Study of the electrodeposition of conductive polypyrrole doped saccharin coatings on 316L stainless steel plate for PEMFC application

Study of the electrodeposition of conductive polypyrrole doped saccharin coatings on 316L stainless steel plate for PEMFC application S. Ben Jadi a,b , K. Bahend a , M. El Fazdoune a , A. Iranzo c,d,* , F.J. García-García e , M. Bazzaoui a a Laboratory of Advanced Materials and Process Engineering, Faculty of Sciences, Ibn Tofail University, 14000, Kenitra, Morocco b LME, Faculty of Sciences, Ibn Zohr University, 80000, Agadir, Morocco c Thermal Engineering Group, School of Engineering, Universidad de Sevilla, Camino de los Descubrimientos, s/n, 41092, Sevilla, Spain d ENGREEN Laboratory of Engineering for Energy and Environmental Sustainability, University of Seville, Spain e Departamento de Ingeniería y Ciencia de los Materiales, Escuela Polit´ ecnica Superior, Universidad de Sevilla, Calle Virgen de ´ Africa, 7, 41011, Sevilla, Spain ARTICLE INFO Handling Editor: Dr Mehran Rezaei Keywords: Organic coating Bipolar plate Corrosion Sodium saccharin Polypyrrole AISI 316L PEMFC ABSTRACT The study investigates the electrosynthesis of pyrrole doped with sodium saccharin as a protective coating for AISI 316L bipolar plate. Electropolymerization durations are systematically varied to optimize corrosion inhibition under different concentrationa of the aggressive environment of typical of PEMFC operation. Weight loss measurements indicated that films synthetized during 30 min period exhibit superior corrosion resistance compared to those produced under shorter durations. Electrochemical assessments, including potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS), demonstrate that the sodium saccharin doped PPy coating significantly reduces the polarisation density of 316L. detailed structural and morphological analyses were conducted using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). XPS confirmed the successful incorporation of the saccharin dopant in the PPy matrix with dopant rates increasing under aggressive acidic condition. While SEM reveals that PPy coatings formed through extended electropolymerization enhance corrosion resistance on AISI 316L through their densely cauliflower structure. The enhanced long-term corrosion protection observed can be attributed to the robust physical barrier and source of anodic protection compared to uncoated 316L. 1. Introduction During the last decades, the development of proton exchange membrane fuel cells has become increasingly attractive due to their high energy density, low operating temperature and weight, and no emission of pollutants. Furthermore, PEMFC presents widespread field application to powering vehicles, trains, and ships, and distributed power generation. In addition to that, PEMFC is considered an environmentally friendly power source compared to conventional fossil fuels [1–3]. Bipolar plates (BPPs) are used in PEMFC to conduct electricity between the anode and the cathode, accelerate water management from the cell, ensure structural support between cell components and separate fuel and oxidant [4,5]. Practically, a BPP must meet a set of requirements such as high electrical conductivity and corrosion resistance, high mechanical strength, low-cost materials, and low gas permeability [6]. So far, several materials have been used as bipolar plates. Graphite BPP has shown good electrical conductivity and high corrosion resistivity. Although its low mechanical strength makes it necessary to use a heavier BPP and therefore resulting in higher costs and a more complicated fabrication procedure [7,8]. Metallic bipolar plates are supposed to be an alternative to graphite BPP as they meet the major requirements of a BPP [9]. Generally, various metals and alloys are used as BPPs, such as aluminum, titanium, stainless steel (SS), copper, and AISI 316L, among others. However, the major inconvenience of metallic BPP is their corrosion behaviour. In fact, long contact of the metal with the acidic and humid environment of PEMFC leads to the formation of a passivation oxide layer due to the release of metal ions; therefore, the electrolyte and membrane electrode are contaminated causing an increase in contact resistance, lowering the ionic conductivity, altering cell performance, and shortening cell life [9–12]. Therefore, several attempts have been made by researchers on the * Corresponding author. Thermal Engineering Group, School of Engineering, Universidad de Sevilla, Camino de los Descubrimientos, s/n, 41092, Sevilla, Spain. E-mail address: [email protected] (A. Iranzo). Contents lists available at ScienceDirect International Journal of Hydrogen Energy journal homepage: www.elsevier.com/locate/he https://doi.org/10.1016/j.ijhydene.2025.05.122 Received 18 June 2024; Received in revised form 6 May 2025; Accepted 8 May 2025 International Journal of Hydrogen Energy 138 (2025) 1066–1076 Available online 22 May 2025 0360-3199/© 2025 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY-NC license ( http://creativecommons.org/licenses/by-nc/4.0/ ). basis of