1 Introduction

Martensitic stainless steel grades (MSS) such as 13Cr-4Ni and 16Cr-5Ni are used in the fabrication of hydro-turbine components such as runner blades, guide vanes, facing plates, and other hydraulic machinery components (Ref 1). These MSS grades are preferred owing to their high strength, high hardness, good low temperature ductility, and excellent toughness (Ref 1, 2). The high hardness of these grades of steel is attributed to the metastable martensitic phase which is obtained by quenching from the austenite phase. In an environment composed of anions such as Cl−, MSS is susceptible to corrosion by pitting due to the attack by these aggressive anions. These anions can penetrate the naturally formed thin and protective passive film developed on the surface, causing localized corrosion (Ref 2). In the case of hydro-turbine blades, the situation is worse as the repeated impact of silt particles present in the fresh water ruptures the reformed passive surface layer (Ref 3). Additionally, the microstructural heterogeneities and pores aggravate the situation causing degradation of the material and failure of the component (Ref 2). Most studies focus on increasing the surface hardness to combat erosion and cavitation. However, it is of utmost importance to understand the electrochemical dissolution effects as the corrosive medium can greatly enhance the material degradation exclusively as well as in synergy with the cavitation-erosion phenomena (Ref 4, 5).

Surface hardening can enhance both hardness and corrosion resistance of metals (Ref 6, 7). The most common surface hardening methods include plasma nitriding and boronizing (Ref 8,9,10). Among these, boronizing imparts greater hardness and corrosion resistance (Ref 11, 12). Boronizing of steels leads to the formation of Fe2B and FeB phases in the substrate surface layers (Ref 12, 13). However, in the case of stainless steels, additional Cr2B, CrB, Ni2B and NiB phases form due to their high alloy content (≥ 12 wt.% Cr and ≥ 4 wt.% Ni) (Ref 14, 15). There are several methods of boronizing including pack, paste, liquid, gas, plasma and electrochemical boronizing. Of these, the pack boronizing method is preferred over other methods as it is relatively simple to operate and is cost-effective (Ref 16,17,18,19). For stainless steels, pack boronizing is done in the temperature range of 850-1000 °C for 4 to 8 h. The chosen combination of boronizing temperature and duration influences the microstructure, layer composition, and properties of the boronized layer (Ref 20).

The effect of boronizing in improving the corrosion resistance of different stainless steel grades has been investigated by many. Campos et al. investigated the corrosion behavior of boronized AISI 304 steel in 0.1 M deaerated NaCl solution (Ref 21). It was observed that higher boronizing temperatures resulted in compact and even boronized layer with less porosity, and it exhibited higher polarization resistance (Rp). The compact layer inhibited the diffusion of aggressive species through the coating and prevented localized and galvanic corrosion. In another work, corrosion behavior of boronized AISI 304 steel was evaluated in 0.1 M NaCl solution, and low passive current density and high stability of the boronized layer obtained using specific boronizing parameters (4 mm paste thickness, 1273 K and 4 h) was attributed to fewer micro-porosities and cracks in the boronized layer (Ref 22). On the other hand, Çetin et al. studied the electrochemical characteristics of boronized AISI 904L stainless steel and reported that corrosion resistance of steel degraded after boronizing due to the presence of micro-cracks and porosity in the boronized layer (Ref 18). Caballero et al. also reported a decline in the corrosion resistance of AISI 316L steel in a simulated body fluid (SBF) solution after boronizing. This was attributed to complex chemical interactions between the borides FeB, Fe2B, CrB, Cr2B, MoB, Ni2B, and Ni3B of the boronized layer, and the sulfates and phosphates of the SBF solution (Ref 23). Sury investigated the corrosion behavior of boronized and nitrided steels in acidic and NaCl media and reported a decline in the corrosion characteristics due to the incorporation of molybdenum and chromium in the boride layers (Ref 24). The work of Çakir et al. on the structural, tribological and electrochemical properties of boronized and nitrided AISI 316 stainless steel also reports a decline in the corrosion resistance in SBF solution (Ref 25). Thus, based on the reported literature, it can be concluded that in boronized stainless steels, both increase and decrease in the corrosion resistance are reported. The effectiveness of boronized layer with regard to the improved corrosion resistance largely depends on the boronizing temperature and time, exposure medium, and substrate material chemistry. The interaction of the heterogeneous boronized layer containing several borides and the electrochemical media is a complex phenomenon which directly influences the corrosion behavior of boronized stainless steels. Studies so far majorly focused on the boronizing of austenitic grades of stainless steel (Ref 23,24,25,26), while studies on martensitic grades are scarce (Ref 12, 20). The corrosion behavior of boronized 13Cr-4Ni and 16Cr-5Ni martensite stainless steel grades is not reported.

