1 Introduction

Renewable hydrogen produced by water electrolysis is expected to play a key role in transitioning from an entirely fossil fuel-based economy to zero-emission technologies, with the goal of decarbonizing the transport and energy sector [1, 2]. To maximize the role of hydrogen in decarbonizing the energy sector, it is essential to invest in the development of cost-effective and sustainable production methods, such as water electrolysis via renewable solar electricity [3]. Amongst different technologies, proton exchange membrane water electrolysis (PEMWE) represents a promising technology for the sustainable production of hydrogen [4] and the current status, research trends and perspectives of key materials for PEMWE are recently reviewed in literature [5]. The most important limitation identified is related to the use of expensive electrocatalysts and construction materials that leads to high hydrogen production costs [5]. Since the main cost drivers in PEMWE technology are the bipolar plates (BPPs) and gas diffusion layers (GDLs), accounting for as much as 51% and 17%, respectively, of the total stack costs [6], the development of cheap alternative materials is urgently required. Many research efforts are devoted to replace currently used titanium but the available options are strongly limited by the harsh operating conditions and by the competing demands for excellent conductivity and corrosion resistance. A commonly investigated replacement material is stainless steel covered with different types of dual-function layers such as Pt/Ti [7, 8] and Nb/Ti coatings [9] to increase corrosion resistance and lower interfacial contact resistance. It has been proposed that even a material prone to corrosion (e.g. copper), but offering excellent thermal and electrical conductivity, may be used to replace titanium as base material, provided a fully dense anticorrosive layer protects it [10].

The corrosion resistance of potential replacement materials is usually tested by conventional electrochemical methods such as potentiodynamic polarization curves, potentiostatic stress tests at high anodic potentials and electrochemical impedance spectroscopy [11,12,13] that are only able to provide an average, macro-scale assessment of corrosion phenomena occurring at the surface. To gain a micro-scale insight into localized corrosion processes that take place on metal surfaces, scanning electrochemical microscopy (SECM) has emerged as a crucial technique providing high-resolution imaging of the local electrochemical activity on the surface of the material [14,15,16]. SECM studies proved to be very suitable in investigating the pitting corrosion behaviour of stainless steel in the presence of chloride ions, detecting different pitting stages from metastable to stable states [17] and even increasing the spatial resolution in pit mapping at submicron scale [18]. High sensitivity localization of corrosion sites was possible using a multi-electrode array combined with SECM [19], which allowed to reduce the analysed area and to increase the time resolution significantly. Using simultaneous imaging of locally generated iron (II) ions and pH distribution, it was possible to observe the pH decrease at the active anode and, respectively, increase at cathode sites [20]. Monitoring the local release of iron species during anodic polarization of stainless steel in 0.1 M HCl showed that pitting occurs with the formation of iron (II) species only, while detection of iron (III) species occurred at high positive overpotentials on the passive surface surrounding the pit [21, 22]. Typically, most of the research regarding localized corrosion of stainless steel using SECM focuses on pitting corrosion induced by chloride ions and it is limited to open-circuit potential or relatively low anodic potentials. Information about corrosion behaviour of ferritic stainless steel at higher anodic potentials, in the absence of chloride ions, relevant for PEMWE applications, is scarce.

In this study, we use SECM to investigate the localized corrosion behaviour of ferritic stainless steel AISI 446 and Ti-coated AISI 446, as candidate materials to replace titanium in BPPs construction for PEMWE. We used the SECM feedback mode with a redox mediator to evidence the passive layer formed at open-circuit potential on the surface of bare and Ti-coated AISI 446. Since BPPs experience high anodic potentials during operation, the samples were also anodically polarized to evidence the localized corrosion phenomena using the substrate generation–tip collection mode of SECM.

2 Materials and Methods

2.1 Preparation and Physicochemical Characterization of the Coating

The investigated samples (square shaped, 10 mm × 10 mm) were made of bare AISI 446 stainless steel and Ti-coated AISI 446. First, the AISI 446 samples were embedded in a resin and a contact copper stripe was attached on the backside. The free surface of the AISI samples was mirror polished using different granulation SiC papers, up to grit #4000. Then, a 500 nm Ti coating was deposited by magnetron sputtering, using and Orion 8 (AJA International, Nord Scituate, USA) magnetron sputtering system. A 2ʺ Ti target (99.995% pure) was used to deposit a film on the AISI 446 sample. The preliminary vacuum was 6 × 10–8 torr, and the deposition was made in 3 mtorr pressure of Ar (5.3 purity, supplied by Linde Gas), using 100 W DC power. The thickness of the deposited film was measured with a Sloan Dektak II Profilometer (Sloan).

