Yosef Thio Widyawan, Dirk L Engelberg
Metallurgy and Corrosion, Department of Materials, School of Natural Sciences,
The University of Manchester, United Kingdom

Yosef Thio Widyawan, BEng, ST, MSc, is a recently graduated engineering professional with academic and practical expertise in naval architecture and corrosion control engineering.He holds a Master’s degree with Distinction in Corrosion Control Engineering from the University of Manchester and a joint Bachelor’s degree in Naval Architecture from Institut Teknologi Sepuluh Nopember, Indonesia and Mokpo National University, Korea. His experience spans roles in ship structural design, welding engineering, and protective organic zinc coating/painting site coordination at Hyundai Samho Heavy Industry, PaxOcean Shipyard, and PT. NOV Profab. Certified as an AMPP Coating Inspector and holding a Welding Engineering Diploma, Yosef demonstrates a strong commitment to integrity, sustainability, and professional development in the marine and offshore industries.

Dirk Engelberg is a professor in materials performance and corrosion at the University of Manchester. He obtained a Dipl.-Ing. (FH) in Surface Engineering and Materials Science from Aalen University (Germany) before moving to Manchester in 2000 for an M.Sc. in Corrosion Science & Engineering and a PhD in Metallurgy & Materials Science. He joined the Corrosion & Protection Centre as an academic lecturer in 2010, which is now part of Metallurgy & Corrosion (Corrosion@Manchester) in the Department of Materials.  Dirk’s research is centred on (i) understanding material degradation related to the storage, disposal, and decontamination of nuclear waste, (ii) applied electrochemistry and high-throughput screening techniques, (iii) development of innovative solutions for net-zero engineering, and (iv) localised corrosion, stress corrosion cracking and hydrogen embrittlement. Dirk is also an expert in microstructure engineering and leads several cross-disciplinary projects combining mechanical, civil, and chemical engineering with chemistry, physics and materials-based research.

(*The work reported here is based on Yosef’s dissertation for the MSc in Corrosion Control Engineering (CCE) at The University of Manchester).

1. Introduction

Austenitic stainless steels are commonly used for applications in the energy, medical and chemical process industries due to their enhanced corrosion resistance, ease of availability and beneficial physical properties [1]. However, the material is not immune to corrosion, which is frequently caused by chlorides and other halide ions that may be present in petrochemical refinery systems or marine atmospheres [2]. Such corrosion damage can lead to serious material loss, crevice or localised corrosion or even stress corrosion cracking (SCC). Thus, monitoring, inspecting, predicting and investigating the corrosion behaviour and its patterns are most important to avoid the risk of material failure and to ensure the materials are performing accordingly.

Passivation treatments are viable options to enhance the corrosion resistance of stainless steels, providing an additional layer of protection

via enhancing surface passive film properties. These treatments are either applied via exposure to citric or nitric acids [3,4,5] or via electrochemical treatments [6]. Passivation treatments strengthen the existing passive film of stainless steel surfaces, but there will likely often remain some residual issues at sites of jointing and particularly welding.

A novel method for screening materials for their corrosion resistance is bipolar electrochemistry. The technique has been used for high-throughput corrosion screening and to obtain information about microstructures that might provide enhanced protection against material degradation [7,8].

The idea of this dissertation is to explore bipolar electrochemistry for modifying passive film properties on type 316L stainless steel.

The set-up does not require a direct ohmic contact to the test sample, and the set-up is quite simple and easy to replicate. A bipolar electrode (BPE) is an electrically conductive substance that encourages electrochemical reactions at its extremities (poles) when exposed to an electrical field between two feeder electrodes [9]. Through bipolar electrochemistry [10], the material’s surface properties can be either modified or assessed.

Differing from the conventional three-electrode experimental corrosion test, the bipolar set-up can be used to conduct corrosion assessment across a far wider range of applied potentials and materials. Bipolar electrochemistry has even been used to screen 2707 Hyper-duplex stainless steels for their corrosion resistance [11]. This is possible through scan rate independent, directly acting potential gradients along the BPE. In addition, one of the studies related to the oil and gas industries is the employment of cathodic protection, where stray current can be further studied via bipolar electrochemistry utilisation or the formation of inhibitor films under anodic or cathodic polarisation control. These test set-ups are currently being further developed for corrosion screening.