Therefore, in the present work, comprehensive understanding of the corrosion behavior of these boronized MSS is attempted. The interactions between environmental chloride species and boronized MSS in comparison with their bare counterparts are studied using cyclic polarization. Characterization of the samples after corrosion is done using SEM and EDS techniques. Further, pack boronizing can introduce carbon in these steels and therefore the possibility of sensitization during boronizing and its effects on electrochemical degradation when the samples are exposed to 0.6 M NaCl is discussed. It is observed that pack boronizing resulted in the sensitization of the boronized MSS that resulted in active dissolution during anodic polarization. The EIS results after longer periods of exposure show a decrease in corrosion resistance of boronized steels with time.

2 Experimental Procedure

2.1 Materials and Methods

As-cast 13Cr-4Ni and 16Cr-5Ni MSS were procured from BHEL, Haridwar, India. Their composition was assessed using wavelength dispersive X-ray fluorescence spectroscopy and is given in Table 1. Powder pack boronizing (pack composition: silicon carbide 90 wt.%, potassium tetra fluoroborate 5 wt.%, and boron carbide 5 wt.%) technique was used to boronize the samples at 950 and 1000 °C for 6 h followed by tempering at 600 °C for 2 h.

Table 1 Chemical composition (in wt.%) of 13Cr-4Ni and 16Cr-5Ni MSS
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2.2 Corrosion Studies

Corrosion behavior of boronized 13Cr-4Ni and 16Cr-5Ni MSS was evaluated using cyclic polarization, electrochemical impedance spectroscopy (EIS) and immersion tests. Open-circuit potential (OCP), cyclic polarization, and ElS tests were performed using Gamry 1000® potentiostat at ambient temperature in 0.6 M NaCl solution. The common flat cell configuration was used, wherein saturated calomel electrode (SCE) and platinum mesh served as the reference electrode and counter electrode, respectively. Bare and boronized specimens of MSS with an exposed circular area of 0.785 cm2 were used as working electrode which were tightened adequately against a tapered Teflon gasket to minimize the gaps. For the OCP measurements, a sampling rate of 1 s−1 was employed. Cyclic polarization tests were performed as per ASTM G61 at standard scan rate of 0.167 mVs−1 (both forward and backward scan), commencing from a potential of − 0.4 to 0.4 V versus OCP, a peak current density of 10 mA/cm2, and after an initial delay of 3600 s. EIS tests were carried out as per ASTM G106 in the frequency range of 100-0.01 Hz measured from higher frequencies to lower with an applied AC voltage range of ± 10 mV. The EIS data were collected after immersion for (i) 24 h, (ii) 48 h and (iii) 72 h. All the corrosion tests were repeated at least thrice in order to ensure repeatability.

2.3 Microstructural Characterization

FEI® Quanta 200F scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) was used for micrography of both bare and boronized specimens subjected to cyclic polarization tests.

3 Results and Discussion

3.1 Microstructural Characteristics After Boronizing

Figure 1 shows the cross section of a representative specimen of 13Cr-4Ni MSS boronized at 950 °C. The important features are described as follows. The cross section can be separated into three zones i.e., the boronized layer, intermediate diffusion zone, and the substrate steel. The FeB phase is observed on the top of specimen surface, while the Fe2B is observed toward the interior of the boronized layer. Pores are visible (shown by arrows) but no micro-cracks are present in the boronized layer. The process parameters of boronizing temperature and duration were chosen to achieve optimum combination of layer thickness, morphology, and mechanical properties of the boronized layer (Ref 20), and the details of layer thickness and composition are given in Table 2. The thickness of the boronized layer is non-uniform. For different specimens summarized in Table 2, the average thickness ranged from 35 to 90 μm.