The chemical compositions of the uncoated and Ti-coated AISI 446 samples were determined by energy-dispersive X-ray (EDX) analysis, and the surface topography was investigated by scanning electron microscopy (SEM) using a TESCAN Vega 3 LM scanning electron microscope (TESCAN Brno, s.r.o., Brno, Czech Republic) equipped with a Bruker Quantax 200 energy-dispersive X-ray spectroscopy system with a Peltier-cooled XFlash 410 M silicon drift detector (Bruker, Billerica, MA, USA).

2.2 SECM Measurements

The SECM measurements were performed with a scanning electrochemical microscope (M370, UNISCAN Instruments Ltd., UK). The corrosion resistance of the samples was determined in two test solutions, 0.1 M Na2SO4 (99%, ACS Reagent, Riedel-de Haën, Germany) and 0.1 M KCl (99.5% for analysis EMSURE, Merck KGaA, Germany). Measurements were carried out in the feedback mode using potassium ferricyanide K3Fe(CN)6 (99%, ACS Reagent, Riedel-de Haën, Germany) as redox mediator added to the electrolyte, and also in the substrate generation–tip collection (SG-TC) mode, in the absence of the redox mediator. Both line scan and area scan measurements were performed, using a disc-shaped Pt ultra-microelectrode (UME) with a diameter of 25 μm. The SECM measurements were performed with the tip at a height of 20 μm over the AISI 446 surface, unless otherwise specified. The electrochemical cell contained the resin embedded samples of uncoated AISI 446 or Ti-coated AISI 446 as working electrode, a Pt plate as counter electrode, and a screen-printed Ag/AgCl reference electrode.

3 Results

3.1 Physicochemical Characterization of the Coating

The elemental composition of the tested AISI 446 specimen, determined by EDX analysis, is listed in Table 1, as well as EDX analysis of a standard AISI 446 stainless steel. The results indicate a high chromium content, specific to ferritic stainless steels.

Table 1 Elemental composition of tested and standard AISI 446 stainless steel
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The surface morphology of the Ti coating on AISI 446 substrate can be observed in the SEM image given in Fig. 1, together with the thickness measurement of the Ti layer. The deposited Ti layer was in the range of 500 nm and showed a compact structure, well adhering to the polished substrate. The composition determined by EDX indicates only the presence of Ti in the coating.

Fig. 1
figure 1

SEM image of the Ti film deposited on the surface of the AISI 446 ferritic stainless steel a; thickness measurement of the Ti layer with the Dektak II profilometer showing a thickness of about 5675 Å b

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3.2 SECM Measurements

The detection potentials applied in the feedback and SG-TC modes were derived from the cyclic voltammogram recorded on the Pt probe in 0.1 M Na2SO4 and 0.1 M KCl solutions containing the redox mediator, potassium ferricyanide. Figure 2 gives a typical example of a cyclic voltammogram obtained in 0.1 M Na2SO4 + 5 mM K3Fe(CN)6, with the Pt probe far away from the AISI 446 substrate. According to Fig. 2, a tip potential of − 0.25 V vs. Ag/AgCl was selected for the reduction of Fe3+ ions in the feedback mode and a tip potential of 0.6 V vs. Ag/AgCl for the oxidation of Fe2+ ions in the SG-TC mode.

Fig. 2
figure 2

Cyclic voltammogram of Pt probe in 0.1 M Na2SO4 + 5 mM K3Fe(CN)6 solution far away from the AISI 446 substrate. Scan rate 20 mV/s

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When the SECM measurements are conducted in the feedback mode, with the redox mediator present in the solution, the sample potential is maintained at OCP and the tip is biased at − 0.25 V vs. Ag/AgCl. No reaction takes place on the substrate and the recorded current on the tip corresponds only to Fe3+ reduction on the Pt probe. The schematic representation of the reaction model for this case, depicted in Fig. 3, shows that when the Pt probe is far away from the substrate, the reduction current on the tip is determined by the diffusion of Fe3+ towards the Pt tip. When the Pt probe is very close to the substrate, the diffusion of Fe3+ ions is hindered. Depending on whether the substrate surface is conductive or insulating, a positive or negative feedback effect is generated, and the tip current will correspondingly increase or decrease, compared to its value in the solution.