2. Methodology

Material and Sample Preparation

Type 316L sheet samples with dimensions of 10 mm x 25 mm were mechanically prepared with SiC paper from 400 up to 2500 grit, then polished with diamond paste to a ¼ micrometre finish. The final mirror-finished surface is required for passivation and corrosion test experiments—the desired quality of sample finish is also needed for elemental depth profile analysis with Glow Discharge Optical Emission Spectroscopy (GDOES). Figure 1 gives a flow chart of the research methodology and analysis steps applied. The prepared samples were first passivated using a novel bipolar electrochemistry approach, followed by assessment of the passivated surfaces in HCl solutions using standard 3-electrode electrochemistry. The sample surfaces were then analysed after corrosion testing using optical microscopy. One additional sample was also prepared under bipolar passivation treatment (in citric acid and boric acid) in order to observe the GD-OES depth profiling effect of bipolar passivation treatment.

Bipolar Electrochemistry Passivation Treatment

The passivation treatment utilised citric acid (C₆H₈O₇), and boric acid (H3BO₃) as passivating agents in order to facilitate the formation of a gradient passivation layer along the BPE surface. Employing bipolar electrochemistry to induce passivation at the anodic side, with the cathodic side expected to weaken the passive film. The expected outcome here was to generate one sample surface containing the full spectrum, starting with a strong passive film at the anodic side, to surfaces experiencing OCP conditions close to the central region, to a cathodically cleaned surface at the cathodic end of the BPE. Figure 2 below shows the bipolar test set-up, with the sample exposed to the electric field between both feeder electrodes, but without any electrical connection to an outside power source. Here it was expected that the negative pole of the feeder electrode would induce a positive reaction at the one end of the BPE surface, with the positive feeder inducing a negative pole at the BPE. The set-up was similar to experiments to test stray current corrosion of different metals related to cathodic protection as well as buried power line corrosion [12,13].

The effect of cathodic polarisation on passive film stability has been demonstrated previously under different applied potentials on super duplex stainless steel [14].

The bipolar parameters used for the passivation treatment were 8V with 10-minute or 30-minute polarisation in 10% (wt.) citric acid or 1 M boric acid. A Keysight power source was was used for applying the voltage. For surface film characterisation GDOES depth profile measurements were carried out, and each experiment was conducted 3x for each parameter variable.

Corrosion Test

Directly after the bipolar passivation treatment, standard 3 electrode potentio-dynamic corrosion tests were carried out to assess the effect of the surface passivation treatment. An 0.01 M, 0.1 M and 1 M hydrochloric acid (HCl) solution was used with Ag/AgCl/saturated KCl reference electrode.

Open circuit potential (OCP) measurements were recorded for 180 minutes, to see whether changes occurred at the surface that might be reflected in a shift of the OCP. The assessed samples were then observed under an optical microscope to analyse the impact of the corrosion testing on the treated sample surfaces.

Glow Discharge Optical Emission Spectroscopy (GDOES)

The depth profiles of the sample surface were measured using a GD-OES instrument. The power output used was 30 W and 650 Pa. with the sample very flat and the backside of the sample parallel to the front side to enable the vacuum to work properly. GD-OES analysis was carried out at both ends and the centre of the sample after passivation.

3. Results and Discussion

Figure 3 below shows the OCP versus time for the untreated type 316L surface. The test in 0.01 M HCl shows a steady increase of OCP from -0.2 V at the start to about -0.05 V after 3 hours, with the 0.1 M HCl giving a final rest potential close to -0.1 V, and the 1 M HCl sample somehow stabilising at -0.3 V. These long OCP measurements were employed to understand whether the bipolar passivation treatment caused changes in the OCP response.

