nature of chromium, molybdenum and nickel. Addition of molybdenum and nitrogen has shown to increase the corrosion resistance of steels w37x. Therefore, the low corrosion current densities obtained for these coatings, which are similar to the stainless steels, is not surprising. The anodic tafel slope, ba, of all the systems (except for CrNi02) was determined to be higher than the cathodic tafel slope, bc. These systems are hence under anodic control. This is expected as they were carried
Table 5 OCP measurements and potentiodynamic polarization results S. No.
Specimen
OCP vs. SCE (mV)
Ecorr vs. SCE (mV)
Icorr (Aycm2)
Rp (V cm2)
1 2 3 4 5 6 7 8 Control
CoCr01 CrNi01 NiCr01 CoMo01 FeCr01 NiCr03 CoCr02 CrNi02 SS316
y290 y80 y150 y55 y325 y170 y290 y100 y125
y335 y180 y270 y164 y341 y327 y317 y174 y284
1.02Ey05 9.10Ey07 2.65Ey07 1.98Ey05 4.36Ey05 4.62Ey06 9.41Ey06 1.79Ey06 5.29Ey07
3.11E03 3.90E04 3.15E04 1.85E03 9.76E02 1.74E03 2.49E03 1.33E04 5.93E04
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Fig. 3. Potentiodynamic polarization curves for the three cobalt-based coatings in 0.1 M HCl acid solution.
out under the exclusion of oxygen, inclusion of which in some cases could have brought the system under mass transfer control or diffusion control. Polarization resistance (Rp ) values as calculated using the Stern–Geary equation w30x varies from 9.76E02 V cm2 (FeCr01) to 3.9E05 V cm2 (CrNi01). The polarization resistance can be considered as an indicator of the corrosion resistance of the material, with a higher value denoting a highly corrosion-resistant material. AISI 316 has Rp of 5.93E05 V cm2, which is slightly higher than the most corrosion-resistant coating. However, given that these coatings have values close to bulk AISI 316, the coating performance can be considered to be excellent. The passivation characteristics of the coatings were deduced from the anodic polarization plots. Figs. 3–5 provide these plots for the eight coatings and bulk AISI 316 alloy. All systems underwent passivation over a range of at least 600 mV before transpassive breakdown, except FeCr01, which was passive for only approximately 50 mV, and breakdown occurred due to pitting. The pitting could have been due to chloride ions. The amount of pitting corrosion can only be evaluated by other studies. The corrosion rates from gravimetric studies and corrosion currents from polarization studies are plotted in Fig. 6. It can be seen that these two curves have similar trends. The cobalt-based alloys, namely, CoCr01, CoCr02 and CoMo01, exhibited similar corrosion characteristics as illustrated in Fig. 3. CoCr01 performed fairly well in
both the tests. It exhibited a moderate corrosion rate of 2.2 mpy in immersion studies and a corrosion current of 10.23 mAycm2 in polarization studies, exhibited passivity in the region 100–950 mV vs. SCE. CoCr02 is also a moderately resistant system as CoCr01. During weight loss it underwent corrosion at the rate of 9.734 mpy, and during polarization had a corrosion current of 9.4 mAycm2, the passive region extended from y100 to 900 mV with a moderately high current of 1000– 1500 mAycm2. OCP is also in the active range at y275 mV vs. SCE. The digimicrograph, shown in Fig. 7, indicates uniform corrosion. However, the preferential leaching or dissolution of cobalt as determined by the qualitative analysis w38x indicates a homogeneous coating. CoMo01 also exhibited a moderately good corrosion behavior as CoCr01. The corrosion rate was 8 mpy as determined by gravimetric studies and corrosion current of 19.8 mAycm2 from polarization studies. It was also passive in the range 175–900 mV vs. SCE. However, CoMo01 had a more noble OCP (y55 mV vs. SCE) as compared to CoCr01 (y290 mV vs. SCE). The iron-based alloy coating based on AISI 316, FeCr01, exhibited the least corrosion resistance in this medium. The corrosion rate was approximately 20 mpy in gravimetric tests, the corrosion current obtained from polarization was 43.6 mAycm2, passivity was exhibited only over a very narrow range of 50 mV and the OCP was comparatively active at y325 mV vs. SCE. The OCP and the corrosion current values obtained here are in agreement with values obtained by Simard et al. w34x. The corrosion current is almost two orders of magnitude
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Fig. 4. Potentiodynamic polarization curves for the two nickel-based coatings and AISI 316 coating in 0.1 M HCl acid solution.
