Corrosion Inhibition of Honeycomb Waste Extracts for 304 Stainless Steel in Sulfuric Acid Solution

The extract of honeycomb waste was studied as a corrosion inhibitor on 304 stainless steel in H2SO4 solutions. The honeycomb waste was obtained from beekeeping at Lawang-Malang, East Java, Indonesia. Electrochemical and scanning electron microscopy methods were used to investigate the performance of the corrosion inhibition process. The inhibition efficiency of the inhibitor (2000 mg/L) reached 97.29% in 0.5 M H2SO4 and decreased with the acid concentration. Kinetic parameters were calculated to explain the effect of acid concentration on the inhibition process. The study on the adsorption behavior of the extracts followed the Frumkin isotherm model. The adsorption of the inhibitor on the 304 stainless steel surface was confirmed by the negative and lower values of Gibbs free energy. The obtained scanning electron microscopy (SEM) images were confirmed by comparing the surface of the specimens with and without inhibitor after corroding for one week. The results indicated that the extract acted as a good inhibitor for 304 stainless steel in acid corrosion.


Introduction
Honey industries generally extract honey by squeezing the honeycomb; at this point, the squeezed honeycomb is damaged and becomes an organic waste that has no market value. This honeycomb waste contains bees wax and honey sugars (i.e., fructose, glucose and sucrose). It has a refractive index of 1.4398-1.4451 at 75 • C, a melting point of 61-65 • C, an acid number of [17][18][19][20][21][22], an ester number of 70-90, and a saponication number of 87-102. Water and hydrocarbon content are less than 1% and lower than 14.5%, respectively [1]. However, the honeycomb waste still contains relatively high flavonoids since a small amount of the honey remains after the squeezing process [2]. It is reported that the organic flavonoids structures have an electronegative atom, conjugated double bonds or aromatic rings that can be exploited as a corrosion inhibitor [3][4][5][6]. Therefore, in this work, we evaluated the potential of honeycomb waste as a corrosion inhibitor for structural steel materials.
Because of its formability and corrosion resistance, 304 SS is one of the materials which is widely applied in industries [7][8][9]. The corrosion resistance property is due to the protective film of chromium oxyhydroxide [10]. However, particularly in acid solutions, 304 SS is very susceptible to corrosion due to the breakage of the passive film. The most effective and economical measure to overcome IE (%) = I corr − I corr(i) I corr × 100 (1) where I corr and I corr(i) represented the corrosion current density with and without inhibitor, respectively. Electrochemical impedance spectroscopy (EIS) was performed at open-circuit potential (OCP) in the frequency range of 1000 to 0.1 Hz using the signal amplitude of 15 mA. IE% from the EIS method was calculated using the following equation (Equation (2)): where R ct and R ct(i) were the charge transfer resistance with and without inhibitor, respectively.

Surface Analysis
The specimens were immersed in 1.0 M, 1.5 M and 2.0 M H 2 SO 4 solution with and without inhibitor (2000 mg/L) for one week at ambient temperature. Then, the specimens were washed with demineralized water and dried at room temperature. The morphological structure of the 304 SS surface was observed using a scanning electron microscope (SEM; FEI Inspect S-50, Tokyo, Japan). Before observation, all specimens were sputtered with a 10 nm layer of gold. where Icorr and Icorr(i) represented the corrosion current density with and without inhibitor, respectively.

Characterization of Honeycomb Waste Extract
Electrochemical impedance spectroscopy (EIS) was performed at open-circuit potential (OCP) in the frequency range of 1000 to 0.1 Hz using the signal amplitude of 15 mA. IE% from the EIS method was calculated using the following equation (Equation (2)): where Rct and Rct(i) were the charge transfer resistance with and without inhibitor, respectively.

Surface Analysis
The specimens were immersed in 1.0 M, 1.5 M and 2.0 M H2SO4 solution with and without inhibitor (2000 mg/L) for one week at ambient temperature. Then, the specimens were washed with demineralized water and dried at room temperature. The morphological structure of the 304 SS surface was observed using a scanning electron microscope (SEM; FEI Inspect S-50, Tokyo, Japan). Before observation, all specimens were sputtered with a 10 nm layer of gold.  Chromatograph of the extract displays nine peaks, which indicates nine different compounds ( Figure 2). A major compound is found at a retention time of 3.59 min (peak 3) with an area of 43.90%, which is identified as quercetin ( Figure 3). Identification of the other peaks gives the following results: luteolin, 1; vitexin, 2; fisetin, 4; isohamnetin, 5; isoferulic, 6; apigenin, 7; pinobanksin, 8; and kaempferol, 9. The detailed analysis of these compounds is summarized in Table 1.  Chromatograph of the extract displays nine peaks, which indicates nine different compounds ( Figure 2). A major compound is found at a retention time of 3.59 min (peak 3) with an area of 43.90%, which is identified as quercetin ( Figure 3). Identification of the other peaks gives the following results: luteolin, 1; vitexin, 2; fisetin, 4; isohamnetin, 5; isoferulic, 6; apigenin, 7; pinobanksin, 8; and kaempferol, 9. The detailed analysis of these compounds is summarized in Table 1. The active sites of the main compound in the extract can interact with the vacant d orbital of Fe and form a thin protective layer [16,17]. The presence of an inhibitor gives two specific adsorbed intermediates to determine the anodic dissolution of Fe by the following mechanism:

