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Phosphate conversion coatings on 35CrMnSi steels subjected to different heat treatments
Electrochemistry Communications 110 (2020) 106636
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Phosphate conversion coatings on 35CrMnSi steels subjected to different heat treatments Congcong Jiang, Xiuzhi Zhang, Dan Wang, Lina Zhang, Xin Cheng
T
⁎
University of Jinan, Jinan 250022, China
ARTICLE INFO
ABSTRACT
Keywords: Heat treatment 35CrMnSi steel Phosphate conversion coating Microstructure Corrosion resistance
Phosphate chemical conversion (PCC) coatings are widely used to improve the corrosion resistance of steels. However, the effects of different substrate heat treatments on the preparation and properties of PCC coatings have rarely been investigated. This research focuses on the influence of rolled (R), normalizing (N), quenching (Q), low-temperature tempering (L-T) and tempering (T) treatments of 35CrMnSi steels on subsequent PCC coating onto these steels. The results showed that the heat treatments played a vital role in coating formation rate, phosphate crystal size and coating anti-corrosion properties. The coating formation rate for heat-treated substrates was as follows: Q > T > L-T > N > R. The order of corrosion resistance of substrates was: R > N > LT > T > Q. The anti-corrosion resistance of PCC-coated steels after different heat treatments was as follows: T > L-T > Q. The PCC-coated tempered sample offered the best corrosion resistance due to the homogeneous and fine-crystal microstructure of the coating.
1. Introduction Alloy steels are excellent manufacturing materials and are used in welded and riveted structures in aerospace and appliance industries, due to their outstanding mechanical properties and workability [1–3]. However, these steel materials are still susceptible to a small degree of corrosion [4]. Phosphate chemical conversion (PCC) coatings were originally developed to protect iron and steels in corrosive environments [5]. PCC coatings provide an electrical isolation layer of insoluble crystalline phosphates [6–11] on the surface of metals and alloys by forming a continuous and complete layer, and also adhere to the metal matrix [12,13]. The first report of a PCC coating to prevent the corrosion and rusting of iron and steel was in 1869, and since then PCC treatments have played a significant role in many fields, including automobile manufacture, cold processing, and appliance industries [6,14]. Most PCC studies have focused mainly on process parameters (pH, treatment temperature, and time) and bath compositions [8,9,15–19]. However, reports on the effect of the substrate on the PCC coating, especially the effect of different heat-treatments on the substrate, are not common in the literature. There are only a few reports on the effect of heat-treated substrates on PCC coatings. Ilaiyavel et al. [20,21] revealed the effect of different heat-treatment processes (mill annealing, full annealing, and air hardening, followed by low-temperature
tempering) on manganese-phosphate (Mn-P) coatings on AISI D2 steel. Manna et al. [22,23] obtained acicular phosphates on tempered martensitic steels with oxide scale (T-M-O) and tempered martensitic (T-M) steels, while coarser phosphate was formed on ferritic-pearlitic (F-P) substrates. The thickness of coatings on T-M-O, T-M, and F-P substrates decreased in this sequence. Totik [24] investigated the effect of quenching and tempering on Mn-P coatings on AISI 4140 steel, and revealed that the coated quenched sample showed superior corrosion resistance compared with a tempered sample. However, further studies are needed on the influence of the microstructure and properties of substrate steels on the quality of PCC coatings. In this study, the effects of different heat treatments applied to 35CrMnSi steels on the microstructure, mineral components, surface roughness, and the corrosion properties of subsequent PCC coatings were examined in detail. The aim was to contribute a theoretical guide to the protective treatment of steels for different applications. 2. Experimental 2.1. Heat treatments of steel substrates Heat treatments were carried out on cubic 35CrMnSi steel substrates with a volume of 10 × 10 × 10 mm3. Their chemical compositions are
⁎ Corresponding author at: Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, No. 336 Nanxinzhuang West Road, Jinan 250022, China. E-mail address: [email protected] (X. Cheng).