surface modification technologies using affordable and inexpensive materials capable of enhancing metallic BBP performance in fuel cell environments. Metallic coatings have been studied as Titanium Nitride Oxide [13], Silver [14], Chromate [15], Chromate nitride [16], etc. However, the major concern for metallic coating is the cost of deposition process as they require sophisticated equipment such as chemical/physical vapour deposition, electrophoretic deposition, pack cementation, etc., which are less suitable for mass production. In recent years, most BPs are made up of metals. Specifically, Austenite stainless steel 316L is widely recongnised as a potential material for BPs. This is attributed to its high thermal and electrical conductivity, high mechanical strength and low manufacturing cost [10]. However, the superior corrosion resistance of AISI 316L is due to the chromium content, which forms a thin film of Cr 2 O 3 . Studies have shown that higher chromium and nickel content ns the steel result in thinner passive film, which contributes in decreasing interfacial contact resistance. However, a major concern when using AISI 31L as bipolar plate is the leaching of metal ions caused by corrosion [11]. While AISI 316L is more expensive than other steels. However, it is still more cost-effective compared to precious metals such as platinum and gold [12]. In particular, the limited corrosion resistance of AISI 316L in highly acidic environments is attributed to the non-uniformity of its passive oxide film, characterised by chromium-depleted areas. This factor contributes to its poor performance under such conditions, restricting its applications in PEMFCs [13]. In recent years, conductive organic coatings have received much interest as a protection film for BPP corrosion. Typically, conductive polymers are easy to synthesise, provide good electrical conductivity, good stability, and good anti-corrosion performance [14–17]. In particular, polyaniline has been electrochemically deposited on 316L SS by Le et al. [18]. The authors reported an improvement in corrosion resistance. The polyaniline passivation film persists after seven days of exposure to the simulated environment of PEMFC (sulfuric acid) yet 316L SS was broken after three days. Polypyrrole is widely studied as a conducting polymer due to its excellent electrical conductivity, ease of synthesis, and environmental stability. PPy coating was also studied by several researchers using different electrodeposition methods (galvanostatic, potentiostatic, cyclic voltammetry), and different bipolar plate metals such as 304SS [19–21], aluminium [22], carbon steel, copper [26], and others. The properties of PPy coatings are significantly influenced by the choice of dopants which are primarily integrated in the polymer chain during pyrrole electrodeposition to improve the electrical, chemical and mechanical characteristics of PPy coating. Sodium saccharin, a well-known artificial sweetening agent, is commonly used in electrodeposition due to its excellent adsorptive properties [20]. Acting as an inhibitor of the hydrogen evolution reaction and water reduction at high potentials, thus improving the polymer’s structure. When added in small amounts to the pyrrole solution, saccharin interacts with pyrrole, leading to enhanced mechanical properties of the coating in terms of resistance to physical damage, durability, end strength. Consequently, the path of corrosive substances becomes slower, more complex, and longer. Furthermore, the addition of sodium saccharin during pyrrole electrodeposition enhances corrosion resistance by forming a protective layer that effectively inhibits the corrosion of the underlying metal, following this initial adsorption, the electrodeposition reaction proceeds, leading to the formation of PPy coating [21,22]. Sodium saccharin and PPy exhibit enhanced stability under various conditions. This stability is crucial for bipolar plates, which are subject to fluctuating temperatures, humidity levels, and chemical exposures in fuel cell operations [23,24]. The enhanced stability ensures that the protective and conductive properties of coating are maintained over prolonged periods [25]. The corrosion behaviour of polypyrrole is influenced by various factors, such as its structure and electronic properties. These factors are closely related to the process and techniques for making the film, the type of material to which they are applied, the chemicals involved and the temperature. One of the significant advantages of using sodium saccharin is its ability to prevent metal corrosion [24]. During the electrodeposition of PPy, sodium saccharin adsorbs onto the electrode substrate forming a protective layer which effectively inhibits the corrosion of the underlying metal, following this initial adsorption, the electrodeposition reaction proceeds, leading to the formation of the desired conducting polymer [25]. To the best of our knowledge, polypyrrole coatings have never been studied in stimulated PEMFC environment on an AISI 316L bipolar plate. In this work, polypyrrole films have