Synergistic inhibition of Q235 steel (QCS) corrosion in H₂SO₄ by silkworm sand extract (Sse) and KI

Xinhua Liu; Siyu Liu; Yuan Zhang; Baojing Luo; Xiaoyu Shi; Xiuge Wang; Ying Wang; Boxi Yang; Jiarui Du; Xinyuan Zhang; Xiaodan Liu; Taoyi Zheng

doi:10.25259/AJC_853_2025

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Original Article

ARTICLE IN PRESS

doi:

10.25259/AJC_853_2025

Synergistic inhibition of Q235 steel (QCS) corrosion in H₂SO₄ by silkworm sand extract (Sse) and KI

Xinhua Liu^a,*, Siyu Liu^b,, Yuan Zhang^a, Baojing Luo^a, Xiaoyu Shi^a, Xiuge Wang^a,, Ying Wang^a,, Boxi Yang^a, Jiarui Du^a, Xinyuan Zhang^a, Xiaodan Liu^a,, Taoyi Zheng^a

aDepartment of Chemistry, Tangshan Normal University, Tangshan, Hebei, China

bDepartment of Materials Science and Engineering China University of Petroleum (East China) Qingdao, China

*Corresponding author: E-mail address: hualiyiwang@163.com (X. Liu)

Received: 2025-07-22, Accepted: 2025-12-09, Epub ahead of print: 2026-03-25,

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Abstract

In recent years, plant extracts had emerged as a promising alternative to conventional corrosion inhibitors, gaining significant attention in green chemistry research. However, using plant extracts alone resulted in the lower corrosion suppression efficiency (ƞ). In order to improve the ƞ of plant extract, the synergistic corrosion suppression behavior and mechanism of silkworm sand extract (Sse) and potassium iodide (KI) on Q235 steel (QCS) were studied for the first time in 0.5 M H₂SO₄. Both weightlessness and electrochemical method demonstrated that ƞ of QCS was optimal and exceeded 90% at 30°C in a 0.5 M H₂SO₄ containing 0.9 g/L Sse and 0.06 g/L KI (Sse/KI). When used alone of 0.9 g·L^-1 Sse and 0.06 g·L^-1 KI, ƞ was 71.12% and 40.16% respectively. Therefore, the synergetic effect of Sse and KI gave the corrosion inhibitor excellent corrosion suppression properties. In addition, scanning electron microscopy (SEM), atomic force microscopy (AFM) and contact angle (CA) demonstrated that Sse/KI could significantly decrease QCS corrosion in acidic environments. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) confirmed that Sse/KI were adsorbed onto the QCS to form a protective film, reducing the QCS damage in acidic environments.

Keywords

KI

Q235 steel (QCS)

Sse (Silkworm sand extract)

Synergistic effect

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1. Introduction

In industrial and manufacturing applications, techniques such as pickling and acid treating are employed to eliminate impurities or oxides on metal surfaces, thereby improving the service life of equipment [1]. Nevertheless, these processes often carry the danger of inducing metallic corrosion, which can lead to equipment failures, heightened costs, and potentially negative influences on both industrial operations and the environment [2]. To solve this problem, corrosion inhibitors (CI) have been widely used to decrease corrosion in the above processes [3]. Traditional CIs can be classified into organic and inorganic inhibitors.[4]. Due to its environmental pollution and toxicity, inorganic CIs have limited use in the field of metallic corrosion prevention. Organic CIs can be divided into synthetic and natural organic CIs. Synthetic organic CI was obtained through artificial synthesis, and the production process was complicated, bearing the disadvantages of difficult synthesis and high cost [5-6]. Therefore, there was an urgent need to discover an efficacious, eco-friendly, and renewable CI as a replacement for conventional organic CIs. The natural organic CIs extracted from roots, stems, leaves, etc., were non-toxic and environmentally friendly, making them a substitute for traditional CIs in the anticorrosion field.

Plant extracts contain biologically active compounds (e.g., phenols, alkaloids, and flavonoids) and could serve as green, environmentally friendly, and renewable CIs [7]. Phenols, alkaloids, and flavonoids contained OH, -COO-, -CHO, and long alkyl chains. They were able to prevent the QCS from corroding in acidic solutions by interacting with iron and forming an adsorption layer on the QCS [8-9]. However, plant extracts utilized as CIs had certain limitations, including corrosion suppression inefficiencies and the risk of degradation when placed for too long [10]. Research indicated that inorganic salts such as sodium iodide (NaI), potassium iodide (KI), and potassium bromide (KBr) could synergistically interact with plant extracts to prevent metal corrosion [11]. The primary reason was that these inorganic salts enhanced the solubility and stability of active molecules of plant extracts in aqueous solutions [12]. The adsorption capacity and reduction activity of I⁻ were stronger than those of Br⁻, and its synergistic effect with plant extracts was more significant. Moreover, potassium iodide had good solubility and lower toxicity than some halides, and its application cost and safety were more balanced. Wan et al. [13] evaluated the synergetic property of Kapok leaves extract (KLE) and I⁻ to carbon steel in H₂SO₄, and demonstrated that I⁻ promoted the adsorption of KLE on the steel surface. Wang et al. [14] studied that KI promoted the adsorption of Eupatorium adenophora spreng stems extract (EASSE), demonstrating the synergetic effect between EASSE and KI. Wang et al. [15] evaluated the synergetic corrosion suppression of Michelia alba leaf extract (MALE) and KI to carbon steel, and the results showed that the combination of MALE and KI showed good corrosion resistance through adsorption. Sun et al. [16] demonstrated that the Metasequoia glyptostroboides leaf (MGL) had excellent corrosion suppression performance to Q235 steel at 1M HCl. This was because MGL contained the terpenes and polyphenol phytochemicals that constituted a protective barrier on the metallic surface.