more than that of the bulk AISI 316, thus agreeing with similar work performed on these materials in H2SO4 medium w12x. Moreover, extensive amounts of iron were observed by qualitative analysis after gravimetric tests w38x. The digimicrographs shown in Fig. 8 indicate extensive corrosion damage by pitting. There were numerous pits seen on the coatings surface. It should be noted that this coating was sprayed using a powder composition similar to that of AISI 316. This appears to have the worst corrosion performance when compared to AIS 316 bulk material. There are inherent problems in spraying stainless steel coatings and hence these studies were conducted to identify coatings with corrosion characteristics similar or better than AISI 316. The electrochemical behavior of FeCr01 justifies this work. Both chromium-based alloys, namely, CrNi01 and CrNi02, have similar corrosion characteristics. CrNi01 exhibited a desirable corrosion behavior with low corrosion rate of 2 mpy in immersion studies and a low corrosion current of 0.9 mAycm2, passivity from 25 to 925 mV vs. SCE and a passive current of 100–200 mAycm2. Moreover, the OCP was comparatively noble at y85 mV vs. SCE. CrNi02 also appears to offer a good corrosion resistance with low corrosion rate of 3.2 mpy and a corrosion current of 1.7 mAycm2 in polarization studies. It also exhibits passivity in the region of 100–900 mV with current in the range of 50–250 mAy cm2. The OCP is on noble side at y100 mV vs. SCE. Nickel analyzed by qualitative analysis could have been due to normal uniform corrosion. The nickel-based alloys, NiCr01 and NiCr03, have similar and highly corrosion-resistant behavior. NiCr01
exhibited excellent corrosion resistance with corrosion rate of 1.1 mpy in immersion studies and a corrosion current of 0.26 mAycm2. Passivity was exhibited over a wide range of potential from y250 to 800 mV vs. SCE with the passive current being as low as 6 mAycm2. The OCP was also towards a noble end at y90 mV vs. SCE. NiCr03 also exhibited good corrosion resistance in this medium with a corrosion rate of 1.6 mpy as obtained from gravimetric studies and a corrosion current of 4.6 mAycm2. The coating was also passive over a wide range of potentials (y325 to q850 mV vs. SCE) with very low passivation currents of 6–50 mAy cm2. The OCP was approximately y150 mV vs. SCE. In comparison, AISI 316 had a corrosion rate of 0.2 mpy in gravimetric tests and a corrosion current of 0.53 mAycm2, exhibited passivity in the range y300 to 200 mV vs. SCE. It exhibited an OCP of y125 mV. The results are in agreement with values obtained earlier w34x. Hence, based on the above discussion, NiCr01, with a corrosion current lower than that of AISI 316 and a comparable corrosion rate from immersion studies appears to offer the best corrosion resistance in 0.1 M HCl amongst all the coatings, followed by NiCr03 and CrNi01. All these coatings also have a comparable polarization resistance as indicated in Table 5. 4. Conclusions The HVOF-sprayed alloy coatings were deposited using optimal spray parameters and thus have very low porosities. The porosities of these coatings are in general
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315
Fig. 5. Potentiodynamic polarization curves for the two chromium-based coatings and bulk AISI 316 in 0.1 M HCl acid solution.
inversely proportional to their densities. Based on the performance of the FeCr01 coating, we can understand that TSCs with a same starting powder composition as AISI 316 exhibit poor corrosion resistance. Hence, there is a need to develop and evaluate coatings offering similar or better performance than AISI 316. Of the eight different coatings studied, the high nickel- and chromium-containing coatings were observed to offer
good corrosion resistance in 0.1 M HCl. Also it is noted that chromium and nickel alone do not impart the corrosion resistance as evident from the behavior of CrNi02, which is a 50% Cr–50% Ni coating. Alloying elements like Mo are also essential for good corrosion resistance. Based on these studies we can conclude that the nickel- and chrome-based alloys tested here exhibit a corrosion behavior better than or comparable to that
Fig. 6. Graph of corrosion rates (in mpy) from gravimetric studies and corrosion currents (in Aycm2) from polarization studies.
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Fig. 7. Digimicrograph of CoCr02 coating after 200 h of exposure to 0.1 M hydrochloric acid solution (gravimetric analysis). Absence of pits and a uniform degradation indicate uniform corrosion. Line marker indicates 100 mm in each case.
Fig. 8. Digimicrograph of FeCr01 coating after 200 h of exposure to 0.1 M hydrochloric acid solution (gravimetric analysis). The coating had undergone extensive pitting, as can be seen, characteristic of stainless steels exposed to chloride media. Well-developed pits over 100 mm in size are observed. Line marker indicates 100 mm in each case.
of AISI 316. Therefore, alloys with high levels of Ni– Cr can be said to be highly corrosion-resistant to hydrochloric acid. In particular, the NiCr01 (Inconel 718 type) coating, containing 21% Cr, 8% Mo, 3% Fe, 0.5% C and 67.5% Ni, exhibited a corrosion resistance comparable to AISI 316 in 0.1 M hydrochloric acid and hence is recommended in the medium. Acknowledgments 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 (US) Inc., for assistance in preparation of the samples and determination of porosity of the coatings. References w1x L.N. Moskowitz, Thermal Spray: International Advanced Coating Technology, Proceedings of the International Thermal Spray Conference, 1992, p. 611. w2x J.K. Knapp, H. Nitta, Tribol. Int. 30 (1997) 225. w3x A. Neville, T. Hodgkiess, Proceedings of the International Thermal Spray Conference and Expo., ASM International, Nice, France, 1998.
D. Chidambaram et al. / Surface and Coatings Technology 176 (2004) 307–317 w4x A. Neville, T. Hodgkiess, Thermal Spray: United Forum Science and Technology Advancement, Proceedings of the United Thermal Spray Conference, vol. 1, 1998, p. 161. w5x A. Neville, T. Hodgkiess, Surf. Eng. 12 (1996) 303. w6x P. Siitonen, S.L. Chen, K. Niemi, P. Vuoristo, Thermal Spray: International Advanced Coating Technology, Proceedings of the International Thermal Spray Conference, vol. 13, 1992, p. 853. w7x P. Siitonen, T. Konos, P.O. Kettunen, Thermal Spray Industrial Applications, Proceedings of the National Thermal Spray Conference, vol. 7, 1994, p. 105. w8x T. Kinos, P. Siitonen, P. Kettunen, V.J. Laurila, Thermal Spray Industrial Applications, Proceedings of the National Thermal Spray Conference, vol. 7, 1994, p. 537. w9x P. Gu, B. Arsenault, J.J. Beaudoin, J.-G. Legoux, B. Harvey, J. Fournier, Am. Concr. Inst., SP SP-171 (1997) 389. w10x P. Gu, B. Arsenault, J.J. Beaudoin, J.G. Legoux, B. Harvey, J. Fournier, Cem. Concr. Res. 28 (1998) 321. w11x M.R. Dorfman, J.A. DeBarro, Thermal Spraying: Current Status Future Trends, Proceedings of the International Thermal Spraying Conference, vol. 14, 1995, p. 567. w12x R. Hofman, M.P.W. Vreijling, G.M. Ferrari, J.H.W. de Wit, Electrochem. Methods Corrosion Res. VI 289-2 (Parts 1 and 2) (1998) 641. w13x A.R. Srivatsa, T. Tamagawa, Y. Di, et al., Surface Modification Technologies IX, Proceedings of the International Conference, vol. 9, 1996, p. 167. w14x C.R. Clayton, G.P. Halada, M.E. Monserrat, et al., Advanced Coating Technology of Surface Engineering, Proceedings Symposium, 1997, p. 353. w15x P.E. Arvidsson, Powder Metall. Int. 24 (1992) 176. w16x L. Gil, M.H. Staia, Surf. Coat. Technol. 121 (1999) 423. w17x L. Gil, M.H. Staia, Surface Modification Technologies XIV, Proceedings of the International Conference on Surface Modification Technologies, vol. 14, Paris, France, September 11– 13, 2000 (2001), p. 633. w18x O. Knotek, E. Lugschei, J. Vac. Sci. Technol. 11 (1974) 798. w19x O. Knotek, E. Lugscheider, H. Reimann, J. Vac. Sci. Technol. 12 (1975) 770. w20x O. Knotek, E. Lugscheider, W. Wichert, Thin Solid Films 53 (1978) 303.