been electrodeposited on AISI 316L bipolar plate by cyclic voltammetry and galvanostatic modes. The corrosion study was performed using open circuit (OCP), Tafel method, and electrochemical impedance spectroscopy. In our study, we opt for the electrosynthesis of polypyrrole with sodium saccharin, which is a relatively large and immobile molecule that has a special ability to attract positive ions while repelling negative ones. This research focusses primarily on conducting a comparative analysis of the corrosion resistance exhibited by the PPy coating under various electrodeposition durations. The study aims at studying the impact of the morphological structure of the film on its corrosion behaviour. This was especially useful when we applied PPy doped with saccharin as a protective coating on austenitic stainless steel 316l to protect against corrosion, especially in the presence of sulfuric acid and hydrofluoric acid. 2. Experimental details 2.1. Chemicals Pyrrole (>98 %) was purchased from Sigma-Aldrich and distilled prior to use. Sodium saccharin salt (C 7 H 4 NNaO 3 S.2H 2 O) was purchased from Tokyo chemical industry. Sulfuric acid H 2 SO 4 (>99 %) and Hydrofluoric acid (40 %) were purchased from PanReac AppliChem. 2.2. Electrochemical apparatus In this study, the 316L austenitic stainless steel sheet used as a bipolar plate was selected as the substrate and its chemical composition is given in Table 1. The steel sheet with an exposed surface area of 1 ×1 cm 2 is degreased by ultrasonic vibration with acetone and cleaned with distilled water for 15 min, and then finally dried. De-ionized water was used for the solution preparation. Polypyrrole electrochemical polymerization was carried out in a single compartment with AISI 316L as the working electrode (WE), graphite as the counter electrode (CE), and Ag/AgCl as the reference electrode. The electrodes were connected to a Gamry 3000 potentiostat/galvanostat monitored by the Gamry software framework. PPy electropolymerization was performed on AISI 316L at room temperature in an aqueous electrolyte medium containing 0.5 M pyrrole and 0.1 M sodium saccharin in galvanostatic mode by applying 2 mA/cm 2 during 30 min. The prepared sample was labelled PPy/316L. 2.3. Corrosion test Corrosion tests were performed on uncoated AISI 316L and PPy/ 316L electrodes in a simulated corrosive medium of PEMFC. According to the literature, different acid concentrations could be used as a simulated corrosion medium such as 0.1 M H 2 SO 4 [26], 0.5 M H 2 SO 4 [27], 0.1 M H 2 SO 4 +2 ppm [30], and 0.5 M H 2 SO 4 +2 ppm [28]. Table 1 Chemical composition of AISI 316L used in this investigation (weight %). Element C Si Mn Cr Ni Mo %<0.03 0.50 1.50 17.80 12.70 2.60 S. Ben Jadi et al. International Journal of Hydrogen Energy 138 (2025) 1066–1076 1067 The corrosion behaviours of the uncoated AISI 316L and film coated electrodes were studied by potentiodynamic polarisation and electrochemical impedance spectroscopy. Electrochemical measurements were performed with the Gamry 3000 Potentiostat/Galvanostat instrument. Potentiodynamic curves were recorded starting from ±150 mV relative to the equilibrium potential of the electrodes with a scan rate of 1 mV/s after 2 h of exposure to corrosive media. AC impedance measurements were performed at an open circuit potential measured between 100 kHz and 0.01 Hz and at a constant AC amplitude of 5 mV for different immersion times (2, 96, and 192 h). 2.4. SEM, XPS, and confocal analysis The structure of PPy coating was investigated by scanning electron microscopy (SEM) using a FEO TENEO instrument, with the distance between the sample and the objective lenses was approximately 15 mm. X-Ray photoelectron spectroscopy (XPS) was performed using a Shimadzu Co: AXIS ULTRA equipped with an Mg K α x-ray source operated at 15 kV. The operating pressure in the analysis chamber is less than 10 −7 Pa, and the analysis area was 2 mm 2 . All spectra were corrected according to the binding energy of carbon C1s at 285 eV. Spectra analyses were performed using CasaXPS software. Thickness measurements were taken using confocal microscopy with Sensofar S-Neox. 3. Results and discussions 3.1. Polypyrrole electrodeposition PPy electrodeposition was performed directly on an AISI 316L electrode in an aqueous solution using sodium saccharin as the dopant. Fig. 1(a) depicts the cyclic voltammetry of PPy electropolymerization during 10 cycles in the potential window of −1 to 1.5 V (vs Ag/AgCl). Based on, the voltage scanning in the first cycle, an increase in the current density is observed at 0.75 V (vs Ag/AgCl) corresponding to the pyrrole oxidation potential and the formation of PPy. During successive scans, an increase in the current density is observed from the first cycle to the seventh, indicating the formation of thicker PPy. The voltametric profile is similar to the voltammograms obtained for PPy in aqueous media [29,30]. Chronopotentiometric mode was investigated to electrodeposit the PPy film on the AISI 316L. Fig. 1(b) presents the potential versus time curve for PPy electrodeposition at