1 Introduction

Concrete is a composite material consisting of coarse and fine granular aggregates contained in a hard matrix of binders. Reinforced concrete is extensively used for building and construction. The reinforcements give higher tensile strength to the concrete, but their relatively weak resistance to corrosion endangers the durability of the concrete structure. Deterioration of concrete frameworks and infrastructures due to reinforcement corrosion is a problematic phenomenon worldwide resulting in huge damage, safety issues, economic loss, and enormous repair and replacements costs [1,2,3,4,5]. There are several methods in place to increase the life span of reinforced concrete structures such as cathodic protection, high chromium steel, corrosion inhibitors, rebar coatings, re-alkalization, high performance concrete etc. [6, 10]. The demand for novel techniques to reduce the rate reinforced material corrosion in concrete cannot be over emphasized as there are currently limited cost-effective and economical techniques available [11,12,13]. Reinforcing steel in concrete is usually shielded from corrosion because of the formation of a protective oxy-hydroxide on the steel in the presence of adequate O2 which passivates its surface when the alkalinity (pH 12.5–13.5) of the concrete pore solution is high [14,15,16]. Several factors are responsible for the formation of protective oxide, e.g., type of aggregates, temperature, humidity, moisture content, etc. [17, 18]. Basically, there are two ways by which the protective oxy-hydroxide layer of carbon steel reinforcement can be destroyed. (a) Accumulation of chloride anions close to the reinforcing steel. This is due to ingress of chloride anions from mixture of concrete impurities with moisture through pores, holes and cracks within the concrete which eventually leads to competitive adsorption with OH− on the oxide surface [19]. (b) Decrease in concrete pH due to dilution of concrete pore solution by CO2 from the atmosphere resulting in carbonation whereby the numbers of defects in passive layer on rebar increase [20,21,22]. The corrosion products from the processes mentioned above increase the pressure within the concrete, creating more cracks, causing pitting of the reinforced steel and expansion of the rust layer. These eventually weaken the concrete structure [23, 24]. Currently, carbon steel is still the dominant rebar in concrete structures. Carbon steel passivates in concrete due to incidental electrochemical reduction reaction between the steel surface and alkaline concrete pore solution [25]. Previous researches on the electrochemical behavior of carbon steel in concrete pore solution and the nature and the composition of the passive film conclude that the passivity of the steel is influenced by their chemical compositions and solution environment [26,27,28,29]. Research on concrete pore solution is important because of the mobility of trace elements which accounts for ion exchange of concrete in the hardened state. In the pursuit of cost-effective, sustainable and eco-friendly concrete, recent research in concrete technology now focuses on partial replacement of cement using some sustainable admixtures and wastes such as aluminum dross (Al-D). Al-D obtained from aluminum smelting process can be non-salt containing (white dross) or salt containing (black dross) [30,31,32]. Million tons of these waste is generated daily. The reuse of this material is important in a bid to reduce solid waste and promote trash to treasure initiative in line with sustainable development goals. Busari et al. [33] assessed the utilization of Al-D as a stabilizer in road construction for the improvement of pavement interlayer properties. The use of this material in concrete production both as a filler, aggregate replacement and cement replacement has been assessed by several authors [34,35,36,37]. Results showed the material proved to be a good and sustainable material in concrete production. However, the dearth of literature exists on the effect of this material on the corrosion of reinforcing steel in concrete. In view of the above, this research aims to study the influence of non-salt containing Al-D in simulated concrete pore solution on the corrosion resistance and active–passive behavior carbon steel bars.

2 Experimental Methods

2.1 Material

Low carbon steel rods were purchased from the open market in Lagos, Nigeria, and analyzed with Phenom proX scanning electron microscope (Model No. MVE0224651193) at the Materials Characterization Laboratory, Department of Mechanical Engineering, Covenant University, gave the nominal wt% composition in Table 1. The steel rods were cut and sectioned to average dimensions of 1.1 cm by 1.15 cm (length by diameter). They were subsequently grinded with silicon carbide abrasive papers (grits of 120, 220, 320, 600, 800 and 1000) and polished to 6 μm with diamond polishing paste. Portland lime stone cement conforming to NIS 444-1:2003 [38] standard was the cement used in the research; in addition, fine aggregate conforming to NIS 13:1974 [39] obtained from Ota, Ogun State, Nigeria, was also used. Portable H2O was used in the batching of the concrete [40]. Al-D, obtained from Aluminum Rolling Mill Factory in Ota, Ogun State, Nigeria, was dried and pulverized to reduce the particle size of the waste sample due to its effect on the structural properties and characteristic bond of admixtures [41].

