A rapid corrosion screening technique for grade 2707 hyper‐duplex stainless steel at ambient temperature

The pitting corrosion behaviour of 27%Cr–7%Ni (2707) hyper‐duplex stainless steel (HDSS) at ambient temperature has been assessed using a novel bipolar electrochemistry test set‐up. Application of this technique generates a linear potential gradient at the surface of the HDSS 2707 test piece, which allows assessment of the pitting corrosion behaviour at ambient temperature. Pit nucleation and associated growth kinetics can be obtained and characterised. To assess pitting corrosion in high‐alloyed HDSS, the normal bipolar test method had to be modified by superimposing an auxiliary potential. This new test set‐up was demonstrated by characterising the nucleation of pits and associated pit growth kinetics. The latter occurred either at austenite/ferrite interphase or inside ferrite phase, with pit growth within the ferrite phase in the HCl environment.


| INTRODUCTION
Solution-annealed duplex stainless steel (DSS) consists of austenite (γ) and ferrite (α), combining the benefits of the austenite and ferrite phases in the alloy.The alloys gain excellent toughness from the austenite phase and high strength/stress corrosion resistance from the ferrite phase. [1][4] Compared to standard DSS 2205, SDSS 2507 and HDSS 2707 have higher Cr and Ni contents, which increases the pitting corrosion resistance, for example, the critical pitting temperature (CPT) in DSS 2205 is 61°C, [5] compared to SDSS 2507 (96°C) and HDSS 2707 (>100°C). [6,7]0] Bipolar electrochemistry is an innovative method used for evaluating corrosion.It involves placing a metal sample between two feeder electrodes, one positive and one negative.By applying a power supply to create a potential difference between the feeder electrodes, an electric field is generated.This electric field induces electrochemical reactions on the bipolar electrode (BPE).A quasilinear potential gradient is formed, starting with a positive potential at the anodic side and transitioning to a highly negative potential at the cathodic side of the BPE.The anodic reactions occur near the negative feeder electrode, while the cathodic reactions occur near the positive feeder electrode.[22] The size and ratio of gamma (γ) to alpha (α) phases have an impact on the frequency of the pit nucleation and growth stability. [14,23]25] In this manuscript, a standard three-electrode potentiodynamic corrosion test is compared to a traditional bipolar and modified bipolar electrochemistry experimental set-up to test HDSS 2707 at room temperature in 0.1 M HCl.The critical pitting potential and pit evolution in HDSS 2707 are investigated and compared via three-dimensional (3D) laser surface scanning confocal microscopy.The relationship between pitting corrosion and microstructure is observed by scanning electron microscopy combined with chemical testing using energy-dispersive X-ray (EDX) analysis.