https://doi.org/10.1016/j.elecom.2019.106636 Received 3 November 2019; Received in revised form 5 December 2019; Accepted 5 December 2019 Available online 13 December 2019 1388-2481/ © 2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Electrochemistry Communications 110 (2020) 106636
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martensite (Fig. 2c), tempered martensite (Fig. 2d) and sorbites (Fig. 2e), respectively. Fig. 2f shows the corresponding XRD patterns, which revealed that the phase results were consistent with the above microstructure analysis. Every metallurgical structure has a unique energy state, inner stress and potential value, thus for various metallurgical structures, the susceptibility to corrosion in the acidic PCC bath was different [22,23]. In fact, substrate corrosion has been reported to be a prerequisite for the deposition of a PCC coating onto a substrate [5,27,28]. Hence, the different microstructures of the substrates will affect the formation of PCC coatings on their surface. The FE-SEM images of PCC coatings treated for 1 min on different heat-treated 35CrMnSi substrates in Fig. 3 show some relatively significant differences in terms of the number of crystals. Sparse, plate-like crystals were deposited on R and N substrates (Fig. 3a–d), while the L-T and T substrates contained more plate-like crystals on their surfaces (Fig. 3g–j). The Q substrate showed the largest number and most dense arrangement of crystals (Fig. 3e–f), which was mainly due to the nonequilibrium microstructure of this substrate. This unstable high-surfaceenergy microstructure promotes the formation of phosphates and thus contributes to the development of the PCC coating [26]. The rate of formation of the coating on heat-treated substrates was as follows: Q > T > L-T > N > R. In practice, coating formation is related to the potential of a substrate surface, and a negative potential is beneficial to the formation of a PCC coating [28,29]. Fig. 4 depicts micrographs of PCC coatings treated for 20 min on 35CrMnSi substrates which had been subjected to different heat treatments. All substrates were covered by large, close-packed, lath-like crystals after 20 min immersion in the bath, and different heat treatments changed the crystal size of the PCC coatings on the substrates. Obviously, the tempered samples presented the finest crystals, with sizes of about 2–6 μm (Fig. 4i–j). This indicates that the microstructure of the substrate surface affects the size of the crystals in the resulting PCC coating. The XRD results of PCC coatings treated for 20 min on different heat-treated 35CrMnSi substrates (Fig. 5A) shows no noticeable differences in diffraction peaks between the different samples. All PCC coatings consisted of hopeite (Zn3(PO4)2·4H2O) and minor phosphophyllite (Zn2Fe(PO4)2·4H2O). The patterns of the various samples showed diffraction peaks of varying intensities due to the different coating thicknesses and/or crystal sizes produced by the different heat treatments [26]. The rolled and normalized samples exhibited higher diffraction intensities due to the presence of coarser crystals in the coatings. The tempered samples showed the lowest diffraction intensities due to the fine-crystal microstructure of the PCC coating. These results are consistent with the above SEM observations. Surface roughness, measured in Ra of the arithmetical mean deviation of the assessed profile, is an important parameter of the surface because it affects the resulting properties, such as corrosion resistance or paint adhesion. Fine microstructures with a low surface roughness are conducive to the surface protection of a coating as an anticorrosive layer, while a coarse microstructure with high surface roughness is beneficial to the paint adhesion of a coating [4]. The surface roughness of PCC-coated specimens with substrates subjected to different heat treatments is shown in Fig. 5B. Moreover, the surface roughness was closely related to the surface microstructure of the PCC coating. The surface roughness of PCC-coated heat-treated substrates was as follows: R > N > Q > L-T > T. This result will help in the application of PCC to substrates subjected to different heat treatments for use in corrosion protection or painting.
