1. Introduction
Corrosion is the degradation of a material and its properties caused by a chemical or electrochemical reaction between the material and its environment [
1]. Carbon steel is the most widely used alloy in the metallurgical industry because it offers many advantages. On one hand, it is less expensive relative to other alloys and, on the other, it has suitable mechanical characteristics [
2,
3,
4]. Yet, a chemical pickling step is required in its industrialization process to eliminate metallic impurities and standardize its surface [
5,
6,
7,
8,
9,
10]. Hydrochloric acid (HCl) is one of the most widely used agents in the industry for steel pickling, cleaning, etc. [
11]. However, this chemical is known to cause severe degradation of metals and metal alloys, either through chemical or electrochemical reactions. Fortunately, several protection measures can be used to inhibit the effects of corrosion, such as cathodic protection [
12] , inhibitor additives [
13], passivation [
14], and protective coatings [
15].
Chemical pickling consists of removing grease by chlorinated solvents and rust by acids (acid pickling). The main disadvantage of this method is the formation of gaseous hydrogen that tends to penetrate the metal and weakens it.
In order to slow or stop this corrosion process, some chemical inhibitors are added in low concentrations to the system [
16,
17,
18]. The main role of the corrosion inhibitor in chemical pickling is to minimize the acid attack on the steel and to reduce the loss of mass during this operation [
19,
20]. Inhibitors are generally classified according to the nature of the electrochemical reactions they trigger [
21,
22]:
They are also classified according to their mode of action:
Absorption inhibitor.
Passivating inhibitor.
Some solid materials have been extensively researched and used as corrosion inhibitors due to their excellent stability and efficiency. These substances are often applied to steel surfaces either as coatings or in solution. Metal oxides, which create a barrier on the surface of steel, like iron and zinc oxides, are among the most often employed solid inhibitors [
23,
24,
25]. Other substances, like calcium carbonate and magnesium hydroxide, are also efficient inhibitors due to their capacity to neutralize acids [
26]. Organic inhibitors, such as quaternary ammonium compounds and imidazolines, have proven to be extremely efficient in preventing corrosion due to their adsorption capacity on the steel surface and subsequent creation of a protective layer. Overall, a potential strategy for reducing corrosion and lengthening the lifespan of metal and alloy structures is the use of solid chemical compounds as corrosion inhibitors [
27,
28].
Among the corrosion inhibitors commonly used as electrolytes for industrial water treatment are numerous mineral substances such as chromates, molybdates, nitrites, sulfites, phosphates, and polyphosphates. However, their use is limited either for technical reasons, such as the hydrolysis temperature for polyphosphates, or for their toxicity , especially with regard to chromates and nitrites [
29,
30,
31].
Layered double hydroxides (LDHs), also referred to as anionic clays, are compounds characterized by a two-dimensional structure formed by a stacking of brucite-type sheets in which part of the divalent metal cations are replaced by trivalent metal cations (
Figure 1). M
2+ represents a divalent metal (Zn
2+, Mg
2+, Ni
2+, Fe
2+, Mn
2+ ...), M
3+ represents a trivalent metal (Al
3+, Fe
3+, Co
3+, Cr
3+, Mn
3+ ...), A
m- is the compensating anion (Cl
-,
NO−,
ClO2−,
CO2−...), and n is the number of water molecules located in the interlamellar space with the anion [
32,
33,
34]. The coefficient, x, is the M
3+/M
2+ mole fraction.
LDHs have many applications in industrial, medical, ecological and scientific fields due to their lamellar structure, varied compositions, ease of synthesis, low cost, low toxicity, significant anion exchange capacity, and redox or acid-base behavior [
36,
37,
38] LDHs have recently attracted attention as corrosion inhibitors, as they offer exceptional corrosion prevention capability [
39,
40,
41,
42].
We propose in the context of this work to use [Zn-Al-Cl], [Mg-Al-Cl], [Ni-Fe-Cl], and [Co-Fe-Cl] LDHs as inhibitors in order to improve the electrochemical corrosion resistance of mild steel in HCl. Therefore, the steel plates were placed in the electrolytic solutions obtained by dissolving the LDHs in the corrosive environment. The inhibitory efficiency was determined by the gravimetric and electrochemical methods. The mode of the electrolyte action and certain corrosion parameters were identified.
2. Materials and methods
2.1. Preparation of the LDHs
The [Zn-Al-Cl], [Mg-Al-Cl], [Ni-Fe-Cl], and [Co-Al-Cl] LDHs were prepared by coprecipitation [
43,
44,
45,
46]. In a three-vial flask containing 250 mL of water, a mixture of 1 M MCl
2 and 1 M MCl
3 salts is added dropwise in a 2:1 molar ratio under moderate stirring. The pH of the solution was maintained at 9 by the addition of NaOH, using an automatic system. The coprecipitation is carried out under a nitrogen atmosphere to minimize the contamination by carbonate ions from the air. The precipitate obtained is left under moderate stirring at room temperature for 80 hours. The solid phase is obtained after repeated centrifugations and washings with demineralized water (4 times). It is then dried at room temperature (25°C). The materials were characterized by XRD, FTIR, and thermal analyses.