| MATERIALS AND METHODS
The HDSS 2707 was obtained from Sandvik/Aleima, with the chemical composition given in Table 1.The pitting resistance equivalent number (PREN) is calculated from PREN = %Cr + 3.3 × %Mo + 30 × %N, and estimated CPT in 6% FeCl 3 can be calculated from ASTM G48 Method 5.
A CS2350 Bipotentiostat and CS Studio-5 software was used for the three-electrode potentiodynamic polarisation experiment.After welding the back of the HDSS 2707 with a copper wire, it was mounted in resin, then ground to a 1500 grit finish and finally polished with a 1-µm polishing paste.The exposed area was 0.5 mm × 0.5 mm, with the rest area was covered by 3M 60 Black PTFE Tape.A platinum electrode and saturated calomel reference electrode (SCE) were used.For the potentiodynamic polarisation test, the open circuit potential (OCP) was measured for 1800 s, and then the potentiodynamic polarisation test was run from −0.2 to + 1.6 V OCP with a scan rate of 1 mV/s in 0.1 M HCl at room temperature.
For the bipolar electrochemistry test, a UTP1003S DC power supply was used.The BPE had an exposed area of 10 mm × 30 mm (width × length), with the same surface finish as the potentiodynamic sample.Figure 1a shows the bipolar electrochemistry set-up; each feeder electrode consists of 4-cm 2 Pt foils.The distance between the feeder electrode is 5 cm, and the BPE is at the centre.The bipolar electrochemistry tests run for 300 s with 10 or 18 V applied on the feeder electrode.
Figure 1b displays the schematic diagram of the modified bipolar electrochemistry set-up. [26]To further increase the acting potential on the BPE, a copper wire was welded to the rear side of the sample and then connected to an auxiliary power supply via a Pt electrode with an area of 4 cm 2 .A potential of +2 V is applied to the BPE from the auxiliary Pt electrode, which was located at different distances away, including 15, 20, and 25 mm.The auxiliary circuit increases the overall potential on the BPE, which can provide more confidence in the corrosion response on a wider potential range. [26,27]The feeder electrodes and auxiliary Pt electrodes are switched on simultaneously during the bipolar electrochemistry tests.
Figure 1c gives the potential response on the BPE, which occurs only when switching on the auxiliary power supply.A Luggin probe is located at the centre line of the BPE, with a CHI600E potentiostat used to record the potential after switching on the auxiliary power supply.The average potential and standard deviation are calculated from a 300-s experimental period.The recorded potential is slightly reduced from 0.88 to 0.84 V SCE after increasing the distance between BPE and the auxiliary Pt electrode.Figure 1d gives the potential distribution of the standard and modified bipolar set-up.For measuring the potential along the BPE, a Luggin capillary was set centreline and connected to an SCE at a set distance of 1 mm above the BPE surface.The OCP was first stabilised and then the bipolar system power supply was switched on.The reported potential is the difference in measured potential to the OCP.The potentials were measured along the BPE surface in increments of 5 mm, with the average potential and its standard deviation at each point calculated from a 300-s exposure.For the modified BPE, the potential at each point is the sum of the potential from the feeder electrode and the auxiliary potential (25 mm).The maximum potential on the BPE (18 V) and modified BPE (10 V from the feeder electrode and +2 V from the auxiliary electrode) near the sample oxidation edge (<5 mm) is similar.However, the potential gradient on the modified BPE is smoother than that for the standard bipolar set-up.
After the bipolar electrochemistry experiment, the BPEs were removed from the electrolyte, washed and rinsed in soap water, followed by drying in hot air.A Keyence VK-200 K laser confocal scanning microscope was used to determine the corrosion morphology.A FEG-FEI Quanta 250 SEM was used to image the microstructure and the EDX analysis was carried out at 20 kV, to study the dissolution phase during pitting propagation.

| RESULTS AND DISCUSSIONS
Figure 2 displays the potentiodynamic polarisation curves of HDSS 2707 in 0.1 M HCl at room temperature.HDSS 2707 contains a high concentration of Cr, Ni and Mo, which makes this material very corrosion-resistant. [28,29] The OCP in 0.1 M HCl is around −0.1 V SCE , with the passive region between +0.1 and +0.9 V SCE .After +0.9 V SCE , the current density suddenly increases due to water decomposition and oxygen evolution. [30,31]The sample surface did not show pitting corrosion, even when the sample was polarised to +1.5 V SCE , indicating that no harmful precipitates were present to reduce the pitting corrosion performance.areas to the left on the HDSS 2707 samples were areas covered by PTFE tape during the bipolar experiment.The dark area next to the BPE oxidation edge is the site where crevice corrosion has occurred; next to it, pitting corrosion surrounded with general corrosion is observed.Figure 3a shows the BPE oxidation edge with a corroded crevice of serval hundred micrometres in length.However, no pits are detected on the BPE with the application of 10 V. Increasing the applied potential on the feeder electrode to 18 V results in an increase in the corrosion-covered length.In Figure 3b, the arrows are used to indicate the presence of corrosion pits, which are further analysed in Figure 3c, where an optical and 3D topography image of region 1 is shown.Pits are not so obvious in the optical image, whereas some holes with a maximum depth of about 15 μm are found in the topography image.Figure 3d displays the regions suffering from general corrosion adjacent to the crevice in region 2, while the 3D image confirms the presence of several pitting corrosion sites.Phases with different heights are seen on the 3D images, indicating that general corrosion is associated with selective phase dissolution.

| Bipolar electrochemistry
These observations show that traditional bipolar electrochemistry can be used to test the occurrence of pitting corrosion in HDSS 2707 at ambient temperature.However, pits cannot be generated if the applied potential is not large enough (10 V), with pits and a large crevice then nucleated by increasing the potential output to the feeder electrodes (18 V).However, the superimposed potential on the feeder electrode then dramatically increases the crevice corrosion length on the BPE, resulting in competition between pit nucleation and crevice corrosion.Only limited numbers of relatively small pits are measured, which makes it challenging to study pitting corrosion kinetics.