Table 1 Percentage nominal composition of LCS
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2.2 Concrete Mix Design

The pulverized Al-D was added in wt% concentrations of 0%, 5%, 10%, 15%, 20% and 25% Al-D per the weight of cement used. The H2O/cement ratio of 0.68 was applied based on the assertion of Kulakowski [42]. The concrete mix design is shown in Table 2.

Table 2 Mix design of concrete aggregates, cement Al-D and H2O
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2.3 Pore Electrolyte Extraction

The concrete pore electrolyte used in the analysis was extracted after 4 h from the onset of the hydration process of the cement in the fresh concrete mix. Further extraction was done manually. Additionally, load was applied on the specimen with the intention to promote confinement tension and expel the pore electrolyte [43]. The process of obtaining the pore electrolyte is extremely difficult; as a result, relatively small quantity of the pore electrolyte was obtainable to perform only a single set of experiments (n = 1). The pH values of the concrete pore electrolytes shown in Table 3 were determined using Hanna calibration check pH meter HI223.

Table 3 pH of concrete pore electrolyte with respect to Al-D concentration
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2.4 Electrochemical Tests

Potentiodynamic polarization test was conducted using a three-electrode system within a glass cell containing 200 mL of H2SO4/Al-D solution at specific Al-D concentrations with Digi-Ivy 2311 potentiostat connected to a computer. LCS mounted in hardened versocit acrylic resin with exposed surface area of 1.13 cm2 is the working electrode, while platinum rod was used as the counter electrode and silver chloride electrode (3 molar concentrations at pH of 6.5) as the reference electrode. Anodic–cathodic polarization curves were plotted at scan rate of 0.0015 V/s between − 1.25 V and 1.75 V. The potential was stabilized for 10 min before polarization to attain quasi-steady state. Corrosion current density Ccd (A/cm2) and corrosion potential Cp (V) were determined from the Tafel extrapolation method.

Corrosion rate Cr (mm/year) was determined from the relationship below;

$${C_{\text{r}}}=\frac{{0.00327 \times {C_{{\text{cd}}}} \times {E_{{\text{qw}}}}}}{D}.$$
(1)

E qw is the equivalent weight (g) of LCS, 0.00327 is a corrosion rate constant, and D is the density (g). Experimental analysis was performed on the carbon steel in the pore electrolyte. The electrochemical system was checked for possible causes of systematic errors. The uncertainty of single measurement is limited by the precision and accuracy of the measuring instrument. As a result, calibration of the instrument and hardware test was performed with the results shown in Table 4. Test for reproducibility of consistent results was also performed.

Table 4 Results of calibration and hardware test
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Open circuit potential (OCP) measurement was taken at 0.1 V/s step potential for 5400 s to obtain information on active–passive behavior and electrochemical equilibrium of LCS without applied potentials. Images of the LCS surfaces (before corrosion and after corrosion from the concrete pore electrolyte at 5% and 25% Al-D) from optical microscopy were analyzed after polarization test with Omax trinocular metallurgical microscope through the aid of ToupCam analytical software.

3 Results and Discussion

3.1 Potentiodynamic Polarization Studies

Potentiodynamic polarization plots of the active–passive behavior of LCS in Al-D contaminated concrete pore electrolyte are shown in Fig. 1. Results of the polarization parameters are shown in Table 5. Comparison of the corrosion rate values (Table 5) shows Al-D has slight influence on the electrochemical behavior and corrosion resistance of LCS in the concrete pore electrolyte. At 0% Al-D concentration, LCS corrodes under applied potential, attaining a corrosion rate value of 3.00 × 10− 2 mm/year, which corresponds to a corrosion current density of 2.59 × 106 A/cm3. The electrochemical mechanisms occurring within the concrete pore electrolyte are responsible for the corrosion of LCS. LCS oxidizes in the electrolyte losing electrons and releasing Fe2+ cations which go into the electrolyte. This process gradually deteriorates the microstructural and mechanical properties of the steel according to Eq. 2. Cathodic reduction reactions also take place resulting in oxygen reduction and to a lesser extent hydrogen evolution reaction when the previously released electrons combine with H2O and O2 in the concrete pore electrolyte according to Eq. 3. The corrosion product resulting from the combined electrochemical half-cell reactions is hydrated 2Fe(OH) known commonly as rust according to Eq. 4.