Fig. 1.
figure 1

SEM micrograph of 13Cr-4Ni MSS boronized at 950 °C

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Table 2 Layer thickness and composition of boronized 13Cr-4Ni and 16Cr-5Ni MSS (20)
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3.2 Electrochemical Behavior

The variation of open-circuit potential (OCP) of bare and boronized specimens with time in 0.6 M NaCl solution for up to 3600 s are presented in Figure 2. The OCP values stabilized after about 1800 s immersion for all the specimens. The stable values of OCP indicate that dynamic equilibrium between the dissolution and formation of oxide layers is achieved (Ref 27). The stable OCP values of bare 13Cr-4Ni and 16Cr-5Ni MSS are − 374 mVSCE and − 385 mVSCE, respectively, while the values for 13Cr-4Ni and 16Cr-5Ni MSS boronized at both 950 and 1000 °C ranged between − 629 and  − 613 mVSCE. Thus, the boronized samples show more active values of OCP, which suggests that these are thermodynamically more prone to corrosion as compared to the bare MSS grades.

Fig. 2.
figure 2

Chronopotentiometry of the bare and boronized specimens of martensitic stainless steels in 0.6 M NaCl solution

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Cyclic polarization tests of 13Cr-4Ni and 16Cr-5Ni MSS were conducted in 0.6 M NaCl solution, and the results are shown in Figure 3(a-b). From these curves, the values of the corrosion potential (Ecorr) and corrosion current density (icorr) are obtained which are tabulated in Table 3. The values of Ecorr reflect the thermodynamic tendency toward corrosion, whereas the icorr values represent the kinetics of dissolution of the specimen surface. The boronized MSS grades have Ecorr values that are active by at least 180 mVSCE and as much as 270 mVSCE as compared to their bare counterparts and their corrosion current density (icorr) also increased after boronizing. Similar studies were undertaken by Cakir et al., where they conducted potentiodynamic polarization tests on boronized 316L SS and have reported that boronized samples showed active Ecorr values and higher corrosion current density compared to their bare counterpart (Ref 25). The OCP values for 13Cr-4Ni MSS boronized at 950 °C and 1000 °C are − 613 ± 15, − 620 ± 21 mVSCE, respectively, and the corresponding OCP values for 16Cr-5Ni MSS are − 625 ± 11 and − 628 ± 16 mVSCE, respectively. These are more negative in comparison with the corresponding Ecorr values of the two steels listed in Table 3.

Fig. 3.
figure 3

Cyclic polarization curves for (a) 13Cr-4Ni and (b) 16Cr-5Ni MSS tested in 0.6 M NaCl solution

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Table 3 Corrosion characteristics of bare and boronized 13Cr-4Ni and 16Cr-5Ni MSS in 0.6 M NaCl solution
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The cyclic polarization curves of bare MSS grades demonstrate a typical hysteresis loop as seen in the anodic region indicating localized corrosion attack on the substrate (Figure 3a-b). The electrochemical noise observed in the polarization curves in the anode regions may be due to the nucleation and repassivation of metastable pits (Ref 28). Figure 3(a) shows the cyclic polarization curve of 13Cr-4Ni alloy. The bare specimen exhibited much nobler corrosion potential as compared to the corresponding boronized specimens, suggesting its lower electrochemical activity, whereas there is an insignificant difference in Ecorr values between the MSS boronized using two different boronizing conditions. Another noteworthy observation is that all the boronized specimens corroded actively without forming any protective passive film. Moreover, the current density in bare specimen is large during reverse scanning as compared to forward scanning forming a larger loop, which could be attributed to intense surface damage during forward scanning and its susceptibility to pitting. In contrast, the cyclic polarization curves of boronized MSS specimens are rather uncharacteristic of the behavior of stainless steels in 0.6 M NaCl solution. The anodic current and potential constantly increased, and there is no tendency for passivation. These show less severe localized damage and thus exhibit less deviation or almost overlapping of current density curves during reverse scanning.

Figure 3(b) shows the cyclic polarization curves for 16Cr-5Ni MSS. The bare specimen shows nobler corrosion potential along with passive behavior during anodic polarization, indicating a formation of protective oxide film at free corrosion potential in the test environment. The hysteresis loop observed in the bare samples suggests that the passive oxide film formed on the stainless steel substrate is destabilized under the continuous attack of Cl− ions of the 0.6 M NaCl solution (Ref 2). This creates several specific active areas where the passive film ruptures and enables pitting. Al Ameri et al. evaluated the pitting initiation and growth of AISI 316 stainless steels in 0.6 M NaCl solution and have reported similar results (Ref 29). However, the boronized 16Cr-5Ni MSS specimens do not exhibit passivity characteristics during anodic polarization, and the hysteresis loop is absent. Besides, all the boronized specimens exhibit relatively steeper slopes during cathodic polarization. Therefore, in the tested aerated solution, oxygen reduction kinetics is rate limiting and thus determines the icorr values. Among the boronized steels, both 13Cr-4Ni and 16Cr-5Ni MSS grades show higher values of icorr after boronizing at 950 °C as compared to those boronized at 1000 °C.

