Corrosion inhibition mechanism of steel reinforcements in mortar using soluble phosphates: A critical review

materials Review Corrosion Inhibition Mechanism of Steel Reinforcements in Mortar Using Soluble Phosphates: A Critical Review David M. Bastidas 1,* , Ulises Martin 1, Jose M. Bastidas 2and Jacob Ress 1   Citation: Bastidas, D.M.; Martin, U.; Bastidas, J.M.; Ress, J. Corrosion Inhibition Mechanism of Steel Reinforcements in Mortar Using Soluble Phosphates: A Critical Review. Materials 2021,14, 6168. https://doi.org/10.3390/ma14206168 Academic Editor: Frank Collins Received: 18 August 2021 Accepted: 13 October 2021 Published: 18 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1National Center for Education and Research on Corrosion and Materials Performance—NCERCAMP-UA, Department of Chemical, Biomolecular and Corrosion Engineering, The University of Akron, 302 E Buchtel Ave., Akron, OH 44325-3906, USA; [email protected] (U.M.); [email protected] (J.R.) 2 National Center for Metallurgical Research—CENIM, Consejo Superior de Investigaciones Científicas—CSIC, Ave. Gregorio del Amo 8, 28040 Madrid, Spain; [email protected] *Correspondence: [email protected] Abstract: The corrosion inhibition mechanism of soluble phosphates on steel reinforcement embedded in mortar fabricated with ordinary Portland cement (OPC) are reviewed. This review focuses soluble phosphate compounds, sodium monofluorophosphate (Na 2 PO 3 F) (MFP), disodium hydrogen phosphate (Na 2 HPO 4 ) (DHP) and trisodium phosphate (Na 3 PO 4 ) (TSP), embedded in mortar. Phosphate corrosion inhibitors have been deployed in two different ways, as migrating corrosion inhibitors (MCI), or as admixed corrosion inhibitors (ACI). The chemical stability of phosphate corrosion inhibitors depends on the pH of the solution, H 2 PO 4− ions being stable in the pH range of 3–6 , the HPO 42− in the pH range of 8–12, while the PO 43− ions are stable above pH 12. The formation of iron phosphate compounds is a thermodynamically favored spontaneous reaction. Phosphate ions promote ferrous phosphate precipitation due to the higher solubility of ferric phosphate, thus producing a protective barrier layer that hinders corrosion. Therefore, the MFP as well as the DHP and TSP compounds are considered anodic corrosion inhibitors. Both types of application (MCI and ACI) of phosphate corrosion inhibitors found MFP to present the higher inhibition efficiency in the following order MFP > DHP > TSP. Keywords: steel reinforcements; concrete; migrating corrosion inhibitors; phosphate penetration; thermodynamics; reactivity 1. Introduction Due to its comparatively low cost and versatility, reinforced concrete is commonly used in the construction industry [ 1 – 6 ]. Despite its excellent compressive force, concrete alone is unable to withstand the necessary tensile load and reinforcements are necessary. The combined properties of concrete and steel reinforcements provide high compression strength as well as increased mechanical properties, thus making it an ideal composite material for a multitude of applications and structures [ 2 ]. The corrosion of these steel reinforcements is considered to be the greatest threat to the integrity of these structures and their service life [ 3 , 4 ]. Various solutions have been implemented to deter corrosion, such as corrosion inhibitors and many others [6–10]. Corrosion inhibitors for steel in concrete can be used by addition to the cement paste, called admixed corrosion inhibitor (ACI) [ 11 , 12 ], or by applying with brush or spray to the hardened concrete surface diffusing through the pores of the concrete, known as migrating corrosion inhibitor (MCI), see Figure 1[ 13 – 15 ]. Examples of ACIs are amines and fatty acid esters [ 16 ], which act through a double mechanism, first, by reducing the