INTRODUCTION
R410Al99 is a commercially pure aluminum alloy containing at least 99 wt.% Al. Its low density, ductility, conductivity, and inherent oxide film support its use in electrical, packaging, architectural, and non-structural applications. Strong mineral acids, however, can destabilize the native film and accelerate both uniform and localized dissolution. Plant-derived inhibitors have therefore been examined as renewable alternatives for protecting aluminum in acidic media, including sulfuric-acid systems [1-10].
Citrullus colocynthis contains oxygen- and nitrogen-bearing phytochemicals capable of interacting with metallic surfaces. Earlier work reported inhibition by methanolic C. colocynthis extracts and related fractions on aluminum, copper, and carbon steel [11-15]. In acid solution, iodide can alter the interfacial charge distribution and promote the retention of organic inhibitor species. Similar KI-assisted effects have been reported for plant extracts on aluminum and other metals [16-18].
The present work evaluates a combined system comprising C. colocynthis leaf extract and 2 M KI for R410Al99 aluminum alloy in 1 M H₂SO₄. Extract concentrations of 3, 6, 9, 12, and 15 mL/L were examined at 10, 20, 30, and 40 °C. Electrochemical measurements were integrated with FTIR, EDS, AFM, and FESEM to relate inhibition performance to the chemical composition and morphology of the surface film.

MATERIALS AND METHODS
Materials and alloy composition
Analytical-grade sulfuric acid (96%, density 1.84 g/mL) was diluted with distilled water to prepare 1 M H₂SO₄. Methanol (≥99.8%, Merck, Germany) was used for extraction, acetone (99.9%, ROMIL) was used for degreasing, and potassium iodide (KI; molar mass 166.00 g/mol) was used to prepare a 2 M solution. R410Al99 aluminum alloy served as the working material. Its optical-emission-spectroscopy specification is summarized in Table 1.

Preparation of the plant extract
Leaves of C. colocynthis were repeatedly washed with distilled water, air-dried under ambient conditions for five days, and ground. A 150 g portion of the powdered material was continuously extracted with 750 mL methanol in a Soxhlet apparatus at 65 °C for 8 h. The resulting methanolic extract was used as the inhibitor stock and was characterized by FTIR to identify the functional groups available for surface interaction [19].

Specimen preparation
The alloy was sectioned by wire cutting into specimens measuring approximately 1 cm × 1 cm × 1.5 mm. Exposed faces were mechanically polished to a uniform finish, rinsed with distilled water, degreased with acetone, dried, and stored in sealed containers before testing. The electrochemically exposed working area was 1 cm².

Electrochemical measurements
Measurements were performed with a Corrtest CS350M potentiostat/galvanostat in a three-electrode cell. R410Al99 was the working electrode, a saturated calomel electrode (SCE) was positioned approximately 1 mm from the exposed surface, and platinum served as the counter electrode. The open-circuit potential was monitored until a quasi-steady interfacial response was obtained. Tafel and cyclic polarization measurements were then recorded for the uninhibited 1 M H₂SO₄ blank and for solutions containing 2 M KI plus 3, 6, 9, 12, or 15 mL/L extract at 10, 20, 30, and 40 °C. [scan rate=2v/s, potential limits=-+3v, stabilization time=900s, and number of replicate measurements=3].
The Tafel-derived inhibition efficiency was calculated from the corrosion-current density using Eq. 1. Polarization resistance was recalculated using the Stern–Geary relationship in Eq. 2, with βa and βc converted from mV/dec to V/dec before calculation.

IE (%) = [1 − icorr,inh / icorr,blank] × 100                          (1)

Rp = βa βc / {2.303 icorr (βa + βc)}                                 (2)

where icorr is the corrosion-current density, βa and βc are the anodic and cathodic Tafel slopes in V/dec, and Rp is expressed in Ω·cm² [20]. 

Surface characterization
FTIR spectra were recorded using a Shimadzu IRAffinity-1 instrument (laboratory code Fou24116). Surface morphology and elemental composition were examined using an Inspect F50 FESEM coupled with an Axia ChemiSEM/EDS system. AFM measurements were obtained with a Nanosurf CoreAFM. The principal surface conditions evaluated were the polished alloy, the uninhibited alloy after exposure to 1 M H₂SO₄, and the alloy protected with 9 mL/L extract + 2 M KI at 30 °C. The plant extract itself was also characterized by FTIR.