Fig. 3
figure 3

Schematic representation of the reaction model for AISI 446 during SECM measurement in the feedback mode, in the presence of ferricyanide redox mediator, depending on the tip–substrate distance

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For SECM measurements in the SG-TC mode, the schematic representation of the reaction model is given in Fig. 4. In this case, the Pt probe is biased at 0.6 V vs. Ag/AgCl to oxidize Fe2+ ions and moved at constant height above the sample. The substrate potential is changed from OCP, where no Fe2+ is generated and no current is recorded on the Pt probe, to anodic potentials where Fe2+ is generated on the substrate and detected on the Pt probe.

Fig. 4
figure 4

Schematic representation of the reaction model for AISI 446 during SECM measurement in the SG-TC mode, depending on the substrate potential

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Figure 5 shows the experimentally obtained approach curves at open-circuit potential in 0.1 M Na2SO4 and 0.1 M KCl solutions containing the redox mediator, recorded as the tip was progressively moved on the Z-axis towards the uncoated and, respectively, Ti-coated AISI 446 substrate.

Fig. 5
figure 5

SECM normalized Z-approach curves measured on the 25 μm Pt tip above: a the uncoated AISI 446 and b the Ti-coated AISI 446 substrate. Conditions: 0.1 M Na2SO4 and 0.1 M KCl solutions with 5 mM K3Fe(CN)6, tip potential Etip = − 0.25 V vs. Ag/AgCl, sample potential, Esample = OCP, scan velocity 50 μm/s, step size 10 μm

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It can be observed that far away from the substrate the Pt tip current remains at a stable value, corresponding to the background current given by the reduction reaction of ferricyanide to ferrocyanide. As the tip approaches the AISI 446 substrate, the tip current starts to decrease because the diffusion of the mediator to the electrode is hindered, generating the so-called negative feedback effect. This proves that the surface film covering both the AISI 446 and Ti-coated AISI 446 substrate has low conductivity and limits the diffusion of ferricyanide ions towards the SECM tip according to a negative feedback curve, as shown in Fig. 5. A similar negative feedback effect is usually observed also for austenitic stainless steels, due to their native, insulating, oxide film [18].

Figure 6 a and b shows SECM area scans obtained in the feedback mode, while the Pt tip scanned an area of 1 mm × 1 mm along the X- and Y-axis. The Pt tip was biased at − 0.25 V vs. Ag/AgCl and placed at a large Z distance away from the AISI 446 substrate, to record the background current in the solution. To observe the effect of the AISI 446 substrate on the tip current, the SECM area scan was repeated with the probe placed at a distance of 20 µm from the substrate.

Fig. 6
figure 6

SECM area scans obtained in: a 0.1 M Na2SO4 + 5 mM K3Fe(CN)6 solution and b 0.1 M KCl + 5 mM K3Fe(CN)6 solution, with the Pt tip placed at a large distance away from the AISI 446 substrate and at 20 μm distance from AISI 446. Conditions: tip potential Etip = − 0.25 V vs. Ag/AgCl, sample potential Esample = OCP, scan velocity 50 μm/s, step size 50 μm. Feedback mode

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The results showed that the background reduction current of Fe3+ in 0.1 M Na2SO4 solution is around 16 nA and reduces to 5 nA when the probe is close to the substrate (Fig. 6a). This indicates that the AISI 446 substrate limits the diffusion of the redox mediator due to its surface passive film. The same area scans obtained in 0.1 M KCl solution (Fig. 6b) indicate that the background reduction current of Fe3+ ions has almost the same value as in 0.1 M Na2SO4 solution. However, comparing the SECM area scans obtained at 20 μm above the AISI 446 substrate, in 0.1 M Na2SO4 (Fig. 6a) and 0.1 M KCl (Fig. 6b) solutions, it is obvious that the tip current in 0.1 M Na2SO4 is below 5 nA, while it increases to 8–12 nA in 0.1 M KCl. This result is an indication that the surface passive film is more stable and insulating in Na2SO4 solution and it becomes more reactive in KCl solution. It is generally agreed that a thin passive layer, mainly composed of chromium and iron oxides and hydroxides, covers the high chromium ferritic stainless steels and protects the substrate from corrosion [20, 23]. Depending on the pH, the composition of the passive layer can be described as a bilayer structure, predominantly containing iron enriched hydroxides in the outer part and chromium enriched oxides in the inner part [24], with hydroxides more favoured at higher pH [25] and increased chromium content at lower pH. The higher the amount of chromium in the steel, the better the corrosion resistance due to a more stable passivation layer [26, 27]. However, the passive film is prone to local breakdown, particularly in the presence of chloride ions. Chloride ions can break the passive layer even at low anodic potentials and generate single pits [28].