Table 1 Chemical composition of 35CrMnSi steel (wt. %). Element
C
Si
Mn
Cr
S
P
Fe
Content
0.35
1.16
0.82
1.17
0.01
0.022
balance
Table 2 Heat treatment procedures for 35CrMnSi steel. Items
Heat treatment procedure
Rolling (R) Normalizing (N) Quenching (Q) Low temperature tempering (L-T) Tempering (T)
– 900 °C/15 min austenited + air quenched 880 °C/15 min austenited + oil quenched 880 °C/15 min austenited + oil quenched + 240 °C/1h tempered + air cooling 880 °C/15 min austenited + oil quenched + 600 °C/1h tempered + air cooling
given in Table 1, and descriptions of the different heat treatment procedures [Rolling (R), Normalizing (N), Quenching (Q), Low-temperature tempering (L-T) and Tempering (T)] applied to the steels are provided in Table 2. 2.2. Preparation of PCC coatings Heat-treated samples were pretreated in a process that included abrading using 180, 400, 600, and 1000 emery papers, degreasing in NaOH solution, pickling in H3PO4, and activation in a colloidal Ti solution [25,26]. The pretreated steels were then soaked in a PCC bath whose composition is given in Table 3. Lastly, the coupons were cleaned with deionized water and then dried with an air gun. A schematic illustration of the heat treatments and PCC processing steps is shown in Fig. 1. 2.3. Measurements and characterization Optical observations of etched 35CrMnSi steels after different heat treatments were performed using a Nikon EPIPHOT-300 optical microscope. The micro morphologies of the coatings were observed using a SUPRA™ 55 thermal field emission scanning electron microscope (FESEM). Mineral components were detected using an X-ray diffractometer (XRD, Rigaku D/max-γB) with CuKα radiation at 40 kV and 100 mA. Surface roughness measurements were performed using a VK-X200 3D laser scanning microscope. The corrosion behavior of PCC-coated samples was characterized using a CHI600E electrochemical workstation using a 3.5 wt% sodium chloride solution as the corrosive medium. Potentiodynamic polarization experiments were carried out at a constant voltage scan rate of 1 mV/s and a polarization potential range of ± 250 mV. 3. Results and discussion 3.1. Characterization The optical microscopy images of the microstructures of 35CrMnSi steel after different heat treatments are shown in Fig. 2. The microstructure of the R steel contained thick ferrite and pearlite phases (Fig. 2a). Meanwhile, the microstructures after N, Q, L-T, and T treatments were transformed into relatively fine pearlite (Fig. 2b), quenched Table 3 The PCC bath components and parameters. Bath constituents
Quantity
Parameters
Quantity
ZnOHNO3H3PO4NaClO3C6H8O7
25 g/L30 ml/L15 ml/L2 g/L5 g/L
pHTimeTemperature
2.75 ± 0.051/20 min25 ± 5 °C
2
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Fig. 1. Schematic diagram of experimental procedures.
3.2. Corrosion performance
there is a close relationship between icorr and corrosion rate as follows:
Based on the polarization curves of blank and PCC-coated 35CrMnSi samples shown in Fig. 5C, the corrosion potential (Ecorr) and corrosion current densities (icorr) were calculated by Tafel extrapolation and are listed in Table 4. The results suggest that Ecorr reflects the corrosion trend and/or possibility of corrosion in a particular medium, and that
v=
Micorr nF
(1)
where v is the corrosion rate, M is the molecular weight, n is the valency, and F is Faraday’s constant. According to Eq. (1), v is proportional to icorr. Thus, a high value of Ecorr and a low value of icorr may
Fig. 2. Optical microscope images of polished and etched 35CrMnSi steels after (a) R, (b) N, (c) Q, (d) L-T, and (e) T treatments and (f) the corresponding XRD patterns (F: Ferrite, P: Pearlite, M: Martensite, S: Sorbite). 3
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Fig. 3. FE-SEM images of PCC-coated 35CrMnSi steel for 1 min after (a) R, (c) N, (e) Q, (g) L-T, and (i) T treatments. (b), (d), (f), (h) and (j) show the corresponding high-magnification images.
indicate a sample with good corrosion resistance [8,9,30]. From Fig. 5C1 and Table 4, Ecorr follows the order: R > N > L-T > T > Q. Additionally, Ecorr and icorr showed opposite trends, and the Q substrate with a low Ecorr and high icorr showed the worst corrosion resistance. The L-T and T substrates showed a positive potential compared with the Q substrate, while the R and N substrates exhibited outstanding anti-
corrosion properties. Therefore, the heat treatment significantly affected the corrosion behavior of the steels studied. The corrosion resistance of the PCC-coated samples was related to both the heat treatments and also the coating morphology and defects, because the defects and porosity of the coatings produced gaps between the corrosive environment and the less-noble substrate [3,31]. From 4
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Fig. 4. FE-SEM images of PCC-coated 35CrMnSi steel for 20 min after (a) R, (c) N, (e) Q, (g) L-T, and (i) T treatments. (b), (d), (f), (h) and (j) are the corresponding high-magnification images.