Figure 4 further summarises the OCP response of the citric and boric acid treated BPE surfaces, with the same trend apparent for all samples. The OCP test for citric acid passivation exposed to 0.1 M HCl started at -0.2 V and stabilised at -0.13 V, with the only marked difference observed for the boric acid treated surface giving an initial potential of +0.3 V. Although the final potential was slightly less positive than the 0.1 M HCl test of the untreated type 316L surface, it still showed the material’s resistance was in a similar OCP range.

Furthermore, the OCP test for both passivation treated surfaces exposed to 1 M HCl started at -0.35 V and gradually rose to about -0.28 V, then remained steady until the end of the test at 3 hours. Both passivation treatments showed a distinct OCP stabilisation hump after 1 hour of exposure. This shows the system reaches a stable state, but at a far more negative potential compared to the lower HCl concentration.

The bipolar passivation treatment was expected to enhance the passive layer at the anodic side of the bipolar electrode, leading to a more corrosion resistant surface, compared to the cathodic side. However, since the full sample was immersed in the passivating solutions, even the central part would be expected to be more resistant than the cathodic, with the later cathodic BPE side is expected to have a far weaker passive film [14].

Although all the results show a similar behaviour, when the corrosion test was deployed in 1 M HCl, the treated sample obtained a constant OCP far earlier than the reference sample test; this could be due to the effect of passivation treatment, which enabled the samples to obtain their equilibrium earlier than the reference sample without passivation treatment [15].

Comparing the results of the samples that have been treated with bipolar passivation in 10% citric acid and 1 M boric acid in Figure 4, the OCP data showed no particular difference between them. The application of bipolar electrochemistry on the citric acid and boric acid passivation showed a similar behaviour when immersed in the 0.1 M HCl and 1 M HCl.

Figure 5 above summarises the optical microscopy observations of both sample surfaces that underwent bipolar electrochemistry passivation treatment in 10% citric acid and 1 M boric acid, which were corrosion in both 0.1M and 1M HCl solutions. The optical assessment showed no significant corrosion attack variation between the anodic and the cathodic pole of the BPE sample. The difference in 0.1M to 1M HCl exposure clearly showed more general surface roughening, with the appearance changing into a “wrinkle-like” roughened contour after 1M HCl exposure. Type 316L is known to readily corrode in 1M HCl.

The idea of this OCP exposure test in HCl was to understand whether differences exist between the anodically and cathodically treated sides after the bipolar passivation treatment. No visual differences were actually observed along the sample surface after both 0.1M and
1M HCl exposure.

Characterisation

GD-OES application for surface chemical analysis is described in more detail in Ref. [16,17]. The plasma generated during the measurement will create a circular crater of 4 mm in diameter, as shown in Figure 6 below. The scan rate was 1 m/s to obtain the depth profiles with a sputter duration of 75 seconds, with measurements taken at different locations. The collision between the argon ions and the sputtered material excites atoms and releases the photon energy as a characteristic light spectrum. The data obtained from the measurements shows the emission intensity versus time (s). GD-OES is frequently used for the depth profiling of thin surface films and layers, such as passive surface films, galvanised materials, or PVD/CVD coatings [18].

All data obtained showed carbon contamination at the surface, which is expected with sample storage under standard ambient conditions in the laboratory. Figure 7 above shows the resulting GD-OES depth profile of the reference Type 316L sample, with carbonaceous surface contamination, followed by enriched chromium, with the iron (Fe) and nickel (Ni) then indicating the bulk composition. The oxygen signal was found slightly raised in the surface region, with the horizontal line then indicating a drop in this signal, indicative of the interface between surface oxide and the bulk.

Finally, Figure 8 below compares the GD-OES depth profiles of the specimens treated in 10% citric acid and 1 M boric acid. The application of the bipolar passivation treatment seemed to influence to some degree the formation of the passive film of the Type 316L sample. The passive film formation of the sample under passivation treatment in 10% citric acid showed some minor change in chromium signal between the middle region and both anodic and cathodic regions. The middle region shows the presence of iron on the outer surface, while chromium dominates both the anodic and cathodic poles [19].