In order to further understand the polarization behavior of the boronized samples, the cross section of samples subjected to cyclic polarization were cut and analyzed in SEM. The results are shown in Figure 4(a-d). The boronized layer comprising mainly of FeB, Fe2B, and the intermediary diffusion zones is seen intact. However, in all the micrographs, a narrow, deep pit-like attack can be seen originating from the surface and propagating toward the substrate. The cross section of boronized specimen of Figure 1 contains pores and these served as active sites that facilitated the attack of Cl− ions on the boronized substrate. Some researchers have further discussed the possibility of a channel between these pores and substrate steel matrix through which the NaCl solution seeps and leads to galvanic corrosion (Ref 21). Although the pores are distributed randomly in the boronized layer, the localized crack-like features are observed in Figure 4. Hence, the possibility of alloy sensitization was explored. While in service or during processing, it is reported that stainless steels get sensitized upon extended exposures in the temperature range of 450-800 °C. Dissolved chromium depletes from near the grain boundaries and precipitates to form chromium carbide (Cr23C6) which leads to intergranular corrosion (Ref 30). Although the steels are too lean in their carbon content (~ 0.02 wt.%) to produce any sensitization, the following boronizing reaction during pack carburizing provides the carbon content necessary for sensitization of the surface layers.

$${\text{8Fe}}\left( {\text{s}} \right) \, + {\text{ B}}_{{4}} {\text{C}}\left( {\text{s}} \right) \, \to {\text{ 4Fe}}_{{2}} {\text{B}}\left( {\text{s}} \right) \, + {\text{ C}}\left( {\text{s}} \right)$$
(1)
Fig. 4.
figure 4

SEM micrographs of cross section of (a) 13Cr-4Ni MSS, 950 °C, (b) 13Cr-4Ni MSS, 1000 °C, (c) 16Cr-5Ni MSS, 950 °C and (d) 16Cr-5Ni MSS, 1000 °C, after cyclic polarization tests

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In order to identify the sensitization phenomenon, the cross section of the boronized samples were polished and subjected to oxalic acid etch test as per ASTM 262A standard. A solution of oxalic acid was prepared by dissolving 100 g of oxalic acid in 900 ml of distilled water, and the samples were electro-etched at 1 A/cm2 for 90 s. Figure 5(a-d) shows the SEM micrographs of the boronized samples subjected to oxalic acid etch test. Oxalic acid seems to have attacked the boronized layer and the porosity contained in it, severely. It also reveals a typical dual microstructure in the substrate characteristic of the partially sensitized microstructure, wherein the grain boundaries are attacked discontinuously. In the present study, both the grades of MSS contain more than 13% chromium and the processing temperature of boronizing is 950 and 1000 °C. Although the boronizing temperatures lie outside the sensitization temperature range, rather these correspond to solutionizing range, subsequent tempering treatment at 600 °C for 2 h most likely resulted in precipitation of chromium carbide leading to partial sensitization. Dissolved chromium also combined with dissolved boron to form CrB and Cr2B in the boronized layer as confirmed by the XRD and XPS analyses presented in an earlier work (Ref 20), leading to further depletion of dissolved chromium.

Fig. 5.
figure 5

Cross-section SEM micrographs of: (a) 13Cr-4Ni MSS, 950 °C, (b) 13Cr-4Ni MSS, 1000 °C, (c) 16Cr-5Ni MSS, 950 °C and (d) 16Cr-5Ni MSS, 1000 °C subjected to oxalic acid etch test

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The SEM micrographs of Figure 5 when seen together with Figure 4 micrographs reveal that corrosion initiated most likely by localized attack in the pores of the boronized layer, and subsequently crack-like features propagate into the substrate by following paths created by anodic dissolution along the grain boundaries of sensitized dual microstructure. Further, due to precipitation of chromium into the boronized layer in the form of borides, when the boronized MSS are exposed to NaCl solution, the passivation tendency is inhibited in the presence of chlorides. This mechanism is in harmony with the cyclic polarization curves. Figure 3(a-b) clearly demonstrates limited/no pitting tendency indicating that the boronized stainless steels exhibit lower corrosion resistance than their bare counterparts.