Fig. 5C2 and Table 4, PCC-coated specimens showed higher Ecorr values than the corresponding bare substrates. On the other hand, PCC-coated samples showed a lower corrosion rate compared to the bare substrate only for Q, L-T and T samples. For R and N samples before and after PCC, the icorr is similar within the margin of error. Therefore, for rolled and normalized materials, the PCC seemingly conferred no benefit with
regards to corrosion rate linked to Icorr. This indicates that the PCC coating imparted better corrosion resistance to bare steels after Q, L-T and T heat treatments. Furthermore, the PCC-coated tempered sample showed the highest Ecorr (−0.06 ± 0.03 V) and the lowest icorr (0.11 ± 0.02 μA cm−2), which produced the best anti-corrosion properties among the samples studied. This result was due to the 5
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Fig. 5. (A) XRD patterns, (B) surface roughness, and (C) polarization curves of (C1) bare substrates and (C2) PCC-coated samples after R, N, Q, L-T, and T treatments.
4. Conclusions
Table 4 Fitting values of electrochemical parameters of bare substrates and PCC-coated samples after R, N, Q, L-T, and T treatments.a. Sample
Ecorr (VSCE)
icorr (μA·cm−2)
Bare substrates
R
−0.34 ± 0.06
0.18 ± 0.05
PCC coated samples
N Q L-T T R N Q L-T T
−0.38 −0.62 −0.46 −0.53 −0.24 −0.20 −0.15 −0.11 −0.06
0.25 1.20 0.32 0.40 0.20 0.26 0.12 0.13 0.11
a
± ± ± ± ± ± ± ± ±
0.08 0.13 0.10 0.11 0.04 0.02 0.01 0.01 0.03
± ± ± ± ± ± ± ± ±
The application of phosphate chemical conversion (PCC) coatings to 35CrMnSi steels subjected to different heat treatments was investigated. Prior to coating, normalizing (N), quenching (Q), low-temperature tempering (L-T), and tempering (T) treatments were applied to the rolled (R) 35CrMnSi steels. It was found that the heat treatments affected the microstructure and anti-corrosion properties of the PCC coatings applied to the steel. The coating formation rate for heat-treated substrates was as follows: Q > T > L-T > N > R. Meanwhile, heat treatments also affected the size of crystals within the coating, with the tempered sample presenting the finest crystals. Electrochemical analysis showed that the order of corrosion resistance of substrates was: R > N > L-T > T > Q, while the corrosion resistance of PCC-coated steels after different heat treatments was as follows: T > L-T > Q. The PCC-coated tempered sample shows the best corrosion resistance due to the homogeneous, fine-crystal microstructure of the coating on the tempered substrate.
0.06 0.24 0.03 0.12 0.01 0.03 0.02 0.03 0.02
Entries are average values, N = 3.
homogeneous and fine-crystal microstructure of the PCC coating formed on the tempered substrate. Manna et al. [22,23] obtained a thick coating with good corrosion resistance on tempered martensitic steel substrates, while a thin coating with coarser phosphates was formed on a ferritic-pearlitic steel substrate, which is consistent with the above results. However, Totik [24] investigated the influence of quenching and tempering on Mn-P coatings on AISI 4140 steel and revealed that the coating on a quenched substrate had a better anti-rust property than a tempered sample. This was the opposite to the results obtained in this study, which may be due to the differences in crystal shape and preferred growth directions between zinc phosphate with plate-like crystals and manganese phosphate with cylindrical crystals. To summarize, the heat treatment of steel substrates can affect the microstructure and corrosion behavior of the coatings.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by Shandong Provincial Natural Science Foundation of China (ZR2018LE004), Major Basic Research Program of Shandong Provincial Natural Science Foundation of China (ZR2018ZC0741) and National Key Research and Development Program of China (2017YFC0703100) as well as National Natural 6
Electrochemistry Communications 110 (2020) 106636
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Science Foundation of China (51632003), the Taishan Scholars Program, Case-by-Case Project for Top Outstanding Talents of Jinan.
