Triton X-100 as a Non-Ionic Surfactant for Corrosion Inhibition of Mild Steel During Acid Cleaning

Abstract

Triton-X 100 (Octylphenolpoly(ethyleneglycolether)9–10), was investigated as corrosion inhibitor during the acid cleaning for mild steel in 5% HCl (~ 0.0137 M) by gravimetric and electrochemical polarization methods in the presence of different concentrations of Triton-X 100 of 5, 10, 20 and occasionally by 50 ppm at different of temperatures of 30, 45, and 60 °C. The gravimetric results showed maximum inhibition efficiency of ~ 77% when 50 ppm of Triton-X 100 used at 30 °C, while the potentiodynamic polarization measurements showed a maximum inhibition efficiency of ~ 63% when 20 ppm were used. The adsorption isotherms followed the Langmuir isotherm. The adsorption of TX-100 is a spontaneous process; accompanied by a decrease in entropy upon increasing the concentration of TX-100. Electrochemical polarization measurements support the finding of weight loss data. The results from this work can be useful to relevant industrial sectors of using this surfactant as corrosion inhibitor during acid cleaning.

Graphic Abstract

Introduction

Corrosion has caused huge damage to many metallic industrial constructions such as boilers, distillers, pipelines, and storage tanks. The damage is not only physical, but it is also due to the time taken to replace corroded structure units but also due to loss in production during shutting down or maintenance of these production units. The cost of corrosion has been reported from many studies to be in the order of 1%–5% of GNP for any country [1].

Steel is considered the most common constituent in metallic constructions and in industrial applications; therefore, intensive studies related to steel and how to protect steel in different environments were reported after being aware of the damage caused by corrosion. During cleaning or pickling of metallic structures, mineral acids are of the first choice over organic acid where it accomplishes the effective task of cleaning in a shorter time resulted in scale removal or other inorganic/organic deposits. However, the use of organic acids such as formic, citric and others might be preferred since they are less aggressive compared to mineral acids so that no deterioration of metallic structures. The need to reduce the maintenance time and the time of shutting down during this cleaning, mineral acids are used while adding inhibitors as recommended practice to protect different steels while removing scales or deposits. Most of these corrosion inhibitors are of organic compounds where they take their action owing to the presence of elemental nitrogen, oxygen, sulfur or other organic compounds.

Most of these inhibitors used in aqueous heating and cooling systems are synthetic chemicals which are expensive and of very hazardous to both human and surrounding environment, therefore; the need to alternative of lower costs and nontoxic properties. As alternative, surfactants have been used as corrosion inhibitors of metals and alloys to substitute organic inhibitors where they demonstrated high inhibition efficiency, low price and low toxicity [2,3,4]. Different types of surfactants have been reported as corrosion inhibitors studied extensively. These have included cationic [5,6,7,8,9,10], anionic [11,12,13] and non-ionic surfactants [14,15,16,17,18,19,20] which have been reported to mitigate the corrosion of steel in acidic media.

Triton-X-100 inhibited ferritic stainless steel at elevated temperatures in sulfuric acid [17] and to enhance the inhibition efficiency of L-Cyctine for mild steel in HCl [21]. Amin et al. [18, 19, 22] studied the effect of Triton-X series such as Triton-X 100, Triton-X 165 and Triton-X 305 as corrosion inhibitors to lessen the corrosion of iron in 1 M HCl.

The aim of the present study is to evaluate the inhibition properties of different concentrations of Triton X-100 in 5% HCl. This included investigation at moderate temperatures and at different cleaning time. We approached also how this inhibition took its action.

Experimental

Inhibitor

The inhibitor investigated in this work was Octylphenolpoly(ethyleneglycolether)9–10 which is available commercially under the name of Triton-X 100 (TX-100) and it has the molecular formula C34H62O11 for x = 9–10. TX-100 obtained from Sigma Aldrich and it was used after making a stock solution, from this stock solution, the desired concentration is used by further dilution. The chemical structure of (TX-100) is given below in Fig. 1.

Fig. 1
figure1

Chemical structure of Triton-X 100

Acid Cleaning Solution

The acid cleaning solution composed of 5% (v/v) HCl (~ 0.0137 M) prepared by dilution of analytical grade 32% HCl (Fluka) with distilled water. The concentration range of TX-100 inhibitor used was 5, 10, and 20 and was occasionally 50 mg l−1 (ppm).

