1 Introduction

Aluminium is a metal with many important applications. It is second to iron in terms of production and consumption [1]. As a result of its relevance, the studies of its corrosion behaviour have been attracting considerable attention in various aggressive environments [2,3,4,5]. However, aluminium and its alloy exhibit relatively good corrosion resistance compared to iron upon exposure to the atmosphere, which was attributed to a thin oxide film that forms on them [5,6,7]. However, numerous studies related to the electrochemical behaviour of aluminium and its alloy in sodium chloride solution reveal that they are prone to localized corrosion on exposure to chloride ion [8,9,10,11,12]. Hence, effective corrosion protection measures should be applied. One of these efficient measures is the use of inhibitors. A lot of researches have been carried out to present inhibitors fit for different corrosive media of aluminium and its alloy [13,14,15,16,17,18,19].

Researches have also shown that some of inhibitors such as chromates have undesirable effect on the environment [20]. Therefore, it is imperative to study the composition of an inhibitor before usage. However, a large number of inorganic inhibitors have been found to possess good inhibitive performance with low toxicity, but most of them are expensive compared to the organic corrosion inhibitors. Organic corrosion inhibitors, especially those from natural source like chitosan, have become of practical interest nowadays because of their eco-friendly nature. Metal protection by these organic inhibitors comes from the fact that they adsorb on the metal surface forming protective layers against corrosive species in the media [21]. Organic inhibitors effectiveness in corrosion mitigation is a function of several factors such as the nature of the metal surface, the type of corrosive media and the chemical structure of the inhibitor [22, 23]. These factors make it essential for careful choice of inhibitors for metals.

Chitosan’s corrosion protection ability is a function of its molecular structure. Naturally, chitosan has electron-rich hydroxyl and amino groups as shown in Fig. 1. These hydroxyl and amino groups have the ability to form bonding on the surface of metals [24, 25] resulting in corrosion protection through coordinate bonding as these electrons are given out freely to the empty or partially occupied Fe orbitals. The choice of chitosan as an inhibitor is drawn from its eco-friendly and adsorption ability on metals [26, 27]. This research aimed at examining the inhibitive ability of chitosan on the corrosion of aluminium alloy in seawater-simulated environment (3.65% NaCl). The effect of chitosan on corrosion behaviour of the aluminium alloy was observed using electrochemical potentiodynamic polarization techniques, mass loss measurements and computational studies.

Fig. 1
figure 1

Molecular structure of chitosan [28]

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2 Experimental Procedures

2.1 Sample Preparation

The aluminium alloy coupons used for these studies are of the dimension of (20 × 20 × 2) mm. The spectrometer chemical composition in wt% is shown in Table 1. These coupons were then polished with emery papers of different grades and rinsed with distilled water. Each specimen’s weight was recorded and labelled appropriately. 3.65% NaCl solution was prepared using double distilled water which acted as the seawater (corrosive medium). The mass of the chitosan whose molecular structure is shown in Fig. 1 was varied in 3.65% NaCl solution for every potential dynamic polarization measurement taken. For all the electrochemical measurement and mass loss, the volume of prepared solution was 200 ml.

Table 1 Chemical composition of aluminium alloy (wt%)
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2.2 Electrochemical Measurement

Autolab PGSTAT 101 Metrohm potentiostat/galvanostat with NOVA software of version 2.1.2 was used for the electrochemical measurements. The aluminium alloy coupon was welded to wire and mounted on resin. Aluminium alloy acted as the working electrode. Graphite rod was used as the counter electrode, and silver chloride electrode (SCE) functions as the reference electrode. Potentiodynamic polarization curves were obtained from potential of − 2.0 to − 0.5 V versus open circuit potential at a sweep rate (scan rate) of 0.005 V/s.

The working electrode (aluminium alloy) was allowed to immerse in the electrolyte solution (3.65% NaCl) for about 10 min to attain the steady-state potential. The same procedure was carried out with four other aluminium alloy samples varying the concentration of chitosan nanoparticles in four different 200 ml of 3.65% NaCl solution, and the effects were noted. To check for reproducibility, each experiment was repeated four times. The polarization potential (Ecorr) and current density (Icorr) data were evaluated from the Tafel plots. The surface coverage (θ) and the percentage inhibition efficiency (IE%) were calculated from Eqs. (1) to (2) [29,30,31,32,33,34].

$$\theta =1 - \frac{{{i_{{\text{corr}}}}}}{{{i_{{\text{ocorr}}}}}}$$
(1)
$$IE\% =1 - \frac{{{i_{{\text{corr}}}}}}{{{i_{{\text{ocorr}}}}}} \times ~100$$
(2)

where \({i_{{\text{corr}}}}\) is the inhibited corrosion current densities and \({i_{{\text{ocorr}}}}\) is the uninhibited corrosion current density.

3 Results and Discussion

3.1 Potentiodynamic Polarization Measurement

Potentiodynamic parameters of aluminium alloy in 3.65% NaCl in the absence and presence of different concentrations of the inhibitor are shown in Table 2, and their corresponding polarization curves are shown in Fig. 2, suggesting that polarization had taken place because of the presence of both cathodic and anodic branches [28, 35].