Observation of the cathode side of the passivated sample (both in citric and boric acid – Figure 8) showed a higher intensity level of chromium at the very beginning of the time axis. This may indicate that the chromium exists at the very outer layer of the treated sample’s cathode side; therefore, the treated sample had higher chromium content on the passive layer than the untreated sample. This behaviour might be influenced by the application of bipolar passivation treatment on the sample, which strengthens the formation of a passive layer on the sample’s cathode side.

The chromium intensity of the sample treated in 10% citric acid fell before it reached the bulk interface. On the other hand, the sample treated in 1 M boric acid underwent a falling of chromium intensity before it went constant at the bulk layer. According to Ref. [36], the peak of the elements chromium and iron showed that the elements were the major cationic alloys in the redox reaction. Thus, it can be assumed that applying 1 M boric acid was a more susceptible, favourable environment for the passivation of stainless steel 316L compared to the treated sample in 10% citric acid.

Conclusion

The overall bipolar passivation treatment did not show the expected behavior regarding the formation of a gradient passive film, although some interesting response was observed conducting OCP measurements in 1 M HCl solution.

It seems though that the application of bipolar passivation treatments influences somehow local passive layer compositions. From GD-OES characterisation of the sample passivated in 10% citric acid, the anodic and cathodic regions seemed Cr enriched in the oxide layers, while the middle part of the surface remained covered with iron oxide. On the other hand, on the sample exposed to 1 M boric acid, the anodic and centre part of the sample have iron oxide at the outer film surface, while the cathodic region was enriched with chromium. More work is certainly needed to further investigate the successful application of bipolar passivating treatments.

References

[1] P Kangas and G C Chai, “Use of Advanced Austenitic and Duplex Stainless Steels for Applications in Oil & Gas and Process Industry,” AMR, vol. 794, pp. 645–669, Sep. 2013, doi: 10.4028/www.scientific.net/AMR.794.645.

[2] A H Al-Moubaraki and I B Obot, “Corrosion Challenges in Petroleum Refinery Operations: Sources, Mechanisms, Mitigation, And Future Outlook,” Journal of Saudi Chemical Society, vol. 25, no. 12, p. 101370, 2021, doi: https://doi.org/10.1016/j.jscs.2021.101370.

[3] ASTM A380-99E1  Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems, 2017. doi: 10.1520/A0380_A0380M-17.

[4] ASTM A967-05E2  Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts, 2013. doi: 10.1520/A0967-05E02.

[5] BS EN 2516:2023 Aerospace Series – Passivation of Corrosion Resisting Steels and Decontamination of Nickel or Cobalt Base Alloys, BS EN 2516:2023, Feb. 06, 2024. doi: 978 0 539 30425 1.

[6] ASTM B912-02(2018)  Standard Specification for Passivation of Stainless Steels Using Electropolishing, 2018. doi: 10.1520/B0912-02R18.

[7] Y  Zhou, J Qi, and D L Engelberg, “A Novel High Throughput Electrochemistry Corrosion Test Method: Bipolar Electrochemistry,” 2023.

[8] Y Zhou, J Qi, and D L Engelberg, “On The Application of Bipolar Electrochemistry For Simulating Galvanic Corrosion Behaviour of Dissimilar Stainless Steels,” Electrochemistry Communications, vol. 126, p. 107023, 2021, doi: 10.1016/j.elecom.2021.107023.

[9] S E  Fosdick et al., “Bipolar Electrochemistry,” Angew Chem Int Ed, vol. 52, no. 40, pp. 10438–10456, Sep. 2013, doi: 10.1002/anie.201300947.

[10] R M Crooks, “Principles of Bipolar Electrochemistry,” ChemElectroChem, vol. 3, no. 3, pp. 357–359, Mar. 2016, doi: 10.1002/celc.201500549.

[11] Y Zhou et al., “A Rapid Corrosion Screening Technique for Grade 2707 Hyper Duplex Stainless Steel At Ambient Temperature,” Materials and Corrosion, vol. 75, no. 2, pp. 227–234, 2023, doi: 10.1002/maco.202313943.