RESULTS AND DISCUSSION
Open-circuit potential behavior
The OCP profiles in Fig. 1 show strongly condition-dependent interfacial behavior. Most KI-containing solutions at 10 and 20 °C shifted toward more negative potentials than the blank, whereas the 30 and 40 °C profiles displayed concentration-specific positive and negative shifts. The absence of a monotonic concentration trend suggests dynamic competition among oxide dissolution, iodide adsorption, extract adsorption, and film reorganization during immersion.
By the end of the 600 s OCP record, the blank and mixed-inhibitor systems had clearly separated potential levels. At 10 °C, the blank approached approximately −0.39 V vs SCE, whereas the 3, 6, 9, 12, and 15 mL/L solutions ended near −0.54, −0.90, −0.87, −0.88, and −0.56 V, respectively. At 20 °C, the blank settled near −0.68 V after a pronounced transient; the corresponding inhibited potentials were approximately −0.76, −0.87, −0.82, −0.86, and −0.64 V. At 30 °C, the blank moved to approximately −0.23 V, while all combined inhibitor solutions remained substantially more negative at about −0.85, −0.86, −0.65, −0.82, and −0.85 V. At 40 °C, the blank remained near +0.28 V, whereas the inhibited curves ended at approximately −0.26, −0.72, −0.85, −0.83, and −0.78 V.
The transient excursion of the 12 mL/L trace at 10 °C, the large blank excursion at 20 °C, and the gradual negative drift of the 9 mL/L trace at 30 °C indicate continuing surface-film nucleation, disruption, and reorganization during immersion. OCP displacement alone does not quantify inhibition efficiency; nevertheless, the end-of-record potentials for the Tafel-optimum conditions were close to the independently fitted Ecorr values: approximately −0.56 versus −0.58279 V at 10 °C/15 mL/L, −0.82 versus −0.82606 V at 20 °C/9 mL/L, −0.65 versus −0.59813 V at 30 °C/9 mL/L, and −0.72 versus −0.77457 V at 40 °C/6 mL/L. This agreement supports the internal consistency of the potential-time and polarization measurements [21].

Tafel polarization
Fig. 2 and Table 2 demonstrate that the combined inhibitor markedly reduced icorr at selected concentrations, but the response was non-monotonic. The best efficiencies at 10, 20, 30, and 40 °C were 98.55% (15 mL/L), 93.12% (9 mL/L), 98.11% (9 mL/L), and 99.77% (6 mL/L), respectively. 
At the temperature-specific optimum, icorr decreased from 1.3631 × 10⁻⁴ to 1.9813 × 10⁻⁶ A/cm² at 10 °C (15 mL/L), from 5.8338 × 10⁻⁴ to 4.0137 × 10⁻⁵ A/cm² at 20 °C (9 mL/L), from 2.1943 × 10⁻³ to 4.1399 × 10⁻⁵ A/cm² at 30 °C (9 mL/L), and from 2.5128 × 10⁻³ to 5.6875 × 10⁻⁶ A/cm² at 40 °C (6 mL/L). The corresponding recalculated Rp values were 5277.154, 237.408, 245.961, and 10891.795 Ω·cm², respectively. Thus, the optimum extract concentration shifted from 15 mL/L at 10 °C to 9 mL/L at 20–30 °C and 6 mL/L at 40 °C, confirming that inhibition performance depends on both concentration and temperature rather than on extract dose alone.
At 30 °C, the surface-characterization condition of 9 mL/L extract + 2 M KI shifted Ecorr from −1.1478 V for the blank to −0.59813 V, decreased icorr from 2.1943 × 10⁻³ to 4.1399 × 10⁻⁵ A/cm², increased Rp from 3.449 to 245.961 Ω·cm², and produced 98.11% inhibition. Negative potential shifts predominated at 10 and 20 °C, positive shifts dominated at 30 °C, and both directions occurred at 40 °C. The combined system is therefore best described as condition-dependent mixed inhibition with anodic or cathodic predominance [22].