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w21x O. Knotek, E. Lugscheider, H. Reimann, Thin Solid Films 64 (1979) 365. w22x G. Fillion, Weld. J. 74 (1995) 45. w23x A. Neville, J.M. Perry, T. Hodgkiess, H.P. Chua, Proceedings of the Institute of Mechanical Engineering, Pt. L-J. Mater. Des. Appl. 214 (2000) 41. w24x A.H. Dent, A.J. Horlock, S.J. Harris, D.G. McCartney, Trans. Inst. Metal Finish. 77 (1999) 60. w25x A.H. Dent, A.J. Horlock, D.G. McCartney, S.J. Harris, J. Therm. Spray Technol. 8 (1999) 399. w26x J.A. DeBarro, M.R. Dorfman, Thermal Spraying: Current Status Future Trends, Proceedings of the International Thermal Spraying Conference, vol. 14, 1995, p. 651. w27x J.A. Debarro, M.R. Dorfman, E. Novinski, M. Nestler, A.R. Nicoll, Oberflaechen Polysurf. 38 (1997) 6. w28x C.H. Lee, K.O. Min, Surf. Coat. Technol. 132 (2000) 49. w29x M.C. Nestler, G. Prenzel, T. Seitz, Thermal Spray: Meeting the Challenges of the 21st Century, Proceedings of the International Thermal Spray Conference, vol. 15, Nice, May 25–29, 1998, p. 1073. w30x M. Stern, A.L. Geary, J. Electrochem. Soc. 104 (1957) 56. w31x M. Rodriguez, M. Staia, L. Gil, F. Arenas, A. Scagni, Surf. Eng. 16 (2000) 415. w32x V. Higuera, F.J. Belzunce, A. Carriles, S. Poveda, J. Mater. Sci. 37 (2002) 649. w33x Properties and Selection: Stainless Steels, Tool Materials and Special Purpose Metals, ASM, OH, 1980. w34x S. Simard, B. Arsenault, K. Laul, M.R. Dorfman, Thermal Spray: Surface Engineering Via Applied Research, Proceedings of the International Thermal Spray Conference, vol. 1, Montreal, QC, Canada, May 8–11, 2000, p. 983. w35x D. Chidambaram, C.R. Clayton, M.R. Dorfman, submitted for publication. w36x D. Chidambaram, M.S. Thesis, Materials Science and Engineering, State University of New York, Stony Brook, 2000, p. 137. w37x R.D. Willenbruch, C.R. Clayton, M. Oversluizen, D. Kim, Y. Lu, Corrosion Sci. 31 (1990) 179. w38x A.I. Vogel, Vogel’s Qualitative Inorganic Analysis, Longman Scientific and TechnicalyWiley, Harlow, Essex, EnglandyNew York, 1987.
Evaluation of the electrochemical behavior of HVOF-sprayed alloy coatings 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, USA b Sulzer-Metco (US), Inc., Westbury, NY 11590-0201, USA Received 7 October 2002; accepted in revised form 27 May 2003
Abstract High-velocity oxy-fuel (HVOF) spraying is a versatile technique that can yield high-density coatings with porosities less than 1% by optimization of the process variables. Oxidation of the coating material can be greatly reduced by inert gas shrouding during the spraying process. This work evaluates the electrochemical behavior of HVOF-sprayed coatings aimed to have better or similar corrosion behavior as stainless steel AISI 316 in acidic medium. A total of eight different chromium-containing coatings with varying proportions of alloying elements such as Ni, Mo, Si, Fe, Co, W, B and C have been studied in hydrochloric acid medium. Porosity measurements, gravimetric analysis, open-circuit potential measurements, potentiodynamic polarization and optical microscopy have been performed. The electrochemical behavior of each coating has been compared with that of bulk AISI 316. The results indicate that HVOF-sprayed AISI 316 coating offers a lower corrosion resistance compared to bulk AISI 316. High nickel- and chromium-containing coatings appear to offer a corrosion resistance comparable to bulk AISI 316. However, results indicate that other alloying elements like molybdenum might be essential for obtaining higher corrosion protection. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Corrosion; Immersion test; Thermal spray; HVOF; Chromium alloy
1. Introduction In the past decade, increased interests in surface engineering of systems using advanced thermal-spray processes for applications in aggressive aqueous corrosive environments have been observed w1,2x. With the advent of the high-velocity oxy-fuel (HVOF) spray process, thermal-sprayed coatings (TSCs), which had limited 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 corrosion-resistant properties. TSCs are widely used for providing wear resistance. However, many of these coatings have to function in an aggressive environment and hence, their corrosion behavior becomes important. For reclaiming a wide range of petrochemical-process components such as *Corresponding author. Tel.: q1-631-632-9272; fax: q1-631-6328052. E-mail address: [email protected] (C.R. Clayton).