Silkworm sand is the dried particulate matter of silkworm feces. Generally, only a small part of it was applied in the pharmaceutical and agricultural industries, and most of it was disposed of as waste. The silkworm sand extract (Sse) mainly contains chlorophyll and a small amount of compounds such as pectin, lutein, carotene, phytol, and alkaloids [17]. The -OH and C=C in these compounds could interact with the metal, giving Sse the potential to act as a CI. Meanwhile, since I⁻ was beneficial for plant extracts to adsorb onto metals, Sse and KI were combined to enhance the inhibitory effect of Sse to QCS in the corrosion solution in the work.

Based on the concept of environmental protection, we extracted Sse. Further, using the weight loss test and electrochemical techniques, we intensely studied the influence of the synergetic effect of Sse and KI in mitigating QCS corrosion in a H₂SO₄ environment. The Fourier transform infrared (FTIR) and UV-Vis spectra were employed to conduct a comparative analysis on the molecular structures of SSE, chlorophyll, and QCS surface reaction products. The surface morphology and its major components of corrosion products were analyzed by scanning electron microscopy (SEM), atomic force microscopy (AFM), CA, and X-ray photoelectron spectroscopy (XPS). This study also explored the adsorption mechanism of Sse on the QCS surface via theoretical calculations. This would help to expand our understanding of the corrosion suppression effect of plant extracts on QCS and offer a reference for corrosion research on other metals.

2. Materials & Methods

2.1. Prepare materials and samples

The materials used in the experiment included mulberry silkworm excrement and Q235 steel [C (0.45%), Mn (0.5%), Si (0.18%), P (0.035%), S (0.035%), Cr (0.25%), Ni (0.30%), Cu (0.25%), and Fe (98%)]. The 0.5 M H₂SO₄ was made by diluting 98% H₂SO₄ with ultrapure water. KI (99% purity) was obtained from Zhiyuan Chemical Reagent Co., Ltd., Tianjin, China. KI solutions of different concentrations were prepared with ultrapure water as the synergistic corrosion inhibitor of Sse.

2.2. Extraction of Sse

Firstly, 90 g of mulberry silkworm excrement was weighed, then ground into powder with a crusher, and the mulberry silkworm excrement powder was soaked in 150 mL of absolute ethanol and acetone in a 1:1 volume ratio. The solution was placed in a water bath at 50°C and heated for reflux for 2 h. The residue was filtered, the supernatant was taken, and concentrated to 10 mL in a rotary evaporator for later use. The yield of extract (Sse) was approximately 2.0%.

2.3. Characterization of Sse

The radical groups of Sse were analyzed using FTIR (A Model VERTEX7 spectrometer, made in Germany) in 400 ∼ 4000 cm^-1. The UV absorption spectra of Sse in the range of 350-800 nm were determined by UV-VIS (UV-2550, China).

2.4. Weight loss experiment

A QCS with dimensions of 7.24 cm × 1.15 cm × 0.2 cm was used for the weightlessness experiment. QCS were soaked for 6 h in 500 mL 0.5 M H₂SO₄ containing Sse with various contents. Then, the QCS was cleaned, dried, and weighed using an electronic balance. In equation (1), W/g represented mass loss, T/h denoted corrosion time, and A/cm₂ indicated the QCS area. In equation (2), (X₀, X) represented the corrosion rates of the blank sample and the sample with added corrosion inhibitor, respectively, while η denoted the corrosion inhibition efficiency [18]:

(1)

X = \frac{W}{A \times T}

(2)

η = \frac{X_{0} - X}{X_{0}} \times 100 %

2.5. Electrochemical test

Electrochemical tests were undertaken utilizing the workstation Zennium Pro (ZAHNER potentiostat GmbH & Co. KG, Germany), and the traditional three-electrode system was adopted to make the test. The working electrode (WE) was a QCS electrode (1.0 cm₂ exposed area), the counter electrode was a platinum sheet (1.0 cm₂), and the reference electrode was a saturated calomel electrode. The WE was placed in the corrosive environments for 30 min to establish a stable open circuit potential (OCP). Measuring parameter: Scanning speed: 0.1 mV·s^-1, voltage range: ±100 mV, test temperature: 30°C. Electrochemical impedance spectroscopy (EIS) tests were conducted using sinusoidal voltages with a 10 mV amplitude and frequencies from 1 kHz ∼ 0.01 Hz. The ƞ was determined according to eq. (3)[18]:

(3)

η = \frac{R_{c t (i n h)} - R_{c t}}{R_{c t (i n h)}} \times 100 %

The R_ct(inh) (Ω·cm²) and R_c_t (Ω·cm²) correspond to the charge transfer resistance measured in systems containing and lacking CIs, respectively.

The Tafel test was recorded by scanning from ±250 mV versus OCP at 0.1 mV s^-1. The ƞ was determined according to eq. (4) :

(4)

η = \frac{i_{c o r r} - i_{c o r r (i n h)}}{i_{c o r r}} \times 100 %

The i_corr(inh) (mA·cm^-2) and i_corr (mA·cm^-2) correspond to the corrosion current density measured in systems containing and lacking CIs, respectively.

2.6. Surface analysis

FTIR, SEM (Dimension Icon, Bruker, Germany), AFM (MFP-3D Origin, Oxford, United Kingdom), CA (JC2000DM, China), and XPS (Axis Supra Shimadzu) were utilized to characterize the surficial morphology and the surficial elementary components of QCS after soaking in the corrosive solution containing CIs.

2.7. Theoretical calculation

To gain a theoretical understanding of how chlorophyll inhibits QCS corrosion in 0.5M H₂SO₄, quantum chemical calculations on chlorophyll molecules were performed using density functional theory (DFT) via the ORCA 4.0 software package [19]. The B3LYP functional, a Becke three-parameter hybrid exchange-correlation functional with Lee-Yang-Parr non-local correlation, was chosen as the exchange-correlation potential [20]. To account for intramolecular interactions, Grimme’s empirical dispersion with Becke-Johnson damping was applied [21]. The def-TZVP basis set, including split valence and triple zeta functions, was selected as the complete basis set [22]. The optimal geometry was determined by the vibration analysis with no imaginary frequency modes.