Further measurements were conducted in the SG-TC mode, in the absence of the redox mediator. In this case, the potentials of both probe and sample are controlled using a bi-potentiostat. The AISI 446 substrate is biased at different anodic potentials to induce corrosion according to the reaction Fe → Fe2+  + 2e–, and the Pt tip is biased to 0.6 V vs. Ag/AgCl to further oxidize the Fe2+ ions generated on the substrate to Fe3+. Hence, the tip can detect Fe2+ ions locally generated on the substrate and the tip current will increase over the areas of the substrate where corrosion takes place. Generally, the more corrosion products result by active dissolution, the higher the Faradaic currents recorded on the tip and the higher the electrochemical activity [17]. Based on the potentiodynamic polarization curves, several potentials of the substrate were selected for the SG-TC mode, as shown in Fig. 7. SECM line scans were obtained first with the substrate at OCP, then potentiostatically polarized to an anodic potential situated in the passive range, and finally at higher anodic potentials, where the current starts to increase. SECM area scans were conducted at OCP, then at two anodic passivation potentials (0.2 and 0.5 V), and lastly at 1.0 V for AISI 446 and 1.2 V for Ti-AISI 446, respectively.

Fig. 7
figure 7

Potentiodynamic polarization curves of AISI 446 and Ti-coated AISI 446 in 0.1 M Na2SO4. Scan rate 1 mV/s

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Figure 8a–d gives SECM line scans in 0.1 M Na2SO4 solution in the SG-TC mode for both uncoated and Ti-coated AISI446 samples. First, the uncoated AISI 446 sample is held at OCP, while the probe is biased at 0.6 V vs. Ag/AgCl and moved across the sample along the Y-axis. Figure 8a gives the probe current recorded as a function of distance on the Y-axis, while Fig. 8b shows the sample current measured at the same time. The measurements are then repeated with the uncoated AISI 446 sample successively biased at different anodic (0.5; 1.2 and 1.4 V vs. Ag/AgCl) and cathodic ( − 1.3 V vs. Ag/AgCl) potentials. For each potential of the sample, the probe is moved across the sample along the Y-axis and the probe and sample currents are measured simultaneously. Similar measurements are performed on the Ti-coated AISI 446 sample, first at OCP and then at various anodic potentials (0.5; 1.2; 1.4; and 1.6 V vs. Ag/AgCl). Figure 8c shows the change of the probe current along the Y-axis, whereas Fig. 8d shows the sample current, as measured at different potentials of the Ti-coated AISI 446 substrate.

Fig. 8
figure 8

SECM line scans obtained in 0.1 M Na2SO4 at 20 μm distance from the substrate: a, b AISI 446 and c, d Ti-coated AISI446. Probe current (left) and sample current (right) were simultaneously recorded at different potentials of the substrate. Conditions: tip potential Etip = 0.6 V vs. Ag/AgCl, scan velocity 50 μm/s, step size 25 μm. Substrate generation–tip collection mode