The possible depletion of chromium on the surface was verified using EDS elemental mapping performed on the cross section of a representative specimen of 16Cr-5Ni MSS boronized at 950 °C. Figure 6 shows the SEM micrograph and corresponding EDS elemental maps obtained from the cross section. The substrate steel shows uniform presence of Fe, and dissolved Cr and Ni. On the other hand, it can be clearly seen that within the boronized layer the chromium content is less in the top region of the boronized layer, while it is enriched in the subsurface region followed by a Cr-depleted intermediary diffusion zone under it. This depletion of chromium in the top region of the boronized layer explains the inhibited passivation tendency of boronized MSS in chloride solution. The depletion of chromium is boronized layer in a supermartensitic 13Cr5Ni2Mo stainless steel is also reported by Tlili et al (Ref 31).

Fig.6.
figure 6

SEM micrographs and EDS elemental maps showing elemental distribution in 16Cr-5Ni MSS boronized at 950 °C

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The EIS spectra of the bare and boronized samples are studied in order to understand the electrochemical interaction between the boronized layer and 0.6 M NaCl solution (Figure 7). The Nyquist plots shows two depressed capacitive arcs, one at high frequencies and another at lower frequencies for both bare and boronized samples. However, the capacitive arcs at higher frequencies are rather diminutive, which suggests that the impedance response is insignificant. The two crests in the Bode phase plots of Figure 7(b and d) confirm that the corroding system exhibits two-time constants. The capacitive arc of bare 16Cr-5Ni MSS is the largest among all the tested samples, which is consistent with the high modulus of impedance values, whereas boronized samples show lower low-frequency impedance values suggesting their inferior corrosion resistance as compared to bare MSS grades. Besides, the boronized samples show some disturbance at lower frequencies, which is suggestive of enhanced localized attack, possibly inside the pores (Figure 4), resulting from longer exposure time (Ref 32).

Fig. 7.
figure 7

Nyquist, Bode magnitude, and Bode phase plots of bare (a-b) and boronized (c-d) MSS after 1 h of exposure

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From practical standpoint, in order to further understand the long-term electrochemical behavior of the boronized layer in martensitic stainless steels, the EIS tests were carried out after 24, 48 and 72 h exposures in 0.6 M NaCl solution. The results are shown in Figure 8(a-f). After 24 h of exposure, the Bode magnitude plots at higher frequencies (≥ 10 Hz) reveal minimum response to the changes in frequency which means the system exhibits a purely resistive behavior with the impedance being constant at about 10 Ωcm2. The phase angle values in this frequency range are close to 0° indicating that the impedance behavior in this range is dominated by electrolyte resistance. At lower frequencies (0.01 to 10 Hz), capacitive behavior is observed and a maximum phase angle of 50° is observed. It is interesting to note that the phase shift for the boronized material occurs at different frequencies. Also, in comparison with other boronized steels, the phase angle changes at relatively higher frequency of 0.31 Hz in 16Cr-5Ni boronized at 950 °C, and it records the highest capacitive impedance (Z) of 243 Ωcm2 at a phase angle of 50°, suggesting better performance of the boronized layer. The Nyquist plots demonstrate capacitive arcs for all the boronized MSS. The capacitive arcs of all the boronized MSS are similar except that of 16Cr-5Ni boronized at 950 °C which indicates that the corrosion protection of boronized layer is marginally better in this case, after 24 h of exposure.

Fig. 8.
figure 8

Nyquist, Bode magnitude, and Bode phase plots after (a-b) 24 h (c-d) 48 h (e-f) 72 h exposure to 0.6 M NaCl solution

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After 48 h of exposure, at frequencies higher than 100 Hz, the change in magnitude of impedance is minimal depicting resistive behavior, whereas at lower frequencies of 0.01 Hz, the Bode magnitude plots demonstrate capacitive behavior. The highest magnitude of Z, 333 Ωcm2 is observed in 16Cr-5Ni boronized at 950 °C at a phase angle of 41°. The Nyquist plots depict two capacitive arcs with the arcs at higher frequency being more pronounced after 48 h in comparison with testing condition of 24-h exposure. Figure 8d shows that all the boronized specimens show higher low-frequency impedance values after 48 h as compared to corresponding values after 24-h exposure. The 16Cr-5Ni MSS boronized at 950 °C shows the highest low-frequency impedance, after 48-h exposure, too, indicating its superior global corrosion resistance.

