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Heat treatment effects on the corrosion resistance of some HVOF-sprayed metal alloy coatings
Surface & Coatings Technology 202 (2008) 4839–4847
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Heat treatment effects on the corrosion resistance of some HVOF-sprayed metal alloy coatings Giovanni Bolelli a,⁎, Luca Lusvarghi a, Massimiliano Barletta b a b
Department of Materials and Environmental Engineering, Università di Modena e Reggio Emilia, Via Vignolese, 905 — 41100 Modena (MO), Italy Department of Mechanical Engineering, Università di Roma — Tor Vergata, Via del Politecnico, 1 — 00133 Roma, Italy
A R T I C L E
I N F O
Article history: Received 21 September 2007 Accepted in revised form 15 April 2008 Available online 25 April 2008 PACS: 81.15.Rs 81.65.Kn 81.40.-z Keywords: High velocity oxyfuel (HVOF) Nickel alloy Immersion test Scanning electron microscopy (SEM) Electrochemical polarization test
A B S T R A C T The present study evaluates the effects of a 600 °C, 1 h heat treatment on the corrosion resistance of three High Velocity Oxygen Fuel (HVOF) flame-sprayed alloy coatings: a Co–28Mo–17Cr–3Si (similar to Tribaloy800) coating, a Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe (Diamalloy-4006) coating and a Ni–32Mo–16Cr–3Si– 2Co (similar to Tribaloy-700) coating. Electrochemical polarization tests and free corrosion tests were performed in 0.1 M HCl aqueous solution. The corrodkote test (ASTM B380-97R02) was also performed, to evaluate the coatings qualitatively. The heat treatment improves the corrosion resistance of the Co–28Mo– 17Cr–3Si coating and of the Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe coating by enhancing their passivation ability. The precipitation of sub-micron sized secondary phases after the treatment may produce galvanic microcells at intralamellar scale, but the beneficial contribution provided by the healing of the very small but dangerous interlamellar defects (normally present in thermal spray coatings but not detectable using ordinary scanning electron microscopy) prevails. The effect on Ni–32Mo–16Cr–3Si–2Co coatings is more ambiguous: its sensitivity to crevice corrosion is worsened by the heat treatment. © 2008 Elsevier B.V. All rights reserved.
1. Introduction High Velocity Oxygen Fuel (HVOF) flame-sprayed coatings are increasingly being considered for wear- and corrosion-resistant applications, thanks to the low defectiveness resulting from the supersonic gas velocity and limited flame temperature (compared to other thermal spraying techniques, like plasma-spraying) [1]. They are therefore also being proposed as an alternative to hard chrome plating, as they are able to offer comparable or superior technical performance [2–10] with a significantly lower environmental impact [2,11]. More specifically, much research has recently focused on HVOFsprayed cermets, which have already been shown to possess excellent tribological properties [12–14] as well as good corrosion resistance with a suitable carbide and metal matrix choice [15–17]. Fewer papers focus on HVOF-sprayed metal alloy coatings, which, although less wear resistant than cermets [8,12], can be very interesting on account of their higher deposition efficiency, far lower machining (grinding/ polishing) costs, and (usually) lower powder cost [18]. They may also possess some peculiar properties like high hot hardness and oxidation resistance.
⁎ Corresponding author. Tel.: +39 059 2056281; fax: +39 059 2056243. E-mail addresses: [email protected] (G. Bolelli), [email protected] (L. Lusvarghi), [email protected] (M. Barletta). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.074
For these reasons, the authors have recently concentrated their attention on HVOF-sprayed metal coatings, performing comparative studies on the wear and corrosion resistance of a number of these materials [19–22], in order to provide an improved understanding of their behaviour, find the best performing coatings under different operating conditions, and assess their technical viability as a hard chrome replacement. Specifically, comprehensive corrosion testing [22] enabled the authors to single out three metal alloy coatings with very promising characteristics, similar or even superior to electrolytic hard chrome: Co– 28Mo–17Cr–3Si (similar to Tribaloy®1-800), Ni–32Mo–16Cr–3Si–2Co (similar to Tribaloy®-700), Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe (Diamalloy-4006). Since industrial applications of HVOF-sprayed metal coatings can probably involve both corrosion and wear degradation [16,23], research on the tribological performance of these three alloys was also performed [19–21]. Interesting tribological properties were found and a heat treatment performed at relatively low temperature (600 °C, 1 h) was shown to be highly beneficial, especially against adhesive wear. Indeed, whereas as-sprayed coatings mainly consist of super-saturated solid solutions [19–21,24], heat treatments produce the precipitation of secondary hard phases: respectively, Co–Mo–Si Laves’ phases in the Co–Mo–Cr–Si coating [25]; Ni–Si or Mo–Si intermetallics
1
Tribaloy is a trademark of Deloro-Stellite.
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Table 1 Characteristics of the commercial spray powders used in this study, and short designation used in the paper Commercial name
Manufacturer
Designation
Nominal chemical composition (in weight%)
Nominal particle size range
Manufacturing method
Co-111 Ni 700 Diamalloy 4006
Praxair Sandvik-Osprey Sulzer Metco
Co800 Ni700 D4006
Co–28Mo–17Cr–3Si (Similar to Tribaloy-800®) Ni–32Mo–15Cr–3Si–2Co (Similar to Tribaloy-700®) Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe
−45 + 10 μm −53 + 10 μm −53 + 11 μm
Gas atomized Gas atomized Water atomized
in the Ni–Mo–Cr–Si coating; W, Mo, Cr-base carbides in the Diamalloy4006 coating. These phases were found to strengthen the coatings and enhance their tribological characteristics. It is therefore also very important to determine the effects of this heat treatment on the corrosion resistance of the coatings. In actual fact, the corrosion behaviour of a metal alloy is highly sensitive to its microstructure and it is reasonable to suspect that changes can occur in the chemical and electrochemical response of these coatings after the treatment [24]. The aim of this paper is to assess these effects using electrochemical and chemical corrosion tests. It is useful to remember that the available literature contains various studies dealing with the corrosion resistance of thermal spray coatings subject to heat treatments; however, they mainly deal with somewhat complex high-temperature treatments, such as high-temperature vacuum furnace treatments or furnace remelting [26,27]. These processes can effectively improve the corrosion resistance of a thermally-sprayed metal coating by changing its microstructure from a lamellar one to a bulk-like one, removing defects, such as pores and lamellae boundaries, which are known to impair performance and limit passivation capacity. However, their applicability is limited, as they significantly increase processing costs and can cause unacceptable microstructural alterations or geometrical distortions in the substrate. Laser treatments are another option in order to densify thermally-sprayed metal alloys. According to the laser processing conditions, a coating can be simply heat treated, in order to modify the microstructure of its top layer without affecting the bottom layers and the substrate [28], or it can be fully re-melted, in order to alter very deeply its microstructure and to promote metallurgical bonding with the substrate [28–32]. These treatments were reported to be very effective in enhancing the corrosion resistance [28–32], but, apart from their high costs, they can also pose certain drawbacks. Depending on the composition of the coating and on the specific type of laser, cracking of the treated surface can occur more or less easily [28,29,31], so that, in some cases, the optimal processing window is very narrow, and the proper adjustment must be found for each coating material [28], through long and costly experiments. Moreover, when full laser remelting is performed, the substrate is also affected and dilution inside the coating can occur [28]. In this study, by contrast, a simple heat treatment in air, performed at a lower temperature (600 °C) than those considered in the above cases, is examined.