Weight Loss Measurements

Two parallel coupons of the steel of 4.8 cm x 4.8 cm with total surface area of 23.04 cm2 were polished with emery paper to the finest grade (down to 1200 grit) and then washed with distilled water and thereafter degreased with acetone. After weighing precisely, the coupons were immersed in 200 ml closed glass container that contained the desired concentration of TX-100 in the electrolytic solution of 5% (v/v) HCl (~ 0.0137 M) and compared to that without TX-100. All experiments were thermostated at the desired temperature of 30°C, 45°C and 60°C. All thermostated temperatures studied having a precision of ± 1°C. After a specific time of 1, 2, 3, 4 and 5 h, the specimens were removed, washed, dried and then reweighed accurately.

Potentiodynamic Polarization Measurements

Electrochemical polarization experiments were carried out in a conventional three-electrode cell with a platinum sheet auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode. The working electrodes for polarization measurements were sealed with epoxy resin so that the cross section area exposed to an electrolytic solution has a total apparent area of 2 cm2. Before sealing the electrodes, each electrode was polished on a series of emery paper down to 1200 grit. Before running the experiments, the electrodes washed thoroughly with acetone followed by distilled water, and finally washed just prior to immersion in 5% (v/v) HCl (~ 0.0137 M) containing the desired concentration of inhibitor. All experiments were thermostated at the desired temperature at 30, 45 and 60°C. All thermostated temperatures studied having a precision of ± 1°C.

The potentiodynamic polarization measurements were recorded using PS6 Meinsberger potentiostat galvanostat (Germany) system by using the conventional three-electrode cell described above. The scan rate was 1 mV s−1 in all experiments.

Results and Discussion

Weight Loss

Figure 2 shows the weight loss of mild steel in 5% HCl (~ 0.0137 M), where specimens immersed in the presence and in the absence of TX-100 (concentrations of 0, 5, 10, 20 and 50 ppm) at temperature of 30 °C (a), 45 °C (b) and 60 °C (c). A close comparison among the curves of each group showed that the weight loss decreased substantially after the addition of TX-100 compared to the control (blank). Simultaneously, the weight loss was inhibited maximally at temperatures 45 °C and 30 °C compared to weight loss inhibition at 60 °C, the data confidence was excellent since we obtained lines of regressions of minimum of at least 0.99. Increasing the concentration of TX-100 in 5% HCl (~ 0.0137 M) lead to more weight loss inhibition at all studied temperature.

Fig. 2
figure2

Weight loss of mild steel coupons during acid wash in 5% HCl at 30 °C, 45 °C, and at 60 °C at different concentrations of TX-100 inhibitor of (filled square) 0 ppm, (open square) 5 ppm, 10 ppm (circle), 20 ppm (triangle) and 50 ppm (inverted triangle)

From the weight loss data; surface coverage, inhibition efficiency and corrosion rates were calculated respectively according to Eqs. (1), (2) and (3) [23,24,25]:

$$\theta = \frac{{W_{o} - W}}{{W_{o} }}$$
(1)
$$IE\left( \% \right) = \frac{{W_{o} - W}}{{W_{o} }} \times 100$$
(2)
$$CR\left( {mpy} \right) = \frac{3445.15W}{A.d.t}$$
(3)

where θ is the degree of surface coverage, Wo and W are the weight loss in the absence and in the presence of TX-100, respectively in mg, IE is the inhibition efficiency in %, CR is the corrosion rate in mpy, A is the total area of the specimen in cm2, t is the corrosion time in hours and d the specimen density (g cm−3); of 7.85 g cm−3 for mild steel. From the weight loss data, surface coverage, inhibition efficiency and corrosion rate were summarized in Table 1, the inhibition efficiency at 30 °C, 45 °C, and 60 °C were represented in Fig. 3.