After 72 h of exposure, the boronized MSS grades demonstrate similar capacitive behavior in the lower frequency range. The Nyquist plots after 72 h depict depressed region at higher frequencies, and beyond a frequency of 1 Hz, a capacitive arc is seen. 13Cr-4Ni MSS boronized at 950 °C stands out and records the highest Z of 324 Ω.cm2 at a phase angle of 35° and frequency of 1 Hz. However, two-time constants are noticed first at frequency of < 0.1 Hz and the other at 1 Hz which may be due to the presence of surface inhomogeneity (pores). In the other boronized MSS grades, phase change occurs at lower frequencies of 0.01 Hz indicating inferior corrosion resistance of the boronized layer.

In order to obtain quantitative information from EIS data, an electrical equivalent circuit (EEC) (Figure 7a inset) is used. The EEC comprises of the following elements: solution resistance (Rs), charge transfer resistance (Rct), the constant phase element (CPE) of the double layer (Qdl), the resistance of the boronized layer for boronizing specimen or the passive film resistance for bare steel (Rf), and the CPE of the boronized layer or the passive film (Qf) (Ref 18).

For the boronized samples, CPE is used instead of an ideal capacitor due to heterogeneity at the surface of electrodes (Ref 32, 33), which results in depressed semicircles in the Nyquist plots of Figures 7 and 8. The impedance of CPE is calculated using Eq 2:

$${Z}_{\text{CPE}}=[Q(j{\omega )}^{n}{]}^{-1} (\text{Ref} 34)$$
(2)

where Q is the magnitude of CPE, j is current, ω is frequency, and n is a constant between 0.5 and 1. The value of n depicts specific inferences; when n = 1, CPE depicts an ideal capacitor, for 0.5 < n < 1, the CPE describes dielectric relaxation times in the frequency domain, and n = 0.5 of the CPE represents Warburg impedance with diffusion control characteristics (Ref 33). The polarization resistance (Rp) is evaluated using Eq 3:

$${R}_{\text{p}}={R}_{\text{f}}+{R}_{\text{ct}}$$
(3)

The effective capacitance values (Ceff) are obtained by using 4.

$${C}_{\text{eff}}={Q}^{\left(\frac{1}{n}\right)} {(\frac{1}{{R}_{\text{s}}}+\frac{1}{{R}_{\text{p}}})}^{(\frac{1-n}{n})}$$
(4)

The electrochemical parameters obtained after fitting the experimental data of all the samples are tabulated in Tables 4 and 5.

Table 4 Electrochemical parameters obtained after fitting the experimental data for 13Cr-4Ni MSS boronized at 950 and 1000 °C
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Table 5 Electrochemical parameters obtained after fitting the experimental data for 16Cr-5Ni MSS boronized at 950 and 1000 °C
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The main inferences that can be drawn from the curve fitting data is that after 24 h of exposure, Rct and Rf values indicate good barrier to protection. However, Rct, Rf, Rp, and Ceff,1 values decrease over prolonged exposures of 48 and 72 h due decline in corrosion resistance. These results also justify the possibility of micro-galvanic effect and chromium depletion mechanisms proposed earlier. The Ceff,2 values are significantly lower demonstrating that the boronized layer characteristics dominate the electrochemical behavior. Also, boronizing temperature has minimal effects in enhancing the corrosion resistance, and general decline in the corrosion resistance is observed over longer periods of exposure. The prominent cause emerging from this work for the inferior corrosion resistance of boronized MSS seems to be their sensitization occurring during the tempering treatment. Therefore, in future, optimizing the tempering process after boronizing by employing low tempering temperatures and their effect on the corrosion behavior can be explored, since this is expected to avoid sensitization of the MSS.

4 Conclusions

The corrosion resistance of boronized MSS grades was evaluated and compared with that of the bare MSS samples. The important findings are as follows.

  1. (i)

    After boronizing, the electrochemical potential became more active. This was attributed to their inability to passivate as evident from active dissolution observed during anodic polarization in the potentiodynamic polarization tests.

  2. (ii)

    Boronized MSS do not indicate any passivating behavior followed by lack of pitting corrosion in the cyclic polarization tests. Sensitization of the boronized samples aided their active dissolution during anodic polarization.

  3. (iii)

    The impedance response indicates that the bare specimen has superior uniform corrosion resistance as compared to the boronized specimen, which is attributed to protective passive film and the absence of chemical heterogeneity. The latter was observed in boronized specimens due to chromium depletion resulting from the formation of chromium borides and sensitization of the boronized martensitic stainless steels.