3. Results and Discussion

3.1. Weightlessness analysis

In Table 1, the data showed that when the test temperature was 30°C, and the concentrations of SSE and KI were 0.3-0.9 g·L^-1 and 0.03-0.2 g·L^-1, respectively, η increased with the rising concentration of the CIs. This might be because, as the concentration of CIs increased, the protective film’s surface coverage on the metallic surface increased. As shown in Table 1, Sse and KI both have certain corrosion suppression properties. To further improve the usage of Sse as a corrosive inhibitor in the metal protection field, this paper combined Sse and KI to improve the utilization rate of Sse through the synergistic corrosion suppression performance of Sse and KI. Because of the high expense of KI, its content should also be as low as possible [13]. The content of KI was selected as 0.06 g·L^-1 according to the data of corrosion suppression performance of a single KI under 30°C in this work and the data provided in literature [13]. From Table 1, η of Sse combined with KI was significantly higher than that of Sse alone. This was because KI enhanced the Sse adsorption on QCS, which effectively slowed down the QCS damage [13]. As the content of Sse increased, the η further improved. When the content of Sse reached 0.9 g L^-1, the η achieved 90.15%. Therefore, the corrosion suppression capability was optimized at a CI content of 0.9 g·L^-1Sse/0.06 g·L^-1 KI.

Table 1. Weightlessness test results of QCS soaked in 0.5 M H₂SO₄ at various contents of Sse and/or KI for 6 h.

Sample	C(g·L^-1)	X(g·(cm^-2∙h^-1))	η(%)
Blank	0	0.003531	—
Sse	0.3	0.002570	27.20
	0.4	0.002262	35.94
	0.6	0.001718	51.35
	0.8	0.001289	63.50
	0.9	0.001019	71.12
KI	0.03	0.002439	30.93
	0.06	0.002113	40.16
	0.1	0.00179	49.30
	0.2	0.001244	64.77
Sse + 0.06 g·L^-1KI	0.3	0.001188	66.35
	0.4	0.0009294	73.68
	0.6	0.0007115	79.85
	0.8	0.0005992	83.03
	0.9	0.0003478	90.15

Table 2 shown that the excellent corrosion suppression capability of Sse in the presence of KI could be attributed to the synergetic effect between them. So the synergistic parameter S could be calculated via eq. (5) to verify this synergy [23]:

(5)

S = \frac{1 - η_{A} - η_{B} + η_{A} η_{B}}{1 - η_{A B}}

Table 2. The Synergistic parameters of Sse at different concentrations with 0.06 g·L^-1 KI.

*C(g·L^-1)*	η _A	η _B	η *_AB*	S
0.3	0.2720	—	0.6635	1.2946
0.4	0.3594	—	0.7368	1.4564
0.6	0.5135	0.4016	0.7985	1.4448
0.8	0.6350	—	0.8303	1.2871
0.9	0.7112	—	0.9015	1.7545

Where, η_A and η_B were respectively the corrosion suppression efficiency of Sse and KI in 0.5M H₂SO₄. η_AB was the corrosion suppression efficiency of Sse and 0.06 g·L^-1 KI in 0.5M H₂SO₄. When S was greater than 1, Sse and KI showed a synergistic inhibition effect on reducing metal corrosion. S was less than 1, revealing that Sse and KI had an antagonistic effect on decreasing the QCS corrosion in a 0.5M H₂SO₄. From Table 2, the synergistic parameters used in the combination of Sse and KI were all greater than 1, indicating that Sse and KI had an excellent synergetic inhibitory impact on reducing the QCS corrosion in 0.5M H₂SO₄.

Figure 1(a) displays the correlation among CI content, temperature, and corrosion rate. Figure 1(b) presents the correlation among CI content, temperature, and corrosion suppression efficiency. Rising temperature accelerated corrosion and dropped corrosion suppression efficiency, suggesting higher temperatures hindered the adsorption of CI onto the QCS surface. At constant temperature, as Sse concentration grew, corrosion rate dropped, and corrosion suppression efficiency rose. This indicated that a greater content of Sse was more conducive to forming an adsorption film on QCS, resulting in effective corrosion reduction.

(a) Sse+0.06 g·L-1KI corrosion rate; (b) corrosion suppression efficiency in the 20∼50°C.

3.2. Sse/KI corrosion kinetics

Adsorption isotherms assess the adsorption capacity of CI on QCS surfaces, reflecting their corrosion suppression performance. The correlation coefficients (R²) of the fitted lines are all above 0.99 at various temperatures in Figure 2, indicating that Sse/KI adsorption on QCS followed the Langmuir isothermal adsorption. Equation (6) is the Langmuir equation,

(6)

\frac{C_{i n h}}{θ} = \frac{1}{K_{a d s}} + C_{i n h}

Fitting outcomes of C and C/θ for various temperatures.

C_inh(g·L^-1) was the content of CI. θ and K_ads were surface coverage (which could be replaced by ƞ) and adsorption equilibrium constant (L·g^-1), respectively. The K_ads were obtained from the intercept between C_inh/θ and C in Figure 2. According to K_ads and formula (7), the standard Gibbs free energy (ΔG0 ads) was obtained:

(7)

Δ G_{a d s}^{0} = - R T \ln (1 \times 10^{3} K_{a d s})

Where R and T were ideal gas coefficient and thermodynamic temperature respectively. The K_ads values calculated from eq. (7) were 11.03, 6.62, 4.20, and 3.13 L·g^-1, and the ΔG0 ads values were -22.68, -22.16, -21.71, and -21.61 kJ·mol^-1. The -ΔG0 ads showed that Sse/KI adsorption on the QCS surface occurred spontaneously. As ΔG0 ads was -20 kJ·mol^-1 ∼ -40 kJ·mol^-1, this suggested that the Sse/KI adsorption on QCS involved both physical adsorption and chemical adsorption processes [18].