Characterization of Honeycomb Waste Extract
FeM ↔ FeM + e (3e) where M represents the inhibitor species.  The active sites of the main compound in the extract can interact with the vacant d orbital of Fe and form a thin protective layer [16,17]. The presence of an inhibitor gives two specific adsorbed intermediates to determine the anodic dissolution of Fe by the following mechanism: where M represents the inhibitor species.   The active sites of the main compound in the extract can interact with the vacant d orbital of Fe and form a thin protective layer [16,17]. The presence of an inhibitor gives two specific adsorbed intermediates to determine the anodic dissolution of Fe by the following mechanism: where M represents the inhibitor species.
There are some active sites on the surface of corroded metal that are able to absorb activation energy. In this case, the inhibitors molecule can be adsorbed easily on the active sites of the surface with matched adsorption enthalpies. According to Equation (3a-g), water molecules on the metal surface are replaced to yield the adsorption of intermediate FeM ads by inhibitor species (Equation (3e)). This reduces the amount of the species FeOH ads which causes retardation of Fe anodic dissolution [16,18,19].

Potentiodynamic Polarization
The  [20]. This is also confirmed by the value of the anodic current density, which is lower than the cathodic. energy. In this case, the inhibitors molecule can be adsorbed easily on the active sites of the surface with matched adsorption enthalpies. According to Equation (3a-g), water molecules on the metal surface are replaced to yield the adsorption of intermediate FeMads by inhibitor species (Equation (3e)). This reduces the amount of the species FeOHads which causes retardation of Fe anodic dissolution [16,18,19].

Potentiodynamic Polarization
The Ecorr of 304 SS shifts to a higher potential with a similar pattern in all acid concentrations in comparison with the blank. The 2000 mg/L inhibitor gives the maximum shift difference (Figure 4) with the shift to the blank, which are 28.5 mV, 98.0 mV, 115.0 mV and 104 mV in 0.5 M, 1.0 M, 1.5 M and 2.0 M H2SO4 solutions, respectively. It confirms that the inhibitor acts as an anodic inhibitor in 1,0 M, 1.5 M and 2,0 M H2SO4 solutions (the displacement of the Ecorr is more than 85.0 mV) but in 0.5 M the inhibitor acts as a mixed inhibitor [20]. This is also confirmed by the value of the anodic current density, which is lower than the cathodic.
The maximum concentration that gives the highest number in IE% is 2000 mg/L ( Table 2). The corrosion rate of 304 SS decreases with an inhibitor of 1000-2000 mg/L and increases with a 3000-4000 mg/L inhibitor. It can be attributed to the adsorption behavior of the inhibitor on 304 SS/acid solution interface [9,21]. The increase of inhibitor concentration beyond 2000 mg/L leads to diminished corrosion protection. This may be caused by the withdrawal of inhibitor back into the bulk solution when the inhibitor concentration is close to or beyond the critical concentration. This leads to a weakening of the metal-inhibitor interactions and causes the replacement of the inhibitor with water or SO4 2− , reducing its IE% [21].   The maximum concentration that gives the highest number in IE% is 2000 mg/L ( Table 2). The corrosion rate of 304 SS decreases with an inhibitor of 1000-2000 mg/L and increases with a 3000-4000 mg/L inhibitor. It can be attributed to the adsorption behavior of the inhibitor on 304 SS/acid solution interface [9,21]. The increase of inhibitor concentration beyond 2000 mg/L leads to diminished corrosion protection. This may be caused by the withdrawal of inhibitor back into the bulk solution when the inhibitor concentration is close to or beyond the critical concentration. This leads to a weakening of the metal-inhibitor interactions and causes the replacement of the inhibitor with water or SO 4 2− , reducing its IE% [21].