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According to the SECM line scans from Fig. 8a and b, when the AISI 446 substrate is left at OCP or anodically polarized at 0.5 V vs. Ag/AgCl, both probe and sample currents stay close to zero, indicating that no reaction takes place on the Pt tip because no Fe2+ ions are generated on the substrate. If the AISI446 potential is increased to 1.2 and 1.4 V vs. Ag/AgCl, a positive current is measured on the substrate, corresponding to an oxidation reaction. Contrary to expectations, the probe current is negative, suggesting that a reduction reaction takes place on the tip. This indicates that the oxidation reaction on the substrate does not generate Fe2+ ions, but it is the oxygen evolution reaction. Since the tip potential is set to 0.6 V, the oxygen generated on the substrate will be reduced on the tip. The negative reduction currents observed for the probe means that oxidation of the AISI 446 generates oxygen, which is detected and reduced on the probe and no corrosion of AISI 446 is evidenced in 0.1 M Na2SO4 at 1.2 and 1.4 V vs. Ag/AgCl. In contrast, applying a negative potential to the substrate, when hydrogen evolution takes place, the measured probe current is positive, indicative of on oxidation reaction on the tip; specifically the hydrogen generated at the substrate oxidizes back to protons on the Pt tip. The proton/hydrogen mediator system is often used to investigate the electrocatalytic activity of electrocatalysts for hydrogen oxidation reaction [29, 30].

For the Ti-coated AISI 446 sample, the SECM line scans from Fig. 8c and d indicate a similar behaviour, with no current flowing through the probe or the sample, when the substrate is kept at OCP or at 0.5 V vs. Ag/AgCl. If the potential is further increased to 1.2 V vs. Ag/AgCl, the sample current stabilizes at around the same value as for the uncoated AISI446 and the probe current remains close to zero. The difference appears at 1.4 V vs. Ag/AgCl, when the sample current is one third of that for the uncoated AISI446 but the probe current still remains close to zero, meaning that the species generated by the substrate oxidation cannot be detected on the Pt probe biased at 0.6 V vs. Ag/AgCl. This might be the case of Ti coating passivation. At even higher potential of 1.6 V vs. Ag/AgCl, significant oxygen evolution occurs on the passive Ti coating, and the sample current reaches about 20 mA, while the probe current becomes negative. This implies that a reduction reaction occurs on the probe, corresponding to the reduction of oxygen generated on the substrate.

To gain a more detailed insight on the surface corrosion behaviour of the uncoated and Ti-coated AISI446 samples, SECM area scans were recorded in 0.1 M Na2SO4 in the SG-TC mode. During the measurements, the uncoated AISI 446 substrate was firstly maintained at OCP and then polarized at 0.2, 0.5, and 1.0 V vs. Ag/AgCl. The Pt tip was biased at 0.6 V vs. Ag/AgCl and moved along the X- and Y-axis to scan an area of 1 mm × 1 mm. At this potential, either the Fe2+ ions formed by the substrate corrosion can be oxidized to Fe3+, or the oxygen generated on the substrate can be reduced. The obtained SECM area scans are represented in Fig. 9a–d for the uncoated AISI 446 and in Fig. 10a–d for the Ti-coated AISI 446 sample.

Fig. 9
figure 9

SECM area scan of uncoated AISI 446 in 0.1 M Na2SO4 solution with the tip biased at E = 0.6 V and the AISI 446 substrate at a OCP, b  0.2 V vs. Ag/AgCl, c 0.5 V vs. Ag/AgCl and d 1.0 V vs. Ag/AgCl. Conditions: substrate tip distance is 20 µm, scan velocity 50 μm/s, step size 50 μm. Substrate generation–tip collection mode

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Fig. 10
figure 10

SECM area scan of Ti-coated AISI 446 in 0.1 M Na2SO4 solution with the tip biased at E = 0.6 V and the Ti-AISI 446 substrate at a OCP, b 0.2 V vs. Ag/AgCl, c 0.5 V vs. Ag/AgCl and d 1.2 V vs. Ag/AgCl. Conditions: substrate tip distance is 20 µm, scan velocity 50 μm/s, step size 50 μm. Substrate generation–tip collection mode

