AISI 420 martensitic stainless steel corrosion resistance enhancement by low-temperature plasma carburizing

Electrochimica Acta 317 (2019) 70e82

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AISI 420 martensitic stainless steel corrosion resistance enhancement by low-temperature plasma carburizing C.J. Scheuer a, b, *, F.A.A. Possoli c, P.C. Borges a, c, R.P. Cardoso a, S.F. Brunatto a a

Plasma Assisted Manufacturing Technology & Powder Metallurgy Group - Department of Mechanical Engineering, UFPR, 81531-990, Curitiba, PR, Brazil Department of Mechanical Engineering, UFSM, 97105-900, Santa Maria, RS, Brazil c Academic Department of Mechanic, UTFPR, 80230-901, Curitiba, PR, Brazil b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 February 2019 Received in revised form 21 April 2019 Accepted 20 May 2019 Available online 29 May 2019

Corrosion resistance of plasma carburized AISI 420 martensitic stainless steel was evaluated by cyclic polarization test in 3.5% NaCl solution. Two distinct treatment groups were studied with the purpose of evaluating the influence of the carburizing temperature and time on the corrosion behavior of treated material (one at 350e500
C temperature range for 8, and 12 h times; and other one at 400
C for 12 e48 h time range, and at 450
C for 4e16 h time range). To support the discussions on electrochemical test results, microstructural characterization was carried out using metallographic analysis and XRD techniques. Microstructural characterization results show that low temperature and short time treatments produce outer layer composed by a0 C and Fe3C phases. High temperature and/or long time treatments promote outer layer Cr7C3 and Cr23C6 phases precipitation. Corrosion test results show improved carburized samples resistance, which is related to chromium-carbides precipitation-free outer layer formation. The Cr-carbides formation promotes a reduction on carburized samples corrosion resistance due to depletion of Cr in the solid solution of carburized layer. Finally, by confronting the behavior of untreated and carburized samples, it is verified the potentialities of the low-temperature plasma carburizing to enhance the corrosion resistance of AISI 420 MSS. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Low-temperature plasma carburizing AISI 420 martensitic stainless steel Corrosion resistance Polarization test

1. Introduction Martensitic stainless steels (MSSs) were created in order to satisfy a need of manufacturing industry for corrosion resistant alloys hardening through heat treatment [3]. MSS corrosion resistance is ensured by formation of a passive oxide layer presenting thickness around 3e5 nm [4], mainly composed of iron and chromium, exhibiting Fe/Cr atomic percent ratio varying among 1.5 to 7.2 depending on the medium that promotes passivation [5]. As pointed in the literature [6,7], MSS passive film structure depends markedly on the steel thermal historical and its formation kinetics,

The main results of this manuscript were presented in two distinct parts (here indicated as ref. [1,2]) at IX Congresso Nacional de Engenharia Mec^ anica (CONEM 2016), August 21e25, 2016, Fortaleza-CE, Brazil. * Corresponding author. Plasma Assisted Manufacturing Technology & Powder Metallurgy Group - Department of Mechanical Engineering, UFPR, 81531-990, Curitiba, PR, Brazil. E-mail addresses: [email protected], [email protected] (C.J. Scheuer). https://doi.org/10.1016/j.electacta.2019.05.101 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

which is influenced by the steel composition and environment conditions. Due to its technical features (high mechanical strength and moderate corrosion resistance), 13% Cr-type martensitic stainless steels (in which the AISI 420 MSS is a reference steel), are largely employed on applications where mechanical properties are the primary requirement [8,9]. However, these alloys find limited applications where a high corrosion resistance is required in addition to mechanical properties [10]. In order to overcome this limitation, several authors have dedicated to the optimization of the 13% Crtype MSS electrochemical properties using surface engineering techniques. Among the treatments available for this purpose, the application of plasma assisted nitriding treatments has resulted in a beneficial effect on corrosion resistance of this steel type, in addition to its increasing mechanical and tribological properties, as shown by Refs. [8,9,11e17]. Table 1 presents a comparative data compilation on some plasma nitrided MSSs corrosion performance. As can be verified through Table 1 analysis, pitting is one of the most important corrosion mechanism operating on nitrided AISI 420 MSS, which tend to present the lowest achieved corrosion

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Table 1 Corrosion resistance data of some plasma nitrided MSSs. References MSS grade

Environment ECORR

EPIT

[6]

AISI 410 3.5% NaCl and 205 1% HCl to 30 mV

[7]

AISI 410 3.5% NaCl

[9]

AISI 420 5.0% 550 e NaCl þ H2SO3 to 50 mV AISI 410 3.0% NaCl 600 e to 250 mVa

[10]

120 mVa

Corrosion Main results mechanisms

600 mV Pitting

600 mVa e Pitting e

[12]

AISI 420 3.0% NaCl

245 to 104 mV

[13]

AISI 420 3.5% NaCl

650 600 mVa Pitting to 300 mVa

[14]

AISI 420 0.1 M NaCl

[15]

AISI 431 0.05 M Na2S04

260 e to 230 mVa e e

a

e

e

Pitting e

Improvement in corrosion resistance of treated material was attributed to the compound layer formation containing g0 -Fe4N and ε-Fe2e3N iron nitrides. The chromium nitrides formation does not affect the corrosion resistance when compound layer containing g0 and/or ε nitrides was formed. Improvement in corrosion resistance is related to the nitrided layer formation, which protects the underlying metal from corrosive attack. Improvement in corrosion resistance is related to the combined effect of the Cr solid solution and the high chemical stable phases of 3-Fe3N and a0 N formation A more noble corrosion behavior was observed for AISI 410 steel treated at 350
C when compared to nitrided at 400 and 500
C, due to the a0 N phase formation. Corrosion resistance reduction is due to the chromium nitrides phases precipitation At 360
C treated temperature, the material corrosion resistance is improved due to ε-Fe2e3N nitride compact phase formation. At nitrided temperatures of 430 and 500
C, CrN precipitates form principally at grain boundaries, leading to a degradation in the corrosion resistance. A more noble corrosion behavior was observed for AISI 420 steel treated at 440 and 480
C when compared to nitrided at 520
C, due to the formation of a layer composed by a0 N, ε and g0 phases. Corrosion resistance reduction is due to the chromium nitrides phases precipitation Improvement in the corrosion resistance arises from the change in the surface chemical composition due to the formation of an oxynitride layer. Nitrided AISI 431 steel presented worst corrosion resistance when compared to untreated ones. This behavior is credited to chromium nitrides precipitation.