Table 1 Effect of TX-100 concentration on mild steel corrosion in 5% HCl at different temperatures. Degree of surface coverage (Ɵ), inhibition efficiency (IE), and corrosion rate (CR) were calculated based on weight loss experiment after 1 h of immersion
Fig. 3
figure3

Inhibition efficiency of different concentrations of TX-100 of 5 ppm (square), 10 ppm (circle), 20 ppm (triangle) and 50 ppm (inverted triangle) in 5% HCl at different temperatures

From Table 1, it is clear that the maximum efficiency after an hour was 77% for the 50 ppm of TX-100 used at 30 °C. This inhibition efficiency is better than that reported of around 73% for 0.1 M for carbon steel in 0.1 N HCl HCl [26]. The adsorption of TX-100 at the metal–solution interface may be represented as a substitution process by the following equation:

$${\text{TX}} - 100_{\text{sol}} + {\text{nH}}_{2} {\text{O}}_{\text{ads}} \to {\text{TX}} - 100_{\text{ads}} + {\text{nH}}_{2} {\text{O}}_{\text{sol}}$$
(4)

where TX-100sol and TX-100ads are TX-100 adsorbate in the solution and adsorbate adsorbed on the metal surface (mild steel in our case), respectively where (n) is the number of water molecules substituted from the metal surface by the TX-100 adsorbate.

The degree of surface coverage (θ) at different temperatures of the inhibitor in the solution has been concluded from the weight loss method. In the present study various adsorption isotherms were considered, namely Langmuir [23, 27], Frumkin [28, 29], Temkin [30, 31], Freundlich [32, 33] and ElAwady [34, 35]. Nonetheless, only Langmuir isotherm was the valid tested isotherm (vide infra).

Langmuir adsorption isotherm may be presented by Eq. (5), and its modified form by rearranging Eq. (5) as represented by Eq. (6) [36, 37],

$$\theta = \frac{{K_{ads} C}}{{1 + K_{ads} C}}$$
(5)
$$\frac{C}{\theta } = \frac{1}{{K_{ads} }} + C$$
(6)

where θ is the degree of surface coverage, C the molar concentration of TX-100 in the bulk solution and Kads is the equilibrium constant of adsorption. When plotting C/θ versus C, a linear fit of unity appeared to all times of immersion and at all studied temperatures. This linearity confirmed the validity of this isotherm (Fig. 4). Langmuir isotherm assumes that: There is a fixed number of adsorption sites on the metallic surface and that each site has only one adsorbate; is the same for all sites and it is independent of surface coverage; there is no interaction between adsorbates, i.e. there is no effect of lateral interaction of the adsorbates on \(\Delta {\text{G}}_{\text{ads}}^{\text{o}}\) [38].

Fig. 4
figure4

Langmuir adsorption isotherm at 30, 45, and 60 °C containing different concentrations of TX-100 (0, 5, 10, and 20 ppm) in 5% HCl on mild steel at different time intervals of immersion, of 60 min. (square), of 120 min. (circle), of 180 min. (triangle), of 240 min. (inverted triangle), and of 300 min. (half filled circle). Data extracted from weight loss

The value of Kads indicated a strong adsorption of surfactant species on the surface of mild steel and then \(\Delta {\text{G}}_{\text{ads}}^{\text{o}}\) is obtained from Eqs. (7), and (9) [6, 39]:

The Kads values were estimated from Fig. 4 by taking the reciprocal of the slope, the Kads found to be 0.774, 0.770 and 0.798 at temperatures of 30, 45, and 60 °C respectively.

One point is worth mentioning here that the Kads was at its maximum at the beginning of immersion; then it is decreased when the time of immersion increased and the average value was taken for all time of immersion used in this work.

The importance of the equilibrium constant of adsorption Kads obtained from Eq. (6), as a reciprocal of the intercept from linear equation, is that it is used to get the values of activation of entropy, enthalpy of adsorption and entropy of adsorption as described below by Eqs. (9), (11).