To study the corrosion kinetics of QCS at varying Sse/KI concentrations, the impact of temperature on corrosion suppression performance was studied through weight loss data. Then, it was fitted, making use of the Arrhenius and transition state equations (8), (9) [24].

(8)

\log X = \frac{- E_{a}}{2.303 R T} + \log A

(9)

\log \frac{X}{T} = \log \frac{R}{N h} + \frac{Δ S_{a}}{2.303 R} - \frac{Δ H_{a}}{2.303 R T}

X was the corrosion rate, and A was the pre-factor. N, h, T, and R denote Avogadro’s constant, Planck’s constant, thermodynamic temperature, and the ideal gas constant, respectively. From equation fitting, activation energy (E_a), activation enthalpy (ΔH_a), and activation entropy (ΔS_a) were determined using equations (8) and (9). Figure 3(a) and (b) showed the outcomes of fitting the Arrhenius and transition state equations for QCS when soaked in varying contents of Sse/KI corrosion solvents. The values of E_a, ΔH_a, and ΔS_a obtained from Figure 3 were as follows: (1) The E_a values obtained from Figure 3 were 42.67, 67.98, 72.75, 72.85, 73.21, and 81.94 kJ·mol^-1. (2) The ΔH_a values were 40.36, 65.67, 70.45, 70.51, 70.90, and 79.63 kJ·mol^-1. (3) The ΔS_a values were -156.23, -88.99, -74.33, -72.85, -64.89, and -48.76 J·(mol·K⁻₁). It could be seen that the E_a in the blank 0.5 M H₂SO₄ was lower than that in the CI samples. This suggested that the presence of the CI elevated the activation energy of the corrosion process. Generally, a higher E_a made the corrosion reaction more difficult. Hence, adding Sse/KI effectively reduced QCS corrosion addition; the positive ΔH_a revealed that the QCS corrosion reaction was heat-absorbing. A rise in temperature promoted the corrosion on QCS. The -ΔS_a suggested a reduced degree of disorder in QCS corrosion.

Fitting outcomes of the Arrhenius equation and transition state equation.

3.3. Electrochemical analysis of Sse/KI composite system

Figure 4 shows the corresponding open circuit potential, EIS, equivalent circuit, and polarization curves (PC) of QCS in 0.5 M H₂SO₄ after adding Sse/KI. The OCP of various contents of CI has been depicted in Figure 4(a), and they basically reached a stable state at 1800s. The OCP of QCS was -0.506V in a blank 0.5 M H₂SO₄, and with the rise of Sse content in 0.5 M H₂SO₄, the OCP gradually shifted in the positive direction. This indicated that Sse/KI bonded to the QCS surface more quickly, enhancing surface stability and offering greater protection through boosting the surface coverage or creating a more effective protective film [25].

(a) Open circuit potential diagram; (b, c, d) EIS diagram; (e) corresponding equivalent circuit; (f) PC of QCS in 0.5 M H2SO4 at various contents of Sse/KI.

It could be seen from Figure 4(b) that QCS presented a small capacitance arc in a blank 0.5M H₂SO₄ solution, while the corresponding capacitance arc diameter grew with the content of Sse/KI added to the 0.5M H₂SO₄. After adding Sse/KI, Nyquist and Bode plots in Figure 4(b, c, d) remained a similar trend as in Figure 4(b). The narrow phase angle peaks in Figure 4(c) suggested only one time constant exists. So, the equivalent circuit in Figure 4(e) fitted the above EIS diagram. R_s, R_ct, and CPE_dl in the circuit represented solution resistance, charge transfer resistance, and double-layer capacitance. Equation (10) could calculate the impedance of constant phase element (CPE) (ZCPE).

(10)

Z_{C P E} = Y_{0}^{- 1} {(j ω)}^{- n}

Where, Y₀, j, and ѡ were CPE constant, imaginary part, and angular frequency, respectively. n was an indicator of surface heterogeneity. When n approaches 0 or 1, it exhibits resistive or capacitive characteristics. The fitting parameters have been shown in Table 3. The n values in Table 3 fell between 0.683 and 0.800, indicating that the non - uniformity of QCS corrosion was mainly charge - transfer - controlled [13]. The inhibitory efficiency of Sse/KI was calculated using equation (3). As could be seen from Table 3, the R_ct value of 0.9 g·L^-1 Sse/KI (281.85Ω·cm²) was much higher than that of blank 0.5M H₂SO₄ (21.66Ω·cm²). The outcomes demonstrated that Sse/KI had excellent corrosion suppression properties. The inhibition effect of Sse/KI on QCS was evaluated using the PC in Figure 4(f). The outcomes of PC fitting have been presented in Table 4. The inhibitory efficiency of Sse/KI inhibitors was calculated by equation (4). From Table 4, with the increase of Sse content, Tafel slope presented a large change, probably owing to the combined effect of adsorption of CI and I^- at the active site [26]. i0 corr (1.98 mA·cm⁻²) of QCS in blank 0.5 M H₂SO₄ was significantly higher than i0 corr (0.193 mA·cm⁻²) of QCS in the 0.9 g·L^-1 Sse/KI solution, indicating that QCS in blank 0.5 M H₂SO₄ was seriously corroded. Sse/KI inhibitors could effectively minimize QCS corrosion. Similar to the EIS and weightlessness method results, the corrosion suppression efficiency reached 90.25% with 0.9 g·L^-1 Sse/0.06 g·L^-1 KI.

Table 3. EIS fitting results corresponding to Figure 4 with Zahner analysis software.