Values estimated from results presented by the referred authors.

potentials when material are treated at temperatures smaller than 400
C. The beneficial effect of the nitriding treatment application on the MSSs corrosion resistance is attributed to formation of 0 0 3-Fe3N, g -Fe4N and/or a N (nitrogen-expanded martensite) phases' compound layers authors, in which the alloying element Cr is maintained solved in the metallic matrix solid solution, at least to Cr content values higher than 10.5 wt%, as indicated by Lippold; Kotecki [18]. So, the corrosion resistance reduction for MSS plasma nitrided at temperatures higher than 400
C is a consequence of the solid solution Cr content depletion to values smaller than 10.5 wt%, which is attributed to the precipitation of chromium nitrides [19,20], in such cases. There is no consensus in the literature on the mechanism that promotes the corrosion resistance increase of stainless steels by the plasma assisted treatment application [21]. According to Flis et al. [22], the mechanism suggested to explain the rise on corrosion resistance of plasma nitrided stainless steel would correspond to the dissolution of nitrogen and consequent alkalization of corrosive solution. Other studies have proposed different passivation mechanisms, including formation of salts passive films [23], and hydrated oxides [24], or due to the combined effect of the Cr solid solution and the nitrided layer formation (which protects the underlying metal from corrosive attack) [8,9,11,12,14e16]. For the carburized stainless steels, it has been proposed that changes on the passive film semiconductor characteristic [25], suppression of the corrosive solution acidification [26], and the suppression of Crcarbides phases precipitation with formation of an protective carburized layer [27,28], correspond to the main reasons of the corrosion resistance increase. On the other hand, for plain-carbon [29], and low-alloy steels [30], it was observed that the increase of the carbon content in solid solution decreases the material dissolution rate due to the presence of chemical bonding of the interstitial carbon (which forms no carbide precipitate) to iron, and manganese on the steel matrix. In contrast regarding the intensive research and development on corrosion behavior of plasma nitrided 13% Cr-type MSS, much less attention has been given to the plasma carburizing application aiming to attend to this same purpose, despite of carbon can be an effective and useful alloying element to increase steels corrosion

resistance, as above mentioned. Not so far in the past, only one work published by Li and Bell [9] was found, where non-promising results (regarding the mechanical, tribological and electrochemical properties) were presented. Nevertheless, it was recently demonstrated by Scheuer et al. [31e33] and Angelini et al. [34], that with an adequate choice of electrical discharge and treatment parameters, interesting results on mechanical properties and tribological behavior were achieved by low-temperature plasma carburizing application. Likewise, Heuer et al. [35] showed that the application of gaseous carburizing treatment promotes significant increase of the PH13-8 Mo precipitation-hardened martensitic stainless steel corrosion resistance (in this case, should be considered the effect of steel composition on the diffusivity and solubility of the interstitial elements as demonstrated in Refs. [36e38], and on corrosion behavior [39]. Despite these recent discoveries on this subject, the effect of the plasma carburizing treatment application on the AISI 420 steel corrosion resistance remains unknown. Selection of appropriate carburizing parameters appears as an important issue when ensuring optimal performance characteristics of 13% Cr-type MSSs. Inadequate choice of the stainless steels processing conditions can lead to precipitation of chromium-rich carbides in the grain boundaries vicinity and martensitic laths [40e45]. Thus, chromium depleted zones are formed adjacent to the precipitated [46]. As a result, the chromium-depleted region of the carburized layer cannot form a protective passive film. Therefore, as previously indicated, plasma nitriding of 13% Crtype MSS is usually performed at temperatures below 400
C (for short-term treatment), in order to avoid chromium nitride precipitation [8,9,11e17]. In the other hand, for 13% Cr-type MSS lowtemperature plasma carburizing, Scheuer et al. [33] demonstrated that the use of temperatures below 450
C (for up to 12 h treatment times) promotes the obtainment of chromium-carbides precipitation-free layers. It has been established in earlier studies [33,34] that when the plasma carburizing is carried out at temperatures sufficiently low and for sufficiently short times, a carbon expanded martensite (also known as a0 C phase) is formed, which provides a significantly increased surface hardness. It is to be noted that the maximum temperature and time available to be used aiming to avoid the alloy corrosion resistance reduction, like in the same