$$\Delta {\text{G}}_{\text{ads}}^{\text{o}} = - {\text{RTln}}\left( {55.5 {\text{K}}_{\text{ads}} } \right)$$
(7)
$$\because \;\;\Delta {\text{G}}_{\text{ads}}^{\text{o}} = \Delta {\text{H}}_{\text{ads}}^{\text{o}} - {\text{T}}\Delta {\text{S}}_{\text{ads}}^{\text{o}}$$
(8)
$$\ln K_{{ads}} = \ln \left( {\frac{1}{{55.5}}} \right) - \frac{{\Delta G_{{ads}}^{o} }}{{RT}}$$
(9)

then we get

$${\text{lnK}}_{\text{ads}} = { \ln }\left( {\frac{1}{55.5}} \right) - \frac{{\Delta {\text{H}}_{\text{ads}}^{\text{o}} }}{\text{RT}} + \frac{{\Delta {\text{S}}_{\text{ads}}^{\text{o}} }}{\text{R}}$$
(10)
$${\text{lnK}}_{\text{ads}} = \frac{{ - \Delta {\text{H}}_{\text{ads}}^{\text{o}} }}{\text{RT}} + {\text{constant}}$$
(11)

The value of 55.5 appeared in Eq. (9) represented the concentration of water in the solution in mole [40].

The spontaneity of the reaction was defined since the entropy value was less than zero. As shown in Tables 2 and 3, the adsorption of TX-100 was spontaneous and it was physiosorption as defined by the amount of enthalpy of activation (from 5 to 50 kJ mol−1) while above of 50–500 kJ mol−1 the nature of adsorption defined as chemisorptions.

Table 2 Values of Kads and \(\Delta \text{S}_{{\text{ads}}}^{\text{o}}\) at different temperatures
Table 3 Calculated values of kinetic/thermodynamic parameters for mild steel corrosion in 5% HCl in the absence and in the presence of TX-100 from weight loss measurements

On the other hand, activation parameters, Ea, ΔHo, and ΔSo were calculated from Arrhenius-type plot according to Eq. (12):

$$\log \left( {CR} \right) = \left[ {\frac{{ - E_{a} }}{2.303RT} + Intercept} \right]$$
(12)

Activation energy (Ea) is the amount of energy required to complete a specific reaction, CR is the corrosion rate obtained from weight loss in mpy, T is the absolute temperature in K, R is the universal gas constant (8.314 J mol−1 K−1) which in our case required accomplishing the adsorption process. As shown above in Eq. (12), corrosion rate is related to activation energy; on the other hand it is related to the rate of adsorption by Eq. (13):

$$Rate \;of\;adsorption = \left( {\frac{dn}{dt}} \right)\left( {1 - \theta } \right)exp\left( {\frac{{ - E_{a} }}{RT}} \right)$$
(13)

where n represents number of adsorbed species which may include the atomic hydrogen; chloride or TX-100, as will be discussed later; the term (1 − ϴ) represented the covered surface which is equivalent to the vacant surface to the total surface [41].

A plot of transition-state based on Eq. [40, 42], was shown in Fig. 5,

$$log\left( {\frac{CR}{T}} \right) = log\left( {\frac{R}{Nh}} \right) + \frac{{\Delta S^{o} }}{2.303R} - \frac{{\Delta H^{o} }}{2.303RT}$$
(14)

was shown in Fig. 5, where N is the Avogadro’s number (6.022 × 1023 mol−1), h is Planck’s constant (6.626 × 10−34 J s−1), ΔSo the entropy of activation, and ΔHo the enthalpy of activation.

Fig. 5
figure5

Arrhenius plot (top) and transition state plot (bottom) for mild steel corrosion in 5% HCl at different concentrations of TX-100 of (filled square) 0 ppm, (open square) 5 ppm, 10 ppm (circle), 20 ppm (triangle) and 50 ppm (inverted triangle)

Figure 5 represents the plot of log CR/T versus 1000/T for TX-100 on mild steel. The lines obtained represent a slope of (\(- \Delta H^{o} /2.303R)\) and an intercept of [log (R/Nh) + \(\Delta S^{o} /2.303R\)] as deduced from Eq. (15) shown above. The values of Kads, \(\Delta {\text{H}}_{\text{ads}}^{\text{o}} ,\) \({\text{and }}\;\Delta {\text{G}}_{\text{ads}}^{\text{o}}\) were calculated and listed in Table 2. The large negative values of \(\Delta {\text{G}}_{\text{ads}}\) is an indication of the spontaneity of adsorption process, and the stability of the adsorbed layer on the mild steel surface. Normally, the values of \(\Delta {\text{G}}_{\text{ads}}\) were reported to be around 20 kJ mol−1 or lower which are consistent with electrostatic interaction between the charged molecules and the metal (physisorption). Other values are reported to be around 40 kJ mol−1 or higher, these involved charges sharing or transfer of inhibitor molecules to the metal surface to form a coordinate bond (chemisorption) [43].