Fig. 1
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Potentiodynamic polarization plots for LCS corrosion in Al-D contaminated concrete pore solution at 0–25% Al-D concentration

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Table 5 Potentiodynamic polarization data for LCS corrosion in Al-D contaminated concrete pore solution at 0–25% Al-D concentration (n = 1)
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$$2{\text{Fe}} \to 2{\text{F}}{{\text{e}}^{2+}}+4{{\text{e}}^ - },$$
(2)
$$2{{\text{H}}_2}{\text{O}}+{{\text{O}}_2}+4{{\text{e}}^ - } \to 4{\text{O}}{{\text{H}}^ - },$$
(3)
$$2{\text{F}}{{\text{e}}^{2+}}+4{\text{O}}{{\text{H}}^ - } \to 2{\text{Fe}}({\text{OH}}).$$
(4)

An increase in Al-D concentration to 5% caused a mild increase in corrosion rate to 3.41 × 10− 2 mm/year which indicates that the characteristics of the oxide film formed with respect to Al-D differ. Beyond 5% Al-D, the corrosion rate of LCS increased with respect to Al-D concentration, attaining a peak value of 8.39 × 10− 2 mm/year at 25% Al-D concentration. The increase in corrosion rate occurs due to an increase in concentration of Al-D within the concrete pore electrolyte. Al being an amphoteric metal chemically reacts with the alkaline solutions prevalent in the concrete pore. According to Darwin et al. [44], the reaction results in the release of H2 before possible passivation of Al. Auxiliary anions present within the concrete also influence the rate of Al reaction [45,46,47,48,49]. Generally, Al mainly occurs as aluminate complexes [\({\text{Al}}({\text{OH}})_{4}^{ - }\) (aq)] in alkaline solutions. However, despite the incremental value of corrosion rate with respect to Al-D concentration, observation of the passivation behavior on the polarization plots shows Al-D could possibly have a positive influence of the electrochemical characteristics of LCS in concrete pore electrolyte. The shifts in corrosion potential value may not confirm this assertion. The corrosion potential value of − 0.746 VAg/AgCl (0% Al-D) shifts to − 0.775 VAg/AgCl at 25% Al-D concentration. The cathodic shift is due to destruction of the passive film resulting in selective deterioration of the steel. At the same time, it could be as a result of superficial electrochemical reactions occurring on the steel surface as a result of the presence of Al-D. Nevertheless, bridging the increase in corrosion rate values, cathodic shift in corrosion potential and extended passivation behavior of LCS in the concrete pore electrolyte with respect to Al-D, it can be surmised that the presence of Al-D improves the passivation behavior and briefly the corrosion resistance of LCS. However, upon collapse of the passive film corrosion accelerates on LCS.

3.2 Passivation and Pitting Corrosion Studies

Potentiodynamic polarization plots in Fig. 1 confirm the formation of protective oxide film formed on LCS surface during potential scanning. The plots display a passivated region where the corrosion current density is relatively low till the point where passivity transitions to transpassivity. This is the region where the passive protective film becomes unstable and rapidly collapses. During this period, metastable pitting corrosion dominates before stable pit propagation. During passivation, LCS is protected from corrosion due to the relative strength of the protective oxide on the steel surface. At the onset of pitting corrosion, the corrosion current density rises significantly leading to progressive breakdown and collapse of the passive oxide film. The passivity of LCS is due to the high alkalinity of concrete pore electrolyte as earlier discussed [50, 51]. Potentiostatic data from LCS polarization plots are shown in Table 6, while Fig. 2 is a close-up view of region where localized corrosion reaction sets in. Observation of the table shows the presence of Al-D slightly improves the passivation and localized corrosion resistance properties of LCS. At 0% Al-D, the passivation range value is 1.456 V, while in the presence of varying concentration of Al-D the high and low points of the passivation range are 1.555 V and 1.534 V. The potential before the onset of localized corrosion reaction phenomena, i.e., metastable pitting activity, also shows the influence of Al-D on the electrochemical reaction processes. Al-D extends the potential at which the passive oxide begins to breakdown from 0.71 V at 0% Al-D to potentials between 0.77 and 0.78 V. The plot in Fig. 2 shows the metastable pitting activity of LCS at 0% Al-D occurs at higher corrosion current density and lower corrosion potential. Within this region, the rate of repassivation is slower and eventually overwhelmed by the film breakdown mechanism [52]. The degree of electrochemical activity at this concentration (0% Al-D) is much higher resulting in accelerated corrosion compared to the plots at varying concentrations of Al-D. Aggressive anions within the concrete pore electrolyte displace the anions responsible for passivation of LCS surface [53].