storage vessels, heat exchangers, pipe end fittings and valves, which are subjected to severe erosive wear and corrosive conditions, Amoco Oil Company routinely employs the HVOF process to apply AISI 316L and Hastalloy C-276 coatings w1x. Neville and Hodgkiess have studied various TSCs for aqueous corrosion and wear properties w3x. The influence and role of galvanic interactions in the corrosion behavior of cermet coatings such as WC–Co–Cr and WC– Ni–Cr, metallic coatings such as Inconel 625 (Cr–Mo– Si–Ni) and Ni–Cr–C–B coatings on mild steel substrates have been studied in simulated seawater w4,5x. Siitonen et al. have investigated the corrosion behavior of various coatings w6–8x. In one of the studies w6x corrosion resistance and connected porosity of alumina and metallic nickel-base alloy were studied. Another study w8x by the same authors have also shown that the corrosion resistance of these coatings cannot be improved by any post treatments, however, shrouding of the plasma jet with an inert gas seemed to reduce the amount of oxide in the coating. Stainless steels have
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972Ž03.00809-0
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been arc-deposited and HVOF-sprayed earlier w9,10x. The electrochemical behavior of wrought stainless steels has been compared with their thermal-sprayed counterparts w11,12x. Corrosion resistance of jet-vapor deposited stainless steel coatings has been studied in hydrochloric acid medium w13,14x. Ni superalloy (Anval 625 and Anval 718) coatings have been HVOF-sprayed and studied w15x. HVOFsprayed Ni–W–Cr–B–Si coatings have been studied for various properties including corrosion resistance in NaCl solution w16,17x. Self-fluxing nickel-based alloys that were extensively studied by Knotek et al. w18–21x have been applied by HVOF spraying w22x. Corrosion behavior of the Ni-based Inconel 625 alloy coatings sprayed using HVOF method has been studied w23x. Metallic coatings with alloying elements similar to ones studied here have been characterized w24,25x and their performance evaluated w26,27x. Effects of heat treatment on HVOF-sprayed Ni–Cr–W–Mo–B coatings have been studied in sulfuric acid w28x. The newest HVOF guns generate an internal combustion jet with gas velocities that can exceed 2100 mys (7000 feetys), whereas the older systems could only approach 1360 mys (4500 feetys). The high gas velocities are capable of producing particle velocities of approximately 400–800 mys (1320–2600 feetys), which are significantly higher than low-velocity combustion processes w29x. However, combustion temperatures are similar to those of low-velocity processes; combustion fuels include propylene, propane, natural gas, hydrogen, acetylene and kerosene. The major advantage of this higher kinetic-energy process is that coatings with greater density are possible. Other benefits include increased thickness capability, smoother surface finishes, lower oxide levels and less effect of the environment (reduced decarbonization, oxidation and loss of key elements by vaporization) during the spray process. HVOF processes are suitable not only for applying tungsten carbide–cobalt and nickel chromium–carbide systems, but also for depositing wear- and corrosion-resistant alloys such as Inconel (NiCrFe), Triballoy (CoMoCr) and Hastalloy (NiCrMo) materials. HVOF-sprayed MCrAlY coatings are also replacing some low-pressure plasma-sprayed coatings for high temperature oxidationyhot corrosion and TBC bond-coat applications for repair and restoration of existing components. Low melting-point ceramics such as alumina and alumina–titania are also applied via some HVOF processes for abrasive wear and dielectric applications. HVOF systems operate by injecting powder into a stream of burning gases. The material melts or softens, and is propelled against the substrate. A characteristic of most HVOF guns is the multiple shock-diamond pattern that is visible in the flame. Upon exiting the nozzle, compressed flame gases undergo free expansion,
thereby generating a supersonic jet. The high gas velocity results in high sound levels of over 120 dB, which implies that proper soundproofing and ear protection are needed. The benefits of HVOF technology are many. Applications that had shown only marginal benefits from TSCs years ago, are now achieving success through HVOF technology. One such application is in the replacement of hard chrome plating. Environmental restrictions and the cost of waste disposal of chromium have made HVOF technology more attractive. Today, HVOF materials are being applied to hydraulic rods, landing gears and the internal diameter of large bore cylinders as hard chrome replacements. Boeing has approved HVOF spraying of carbide materials on the landing gears of 737 and 757 commercial airliners. In this work, we evaluate the electrochemical behavior of eight different HVOF-sprayed Cr-based coatings with varying proportions of alloying elements such as Ni, Mo, Si, Fe, Co, W, B and C. The coatings have been subjected to porosity measurements, gravimetric analysis, open-circuit potential (OCP) measurements, potentiodynamic polarization and optical microscopy and these results have been compared with that of bulk AISI 316. 2. Experimental 2.1. Materials used The materials used in this research consist of eight different TSCs on mild steel substrate and stainless steel AISI 316 as a control. The compositions of the coatings and their designations are given in Table 1. The two major elements 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. It should be noted that of the coatings tested were— AISI 316 type (FeCr01), Hastalloy C-22 type (NiCr03), Inconel 718 type (NiCr01), high chromium self-fluxing alloy and cobalt-based alloy coatings. The coatings were obtained using a Sulzer-Metco Diamond Jet䉸 2600 Hybrid HVOF system. The DJ2600 hardware was used to spray coatings using hydrogen as the fuel gas. Fuel gases are mixed in a proprietary siphon system in the front portion of the gun. Mixed gases are ejected from the nozzle and are ignited external to the gun. Ignited gases form a circular flame configuration, which surrounds the axially injected powder. The combustion gases, along with the heated powder, are accelerated further through the converging–diverging nozzle to hypersonic velocities. In contrast to most other guns, that use water as the only cooling medium, in this process air is used to cool the combustion portion of the air cap, while water is used to cool the rest of the gun. This greatly reduces the amount of water-