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manner that is verified for the plasma nitriding, comprise important limitations for low-temperature plasma carburizing process.1 Considering the extensive literature regarding the corrosion behavior of low-temperature plasma nitrided 13% Cr-type MSSs, and considering the lack of information about the role of the lowtemperature plasma carburizing on the corrosion behavior of such steels, the aiming of this work is evaluating the influence of the treatment temperature and time on the corrosion resistance of plasma carburized AISI 420 martensitic stainless steel. 2. Experimental procedure Samples of 50.8 mm in diameter and 10 mm in height were machined from an AISI 420 stainless steel commercial bar (0.04% P, 0.36% Si, 13.68% Cr, 0.31% C, 0.06%Cu, 0.17%Ni, 0.75% Mn, 0.03% S, and Fe balance, in wt.%). Machined samples were austenitized at 1050
C for 0.5 h and air-cooled (the hardening treatment). Posteriorly, samples were ground using SiC sandpaper from 120 to 1200 grade and mirror polished using 1 mm Al2O3 abrasive suspension. Ultimately, samples were alcohol cleaned in ultrasonic bath, dried in a heated airflow, and then introduced into the discharge chamber. In this work tempering is performed simultaneously to the plasma carburizing treatment. In order to compare carburized and non-carburized samples corrosion behavior, a reference samples was adopted (untreated), which was tempered at 300
C for 1 h after hardening. DC plasma carburizing treatment was carried out in a conventional plasma system, which is explained in details in Ref. [33]. The sputter-cleaning stage, in order to remove the superficial oxide layer of the samples, and the plasma parameters comprising the gas mixture, gas flow rate, pressure and applied peak voltage were the same previously presented in Ref. [32]. Treatments were divided in two groups: the first, in order to study the effect of treatment temperature on the treated material corrosion behavior, samples were carburized at 350, 400, 450, and 500
C, for two different treatment times, in the case 8, and 12 h; and in the second, intending to determine the influence of the treatment time for two distinct temperatures, samples were carburized at the 400
C fixed temperature for times of 12, 24, 36, and 48 h; and at 450
C for times of 4, 8, 12, and 16 h. Carburized samples were cross-sectioned and prepared for microstructural analysis by conventional metallographic procedure. After polishing, specimens were etched using Marble's reagent (4 g of Cu4SO4 þ 20 ml de HCl þ 20 ml de H2O), and analyzed in an Olympus BX51M optical microscope. The outer layer thickness was determined by taking the mean of ten measurements using optical microscopy images. The recognition of phases present in the carburized layer was made by X-ray diffractometry (XRD) technique, employing a Shimadzu XDR7000 X-ray diffractometer with a Cu Ka X-ray tube in the Bragg-Brentano configuration. Diffraction lines were obtained with 2q angles in the range of 30e60
applied a scan speed of 0.1
/min. Electrochemical corrosion behavior of untreated and carburized AISI 420 steel samples were studied using potentiodynamic polarization technique. Cyclic potentiodynamic polarization curves were recorded according to the ASTM G61 [50] standard, which is recommended for evaluating the pitting corrosion tendency of materials in chloride-containing media. The set up consisted of an Iviun Potentiostat/Galvanostat Model IviuneneStat interfaced to a

1 At this point, it is important to mention recent research results showing that the use of thermochemical treatments under high temperature and short time conditions also promote improvements in the stainless steels surface properties [47e49].

personal computer using the IviumSoft software, for data acquisition and analysis (using these software, the OCP and potentiodynamic polarization curves were plotted and both the corrosion current density (iCORR) and the zero current potential (ECORR) were estimated), and an electrochemical corrosion cell (Fig. 1 show the schematic arrangement off electrochemical cell designed according to Qvarfort [51] and ASTM G150 [52]). Experiments were carried out in an aerated medium 3.5% NaCl solution at 25
C ± 2, using a graphite rod as counter electrode (CE) and a Ag/AgCl in saturated KCl reference electrode (RE), and the samples (AISI 420 steel disc) as the working electrode (WE). All the potentials mentioned in the text refer to the normal hydrogen electrode (NHE). Prior to each experiment, the electrode surface was degreased with acetone, rinsed with deionized water, and dried in flow of heated air. The used electrolyte was 3.5 wt % NaCl (pH ¼ 6.9), obtained from high-purity NaCl (99.0%), mixed with (Type 1) high purity laboratory water. The samples was clamped on the cell, sealing against a silicone gasket and a paper filter, with an exposed area of 50.24 mm2 to the electrolyte. To prevent the electrolyte stagnation in the silicone gasket vicinity (which could cause the crevice corrosion mechanism), the electrochemical cell contains a channel for water circulation (see detail in Fig. 1). The water supply was taken by a peristaltic pump, providing the fluid at a rate of 2 mL h1, as recommended by Qvafort [51] and ASTM G150 [52]. After stabilizing the open circuit potential (OCP) for 60 min, cyclic polarization measurement was performed potentiodynamically at a scan rate of 1 mV s1, by setting a potential value of 100 mV below the OCP, in the positive direction until a threshold current density of 5.0 mAcm2 was reached. At this point, the potentiodynamic scan was reversed to the negative direction, and continued down to starting point. The pitting potential (EPIT) was taken as the value on the potentiodynamic polarization curve at which the current sharply increased during the positive scan. Each experiment was repeated at least three times to check the reproducibility and the scatter in the reported data. The samples area exposed to the corrosive solution and was subsequently analyzed using an Olympus LEXT OLS 3000 confocal laser scanning microscope. This same equipment was used to estimate the pits size. 3. Results and discussion 3.1. Microstructural characterization Cross-section micrographs of AISI 420 MSS specimens plasma carburized at temperatures of 350, 400, 450 and 500
C for 12 h are shown in Fig. 2(a-d), respectively. A thin and continuous carbonenriched layer (termed in Ref. [31] as outer layer) was observed for all treatment studied conditions. According to XRD data presented on Fig. 3., this layer is composed by a carbon supersaturated solid solution on martensite lattice, also well known as carbonexpanded martensite (a0 C), and Fe3C iron carbide (or Fe-rich M3C; being M: metal, mainly Fe and Cr, but respecting the original alloy composition wt.% Fe/wt.% Cr ratio referred to the steel substrate bulk), for specimens treated at temperatures up to 450
C; and additionally by chromium carbides (Cr7C3 and Cr23C6) for specimen carburized at 500
C. Similar results to those presented in Fig. 2 were obtained on the treatments performed for 8 h at the same treatment temperature range (as shown in previous paper [33]), id. est, the occurrence of a thin and continuous (outer) carbon-enriched surface layer. In this case, for both the treatment series, the outer layer thickness increases with the carburizing temperature (see Table 2), being dependent on the interstitial carbon diffusivity [53], and thus a thermal-activated process. Additionally, it was also verified the occurrence of diffusion layer in the treated layers, a result

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Fig. 1. Schematic representation of the experimental setup applied in the electrochemical tests.