Electrochemical Impedance Spectroscopy (EIS) Measurement
The performance of honeycomb waste extract as an inhibitor at corroding 304 SS in sulfuric acid was also studied by electrochemical impedance spectroscopy (EIS). Figure 5 shows the Nyquist plots of 304 SS with and without inhibitor. According to Figure 5, there is a semicircle at high frequencies and a straight line at low frequencies. High frequency semicircles are generally associated with the charge transfer at the electrode/electrolyte interface, such as an electrical double layer. A straight line at low frequencies indicate Warburg impedance (W) [22]. W attributed to the anodic diffusion process of oxygen transport from the bulk solution to the electrode surface [23]. Figure 6a,b are equivalent circuits used for the EIS spectra for 304 SS in both the absence and the presence of inhibitor, respectively. Figure 6a is the standard equivalent circuit with 4 parameters, that is, R s , R ct , CPE and W. R s represents the solution resistance, R ct is correlated with the charge transfer resistance of metal, CPE is the constant phase element and W is the Warburg impedance. A more complicated equivalent circuit occurred with the addition of the inhibitor (Figure 6b). According to Figure 6b, there are two parts of equivalent circuit when the first part (R s , R ct and CPE) is the standard and the second part (R 1 , R 2 , C 1 and C 2 ) is the additional circuit. The first part indicates that the inhibitor attaches to the metal surface, which is shown by the increasing in the value of R ct . The R ct increases along with the increased inhibitor concentration which is up to 2000 mg/L; a further increase of the concentration leads to a decrease in the value of R ct ( Table 3). The R ct is inversely proportional to the corrosion rate and related to IE% [21,23]. The higher R ct , the lower the 304 SS corrosion rate. Therefore, the inhibition process is more efficient. It is in line with the result of potentiodynamic polarization. Moreover, additional parameters appeared in the second part of the equivalent circuit that give more information about the effect of the inhibitor. These parameters are related to the formation of the other passive films on the metal surface [24]. It is probably due to the reaction between the inhibitor and the metal surface. The fitting results for electrochemical impedance spectroscopy (EIS) data for 304 SS in several H 2 SO 4 concentration with and without inhibitor was summarized in Table 3.
The Bode plots consist of a one loop capacitive (Figure 7a-d). It indicates that the inhibitor is adsorbed on the 304 SS surface by the gradual replacement of water molecules and ions.       The Bode plots consist of a one loop capacitive (Figure 7a−d). It indicates that the inhibitor is adsorbed on the 304 SS surface by the gradual replacement of water molecules and ions. Figure 7 shows that increasing inhibitor concentration up to 2000 mg/L results in a more negative value of the phase angle. It showed that there is greater surface coverage and transfer charge resistance [25][26][27].

The Effect of Acid Concentration
The relationship of the corrosion rate (C R ) against acid concentration (C) obeys the kinetic equation: where k is the rate constant and B is the reaction constant. The relationship lnC R versus C gives straight lines as shown in Figure 8. The slopes and intercept of these lines represent the B constant and lnk, respectively.
The relationship of the corrosion rate (CR) against acid concentration (C) obeys the kinetic equation: where k is the rate constant and B is the reaction constant. The relationship lnCR versus C gives straight lines as shown in Figure 8. The slopes and intercept of these lines represent the B constant and lnk, respectively. k can be deemed a commencing rate at zero acid concentration. Therefore, k means the corrosion ability of acid for metal. B represents the difference of the corrosion rate at the acid concentration. The decreased k with inhibitor (Table 4) means that the extracts inhibit the corrosion process of 304 SS [28]. The decreased B with inhibitor (Table 4) indicates that the changes of the corrosion rate in inhibited acid are larger than in uninhibited acid [28].

Adsorption Isotherm and Thermodynamic Calculations
The corresponding plots of the adsorption isotherm are shown in Figure 9. These are the Langmuir (Figure 9a-d), Freundlich (Figure 9e-h), Temkin (Figure 9i-l) and Frumkin isotherms (Figure 9m-p). The best fit is shown by R−square (Table 5), for this study follows the Frumkin isotherm (Equation (5)).
Frumkin equation: where C is the inhibitor concentration, θ is the surface coverage, Kads is the adsorption equilibrium constant and a is an interaction parameter. The value of a indicates attraction or repulsion between adsorbed species for a > 0 and a < 0, respectively. When a = 0, it means no interaction and it becomes equivalent to the Langmuir isotherm [29]. The Frumkin isotherm considers lateral interactions between adsorbed inhibitor molecules indicating that the inhibitor displaces the water molecules from the 304 SS surface [30]. The interaction parameters were calculated from the slope of Figure 9m-p. The positive value of a (Table 6) indicates highly attractive lateral interactions in the adsorbed layer. k can be deemed a commencing rate at zero acid concentration. Therefore, k means the corrosion ability of acid for metal. B represents the difference of the corrosion rate at the acid concentration. The decreased k with inhibitor (Table 4) means that the extracts inhibit the corrosion process of 304 SS [28]. The decreased B with inhibitor (Table 4) indicates that the changes of the corrosion rate in inhibited acid are larger than in uninhibited acid [28].