| Application of a modified bipolar electrochemistry set-up
In our previous work, the modified bipolar electrochemistry set-up was implemented by applying an auxiliary potential on the BPE.This is used to control the overall potential distribution on the BPE. [26]In traditional bipolar electrochemistry set-ups, pits with lacy covers are obtained in type 316 L stainless steels.However, the application of an auxiliary potential increases the overall potential on type 316L stainless steel, leading to a wider pit-covered length being observed compared to the unpolarised state.Additionally, open pits and transpassive corrosion on the 316L BPE are also observed and measured.The same methodology is now applied here to assess the pitting corrosion of HDSS 2707.A potential of 10 V was chosen for the feeder electrodes of the BPE system, with another +2 V applied to the sample via the secondary circuit of the modified bipolar set-up.The corrosion response is summarised in Figure 4a-c as a function distance between the auxiliary Pt feeder electrode distance and BPE.A pit-covered region in excess of 5 mm was found.When the auxiliary Pt electrode is located 15 mm from the BPE, the pitcovered regions are merged with the crevice, forming elongated pit lines and shapes, with pits at the lower applied potential region (close to the BPE centre) not connected.Increasing the distance to 20 mm, the length of the connected pitting corrosion response decreases, and more circular pits are observed.When the distance between the BPE and auxiliary Pt electrode increases to 25 mm, individual pits are outlined rather than these elongated shapes.Most pits are circular, and pits at the higher applied potential region (near the BPE oxidation edge) are larger than those in the lower applied potential area.The crevice width dramatically increases as the auxiliary Pt electrode is located 25 mm away in the modified BPE set-up.A possible explanation here is that these elongated and connected corrosion channels with pits indicate a low-resistance pathway for the growth of local dissolution along the BPE potential gradient.
The critical potential to induce pits with different distances between auxiliary potential and BPEs can also be calculated.The pit-covered length is measured from Figure 4a-c, with the relationship between the potential distribution on the BPE having been obtained in Figure 1d.The critical pitting potential for the modified BPE with a 25 mm distance to the auxiliary Pt electrode is highest (1.86 V OCP ) as the potential is lower from the auxiliary electrode.
Figure 5a summarises the critical pitting potential on the BPE with different distances between the sample and auxiliary electrode.The critical potential for inducing pits on the modified BPE (15 mm) is 1.61 V OCP , which is higher than the modified BPE potential of 1.55 V OCP with 20 mm distance.To compare the pit depth and pit volume in these three modified BPE arrangements, the 10 deepest pits were measured, shown in Figure 5b.The pit depth of all samples is around 40 μm, indicating that the pit depth is independent of the distance between BPE and auxiliary electrode, which offer slightly difference potentials.These pits are possibly growing under diffusion-controlled conditions for all three different distances. [22,29]Figure 5c gives the associated average pit volume of the 10 deepest pits with the standard deviation.A slightly larger volume was measured for the 20 mm distance sample, but with far larger error bars.Interestingly, the pit volume is not increasing with longer distance to the secondary Pt electrode, as the offered is slightly reduced.The presence of crevice corrosion seems to affect the expansion of the pit volume.Also, the largest pits on the BPE (15 mm distance to the Pt electrode) are connected to crevice corrosion, which was not included in pit volume observations.The modified bipolar electrochemistry set-up here is more suitable to carry out pitting corrosion tests of HDSS 2707 at room temperature, as the applied auxiliary potential enhances the overall potential acting on the BPE without raising the potential gradient.With an auxiliary potential of +2 V and a reasonable distance between the BPE and auxiliary Pt electrode (25 mm), the competition between pitting and crevice corrosion can be minimised, which is helpful for pitting corrosion research.Figure 6 shows the SEM image of the crevice corrosion sites in Figure 4c.Some small circular holes (marked by arrows) are present, these are most likely metastable pits, and the nucleation of the metastable pit inside the crevice is representative of one of the crevice growth mechanisms. [32]The three stages of pitting corrosion from nucleation, metastable growth, to stable growth are shown in Figure 6b-d  shows regions with different heights.Several random point EDX tests were carried out for both phases; the phase with less corrosion (higher height) contains 25.4 ± 0.2 wt% Cr, 7.3 ± 0.4 wt% Ni, and 3.91 ± 0.5 wt% Mo, indicating austenite.In contrast, the phase that showed corrosion has 27.5 ± 0.7 wt% Cr, 5.2 ± 0.7 wt% Ni, and 5.6 ± 0.3 wt% Mo.This indicates that the ferrite phase is preferentially corroded.The observed pits nucleated at the interface of the two phases or inside the ferrite, with the surface of the austenite appearing relatively clean.The two pits nucleating at the interface of the two phases are more significant than those inside the ferrite.This indicates that the pits that nucleate inside the ferrite are more accessible to undergo repassivation.Figure 6c displays a pit with phase remnants sticking out along the mouth.The retained phase is the austenite phase, which cannot be removed during pitting corrosion.The pit is under metastable growth kinetics, which might be repassivated or become stable at growth.Here, the size/shape of ferrite/austenite plays an important role for stable pit growth, determining the stability and porosity of the pit lacy cover.The pit lacy cover maintains the pit electrolyte inside of the pit. Figure 6d gives a large stable pit, and the retained austenite can still be measured at the pit mouth.Whereas the austenite phase is relatively small, limiting the size of the giant pit close to the circle.Only tiny pits are found inside the ferrite, meaning pits nucleated inside the ferrite have less stability to become stable pits.The tiny austenite acts as the diffusion barrier for the pit nucleated at the interface of austenite and ferrite at the early stage of metastable growing.The diameter of the pit is over 50 μm, which should be a stable pit, where the pit depth/volume acts as a diffusion barrier.Here, the pit propagation is strongly related to the geometry of ferrite and austenite, it was reported that suitable design of size/ ratio of ferrite and austenite increases the pitting propagation resistance, the pitting growth kinetics in LDSS 2101 can be even lower than DSS 2205. [33]n conventional corrosion testing methodologies, the detection of pitting corrosion on HDSS 2707 at room temperature is not possible.The use of bipolar electrochemistry induces the nucleation of pitting corrosion at room temperature, and pitting potential on the BPE is time dependent, same as traditional potentiodynamic polarisation test. [34]However, the pronounced competition between localised corrosion and pitting in a narrow region have made it difficult to examine pitting corrosion behaviour.By employing a modified bipolar electrochemistry approach, with an appropriate distance between the auxiliary electrode and BPE, effective evaluation of pitting corrosion on HDSS 2707 has been achieved.The significant reduction in experimental duration and the prevalence of numerous pits without serious competition from crevice corrosion are the primary factors contributing to the success of this methodology for pitting corrosion testing of high alloy DSS.