Fig. 2. Cross-section micrographs of specimens treated at: (a) 350, (b) 400, (c) 450 and (d) 500
C (treatments carried out for 12 h, using a gas mixture composition of 99.5% (80% H2þ20%Ar)þ0.5% CH4 at a flow rate of 1.67  106 Nm3 s1, and pressure of 400 Pa).

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Fig. 3. XRD patterns for as-quenched (untreated) specimens and for samples treated at 350, 400, 450 and 500
C for 12 h (treatments carried out using a gas mixture composition of 99.5% (80%H2þ20% Ar)þ0.5% CH4 at a flow rate of 1.67  106 Nm3 s1, and pressure of 400 Pa).

confirmed by hardness profiles (data reported in Ref. [33]), and evidenced by a more pronounced chemical etching of the region just below the outer layer (as recently shown in Ref. [54]), here mainly perceptible in the micrograph of the sample carburized at 500
C (Fig. 2d). As presented in Ref. [33], the activation energy for outer layer growth was 29 kJ mol1, for both the treatment series, and the estimated activation energy for the diffusion layer growth was 85 kJ mol1, confirming the diffusive character of the referred layer growth processes. The diffusion layer depth (here defined by the depth for which the substrate bulk hardness is attained) values, determined by hardness profile measurements, are presented in Table 2. The X-ray diffraction patterns of untreated (as-quenched) and plasma carburized specimens treated at different temperatures for 12 h are shown in Fig. 3. For the applied 30e60
2q scan range, the as-quenched specimen exhibits 44.18
2q angle (110) peak reflection, which was attributed to the martensite phase (a0 ) (JCPDS number 44-1290). The XRD patterns of specimens carburized at 350e450
C for 12 h revealed the enlargement of the referred martensite peak, and slight shift (up to 0.2
) to smaller 2q angles, indicating the carbon-expanded martensite (a0 C phase) layer formation. In addition, peaks relating to the Fe-rich M3C carbide phase to 37.76, 39.78, 40.62, 43.73, 45.84, and 55.96
2q angles (JCPDS number 03-1012) can also be observed. This same pattern was observed for specimens carburized at 400
C for 12e36 h as shown in Fig. 4. Differently, the XRD pattern of the sample treated at 500
C for 12 h and at 400
C for 48 h (Figs. 3 and 4, respectively) showed

peak reflections related to Cr7C3 (39.49, and 48.10
2q angles e JCPDS number 11-0550), and Cr23C6 (37.76, 48.10, 51.59, and 57.55
2q angles e JCPDS number 03-1172) chromium carbide phases (this assertion is confirmed by the dark spots observed along the outer layer on Figs. 2d and 5.d). It is also to be observed from XRD patterns that at 500
C for 12 h and at 400
C for 48 h, the a0 C diffraction peak has disappeared, giving place to (110) a-Fe phase (44.28
2q angle e JCPDS number 85-1410), which indicates the a0 C phase decomposition at the expense of the intense chromium carbide(s) precipitation. Similar result was observed in Ref. [33] to treatments performed at the same temperature range for 8 h time. As previously shown in Ref. [33], for specimens carburized at 450
C for 4e12 h times, precipitate-chromium-carbide-free-layers were formed, being constituted of a0 C and Fe-rich M3C. In that work, no evidence of chromium carbide precipitation occurrence was observed for 16 h treated specimens. In this case, it is believed that chromium carbide precipitation has only occurred in its initial stage, since no clear Cr depletion was observed in outer layer micrographs, a result supported by the respective XRD patterns. As indicated on Table 3 results, as expected, both the outer and diffusion layer thickness increased with the carburizing time, confirming that the carbon transport into the steel surface is diffusion-controlled, being the carbon diffusion depth proportional to the square root of the treatment time. In accordance with Sun [53], predicting the initiation of chromium carbide precipitation as a function of both the thermochemical treatment temperature and time is a key parameter in the

Fig. 4. XRD patterns for as-quenched (untreated) specimens and for samples treated at 400
C for 12, 24, 36 and 48 h (treatments carried out using a gas mixture composition of 99.5% (80%H2þ20% Ar)þ0.5% CH4 at a flow rate of 1.67  106 Nm3 s1, and pressure of 400 Pa).

Table 2 Carburized layer characteristics and electrochemical parameters obtained for AISI 420 MSS treated at 350, 400, 450 and 500
C, for 8, and 12 h. Treatment condition Carburized layer characteristics Time Temperature Outer layer thickness (mm) (h) (
C) 8 350 1.5 ± 0.15 400 1.8 ± 0.20 450 2.2 ± 0.20 500 3.4 ± 0.22 12 350 1.8 ± 0.13 400 2.2 ± 0.16 450 2.6 ± 0.21 500 3.5 ± 0.15 Untreated e (reference)

Diffusion layer depth (mm) 25 40 55 65 35 50 70 75 e

Electrochemical parameters Precipitation occurrence No No No Yes No No No Yes e

OCP (V vs. NHE) 0.134 ± 0.012 0.063 ± 0.016 0.008 ± 0.015 0.185 ± 0.034 0.144 ± 0.019 0.044 ± 0.030 0.058 ± 0.009 0.181 ± 0.023 0.175 ± 0.037

ECORR (V vs. NHE) 0.125 ± 0.017 0.039 ± 0.019 0.042 ± 0.022 0.219 ± 0.041 0.129 ± 0.023 0.042 ± 0.034 0.014 ± 0.011 0.293 ± 0.024 0.176 ± 0.024