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Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Phosphate conversion coatings on 35CrMnSi steels subjected to different heat treatments Congcong Jiang, Xiuzhi Zhang, Dan Wang, Lina Zhang, Xin Cheng
T
⁎
University of Jinan, Jinan 250022, China
ARTICLE INFO
ABSTRACT
Keywords: Heat treatment 35CrMnSi steel Phosphate conversion coating Microstructure Corrosion resistance
Phosphate chemical conversion (PCC) coatings are widely used to improve the corrosion resistance of steels. However, the effects of different substrate heat treatments on the preparation and properties of PCC coatings have rarely been investigated. This research focuses on the influence of rolled (R), normalizing (N), quenching (Q), low-temperature tempering (L-T) and tempering (T) treatments of 35CrMnSi steels on subsequent PCC coating onto these steels. The results showed that the heat treatments played a vital role in coating formation rate, phosphate crystal size and coating anti-corrosion properties. The coating formation rate for heat-treated substrates was as follows: Q > T > L-T > N > R. The order of corrosion resistance of substrates was: R > N > LT > T > Q. The anti-corrosion resistance of PCC-coated steels after different heat treatments was as follows: T > L-T > Q. The PCC-coated tempered sample offered the best corrosion resistance due to the homogeneous and fine-crystal microstructure of the coating.
1. Introduction Alloy steels are excellent manufacturing materials and are used in welded and riveted structures in aerospace and appliance industries, due to their outstanding mechanical properties and workability [1–3]. However, these steel materials are still susceptible to a small degree of corrosion [4]. Phosphate chemical conversion (PCC) coatings were originally developed to protect iron and steels in corrosive environments [5]. PCC coatings provide an electrical isolation layer of insoluble crystalline phosphates [6–11] on the surface of metals and alloys by forming a continuous and complete layer, and also adhere to the metal matrix [12,13]. The first report of a PCC coating to prevent the corrosion and rusting of iron and steel was in 1869, and since then PCC treatments have played a significant role in many fields, including automobile manufacture, cold processing, and appliance industries [6,14]. Most PCC studies have focused mainly on process parameters (pH, treatment temperature, and time) and bath compositions [8,9,15–19]. However, reports on the effect of the substrate on the PCC coating, especially the effect of different heat-treatments on the substrate, are not common in the literature. There are only a few reports on the effect of heat-treated substrates on PCC coatings. Ilaiyavel et al. [20,21] revealed the effect of different heat-treatment processes (mill annealing, full annealing, and air hardening, followed by low-temperature
tempering) on manganese-phosphate (Mn-P) coatings on AISI D2 steel. Manna et al. [22,23] obtained acicular phosphates on tempered martensitic steels with oxide scale (T-M-O) and tempered martensitic (T-M) steels, while coarser phosphate was formed on ferritic-pearlitic (F-P) substrates. The thickness of coatings on T-M-O, T-M, and F-P substrates decreased in this sequence. Totik [24] investigated the effect of quenching and tempering on Mn-P coatings on AISI 4140 steel, and revealed that the coated quenched sample showed superior corrosion resistance compared with a tempered sample. However, further studies are needed on the influence of the microstructure and properties of substrate steels on the quality of PCC coatings. In this study, the effects of different heat treatments applied to 35CrMnSi steels on the microstructure, mineral components, surface roughness, and the corrosion properties of subsequent PCC coatings were examined in detail. The aim was to contribute a theoretical guide to the protective treatment of steels for different applications. 2. Experimental 2.1. Heat treatments of steel substrates Heat treatments were carried out on cubic 35CrMnSi steel substrates with a volume of 10 × 10 × 10 mm3. Their chemical compositions are
⁎ Corresponding author at: Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, No. 336 Nanxinzhuang West Road, Jinan 250022, China. E-mail address: [email protected] (X. Cheng).