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The obtained results are in line with the SECM line scans from Fig. 8a, indicating very low tip currents when the sample is maintained at OCP (Fig. 9a) or polarized at 0.2 V (Fig. 9b) and 0.5 V vs. Ag/AgCl (Fig. 9c). The absence of any current spikes means that no species are generated on the substrate that can be detected by the tip. However, even at these low currents, differences are noticeable in the surface behaviour, regarding the current distribution. A less homogeneous current distribution is observed for the sample at OCP, whereas at 0.2 V vs. Ag/AgCl the tip current further decreases and the distribution becomes more uniform. This suggests that the surface becomes more insulating when the sample potential increases, due to passivation. At 0.5 V vs. Ag/AgCl, the surface is still passive, but some small, more reactive zones start to emerge. At 1.0 V vs. Ag/AgCl, as anticipated, based on the preliminary findings from the line scans, the SECM area scan in Fig. 9d shows a negative current spike corresponding to the reduction of oxygen produced on the substrate when the sample potential is 1.0 V. The area scan, however, also reveals a positive current spike that relates to the oxidation of Fe2+ generated by the substrate corrosion. The fact that both oxygen evolution reaction (OER) and Fe2+ generation on the substrate are detected in the same area can be explained considering that OER leads to a local decrease in the pH close to the substrate. Recent studies have shown that during OER, the local pH deviates significantly, dropping by orders of magnitude as the current density of OER increases, with this effect being particularly pronounced for near neutral pH solutions [31]. The local generation of H+ near the surface causes a lower pH and, as a result, a more corrosive solution. This also affects the chemical stability of the oxides in the passive layer and may result in passivity breakdown [32]. Studies on the corrosion behaviour of different constituents in equiatomic alloys indicated that the protective performance of passive film is mainly determined by the constituent with the highest corrosion resistance, such as chromium, and the breakdown of passive film is mainly dominated by the elements with the highest corrosion rate, like iron [33]. Therefore, it is expected that during OER, the local acidification near the substrate induces passivity breakdown and release of Fe2+ ions. Additionally, area scan has a higher probability in detecting localized corrosion processes, as seen by the difference in the results obtained by line scan and area scan. This proves that the uncoated AISI 446 substrate may experience some localized corrosion even at 1.0 V vs. Ag/AgCl, undetected by a simple line scan.

Concerning the Ti-coated AISI 446 sample, even at OCP, the surface is more insulating than the uncoated AISI 446 surface, as shown in Fig. 10a. The current distribution uniformity slightly improves as the potential increases from OCP to 0.2 (Fig. 10b) and, respectively, 0.5 V vs. Ag/AgCl (Fig. 10c). Even at higher potentials than for the uncoated substrate, at 1.2 V vs. Ag/AgCl (Fig. 10d), the surface remains passive and only negative, low intensity current spikes appear, indicating that the substrate oxidation produces only molecular oxygen. The SECM area scan does not reveal any evidence of localized corrosion.

AISI 446 corrosion behaviour was further investigated in 0.1 M KCl, considering the capability of chlorine ions to induce pitting corrosion. SECM line scans were recorded, according to Fig. 11 which shows successive line scans obtained when the Pt UME moves over the AISI 446 substrate on the X-axis, with on offset of 200 µm on the Y-axis. The results clearly demonstrate that when the substrate is maintained at OCP, no Fe2+ ions are generated and no current is recorded on the Pt tip. If the substrate is polarized to more positive potential values, i.e. at 0.2 V vs. Ag/AgCl, the tip current shows higher or lower intensity positive current spikes above distinct regions of the substrate, indicating the spots where the substrate starts to corrode and to generate the Fe2+ ions. The current spikes correspond to pitting corrosion of the substrate. The situation is completely different from that observed in 0.1 M Na2SO4, where at the same potential of 0.2 V vs. Ag/AgCl (Fig. 9 d) the surface is passive and no electrochemical activity is detected. Moreover, in 0.1 M Na2SO4 solution, the first signs of corrosion are observed starting from a much higher potential of 1.0 V vs. Ag/AgCl.

Fig. 11
figure 11

SECM line scans of uncoated AISI 446 in 0.1 M KCl solution with the tip biased at E = 0.6 V and the AISI 446 substrate at a OCP and b polarized at 0.2 V vs. Ag/AgCl. Conditions: substrate tip distance is 20 µm, scan velocity 50 μm/s, step size 25 μm. Substrate generation–tip collection mode

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Additionally, SECM area scan was recorded for the uncoated AISI 446 substrate in 0.1 M KCl, first with the substrate maintained at OCP and after polarized at 0.2 V vs. Ag/AgCl. The obtained area scans are detailed in Fig. 12a–b. The results point to a stable surface when the substrate is maintained at OCP (Fig. 12a). Notable differences appear, however, when the substrate is polarized from OCP to an anodic potential of 0.2 V vs. Ag/AgCl (Fig. 12b). The onset of corrosion phenomena is indicated by several current spikes on the SECM area scan, having different intensities, depending on the amount of Fe2+ ions generated on the substrate. Generally, the larger the amount of corrosion products resulted by active dissolution, the higher the current intensities on the tip, implying higher electrochemical activities. This is evidenced by comparing the results from Figs. 9d and 12d, which show that in 0.1 M Na2SO4, even at much higher anodic potentials, the current intensity on the tip is about 0.15 nA, 100 times lower than in 0.1 M KCl. It can be concluded that AISI 446 substrate corrosion resistance is affected by the test solution, revealing enhanced corrosion resistance in 0.1 M Na2SO4 solution compared to 0.1 M KCl solution.