(Metcolite-C, Sulzer Metco). Full details on powder characterisation and coating deposition parameters can be found in [22]. Part of the coated plates was heat treated in an electric kiln in air at 600 °C. The heat treatment program involved heating at a 15 °C/min rate, 1h isotherm, and natural cooling inside the kiln (no external control of cooling rate). Coatings were characterised by scanning electron microscopy (SEM: Philips XL-30 and Leo Supra 35 high-resolution field emission gun (FEG) microscope) observations of polished cross-sections (hotmounted in phenolic resin) and by X-ray diffraction (XRD, PANAlytical X’Pert Pro) analysis on polished top surfaces. Polishing was performed using SiC papers (up to 2000 mesh), diamond slurry (up to 0.5 μm particle size) and alumina slurry (0.2 μm particle size). 2.2. Corrosion testing Before each corrosion test, all samples were ground to a coating thickness of about 300 μm, polished to Ra ≈ 0.5 μm and ultrasonically cleaned and degreased with acetone. The thin oxide layer produced by the heat treatment on the coating surface was therefore completely removed and had no influence on test results. Anodic and cathodic electrochemical polarization tests were performed in contact with a 0.1M HCl aqueous solution in equilibrium with the environment at room temperature, using a three-electrode cell (K0235 flat cell, Princeton Applied Research) where the working electrode is the sample, pressed against a Teflon gasket leaving a 1cm2 exposed surface, the counter-electrode is a Pt grid; the reference electrode is an Ag/AgCl electrode. A PC-programmed EG&G 270 potentiostat/galvanostat (Princeton Applied Research, UK) was employed. The scanned potential range is − 300 mV/+ 1400 mV from rest potential, at a 0.5 mV/s scan rate. The tests started after 30 min of free corrosion (to allow full wetting of the coating surface and open circuit potential stabilization). Two tests were performed on each sample. The corrosion current density Icorr (A/cm2) and corrosion potential Ecorr (mV) were determined by Tafel analysis: linear portions of the anodic and cathodic curves were fitted by Tafel's law (1) E ¼ aFb:logðIÞ
ð1Þ
and the resulting linear system (2) is solved to find Ecorr and Icorr:
E ¼ aa þ ba :logðIÞ E ¼ ac bc :logðIÞ
ð2Þ
2. Experimental 2.1. Coating manufacturing and characterisation Three commercially available thermal spray powders were employed. Their characteristics, including nominal chemical composition, manufacturing method and designation used hereafter, are listed in Table 1. The powders were HVOF-sprayed using a Praxair-Tafa JP5000 kerosene-fuelled torch2 onto AISI 1040 steel plates (100 × 100 × 5)mm3, grit-blasted immediately prior to deposition using a manual vacuumoperated Norblast blasting machine with 500 μm angular alumina grits
2
Owned by Centro Sviluppo Materiali S.p.A., Roma, Italy.
where: aa, ba = anodic curve coefficients; ac, bc = cathodic curve coefficients. At least two samples were tested for each coating. The top surface and polished cross-section of corroded samples were also observed by SEM. The results obtained on an electrolytic hard chrome plating under the same conditions in former research [22] are also listed for comparison. Free corrosion tests were conducted for 2 days, in contact with 0.1M HCl aqueous solution in equilibrium with the environment at room temperature, using the K0235 flat cell, with the same experimental arrangement described above. Two tests were performed on each sample. The polarization resistance Rp was periodically measured by electrochemical polarization tests (EG&G 270 potentiostat/galvanostat) according to the Stern–Geary method [33]: a ± 20 mV overpotential range (from rest potential) was
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scanned (0.5 mV/s scan rate) and the slope of the linearly fitted E vs. I data is assumed as Rp. For each coating, the inverse of the polarization resistance 1/Rp is proportional to the corrosion current density according to Eq. (3) [33]. Icorr ¼
ba :bc 1 d 2:3:ðba þ bc Þ Rp
ð3Þ
However, a direct comparison between the corrosion current density values of the various coatings cannot be obtained by comparing 1/Rp values (i.e.: the coating displaying the lowest 1/Rp value is not necessarily the one with the lowest Icorr), since the proportionality coefficient between 1/Rp and Icorr depends on the anodic and cathodic Tafel slopes (as shown in Eq. (3)), which are specific to the system being tested, as also noted in [34]. Qualitative evaluation of the corrosion resistance of the coatings was also performed by means of the corrodkote test [35], specifically developed for Ni- and Cr-based coatings. A thick corrosive suspension must be uniformly spread on the coated sample surface with a paintbrush. The suspension consists of 60 g of kaolin + 100 ml of a solution containing NH4Cl (20 g/l), FeCl3·6 H2O (3.3 g/l), Cu (NO3)2·3 H2O (0.7 g/l). The sample is then kept for 20 h at 40 °C, 90% relative humidity. The suspension is then removed using water and a cloth, and the sample is visually inspected for a qualitative evaluation of corrosion. A new, fresh suspension is then reapplied for a new 20 htest cycle. This test is considered as being 100 times more accelerated than the salt spray test [36]. In this study, two 20 h-test cycles were performed. For each coating, two 25 mm × 25 mm samples, cut from the larger plates, were tested. The sides of the samples were protected with silicone resin to prevent the infiltration of corrosive agents. Digital photographs of the coatings were taken after each cycle. A better inspection of the corrosion morphology was also obtained by means of optical microscopy. 3. Results and discussion 3.1. Microstructural characterisation of coatings A complete microstructural characterisation of as-sprayed and heat-treated coatings has already been provided by the authors in former papers [19–22] and therefore only a brief overview is given here. As shown in those papers, as-sprayed Co800 and Ni700 coatings possess an amorphous or nanocrystalline metal matrix, producing a large hump in X-ray diffraction patterns, and display few, large-sized secondary phases (Fig. 1A, arrow), most probably coming from the microstructure of the original spray powder [22]. After the heat treatment, apart from those few large-sized secondary phases (Fig. 1B, arrow), numerous small-sized (sub-micrometric) secondary phases appear (Fig. 1B, circle. Detail of the sub-micrometric phases in Fig. 1C). In the case of Co800, XRD reveals the newly-formed phases are Co3Mo2Si, Co7Mo6 and CoSi2 intermetallics; in Ni700, they are Mo5Si3 and Ni3Si2 [20]. The as-sprayed D4006 coating displays no remarkable phase contrast in backscattered electrons (Fig. 2A), but, after etching (75 g HCl + 25 g HNO3 + 2 g CuCl2, 30s), it reveals a two-phase microstructure (Fig. 2B). These micrographs perfectly agree with a former literature study on HVOF-sprayed D4006 [24], where these two phases were identified as two solid solutions, one richer in Ni, the other richer in Cr and W. After the heat treatment, fine-grained carbides and borides (Cr23C6, WC, NiB2 [22,24]) appear (overview in Fig. 2C and detail in Fig. 2D: bright WC phases are labelled as 1 and darker Cr23C6, NiB2 are labelled as 2). The coatings look rather dense, though some closed porosity is recognizable (Fig. 3A, circles), and contain oxide inclusions, mainly in the form of dark stringers along lamellae boundaries (Fig. 3B). The
Fig. 1. SEM (backscattered electrons) micrograph of as-sprayed (A) and heat-treated (B) Ni700 coating, and high-magnification detail of the latter (C).
porosity and the oxide inclusion contents, as determined by image analysis, do not change significantly after the heat treatment (Table 2). However, as indicated in pertinent literature, image analysis based on ordinary SEM images is not sufficiently accurate in order to differentiate between the porosity values of these highly dense coatings [34]. More specifically, it seems inadequate at capturing those tiny interlamellar defects [37], which are potentially extremely dangerous because they may offer penetration paths to corrosive agents [38]. For instance, Zhang et al. [37] used the same HVOF torch in order to spray two Inconel-625 powders having different particle size distributions, and found that the resulting coatings had completely different corrosion behaviours, although their porosity values as determined from image analysis were similar and very low. They reported that, by performing image analysis on SEM micrographs of polished cross-sections, small interlamellar pores, dramatically affecting the coatings’ performances, were overlooked. This finding was also confirmed by the authors in former corrosion studies on
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Fig. 2. SEM micrographs of the D4006 coating: A: as-sprayed; B: as-sprayed, detail after etching; C: heat-treated; D: heat-treated, detail (label 1: bright WC phase; Label 2: dark Cr23C6 and Ni2B phases).
these same HVOF-sprayed alloy coatings [22]. So, image analysis results cannot rule out possible modifications to interlamellar cohesion. 3.2. Electrochemical corrosion tests The heat treatment relevantly modifies the anodic polarization curves of the coatings (Fig. 4A). The as-deposited Co800 coating shows a remarkable lack of passivation, although the high Cr and Mo content of this alloy should promote good passivating ability even in chlorinecontaining environments [25]. This lack of passivation in thermallysprayed metal alloys, resulting in a poorer corrosion resistance than in bulk form, is well-documented in pertinent literature [9,26,34,39,40]
and is thought to be due to a combination of factors, mainly involving the presence of lamellae boundaries. Indeed, when very small defects exist along lamellae boundaries, infiltration by corrosive agents is possible, thus impairing the coating’s protectiveness [37,38]. Moreover, if oxides are formed along those boundaries during coating deposition, the alloy composition, in their proximity, is deprived of those oxidizable elements (like Cr) which should provide passivation and corrosion protection [37,41]. Finally, chemical inhomogeneities like oxide inclusions can represent preferential sites for corrosion initiation. After the heat treatment, by contrast, the whole anodic polarization curve shifts to lower current density values, and its trend becomes less active, although there is no true passivation. The
Fig. 3. Low-magnification SEM overview of as-sprayed Ni700 coating: secondary electron micrograph (A, circles indicate closed porosity) and backscattered electron micrograph (B).