Table 6 Potentiostatic data for localized corrosion evaluation for LCS in concrete pore electrolyte at 0–25% Al-D concentration (n = 1)
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Fig. 2
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Polarization plot of the pitting region of LCS corrosion during potential scanning in concrete pore solution at 0–25% Al-D concentration

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3.3 Open Circuit Corrosion Potential Measurement

LCS was exposed to concrete pore electrolytes at 0%, 5% and 25% Al-D and the corrosion potential was measured with respect to exposure time. Generally similar OCP plot is displayed in Fig. 3 by LCS in the electrolytes studied signifying similar active–passive corrosion thermodynamic behavior of the steel though differences do exist which helps to understand the nature of the electrochemical reactions occurring at rest potential. The corrosion potential values of LCS at 0% Al-D briefly decreased for the first 17 s of exposure from − 0.506 VAg/AgCl (0 s) to − 0.518 VAg/AgCl, after which a steep increase in corrosion potential occurred to − 0.399 VAg/AgCl at 300 s. Beyond 300 s, the rate of increase in corrosion potential reduced substantially causing a parabolic curvature until 1800 s at − 0.209 VAg/AgCl where quasi-steady equilibrium state was achieved due to the spontaneous growth of the protective passive oxide on LCS surface. The interaction of adsorbed O2 with the ionized LCS surface is responsible for the formation of the oxide film. The corrosion potential of LCS at 0% Al-D peaks at − 0.164 VAg/AgCl which greatly differs from its value at 0 s. This phenomenon increases the activation energy for the surface deterioration of LCS as a result the leaching of LCS cations away from the steel surface is effectively controlled. At 0 s the corrosion potential of LCS at 5% and 25% Al-D initiated at values of − 0.667 VAg/AgCl and − 0.793 VAg/AgCl which are significantly more electronegative than the corrosion potential of LCS at 0% Al-D. As earlier mentioned, similar OCP plot was displayed by LCS at 5% and 25% Al-D compared to the plot at 0% Al-D. The lower pH values of the concrete pore electrolyte at 5% and 25% Al-D mean the hydration, and diffusion of LCS cations in the concrete pore electrolyte is more prevalent. However, at 4900 s the OCP plot of LCS at 5% Al-D links with the plot at 0% Al-D (− 0.175 VAg/AgCl) to peak at − 0.166 V at 5400 s while the plot 25% Al-D peaked at − 0.187 VAg/AgCl (5400 s). This shows that Al-D has negligible effect to the passivation properties of LCS in concrete pore electrolyte without applied potential.

Fig. 3
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Variation in OCP plot of LCS (0%, 5% and 25% Al-D) versus exposure time

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3.4 Morphological Studies

Optical images of LCS morphology (mag. ×10 and ×40) before corrosion and after corrosion from the concrete pore electrolyte at 0%, 5% and 25% Al-D are shown in Figs. 4, 5, 6 and 7. Severe morphological deterioration in the form of corrosion pits of different sizes occurred in Fig. 5 due to the electrochemical action of corrosive anions present within the electrolyte. Figure 6 shows the presence of fewer, smaller and comparatively shallower corrosion pits, while in Fig. 7 numerous microscopic corrosion pits are visible. This observation contrasts the corrosion rate results in Table 5. LCS at 0% Al-D had the lowest corrosion rate from potentiodynamic polarization test and yet has the most severe morphological deterioration, while Fig. 7 has the highest corrosion rate value with the mildest morphological deterioration. Comparing the optical images with the results from passivation and pitting corrosion evaluation, it is clearly visible that Al-D improves the localized corrosion resistance of LCS despite minimal increase in general corrosion rate. At 25% Al-D, the effect of the corrosive anions on the pitting corrosion resistance of LCS has been significantly curtailed. Under the influence of applied potential, the passive film will eventually breakdown; however, the small microscopic pits show the breakdown likely occurred at sites or regions where microscopic impurities are prevalent.

Fig. 4
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Optical microscopy images of LCS morphology before corrosion

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Fig. 5
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Optical microscopy images of LCS morphology after corrosion from concrete pore electrolyte/0% Al-D

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Fig. 6
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Optical microscopy images of LCS morphology after corrosion from concrete pore electrolyte/5% Al-D

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Fig. 7
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Optical microscopy images of LCS morphology after corrosion from concrete pore electrolyte/25% Al-D

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4 Conclusion

Al-D in concrete enhanced the passivation and pitting corrosion resistance of carbon steel in concrete pore electrolyte. The passivation range of the polarization plots and pitting potential increased with an increase in Al-D concentration. The morphology of the carbon steel in the concrete pore without Al-D showed the presence of localized corrosion damage in addition to a severely pitted surface. This contrasts the morphology of the steel from the pore electrolyte at specific Al-D concentration which showed very limited surface deterioration.