Fig. 12
figure 12

SECM area scan of uncoated AISI 446 in 0.1 M KCl solution with the tip biased at E = 0.6 V and the AISI 446 substrate at a OCP and b polarized at 0.2 V vs. Ag/AgCl. Conditions: substrate tip distance is 20 µm, scan velocity 50 μm/s, step size 50 μm. Substrate generation–tip collection mode

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Similar SECM area scans obtained for the Ti-coated AISI 446 substrate in 0.1 M KCl solution are represented in Fig. 13a–d. First, the substrate was maintained at OCP, and the successively polarized at 0.2, 0.5, and, respectively, 1.2 V vs. Ag/AgCl. According to the SECM area scans, no electrochemical activity of the surface of Ti-coated AISI 446 is detected at OCP (Fig. 13a) 0.2 V (Fig. 13b) and 0.5 V (Fig. 13c). By shifting the potential anodically up to 1.2 V vs. Ag/AgCl, the surface still remains in the passive state and only one negative current spike is observable, that indicates local oxygen evolution reaction on the passive surface.

Fig. 13
figure 13

SECM area scan of Ti-coated AISI 446 in 0.1 M KCl solution with the tip biased at E = 0.6 V and the Ti-AISI 446 substrate at a OCP, b  0.2 V vs. Ag/AgCl, c 0.5 V vs. Ag/AgCl and d 1.2 V vs. Ag/AgCl. Conditions: substrate tip distance is 20 µm, scan velocity 50 μm/s, step size 50 μm. Substrate generation–tip collection mode

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In summary, the results point to the effectiveness of electrochemical probe techniques, i.e. SECM to detect localized corrosion phenomena at earlier stages than by conventional electrochemical techniques. It has also been demonstrated that a thin Ti layer deposited by magnetron sputtering can protect the underlying ferritic steel substrate from corrosion in sodium sulphate solution and even in a more aggressive potassium chloride solution.

4 Conclusions

SECM was used to evaluate the localized corrosion processes of uncoated and Ti-coated AISI 446 ferritic stainless steel, a promising material for BPPs fabrication in PEMWE. The electrochemical activity in sodium sulphate and potassium chloride solutions was measured at open-circuit potential and during anodic polarization. Due to its low conductivity, the surface passive oxide layer generates a negative feedback effect in the presence of a redox mediator at open-circuit potential in the SECM feedback mode. This effect is more pronounced in sodium sulphate solution than in potassium chloride solution, showing that the passive layer is more stable and insulating when chloride ions are absent and becomes more reactive when they are present. The substrate generation-tip collection mode of SECM allows for the observation of distinct localized corrosion behaviour during anodic polarization, depending on the substrate and electrolyte solution. The uncoated AISI 446 substrate is corrosion resistant in sodium sulphate solution up to an anodic potential of 1.0 V vs. Ag/AgCl, where the onset of localized corrosion is detected simultaneously with oxygen evolution reaction. In contrast, in the presence of 0.1 M chloride ions, AISI 446 experiences significant pitting corrosion at 0.2 V vs. Ag/AgCl, independently on the oxygen release. The Ti-coated AISI 446 substrate polarized at 1.2 V vs. Ag/AgCl exhibits enhanced corrosion resistance, without any signs of localized corrosion neither in the absence, nor in the presence of chloride ions. The only electrochemical activity detected under these conditions corresponds to oxygen generation on the passive substrate. The obtained results demonstrate that AISI 446 is susceptible to localized corrosion at lower anodic potentials than previously detected by conventional electrochemical methods. A thin Ti coating is very effective in providing enhanced corrosion resistance in the absence and presence of chloride ions, allowing AISI 446 to be used for manufacturing BPPs.