However, other authors defined physisorption adsorption as those having less activation energy than 40 kJ mol−1 while those of chemisorption processes are those of having activation energy greater than 80 kJ mol−1 [44]. Other scientists define the physisorption to be lower than 41.86 kJ mol−1 while the heat of adsorption of chemisorption processes approaching 100 kJ mol−1 [45].

Table 3 showed some of kinetics and thermodynamic parameters which are showing a difference between the mild steel in the absence of TX-100 and in the presence of different concentrations of TX-100. It is clear that when comparing the results, the Ea, \(\Delta {\text{H}}_{\text{ads}}^{\text{o}}\), and \(\Delta {\text{G}}_{\text{ads}}^{\text{o}}\) increased negatively compared to the values of mild steel before any addition of TX-100. This clearly indicated that the adsorption is still proceeding where the reaction is exothermic, the spontaneity is still active.

Potentiodynamic Polarization Studies

The potentiodynamic polarization curves of mild steel in 5% HCl (~ 0.0137 M) in the absence and in the presence of TX-100 were shown in Fig. 6 at 30, 45 and 60 °C. In general, the Ecorr was not showing a trend upon increasing the TX-100, the maximum shift from the uninhibited sample was around 15 mV to the more negative potential at 30 °C, while at 45 °C, the maximum shift was only 4 mV to the more negative potential and 22 mV towards the more positive potential. At the 60 °C, the potential reached a maximum value of 2 mV to the more active value while it reached a maximum shift of 16 mV towards the nobler value. For all concentrations and at all temperature studied, a protection was noticed compared to that of uninhibited samples. The electrochemical parameters are summarized in Table 4.

Fig. 6
figure6

Potentiodynamic polarization curves of mild steel in 5% HCl in the absence of TX-100, and in the presence of 5, 10, and 20 ppm of TX-100 at different temperatures of 30 °C (a) 45 °C (b) and 60 °C (c)

Table 4 Potentiodynamic polarization parameters for mild steel in 5% HCl in the absence and in the presence of TX-100 at 30 °C, 45 °C and 60 °C

The little change in the Ecorr might be attributed to the evolution of hydrogen gas and to the adsorption of atomic hydrogen on the surface of mild steel. Hence we may deduce that the Ecorr change is insignificant with the increase of the TX-100 on the range studied in this work.

The surface coverage from potentiodynamic polarization may be measured by using Eq. (15), inhibition efficiency (IE, %) calculated by using Eq. (16), and the corrosion rate (CR, mpy) calculated by using Eq. (17)

$$\theta = \frac{{I_{corr} - I_{{corr\left( {inh} \right)}} }}{{I_{corr} }}$$
(15)
$$IE\left( \% \right) = \frac{{I_{corr} - I_{{corr\left( {inh} \right)}} }}{{I_{corr} }} \times 100$$
(16)
$$CR = 0.129\left( {\frac{{I_{corr} \cdot EW}}{D}} \right)$$
(17)

The polarization resistance (Rp) was calculated from the Stern–Geary equation as shown in Eq. (18) [46]:

$$R_{p} = \frac{{b_{a} b_{c} }}{{2.303I_{corr} \left( {b_{a} + b_{c} } \right)}}$$
(18)

where θ is the degree of surface coverage, Icorr is the corrosion current in µA cm2 for the uninhibited electrode, Icorr(inh) is the corrosion current in µA cm−2 after adding different concentrations of the inhibitor, CR is the corrosion rate, in mpy, EW is the equivalent weight of mild steel, D is the density of the mild steel, Rp is the polarization resistance, bc is the cathodic Tafel constant and ba is the anodic Tafel constant.

The inhibition efficiency depicted in Fig. 7 showed that a maximum efficiency was around 63% at 45 °C for the 20 ppm concentration of TX-100 while it was a little smaller than this value at 60 °C (~ 61%).