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cooling required, and increases the overall thermal efficiency of the gun. The equipment was specifically designed to allow high particle temperatures and velocities. Table 2 lists the spray parameters used to obtain various coatings. Nitrogen was used as a cooling gas instead of air to minimize the oxidation of the powder particles during the spray process. The powders used were (99% or more) pure. The coatings were sprayed on to mild steel coupons of dimension 25=75 mm2 (1 inch=3 inch) to obtain 400–500 mm thick coatings. The coupons were then cut into sections of 25=37 mm2 and lapped using a 150grit diamond wheel. These coupons were used for potentiodynamic and OCP studies. Freestanding coatings were obtained for use in porosity measurements and gravimetric studies. The tests were conducted in 0.1 M hydrochloric acid supplied by Fisher scientific. The hydrochloric acid was diluted to 0.1 M using de-ionized water. The pH of the acid was approximately 0.7, as measured using a Agy AgCl Sure-Flow䉸 electrode (model 9165BN) interfaced with a bench top pHyISE meter (model 410A), both supplied by Orion Research Inc. 2.2. Porosity measurements The porosities of the coatings were determined by the oil-impregnation method, which is the standard practice of measuring porosity in industry. This technique is based upon the Archimedes principle. The weight of a dry freestanding coating is noted (Cdry). The coating is then impregnated with oil. The weight of the coating is noted again (Coil). The coating is then immersed in water and weighed (Cwater). With these measured values the porosity (connected to surfaceyinterface) of the coating can be determined by a simple calculation procedure, assuming that the density of oil (roil) and the density of water (rwater) is known, as given below: Weight of oilsCoilICdry Volume of oils(CoilyCdry)yroilsVo Volume of coatings(CoilyCwater)yrwatersVc Density of coatingsCdry yVc Porosity (%)s(Vo yVc)=100
309
Table 2 Spray parameters used to obtain the coatings Carrier gas Pressure (MPaypsi) FMR Feed rate Spray distance
Nitrogen 1.03y110 55 40 gymin 230 mm
Gases
Oxygen
Hydrogen
Nitrogen
Pressure (MPaypsi) FMR Flow (scfh)
1.16y170 22 327
0.96y140 62 1450
0.75y110 66 1250
2.3. Optical microscopy Coatings with the substrate were mechanically polished across the cross-section. Samples were prepared as recommended by ASTM E 1920–97, ‘Standard Guide for Metallographic Preparation of Thermal-Sprayed Coatings’. These were then analyzed metallographically under an optical microscope. The optical microscope supplied by MEIJI was interfaced with a Nikon digital camera (model DXM 1200) capable of a resolution of 3840=3072 pixels (12-million pixel resolution). All images (digimicrographs) were taken with maximum resolution and at high sensitivity. 2.4. Electrochemical studies 2.4.1. Gravimetric studies Thick freestanding coatings (f2 mm in thickness), which were delaminated from the surface, were used for gravimetric studies. These ‘as-sprayed’ samples were degreased using isopropanol and dried. The samples were then weighed using an analytical balance (model AB104) with a readability of 0.1 mg supplied by Mettler Toledo Inc. Coatings of dimensions 2.5=2.5 cm2 were then immersed in 250 ml of the 0.1 M HCl in glass beakers. These electrolytes were open to atmosphere. The changes in the appearance of the solution and the coating were monitored every day for 8 days (f200 h). The volume of the solution was kept constant by adding de-ionized water to account for the loss of water through evaporation. At the end of the period the
Table 1 Chemical composition of the powder used for thermal spraying S. No.
Material designation
Starting material
Composition (%)
1 2 3 4 5 6 7 8 9
CoCr01 CrNi01 NiCr01 CoMo01 FeCr01 NiCr03 CoCr02 CrNi02 Wrought AISI 316
Co–Cr–Mo–Ni–C High chromium self-fluxing alloy Inconel 718 type Co–Mo–T-800 type SS316 Hastalloy C-22 type Co–Cr–C–W–C Ni–50Cr Bulk material
29Cr; 8.5Mo; 3Ni; 3Fe; 2C; 1.5Si, Co-balance 39Ni; 3Mo; 1Si; 1B; Cr-balance 21Cr; 8Mo; 3Fe; 0.5C; Ni-balance 28Mo; 17Cr; 3Si; Co-balance 17Cr; 12Ni; 2.5Mo; 1Si; 0.1C; Fe-balance 21Cr; 14Mo; 6Fe; 3W; Ni-balance 25Cr; 10Ni; 7W; 0.5C; Co-balance 50Cr; 50Ni 17Cr; 12Ni; 2.5Mo; 1Si; 0.08C; Fe-balance
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coatings were removed and rinsed thoroughly and carefully with de-ionized water so as not to remove any layer formed on the surface of the coating. The coatings were then dried thoroughly and weighed, at constant intervals for a period of 24 h to check for the constancy of their weight. This was necessary as these coatings are porous and hence may have adsorbed water. Using the weight change in gravimetric testing (W), exposed area of the sample (A), density of coating (D) and time of gravimetric analysis or immersion (t), we calculate the corrosion rate using the following equation in ASTM G31: Corrosion rates
KW DAt
where K is a constant. To obtain the corrosion rate in mils per year (mpy), Ks3.45=106 with W in mg, D in gycm3, A in cm2 and t in h. A few coatings were observed using an optical microscope after exposure to the electrolytes for 200 h and compared. Some coatings on which pits had developed were also observed under the optical microscope. 2.5. Electrochemical cell and instrumentation A corrosion flat cell supplied by Princeton Applied Research (K0235) was used for all OCP measurements and potentiodynamic polarization. One-square-centimeter area of the electrode was exposed to the solution. A platinum-coated niobium-mesh was used as the counter electrode. All potentials were measured with respect to a saturated calomel electrode (SCE), which was used as the reference electrode. Electrochemical polarizations and measurements were performed using PC3 potentiostatygalvanostatyZRA and PC4yFAS1 model potentiostats supplied by Gamry Instruments Inc., which have maximum sensitivities up to 0.01 fA and down to 1 mV resolution. The DC corrosion techniques software, DC105, supplied by Gamry Inc. was used to control the potentiostats. The Gamry potentiostats and their controlling software use control loop algorithms to accurately measure and correct for uncompensated resistance thus eliminating the manipulation of the supporting electrolyte or the cell geometry. 2.6. OCP and potentiodynamic polarization The coatings bonded to the substrate were used for both the OCP and polarization experiments. The polished coatings were rinsed with de-ionized water and degreased using isopropanol. The electrolyte was deaerated for 30 min prior to the experiments by vigorously bubbling high purity nitrogen. The sample was exposed to the electrolyte and the OCP was monitored for 500 s. Then, the sample was potentiodynamically polarized
Table 3 Porosities of the coatings as measured by oil-impregnation method S. No.