a current density of 2 mA/cm 2 . The electropolymerization of pyrrole in the presence of sodium saccharin is a complex process involving several steps, as described in the following detailed mechanisms. In the initial stage of electropolymerization, it is observed in the initial stage of electropolymerization that the potential increases with oxidation of pyrrole monomers to oligomers. In the second stage, an instantaneous decrease in potential is observed due to the electropolymerization of pyrrole on AISI 316L. In fact, the significant reactivity of generated pyrrole radical cations leads to the formation of dimer and oligomers species by interacting with one another or with neutral pyrrole monomers, as follow. Afterwards, the PPy coating grows until the process is completed, resulting in a slow potential decrease. During electropolymerization, sodium saccharin acts as a dopant by integrating into the polypyrrole matric to maintain charge neutrality, according to the reaction bellow [31,32]. In the electropolymerization of pyrrole, sodium saccharin can strongly adsorb onto the metal electrode surface via its sulfur atom, which enhances the interaction between pyrrole monomers and the Fig. 1. (a) Voltammetric profile for the potentiodynamic synthesis of PPy; scan rate: 100 mV/s. (b) Chronoamperograms recorded during PPy electrosynthesis at 2 mA/cm 2 in aqueous medium (0.5 M Pyrrole +0.1 M C 7 H 4 NNaO 3 S) aqueous medium. S. Ben Jadi et al. International Journal of Hydrogen Energy 138 (2025) 1066–1076 1068 electrode. This adsorption facilitates electron transfer, promoting the polymerization process. The inductive effect of saccharin further stabilizes the intermediate species, leading to enhanced electrical and mechanical properties of polypyrrole film [33]. Based on the results of the voltametric profile, we can conclude that the optimal potential for pyrrole electropolymerization is 0.7, vs Ag/ AgCl and the applied current density of 2 mA/cm 2 leads to the formation of an adherent PPy coating as the constant potential reached is high enough for complete polymerization [34]. It is usually assumed that the initial high potential in constant current polarisation is due to the polarisation of constant currents. To calculate the faradaic efficiency, we synthesised PPy electrochemically using the galvanostatic method. We applied a current density j =2mA/cm 2 during different times t 1 =5min, t 2 =10 min and t 3 =30 min to the AISI 316L working electrode. For each trial, we weighed the electrode before and after the deposition. Before measurement, the samples were dried for 1 h at 100 ◦C and then cooled in a desiccator. We determined the electropolymerization efficiency (γ) using the following equation: γ=(2+y)(ΔmF + τ jMmet/Z) Q1(MM+yMA)(4) Where Δm is the mass variation, j is the current density, where j =Q/ τ , M met is the atomic mass of the substrate, M M is the molecular mass of the monomer, M A is the molar mass of the doping anion, τ is the polarisation time, and Z is the cation metal charge. The expression for the thickness of the PPy coating is defined by the following equation: θ=mth γ A ρ (5) And the m th is defined by the following equation: mth =M1(MM+yMA) × 1 (2+y)F(6) Where A is the surface of the working electrode and ρ is the polymer density of PPy. The combination of Eq. (1) and Eq. (2) yields the following equation: θ=Q1(MM+yMA)γ (2+y)F A ρ (7) Table 2 presents the results of the theoretical thickness of PPy calculated at different electropolymerization times. 3.2. SEM analysis The coverage of the substrate surface is one of the most important factors in preventing corrosion. In this regard, the coverage of PPy on stainless steel was examined by SEM during different electroception periods. Fig. 2(a–c) corresponds to saccharin doped samples during 5, 10, and 30 min (the inserts are low-magnification micrographs of the corresponding samples). The typical globular morphology is shown for all samples with a velvety surface similar to that of cauliflower. Several studies have reported the same morphology of polypyrrole synthesised in various electrolytic media [29,35,36]. It has been reported that cauliflower-like morphology is related to the difficulty of dopant intercalation in the disordered polymeric matrix [37]. Micro-spherical grains with diameters ranging from 0.5 to 15 μ m were observed. These spheres are interconnected. The size of the formed spherical structures is noticeably affected by the increase in the electrodeposition time. Thus, the increase in the electropolymerization time resulted in thicker surface and greater compaction. Therefore, the 30 min coating can provide long-term corrosion prevention due to the stronger physical barrier effect. Fig. 3 shows the surface morphologies of bare steel and of PPy coated steel after potentiodynamic polarisation in different media. After polarisation, numerous pits could be seen in SEM images of the uncoated surface (Fig. 3(a)), while coated steel does not undergo pitting corrosion (Fig. 3(b-e)). The morphology of PPy in 0.1 M H 2 SO 4 and 0.5 M H 2 SO 4 media, characterised