C(g·L^-1)	*R_s*(Ω·cm²)	Y₀(10^-4·Sⁿ ·Ω·cm^-2)	*R_ct*(Ω·cm²)	n	*C_dlt*(uF·cm^-2)	Χ ²	η(%)
0	0.48	52.3	21.66	0.683	187.91	6.26×10^-3	——
0.3	050	17.7	58.60	0.8	100.4	3.07×10^-3	63.03
0.4	0.62	15.6	99.36	0.769	89.17	8.28×10^-3	78.20
0.6	0.78	10.3	106.05	0.782	55.41	6.89×10^-3	79.66
0.8	0.88	9.36	178.98	0.736	49.20	5.21×10^-3	87.89
0.9	0.89	7.39	281.85	0.752	44.04	8.69×10^-3	92.32

Table 4. Corresponding fitting results PC(f) in Figure 4.

Inhibitor	C (g·L^-1)	$β_{a}$ (mV·dec⁻¹)	$β_{c}$ (mV·dec⁻¹)	*i_corr* (mA·cm⁻²)	*E_cor*_r (mV)	η (%)
SSE/KI	0	0.482	0.715	1.98	-491	——
	0.3	0.403	0.242	0.673	-490	66.01
	0.4	0.337	0.146	0.553	-488	72.07
	0.6	0.222	0.142	0.435	-485	78.033
	0.8	0.218	0.141	0.274	-483	86.16
	0.9	0.214	0.104	0.193	-477	90.25

3.4. Characterizations of Sse, chlorophyll, and the surface of QCS

Figure 5(a) shows the FTIR spectra of Sse, chlorophyll, and the surface of QCS immersed in 0.5 M H₂SO₄. The study revealed that Sse and surfaces of QCS had very similar absorption peaks at 3403 cm^-1(3407 cm^-1), 2902 cm^-1(2902 cm^-1), 1660 cm^-1(1636 cm^-1), 1447cm^-1(1430 cm^-1), and 1061 cm^-1(1059 cm^-1). In the FTIR spectra of chlorophyll, absorption peaks appeared at 3404 cm^-1, 2926 cm^-1, 1636 cm^-1, 1438 cm^-1, and 1069 cm^-1, respectively. A broad band near 3425 cm⁻₁ (3401 cm⁻₁, 3404 cm⁻₁) indicated O−H stretching vibrations. The adsorption peak at 2926 cm⁻₁ (2901 cm⁻₁, 2926 cm⁻₁) represented to C−H stretching vibrations. The absorption peaks of 1374 cm^-1(1374 cm^-1, 1438 cm^-1) and 1638 cm^-1(1638 cm^-1, 1636 cm^-1) demonstrated the inclusion of bond groups such as C=O, C=C, C=N. The absorption peak at 1069 cm^-1(1059 cm^-1, 1069 cm^-1) demonstrated the inclusion of pyrrole rings. The outcome indicated that FTIR spectra of chlorophyll and Sse were basically the same, proving that the major composition of Sse was chlorophyll. And the major composition of Sse (chlorophyll) could form complexes on the QCS surface, which could minimize the QCS corrosion [27].

FTIR spectra of (a) Sse, chlorophyll, and QCS soaked in 0.5 M H2SO4 with Sse/KI; (b) UV-vis spectra of Sse and chlorophyll.

Figure 5(b) showed the ultraviolet-visible spectra (UV-vis) of chlorophyll and Sse. Chlorophyll and Sse had absorption peaks at 412 nm (413 nm) and 665 nm (664nm), respectively. The UV - vis spectra of chlorophyll and Sse showed an absorption peak at 412 nm (413 nm), resulting from the π - π* transition in the conjugate system (e.g., C = O, C = C, C = N). The absorption peak at 665 nm (664 nm) was a transition from the π orbital to the π* antibonding orbital in the conjugated system. Figure 5(b) demonstrated that the major composition of Sse was chlorophyll.

3.5. Surface topography of QCS

3.5.1. SEM analysis

The superficial morphology of the QCS was characterized by SEM. Figures 6(a₁-a₂) displayed the smooth, polished surface of the pristine sample. In contrast, Figures 6(b₁-b₂) showed that after soaking in 0.5 M H₂SO₄, the QCS exhibited severe surface corrosion, with numerous pits and corrosion products. Figures 6(c₁-c₂) was the SEM imagery of QCS after soaking in 0.5 M H₂SO₄ containing Sse. Compared with the QCS sample in the blank 0.5 M H₂SO₄, the QCS surface remained smooth, and there were some slight polishing scratches. As shown in Figures 6(d₁-d₂), the QCS surface soaked in 0.5 M H₂SO₄ containing Sse/KI was much smoother with minimal corrosion products, and significant mechanical polishing marks could be observed. Thus, Sse/KI significantly reduced QCS corrosion, aligning with weight loss and electrochemical results. These test results additionally reinforced the conclusion of the synergy effect from combining Sse/KI.

SEM micrographs of QCS samples: (a1-a2) polished surface, (b1-b2) after 6 h exposure to 0.5 M H2SO4, (c1-c2) with Sse addition and, (d1-d2) with Sse/KI mixture in 0.5 M H2SO4 (6 h).

3.5.2. AFM and CA analysis

Figure 7 showed the 3D topography images, corresponding surface roughness, and CA of QCS samples under 2D surface morphologies of QCS samples after polishing. The polished QCS surface was very smooth. The roughness (R_a) of the QCS surface (15×15μm²) was equal to 0.111 nm as shown in Figure 7(a₃). Figures 7(b1-b2) showed the 3D and 2D morphologies of QCS soaked in blank 0.5M H₂SO₄ for 6 h. The whole surface of the QCS sample was seriously corroded by sulfuric acid. The R_a value was 17.74 nm as shown in Figure 7(b₃). Figures 7(c₁-c₂) showed the three-dimensional and two-dimensional surface morphologies of the QCS sample after soaking for 6 h in 0.5 M H₂SO₄ medium containing Sse/KI. It could be clearly found that the mean surface roughness was 1.394 nm in Figure 7(c₃). In comparison with the blank 0.5 M H₂SO₄ medium, the height of the crest and trough decreased significantly. As a result, Sse/KI compounds could exhibit a high level of anti-corrosion properties.