Coating
Density of coating (gycm3)
Porosity of coating (%)
1 2 3 4 5 6 7 8
CoCr01 CrNi01 NiCr01 CoMo01 FeCr01 NiCr03 CoCr02 CrNi02
7.6059 6.9816 8.077 7.982 7.610 8.217 7.962 7.2006
2.263 3.536 0.452 2.906 2.098 1.023 1.279 2.62
at a scan rate of 1 mVys from approximately 250 mV below OCP to ;100 mA current or damage of the coating, whichever was earlier. The electrolyte was deaerated throughout the experiment by constant bubbling of high purity nitrogen. The vigorous bubbling of the gas stirred the solution and hence the process was not under diffusion control. All electrochemical tests were repeated to assure the reproducibility of the data. The potentiodynamic polarization parameters were obtained by analyzing the data using the DC105 corrosion measurement software. The corrosion current was calculated using the Stern–Geary equation. If the potential (E), current (I), corrosion current (Icorr) and corrosion potential (Ecorr) are known, then using the anodic tafel slope (basslope in the tafel region of the anodic polarization curve) and the cathodic tafel slope (bcs slope in the tafel region of the cathodic polarization curve), we can find the polarization resistance (Rp) as follows w30x: Polarization resistance: RpsZdEydIappZEyEcorr;0 Corrosion current (Stern–Geary equation w30x): z Icorrswy1yŽ2.303Rp.zw ~ybabcyŽbaqbc.~ x
|x
|
3. Results and discussion 3.1. Porosity measurements The porosities and the densities of the eight coatings as determined by impregnation tests are provided in Table 3. They range from 0.452 to 3.536%. These low values are a result of HVOF spraying, as mentioned earlier. NiCr01 (21Cr; 8Mo; 3Fe; 0.5C; Ni-bal) is the least porous at 0.452%. In general, we observe the density to be higher when the coating has a lower porosity and vice versa. This is illustrated in Fig. 1. However, there is a discrepancy in the case of CoMo01, which could be due to the variations in alloying elements and their molecular masses. The porosities ranged from
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311
Fig. 1. Graph showing the relationship between density and reciprocal of porosity of the coatings. In general, density is observed to be inversely proportional to porosity, except for CoMo01.
3.6 to 6.2% w16x and 2.68% w31x for NiWCrBSi coatings, ;2.0% for NiCrWMoB coatings w28x, ;0.7% for Ni–Cr alloy coating w32x and ;1.0% for stainless steel w10x all formed using HVOF spraying. The coatings thus having similar or lower porosities comparatively, confirm that the spray parameters used were optimum. Digimicrographs showed that the surface of all the coatings was dull and rough. Representative digimicrographs of the cross-sections of a coating are shown in Fig. 2. In general, these coatings had fewer voids than observed normally in coatings sprayed through alternative processes. There were relatively more voids towards the surface of the coating than at the coating–substrate interface. This is due to the repeated impingement experienced by the inner layers. Porosity determined from micrographs of these coatings indicated that these coatings had lower than 5% porosities, in agreement with the oil-impregnation method. 3.2. Gravimetric analysis The results of the gravimetric studies are provided in Table 4. These are the average corrosion rates as calculated from gravimetric tests conducted on duplicate specimens. Gravimetric measurements indicate extremely low corrosion rates with just three systems having a corrosion rate of more than 3 mpy. The corrosion rate ranges from 1.11 to 20.44 mpy, with the majority of the systems exhibiting a rate in the range of 1–2 mpy. AISI 316 control sample has a corrosion rate of 0.2 mpy.
These results are in agreement with other work done on similar system w33,34x. Therefore the least corroding coating is approximately five times more corroding than bulk AISI 316. Still, an absolute corrosion rate of approximately 1–2 mpy is considered to be extremely corrosion resistant. 3.3. OCP polarization
measurements
and
potentiodynamic
The results of the OCP and potentiodynamic polarization are provided in Table 5. The OCP of the systems can be seen to vary between y325 and y55 mV vs. SCE. These are the OCP after 500 s of exposure to the electrolyte. In all cases the potential after 500 s was more active than immediately after immersion. This is expected as the electrolyte was deaerated and thus the formation of a passive film is difficult. In comparison, bulk AISI 316 has an OCP of y125 mV, which falls in the range of the OCP values of the coatings. OCP values also agree with reported values w34x. The corrosion potential, Ec, is seen to fall in the range y335 to y164 mV. In all cases the corrosion potential of a coating was observed to be more noble in nitric acid compared to hydrochloric acid w35,36x. This might be due to formation of passive nitride films on the metallic electrodes when polarized in the nitrate medium. AISI 316 has a corrosion potential of y284 mV. CrNi01, CrNi02, NiCr01 and CoMo01 coatings all have a more noble corrosion potential than AISI 316.