by the formation of spherical aggregates, remains almost intact. A close view reveals that the diameter decreases slightly with increasing concentration of sulfuric acid [38,39]. This result shows the effectiveness of the PPy coating in inhibiting the diffusion of corrosive SO 4 2− anions to the substrate, preventing anodic dissolution and cathodic reaction during corrosion. The corrosion rate accelerates after adding hydrofluoric acid, and only a few small particles appear to have remained on the surface (Fig. 3 (d)). The disappearance of large polypyrrole particles, such as cauliflower-like structures, after immersion in a high-concentration acid medium is primarily due to the breakdown and dissolution of the larger PPy particles, resulting in their disappearance from the solution. Less damage is observed on the surface of the AISI 316L with 0.1 M H 2 SO 4 than with 0.5 M H 2 SO 4 . Sulfuric acid, a strong acid and oxidizing agent creates a highly acidic environment by donating protons (H + ), which attacks the metal and accelerate metal ion dissolution [40,41]. On the other hand, HF enhances metal dissolution by forming soluble complexes with metal ions via fluoride ions (F ⎯ ) [42,43]. When both H 2 SO 4 and HF are present, their synergetic effect accelerates the corrosion process, leading to a continuous and aggressive attack on the metal surface and resulting in a higher corrosion rate [44]. 3.3. Corrosion protection 3.3.1. Gravimetric study The study focused on investigating how the corrosion of PPy electrosynthetised on AISI 316L at different times (5, 10, and 30 min) in 0.1 M H 2 SO 4 solution was affected. Thus, the weight loss method was used after immersing the samples for 4 days at room temperature. The resulting corrosion rate (CR) and inhibition efficiency η W(%)were calculated using the following equations [45,46]: CR=wb−wa A t (8) η W(%) = (1−wi w0)×100 (9) Where wband wa represent the weight of the specimen before and after immersion in the corrosive solution, and w0and wi present the values of corrosion weight losses in coated and uncoated specimen. A refers to the total area of the specimen (cm 2 ), while t represents the exposure time in hours (h). The values obtained are summarized in Table 3. It is clearly observed that electrosynthetized coatings at 30 min demonstrate superior corrosion resistance, as evidenced by weight loss measurements and inhibition efficiency. Conversely, corrosion prevention relies on impeding diffusion, and a reduction in coating thickness, as well as the appearance of pores, flaws, and cracks, results in an increased corrosion rate [47]. Thus, the following study will focus on PPy/316L (t 3 ). The results of our study indicate that there was no noticeable improvement in corrosion resistance with the coating times beyond 30 min of electropolymerization. Table 2 The theoretical thickness of PPy calculated at different electropolymerization times t 1 =5 min, t 2 =10 min, and t 3 =30 min. Label Theoretical thickness ( μ m) Experimental thickness ( μ m) PPy/316L (t 1 )5.5 5 PPy/316L (t 2 )8 7.5 PPy/316L (t 3 )19.6 20 S. Ben Jadi et al. International Journal of Hydrogen Energy 138 (2025) 1066–1076 1069 3.4. XPS analysis XPS was utilised to characterise the surface chemistry of the PPy, to verify the formation of the coating, examining the level of oxidation, and identify the chemical bonding of the polymer. The high-resolution C1s spectrum of the PPy film after electropolymerization for 30 min in sodium saccharin electrolyte could be divided into four components (Fig. 4). One of the components with a binding energy value of 284 eV corresponds to the β - carbons in the pyrrole ring. The other components are assigned to peaks at 248.92 eV (C α ), 285.9 eV (C–N), 287.02 eV (C – – N, C–O), 288.23 eV (C–N + , C – – N + ), and 289.4 ( π - π interaction) [48–50], which are the result of the groupings that arise during the polymer film formation process. It is possible that during electropolymerization of the PPy, the carbon undergoes oxidation, leading to Fig. 2. Top view SEM micrographs of the PPy film on AISI 316L obtained at a current density of 2 mA/cm −2 vs. Ag/AgCl in a 0.5 M pyrrole and 0.1 sodium saccharin aqueous solution for (a) 5min, (b) 10min and (c) 30 min. Fig. 3. SEM images of bare AISI 316L after polarisation in (a) 0.5 M H 2 SO 4 and PPy/316L coating after polarisation in different media: (b) 0.1 M H 2 SO 4 , (c) 0.5 M H 2 SO 4 , (d) 0.1 M H 2 SO 4 +2 ppm HF, (e) 0.5 M H 2 SO 4 +2 ppm HF solutions at room temperature. S. Ben Jadi et al. International Journal of Hydrogen Energy 138 (2025) 1066–1076 1070 the formation of C–OH groups in the first step, followed by the formation of C – – O in the subsequent step. The N1s peak observed in the PPy film is a multicomponent peak, corresponding to three different nitrogen species (Fig. 4) [51]. Specifically, the peak can be deconvoluted into three components at 398.1, 399.4, 400.9, and 401.9 eV, which correspond to C – – N, –NH, C–N + -, and C – – N + -, respectively [52]. The high binding energy component at, 400.9 and 401.9 which is assigned to