AFM micrographs and CA diagrams of QCS: (a1-a4) polished; (b1-b4) soaked in blank 0.5M H2SO4 for 6 h; (c1-c4) after soaking in 0.5M H2SO4 containing Sse/KI for 6h.

Surface hydrophilicity/hydrophobicity measured by contact angle (CA) characterizes the interaction of adsorbed molecules and corrosive substances with metal interfaces. Figure 7(a₄) showed the CA after QCS polishing. Figures 7(b₄-c₄) showed the CA of QCS after soaking in 0.5 M H₂SO₄ with and without Sse/KI for 6 h. The CA value of the polished QCS sample was 89.0°, as shown in Figure 7(a₄). In a blank 0.5 M H₂SO₄ solution, the average CA value of QCS was 54.5°., as shown in Figure 7(b₄). In the presence of Sse/KI, the average CA value rose to 81.5°, as shown in Figure 7(c₄). A significant change in the CA showed that the protective film from CI adsorption on the QCS surface was less hydrophilic than the blank solution. This finding could be explained by the fact that the CIS and the adsorption of negatively charged iodide ions on the metal surface formed a protective film, which reduced the surface area of QCS exposed to a corrosive environment [28].

3.5.3. XPS analysis

Figure 8 showed the XPS spectra of QCS after soaking in 0.5 M H₂SO₄ containing Sse/KI for 6 h. Figure 8(a) showed that the treated QCS surface contained C, O, S, N, and Fe, and the added I^- content might be low, without an obvious characteristic peak. Based on this, the five elements Fe2p, O1s, S2p, C1s, and N1s were further analyzed, and the function groups or compounds on the QCS surface were obtained. The high-resolution spectra have been shown in Figures 8(b-f). Three peaks of Fe2p spectrum, Figure 8(b) were detected at 703.37 eV, 707.79 eV, and 722.20 eV, respectively. The characteristic peak of 703.37 eV was Fe 2p1/2, the characteristic peak of 707.79 eV was related to the oxide of Fe, and the characteristic peak of 722.20 eV corresponded to FeOOH [29]. In Figure 8(c), the three peaks at 534.79 eV, 533.15 eV, and 530.63 eV corresponded to O-C, O-H, and O-Fe, respectively. The existence of O-Fe demonstrated that Sse was adsorbed on the QCS surface [30]. As the S2P spectrum has been demonstrated in Figure 8(d), the three peaks of 166.86eV, 167.68eV, and 168.88eV corresponded to S-Fe, S-C, and SO₄^2-, respectively, indicating that molecules of corrosive solution existed on the QCS surface [31]. As the C1s spectrum was shown in Figure 8(e), the three characteristic peaks at 284.56 eV, 286.91 eV, and 288.77 eV corresponded to C-C, C-N, and C=O, respectively [32]. In the N1s spectrum, there was a significant peak at 399.37 eV caused by the presence of N-H/C-NH [33]. The C1s and N1s spectra confirmed that Sse acts as a protective layer on the QCS surface. In summary, XPS analysis of the surfaces of the test specimens further demonstrated that Sse effectively inhibited metal corrosion.

XPS spectra of QCS exposed to 0.5 M H2SO4 containing Sse/KI for 6 h: (a) Full-range spectrum; (b–f) High-resolution regions of Fe2p, O1s, S2p, C1s, and N1s, respectively.

3.6. Quantum chemical calculation

According to previous reports [34], combined with the FTIR and UV-vis spectra from this work revealed that Sse contains mainly chlorophyll. Figure 9 shows the structure and frontier molecular orbital density distribution of chlorophyll molecules. The highest occupied molecular orbital’s (HOMO) electron density was mainly focused on the porphyrin ring’s oxygen or the carbonyl group’s oxygen. The electron density of the lowest unoccupied molecular orbital (LUMO) was distributed on heterocyclic porphyrin rings. The quantum chemical parameters of chlorophyll were as follows: E_HOMO = -5.212 eV, E_LUMO = -2.841 eV, ΔE=2.371 eV, δ= 1.186 eV, χ = 4.027 eV, and ω = 1.253 eV. The E_HOMO and E_LUMO values represented the ability to give and receive electrons, respectively [27]. Generally, the smaller the energy gap value (∆E = E_LUMO-E_HOMO) was, the higher the inhibition effect of the inhibitor was [27]. According to Koopman’s theory, electron affinity (A) and ionization energy (I) equate to the negative E_LUMO and E_HOMO values, respectively. The absolute electronegativity (χ), absolute hardness (δ), and electron transfer ratio (ω) of the corrosion inhibitor molecules were derived from the following formula (11), (12) [13,27,35]:

(11)

χ = \frac{I + A}{2} δ = \frac{I - A}{2}

(12)

ω = \frac{χ_{F e} - χ_{i n h}}{2 (δ_{F e} + δ_{i n h})} ​ ​ ​ ​ ​ ​

Molecular structural formula, (a) chlorophyll, (b) HOMO and (c) LUMO orbitals of chlorophyll.

Original theoretical parameters for χ_Fe and δ_Fe were 7.0 eV and 0 eV, respectively [36]. The electron donation tendency was evaluated through the electron transfer ratio (ω). A higher ω value facilitated CI formation of coordination bonds with Fe atoms. When ω was less than 3.6, the larger ω was, the better the corrosion suppression effectiveness of the CI appeared [27]. This theoretically justified the ability of the CI to inhibit QCS corrosion in 0.5 M H₂SO₄.