https://doi.org/10.1016/j.elecom.2019.106636 Received 3 November 2019; Received in revised form 5 December 2019; Accepted 5 December 2019 Available online 13 December 2019 1388-2481/ © 2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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martensite (Fig. 2c), tempered martensite (Fig. 2d) and sorbites (Fig. 2e), respectively. Fig. 2f shows the corresponding XRD patterns, which revealed that the phase results were consistent with the above microstructure analysis. Every metallurgical structure has a unique energy state, inner stress and potential value, thus for various metallurgical structures, the susceptibility to corrosion in the acidic PCC bath was different [22,23]. In fact, substrate corrosion has been reported to be a prerequisite for the deposition of a PCC coating onto a substrate [5,27,28]. Hence, the different microstructures of the substrates will affect the formation of PCC coatings on their surface. The FE-SEM images of PCC coatings treated for 1 min on different heat-treated 35CrMnSi substrates in Fig. 3 show some relatively significant differences in terms of the number of crystals. Sparse, plate-like crystals were deposited on R and N substrates (Fig. 3a–d), while the L-T and T substrates contained more plate-like crystals on their surfaces (Fig. 3g–j). The Q substrate showed the largest number and most dense arrangement of crystals (Fig. 3e–f), which was mainly due to the nonequilibrium microstructure of this substrate. This unstable high-surfaceenergy microstructure promotes the formation of phosphates and thus contributes to the development of the PCC coating [26]. The rate of formation of the coating on heat-treated substrates was as follows: Q > T > L-T > N > R. In practice, coating formation is related to the potential of a substrate surface, and a negative potential is beneficial to the formation of a PCC coating [28,29]. Fig. 4 depicts micrographs of PCC coatings treated for 20 min on 35CrMnSi substrates which had been subjected to different heat treatments. All substrates were covered by large, close-packed, lath-like crystals after 20 min immersion in the bath, and different heat treatments changed the crystal size of the PCC coatings on the substrates. Obviously, the tempered samples presented the finest crystals, with sizes of about 2–6 μm (Fig. 4i–j). This indicates that the microstructure of the substrate surface affects the size of the crystals in the resulting PCC coating. The XRD results of PCC coatings treated for 20 min on different heat-treated 35CrMnSi substrates (Fig. 5A) shows no noticeable differences in diffraction peaks between the different samples. All PCC coatings consisted of hopeite (Zn3(PO4)2·4H2O) and minor phosphophyllite (Zn2Fe(PO4)2·4H2O). The patterns of the various samples showed diffraction peaks of varying intensities due to the different coating thicknesses and/or crystal sizes produced by the different heat treatments [26]. The rolled and normalized samples exhibited higher diffraction intensities due to the presence of coarser crystals in the coatings. The tempered samples showed the lowest diffraction intensities due to the fine-crystal microstructure of the PCC coating. These results are consistent with the above SEM observations. Surface roughness, measured in Ra of the arithmetical mean deviation of the assessed profile, is an important parameter of the surface because it affects the resulting properties, such as corrosion resistance or paint adhesion. Fine microstructures with a low surface roughness are conducive to the surface protection of a coating as an anticorrosive layer, while a coarse microstructure with high surface roughness is beneficial to the paint adhesion of a coating [4]. The surface roughness of PCC-coated specimens with substrates subjected to different heat treatments is shown in Fig. 5B. Moreover, the surface roughness was closely related to the surface microstructure of the PCC coating. The surface roughness of PCC-coated heat-treated substrates was as follows: R > N > Q > L-T > T. This result will help in the application of PCC to substrates subjected to different heat treatments for use in corrosion protection or painting.