positively charged nitrogen in the polymer matrix, requires a negatively charged counter ion, in our case (C 7 H 4 NSO 3 ) - , to maintain charge neutrality in the polymer. Hence, the peaks are associated with the doping level of the PPy coating and are directly related to their electrical conductivity. A higher doping level typically leads to a higher electrical conductivity due to the increased number of charge carriers in the polymer matrix, and it is expressed as the ratio of the positively charged nitrogen area (N + ) to the total area of the N1s peak (N + /N ratio). Typical doping values for PPy range from 0.1 to 0.33. In our study, we found a doping level of 0.2 which means that 20 % of PPy repeat units were doped with sodium saccharin. To allow comparison, XPS spectra were collected from the PPy film synthetized in saccharin sodium, and the surface of the PPy film immersed in a solution containing 0.1 M H 2 SO 4 , 0.5 M H 2 SO 4 , 0.1 M H 2 SO 4 +2 ppm HF, and 0.5 M H 2 SO 4 +2 ppm HF respectively, as shown in.Fig. 5. As expected, the absence of any additional peaks corresponding to the AISI 316L or traces from other impurities demonstrates the high corrosion resistance of the PPy coatings to the aggressive medium of the fuel cell. XPS was employed in this study to evaluate the doping rate of PPy before and after exposure to corrosive medium. In fact, the doping rate of PPy is critical as it is significantly influencing the chemical and electrical properties of the coating. Specifically, PPy doping transform the coating from a neutral state to a conductive film by introducing charge carriers, thereby enhancing its conductivity. The N + /N ration is a key indicator for assessing the doping stability of the film. Particularly, PPy protects AISI 316L through anodic passivation. Where doped PPy maintain a passive oxide layer. A decrease in the doping rate would indicate coating failure [53–55] (see Fig. 6). Table 4 presents the XPS deconvolution of nitrogen peaks in PPy after immersion in various acidic media. The study demonstrates how different concentrations of sulfuric acid and the addition of HF impact the binding energies of nitrogen species and the doping rate of PPy. For each acid treatment, slight shifts to higher binding energies are observed, indicating modifications in the nitrogen environment attributed to the oxidizing effects of H 2 SO 4 and HF. The results show that increasing the concentrations of H 2 SO 4 from 0.1 M to 0.5 M leads to a slight increase in doping rate from 29.88 % to 32.52 %. However, the addition of HF at 0.1 M H 2 SO 4 results in a substantial increase in doping 47.30 %, and the combination of 0.5 M H 2 SO 4 with HF yields the highest doping rate (69,5 %). This enhanced doping and binding energy shift are attributed to the intensified oxidizing environment provided by both acids, which extensively modifies the nitrogen environment of the PPy matrix. Particularly, when sodium saccharin doped PPy is exposed to H 2 SO 4 environment, The coating incorporates the anions from the corrosive medium, thus higher doping level is observed. This process facilitates the stabilization of the oxidized PPy + by providing supplementary counterions. These observations correlate with SEM images, which revealed structural change in PPy. Particularly, When PPy is exposed to H 2 SO 4 medium, PPy incorporate anions from the corrosive medium, thus higher doping level is observed, supplying additional counterions to stabilize PPy + . These findings correlate with SEM images where PPy exhibits swelling due to Table 3 The corrosion parameters of polypyrrole-coated and uncoated AISI 316L in 0.5 M H 2 SO 4 acquired by weight loss measurements. CR (mg cm −2 h −1 ) η W(%) Blank 3.4 - PPy/316L (t 1 ) 1.7 49.9 PPy/316L (t 2 ) 1.35 60.41 PPy/316L (t 3 ) 0.26 92.30 Fig. 4. C1s and N1s deconvolution of PPy film synthesised using the galvanostatic method (2 mA/cm 2 for 30 min) on the AISI 316L electrode in 0.5 M pyrrole and 0.1 sodium saccharin. Fig. 5. XPS survey of polypyrrole coated on AISI 31L after 4 days of immersion in a)0.1 M H 2 SO 4 , b) 0.5 M H 2 SO 4 , c) 0.1 M H 2 SO 4 +2 ppm HF and d) H 2 SO 4 + 2 ppm HF solution. S. Ben Jadi et al. International Journal of Hydrogen Energy 138 (2025) 1066–1076 1071 excessive anions uptake. XPS analysis was also used to evaluate the effect of adding HF to H 2 SO 4 on the elemental composition of the AISI 316L surface after immersion in highly corrosive medium. Fig. 5 displays the corresponding XPS spectra of the AISI 316L after immersion in 0.1 M H 2 SO 4 and 0.1 M H 2 SO 4 +2 ppm HF respectively, and Table 5 provides the atomic concentrations of the elements at the respective surfaces. As presented in the table, oxygen is the dominant element on all the AISI 316L immersed in corrosive medium due to the formation of a passive oxide film. It is also observed that there is an increase in the atomic percentage of chromium with the addition of HF to the corrosive medium. Fig. 7 Also indicates the presence of nickel with the addition of HF, which may be related to possible oxidation of nickel. This