Various electrochemical methods were used to understand the behavior of steel buried in unsaturated artificial soil in the presence of cathodic protection (CP) applied at polarization levels corresponding to correct CP or overprotection. Carbon steel coupons were buried for 90 days, and the steel/electrolyte interface was studied at various exposure times. The coupons remained at open circuit potential (OCP) for the first seven days before CP was applied at potentials of −1.0 and −1.2 V vs. Cu/CuSO₄ for the remaining 83 days. Voltammetry revealed that the corrosion rate decreased from ~330 µm yr⁻¹ at OCP to ~7 µm yr⁻¹ for an applied potential of −1.0 V vs. Cu/CuSO₄. CP effectiveness increased with time due to the formation of a protective layer on the steel surface. Raman spectroscopy revealed that this layer mainly consisted of magnetite. EIS confirmed the progressive increase of the protective ability of the magnetite-rich layer. At −1.2 V vs. Cu/CuSO₄, the residual corrosion rate of steel fluctuated between 8 and 15 µm yr⁻¹. EIS indicated that the protective ability of the magnetite-rich layer deteriorated after day 63. As water reduction proved significant at this potential, it is proposed that the released H₂ bubbles damage the protective layer.

Carbon dioxide (CO₂) injection into geologic formations has gained global traction, including in South Africa, to mitigate anthropogenic emissions through carbon capture, utilisation, and storage technology. These technological and technical developments require a comprehensive and reliable study of CO₂ sorption equilibria under in situ unmineable coal reservoir conditions. This paper presents novel findings on the study of the equilibrium adsorption of CO₂ on two South African coals measured at four temperatures between 30 and 60 °C and pressures up to 9.0 MPa using the volumetric technique. Additionally, the sorption mechanism and thermodynamic nature of the process were studied by fitting the experimental data into Langmuir–Freundlich (Sips), Tóth, and Dubinin–Astakhov (DA) isotherm models, and the Clausius–Clapeyron equation. The findings indicate that the sorption process is highly exothermic, as presented by a negative temperature effect, with the maximum working capacity estimated to range between 3.46 and 4.16 mmol/g, which is also rank- and maceral composition-dependent, with high-rank vitrinite-rich coal yielding more sorption capacity than low-rank inertinite-rich coal. The experimental data fit well in Sips and Tóth models, confirming their applicability in describing the CO₂ sorption behaviour of the coals under the considered conditions. The isosteric heat of adsorption varied from 7.518 to 37.408 kJ/mol for adsorbate loading ranging from 0.4 to 3.6 mmol/g. Overall, the coals studied demonstrate well-developed sorption properties that characteristically make them viable candidates for CO₂ sequestration applications for environmental sustainability.

This study was aimed at evaluating the effects of exposure time and SO₂ as one of the impurities on the corrosion behavior of steel pipeline transporting supercritical CO₂ during carbon capture and storage. Steel coupons were exposed to supercritical CO₂ with various concentrations of SO₂ for durations up to 1512 hours. Weight loss measurements were performed to evaluate the steel corrosion behavior. The weight loss results revealed a decrease in corrosion rate with an increase in exposure time which was mainly attributed to the gradual increase of the mineral layer on the steel surface. The increase in SO₂ concentration within supercritical CO₂ increased the corrosion rate from 0.0109 mm/yr to the highest value of 1.396 mm/yr. X-ray diffraction analysis revealed that the mineral layer formed on the steel surface under pure supercritical CO₂ mainly consisted of siderite while iron sulfite hydrate was observed under in the presence of SO₂.

Various electrochemical techniques were used to study the corrosion behaviour of the CNT-reinforced NiAl alloys under NaCl and Na₂SO₄ environments. The potentiodynamic polarization curves revealed activation Tafel behaviour when the NiAl alloy specimens were immersed in NaCl solution while passivation followed by slight development of transpassivation resulted under Na₂SO₄ environment. The reinforcement of pure NiAl alloy with up to 1.0 wt.% CNT increased the corrosion rate from 0.10 to 0.63 mm/yr under NaCl environment, while under Na₂SO₄ environment, the corrosion rate increased from 0.04 to 0.12 mm/yr. The observed increase in corrosion rate with an increase in CNT reinforcement (under both NaCl and Na₂SO₄ environments) suggested that the reinforcement of NiAl alloys with CNT reduced the corrosion resistance of NiAl. The X-ray diffraction analysis revealed that the corrosion products consisted of oxides including Al₂O₃ under NaCl environment, while scanning electron microscope analysis showed a porous passive layer on the surface of the alloy specimens immersed in Na₂SO₄ solution. Graphical abstract

Effective techniques are crucial for enhancing the durability of underground steel pipelines. Cathodic protection (CP) is commonly used to prevent corrosion, but long‐term monitoring is limited by intrusive methods. This study evaluates electrochemical impedance spectroscopy (EIS) to track corrosion product formation on steel exposed to a soil‐mimicking solution under CP. The steel was immersed for 15 days, with CP applied for 11 days, and EIS measurements recorded periodically. Voltammetric measurements showed that CP reduced the corrosion rate from 410 to 7 µm yr⁻¹ between days 5 and 15. EIS results indicate that, under open circuit conditions, the electrochemical response is consistent with mixed activation–diffusion behavior, whereas under CP the impedance response becomes dominated by activation and interfacial film effects within the measured frequency range. This shift is associated with substantial modification of the interfacial environment and the development of a more resistive surface film. X‐ray diffraction identified the formation of calcareous deposits, primarily aragonite and brucite, together with iron oxide corrosion products.