[12] T J Lennox and M H  Peterson, “Stray Current Corrosion of Steel, ”Naval Engineers Journal, vol. 88, no. 1, pp. 45–53, 1976, doi: 10.1111/j.1559-3584.1976.tb03795.x.

[13] J M K Nairn et al., P Wade, and S Thomas, “Cell-Within-Cell Study of Stray Current Copper Corrosion: Impact of Soil Conductivity and Source Distance,” Corrosion Engineering Science and Technology, The International Journal of Corrosion Processes and Corrosion Control, 2025, doi: 10.1177/1478422×241311671.

[14] C  Ornek et al., “Understanding Passive Film Degradation and Its Effect On Hydrogen Embrittlement of Super Duplex Stainless Steel – Synchrotron X-Ray And Electrochemical Measurements Combined With Calphad And Ab-Initio Computational Studies,” Applied Surface Science, vol. 628, p. 157364, 2023, doi: 10.1016/j.apsusc.2023.157364.

[15] S Mischler and A I Munoz, “Tribocorrosion,” in Encyclopedia of Interfacial Chemistry, Elsevier, 2018, pp. 504–514. doi: 10.1016/B978-0-12-409547-2.13424-9.

[16] R Payling and T Nelis, Glow Discharge Optical Emission Spectroscopy: A Practical Guide. Royal Society of Chemistry, 2007. Available:https://books.google.co.uk/books?id=7mwoDwAAQBAJ&dq=glow+discharge+optical+emission+spectroscopy+&lr=&source=gbs_navlinks_s

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[19] M Uemura et al., “Depth Profile Analysis of Thin Passive Films on Stainless Steel By Glow Discharge Optical Emission Spectroscopy,” Corrosion Science, vol. 51, no. 7, pp. 1554–1559, Jul. 2009, doi: 10.1016/j.corsci.2008.11.017.

‘Amber Sykes¹, Danny Burkle², Robert Jacklin^1,2 Joshua Owen¹, R Woollam¹, R Barker¹
¹School of Mechanical Engineering, University of Leeds, UK | ²LBBC Baskerville, UK

Amber Sykes is currently a PhD student at the University of Leeds, within the Institute of Functional Surfaces, where she is researching iron carbonate corrosion product formation in geothermal environments. Prior to her PhD research, Amber gained a Master of Engineering (MEng) degree in Mechanical Engineering at the University of Bath. During 2024 she received an EFC – European Federation of Corrosion Award for Best Oral Presentation by an Early Career Author.

]Dr Danny Burkle, is a Principal Engineer and corrosion testing specialist at LBBC Baskerville. He earned his Master’s Degree in Mechanical Engineering and a PhD in Corrosion from the University of Leeds. With over 10 years’ experience dedicated to the corrosion field, he is a professional member of the Institute of Corrosion (ICorr) and formerly the chair of Young ICorr, Danny now leads the ICorr Corrosion Engineering Division (CED). His contributions to corrosion research have included 6 published journal papers as the lead author and over 10 papers as a named author/contributor. He joined LBBC in 2017 through a KTP – Knowledge Transfer Partnership with the University of Leeds, Danny and has since progressed seamlessly from a Design Engineer to a Principal Engineer. His trajectory has a strong focus on business development and sales related to the research sector.

1. Introduction: Why Collaboration Matters

This article highlights a collaborative approach in developing a test method to obtain high-quality, reproducible electrochemical data within an autoclave operating at elevated pressure and temperature.

The evaluation and understanding of material corrosivity in high-pressure, high-temperature (HPHT) environments is critical to ensuring asset integrity in industries such as Oil and Gas, Carbon Capture and Storage (CCS) and Geothermal Energy to name a few. Carbon steel processing infrastructure in these systems is frequently exposed to carbon dioxide (CO₂)-containing fluids, accelerating corrosion and threatening long-term integrity.

Under specific CO₂ – containing conditions—particularly elevated temperature, pressure, and chemical saturation—the corrosion product iron carbonate (FeCO₃) can precipitate onto the internal surfaces of carbon steel pipes. This layer can slow degradation by blocking active corrosion sites and impeding the diffusion of electrochemically active ions [1]. Understanding the formation, structure, and protective mechanisms of FeCO₃ layers is therefore vital for accurate corrosion modelling and prediction, and hence asset integrity management.