EPIT (V vs. NHE) e e e e 0.307 ± 0.013 e 0.163 ± 0.009 e

iCORR (mA/ cm2) 0.82 ± 0.14 1.13 ± 0.21 0.94 ± 0.10 1.71 ± 0.17 0.98 ± 0.11 1.09 ± 0.14 1.12 ± 0.09 1.53 ± 0.12 1.96 ± 0.22

CR (mm/ year) 0.009 ± 0.004 0.012 ± 0.001 0.010 ± 0.003 0.019 ± 0.003 0.011 ± 0.007 0.012 ± 0.008 0.012 ± 0.003 0.017 ± 0.008 0.022 ± 0.011

Corrosion form Pitting Pitting Pitting Pitting Pitting Intergranular Pitting Pitting Uniform

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Fig. 5. Cross-section micrographs of specimens treated for: (a) 12, (b) 24, (c) 36, and (d) 48 h (treatments carried out at 400
C, using a gas mixture composition of 99.5% (80% H2þ20%Ar)þ0.5% CH4 at a flow rate of 1.67  106 Nm3 s1, and pressure of 400 Pa).

control of the treated layer formation, aiming to obtain a highquality precipitate-chromium-carbide-free-layer. Based on the experimental data previously reported, and inspired on Sun [53] and Bell [55] researches, a temperature vs. time (Txt) precipitation threshold mapping for the plasma carburized AISI 420 martensitic stainless steel has been constructed (see Fig. 6). The development of the Txt mapping was carried out in order to help on selection of optimum conditions for carburizing treatment application in 13% Cr-type MSS, aiming to avoid the treated material corrosion resistance loss. So, when a determined (Txt) treatment parameters pair indicated by the symbol ‘C’ on Fig. 6 is selected for the 13% Cr-type MSS carburizing, precipitate-chromium-carbide-free-carburizedlayers will be produced (being supposedly constituted of a0 C and Fe3C phases only). In addition, in accordance with the corrosion resistance results shown in the next topic, there is a transition region delimited in Fig. 6 by the symbol ‘I’, where the occurrence of

chromium carbide precipitation cannot be totally disregarded, despite no evidence of outer layer Cr depletion (at least for the characterization techniques utilized here), from which the corrosion resistance of the treated material has already been decreased. Finally, for (Txt) treatment parameters pairs indicated by the symbol ‘B’, a Cr depleted outer layer with strongly reduced corrosion resistance is obtained. Naturally, the Txt mapping presented here is to be improved by further studies, as a result of the literature updating on the subject, in order to refine the referred map. 3.2. Corrosion behavior The corrosion behavior of low-temperature plasma carburized AISI 420 MSS is dependent of the previously reported microstructural and phase compositional changes. To measure this effect,

Table 3 Carburized layer characteristics and electrochemical parameters obtained for AISI 420 MSS treated at 400
C for 12, 24, 36 and 48 h, and at 450
C for 4, 8, 12 and 16 h. Treatment condition Carburized layer characteristics Temperature Time (h) (
C) 400 12 24 36 48 450 4 8 12 16 Untreated (reference)

Outer layer thickness (mm) 2.2 ± 0.16 3.0 ± 0.23 3.8 ± 0.17 4.4 ± 0.23 1.5 ± 0.1 2.2 ± 0.20 2.6 ± 0.21 3.0 ± 0.16 e

Diffusion layer depth (mm) 50 65 80 85 40 55 70 85 e

Electrochemical parameters Precipitation occurrence No No No Yes No No No No e

OCP (V vs. NHE) 0.044 ± 0.030 0.051 ± 0.022 0.134 ± 0.017 0.262 ± 0.026 0.134 ± 0.012 0.008 ± 0.015 0.058 ± 0.009 0.203 ± 0.020 0.175 ± 0.037

ECORR (V vs. NHE) 0.042 ± 0.034 0.028 ± 0.019 0.121 ± 0.028 0.263 ± 0.013 0.067 ± 0.016 0.042 ± 0.022 0.014 ± 0.011 0.208 ± 0.014 0.176 ± 0.024

EPIT (V vs. NHE) e 0.323 ± 0.022 e e e e 0.163 ± 0.009 e e

iCORR (mA/ cm2) 1.09 ± 0.14 1.41 ± 0.07 1.34 ± 0.13 1.62 ± 0.11 1.08 ± 0.11 0.94 ± 0.10 1.12 ± 0.09 2.27 ± 0.17 1.96 ± 0.22

CR (mm/ year) 0.012 ± 0.008 0.016 ± 0.006 0.015 ± 0.009 0.018 ± 0.008 0.012 ± 0.005 0.010 ± 0.003 0.012 ± 0.003 0.026 ± 0.008 0.022 ± 0.011

Corrosion form Intergranular Pitting Pitting Pitting Pitting Pitting Pitting Pitting Uniform

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Fig. 6. Temperature vs. time (Txt) precipitation threshold mapping to produce precipitate-free microstructures of plasma carburized AISI 420 martensitic stainless steel.

that the OCP potential increases to nobler values (more positive) with the carburizing temperature in the 350e450
C range. This result evidence that the exposure for 8 h at 450 C does not promote the substitutional alloying element mobility, thus resulting preferentially in iron-rich carbide precipitation (the Fe-rich M3C phase). In contrast, for samples treated at 500
C for 8 h, the OCP potential is less noble (more negative) compared to untreated, which is due to the chromium carbide precipitates formation, as presented in Ref. [33]. Fig. 7(b) presents the cyclic polarization curves of the untreated and carburized samples treated at 350e500
C temperature range for 12 h (these curves were selected as being the most representative of at least three tests carried out for each studied condition, providing a good approximation of the resulting average value). It can be verified a behavior similar to that presented in the OCP analysis, since ECORR become more negative (presenting worse corrosion behavior) for treatment temperatures above and below 400
C (see Table 2). This consonance between the behavior of OCP and ECORR was also observed for samples carburized for 8 h at the same temperature interval (Figure A1b). Confronting the electrochemical parameters presented on Table 2, it is verified that the ECORR increased while iCORR decreased for low-temperatures short times treatment conditions, indicating that the plasma carburizing