Table 1 Chemical composition of 35CrMnSi steel (wt. %). Element
C
Si
Mn
Cr
S
P
Fe
Content
0.35
1.16
0.82
1.17
0.01
0.022
balance
Table 2 Heat treatment procedures for 35CrMnSi steel. Items
Heat treatment procedure
Rolling (R) Normalizing (N) Quenching (Q) Low temperature tempering (L-T) Tempering (T)
– 900 °C/15 min austenited + air quenched 880 °C/15 min austenited + oil quenched 880 °C/15 min austenited + oil quenched + 240 °C/1h tempered + air cooling 880 °C/15 min austenited + oil quenched + 600 °C/1h tempered + air cooling
given in Table 1, and descriptions of the different heat treatment procedures [Rolling (R), Normalizing (N), Quenching (Q), Low-temperature tempering (L-T) and Tempering (T)] applied to the steels are provided in Table 2. 2.2. Preparation of PCC coatings Heat-treated samples were pretreated in a process that included abrading using 180, 400, 600, and 1000 emery papers, degreasing in NaOH solution, pickling in H3PO4, and activation in a colloidal Ti solution [25,26]. The pretreated steels were then soaked in a PCC bath whose composition is given in Table 3. Lastly, the coupons were cleaned with deionized water and then dried with an air gun. A schematic illustration of the heat treatments and PCC processing steps is shown in Fig. 1. 2.3. Measurements and characterization Optical observations of etched 35CrMnSi steels after different heat treatments were performed using a Nikon EPIPHOT-300 optical microscope. The micro morphologies of the coatings were observed using a SUPRA™ 55 thermal field emission scanning electron microscope (FESEM). Mineral components were detected using an X-ray diffractometer (XRD, Rigaku D/max-γB) with CuKα radiation at 40 kV and 100 mA. Surface roughness measurements were performed using a VK-X200 3D laser scanning microscope. The corrosion behavior of PCC-coated samples was characterized using a CHI600E electrochemical workstation using a 3.5 wt% sodium chloride solution as the corrosive medium. Potentiodynamic polarization experiments were carried out at a constant voltage scan rate of 1 mV/s and a polarization potential range of ± 250 mV. 3. Results and discussion 3.1. Characterization The optical microscopy images of the microstructures of 35CrMnSi steel after different heat treatments are shown in Fig. 2. The microstructure of the R steel contained thick ferrite and pearlite phases (Fig. 2a). Meanwhile, the microstructures after N, Q, L-T, and T treatments were transformed into relatively fine pearlite (Fig. 2b), quenched Table 3 The PCC bath components and parameters. Bath constituents
Quantity
Parameters
Quantity
ZnOHNO3H3PO4NaClO3C6H8O7
25 g/L30 ml/L15 ml/L2 g/L5 g/L
pHTimeTemperature
2.75 ± 0.051/20 min25 ± 5 °C
2
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Fig. 1. Schematic diagram of experimental procedures.
3.2. Corrosion performance
there is a close relationship between icorr and corrosion rate as follows:
Based on the polarization curves of blank and PCC-coated 35CrMnSi samples shown in Fig. 5C, the corrosion potential (Ecorr) and corrosion current densities (icorr) were calculated by Tafel extrapolation and are listed in Table 4. The results suggest that Ecorr reflects the corrosion trend and/or possibility of corrosion in a particular medium, and that
v=
Micorr nF
(1)
where v is the corrosion rate, M is the molecular weight, n is the valency, and F is Faraday’s constant. According to Eq. (1), v is proportional to icorr. Thus, a high value of Ecorr and a low value of icorr may
Fig. 2. Optical microscope images of polished and etched 35CrMnSi steels after (a) R, (b) N, (c) Q, (d) L-T, and (e) T treatments and (f) the corresponding XRD patterns (F: Ferrite, P: Pearlite, M: Martensite, S: Sorbite). 3
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Fig. 3. FE-SEM images of PCC-coated 35CrMnSi steel for 1 min after (a) R, (c) N, (e) Q, (g) L-T, and (i) T treatments. (b), (d), (f), (h) and (j) show the corresponding high-magnification images.
indicate a sample with good corrosion resistance [8,9,30]. From Fig. 5C1 and Table 4, Ecorr follows the order: R > N > L-T > T > Q. Additionally, Ecorr and icorr showed opposite trends, and the Q substrate with a low Ecorr and high icorr showed the worst corrosion resistance. The L-T and T substrates showed a positive potential compared with the Q substrate, while the R and N substrates exhibited outstanding anti-
corrosion properties. Therefore, the heat treatment significantly affected the corrosion behavior of the steels studied. The corrosion resistance of the PCC-coated samples was related to both the heat treatments and also the coating morphology and defects, because the defects and porosity of the coatings produced gaps between the corrosive environment and the less-noble substrate [3,31]. From 4
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Fig. 4. FE-SEM images of PCC-coated 35CrMnSi steel for 20 min after (a) R, (c) N, (e) Q, (g) L-T, and (i) T treatments. (b), (d), (f), (h) and (j) are the corresponding high-magnification images.