G. Bolelli et al. / Surface & Coatings Technology 202 (2008) 4839–4847 Table 2 Porosity and oxide inclusion amount in as-sprayed and heat-treated coatings Coating
Porosity %
Oxide inclusions %
Co800 as-sprayed Co800 heat-treated Ni700 as-sprayed Ni700 heat-treated D4006 as-sprayed D4006 heat-treated
1.4 ± 0.1 1.6 ± 0.6 2.2 ± 0.5 2.0 ± 0.4 2.9 ± 0.1 1.6 ± 0.9
8.1 ± 2.0 8.9 ± 1.5 7.1 ± 0.6 7.3 ± 2.3 5.9 ± 0.6 7.9 ± 2.6
corrosion potential therefore shifts to more noble values and the corrosion current density definitely decreases (Table 3). Moreover, the nobler and less active behaviour of the material reflects in a decreased and more stable 1/Rp value during the whole duration of the 2-day free corrosion test (Fig. 4B). The corrosion morphology of the Co800 coating subject to the electrochemical polarization test becomes more uniform after the heat treatment. The large pits found on the as-sprayed coating (Fig. 5A and ref. [22]) largely disappear (Fig. 5B), and, most remarkably, corrosion penetration along interlamellar boundaries (which become deeply marked in as-sprayed condition) would appear reduced (compare the morphology of as-deposited coating in Fig. 5C to that of the heat-treated coating in Fig. 5D). Higher magnification intralamellar-scale micrographs (Fig. 5E) also reveal selective corrosion of the metal matrix (label 2) around the secondary Mo-rich phases (label 1, EDX analysis in Fig. 5F).
Fig. 4. Electrochemical anodic and cathodic polarization curves (A) and inverse polarization resistance 1/Rp trends during free corrosion tests (B) for as-sprayed and heat-treated coatings.
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Table 3 Average corrosion current density (Icorr) and corrosion potential (Ecorr) of HVOF-sprayed coatings from electrochemical polarization tests Coating
Icorr [⁎10− 5 A/cm2]
Ecorr [mV]
Co800 as-sprayed Co800 heat-treated Ni700 as-sprayed Ni700 heat-treated D4006 as-sprayed D4006 heat-treated EHC
3.01 0.0656 1.95 0.182 5.64 2.27 0.149
−315.8 −241.1 − 307.9 −104.5 −312.0 −263.6 −141.6
Results obtained on electrolytic hard chrome (EHC) plating, under the same conditions, in a former research are also listed for comparison.
This phenomenon, which has already been found to occur in crystalline unmelted particles in as-deposited condition [22], is now more widely extended to the whole coating, due to increased intralamellar crystallization. The heat treatment therefore seems to produce two opposite effects on corrosion behaviour. On the one hand, the heat treatment reduces active corrosion along interlamellar boundaries. As explained above, corrosion activation along lamellae boundaries is one of the most remarkable causes of the lack of passivation in the as-deposited coating [9,26,34,39,40]. Reducing it could therefore contribute significantly to the overall decreased anodic activation of the material. This is probably due to an improvement in interlamellar cohesion during the heat treatment, possibly on account of solid state diffusion [24]. Modifications to the extremely small interlamellar defects can only be reliably revealed by corrosion testing, because ordinary image analysis techniques are not adequate for this purpose [34]. On the other hand, changing from a uniform intralamellar microstructure to a multi-phase one can produce galvanic microcells at an intralamellar level. However, interlamellar corrosion makes by far the largest contribution to corrosion penetration inside the coating, as shown in the micrographs in Fig. 5. So, the beneficial effect of the formerly discussed improvement in interlamellar corrosion resistance largely prevails over the onset of intralamellar galvanic microcells. The Ni700 coating also shifts to more noble corrosion potential values (Table 3, Fig. 4A); besides, 1/Rp is once again decreased and more stable (Fig. 4B). However, the overall effect of the heat treatment is not as obvious as in the former case. Whereas the as-sprayed coating exhibits two anodic “passive-like” stages, the first occurring at low anodic overpotential (potential range between − 250 mV and − 150 mV), with a constant current density of about 7 ⁎ 10− 5A/cm2, and the second occurring at high anodic overpotential (potential range between 200 mV and 800 mV), with higher current density (between 1 ⁎ 10− 3A/cm2 and 2 ⁎ 10− 3A/cm2), the heat-treated coating only retains the “passive-like” stage at high overpotential values (potential range between 200 mV and 800 mV). So, the first “passive-like” stage is lost: by contrast, continuous activation takes place at the beginning of the anodic polarization curve of the heat-treated Ni700 coating. However, the current density of the second “passive-like” stage (approximately between 4 ⁎ 10− 4A/cm2 and 6 ⁎ 10− 4A/cm2) is lower than in the as-sprayed condition. It is not therefore immediately clear whether the heat treatment has a favourable or adverse effect on the corrosion resistance of this coating. The term “passive-like” has been adopted on purpose, as the above-described stages cannot be considered true, complete passivation. Indeed, although in such stages current density does not increase, it settles at values that are too high for a perfect passivity state [42]. The lack of perfect passivation can be explained by the above-described existence of defects and lamellae boundaries. In actual fact, corrosion of interlamellar boundaries seems to take place both in the as-sprayed (Fig. 6A) and in the heat-treated (Fig. 6B) coating. Whereas the appearance of the corroded surfaces of as-
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Fig. 5. Surfaces of as-sprayed and heat-treated Co800 coatings after electrochemical polarization test. A: as-sprayed, low magnification; B: heat-treated, low magnification; C: assprayed, higher magnification; D: heat-treated, higher magnification; E: interlamellar detail of heat-treated coating showing metal matrix corrosion (2) between secondary phases (1); E: EDX analyses of Mo-rich emerging phases (1) and corroded matrix (2).
sprayed and heat-treated Co800 is significantly different (Fig. 5), changes to the corrosion morphology of Ni700 seem less substantial (Fig. 6A,B). Moreover, the sensitivity to crevice corrosion during the free corrosion test, which is very moderate in the as-sprayed condition, definitely increases after the heat treatment. Indeed, whereas regions not under the Teflon gasket show almost no damage after the free corrosion test (Fig. 6C), Fe-containing corrosion products appear on those under the Teflon gasket, indicating corrosion penetration down to the substrate in this area (Fig. 6D, EDX analysis in Fig. 6F). Selective corrosion of the metal matrix around the small crystalline phases also occurs under the Teflon gasket (Fig. 6E). D4006 shows the highest Icorr values of the tested coatings (Table 3). The heat treatment decreases its corrosion current density (although it is not as low as for Co800 and Ni700) and
improves its “passive-like” behaviour (Fig. 4A); indeed, at low anodic overpotentials, the current density retains a constant value of about 2 ⁎ 10− 5A/cm2 in the heat-treated coating, whereas it is about 9 ⁎ 10−5A/cm2 in the as-sprayed condition. This anodic polarization behaviour is definitely consistent with the only former literature report on this kind of alloy known to the authors [24], although, in that paper, the corrosive medium was 1M sulphuric acid. The authors, in a past research, showed that the anodic polarization behaviour of some HVOF coatings (also including the Co800 alloy) in acid environments is not particularly sensitive to the nature of the anions [9]: comparison of the present polarization curves to those in [24] may indicate that this observation can be extended to D4006 as well. As mentioned in [9], this would be very interesting, because it would mean that the coating behaviour can be more confidently predicted and relied upon, irrespective of the
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Fig. 6. Surface of Ni700 coatings after corrosion testing. A,B: surface of as-sprayed and heat-treated coating after electrochemical corrosion test; C: surface of heat-treated coating after free corrosion testing, area outside the Teflon gasket; D: surface of heat-treated coating after free corrosion testing, area under the Teflon gasket; E: surface of heat-treated coating after free corrosion testing, area under the Teflon gasket: detail of one of the regions indicated by circles in panel D. F: EDX microanalysis of non-corroded surface and of corrosion products (label 1 in panel D) on heat-treated coating after free corrosion testing.
acid medium; however, further investigation is obviously required before it is reasonable to make such a claim. The D4006 coating is able to retain a constant 1/Rp value throughout free corrosion test duration in as-sprayed condition, and it preserves this feature after the heat treatment. The corroded surface, both in as-sprayed and in heat-treated condition, is covered by a layer of corrosion products rich in O, W, Mo, Cr (Fig. 7A, B label 2; EDX analysis in Fig. 7C). In some areas, Cr,W-rich phases emerge from the surface (Fig. 7B, label 1; EDX analysis in Fig. 7C): these could be the Cr,W-rich solid solution phase which was highlighted by etching (Fig. 2B). The basic corrosion mechanisms therefore remain substantially unaltered; the improvement, as for the Co800 coating, may be due to reduced interlamellar corrosion penetration, as mentioned also in [24].