potentiodynamic polarization test were performed in a 3.5% NaCl solution. Before that, OCP was measured over 60 min, aiming to determinate the tendency of the material to dissolve or to form a passive film in the 3.5% NaCl solution. Results of open circuit potential test (OCP) for untreated (the reference condition) and plasma carburized samples at 350e500
C temperature range for 12 h treatment time are shown in Fig. 7(a). Results show a rapid decrease of OCP values in the initial instants of the samples immersion on 3.5% NaCl solution, with subsequent stabilization. The trend of OCP decreasing values indicates the passive film dissolution on the 3.5% NaCl solution [56]. The posterior stabilization indicates that a stable interface was produced between outer layer and the corrosive solution [57]. The OCP values are strongly dependent on the carburizing conditions (or the obtained outer layer microstructure and its chemical composition), as can be verified in Fig. 7(a) detail (the presented OCP values correspond to the last measured values). The OCP of samples carburized at 400
C stabilized at a more positive (nobler) potential compared to the other conditions, indicating its lower passive film dissolution tendency in the studied environment. By confronting the OCP potentials of the untreated and 500
C treated samples, it appears a decrease of the 500
C carburized samples corrosion resistance, as a consequence of Cr depletion promoted by the chromium carbide precipitation (as previously shown in Fig. 2 and 3.). For other treatment conditions, the outer (precipitate-free) layer possibly acts according to the same passivation protection mechanism of passivating (Cr2O3) film, protecting the untreated material core of aggressive external environment. With the outer layer thickness increase, higher is the effectiveness of the promoted protection, since longer is the time required for its dissolution under the same environmental. However, although at 450
C has shown greater outer layer thickness, this condition presented stabilization in a more negative (less noble) potential when compared to 400
C. It is supposed that the exposure for 12 h to the temperature of 450 C induce the mobility of chromium atoms sufficient to start too slight but non-perceptible chromium carbide phase precipitation, as discussed from Figs. 2, Figure 3, and Fig. 6 results. This argument is also supported by analyzing the OCP curve behavior for samples carburized at 350e500
C temperature range for 8 h (Figure A1a e see Appendix in Supplementary material). In this case, it appears

Fig. 7. (a) Open circuit; and (b) potentiodynamic polarization curves achieved in an aerated medium 3.5% NaCl solution at 25
C ± 2 of untreated and AISI 420 steel samples plasma carburized at 350, 400, 450 and 500
C for 12 h (potentials measured against a saturated Ag/AgCl reference electrode).

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had improved the corrosion properties of the 420 MSS under these conditions. Moreover, by comparing the ECORR of carburized AISI 420 MSS with those measured for nitrided AISI 420 MSS (also measured in a 3.5% NaCl solution) (Table 1), it is concluded that the first treatment is more efficient regarding to the AISI 420 MSS corrosion resistance improvement. It is important to note that, the noblest corrosion potential (more positive) was achieved at 400
C, despite the occurrence of no passivation region, differently from the observed for the 350 and 450
C conditions. Similar behavior (transpassivation without pits) was demonstrated by a previous published works [9,11,12,16,28] referring to plasma nitrided MSS. It is also verified by analysing Fig. 7(b) and Figure A1b, that the potentiodynamic curves of untreated and 500
C carburized samples are displaced upward, i.e. to higher current density values. This result demonstrates the low corrosion resistance of these samples, indicating the occurrence of a higher corrosion rate [57,58], which agrees well with the calculated corrosion rate (CR) values presented in Table 2. Furthermore, the positive hysteresis loop indicates the passive layer collapses (without its recomposition) with consequent pitting corrosion occurrence [46]. It is also to be observed from Fig. 7(b) and Figure A1b that the potentiodynamic curves do not show the occurrence of repassivation phenomenon. This result indicates that pits penetrate through the outer layer, and the passivating film did not regenerate over diffusion layer [35]. The

77

possible reason of this behavior is the aggressiveness of corrosion medium that prevents passivation, and so pitting corrosion continues propagating in an autocatalytic process, i.e., the pitting corrosion process produces the conditions required for increasing the reaction rate. The same behavior (upward displacement of the potentiodynamic curves for Cr depleted samples, positive hysteresis loop and non-repassivation) were also observed for the other analyzed conditions. Fig. 8 (a-d) shows the surface aspect after electrochemical tests of the samples carburized at 350, 400, 450 and 500
C for 12 h, respectively, for which different corrosion behaviors are evidenced. First, noting that for 350 and 450
C treatment conditions (Fig. 8a and c, respectively) large pits are observed (average diameter of about 64.7 and 59.1 mm, respectively) when compared to samples carburized at 400 and 450
C for 8 h (Figure A2b and A2c) (average diameters of about 32.7 and 27.4 mm, respectively). The pits presence indicates that in some points the outer layer acts as the anode, while in the other regions it remains as the cathode (the noblest), which induces to localized corrosion [56]. The pits formation probably occurs due to the defects in outer layer, the presence of less noble and more anodic phases, and/or the located Cr depletion of the metallic matrix as a consequence of the Cr-rich phases formation. According to Ref. [39] regions with highest density of crystalline defects (inclusions, dislocations, grain and phase

Fig. 8. Confocal laser scanning microscopy surface aspect of samples plasma carburized for 12 h at: (a) 350, (b) 400, (c) 450 and (d) 500
C after corrosion testing.