observation suggests that the nickel, which is a component of the AISI 316L, may have undergone oxidation due to its exposure to the aggressive environment created by HF. The oxidation process could cause nickel atoms within the alloy to migrate to the surface and form nickel oxides or other nickel compounds, which not only alters the surface composition but also influence the overall corrosion resistance of the material, potentially impacting its performance in highly acidic and fluoride containing Fig. 6. N1s deconvolution of PPy electrosynthesised on the AISI 316L electrode in 0.1 M H 2 SO 4 , 0.5 M H 2 SO 4 , 0.1 M H 2 SO 4 +2 ppm HF and H 2 SO 4 +2 ppm HF solution. Table 4 XPS deconvolution of nitrogen peaks in PPy after immersion in various acidic media. Acid medium Binding energy (eV) Assignment Doping rate (%) 0.1 M H 2 SO 4 399.14 C – – N 29.88 399.85 C–N 400.84 C–N + 401.96 C – – N + 0.5 M H 2 SO 4 399.00 C – – N 32.52 399.69 C–N 400.51 C–N + 401.64 C – – N + 0.1 M H 2 SO 4 +HF 399.18 C – – N 47.30 400.08 C–N 401.18 C–N + 402.05 C – – N + 0.5 M H 2 SO 4 +HF 398.92 C – – N 69.5 400.31 C–N 401.19 C–N + 402.28 C – – N + Table 5 Atomic percentage of uncoated AISI 316L in sulfuric acid. 316L 0.1 M H 2 SO 4 316L 0.1 M H 2 SO 4 +2 ppm HF C 1s 38.36 9.65 O 1s 28.26 48.00 Cl 2p –6.86 S 2p –1.63 Mo 3d 3.40 0.14 Cr 2p 9.95 28.06 Ni 2p –5.67 Fig. 7. AISI 316L XPS survey after 4 days of immersion in a)0.1 M H 2 SO 4 and b) 0.1 M H 2 SO 4 +2 ppm HF. S. Ben Jadi et al. International Journal of Hydrogen Energy 138 (2025) 1066–1076 1072 environments [56]. 3.4.1. Tafel Polarisation Tafel Polarisation measurements are used to test the corrosion performance of coated and uncoated AISI 316L in 0.1 M and 0.5 M H 2 SO 4 . The corrosion potential (E corr ) and corrosion current (J corr ) were evaluated by the intersection of the corresponding anodic and cathodic polarisation (see Fig. 8). Electrochemical parameters calculated by fitting the potentiodynamic polarisation curves are summarized in Table 6. Lower J corr , and a more positive E corr indicate that the coating is more difficult to corrode by acidic media [14]. The corrosion of AISI 316L in sulfuric acid involves the destruction of the passive film of chromium oxide by the reaction: Cr2O3+3H2SO4→2Cr3++3SO42−(10) The cathodic reaction involves the reduction of hydrogen ions from the sulfuric acid with electrons from the metal as follow: 2H++2e−→H2(11) It is observed that the E corr of uncoated AISI 316L in 0.1 M H 2 SO 4 exhibits s slightly positive corrosion potential indicating that the uncoated substrate exhibits moderate resistance to corrosion in a less concentrated environment. However, when immersed in s highly acidic environment (0.5 M H 2 SO 4 ) a highly negative value was observed indicating a high susceptibility to corrosion in a more concentrated acidic environment thus decreasing the corrosion resistance. Otherwise, the coated metal has a high positive corrosion potential in 0.1 M H 2 SO 4. This significantly positive value suggests excellent corrosion resistance in a less concentrated acidic medium. In regards to a more highly acidic environment, the corrosion potential is negative but it is less negative compared to the uncoated AISI 316L in the same environment (0.5 M H 2 SO 4 ). I corr is another critical indicator in Tafel analysis, representing the rate of corrosion. Lower I corr values generally indicate better corrosion resistance. In summary, we find that the I corr evolves in a noble direction, which means that the polypyrrole coating presents excellent corrosion resistance in a less concentrated acidic medium and retains a considerable level of protection in a more concentrated acid, though the effectiveness diminishes as the acid concentration increases. Wang et al. and Gharbi et al. also obtained similar results for 316 stainless steel in 0.1 M sulfuric acid solution [57] and 0.5H 2 SO 4 [44]. The PPy coating in the article shifts Ecorr positively by about 251 mV compared to bare copper, which is consistent with improved corrosion resistance. Our study demonstrates that the PPy coating significantly enhance the corrosion resistance of AISI 316L in acidic environments, achieving an impressive reduction in Icorr to 4.59 ×10 −4 μ A. cm/cm 2 in 0.1 M H 2 SO 4 . This performance is superior to some PPy based coatings on different substrates, such as the PPy dopped with chromium nitride [58] PPy doped with oxalic acid [59], TnNB and TiNBN coatings on 316L stainless steel [60]. 