3.3. Corrodkote test The Co800 and D4006 coatings suffer significant corrosion deterioration in the as-sprayed condition (Fig. 8A,B). Some reddish (i.e. Fe-containing) corrosion products, clearly witnessing substrate corrosion due to coating break-through, emerge after the first test cycle on as-sprayed Co800 (Fig. 8A, see circle) and D4006 (Fig. 8A, arrows). Coating degradation is worsened after the second cycle (Fig. 8B), when reddish corrosion products spread over large portions of both surfaces. The extent of corrosion damage on heat-treated coatings is certainly reduced (Fig. 8A,B). On the Co800 coating, reddish products are only found in a few pits, both after the first and after the second test cycle (Fig. 8A,B, see circles). Greater detail of these pits, containing
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reddish corrosion products, is seen in Fig. 9A (circle). No red Fecontaining corrosion products emerge on the D4006 surface. In the latter coating, the corrosion morphology changes from large corrosion-affected areas in the as-sprayed condition (like those seen in Fig. 8B) to more numerous and smaller pits in the heat-treated condition (optical micrograph in Fig. 9B). The improved corrosion resistance of Co800 and D4006 coatings after heat treatment is definitely consistent with the less active behaviour and lower interlamellar corrosion penetration displayed by both coatings in electrochemical tests, as discussed above. The electrochemical corrosion test outcomes for the Ni700 coating, instead, were less univocal. Consequently, there is no improvement in the corrodkote test behaviour (Fig. 8A,B). Specifi-
Fig. 8. Photographs of as-sprayed (label: AS) and heat-treated (label: HT) sample surfaces after corrodkote test. A: 1 cycle = 20 h; B: 2 cycles = 40 h.
cally, after 2 cycles, reddish corrosion products emerge in the heattreated coating along the boundaries of the slurry-coated region (circle in Fig. 8B). A detail of these products is provided in Fig. 9C. This phenomenon suggests an increased sensitivity to crevice corrosion after the heat treatment, as observed previously in the free corrosion test. Such crevice corrosion failure is not detected on the as-sprayed coating, so that the Ni700 coating’s overall outcome in the corrodkote test is worsened. 4. Conclusions
Fig. 7. Surface of heat-treated D4006 coating after electrochemical polarization testing. A: general view; circles indicate areas where protruding secondary phases exist; B: detail of an area with protruding secondary phases (label 1); C: EDX microanalysis of non-corroded alloy, dark corrosion products (label 2 in panel B) and protruding phases (label 1 in panel B).
The effect of a 600 °C, 1 h heat treatment on the corrosion performance of three HVOF-sprayed metal alloy coatings was evaluated by electrochemical corrosion tests and corrodkote test. In general, the heat treatment has two major effects on the tested coatings: it improves interlamellar cohesion, reducing active corrosion along interlamellar boundaries, but can also trigger galvanic microcells at intralamellar level, because of the formation of secondary phases. The first, beneficial effect prevails in the case of Co800 and D4006 coatings, so that an overall improvement in their corrosion resistance is found and they have lower corrosion current density, less active corrosion at interlamellar boundaries and improved corrodkote test resistance. The heat treatment is therefore an effective way to improve the overall performance of the Co800 and D4006 coatings. The properties of the heat-treated Co800 coating are particularly significant when compared to those of electrolytic hard chrome (EHC). By coupling the corrosion test outcomes to former results on tribological behaviour [19–21], we find that the corrosion resistance of heat-treated Co800 is comparable to that of EHC and its tribological characteristics far surpass EHC under various contact conditions. By contrast, the effects of the heat treatment on the corrosion resistance of Ni700 are less obvious. Most importantly, after the heat
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Thanks to Galvanica Nobili S.r.l. (Marano sul Panaro, Modena, Italy) and to Mr. Moreno Ghiaroni for the deposition of hard chrome platings. We are particularly grateful to Ing. Elisabetta Parsini and Ms. Elena Ternelli for their precious contribution to the experimental characterisation. Partially supported by PRRIITT (Regione EmiliaRomagna, Italy), Net-Lab “Surface and Coatings for Advanced Mechanics and Nanomechanics” (SUP&RMAN). References
Fig. 9. Optical micrographs of the surfaces of heat-treated Co800 (A), D4006 (B) and Ni700 (C) coatings after 2 corrodkote test cycles.
treatment, the Ni700 coating shows greater sensitivity to crevice corrosion, so that its overall corrosion resistance may seem to be reduced by the heat treatment. Acknowledgements Coating manufacturing by Centro Sviluppo Materiali S.p.A. (Roma, Italy), and in particular by Dr. Fabrizio Casadei, Mr. Edoardo Severini, Mr. Francesco Barulli and Mr. Carlo Costa, is gratefully acknowledged.
[1] H. Herman, S. Sampath, R. McCune, Mater. Res. Soc. Bull. 25 (2000) 17. [2] B. Sartwell, K. Legg, J. Sauer, Validation of WC/Co HVOF Thermal Spray Coatings as a Replacement for Hard Chrome Plating On Aircraft Landing Gear, Joint Test Report by U.S. Department of Defense, Environmental Security Technology Certification Program (ESTCP) and Joint Group on Pollution Prevention (JG-PP). [3] J.M. Guilemany, N. Espallargas, P.H. Suegama, A.V. Benedetti, J. Fernández, J. Therm. Spray Technol. 14 (2005) 335. [4] J.A. Picas, A. Forn, G. Matthäus, Wear 261 (2006) 477. [5] K.O. Legg, M. Graham, P. Chang, F. Rastagar, A. Gonzales, B. Sartwell, Surf. Coat. Technol. 81 (1996) 99. [6] G. Montavon, A. Vardelle, N. Krishnan, P. Ulloa, S. Costil, H. Liao, Building on 100 Years of Success — Proceedings of the International Thermal Spray Conference 2006, ASM International, Materials Park, OH, USA, 2006, Paper s17–10. [7] B. Evans, R. Panza-Giosa, E. Cochien-Brikaras, S. Maitland, Building on 100 Years of Success — Proceedings of the International Thermal Spray Conference 2006, ASM International, Materials Park, OH, USA, 2006, Paper s7–2. [8] G. Bolelli, V. Cannillo, L. Lusvarghi, S. Riccò, Surf. Coat. Technol. 200 (2006) 2995. [9] G. Bolelli, R. Giovanardi, L. Lusvarghi, T. Manfredini, Corros. Sci. 48 (2006) 3375. [10] D.W. Wheeler, R.J.K. Wood, Wear 258 (2005) 526. [11] T. Pearson, Trans. Inst. Met. Finish. 83 (2005) 2. [12] H.M. Hawthorne, B. Arsenault, J.P. Immarigeon, J.G. Legoux, V.R. Parameswaran, Wear 225–229 (1999) 825. [13] E. Celik, O. Culha, B. Uyulgan, N.F. Ak Azem, I. Ozdemir, A. Turk, Surf. Coat. Technol. 200 (2006) 4320. [14] A. Wank, B. Wielage, H. Pokhmurska, E. Friesen, G. Reisel, Surf. Coat. Technol. 201 (2006) 1975. [15] J.E. Cho, S.Y. Hwang, K.Y. Kim, Surf. Coat. Technol. 200 (2006) 2653. [16] V.A.D. Souza, A. Neville, Wear 259 (2005) 171. [17] V.A.D. Souza, A. Neville, J. Therm. Spray Technol. 15 (2006) 106. [18] D.E. Crawmer, in: J.R. Davis (Ed.), Handbook of Thermal Spray Technology, ASM International, Materials Park, OH, USA, 2004, p. 58. [19] G. Bolelli, L. Lusvarghi, J. Therm. Spray Technol. 