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Fig. 2. Digimicrographs of the cross-section of the coatings (a) NiCr01 and (b) NiCr03. The coating surface is on the right side of the image (a) and the coating–substrate interface can be easily seen in both digimicrographs. The digimicrographs show that these HVOF-sprayed coatings have very low porosity in comparison with coatings formed using alternative spray processes. This gives rise to the high densities observed in these coatings. Table 4 Gravimetric analyses experiment results S. No.
Specimen
Corrosion rate in 0.1 M HCl (mpy)
1 2 3 4 5 6 7 8 Control
CoCr01 CrNi01 NiCr01 CoMo01 FeCr01 NiCr03 CoCr02 CrNi02 SS316
2.211 2.078 1.105 1.142 20.437 1.616 9.734 3.255 0.200
The corrosion current density varies from 44 to 0.27 mAycm2. The highest corrosion current density is observed for the HVOF-sprayed AISI 316 alloy, namely,
FeCr01. Both the alloys, NiCr01 and CrNi01 containing high amounts of Cr–Mo–Ni, are observed to have very low corrosion current densities and hence a high resistance to corrosion. In fact, NiCr01 (0.27 mAycm2) has a lower corrosion current than the control AISI 316 (0.53 mAycm2). Once again, this could be explained to be due to the protective nature of chromium, molybdenum and nickel. Addition of molybdenum and nitrogen has shown to increase the corrosion resistance of steels w37x. Therefore, the low corrosion current densities obtained for these coatings, which are similar to the stainless steels, is not surprising. The anodic tafel slope, ba, of all the systems (except for CrNi02) was determined to be higher than the cathodic tafel slope, bc. These systems are hence under anodic control. This is expected as they were carried
Table 5 OCP measurements and potentiodynamic polarization results S. No.
Specimen
OCP vs. SCE (mV)
Ecorr vs. SCE (mV)
Icorr (Aycm2)
Rp (V cm2)
1 2 3 4 5 6 7 8 Control
CoCr01 CrNi01 NiCr01 CoMo01 FeCr01 NiCr03 CoCr02 CrNi02 SS316
y290 y80 y150 y55 y325 y170 y290 y100 y125
y335 y180 y270 y164 y341 y327 y317 y174 y284
1.02Ey05 9.10Ey07 2.65Ey07 1.98Ey05 4.36Ey05 4.62Ey06 9.41Ey06 1.79Ey06 5.29Ey07
3.11E03 3.90E04 3.15E04 1.85E03 9.76E02 1.74E03 2.49E03 1.33E04 5.93E04
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313
Fig. 3. Potentiodynamic polarization curves for the three cobalt-based coatings in 0.1 M HCl acid solution.
out under the exclusion of oxygen, inclusion of which in some cases could have brought the system under mass transfer control or diffusion control. Polarization resistance (Rp ) values as calculated using the Stern–Geary equation w30x varies from 9.76E02 V cm2 (FeCr01) to 3.9E05 V cm2 (CrNi01). The polarization resistance can be considered as an indicator of the corrosion resistance of the material, with a higher value denoting a highly corrosion-resistant material. AISI 316 has Rp of 5.93E05 V cm2, which is slightly higher than the most corrosion-resistant coating. However, given that these coatings have values close to bulk AISI 316, the coating performance can be considered to be excellent. The passivation characteristics of the coatings were deduced from the anodic polarization plots. Figs. 3–5 provide these plots for the eight coatings and bulk AISI 316 alloy. All systems underwent passivation over a range of at least 600 mV before transpassive breakdown, except FeCr01, which was passive for only approximately 50 mV, and breakdown occurred due to pitting. The pitting could have been due to chloride ions. The amount of pitting corrosion can only be evaluated by other studies. The corrosion rates from gravimetric studies and corrosion currents from polarization studies are plotted in Fig. 6. It can be seen that these two curves have similar trends. The cobalt-based alloys, namely, CoCr01, CoCr02 and CoMo01, exhibited similar corrosion characteristics as illustrated in Fig. 3. CoCr01 performed fairly well in
both the tests. It exhibited a moderate corrosion rate of 2.2 mpy in immersion studies and a corrosion current of 10.23 mAycm2 in polarization studies, exhibited passivity in the region 100–950 mV vs. SCE. CoCr02 is also a moderately resistant system as CoCr01. During weight loss it underwent corrosion at the rate of 9.734 mpy, and during polarization had a corrosion current of 9.4 mAycm2, the passive region extended from y100 to 900 mV with a moderately high current of 1000– 1500 mAycm2. OCP is also in the active range at y275 mV vs. SCE. The digimicrograph, shown in Fig. 7, indicates uniform corrosion. However, the preferential leaching or dissolution of cobalt as determined by the qualitative analysis w38x indicates a homogeneous coating. CoMo01 also exhibited a moderately good corrosion behavior as CoCr01. The corrosion rate was 8 mpy as determined by gravimetric studies and corrosion current of 19.8 mAycm2 from polarization studies. It was also passive in the range 175–900 mV vs. SCE. However, CoMo01 had a more noble OCP (y55 mV vs. SCE) as compared to CoCr01 (y290 mV vs. SCE). The iron-based alloy coating based on AISI 316, FeCr01, exhibited the least corrosion resistance in this medium. The corrosion rate was approximately 20 mpy in gravimetric tests, the corrosion current obtained from polarization was 43.6 mAycm2, passivity was exhibited only over a very narrow range of 50 mV and the OCP was comparatively active at y325 mV vs. SCE. The OCP and the corrosion current values obtained here are in agreement with values obtained by Simard et al. w34x. The corrosion current is almost two orders of magnitude
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Fig. 4. Potentiodynamic polarization curves for the two nickel-based coatings and AISI 316 coating in 0.1 M HCl acid solution.