Cyclic polarization
The cyclic polarization curves in Fig. 3 exhibited positive hysteresis over much of the investigated range, confirming that localized corrosion susceptibility was not completely removed. Loop width and irregularity changed with temperature and extract concentration. Selected curves became narrower, indicating improved repassivation and reduced persistence of growing pits, whereas broad or irregular loops at other concentrations indicated unstable passive-film behavior. The combined inhibitor therefore improved localized-corrosion resistance in a concentration- and temperature-dependent manner.
Detailed comparison of the forward and reverse scans reveals distinct concentration effects. At 10 °C, the 6 and 15 mL/L curves showed the largest branch separation, whereas the 3 and 12 mL/L curves were comparatively more compact. At 20 °C, the 3 mL/L curve developed the broadest loop, and the 12 mL/L trace exhibited repeated current oscillations; the higher-concentration curves still showed positive hysteresis but with less extreme leftward displacement than the 3 mL/L condition. At 30 °C, the 3 mL/L curve displayed the closest forward–reverse alignment, while the 9, 12, and 15 mL/L curves were broader and more irregular, especially the oscillatory 12 mL/L trace. At 40 °C, the 3 and 9 mL/L curves were comparatively more compact than the 6, 12, and 15 mL/L curves, although none showed complete overlap of the reverse and forward branches.
These observations show that the conditions giving the lowest uniform-corrosion current were not always those giving the best localized-corrosion response. The 9 mL/L + 2 M KI system at 30 °C produced 98.11% Tafel efficiency but retained a positive, irregular hysteresis loop; similarly, 6 mL/L at 40 °C reached 99.77% Tafel efficiency while a distinct reverse-scan separation remained. The combined inhibitor therefore strongly suppresses the average dissolution rate and can assist repassivation, but it does not provide complete immunity to pit initiation and propagation. The oscillatory segments are consistent with metastable film breakdown followed by partial repair and renewal of the adsorbed extract–iodide layer [23].

FTIR analysis
The extract spectrum contained bands near 3277 cm⁻¹ (O–H/N–H), 2916 cm⁻¹ (aliphatic C–H), 1731 cm⁻¹ (C=O), and 1633 cm⁻¹ (aromatic C=C and/or conjugated C=O), together with C–O/C–O–C bands near 1235, 1147, and 1022 cm⁻¹. These groups provide heteroatom lone pairs and π-electron density that can participate in adsorption.
The polished alloy showed O–H/Al–OH bands over 3545–3235 cm⁻¹ and Al–O-related bands between 1096 and 409 cm⁻¹, consistent with a hydrated native oxide/hydroxide film. After uninhibited acid exposure, bands at 1101, 984, 935, 691, and 606 cm⁻¹ were consistent with sulfate/bisulfate-containing corrosion products. For the 9 mL/L + 2 M KI surface, broad O–H/N–H bands occurred at 3410 and 3205–3206 cm⁻¹; C–H bands appeared at 3030, 2966, and 2940 cm⁻¹; and bands at 1647, 1555, 1537, 1128, and 1099 cm⁻¹ reflected aromatic/conjugated and C–O-containing species. Changes over 941–406 cm⁻¹ indicate reorganization of the oxide/hydroxide–sulfate layer in the presence of the extract and iodide [24].

FESEM analysis
The polished reference displayed relatively fine surface features. Uninhibited acid exposure produced a strongly irregular surface with obvious localized attack and features in the approximate 50–70 nm range. The KI-assisted inhibited specimen showed denser coverage and smaller measured features of approximately 47–52 nm in the imaged region, with fewer obvious corrosion pits than the uninhibited surface. Local heterogeneity remained, consistent with the thick deposit detected by AFM [25].

EDS analysis
The polished alloy contained 90.3 at.% Al and 9.7 at.% O, with no detectable S. Uninhibited exposure decreased the Al signal to 9.9 at.% and increased O and S to 72.9 and 17.1 at.%, respectively, indicating extensive coverage by oxygen- and sulfate-containing corrosion products. In contrast, the surface protected with 9 mL/L extract + 2 M KI contained 64.4 at.% Al and 29.1 at.% O; S was below the detection limit, while K and I were detected at 3.4 and 3.2 at.%. This composition supports reduced sulfate accumulation and retention of KI-derived species in the protective surface region [26].