Fig. 7
figure7

Inhibition efficiency of different concentrations of TX-100 in 5% HCl at different temperatures of 30 °C (square), 45 °C (circle) and 60 °C (triangle), data extracted from potentiodynamic polarization

The overall kinetics of the system under study can be explained based on the Butler–Volmer equation for a one electron transfer process:

$$i = i_{o} \left\lfloor {exp^{{\left( {\frac{\alpha \eta F}{RT}} \right)}} - exp^{{ - \left( {1 - \alpha } \right)\eta F/RT)}} } \right\rfloor$$
(19)

When mild steel immersed in HCl electrolytic solution, the cathodic reaction included water reduction to produce atomic hydrogen. Most of this hydrogen combined to producing the evolved hydrogen gas during the experiment, on the other hand, since oxygen is dissolved in the HCl (not deaerated), it may be reduced to form hydroxyl anions according to Eqs. (20) and (21.

$$2H_{2} O + 2e^{ - } \to H_{2} \uparrow + 2OH^{ - }$$
(20)
$$O_{2} + 2H_{2} O + 4e^{ - } \to 4OH^{ - }$$
(21)

while the anodic reaction was attributed to the oxidation of mild steel according to the reaction:

$$Fe - 2e^{ - } \to Fe^{2 + }$$
(22)

Moreover, the Fe(II) may be losing extra one electron and transformed into Fe(III). This may lead to the complexity of Butler–Volmer (Eq. 19), especially in the presence of TX-100.

Some of the produced atomic hydrogen adsorbed while some were diffused into the mild steel, and the rest may set free to be evolved as molecular hydrogen. The experimental values of Tafel parameters may be affected by complexity of the system where multiple electrons are contributed and the authors also believe that a competition of these processes occurred may cause the same effect.

$$b = \frac{RT}{\alpha zF}$$
(23)

The Tafel slope (b), as shown in Eq. (23), is proportional to the term T (temperature) and inversely to α (transfer coefficient). For simplicity if we consider unity for z (electron charge number included in the reaction), then it is clear that the electron transfer (α) decreased at the 20 ppm and at 60 °C, which we believed was due to increase of surface coverage of the TX-100 on the mild steel surface. TX-100 was formed as it replaced the intermediate of adsorbed hydrogen or hydroxyl anions on the surface of mild steel.

When comparing the Tafel constant of blank sample (see Table 4) to that of after the addition of 20 ppm of TX-100, we noticed that in most cases, the cathodic values were larger than the anodic value which may support our discussion above. This is clearly remarkable at 60 °C where the chance of atomic hydrogen adsorption is less favored. Moreover, the processes of hydrogen evolution, adsorption, migration and physically adsorption of TX-100 on mild evidenced the complexity of our system which in return affected the total current where the mass transfer of TX-100 toward the mild steel surface has its deleterious effect on applicability of Tafel’s relationship.

In 0.5 M H2SO4, the TX-100 was chemically adsorbed on the surface of steel as indicated by ΔGads of − 43.24 kJ mol−1 [47], similarly, at 50 ppm of TX-100, Amin et al. [22] considered the adsorption of TX-100 to be chemisorptions on the pure iron electrode in 1 M HCl. At high temperature of 338 K (64.85 °C) this is obviously true, while at temperature below 338 K, the adsorption was clearly physiosorption as indicated from their reported values of Gibbs adsorption. In this work, within the experimental conditions, it is clear that we have a physisorption as indicated above. This was not in agreement of the work of Amin et al. [22] who claimed pure chemisoption mechanism. This may be ascribed to the low concentration we choose for HCl [48].

From Table 4, the values of βa and βc were increased (with some hysteresis for the reasons discussed above) which meant that the reaction was going to be slower when increasing the concentration of TX-100 while increasing the temperature; consequently, the surface coverage increased. This may be attributed to great occupancy of the active sites.