Table 5 presents the comparative data of Sse extracted in this study and those from the literature. From the perspectives of the dosage of corrosion inhibitor and the corrosion suppression effectiveness, Sse and KI were better in the sulfuric acid system. Therefore, this study provided a new idea for using plant extracts as corrosion inhibitors and had a promising application prospect.

Table 5. Comparison of corrosion inhibitors in sulfuric acid solution.

Corrosion inhibitor	C_inh	Ƞ/%	Ref.
MALE	0.8 g·L^-1 +0.4 g·L^-1KI	95.02	[15]
EASSE	0.2 g·L-1 +0.1 g·L-1 KI	85.05	[14]
Sse	0.9 g·L-1 +0.06 g·L-1 KI	92.32	This work

3.7. Corrosion inhibition mechanism

The synergistic adsorption behavior and Sse/KI corrosion inhibition mechanism on QCS in 0.5 M H₂SO₄ were systematically investigated through combined experimental and theoretical approaches, with results presented in Figure 10. The polished QCS surface, when in contact with 0.5 M H₂SO₄, demonstrated characteristic electrochemical activity at both electrodes with observable gas generation Figure 10(a). According to the following reaction scheme, the anode iron released electrons, while the cathode protons acquired electrons. The electrode reactions are as shown in (13) and (14).

(13)

F e = F e^{2 +} + 2 e^{-}

(14)

2 H^{+} + 2 e^{-} = H_{2}

QCS corrosion inhibition mechanism in 0.5 M H2SO4, which contained Sse/KI.

The Sse adsorption on QCS is physisorption and chemisorption. Firstly, SO₄^2- adhered to the QCS surface and [Sse]⁺ from the corrosion solution are adsorbed on QCS by electrostatic attraction, forming a protective film. Chemisorption was facilitated by the coordination of Sse’s heteroatom lone pair electrons with Fe’s 3d empty orbital, Figure 10(c) [33,37]. The protective film formed on the QCS surface decreased the active sites on the QCS surface interacting with corrosive medium, so that the QCS corrosion in acidic solution was effectively reduced. In a word, in the compounded system of Sse and I^⁻, I^⁻ preferentially adsorbed onto the active sites on the metal surface (such as the anodic dissolution zone), occupying the corrosion reaction sites and simultaneously inhibiting the cathodic reaction of H⁺ discharge. Some I⁻ may also be slightly oxidized by the medium to form I₂, which, combined with the unoxidized I⁻, forms I₃⁻. The interaction between I⁻/I₃⁻ and the active components of Sse promoted the formation of a “physical adsorption film and chemical complex film” on the metal surface, which not only blocked the penetration of corrosive media but also further enhanced the adsorption stability [33,37]. Therefore, the original adsorption mechanism would be changed, and the synergistic corrosion inhibition of Sse and I^- showed superior corrosion suppression effectiveness in the presence of Sse and KI. To summarize, firstly, I^- adsorbed uniformly on the QCS surface, and then attracted more protonated Sse for adsorption, thereby ensuring a larger coverage area and higher inhibition efficiency.

4. Conclusions

Data from weight loss and electrochemical tests showed that when 0.9 g·L^-1 Sse and 0.06 g·L^-1 KI were added as corrosion inhibitors in a corrosive environment, η was as high as more than 90% due to the synergistic effect between Sse and I, compared to the blank sulfuric acid solution. Meanwhile, the Tafel curve was used to verify the corrosion suppression effect of Sse and KI as mixed CI.

Based on the outcomes of K_ads and ΔG_ads⁰, the adsorption of Sse and KI on QCS conformed to the Langmuir adsorption isotherm model. The adsorption manner was mixed physico-chemical adsorption. The SEM, AFM, and CA demonstrated that the 0.9 g·L^-1 Sse/0.06 g·L^-1 KI as CI had good synergetic corrosion suppression effectiveness. The XPS analysis showed that O, C, N, and S were detected on the QCS surface after the addition of 0.9 g·L^-1 Sse/0.06 g·L^-1 KI to 0.5 M H₂SO₄. The outcomes revealed that 0.9 g·L^-1 Sse/0.06 g·L^-1 KI formed an adsorption film on the QCS surface, thus inhibiting the corrosive behavior of QCS.

Theoretical calculation results revealed that chlorophyll, which was the major constituent of Sse, would be adsorbed on QCS to decrease the contact between corrosive medium and active sites on QCS, thus decreasing the metallic corrosion. In future research, we will further combine Sse with other green corrosion inhibitors to enhance the comprehensive performance of Sse through their synergistic effect.

Acknowledgment

This work was financially supported by Natural Science Foundation of Hebei Province of China (D2022105004); Discipline Construction Project of Tangshan Normal University (No. 20266128054); Discipline Construction Project of Tangshan Normal University (No. 20265128054); Hebei Provincial Higher Education Institution Science and Technology Research Project (ZC2022081).

CRediT authorship contribution statement

All authors have contributed to the work presented in this manuscript. Xinhua Liu: conceptualized the study, designed the experimental framework, and drafted the original manuscript. Siyu Liu: performed the synthesis of corrosion inhibitors/scale inhibitors and conducted material characterization experiments. Yuan Zhang, Baojing Luo, Xiaoyu Shi, Xiuge Wang, Ying Wang, Boxi Yang, Jiarui Du, Xinyuan Zhang, Xiaodan Liu, Taoyi Zheng: carried out water treatment performance tests and analyzed the experimental data, and provided revision of the manuscript for intellectual content. All authors have read and approved the final version of the manuscript and agree to be accountable for all aspects of the work.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript, and no images were manipulated using AI.