Adsorption Isotherm and Thermodynamic Calculations
The corresponding plots of the adsorption isotherm are shown in Figure 9. These are the Langmuir (Figure 9a The best fit is shown by R-square (Table 5), for this study follows the Frumkin isotherm (Equation (5)).
Frumkin equation: where C is the inhibitor concentration, θ is the surface coverage, K ads is the adsorption equilibrium constant and a is an interaction parameter. The value of a indicates attraction or repulsion between adsorbed species for a > 0 and a < 0, respectively. When a = 0, it means no interaction and it becomes equivalent to the Langmuir isotherm [29]. The Frumkin isotherm considers lateral interactions between adsorbed inhibitor molecules indicating that the inhibitor displaces the water molecules from the 304 SS surface [30]. The interaction parameters were calculated from the slope of Figure 9m-p. The positive value of a (Table 6) indicates highly attractive lateral interactions in the adsorbed layer. The increase of the inhibitor concentration probably induces desorption of the inhibitor, which is already adsorbed at the 304 SS surface and then it dissolves into solution. It makes the interactions stronger between the inhibitor in the solution and leads to secondary desorption [29,30]. The increase of the inhibitor concentration probably induces desorption of the inhibitor, which is already adsorbed at the 304 SS surface and then it dissolves into solution. It makes the interactions stronger between the inhibitor in the solution and leads to secondary desorption [29,30]. Kads is related to the Gibbs free energy of adsorption (∆ °) by the Equation (6) [14].
where A is the concentration of water (55.5 in M or 1000 in g/L), T is the absolute temperature and R is the universal gas constant. The thermodynamic parameters are summarized in Table 6. The negative and lower values of ∆ ° indicate the inhibition process of the inhibitor on the 304 SS surface, which is spontaneous, and physisorption [13,31]. The more negative value follows the order 0.5 M H2SO4 > 1.0 M H2SO4 > 1.5 M H2SO4 > 2.0 M H2SO4.
The heat of adsorption (∆ °) is calculated using the Van't Hoff equation: The entropy of adsorption (∆ °) can be obtained using Equation (8).
The negative value of ∆ ° confirms the exothermic nature of the metal dissolution process with the inhibitor [31]. The positive value of ∆ ° means the adsorption process is accompanied by an increase in entropy [18].    (6) [14].
where A is the concentration of water (55.5 in M or 1000 in g/L), T is the absolute temperature and R is the universal gas constant. The thermodynamic parameters are summarized in Table 6. The negative and lower values of ∆G • ads indicate the inhibition process of the inhibitor on the 304 SS surface, which is spontaneous, and physisorption [13,31]. The more negative value follows the order 0.
The entropy of adsorption (∆S • ads ) can be obtained using Equation (8).
∆G ads = ∆H The negative value of ∆H • ads confirms the exothermic nature of the metal dissolution process with the inhibitor [31]. The positive value of ∆S • ads means the adsorption process is accompanied by an increase in entropy [18].

Surface Analysis
In order to provide physical evidence, SEM analysis was conducted. The impact of the inhibitor on the microstructure of the top surface of 304 SS is depicted in Figure 10. The surface of 304 SS is relatively smooth before the corrosion process ( Figure 10a). After being corroded in acid solutions in the absence of the inhibitor, some cracks and pits appear on the 304 SS surface ( Figure 10b); meanwhile, in the presence of the inhibitor, there is less damage (Figure 10c-f). It shows that the inhibitor works well at protecting against corrosion.

Conclusions
The honeycomb waste extracts can be used as an effective inhibitor for 304 SS corrosion in H2SO4 solutions. The FTIR spectrum indicates that the extract has several functional groups which are typical for a flavonoid compound. Furthermore, LC−MS analysis shows that the main compound of the extract was quercetin. The IE% maximum in this study was obtained by adding 2000 mg/L inhibitors in 0.5 M H2SO4, that is, 97.29% using potentiodynamic polarization. The extracts adsorption behavior of the extracts follow the Frumkin isotherm model. The corrosion inhibition performance of the extracts on the surface of 304 SS decreases with acid concentration.

Conclusions
The honeycomb waste extracts can be used as an effective inhibitor for 304 SS corrosion in H 2 SO 4 solutions. The FTIR spectrum indicates that the extract has several functional groups which are typical for a flavonoid compound. Furthermore, LC-MS analysis shows that the main compound of the extract was quercetin. The IE% maximum in this study was obtained by adding 2000 mg/L inhibitors in 0.5 M H 2 SO 4 , that is, 97.29% using potentiodynamic polarization. The extracts adsorption behavior of the extracts follow the Frumkin isotherm model. The corrosion inhibition performance of the extracts on the surface of 304 SS decreases with acid concentration.