Phytosubstances of TCFUV–Vis characteristicsThe results of the UV–Vis spectrum of the TCF are shown in Fig. 1. The spectrum of the TCF shows several absorbance peaks in the range of 260–550 nm. The bands at 260 nm and 308 nm indicate the presence of alkaloid and tannin compounds, respectively. Phenolic compounds are seen at the peak of the 348 nm band due to their aromatic hydroxyl groups. Flavonoid compounds are detected at an absorption of around 384 nm due to the presence of a conjugated ring structure. Furthermore, the peak range of 422 nm to 528 nm is considered characteristic of chlorophyll compounds. This chlorophyll absorbance band aligns with the findings reported in the ethanol extraction of Dillenia suffruticosa leaves32.Fig. 1UV–Vis spectra of TCF.Full size imageFT-IR characteristicsFT-IR (Fourier Transform Infrared Spectroscopy) analysis was employed to identify the functional groups present in compounds contained in the TCF, as well as to elucidate the chemical structure of active compounds that may contribute to the corrosion inhibition of carbon steel in seawater media. The results of the TCF spectra measurements are shown in Fig. 2.Fig. 2FT-IR spectra of TCF.Full size imageThe wide band in the valley (3300–3350 cm1) indicates the presence of hydroxyl groups (O–H groups). This band is a characteristic of polyphenol, flavonoid, or tannin compounds that have the potential to be corrosion inhibitors by forming a protective layer on the surface of carbon steel. The C–H group was identified at 2800–3000 cm1, and this band indicates the presence of alkyl compounds or compounds with alkyl groups in the structure. In addition, a carbonyl group (C=O) at 1704 cm−1 was also detected, which can interact with the surface of carbon steel through the adsorption process. In TCF, double bonds in aromatic compounds, such as flavonoids or polyphenols, were also detected in the band 1600 cm−1. Furthermore, in the band 1225 cm−1 (C–O group), it is suspected that the compound is an ester or phenolic compound, essential in increasing the metal surface’s affinity and corrosion inhibition efficiency.FT-IR analysis revealed that the TCF contains bioactive compounds, including phenols, flavonoids, and esters, which possess active functional groups such as –OH, C=O, and C=C, potentially playing a role in inhibiting the corrosion process in carbon steel. These compounds can potentially adsorb on the surface of carbon steel, forming a protective layer that reduces the interaction of the metal with corrosive media. Therefore, the active compounds in Tinospora cordifolia can effectively protect against corrosion, especially in seawater environments.HR-MS analysisHR-MS (High-Resolution Mass Spectrometry) analysis was employed to determine the molecular weight and structure of compounds present in the TCF, as well as to identify bioactive compounds with the potential to act as corrosion inhibitors on carbon steel in seawater media. The results of the TCF spectra with HR-MS are shown in Fig. 3.Fig. 3HR-MS spectra of TCF.Full size imageThe spectral results in Fig. 3 show several peaks, with the prominent peak detected at m/z (mass-to-charge ratio) 314. This peak indicates a possible compound with a molecular weight of around 313 Da. Based on literature data, this may be related to flavonoids or phenolics, often found in Tinospora cordifolia33. There is also a peak at m/z 177, indicating a compound with a smaller structure, most likely a phenolic derivative or a flavonoid compound, which also contributes to the inhibition mechanism.NMR analysisThe NMR results on the dominant compound of the TCF are shown in Fig. 4. Based on the results of specific H-NMR, C-NMR, and 2D-NMR spectra, this compound is suspected to be N-(4-Hydroxy-3-methoxy-E-cinnamoyl)-4-(2-aminoethyl)phenol, also known as 4-(2-aminoethyl) N-trans-feruloyltyramine and Moupinamide. This finding is reinforced by the HRMS results, where the compound with a molecular weight of 313 (moupinamide) is the dominant compound in the TCF.Fig. 4NMR spectra of the compound from TCF.Full size imageMoupinamide can act as an inhibitory agent for corrosion activity on carbon steel in seawater media, considering that amide compounds often interact with metal surfaces34. The amide group (–CONH–) in Moupinamide can interact with metal surfaces through physical or chemical adsorption, forming a protective layer that reduces the interaction between metal and corrosive media. In addition, these compounds, especially those with amide and aromatic groups, can act as antioxidants, reducing oxidation reactions that occur on carbon steel in seawater media35.Electrochemical studyOpen circuit potential (OCP) analysisOpen-circuit potential (OCP) measurements were performed to determine potential stability before applying the current. The results of the OCP graph for 600 s on carbon steel with and without TCF at various temperatures are presented in Fig. 