15 (2006) 802. [20] G. Bolelli, V. Cannillo, L. Lusvarghi, M. Montorsi, F. Pighetti Mantini, M. Barletta, Wear 263 (2007) 1397. [21] G. Bolelli, L. Lusvarghi, F. Pighetti Mantini, M. Barletta, F. Casadei, Surf. Eng. 23 (2007) 355. [22] G. Bolelli, L. Lusvarghi, R. Giovanardi, Surf. Coat. Technol. (in press), doi:10.1016/j. surfcoat.2008.04.056. [23] L. Fedrizzi, S. Rossi, R. Cristel, P.L. Bonora, Electrochim. Acta 49 (2004) 2803. [24] C.H. Lee, K.O. Min, Surf. Coat. Technol. 132 (2000) 49. [25] R.D. Schmidt, D.P. Ferriss, Wear, 32 (1975) 279. [26] L. Gil, M.A. Prato, M.H. Staia, J. Therm. Spray Technol. 11 (2002) 95. [27] C. Navas, R. Colaço, J. de Damborenea, R. Vilar, Surf. Coat. Technol. 200 (2006) 6854. [28] Z. Liu, J. Cabrero, S. Niang, Z.Y. Al-Taha, Surf. Coat. Technol. 201 (2007) 7149. [29] J. Tuominen, P. Vuoristo, S. Ahmaniemi, J. Vihinen, P.H. Andersson, J. Therm. Spray Technol. 11 (2002) 233. [30] M.L. Capp, J.M. Rigsbee, Mater. Sci. Eng., A 62 (1984) 49. [31] C. Navas, R. Vijande, J.M. Cuetos, M.R. Fernández, J. de Damborenea, Surf. Coat. Technol. 201 (2006) 776. [32] B.S. Sidhu, D. Puri, S. Prakash, Mater. Sci. Eng., A 368 (2004) 149. [33] M. Stern, A.L. Geary, J. Electrochem. Soc. 104 (1957) 56. [34] J. Kawakita, S. Kuroda, T. Kodama, Surf. Coat. Technol. 166 (2003) 17. [35] Standard Test Method of Corrosion Testing of Decorative Electrodeposited Coatings by the Corrodkote Procedure, ASTM Standard B380-97/R02. [36] G. Wranglén, An Introduction to Corrosion and Protection of Metals, Institut för Metallskydd, Stockholm, Sweden, 1972, p. 239. [37] D. Zhang, S.J. Harris, D.G. McCartney, Mater. Sci. Eng., A 344 (2003) 45. [38] W.-M. Zhao, Y. Wang, L.-X. Dong, K.-Y. Wu, J. Xue, Surf. Coat. Technol. 190 (2005) 293. [39] M.P. Planche, B. Normand, H. Liao, G. Rannou, C. Coddet, Surf. Coat. Technol. 157 (2002) 247. [40] J. Kawakita, S. Kuroda, T. Fukushima, T. Kodama, Corros. Sci. 45 (2003) 2819. [41] T.E. Lister, R.N. Wright, P.J. Pinhero, W.D. Swank, J. Therm. Spray Technol. 11 (2002) 530. [42] J. Kruger, Corrosion: Fundamentals, Testing, and Protection — ASM Handbook, vol. 13A, ASM International, Materials Park, OH, USA, 2003, p. 61.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Heat treatment effects on the corrosion resistance of some HVOF-sprayed metal alloy coatings Giovanni Bolelli a,⁎, Luca Lusvarghi a, Massimiliano Barletta b a b
Department of Materials and Environmental Engineering, Università di Modena e Reggio Emilia, Via Vignolese, 905 — 41100 Modena (MO), Italy Department of Mechanical Engineering, Università di Roma — Tor Vergata, Via del Politecnico, 1 — 00133 Roma, Italy
A R T I C L E
I N F O
Article history: Received 21 September 2007 Accepted in revised form 15 April 2008 Available online 25 April 2008 PACS: 81.15.Rs 81.65.Kn 81.40.-z Keywords: High velocity oxyfuel (HVOF) Nickel alloy Immersion test Scanning electron microscopy (SEM) Electrochemical polarization test
A B S T R A C T The present study evaluates the effects of a 600 °C, 1 h heat treatment on the corrosion resistance of three High Velocity Oxygen Fuel (HVOF) flame-sprayed alloy coatings: a Co–28Mo–17Cr–3Si (similar to Tribaloy800) coating, a Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe (Diamalloy-4006) coating and a Ni–32Mo–16Cr–3Si– 2Co (similar to Tribaloy-700) coating. Electrochemical polarization tests and free corrosion tests were performed in 0.1 M HCl aqueous solution. The corrodkote test (ASTM B380-97R02) was also performed, to evaluate the coatings qualitatively. The heat treatment improves the corrosion resistance of the Co–28Mo– 17Cr–3Si coating and of the Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe coating by enhancing their passivation ability. The precipitation of sub-micron sized secondary phases after the treatment may produce galvanic microcells at intralamellar scale, but the beneficial contribution provided by the healing of the very small but dangerous interlamellar defects (normally present in thermal spray coatings but not detectable using ordinary scanning electron microscopy) prevails. The effect on Ni–32Mo–16Cr–3Si–2Co coatings is more ambiguous: its sensitivity to crevice corrosion is worsened by the heat treatment. © 2008 Elsevier B.V. All rights reserved.
1. Introduction High Velocity Oxygen Fuel (HVOF) flame-sprayed coatings are increasingly being considered for wear- and corrosion-resistant applications, thanks to the low defectiveness resulting from the supersonic gas velocity and limited flame temperature (compared to other thermal spraying techniques, like plasma-spraying) [1]. They are therefore also being proposed as an alternative to hard chrome plating, as they are able to offer comparable or superior technical performance [2–10] with a significantly lower environmental impact [2,11]. More specifically, much research has recently focused on HVOFsprayed cermets, which have already been shown to possess excellent tribological properties [12–14] as well as good corrosion resistance with a suitable carbide and metal matrix choice [15–17]. Fewer papers focus on HVOF-sprayed metal alloy coatings, which, although less wear resistant than cermets [8,12], can be very interesting on account of their higher deposition efficiency, far lower machining (grinding/ polishing) costs, and (usually) lower powder cost [18]. They may also possess some peculiar properties like high hot hardness and oxidation resistance.
⁎ Corresponding author. Tel.: +39 059 2056281; fax: +39 059 2056243. E-mail addresses: [email protected] (G. Bolelli), [email protected] (L. Lusvarghi), [email protected] (M. Barletta). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.074
For these reasons, the authors have recently concentrated their attention on HVOF-sprayed metal coatings, performing comparative studies on the wear and corrosion resistance of a number of these materials [19–22], in order to provide an improved understanding of their behaviour, find the best performing coatings under different operating conditions, and assess their technical viability as a hard chrome replacement. Specifically, comprehensive corrosion testing [22] enabled the authors to single out three metal alloy coatings with very promising characteristics, similar or even superior to electrolytic hard chrome: Co– 28Mo–17Cr–3Si (similar to Tribaloy®1-800), Ni–32Mo–16Cr–3Si–2Co (similar to Tribaloy®-700), Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe (Diamalloy-4006). Since industrial applications of HVOF-sprayed metal coatings can probably involve both corrosion and wear degradation [16,23], research on the tribological performance of these three alloys was also performed [19–21]. Interesting tribological properties were found and a heat treatment performed at relatively low temperature (600 °C, 1 h) was shown to be highly beneficial, especially against adhesive wear. Indeed, whereas as-sprayed coatings mainly consist of super-saturated solid solutions [19–21,24], heat treatments produce the precipitation of secondary hard phases: respectively, Co–Mo–Si Laves’ phases in the Co–Mo–Cr–Si coating [25]; Ni–Si or Mo–Si intermetallics
1
Tribaloy is a trademark of Deloro-Stellite.