| CONCLUSIONS
A corrosion screening method to test the pitting corrosion resistance of HDSS 2707 is introduced.The standard bipolar electrochemistry set-up can reveal both pitting and crevice corrosion, but difficulties exist to separate both forms of corrosion.For the modified bipolar electrochemistry set-up, pitting and crevice containing regions can clearly be separated using an applied auxiliary potential of +2 V. Pits nucleated at the austenite/ferrite interface and inside ferrite phase, with pit growth then following consuming ferritic regions.

Figure
Figure 3a,b displays the regions where corrosion responses were observed on the BPE when a potential of 10 and 18 V was applied to the feeder electrode.The relatively bright

F I G U R E 2
Potentiodynamic polarisation curve of hyperduplex stainless steel (HDSS) 2707 in 0.1 M HCl at room temperature.SCE, saturated calomel reference electrode.F I G U R E 3 BPE oxidation edge with (a) 10 V and (b) 18 V on feeder electrode.Three-dimensional morphology images of (c) region next to crevice and (d) pitting corrosion region.(c) and (d) are highlighted region 1 and region 2 in (b).[Color figure can be viewed at wileyonlinelibrary.com]

F
I G U R E 4 Localised corrosion response on modified bipolar electrode (BPE) (10 V from feeder electrode and +2 V from auxiliary electrode) with different distances between BPE and auxiliary Pt electrode from (a) 15 mm, (b) 20 mm and (c) 25 mm.[Color figure can be viewed at wileyonlinelibrary.com] . Figure6b

F
I G U R E 5 (a) Critical pitting potential on modified bipolar electrode, (b) maximum pit depth and (c) corresponding average pit volume with different distance between BPE and auxiliary electrode.[Color figure can be viewed at wileyonlinelibrary.com]F I G U R E 6 (a) Crevice corrosion and (b-d) pitting corrosion on modified bipolar electrode with 25 mm to auxiliary Pt electrode.[Color figure can be viewed at wileyonlinelibrary.com] Chemical composition (wt%) of HDSS 2707 with the corresponding PREN.