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C.J. Scheuer et al. / Electrochimica Acta 317 (2019) 70e82

boundaries) become more susceptible to pitting corrosion attack. It is also possible verify that the pitting diameter (average 69.1 mm) of samples treated at 350
C for 12 h was greater than that carburized at 450
C, in the same way as for the samples treated for 8 h at 400 C and 450 C. This results, associated with the low corrosion rate, confirming the superior corrosion resistance of samples treated at 450
C for 8 h. This linked to the greater chromium-carbides precipitation-free outer layer obtained for these treatments conditions. Mittelstadt et al. [59] showed that the corrosion resistance of nitrided chromium-containing alloys is related to the treated layer thickness and homogeneity, and regions with faults or smaller thicknesses corresponding to points of greater corrosion susceptibility. Likewise, the galvanic effect between the substrate and the nitrided layer is an additional factor for pitting formation and growth [59]. On the other hand, for the sample carburized at 400
C, it is evidenced a localized corrosive attack along the grain boundaries, characterizing intergranular corrosion (see Fig. 8b). To justify this behavior the tempering effect to which the material is simultaneously subjected during thermochemical treatment must be considered. As evidenced by several authors [60e63], the effects of the tempering heat treatment on corrosion properties of 13% Crtype MSS is remarkable. The results presented by the aforementioned authors agree that exposure of the 13% Cr-type MSS to temperatures between 300 and 600
C for 2 h leads to Cr-rich M23C6 carbides precipitation promoting the formation of Crdepleted zones close to the boundary matrix/precipitates, reducing corrosion resistance [60,62,63]. On the other hand, 13% Crtype MSS exposure to greater than 600
C for 2 h causes an increasing on it corrosion resistance. This corrosion resistance improvement is credited to two phenomena: i) the formation of reverse austenite, which reduces the extent of Cr-depletion and enhances the stability of passive film [61]; and ii) due the Cr rediffusion from the matrix to Cr-depleted zones, minimizing the Cr-content discontinuity at the interfacial regions [63]. According Panossian [64] for a given tempering temperature there is a critical time range that promotes an increase in the chromium carbides precipitation on AISI 420 MSS and, consequently, the reduction of its corrosion resistance. After which time the corrosion rates decrease becoming almost nil. Based on the data presented by Panossian [65], it is believed that 12 h would be the critical time for maximum corrosion rate at 400
C. This statement is in consonance with the claimed by Corengia et al. [12], for which the intergranular corrosion is a consequence of tempering associated to surface treatment. In contrast, samples carburized at 350
C for 8 h (Figure A2a), at 500
C for 8 h (Figure A2d) and 12 h (Fig. 8d) was severely attacked by large pits (with dimensions up to 116.1 mm). It is possible to note the occurrence of widely corroded areas, confirming the lowest corrosion resistance for these conditions, as previously indicated by Fig. 7(a, b) and Figure A1(a, b). In the first case, the low corrosion resistance is supposedly due to the lower outer layer thickness observed for this condition, whereas in the latter two cases, to the intense Cr-rich M23C6 carbides precipitation. In order to improve understanding of treatment parameters influence on the corrosion behavior of carburized AISI 420 steel, the treatment temperature was set at 400 and 450
C and the effect of treatment time was also evaluated for 4e48 h time range. Corrosion behavior was found again to be dependent on the microstructural changes promoted by the carburizing time variation. Fig. 9(a) presents the OCP testing results for samples carburized at 400
C, for 12, 24, 36, and 48 h, as well as for the untreated sample. A similar behavior to that verified for the temperature study group was observed, comprising generically an initial decrease of the OCP curve to less noble values with subsequent

stabilization, implying that a stable film/solution interface was produced (samples carburized at 450
C for 4e16 h shown the same behavior, as indicated in Figure A3a). Furthermore, it is noticed a reduction on the OCP-values with the treatment time rise of 12e48 h for samples carburized at 400
C, and 8e16 h for samples carburized at 450
C (see Table 3). Also, it may be noted that the 48 h-400
C and 16h-450
C conditions presented OCP values below to those of the untreated samples, indicating a reduced nobleness of samples carburized at this condition (due to Cr-carbide phase formation, as discussed in Fig. 4 and in Ref. [33]). Although the used microstructural characterization techniques have not shown the chromium carbide precipitation occurrence for specimen carburized at 24 and 36 h for 400
C and at 12 and 16 h for 450
C, one cannot rule out a possible nanosized chromium carbide precipitates formation. Nanosized Cr-rich carbides formation would justify the corrosion resistance depletion when compared to 12 h for 400
C and at 8 h for 450
C conditions, respectively. It is clear to the authors that additional microstructural characterization is required to prove this assumption. Nanosized coherent precipitates occurrence on H13 tool steel after plasma nitriding was observed by Zagonel et al. [65] using High-Resolution Transmission Electron Microscopy (HR-TEM). Therefore, further studies using HR-TEM technique is recommended to be performed, aiming to

Fig. 9. (a) Open circuit potential; and (b) potentiodynamic polarization curves in an aerated medium 3.5% NaCl solution at 25
C ± 2 of untreated and AISI 420 steel samples plasma carburized at 400
C for 12, 24, 36 and 48 h (potentials measured against a saturated Ag/AgCl reference electrode).

C.J. Scheuer et al. / Electrochimica Acta 317 (2019) 70e82

79

Fig. 10. Confocal laser scanning microscopy morphologies of samples plasma carburized at 400
C for (a) 12, (b) 24, (c) 36 and (d) 48 h after corrosion testing.