more than that of the bulk AISI 316, thus agreeing with similar work performed on these materials in H2SO4 medium w12x. Moreover, extensive amounts of iron were observed by qualitative analysis after gravimetric tests w38x. The digimicrographs shown in Fig. 8 indicate extensive corrosion damage by pitting. There were numerous pits seen on the coatings surface. It should be noted that this coating was sprayed using a powder composition similar to that of AISI 316. This appears to have the worst corrosion performance when compared to AIS 316 bulk material. There are inherent problems in spraying stainless steel coatings and hence these studies were conducted to identify coatings with corrosion characteristics similar or better than AISI 316. The electrochemical behavior of FeCr01 justifies this work. Both chromium-based alloys, namely, CrNi01 and CrNi02, have similar corrosion characteristics. CrNi01 exhibited a desirable corrosion behavior with low corrosion rate of 2 mpy in immersion studies and a low corrosion current of 0.9 mAycm2, passivity from 25 to 925 mV vs. SCE and a passive current of 100–200 mAycm2. Moreover, the OCP was comparatively noble at y85 mV vs. SCE. CrNi02 also appears to offer a good corrosion resistance with low corrosion rate of 3.2 mpy and a corrosion current of 1.7 mAycm2 in polarization studies. It also exhibits passivity in the region of 100–900 mV with current in the range of 50–250 mAy cm2. The OCP is on noble side at y100 mV vs. SCE. Nickel analyzed by qualitative analysis could have been due to normal uniform corrosion. The nickel-based alloys, NiCr01 and NiCr03, have similar and highly corrosion-resistant behavior. NiCr01
exhibited excellent corrosion resistance with corrosion rate of 1.1 mpy in immersion studies and a corrosion current of 0.26 mAycm2. Passivity was exhibited over a wide range of potential from y250 to 800 mV vs. SCE with the passive current being as low as 6 mAycm2. The OCP was also towards a noble end at y90 mV vs. SCE. NiCr03 also exhibited good corrosion resistance in this medium with a corrosion rate of 1.6 mpy as obtained from gravimetric studies and a corrosion current of 4.6 mAycm2. The coating was also passive over a wide range of potentials (y325 to q850 mV vs. SCE) with very low passivation currents of 6–50 mAy cm2. The OCP was approximately y150 mV vs. SCE. In comparison, AISI 316 had a corrosion rate of 0.2 mpy in gravimetric tests and a corrosion current of 0.53 mAycm2, exhibited passivity in the range y300 to 200 mV vs. SCE. It exhibited an OCP of y125 mV. The results are in agreement with values obtained earlier w34x. Hence, based on the above discussion, NiCr01, with a corrosion current lower than that of AISI 316 and a comparable corrosion rate from immersion studies appears to offer the best corrosion resistance in 0.1 M HCl amongst all the coatings, followed by NiCr03 and CrNi01. All these coatings also have a comparable polarization resistance as indicated in Table 5. 4. Conclusions The HVOF-sprayed alloy coatings were deposited using optimal spray parameters and thus have very low porosities. The porosities of these coatings are in general
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Fig. 5. Potentiodynamic polarization curves for the two chromium-based coatings and bulk AISI 316 in 0.1 M HCl acid solution.
inversely proportional to their densities. Based on the performance of the FeCr01 coating, we can understand that TSCs with a same starting powder composition as AISI 316 exhibit poor corrosion resistance. Hence, there is a need to develop and evaluate coatings offering similar or better performance than AISI 316. Of the eight different coatings studied, the high nickel- and chromium-containing coatings were observed to offer
good corrosion resistance in 0.1 M HCl. Also it is noted that chromium and nickel alone do not impart the corrosion resistance as evident from the behavior of CrNi02, which is a 50% Cr–50% Ni coating. Alloying elements like Mo are also essential for good corrosion resistance. Based on these studies we can conclude that the nickel- and chrome-based alloys tested here exhibit a corrosion behavior better than or comparable to that
Fig. 6. Graph of corrosion rates (in mpy) from gravimetric studies and corrosion currents (in Aycm2) from polarization studies.
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Fig. 7. Digimicrograph of CoCr02 coating after 200 h of exposure to 0.1 M hydrochloric acid solution (gravimetric analysis). Absence of pits and a uniform degradation indicate uniform corrosion. Line marker indicates 100 mm in each case.
Fig. 8. Digimicrograph of FeCr01 coating after 200 h of exposure to 0.1 M hydrochloric acid solution (gravimetric analysis). The coating had undergone extensive pitting, as can be seen, characteristic of stainless steels exposed to chloride media. Well-developed pits over 100 mm in size are observed. Line marker indicates 100 mm in each case.
of AISI 316. Therefore, alloys with high levels of Ni– Cr can be said to be highly corrosion-resistant to hydrochloric acid. In particular, the NiCr01 (Inconel 718 type) coating, containing 21% Cr, 8% Mo, 3% Fe, 0.5% C and 67.5% Ni, exhibited a corrosion resistance comparable to AISI 316 in 0.1 M hydrochloric acid and hence is recommended in the medium. Acknowledgments This work was supported in part by the Strategic Partnership for Industrial Resurgence (SPIR), a program
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