Autoclave technology can facilitate experimental corrosion testing in HPHT environments, in which protective FeCO₃ layers tend to form. In-situ electrochemical monitoring techniques, such as Linear Polarisation Resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS), can provide real-time insight into the rate of FeCO₃ layer formation, and the evolving mechanisms of corrosion protection offered by the layer. However, the application of these techniques in HPHT conditions remains exceptionally challenging. Issues such as reference electrode instability, electrical interference, and difficulty maintaining test equilibrium often lead to poor data reproducibility and limit the utility of these techniques for model development and material evaluation.

To address this gap, a collaborative project between LBBC Baskerville and the University of Leeds has advanced the reliability and precision of electrochemical testing in autoclaves. By combining industrial design expertise with academic electrochemical research, the team has developed a refined methodology capable of delivering high-quality, reproducible electrochemical data under extreme conditions.

This article presents two core outcomes of that collaboration:

1.How a refined testing methodology enables reproducible monitoring of corrosion product formation in autoclave conditions.

2.How this data supports advanced EIS modelling to quantify the layer’s physical properties in an autoclave.

Together, these insights offer a practical foundation for enhancing corrosion prediction in real-world HPHT systems.

2. Experimental Set-Up: Refining the Approach

The collaborative work began by identifying recurring limitations in electrochemical autoclave testing. Robert Jacklin’s PhD research project at University of Leeds played a key role in diagnosing failure points, including pressure instability, signal drift, and inconsistent electrode positioning [2].

The team developed a refined procedure using the LBBC Baskerville CTA-S1300 autoclave, incorporating:

•Improved sealing methods and sample insulation to prevent electrical interference,

•A low-pressure CO₂ sparging method to pre-saturate the brine prior to solution transfer,

•Careful arrangement of working, counter, and reference electrodes to avoid electrical interference,

•Use of a back pressure regulator (BPR) to maintain consistent test pressure during heating.

Electrode Preparation and Assembly

A three-electrode set up was used to take in-situ electrochemical measurements, including an X65 carbon steel working electrode, a Hastelloy C-276 counter electrode and an externally pressure balanced silver/silver chloride (Ag/AgCl) reference electrode. The working and counter electrodes were prepared by wet grinding all surfaces to P600 grit and sealing the back and sides with high-pressure thermosetting resin. An additional X65 carbon steel coupon was prepared for each test to assess mass loss. The arrangement of the electrochemical samples and mass loss coupon in the autoclave is shown in Figure 1.

Brine Preparation and Autoclave Testing Procedure

A 3 wt.% sodium-chloride (NaCl) brine was prepared by dissolving >99 % purity NaCl in deionised water, followed by stirring and continuous CO₂ sparging for a minimum of 12 hours to ensure oxygen removal and full saturation. The 1.3L LBBC Baskerville autoclave was also flushed through with CO₂ for a minimum of 30 minutes before the brine solution was transferred into the autoclave by CO₂ gas displacement.

The autoclave was pressurised to the desired operating pressure (5, 10 or 15 bar) using a high-pressure CO₂ line at room temperature, followed by heating to 80 °C using a hot plate and temperature probe. A BPR maintained the test pressure during the heating phase. Upon reaching the target temperature, the outlet valve to the BPR was closed and the pressure in the autoclave was allowed to evolve naturally throughout the test. The working, counter and reference electrodes were connected to a Gamry Potentiostat to take electrochemical measurements. The autoclave vessel was grounded through a metal hot plate [2].  An image of the autoclave set up is shown in Figure 1.

Electrochemical testing followed a repeatable schedule: Open Circuit Potential (OCP), five cycles of LPR, and EIS scans. This setup significantly improved baseline stability and reduced variability across runs. These methods were carefully documented and refined through repeated testing, highlighting how hardware design, procedural control, and pressure regulation directly influence
data quality.