Mechanism of Inhibition

Bockris and Kita [49] assumed that iron–hydroxyl intermediates are formed during the iron dissolution and that included a two step process.

$$Fe^{ + } + OH^{ - } \rightleftarrows \left( {FeOH} \right)^{ + }$$
(24)
$$\left( {FeOH} \right)^{ + } + e^{ - } \to \left( {FeOH} \right)$$
(25)
$$\left( {FeOH} \right) + e^{ - } \rightleftarrows Fe + OH^{ - }$$
(26)

Mechanism through one-electron transfer may be involved or competed the above proposed mechanisms

$$Fe + OH^{ - } \rightleftarrows FeOH + e^{ - }$$
(27)

The atomic hydrogen adsorption resulted in blockage of metal surface which in turn led to the hysteresis observed during measuring potentiodynamic polarization (Data shown in Fig. 6 were smoothed).

$$Fe + H^{ + } \rightleftharpoons \left( {FeH^{ + } } \right)_{ads}$$
(28)
$$\left( {FeH^{ + } } \right)_{ads} + e^{ - } \to \left( {FeH} \right)_{ads}$$
(29)
$$\left( {FeH} \right)_{ads} + H^{ + } + e^{ - } \to Fe + H_{2}\uparrow$$
(30)

During the formation of atomic hydrogen, some were diffused into the bulk of mild steel, while the rest were combined to form molecular hydrogen that evolved (Eq. 30). The intermediate complexes and their act as rate determining step prior to the adsorption of different species and theoretical approaches were discussed elsewhere [49,50,51,52]. An intermediate complex similar to the intermediates discussed above of (FeCl)ads may be formed [53, 54].

The overall assumed mechanism of inhibition believed to include the replacement of the adsorbed hydrogen atoms or molecules by that of TX-100 (which is different from that shown above in Eq. 4). This is similar to what happen in many organic molecules or neural (non-ionic surfactants). When protons accumulated on the metal surface in excess, the chloride adsorbed to balance this situation, which is completely dependent on the strength of the co-ordination of the metal surface with these specific anions [55].

The TX-100 was adsorbed physically as evidenced by the value of the heat of enthalpy where it is ranged between − 5 and − 50 kJ mol−1 (see Table 3) as it is defined early for physiosorption processes. TX-100 may be adsorbed on mild steel through one of three groups, viz the phenyl ring, octyl group or polyethoxylate chain depending on the charge of active molecules adsorbed on the mild steel. If the hydrogen adsorbed forming the intermediate complex, then TX-100 adsorbed through the polyethoylate group (where electron density is high because of the oxygen atoms). On the other hand, if chloride anions was forming the intermediate complex, then the TX-100 will prefer to adsorb through the alkyl (octyl group or the phenyl ring) (Fig. 8).

Fig. 8
figure8

Optimized molecular structure of TX-100 a, showing a possible interaction of the TX-100 to adsorb on mild steel b showing partial surface coverage c and complete surface coverage

Conclusions

  1. 1.

    The performance of the non-ionic surfactant, TX-100, was evaluated at low concentrations of 5, 10, 20 and occasionally 50 ppm in 5% HCl (~ 0.0137 M) showed that the TX-100 inhibited the corrosion of mild steel during testing for periods ranged from 60 to 300 min as revealed by weight loss method.

  2. 2.

    The weight loss method showed that the maximum efficiency of TX-100 was around 77% at 30 °C, when only 50 ppm used in the presence of 5% HCl (~ 0.0137 M).

  3. 3.

    The estimated adsorption equilibria were 0.774, 0.770, and 0.798 at temperatures of 30, 45, and 60 °C and that the adsorption isotherm followed Langmuir adsorption isotherm.

  4. 4.

    The adsorption of TX-100 is a spontaneous process; the process is an exothermic process, which is accompanied by a decrease in entropy upon increasing the concentration of TX-100.

  5. 5.

    The electrochemical measurements support the weight loss, and the plateau obtained at the 20 ppm is attributed to steric hindrance of the molecules of TX-100 trying to adsorb on the mild steel surface.

  6. 6.

    The adsorption was physisorption, in this study.

  7. 7.

    We assumed that the TX-100 is adsorbed through the phenyl ring or the octyl group if the chloride was forming the intermediate complex while it is adsorbed through the polyethoxylate chain if the intermediate was (FeH+)ads.

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Attia, A., Abdel-Fatah, H.T.M. Triton X-100 as a Non-Ionic Surfactant for Corrosion Inhibition of Mild Steel During Acid Cleaning. Met. Mater. Int. 26, 1715–1724 (2020). https://doi.org/10.1007/s12540-019-00533-7

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Keywords

  • Acid wash cleaning
  • Corrosion inhibition
  • Triton-X 100
  • Mild steel
  • Langmuir isotherm
  • Inhibition mechanism