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Show Sections

[1] Gu T., Xu Z., Zheng X., Fu A., Zhang F., Al-Zaqri N., Chen J., Tan B., Li W.. Lycium barbarum leaf extract as biodegradable corrosion inhibitor for copper in sulfuric acid medium. Industrial Crops and Products. 2023;203:117181. https://doi.org/10.1016/j.indcrop.2023.117181
[Google Scholar]

[2] Mahdi B.S., Abbass M.K., Mohsin M.K., Al-Azzawi W.K., Hanoon M.M., Al-Kaabi M.H.H., Shaker L.M., Al-Amiery A.A., Isahak W.N.R.W., Kadhum A.A.H., Takriff M.S.. Corrosion inhibition of mild steel in hydrochloric acid environment using terephthaldehyde based on schiff base: Gravimetric, thermodynamic, and computational studies. Molecules (Basel, Switzerland). 2022;27:4857. https://doi.org/10.3390/molecules27154857
[Google Scholar]

[3] Luaibi H.M., Al-Tweel S.S.. Hybrid organic–inorganic corrosion inhibitors: bridging performance and eco-safety. AUIQ Technical Engineering Science. 2025;2:132-154. https://doi.org/10.70645/3078-3437.1038
[Google Scholar]

[4] Sun A., Cui G., Liu Q.. Capsule corrosion inhibitor loaded with hyperbranched chitosan: Carbon dioxide corrosion protection for downhole pipelines in oil fields. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2023;664:131106. https://doi.org/10.1016/j.colsurfa.2023.131106
[Google Scholar]

[5] Zhao Q., Guo J., Cui G., Han T., Wu Y.. Chitosan derivatives as green corrosion inhibitors for P110 steel in a carbon dioxide environment. Colloids and Surfaces. B, Biointerfaces,. 2020;194:111150. https://doi.org/10.1016/j.colsurfb.2020.111150
[Google Scholar]

[6] Zeng D., Liu Z., Yi Y., Han X., Dong B., Tong H., Yu Z., Tian G., Yu H.. Optimizing the performance of a composite corrosion/scale inhibitor for CO₂ flooding wells. Journal of Natural Gas Science and Engineering. 2022;102:104555. https://doi.org/10.1016/j.jngse.2022.104555
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[7] R Holla B., Mahesh R., Manjunath H.R., Anjanapura V.R.. Plant extracts as green corrosion inhibitors for different kinds of steel: A review. Heliyon. 2024;10:e33748. https://doi.org/10.1016/j.heliyon.2024.e33748
[Google Scholar]

[8] Koundal M., Singh A., . K., Sharma C.. Study on the effect of imidazolium ionic liquid as a modulator of corrosion inhibition of anionic surfactant sodium dodecyl sulfate (SDS) on mild steel in sodium chloride solution. Journal of Molecular Liquids. 2022;350:118561. https://doi.org/10.1016/j.molliq.2022.118561
[Google Scholar]

[9] Laaroussi H., Aouniti A., Mokhtari O., Hafez B., Sheikh R.A., Sameeh M.Y., Khowdiary M.M., Alderhami S.A., Elhenawy A.A., Azzouzi M.E., Rahhou I., Belbachir C., Hammouti B., Hadda T.B., Elmsellem H.. Experimental and theoretical investigations of Argania spinosa’s extracts on the antioxidant activity and mild steel corrosion’s inhibition in 1 M HCl. Applied Sciences. 2022;12:12641. https://doi.org/10.3390/app122412641
[Google Scholar]

[10] Alrefaee S.H., Rhee K.Y., Verma C., Quraishi M.A., Ebenso E.E.. Challenges and advantages of using plant extract as inhibitors in modern corrosion inhibition systems: Recent advancements. Journal of Molecular Liquids. 2021;321:114666. https://doi.org/10.1016/j.molliq.2020.114666
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[11] Sığırcık G., Yildirim D., Tüken T.. Synthesis and inhibitory effect of N,N’-bis(1-phenylethanol)ethylenediamine against steel corrosion in HCl Media. Corrosion Science. 2017;120:184-193. https://doi.org/10.1016/j.corsci.2017.03.003
[Google Scholar]

[12] Yang H., Deng S., Shao D., Lei R., Du G., Li X.. Eupatorium adenophorum Spreng leaves extract/potassium iodide as a highly sustainable inhibitor for the corrosion protection of steel in citric acid solution. Journal of Molecular Liquids. 2023;387:122614. https://doi.org/10.1016/j.molliq.2023.122614
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[13] Wan S., Zhang T., Chen H., Liao B., Guo X.. Kapok leaves extract and synergistic iodide as novel effective corrosion inhibitors for Q235 carbon steel in H₂SO₄ medium. Industrial Crops and Products. 2022;178:114649. https://doi.org/10.1016/j.indcrop.2022.114649
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Synergistic inhibition of Q235 steel (QCS) corrosion in H₂SO₄ by silkworm sand extract (Sse) and KI

Abstract

Keywords

KI

Q235 steel (QCS)

Sse (Silkworm sand extract)

Synergistic effect

1. Introduction

2. Materials & Methods

2.1. Prepare materials and samples

2.2. Extraction of Sse

2.3. Characterization of Sse

2.4. Weight loss experiment

2.5. Electrochemical test

2.6. Surface analysis

2.7. Theoretical calculation

3. Results and Discussion

3.1. Weightlessness analysis

3.2. Sse/KI corrosion kinetics

3.3. Electrochemical analysis of Sse/KI composite system

3.4. Characterizations of Sse, chlorophyll, and the surface of QCS

3.5. Surface topography of QCS

3.5.1. SEM analysis

3.5.2. AFM and CA analysis

3.5.3. XPS analysis

3.6. Quantum chemical calculation

3.7. Corrosion inhibition mechanism

4. Conclusions

Acknowledgment

CRediT authorship contribution statement

Declaration of competing interest

Declaration of generative AI and AI-assisted technologies in the writing process

References

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