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Table 1 Characteristics of the commercial spray powders used in this study, and short designation used in the paper Commercial name
Manufacturer
Designation
Nominal chemical composition (in weight%)
Nominal particle size range
Manufacturing method
Co-111 Ni 700 Diamalloy 4006
Praxair Sandvik-Osprey Sulzer Metco
Co800 Ni700 D4006
Co–28Mo–17Cr–3Si (Similar to Tribaloy-800®) Ni–32Mo–15Cr–3Si–2Co (Similar to Tribaloy-700®) Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe
−45 + 10 μm −53 + 10 μm −53 + 11 μm
Gas atomized Gas atomized Water atomized
in the Ni–Mo–Cr–Si coating; W, Mo, Cr-base carbides in the Diamalloy4006 coating. These phases were found to strengthen the coatings and enhance their tribological characteristics. It is therefore also very important to determine the effects of this heat treatment on the corrosion resistance of the coatings. In actual fact, the corrosion behaviour of a metal alloy is highly sensitive to its microstructure and it is reasonable to suspect that changes can occur in the chemical and electrochemical response of these coatings after the treatment [24]. The aim of this paper is to assess these effects using electrochemical and chemical corrosion tests. It is useful to remember that the available literature contains various studies dealing with the corrosion resistance of thermal spray coatings subject to heat treatments; however, they mainly deal with somewhat complex high-temperature treatments, such as high-temperature vacuum furnace treatments or furnace remelting [26,27]. These processes can effectively improve the corrosion resistance of a thermally-sprayed metal coating by changing its microstructure from a lamellar one to a bulk-like one, removing defects, such as pores and lamellae boundaries, which are known to impair performance and limit passivation capacity. However, their applicability is limited, as they significantly increase processing costs and can cause unacceptable microstructural alterations or geometrical distortions in the substrate. Laser treatments are another option in order to densify thermally-sprayed metal alloys. According to the laser processing conditions, a coating can be simply heat treated, in order to modify the microstructure of its top layer without affecting the bottom layers and the substrate [28], or it can be fully re-melted, in order to alter very deeply its microstructure and to promote metallurgical bonding with the substrate [28–32]. These treatments were reported to be very effective in enhancing the corrosion resistance [28–32], but, apart from their high costs, they can also pose certain drawbacks. Depending on the composition of the coating and on the specific type of laser, cracking of the treated surface can occur more or less easily [28,29,31], so that, in some cases, the optimal processing window is very narrow, and the proper adjustment must be found for each coating material [28], through long and costly experiments. Moreover, when full laser remelting is performed, the substrate is also affected and dilution inside the coating can occur [28]. In this study, by contrast, a simple heat treatment in air, performed at a lower temperature (600 °C) than those considered in the above cases, is examined.
(Metcolite-C, Sulzer Metco). Full details on powder characterisation and coating deposition parameters can be found in [22]. Part of the coated plates was heat treated in an electric kiln in air at 600 °C. The heat treatment program involved heating at a 15 °C/min rate, 1h isotherm, and natural cooling inside the kiln (no external control of cooling rate). Coatings were characterised by scanning electron microscopy (SEM: Philips XL-30 and Leo Supra 35 high-resolution field emission gun (FEG) microscope) observations of polished cross-sections (hotmounted in phenolic resin) and by X-ray diffraction (XRD, PANAlytical X’Pert Pro) analysis on polished top surfaces. Polishing was performed using SiC papers (up to 2000 mesh), diamond slurry (up to 0.5 μm particle size) and alumina slurry (0.2 μm particle size). 2.2. Corrosion testing Before each corrosion test, all samples were ground to a coating thickness of about 300 μm, polished to Ra ≈ 0.5 μm and ultrasonically cleaned and degreased with acetone. The thin oxide layer produced by the heat treatment on the coating surface was therefore completely removed and had no influence on test results. Anodic and cathodic electrochemical polarization tests were performed in contact with a 0.1M HCl aqueous solution in equilibrium with the environment at room temperature, using a three-electrode cell (K0235 flat cell, Princeton Applied Research) where the working electrode is the sample, pressed against a Teflon gasket leaving a 1cm2 exposed surface, the counter-electrode is a Pt grid; the reference electrode is an Ag/AgCl electrode. A PC-programmed EG&G 270 potentiostat/galvanostat (Princeton Applied Research, UK) was employed. The scanned potential range is − 300 mV/+ 1400 mV from rest potential, at a 0.5 mV/s scan rate. The tests started after 30 min of free corrosion (to allow full wetting of the coating surface and open circuit potential stabilization). Two tests were performed on each sample. The corrosion current density Icorr (A/cm2) and corrosion potential Ecorr (mV) were determined by Tafel analysis: linear portions of the anodic and cathodic curves were fitted by Tafel's law (1) E ¼ aFb:logðIÞ
ð1Þ
and the resulting linear system (2) is solved to find Ecorr and Icorr:
E ¼ aa þ ba :logðIÞ E ¼ ac bc :logðIÞ
ð2Þ
2. Experimental 2.1. Coating manufacturing and characterisation Three commercially available thermal spray powders were employed. Their characteristics, including nominal chemical composition, manufacturing method and designation used hereafter, are listed in Table 1. The powders were HVOF-sprayed using a Praxair-Tafa JP5000 kerosene-fuelled torch2 onto AISI 1040 steel plates (100 × 100 × 5)mm3, grit-blasted immediately prior to deposition using a manual vacuumoperated Norblast blasting machine with 500 μm angular alumina grits
2
Owned by Centro Sviluppo Materiali S.p.A., Roma, Italy.
where: aa, ba = anodic curve coefficients; ac, bc = cathodic curve coefficients. At least two samples were tested for each coating. The top surface and polished cross-section of corroded samples were also observed by SEM. The results obtained on an electrolytic hard chrome plating under the same conditions in former research [22] are also listed for comparison. Free corrosion tests were conducted for 2 days, in contact with 0.1M HCl aqueous solution in equilibrium with the environment at room temperature, using the K0235 flat cell, with the same experimental arrangement described above. Two tests were performed on each sample. The polarization resistance Rp was periodically measured by electrochemical polarization tests (EG&G 270 potentiostat/galvanostat) according to the Stern–Geary method [33]: a ± 20 mV overpotential range (from rest potential) was
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scanned (0.5 mV/s scan rate) and the slope of the linearly fitted E vs. I data is assumed as Rp. For each coating, the inverse of the polarization resistance 1/Rp is proportional to the corrosion current density according to Eq. (3) [33]. Icorr ¼
ba :bc 1 d 2:3:ðba þ bc Þ Rp
ð3Þ
However, a direct comparison between the corrosion current density values of the various coatings cannot be obtained by comparing 1/Rp values (i.e.: the coating displaying the lowest 1/Rp value is not necessarily the one with the lowest Icorr), since the proportionality coefficient between 1/Rp and Icorr depends on the anodic and cathodic Tafel slopes (as shown in Eq. (3)), which are specific to the system being tested, as also noted in [34]. Qualitative evaluation of the corrosion resistance of the coatings was also performed by means of the corrodkote test [35], specifically developed for Ni- and Cr-based coatings. A thick corrosive suspension must be uniformly spread on the coated sample surface with a paintbrush. The suspension consists of 60 g of kaolin + 100 ml of a solution containing NH4Cl (20 g/l), FeCl3·6 H2O (3.3 g/l), Cu (NO3)2·3 H2O (0.7 g/l). The sample is then kept for 20 h at 40 °C, 90% relative humidity. The suspension is then removed using water and a cloth, and the sample is visually inspected for a qualitative evaluation of corrosion. A new, fresh suspension is then reapplied for a new 20 htest cycle. This test is considered as being 100 times more accelerated than the salt spray test [36]. In this study, two 20 h-test cycles were performed. For each coating, two 25 mm × 25 mm samples, cut from the larger plates, were tested. The sides of the samples were protected with silicone resin to prevent the infiltration of corrosive agents. Digital photographs of the coatings were taken after each cycle. A better inspection of the corrosion morphology was also obtained by means of optical microscopy. 3. Results and discussion 3.1. Microstructural characterisation of coatings A complete microstructural characterisation of as-sprayed and heat-treated coatings has already been provided by the authors in former papers [19–22] and therefore only a brief overview is given here. As shown in those papers, as-sprayed Co800 and Ni700 coatings possess an amorphous or nanocrystalline metal matrix, producing a large hump in X-ray diffraction patterns, and display few, large-sized secondary phases (Fig. 1A, arrow), most probably coming from the microstructure of the original spray powder [22]. After the heat treatment, apart from those few large-sized secondary phases (Fig. 1B, arrow), numerous small-sized (sub-micrometric) secondary phases appear (Fig. 1B, circle. Detail of the sub-micrometric phases in Fig. 1C). In the case of Co800, XRD reveals the newly-formed phases are Co3Mo2Si, Co7Mo6 and CoSi2 intermetallics; in Ni700, they are Mo5Si3 and Ni3Si2 [20]. The as-sprayed D4006 coating displays no remarkable phase contrast in backscattered electrons (Fig. 2A), but, after etching (75 g HCl + 25 g HNO3 + 2 g CuCl2, 30s), it reveals a two-phase microstructure (Fig. 2B). These micrographs perfectly agree with a former literature study on HVOF-sprayed D4006 [24], where these two phases were identified as two solid solutions, one richer in Ni, the other richer in Cr and W. After the heat treatment, fine-grained carbides and borides (Cr23C6, WC, NiB2 [22,24]) appear (overview in Fig. 2C and detail in Fig. 2D: bright WC phases are labelled as 1 and darker Cr23C6, NiB2 are labelled as 2). The coatings look rather dense, though some closed porosity is recognizable (Fig. 3A, circles), and contain oxide inclusions, mainly in the form of dark stringers along lamellae boundaries (Fig. 3B). The
Fig. 1. SEM (backscattered electrons) micrograph of as-sprayed (A) and heat-treated (B) Ni700 coating, and high-magnification detail of the latter (C).