3.4.2. Electrochemical impedance spectroscopy (EIS) Electrochemical impedance spectroscopy measurements allow the characterisation of the interface resistance behaviour of the electrolyte and the metal exposed to a corrosive medium with oxides and adsorbed species. Fig. 9 shows the typical Nyquist plot for the AISI 316L electrode over time in the 0.1 M H 2 SO 4 and 0.5 M solutions at room temperature. The depressed semicircle shown in the Nyquist plot is generally attributed to the high roughness, porosity, or inhomogeneity of the electrode surface [61]. At the beginning of immersion, it is observed that the high-frequency semicircle expanded as the immersion time increased from 2 h of immersion to 96 h which can be attributed to the spontaneous growth of the passive film of the AISI 316L electrode. After 192 h of immersion in sulfuric acid, the low frequency semicircle becomes very flattened for both concentrations, which is related to the weakness of the passive film and the appearance of diffusion process through a porous layer [62]. When comparing the EIS plots obtained for both concentrations, it is seen that in the 0.1 M H 2 SO 4 solution there is a higher impedance, indicating that the AISI 316L is less corroded than the one immersed in 0.5 M H 2 SO 4 . These results are consistent with those found by Li et al. [63]. The AISI 316L electrode exposed to sulfuric acid at different concentrations can be simulated by the equivalent circuit presented in Fig. 10. The simulated theoretical impedance parameters are summarized in Table 7. R s represents the electrolyte resistance; R f and CPE f represent the resistance and capacitance of the porous corrosion product layer; R ct represents the transfer resistance; CPE dl presents double-layer capacitance. As a result of a nonideal capacitive behaviour, the CPE interface was used instead of pure capacitance. It can be seen that the R f values increased with time before 96 h of immersion, which is due to the passivation of AISI 316L in an acidic environment. However, this passive film gradually deteriorates due to the penetration of corrosive products, leading to a decreasing trend of R f values after 96 h of immersion. Furthermore, the decrease in R ct after 96 h of immersion indicates that AISI 316L is attacked by the corrosive species. The increase in R ct values during this period is attributed to the accumulated corrosion products on the surface of the AISI 316L. The fitted results for PPy/316L correspond to the equivalent circuit presented in Fig. 10(b). These circuits are consistent with those known in large part in the literature for this type of coating. In this system, Rc is the resistance of the polypyrrole coating pores, CPE C is the constant phase element of the polymeric coating, and Zd is the Warburg impedance. It should be noted that these models take into account the existence of pores in polymeric coatings. Fig. 9(c and d) show for the immersion time of 2 h that the Nyquist plot presents two different behaviors: a single capacitive loop at high frequencies and an inclined line attributed to semi-infinite diffusion process associated in the low frequencies. For the other immersion times (96 and 192 h) the impedance Fig. 8. Potentiodynamic polarisation curves of bare AISI 316L and PPy/AISI in 0.1 M H 2 SO 4 and 0.5 M H 2 SO 4 solutions at room temperature with a scan rate of 1 mV/s. Table 6 Polarisation parameters of the AISI 316L and PPy/316L coating in 0.1 M H 2 SO 4 and 0.5 M H 2 SO 4 solutions at room temperature. E corr (mV) I corr ( μ A) AISI 316L In 0.1 M H 2 SO 4 37 713 4.273 In 0.5 M H 2 SO 4 −304,284 76.133 PPy/316L In 0.1 M H 2 SO 4 187 335 4.59 ×10 −4 In 0.5 M H 2 SO 4 −192 967 8.894 S. Ben Jadi et al. International Journal of Hydrogen Energy 138 (2025) 1066–1076 1073 behaviour is similar to that observed in the case of bare AISI 316L, with capacitive behaviour at high frequency. The fitted R c values presented in Table 8 indicate that the coating exhibits a low initial R c value compared to the uncoated AISI 316L. The low R c value polymeric film is mainly due to its conductive properties. The increase in R c indicates that the PPy coating is reduced and the decreased coating conductivity decreases the conductivity during immersion, taking the scattering behaviour at low frequencies to indicate that the movement of counterions through the polypyrrole backbone. During dedoping (the reduction process), saccharin ions remain trapped in the polymer layer due to their low Fig. 9. Nyquist plot of the bare AISI 316L in (a) 0.1 M H 2 SO 4 , (b) 0.5 M H 2 SO 4 and PPy/316L in (c) 0.1 M H 2 SO 4 , (d) 0.5 M H 2 SO 4 . Fig. 10. Equivalent circuit for the EIS plots of uncoated AISI 316L/sulfuric acid (a) and PPy/AISI 316L (b), Rs, electrolyte resistance; CPE f and R f are attributed to the capacitance and resistance of the oxide film, respectively; CPEdl and Rct are attributed to the capacitance of the double layer and the charge transfer resistance, respectively, and Z d , diffusion impedance. Table 7 Electrochemical parameters values obtained by EIS simulation of uncoated AISI316L after various exposure times in solution with 0.1 M H 2 SO 4 and 0.5 M H 2 SO 4 . 0.1 M H 2 SO 4 0.5 M H 2 SO 4 2 h 96 h 192 h 2 h 96 h 192 h Rs (Ω) 5906 5825 4883 1358 1389 1281 CPE dl (F. s n−1 ) 54,72e6 41,15e6 0,178 7e-3 78,69e-6 96,44e-6 0,145 3e-3 n dl 0,910 5 1 0,823 3 0,886 4 1 0,856 7 Rct (Ω) 806 278 1,52e6 42 798 449 834 176 403 70 675 CPE f (F. s n−1 ) 0,613 9e-3 0,174 1e-3 0,202 9e-3 7,621e-6 98,7e-6 0,245 8e-3 n f 0,701 7 0,826 0,894 6 0,8874e3 0,869 0,843 8 Rf (Ω) 1027 12 585 126 0,085 84 2,905E19 7370 S. Ben Jadi et al. International Journal of Hydrogen Energy 138 (2025) 1066–1076 1074