ingress of the chlorine ion through the hydrophobic property of the esters and second, by forming a protective layer through the ion-dipole interaction (δ+H−Nδ−) [17]. Materials 2021,14, 6168. https://doi.org/10.3390/ma14206168 https://www.mdpi.com/journal/materials Materials 2021,14, 6168 2 of 34 Figure 1. Penetration depth of inhibitors acting as migrating corrosion inhibitors (MCI): ( a ) watersoluble fluoride and ( b ) water-soluble phosphate [ 15 ]. Reproduced with permission from Ngala, V. et al., Corros. Sci.; published by Elsevier, 2003. Sodium nitrite (NaNO 2 ) [ 18 ], disodium stannate (Na 2 SnO 3 ) [ 19 ], disodium molybdate (Na 2 MoO 4 ), sodium borates (NaBO 2 ), trisodium borate (Na 3 BO 3 ), cerium nitrate (Ce(NO 3 ) 2 ) and trisodium phosphate (Na 3 PO 4 ) (TSP) have been used as corrosion inhibitors for steel [ 20 , 21 ]. Thermodynamic studies indicate that adsorption of phosphate inhibitor molecules is a spontaneous process governed by physisorption through the Langmuir isotherm model [ 22 , 23 ]. Nitrite-based inhibitors compete with chloride ions in the reaction with ferric iron, favoring the formation of ferric oxide (Fe 2 O 3 ) and lepidocrocite (γ-FeOOH) [24]: 2Fe2+ + 2OH−+ 2NO2−→2NO + Fe2O3+ H2O (1) Fe2+ + OH−+ NO2−→NO + γ-FeOOH (2) Because the reactions in Equations (1) and (2) are faster than those related to chloride ions, a stable and protective layer of lepidocrocite is generated. This ability to oxidize ferrous to ferric iron produces more insoluble compounds layers, for instance, Ksp,Fe(OH)2= 7.9 ×10−15,Ksp,Fe(OH)3= 6.3 ×10−38 [25], with a thickness of 17–50 Å [16]. Inorganic corrosion-inhibiting compounds are toxic and have low corrosion-inhibitor efficiency if they are not added in the appropriate amount. In the use of nitrite-type inhibitors, the ratio [NO 2− ]/[Cl − ] needs to be near unity, so that the corrosion inhibition efficiency is maximized [ 26 ]. Hybrid inhibitors consisting of inorganic quaternary ammonium salts or phosphates and organic compounds such as imidazole have also been shown Materials 2021,14, 6168 3 of 34 to impart high inhibition efficiency [ 27 , 28 ]. Additionally, tertiary amines for the repair of structures have been used as organic corrosion inhibitors [29]. Another method for enhancing the corrosion protection of the embedded rebars is by the use of coatings, as they not only provide a barrier against the electrolyte but also are able to work as a vehicle for inhibitors to further improve the corrosion resistance [ 30 – 34 ]. The use of phosphate chemical conversion (PCC) coatings causes the phosphates to react with Fe ions, forming insoluble compounds, thus impeding the corrosion process [ 31 ]. Additionally, phosphates are used as corrosion-inhibitor pigments [ 35 – 38 ]. Due to its low-cost and low environmental impact, PCC technology has been widely used to improve corrosion resistance by application to the metal surface in order to receive a liquid, powder, or electrodeposited coating [ 39 ]. PCC primarily is used to form a layer of iron-zinc phosphate, which is insoluble and corrosion resistant, to produce a protective coating [ 40 ]. PCC provides multiple benefits including water resistance and increased adhesion, thus increasing the coating durability. Traditionally, the PCC solution is composed of diluted phosphoric acid (H 3 PO 4 ) (PA) and contains metal ions to form coatings, for instance Zn 2+ , Ni 2+ , Fe 2+ and Mn 2+ and ions of NO 3− or NO 2− as accelerators [ 39 ]. Both a hopeite (Zn 3 (PO 4 ) 2· 4H 2 O) coated specimen at pH 3.00 and the scholzite (CaZn 2 (PO 4 ) 2· 2H 2 O) coated specimen at pH 3.75 displayed improved corrosion resistance due to the high ratios of Zn/P and Ca/P, respectively. Alternatively, Ca 2+ ions have been used as accelerators in the PCC rather than a constituent