This approach was initially used to refine the procedure and ensure high-quality, reproducible measurements at 5 bar, before being applied in tests at 10 and 15 bar to evaluate reproducibility and environmental sensitivity.

3. Reproducibly Tracking FeCO₂ Layer Formation in Autoclaves

The LPR results in Figure 2 were obtained using the refined autoclave testing set up and show how the average inverse polarisation resistance (R_p^-1) varied throughout the 120-hours corrosion tests at 5, 10 and 15 bar. R_p^-1 is directly proportional to the corrosion rate of the underlying carbon steel surface. The error bars represent the range in R_p^-1 values across four repeats at 5 bar and two repeats at 10 and 15 bar.

The evolution of a protective FeCO₃ layer was initially investigated at 80 °C and 5 bar.

At these conditions, a rapid decrease in corrosion rate occurred after the first 40 hours of exposure, aligning with the onset of FeCO₃ precipitation. The EIS Nyquist plots also showed a change in shape from two distinct semi-circles to the formation of a low-frequency linear tail, shown in Figure 5, indicating the transition from active corrosion to a diffusion-limited processes. At this point, the mechanism of protection offered by the FeCO₃ layer transitioned from predominantly surface blocking to effectively restricting the diffusion of ions [1]. This occurred alongside the ‘pseudo-passivation’ phenomenon, between 70 and 80 hours, marked by a sudden anodic shift in OCP, shown in Figure 3, and a plateau in polarisation resistance [4]. The cross-sectional scanning electron microscopy (SEM) images in Figure 4 confirm the evolution of a low-porosity FeCO₃ layer, consistent with the formation of an effective diffusion barrier, supporting the electrochemical interpretations. These findings validated that the new autoclave methodology could reproducibly capture the entire corrosion-product evolution timeline, from initial exposure through to protective film formation.

The LPR results in Figure 2 also show that the refined autoclave testing methodology could capture repeatable electrochemical measurements in more demanding environments, and with accelerated corrosion kinetics. Increasing the operating pressure from 5 to 15 bar resulted in higher initial corrosion rates and a faster rate of formation of a protective FeCO₃ layer [3]. These tests demonstrate the sensitivity of FeCO₃ formation and corrosion characteristics to environmental changes and highlights the importance of experimental testing over a wide range of operating conditions.

4. Quantifying Layer Properties Using High-Precision EIS Modelling

The LPR responses in Figure 2 show that corrosion product formation significantly alters the corrosion rate of the underlying carbon steel and must be considered for the accurate prediction of corrosion rates in HPHT environments. However, the current incorporation of FeCO₃ into CO₂ corrosion models relies on numerous theoretical assumptions regarding the physical properties of the layer, and how these properties contribute towards the mechanisms of corrosion protection [5]. To quantify the protectiveness of corrosion products layers throughout their development, and extract physical properties of FeCO₃, equivalent electrical circuit models can be fit to the experimental EIS data.

] 

High-precision EIS data is required to obtain useful and reliable information from equivalent circuit models. To assess the quality of the experimental EIS data, Kramers-Kronig analysis is used. The Kramers-Kronig transformation assesses the validity of the EIS response by testing for linearity, causality and stability [6].  This assessment is particularly important for the low frequency EIS data, used to characterise diffusion behaviour. The time scales required for low frequency measurements are increased, such that the stability condition may not be achieved. Once the EIS data has been validated, the quality of fit between the modelled and measured EIS data is calculated using chi-squared analysis normalised by the degrees of freedom (x²/v). The circuit models illustrated in Figure 5 both achieve an excellent quality of fit to the experimental EIS data, achieving x²/v values of less than 0.0002.

The quality of the electrochemical data in this study was sufficient to extract the full low-frequency EIS response, enabling the quantification of diffusion characteristics through the FeCO₃ layer. This represents a significant advancement, as such measurements have rarely been achieved under comparable high-pressure, high-temperature conditions. The circuit models illustrated in Figure 5 were used to quantify the extent of surface coverage from FeCO₃, and the diffusion coefficients of ions through the layer. This allows the respective contributions of surface blocking and diffusion restriction towards the overall reduction in corrosion rate to be identified, information that is critical to the advancement of corrosion prediction models.