5. The shift in the OCP value shifted negatively, indicating ongoing corrosion. However, in the presence of TCF, the OCP value stabilized over time, indicating the formation of a protective inhibitor layer on the steel surface. The stabilisation of the OCP over time further confirms the formation of a durable inhibitor layer that minimises the anodic and cathodic reactions responsible for corrosion. A limited range of potential shifts is − 606.9 mV to − 703.7 mV, − 612.9 mV to − 713.3 mV, and − 629.2 mV to − 727.7 mV for 300 K, 310 K, and 320 K, respectively. This limit of OCP may reflect the type of mixed inhibitors with cathode dominance36.Fig. 5Open circuit potential (OCP) curves of carbon steel in seawater with the absence and presence of varying TCF at (a) 300 K, (b) 310 K, and (c) 320 K.Full size imageOn the other hand, it has been suggested that the shift towards more negative corrosion potentials is related to the absorption of inhibitors or desorption of corrosion products from the metal surface37. In addition, increasing temperature also causes a shift in the OCP value to be increasingly negative38. Similar behaviour also occurs in aloe saponaria extract39, where the OCP moves negatively with the addition of extract.Potentiodynamic polarization (PDP) analysisThe potentiodynamic polarization (PDP) measurements were conducted on carbon steel samples in a seawater medium with varying concentrations of TCF as an organic inhibitor, ranging from 25 to 150 mg.L−1, at different temperatures. Polarization curves were recorded (Fig. 6), and key parameters involving corrosion current density (Icorr), Tafel slopes (βc), and corrosion potential (Ecorr) were extracted from the polarization plots as tabulated in Table 2. The PDP curve results show that the corrosion potential (Ecorr) shifts toward negative values as the concentration of TCF increases. This decrease in corrosion potential indicates a tendency for the carbon steel surface to react with increasing concentrations of TCF40. The corrosion potential (Ecorr) also shifts negatively with increasing temperature. This negative shift indicates that the activation energy for the corrosion process decreases, making the process more thermodynamically favorable and effortless to initiate at higher temperatures41.Fig. 6Potentiodynamic polarization (PDP) curves of carbon steel in seawater with the absence and presence of varying TCF at (a) 300 K, (b) 310 K, and (c) 320 K.Full size imageTable 2 Potentiodynamic electrochemical parameters of carbon steel in seawater containing TCF at different temperatures.Full size tableThe cathodic slope (βc) results in Fig. 6 provides information about the rate of oxygen reduction, which in this work tends to be cathodic inhibition. The βc value changes with TCF, indicating a change in the cathode hydrogen evolution mechanism. This case suggests that the active substances in TCF significantly slow down carbon steel corrosion as their inhibition capacity increases with concentration. The βc value changed slightly from 25 mg.L-1 to 100 mg.L− 1, indicating a minimal effect on the cathodic process. However, at a concentration of 150 mg.L− 1, βc tended to increase, indicating that the inhibitor inhibited oxygen reduction, which is essential in reducing the corrosion rate42. The relatively unchanged Tafel slope (although slightly cathodic) suggests that the TCF does not alter the fundamental corrosion mechanism but instead reduces the overall rate of the anode and cathode processes43. This type of inhibition may be due to the chemical structure of the TCF molecule, which contains functional groups that can interact with metal surface atoms and reduce oxygen in solution42. From the polarization curve in Fig. 6 and the data in Table 2, it was also identified that the Tafel beta slope (βc) changed with increasing temperature. Under conditions with the addition of TCF, it tended to be smaller (less sharp) than in the blank solution. This shift in the Tafel slope confirms that the TCF affects the corrosion process (cathode) across all temperature ranges, a characteristic of the inhibitor44.The PDP curve (Fig. 6) also shows a decrease in extensive corrosion current density (Icorr) with TCF. The reduction in corrosion current density (Icorr) and the negative change in corrosion potential (Ecorr) indicate that TCF molecules are adsorbed onto the steel surface45, thereby establishing a protective film46. This film effectively blocks the active sites, inhibiting the electrochemical reactions responsible for metal dissolution46. The inhibition efficiency is moderate at low concentrations of TCF (25 to 100 mg.L−1 TCF). The drop in corrosion current density (Icorr) is apparent under these conditions, implying that the inhibitor may function by adsorbing onto the steel surface and forming a protective layer. However, the effect is less pronounced than at higher concentrations (150 mg.L−1). This result implies that