Fig. 5C2 and Table 4, PCC-coated specimens showed higher Ecorr values than the corresponding bare substrates. On the other hand, PCC-coated samples showed a lower corrosion rate compared to the bare substrate only for Q, L-T and T samples. For R and N samples before and after PCC, the icorr is similar within the margin of error. Therefore, for rolled and normalized materials, the PCC seemingly conferred no benefit with
regards to corrosion rate linked to Icorr. This indicates that the PCC coating imparted better corrosion resistance to bare steels after Q, L-T and T heat treatments. Furthermore, the PCC-coated tempered sample showed the highest Ecorr (−0.06 ± 0.03 V) and the lowest icorr (0.11 ± 0.02 μA cm−2), which produced the best anti-corrosion properties among the samples studied. This result was due to the 5
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Fig. 5. (A) XRD patterns, (B) surface roughness, and (C) polarization curves of (C1) bare substrates and (C2) PCC-coated samples after R, N, Q, L-T, and T treatments.
4. Conclusions
Table 4 Fitting values of electrochemical parameters of bare substrates and PCC-coated samples after R, N, Q, L-T, and T treatments.a. Sample
Ecorr (VSCE)
icorr (μA·cm−2)
Bare substrates
R
−0.34 ± 0.06
0.18 ± 0.05
PCC coated samples
N Q L-T T R N Q L-T T
−0.38 −0.62 −0.46 −0.53 −0.24 −0.20 −0.15 −0.11 −0.06
0.25 1.20 0.32 0.40 0.20 0.26 0.12 0.13 0.11
a
± ± ± ± ± ± ± ± ±
0.08 0.13 0.10 0.11 0.04 0.02 0.01 0.01 0.03
± ± ± ± ± ± ± ± ±
The application of phosphate chemical conversion (PCC) coatings to 35CrMnSi steels subjected to different heat treatments was investigated. Prior to coating, normalizing (N), quenching (Q), low-temperature tempering (L-T), and tempering (T) treatments were applied to the rolled (R) 35CrMnSi steels. It was found that the heat treatments affected the microstructure and anti-corrosion properties of the PCC coatings applied to the steel. The coating formation rate for heat-treated substrates was as follows: Q > T > L-T > N > R. Meanwhile, heat treatments also affected the size of crystals within the coating, with the tempered sample presenting the finest crystals. Electrochemical analysis showed that the order of corrosion resistance of substrates was: R > N > L-T > T > Q, while the corrosion resistance of PCC-coated steels after different heat treatments was as follows: T > L-T > Q. The PCC-coated tempered sample shows the best corrosion resistance due to the homogeneous, fine-crystal microstructure of the coating on the tempered substrate.
0.06 0.24 0.03 0.12 0.01 0.03 0.02 0.03 0.02
Entries are average values, N = 3.
homogeneous and fine-crystal microstructure of the PCC coating formed on the tempered substrate. Manna et al. [22,23] obtained a thick coating with good corrosion resistance on tempered martensitic steel substrates, while a thin coating with coarser phosphates was formed on a ferritic-pearlitic steel substrate, which is consistent with the above results. However, Totik [24] investigated the influence of quenching and tempering on Mn-P coatings on AISI 4140 steel and revealed that the coating on a quenched substrate had a better anti-rust property than a tempered sample. This was the opposite to the results obtained in this study, which may be due to the differences in crystal shape and preferred growth directions between zinc phosphate with plate-like crystals and manganese phosphate with cylindrical crystals. To summarize, the heat treatment of steel substrates can affect the microstructure and corrosion behavior of the coatings.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by Shandong Provincial Natural Science Foundation of China (ZR2018LE004), Major Basic Research Program of Shandong Provincial Natural Science Foundation of China (ZR2018ZC0741) and National Key Research and Development Program of China (2017YFC0703100) as well as National Natural 6
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Science Foundation of China (51632003), the Taishan Scholars Program, Case-by-Case Project for Top Outstanding Talents of Jinan.
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