The physical porosity of the FeCO₃ layer was also estimated from in-situ EIS measurements, showing promising agreement with results from literature [7]. Validating the use of electrochemical techniques to evaluate the porosity of corrosion product layers in-situ would be a landmark finding for this research area.

In this study, the development of a robust autoclave testing methodology enabled the collection of high-precision and meaningful EIS data in HPHT environments. Equivalent electrical circuit models could then be utilized to extract physical properties of the layer, in-situ, throughout the layer’s development – information which no other technique can provide.

Standardisation of electrochemical set-ups and autoclave testing procedures will continue to improve EIS data quality and facilitate innovative EIS studies that can further the development of corrosion prediction models and advance the knowledge of the wider corrosion industry.

5. Conclusion: A Path Forward Through Partnership

This collaboration between LBBC Baskerville and the University of Leeds has delivered two critical advances in autoclave corrosion testing:

• A validated methodology for reproducibly tracking FeCO₃ layer formation under HPHT conditions using LPR, OCP, and EIS.

• The demonstration that precision EIS can be used to quantify key physical properties of corrosion product layers — such as surface coverage and diffusion characteristics.

These outcomes not only advance the science of corrosion but also provide practical tools and benchmarks for researchers and industry alike. Achieving reproducibility in HPHT electrochemical testing has long been a challenge; this work proves it is possible with the right approach. The success of this collaboration also emphasises a pressing need in corrosion testing to the wider community: standardised methodologies for electrochemical testing in autoclaves.

Without clear standards, comparing data across institutions becomes difficult, and model calibration remains speculative.  By publishing detailed methods and validating their approach, this project offers a foundation on which others can build.

The methods and findings shared here are intended to empower others facing similar challenges in autoclave corrosion testing. As CCS, hydrogen, and geothermal technologies continue to expand, the need for reliable, high-precision corrosion data will only grow. This study provides a blueprint for how that data can be generated and used.

Next Steps: The team welcomes further collaboration to extend this methodology to multi-impurity systems and operational-scale environments. For more information or to explore partnership opportunities, contact Dr Danny Burkle (LBBC) and Professor Richard Barker (University of Leeds).

6. References

[1] R De Motte et al., E Basilico, R Mingant, J Kittel, F Ropital, P Combrade, S Necib, V Deydier, D Crusset, S Marcelin, “A Study by Electrochemical Impedance Spectroscopy and Surface Analysis of Corrosion Product Layers Formed During CO₂ Corrosion of Low Alloy Steel”. Corrosion Science 172, (2020): p. 108666.

[2] R Jacklin, Characterising Protective Corrosion Product Development in Demanding CO₂ Environments. Doctor of Philosophy thesis, University of Leeds, 2023.

[3] A Sykes et al., R Jacklin, D Burkle, R C Woollam, J Owen, R Barker, “The Effect of CO₂ Partial Pressure on the Formation and Protective Characteristics of Iron Carbonate Corrosion Products”. AMPP Annual Conference + Expo, paper no. AMPP-2024-20666 (New Orleans, Louisiana, USA: Association for Materials Protection and Performance, 2024).

[4] W Li et al., B Brown, D Young, S Nešić, “Investigation of Pseudo-Passivation of Mild Steel in CO₂ Corrosion”. Corrosion 70, (2014): p.
294-302.

[5] M Nordsveen et al., S Nešić, R Nyborg, A Stangeland, “A Mechanistic Model for Carbon Dioxide Corrosion of Mild Steel in the Presence of Protective Iron Carbonate Films-Part 1: Theory and Verification”. Corrosion 59, 5 (2003): p. 443-456.

[6] A Lazanas, M  Prodromidis, Electrochemical Impedance Spectroscopy – A Tutorial. ACS Measurement Science Au. 2023, 3.

[7] R Barker et al., D Burkle, T Charpentier, H Thompson, A Neville, A review of iron carbonate (FeCO₃) formation in the oil and gas industry, Corrosion Science, Volume 142, 2018.