porosity and the oxide inclusion contents, as determined by image analysis, do not change significantly after the heat treatment (Table 2). However, as indicated in pertinent literature, image analysis based on ordinary SEM images is not sufficiently accurate in order to differentiate between the porosity values of these highly dense coatings [34]. More specifically, it seems inadequate at capturing those tiny interlamellar defects [37], which are potentially extremely dangerous because they may offer penetration paths to corrosive agents [38]. For instance, Zhang et al. [37] used the same HVOF torch in order to spray two Inconel-625 powders having different particle size distributions, and found that the resulting coatings had completely different corrosion behaviours, although their porosity values as determined from image analysis were similar and very low. They reported that, by performing image analysis on SEM micrographs of polished cross-sections, small interlamellar pores, dramatically affecting the coatings’ performances, were overlooked. This finding was also confirmed by the authors in former corrosion studies on
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Fig. 2. SEM micrographs of the D4006 coating: A: as-sprayed; B: as-sprayed, detail after etching; C: heat-treated; D: heat-treated, detail (label 1: bright WC phase; Label 2: dark Cr23C6 and Ni2B phases).
these same HVOF-sprayed alloy coatings [22]. So, image analysis results cannot rule out possible modifications to interlamellar cohesion. 3.2. Electrochemical corrosion tests The heat treatment relevantly modifies the anodic polarization curves of the coatings (Fig. 4A). The as-deposited Co800 coating shows a remarkable lack of passivation, although the high Cr and Mo content of this alloy should promote good passivating ability even in chlorinecontaining environments [25]. This lack of passivation in thermallysprayed metal alloys, resulting in a poorer corrosion resistance than in bulk form, is well-documented in pertinent literature [9,26,34,39,40]
and is thought to be due to a combination of factors, mainly involving the presence of lamellae boundaries. Indeed, when very small defects exist along lamellae boundaries, infiltration by corrosive agents is possible, thus impairing the coating’s protectiveness [37,38]. Moreover, if oxides are formed along those boundaries during coating deposition, the alloy composition, in their proximity, is deprived of those oxidizable elements (like Cr) which should provide passivation and corrosion protection [37,41]. Finally, chemical inhomogeneities like oxide inclusions can represent preferential sites for corrosion initiation. After the heat treatment, by contrast, the whole anodic polarization curve shifts to lower current density values, and its trend becomes less active, although there is no true passivation. The
Fig. 3. Low-magnification SEM overview of as-sprayed Ni700 coating: secondary electron micrograph (A, circles indicate closed porosity) and backscattered electron micrograph (B).
G. Bolelli et al. / Surface & Coatings Technology 202 (2008) 4839–4847 Table 2 Porosity and oxide inclusion amount in as-sprayed and heat-treated coatings Coating
Porosity %
Oxide inclusions %
Co800 as-sprayed Co800 heat-treated Ni700 as-sprayed Ni700 heat-treated D4006 as-sprayed D4006 heat-treated
1.4 ± 0.1 1.6 ± 0.6 2.2 ± 0.5 2.0 ± 0.4 2.9 ± 0.1 1.6 ± 0.9
8.1 ± 2.0 8.9 ± 1.5 7.1 ± 0.6 7.3 ± 2.3 5.9 ± 0.6 7.9 ± 2.6
corrosion potential therefore shifts to more noble values and the corrosion current density definitely decreases (Table 3). Moreover, the nobler and less active behaviour of the material reflects in a decreased and more stable 1/Rp value during the whole duration of the 2-day free corrosion test (Fig. 4B). The corrosion morphology of the Co800 coating subject to the electrochemical polarization test becomes more uniform after the heat treatment. The large pits found on the as-sprayed coating (Fig. 5A and ref. [22]) largely disappear (Fig. 5B), and, most remarkably, corrosion penetration along interlamellar boundaries (which become deeply marked in as-sprayed condition) would appear reduced (compare the morphology of as-deposited coating in Fig. 5C to that of the heat-treated coating in Fig. 5D). Higher magnification intralamellar-scale micrographs (Fig. 5E) also reveal selective corrosion of the metal matrix (label 2) around the secondary Mo-rich phases (label 1, EDX analysis in Fig. 5F).
Fig. 4. Electrochemical anodic and cathodic polarization curves (A) and inverse polarization resistance 1/Rp trends during free corrosion tests (B) for as-sprayed and heat-treated coatings.
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Table 3 Average corrosion current density (Icorr) and corrosion potential (Ecorr) of HVOF-sprayed coatings from electrochemical polarization tests Coating
Icorr [⁎10− 5 A/cm2]
Ecorr [mV]
Co800 as-sprayed Co800 heat-treated Ni700 as-sprayed Ni700 heat-treated D4006 as-sprayed D4006 heat-treated EHC
3.01 0.0656 1.95 0.182 5.64 2.27 0.149
−315.8 −241.1 − 307.9 −104.5 −312.0 −263.6 −141.6
Results obtained on electrolytic hard chrome (EHC) plating, under the same conditions, in a former research are also listed for comparison.
This phenomenon, which has already been found to occur in crystalline unmelted particles in as-deposited condition [22], is now more widely extended to the whole coating, due to increased intralamellar crystallization. The heat treatment therefore seems to produce two opposite effects on corrosion behaviour. On the one hand, the heat treatment reduces active corrosion along interlamellar boundaries. As explained above, corrosion activation along lamellae boundaries is one of the most remarkable causes of the lack of passivation in the as-deposited coating [9,26,34,39,40]. Reducing it could therefore contribute significantly to the overall decreased anodic activation of the material. This is probably due to an improvement in interlamellar cohesion during the heat treatment, possibly on account of solid state diffusion [24]. Modifications to the extremely small interlamellar defects can only be reliably revealed by corrosion testing, because ordinary image analysis techniques are not adequate for this purpose [34]. On the other hand, changing from a uniform intralamellar microstructure to a multi-phase one can produce galvanic microcells at an intralamellar level. However, interlamellar corrosion makes by far the largest contribution to corrosion penetration inside the coating, as shown in the micrographs in Fig. 5. So, the beneficial effect of the formerly discussed improvement in interlamellar corrosion resistance largely prevails over the onset of intralamellar galvanic microcells. The Ni700 coating also shifts to more noble corrosion potential values (Table 3, Fig. 4A); besides, 1/Rp is once again decreased and more stable (Fig. 4B). However, the overall effect of the heat treatment is not as obvious as in the former case. Whereas the as-sprayed coating exhibits two anodic “passive-like” stages, the first occurring at low anodic overpotential (potential range between − 250 mV and − 150 mV), with a constant current density of about 7 ⁎ 10− 5A/cm2, and the second occurring at high anodic overpotential (potential range between 200 mV and 800 mV), with higher current density (between 1 ⁎ 10− 3A/cm2 and 2 ⁎ 10− 3A/cm2), the heat-treated coating only retains the “passive-like” stage at high overpotential values (potential range between 200 mV and 800 mV). So, the first “passive-like” stage is lost: by contrast, continuous activation takes place at the beginning of the anodic polarization curve of the heat-treated Ni700 coating. However, the current density of the second “passive-like” stage (approximately between 4 ⁎ 10− 4A/cm2 and 6 ⁎ 10− 4A/cm2) is lower than in the as-sprayed condition. It is not therefore immediately clear whether the heat treatment has a favourable or adverse effect on the corrosion resistance of this coating. The term “passive-like” has been adopted on purpose, as the above-described stages cannot be considered true, complete passivation. Indeed, although in such stages current density does not increase, it settles at values that are too high for a perfect passivity state [42]. The lack of perfect passivation can be explained by the above-described existence of defects and lamellae boundaries. In actual fact, corrosion of interlamellar boundaries seems to take place both in the as-sprayed (Fig. 6A) and in the heat-treated (Fig. 6B) coating. Whereas the appearance of the corroded surfaces of as-
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Fig. 5. Surfaces of as-sprayed and heat-treated Co800 coatings after electrochemical polarization test. A: as-sprayed, low magnification; B: heat-treated, low magnification; C: assprayed, higher magnification; D: heat-treated, higher magnification; E: interlamellar detail of heat-treated coating showing metal matrix corrosion (2) between secondary phases (1); E: EDX analyses of Mo-rich emerging phases (1) and corroded matrix (2).