Fig. 11. Confocal laser scanning microscopy showing two views of the surface aspect of untreated (reference condition) samples after corrosion testing.

confirm the assumptions here presented. Cyclic polarization curves (Fig. 9b) indicate the maintenance of the same behavior of OCP test results, i.e. decrease of the corrosion

potential to more negative values (less noble) with increasing treatment time (see Table 3). This same pattern where ECORR values follows the same behavior tendency of OCP, were also identified in

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C.J. Scheuer et al. / Electrochimica Acta 317 (2019) 70e82

the polarization curves of samples treated at 450
C for 4e16 h time range (Figure A3b). Confronting the electrochemical parameters presented on Table 3, it is verified that the ECORR and EPIT increased while iCORR decreased after treatment times lower than 12 h for 450
C, and lower than 36 h for 400
C, indicating that the plasma carburizing had improved the corrosion properties of the 420 MSS under these conditions. The surface aspect analysis (Fig. 10a-d and Figure A4a-d) performed on samples after the cyclic polarization testings shows a behavior that is in agreement with the results previously presented for 400
C (Fig. 9a-b), and 450
C (Figure A2a-d) treated samples, respectively. There is an increase of the pits size (average of 48.2, 60.1 and 82.1 mm, respective) with the increment of treatment time from 24 to 48 h at 400
C (Fig. 10b-d, respectively). In contrast, for samples carburized at 450
C there is a reduction in the pits size (average of 52.6 to 27.3 mm, respectively) by rise the treatment time from 4 to 8 h, with subsequent increment with the growth to 12 and 16 h (average of 53.4 and 52.8 mm), confirming it lowest corrosion resistance (Figure A2a-d, respectively). This results agreeing with the result presented by Li; Bell [9]. In these case, possibly the existence of second-phase defects along the outer layer, such as chromium carbide precipitates (and the consequent located Cr-depletion), and the increment in its dispersion or density by increasing the carburizing time is the cause of the most intense corrosive attack [39]. The untreated sample (reference condition) was also subjected to a severe corrosive attack. As can be noted in Fig. 11, the corrosive attack was uniform and generic all over the surface exposed to corrosive solution, being that 126.8 mm average size pits are also verified. In similar way, the aggressiveness of the corrosive medium also prevents passivation, and pittings corrosion propagates in profusion. In addition, after each cyclic polarization test, the electrolyte was collected and analyzed, aiming to determine eventual presence of solid precipitates and measure the solution pH. The remaining solutions of the tests performed on the samples carburized at 350e450
C temperature range for 8 and 12 h showed no trace of precipitated solids, and the maintenance of the original transparency, similar to the exhibited in testing beginning. In this case, the electrolyte pH values after the tests also remained practically unaltered (6.5e7.0). This same pattern was observed for samples treated for 12e36 h at 400
C and for 4e16 h at 450
C. Contrarily, the resulting electrolytes of the tests carried out on the untreated samples, and samples carburized at 500
C for 8 and 12 h and at 400
C for 48 h, demonstrate an orange coloring. Also, a considerable amount of solid particles, suggesting ferrous chloride (FeCl2) formation, and pH values of about 1.3, thus too lower than those measured at the beginning of the tests. As demonstrated by Marcelin et al. [66] the critical condition for the depassivation of MSS is pH < 1.5, which justifies the severity of the corrosive attack that occurred for these conditions.

 The carburized layers produced by low-temperature (below 450
C) and short-time (less than 12 h) treatment conditions are composed of an outer layer constituting of carbon expanded martensite (a0 C) and Fe-rich M3C carbide phases. For treatments performed at the highest treatment temperature (500
C for 8, and 12 h) and the longest time (16 h at 450
C, and 48 h at 400
C), it was also noted the occurrence of chromium carbides phases (Cr7C3 and Cr23C6) precipitation in the treated layer;  Intense Cr depletion was verified for specimen treated at 500
C for 8, and 12 h, and at 400
C for 48 h, which is attributed to a0 C phase decomposition in aeFe þ chromium carbides phases;  Corrosion study comprising the determination of open circuit potential, corrosion potential, corrosion current density, and corrosion rate, in 3.5% NaCl solution, revealed that lowtemperature and short-time plasma carburizing treatment significantly improved the corrosion behavior of the treated samples when compared with the untreated material. The improved corrosion resistance was found to be directly related to carburized layer formation (a0 C þ Fe3C). For high-temperature long-time treatments, worse corrosion behavior is due to Crcarbides phases formation;  The corroded surface of untreated samples showed a general and uniform corrosion all over the exposed surface associated with pits occurrence, while the corroded surface of carburized samples was replaced by pitting corrosion. Otherwise, for samples carburized at 500
C for 8, and 12 h it was also verified the occurrence of an intense corrosion attack along the grain boundaries;  The pits size reduces by increasing the carburizing temperature up to 450 for treatments performed at 8, and 12 h, respectively, and it increases to higher temperatures. In contrast, pits size increases by increasing the treatment time from 12 h to treatments carried out at 400
C, and at 450
C;  Finally, based on the cyclic polarization testing results, samples carburized at 400
C for 12 h, and 450
C for 8 h presented nobler corrosion potentials than the samples treated under other conditions. However, for the first treatment condition, corrosion tends to occur on the grain boundary region (intergranular corrosion), while for the second one, corrosion occurs in the form of small pits. Therefore, it can be stated that the best carburizing condition to treat AISI 420 MSS corresponds to 450
C for 8 h. Acknowledgements This work was supported by CAPES (Coordenaç~ ao de Aperfeiçoamento de Pessoal de Nível Superior - Brasil - Finance Code 001), ~o Arauca ria of the Parana  State, CNPq-Universal CNPq, Fundaça rio Grant N. Grant N. 482380/2012-8, and MCTI/CNPq/CT-Aquavia 456347/2013-5. The authors also wish to express their thanks to the Laboratory of X-ray Optics and Instrumentation (LORXI-UFPR) by the use of the XRD equipment.

4. Conclusions

Appendix A. Supplementary data

The corrosion behavior of plasma-carburized AISI 420 MSS was evaluated and related to the microstructural features produced according to the applied treatment conditions. The main conclusions from the depicted results can be summarized as follows:

Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.05.101.

 The microstructure, constitution and thickness of carburized layers produced on AISI 420 MSS can be controlled by suitably selecting the carburizing temperature and time conditions. These parameters have a direct effect on the carburized AISI 420 MSS corrosion behavior in 3.5% NaCl solution;

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