2.2. Steel Samples
The mild steel samples used in this study were polished before the experiments with various emery paper grades of 220, 400, and 800, rinsed with bi-distilled water, degreased with acetone, and then washed again with bi-distilled water. Finally, the samples were air-dried before use.
The chemical composition of the steel used (GRADD plate: DD13 from Maghreb Steel, Casablanca, Morocco) is given in
Table 1.
2.3. Methods
The XRD equipment used was a Pert-Pro diffractometer (Bruker-AXS). Powder samples were exposed to copper Kα radiation (λ = 0.15415 nm).
IR spectra were recorded on a Perkin Elmer 16 PC spectrophotometer at a resolution of 2 cm-1 and averaging over 100 scans, in the range of 400 to 4000 cm-1. The pellets consist of 100 mg of finely powdered KBr and 2 mg of the sample.
Thermal analyses were performed in air on a Setaram TG-DSC 92 instrument. Curves were recorded between 25 and 900°C at a heating rate of 5°C/min.
The surface morphology of the samples was examined by a Nikon optical microscope and Hitachi SU8020 SEM, coupled with an EDS.
2.4. Evaluation of inhibitory efficiency
The comparative study of the inhibiting efficiencies was achieved by gravimetry and linear voltammetry. The samples were immersed in an 18.5% HCl solution, in the absence and presence of the LDH-based inhibitor (1.2 g.L-1). The samples were removed, rinsed, and dried before examination.
2.4.1. Gravimetric method
The test consists of stripping a sample of steel (area: S = 4 cm × 2 cm, thickness: e = 2.7 mm) in the HCl solution at 60°C followed by its immersing it in a mixture of the acid and the inhibitor at different LDH concentrations (C) ranging from 0 up to 1.2 g.L-1. The LDH-based inhibitors used are soluble under these acidic conditions. The steel-solution contact time is set for 10 min. The sample is then rinsed with demineralized water, dehydrated using filter paper, and dried in the oven at 100°C.
The mass variation of the sample is determined without (Δ
m0) and with (Δ
mf) the inhibitor. The inhibitory efficiency (IE) is calculated by the following formula:
2.4.2. Linear voltammetry
The linear voltammetry method, at low speed of potential sweep, allows to determine the kinetic parameters of the corrosion reaction of steel in the solution. The tests were performed at 60°C in an 18.5% HCl solution using a three-compartment cell with a platinum counter electrode and an Ag/AgCl electrode as a reference. The working electrode is a steel disc of 0.8 cm diameter.
The three electrodes are connected to an SP-150 Biologic potentiostat controlled by a computer via an "Ec-Lab" software. The corrosion potential covers the anodic and cathodic zones (- 0.350 to 0.350
V) at a scan rate of 2
mV/
s. The inhibitory efficiency (IE) is calculated by the following formula. Where I
corr,0 and I
corr,f are the corrosion current densities with and without inhibitor, respectively:
4. Conclusions
The present work was devoted to the study of the improvement of the electrochemical corrosion resistance of mild steel in a concentrated HCl solution by the use of [Zn-Al-Cl], [Mg- Al-Cl], [Ni-Fe-Cl], and [Co-Fe-Cl] HDLs as inhibitors.
The LDH materials were prepared by coprecipitation and characterized by XRD, FTIR, and thermal analyses.
This study of the inhibitory efficiency was carried out by gravimetric and electrochemical methods (polarization curves). The results obtained show that all the four LDHs used have acted as corrosion inhibitors of the steel in the HCl aggressive environment. The results may be summarized as follows.
The best effect is observed with the maximum LDH concentration of 1.2 g.L-1 using [Zn-Al-Cl] and [Ni-Fe-Cl] LDHs which inhibitory efficiency reached 83.65% and 83.33%, respectively.
The displacement of the free potentials towards more anodic values characterizes the formation of a protective layer of the LDHs on the steel surface.
Increasing the concentration of each of the four inhibitors decreases the corrosion current densities, thus the corrosion rates, especially in the cases of [Zn-Al-Cl] and [Ni-Fe-Cl].
The surface morphology analyses of the steel after chemical pickling in HCl in the absence of inhibitor allowed to identify the type of corrosion (pitting corrosion) and the damaged state (cracks). These undesirable effects were absent in the presence of the inhibitors, indicating a good protection of the steel against corrosion.
"The same solutions containing the corrosion inhibitor can be used multiple times for different metal plates."
The main outcome of this work is that the fight against corrosion of steel in a HCl environment is possible using a new generation of inhibitors that have the advantage of being easier to obtain and less toxic than the traditional inhibitors.