sprayed and heat-treated Co800 is significantly different (Fig. 5), changes to the corrosion morphology of Ni700 seem less substantial (Fig. 6A,B). Moreover, the sensitivity to crevice corrosion during the free corrosion test, which is very moderate in the as-sprayed condition, definitely increases after the heat treatment. Indeed, whereas regions not under the Teflon gasket show almost no damage after the free corrosion test (Fig. 6C), Fe-containing corrosion products appear on those under the Teflon gasket, indicating corrosion penetration down to the substrate in this area (Fig. 6D, EDX analysis in Fig. 6F). Selective corrosion of the metal matrix around the small crystalline phases also occurs under the Teflon gasket (Fig. 6E). D4006 shows the highest Icorr values of the tested coatings (Table 3). The heat treatment decreases its corrosion current density (although it is not as low as for Co800 and Ni700) and
improves its “passive-like” behaviour (Fig. 4A); indeed, at low anodic overpotentials, the current density retains a constant value of about 2 ⁎ 10− 5A/cm2 in the heat-treated coating, whereas it is about 9 ⁎ 10−5A/cm2 in the as-sprayed condition. This anodic polarization behaviour is definitely consistent with the only former literature report on this kind of alloy known to the authors [24], although, in that paper, the corrosive medium was 1M sulphuric acid. The authors, in a past research, showed that the anodic polarization behaviour of some HVOF coatings (also including the Co800 alloy) in acid environments is not particularly sensitive to the nature of the anions [9]: comparison of the present polarization curves to those in [24] may indicate that this observation can be extended to D4006 as well. As mentioned in [9], this would be very interesting, because it would mean that the coating behaviour can be more confidently predicted and relied upon, irrespective of the
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Fig. 6. Surface of Ni700 coatings after corrosion testing. A,B: surface of as-sprayed and heat-treated coating after electrochemical corrosion test; C: surface of heat-treated coating after free corrosion testing, area outside the Teflon gasket; D: surface of heat-treated coating after free corrosion testing, area under the Teflon gasket; E: surface of heat-treated coating after free corrosion testing, area under the Teflon gasket: detail of one of the regions indicated by circles in panel D. F: EDX microanalysis of non-corroded surface and of corrosion products (label 1 in panel D) on heat-treated coating after free corrosion testing.
acid medium; however, further investigation is obviously required before it is reasonable to make such a claim. The D4006 coating is able to retain a constant 1/Rp value throughout free corrosion test duration in as-sprayed condition, and it preserves this feature after the heat treatment. The corroded surface, both in as-sprayed and in heat-treated condition, is covered by a layer of corrosion products rich in O, W, Mo, Cr (Fig. 7A, B label 2; EDX analysis in Fig. 7C). In some areas, Cr,W-rich phases emerge from the surface (Fig. 7B, label 1; EDX analysis in Fig. 7C): these could be the Cr,W-rich solid solution phase which was highlighted by etching (Fig. 2B). The basic corrosion mechanisms therefore remain substantially unaltered; the improvement, as for the Co800 coating, may be due to reduced interlamellar corrosion penetration, as mentioned also in [24].
3.3. Corrodkote test The Co800 and D4006 coatings suffer significant corrosion deterioration in the as-sprayed condition (Fig. 8A,B). Some reddish (i.e. Fe-containing) corrosion products, clearly witnessing substrate corrosion due to coating break-through, emerge after the first test cycle on as-sprayed Co800 (Fig. 8A, see circle) and D4006 (Fig. 8A, arrows). Coating degradation is worsened after the second cycle (Fig. 8B), when reddish corrosion products spread over large portions of both surfaces. The extent of corrosion damage on heat-treated coatings is certainly reduced (Fig. 8A,B). On the Co800 coating, reddish products are only found in a few pits, both after the first and after the second test cycle (Fig. 8A,B, see circles). Greater detail of these pits, containing
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reddish corrosion products, is seen in Fig. 9A (circle). No red Fecontaining corrosion products emerge on the D4006 surface. In the latter coating, the corrosion morphology changes from large corrosion-affected areas in the as-sprayed condition (like those seen in Fig. 8B) to more numerous and smaller pits in the heat-treated condition (optical micrograph in Fig. 9B). The improved corrosion resistance of Co800 and D4006 coatings after heat treatment is definitely consistent with the less active behaviour and lower interlamellar corrosion penetration displayed by both coatings in electrochemical tests, as discussed above. The electrochemical corrosion test outcomes for the Ni700 coating, instead, were less univocal. Consequently, there is no improvement in the corrodkote test behaviour (Fig. 8A,B). Specifi-
Fig. 8. Photographs of as-sprayed (label: AS) and heat-treated (label: HT) sample surfaces after corrodkote test. A: 1 cycle = 20 h; B: 2 cycles = 40 h.
cally, after 2 cycles, reddish corrosion products emerge in the heattreated coating along the boundaries of the slurry-coated region (circle in Fig. 8B). A detail of these products is provided in Fig. 9C. This phenomenon suggests an increased sensitivity to crevice corrosion after the heat treatment, as observed previously in the free corrosion test. Such crevice corrosion failure is not detected on the as-sprayed coating, so that the Ni700 coating’s overall outcome in the corrodkote test is worsened. 4. Conclusions
Fig. 7. Surface of heat-treated D4006 coating after electrochemical polarization testing. A: general view; circles indicate areas where protruding secondary phases exist; B: detail of an area with protruding secondary phases (label 1); C: EDX microanalysis of non-corroded alloy, dark corrosion products (label 2 in panel B) and protruding phases (label 1 in panel B).
The effect of a 600 °C, 1 h heat treatment on the corrosion performance of three HVOF-sprayed metal alloy coatings was evaluated by electrochemical corrosion tests and corrodkote test. In general, the heat treatment has two major effects on the tested coatings: it improves interlamellar cohesion, reducing active corrosion along interlamellar boundaries, but can also trigger galvanic microcells at intralamellar level, because of the formation of secondary phases. The first, beneficial effect prevails in the case of Co800 and D4006 coatings, so that an overall improvement in their corrosion resistance is found and they have lower corrosion current density, less active corrosion at interlamellar boundaries and improved corrodkote test resistance. The heat treatment is therefore an effective way to improve the overall performance of the Co800 and D4006 coatings. The properties of the heat-treated Co800 coating are particularly significant when compared to those of electrolytic hard chrome (EHC). By coupling the corrosion test outcomes to former results on tribological behaviour [19–21], we find that the corrosion resistance of heat-treated Co800 is comparable to that of EHC and its tribological characteristics far surpass EHC under various contact conditions. By contrast, the effects of the heat treatment on the corrosion resistance of Ni700 are less obvious. Most importantly, after the heat
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Thanks to Galvanica Nobili S.r.l. (Marano sul Panaro, Modena, Italy) and to Mr. Moreno Ghiaroni for the deposition of hard chrome platings. We are particularly grateful to Ing. Elisabetta Parsini and Ms. Elena Ternelli for their precious contribution to the experimental characterisation. Partially supported by PRRIITT (Regione EmiliaRomagna, Italy), Net-Lab “Surface and Coatings for Advanced Mechanics and Nanomechanics” (SUP&RMAN). References
Fig. 9. Optical micrographs of the surfaces of heat-treated Co800 (A), D4006 (B) and Ni700 (C) coatings after 2 corrodkote test cycles.
treatment, the Ni700 coating shows greater sensitivity to crevice corrosion, so that its overall corrosion resistance may seem to be reduced by the heat treatment. Acknowledgements Coating manufacturing by Centro Sviluppo Materiali S.p.A. (Roma, Italy), and in particular by Dr. Fabrizio Casadei, Mr. Edoardo Severini, Mr. Francesco Barulli and Mr. Carlo Costa, is gratefully acknowledged.
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