AFM analysis
The polished surface had Sa = 17.198 nm, Sq = 23.241 nm, and Ra = 13.889 nm. After uninhibited acid exposure, the corresponding values decreased to approximately 0.763, 0.942, and 0.882 nm. This apparent smoothing is attributed to preferential dissolution of polishing asperities and should not be interpreted as absence of corrosion. The KI-assisted inhibited surface showed much larger values (Sa = 1038.5 nm, Sq = 1380.2 nm, and Ra = 1134.2 nm) and a reported vertical range of 7.3696 μm. The high roughness reflects a thick, heterogeneous adsorbed/deposited layer rather than a smooth exposed substrate [27].
The combined electrochemical results show that inhibition cannot be represented by a simple monotonic concentration law. Plant extracts are multicomponent systems, and increasing dose can improve coverage while also promoting aggregation, competitive adsorption, or desorption of weakly bound species. Temperature further changes oxide dissolution, inhibitor transport, and adsorption equilibria. Comparable concentration- and temperature-dependent behavior has been reported for other plant-derived aluminum inhibitors.
The complete Tafel matrix confirms that the optimum extract concentration changed from 15 mL/L at 10 °C to 9 mL/L at 20–30 °C and 6 mL/L at 40 °C. The direction of the corrosion-potential shift also changed with temperature and concentration, while both anodic and cathodic Tafel slopes varied. The system therefore affects both aluminum dissolution and the cathodic reduction process, with one branch becoming dominant under specific conditions. The pronounced positive shift at 30 °C/9 mL/L indicates anodic predominance for the surface-characterization condition, whereas the negative shifts at lower temperatures indicate stronger cathodic influence. The mixed directions at 40 °C reinforce that inhibition cannot be assigned to one fixed electrochemical mode.
The end-of-record OCP values provide an additional consistency check. For the Tafel-optimum concentration at each temperature, the OCP value differed from the fitted Ecorr by only approximately 0.01–0.06 V. This close agreement strengthens confidence that the observed potential shifts were reproducible features of the interfacial state rather than isolated fitting artifacts. The large negative OCP displacements at 30 and 40 °C also show that the extract–iodide layer substantially altered the mixed potential, although OCP direction by itself should not be used to rank inhibition efficiency.
The surface analyses provide a coherent mechanism for the KI-assisted effect. FTIR identifies O–H/N–H, carbonyl, aromatic/conjugated, and C–O-containing groups capable of coordinating or interacting electrostatically with the interfacial region. Iodide can adsorb specifically and modify the surface charge, facilitating retention of protonated or polar extract species [9,10]. Detection of K and I by EDS, together with the recovery of the Al signal and the absence of detectable S, supports formation of an iodide-containing organic–inorganic barrier that limits sulfate-rich corrosion-product accumulation.
AFM and FESEM describe different aspects of this barrier. FESEM indicates fewer obvious pits and finer features in the mixed system, while AFM records a large vertical range and high roughness. The latter does not imply greater corrosion by itself; it is consistent with a thick, non-uniform surface deposit. Correlating AFM with FTIR and EDS is therefore essential. Cyclic-polarization hysteresis further indicates that the film improves repassivation at selected conditions but does not eliminate localized-corrosion susceptibility across the complete concentration–temperature matrix. The persistence of positive loops for the 30 °C/9 mL/L and 40 °C/6 mL/L high-efficiency conditions is especially important: Tafel current suppression describes average uniform corrosion, whereas loop separation probes localized breakdown and repassivation. Hence, the inhibitor can be highly effective against general dissolution without completely eliminating pitting susceptibility.
[scan rate=2v/s, potential limits=-+3v, stabilization time=900s, and number of replicate measurements=3].
A separate KI-only control would also strengthen quantitative evaluation of cooperative inhibition.

CONCLUSION
A combined inhibitor containing 2 M KI and methanolic C. colocynthis leaf extract effectively reduced the corrosion of R410Al99 aluminum alloy in 1 M H₂SO₄. Tafel efficiency depended strongly on concentration and temperature, reaching 99.77% at 6 mL/L and 40 °C. The 9 mL/L condition at 30 °C, used for detailed surface analysis, achieved 98.11% inhibition and reduced icorr by more than one order of magnitude. End-of-record OCP values agreed reasonably with the fitted Ecorr values at the best-performing concentration for each temperature, providing cross-method consistency. Positive cyclic hysteresis persisted, however, including at the 30 °C/9 mL/L and 40 °C/6 mL/L high-efficiency conditions; the formulation therefore reduces general corrosion and supports repassivation but does not guarantee complete immunity to localized attack. FTIR, FESEM, EDS, and AFM collectively support formation of a chemically modified, iodide-containing, heterogeneous protective layer. The layer reduced sulfate-rich corrosion products, preserved the Al signal, and decreased obvious localized damage in the imaged region. The system acts through condition-dependent mixed inhibition, with anodic or cathodic predominance determined by temperature and extract concentration.

ACKNOWLEDGMENTS
The authors acknowledge the Department of Chemistry, College of Science, University of Diyala; the Nanotechnology Research Center, University of Technology; and the Al-Khora Technology Center, Baghdad, for laboratory and characterization support.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.