Table 2 Polarization parameters for inhibited and uninhibited aluminium alloys
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Fig. 2
figure 2

Potentiodynamic polarization curves for inhibited and uninhibited aluminium alloys

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Evaluation of bc and ba (cathodic and anodic slope, respectively), as well as icorr and the corrosion current densities was done by Tafel extrapolation of the current–potential lines at the corresponding values of corrosion potentials (Ecorr) and is shown in Table 2. The values of corrosion rate (CR) and polarization potential (PR) were also obtained from the Tafel extrapolation. It is worthy of note that the presence of inhibitors causes a decrease in corrosion rate by moving the anodic and cathodic polarization curves in the direction of the lower values of current densities. This prevents the cathodic evolution and anodic metal dissolution reactions of aluminium alloy [36, 37]. The reduction in rate of these reactions increases with an increase in inhibitor concentration. This is an indication that an increase in chitosan nanoparticle from 0.3 g through 0.6 g, 0.9 g and finally 1.2 g drastically hinders the corrosion of aluminium alloy. However, there are little variation in Ecorr values on varying concentration of Chitosan, suggesting that polarization is a mixed type, which shows that the water-soluble chitosan is a mixed-type inhibitor having adsorbed its molecules on the surface of aluminium alloy [31, 38,39,40,41]. But with 1.2 g of chitosan in the salt solution, Ecorr shifted to a more negative value with respect to others indicating that with higher concentration, chitosan will behave predominantly as cathodic inhibitor in 3.65% NaCl at room temperature.

3.2 Open Circuit Potential (OCP) Measurement

Figure 3 shows the open circuit potential (OCP) versus time curves for aluminium alloy in 3.65% NaCl. Careful examination of the OCP versus time curves reveals that the presences of chitosan shift the steady-state potential towards more negative direction. The negative shift suggests that the cathodic reaction is relatively more affected than the anodic reaction. There were notable changes in the features of the curved compared to the uninhibited sample. The values of OCP for the inhibited samples were between − 0.9 and − 0.92 V within the first 5 s but later moved between more negative value of − 0.92 and − 0.85. It is important to note that the OCP versus time curve for the inhibited and uninhibited aluminium alloy was near straight line, indicating that steady-state potential was attained [42].

Fig. 3
figure 3

Evolution of open circuit potential (OCP) versus exposure time for inhibited and uninhibited aluminium alloys in 3.65% NaCl solution

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3.3 Measurement of Mass Loss and Corrosion Rate

Results obtained from mass loss measurements after the electrochemical experiment using OHAUS pioneer ™PA1214 model are shown in Table 3. Mass loss results indicate that introduction of chitosan into the corrosive medium minimizes corrosion. The loss in mass of aluminium alloy decreases as the concentration of chitosan increases. This is an indication that the mass loss rate had been influenced. Figure 4 shows the effect of chitosan on the corrosion rate of aluminium alloy. The rate of corrosion reduces as the concentration of chitosan increases, which is in accordance with the work of author ref [43]. The corrosion rate of the uninhibited aluminium alloy was found to be the highest on the chart.

Table 3 Mass loss, surface coverage and C/θ
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Fig. 4
figure 4

Representation of rate with respect to the inhibited and uninhibited aluminium alloys

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3.4 Mechanism of Inhibition Efficiency and Adsorption Study

The values of corrosion current (icorr) in Table 2 and CR indicated by Fig. 5 were found to reduce with an increase in the concentration of chitosan, indicating the adsorption of chitosan on the surface of aluminium alloy [44]. The corrosion inhibition efficiency of chitosan increases with an increase in concentration.

Fig. 5
figure 5

Variation of inhibition efficiency with an increase in concentration of chitosan

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In order to have a good knowledge of metal inhibitor interaction and the metallic complex activities on the coverage site, an adsorption mechanism facilitated the computation of C/θ and C for potentiodynamic polarization system using Langmuir adsorption isotherm and a linear relationship [45,46,47]. Equation (3) shows the Langmuir adsorption isotherm law. The Langmuir isotherm plot in Fig. 6 on the surface features shows a linear relationship with the increase in the concentration of the inhibitor, indicating the continuous adsorption of the inhibitor on the surface of aluminium alloy. The value of (R2) for Langmuir absorption isotherm was 0.9961. This R2 value is in the same range as those of Ref. [47, 48]. This showed that the corrosion protection of aluminium alloy by chitosan had been achieved since R2 is close to unity.

Fig. 6
figure 6

Langmuir adsorption isotherm for the inhibited samples

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The Langmuir adsorption isotherm law is

$$\frac{C}{\theta }=\frac{1}{{{K_{{\text{ads}}}}}}+C$$
(3)

where C is the concentration of the corrosion inhibitor, θ is degree of surface coverage, and K is the adsorption equilibrium constant.

4 Conclusions

  1. 1.

    Langmuir adsorption isotherm was obeyed by inhibition of aluminium alloy 3.65% NaCl with correlation regression coefficient of R2 = 0.9961. The values of R2 are near unity, indicating the effectiveness of the inhibitor

  2. 2.

    Maximum corrosion inhibition efficiency of 71.45% was obtained for the inhibited aluminium alloy in salt solution.

  3. 3.

    The electrochemical potentiodynamic polarization studies show that chitosan acted as a mixed-type inhibitor.

  4. 4.

    Chitosan inhibition efficiency increases with an increase in the inhibitor at room temperature 3.65% NaCl.

  5. 5.

    The corrosion of aluminium alloy substrate in the chloride environment is mitigated by the effective adsorbent of chitosan molecules on aluminium surface. The results of electrochemical techniques and mass loss were in reasonable agreement.