of the coating [32]. The PCC coating consists of a double layer with an amorphous inner layer and a crystalline phosphate outer layer, according to Jiang et al. [ 33 ]. The four stages Jiang et al. proposed are shown in Figure 2; (Figure 2a) the steel dissolution, (Figure 2b) the deposition of the amorphous phase, (Figure 2c) phosphate growth and crystallization and (Figure 2d) the balance of coating dissolution and formation. Figure 2. Formation of a phosphate chemical conversion coating: ( a ) microgalvanic couple is established immediately, ( b ) rapid precipitation of amorphous ferric phosphate and ferric oxide on the steel surface, ( c ) insoluble phosphate is deposited and then crystallizes onto the PCC coating on the amorphous base layer, and ( d ) complete PCC coating is formed [ 33 ]. Reproduced with permission from Jiang, C. et al., Electrochem. Commun.; published by Elsevier, 2020. By means of XPS analysis, the formation of the amorphous base layer of Fe 2 O 3 and FePO 4 was proven, as seen in Figure 3, where both peaks were recognized in the Fe 2p 3/2 (Figure 3b) and O 1 s (Figure 3d) spectra. Materials 2021,14, 6168 4 of 34 (a) 01s 9000 (b) 80000 Fe2p3 8000 60000 7000 "fil "fil = 6000 5 40000 = 5 u u 5000 20000 4000 3000 200 400 600 800 1000 1200 706 707 708 709 710 711 712 713 714 715 716 717 B.E.(eV) B.E.(eV) 12000 16000 Zn2p,,, (d) Ols 10500 15000 � "fil 7500 = i= 14000 5 6000 5 4500 13000 3000 12000 1500 1016 1018 1020 1022 1024 1026 1028 522 524 526 528 530 532 534 536 538 540 B.E.(eV) B.E.(eV) Figure 3. XPS surface analysis: ( a ) the XPS survey spectrum and the XPS spectrum of ( b ) Fe 2p 3/2 , ( c ) Zn 2p 3/2 , and (d) O 1 s for samples immersed in the PCC bath for 10 s [ 33 ]. Reproduced with permission from Jiang, C. et al., Electrochem. Commun.; published by Elsevier, 2020. The corrosion-inhibition mechanism of phosphates is not fully understood, it is believed that phosphate inhibitors react with the iron ions generated in the corrosion process [41] , or with ions present in the mortar, such as calcium, which forms calcium phosphate (Ca 3 (PO 4 ) 2 ) precipitates, filling the pores and cracks of the mortar, thus impeding the diffusion of aggressive ions [ 42 – 46 ]. It has been found that sodium phosphate (Na 3 PO 4 ) can prevent pitting corrosion of steel in the simulated concrete pore solution if its concentration is equal to the chloride concentration [ 47 ]. The presence of phosphates in the mortar increases the critical period of pitting initiation from 30 to 100 days and significantly reduces the chloride diffusion rate. Moreover, the apparent chloride diffusion coefficient calculated for mortar containing Na 3 PO 4· 12H 2 O which is around 1.03 × 10 −12 m 2 /s is lower than that obtained with the reference mortar (2.2 × 10 −12 m 2 /s) for the same testing period [48]. The concentration of phosphate species inside the pits was higher than the passive film zones without the pit. This indicates that phosphate ions could inhibit the corrosion process through a competitive adsorption mechanism with chloride ions where the chloride attack triggers the phosphate species to further adsorb at the pit locations on the metal surface [ 49 ]. In addition, the presence of phosphate ions stabilizes ferrihydrite, a poorly crystallized FeOOH, which may be a protective layer for steel in Cl − -contaminated concrete simulating solutions [ 8 ]. As a counterpart, the inhibition efficiency of phosphate corrosion inhibitors is decreased in concrete because of the reaction of PO 43− ions with the concrete matrix [50]. Materials 2021,14, 6168 5 of 34 The surface analysis methods demonstrated that the inhibition mechanism of phosphate ions is attributed to the formation of a passive film with a duplex layer on the metal surface, including the inner layer of iron(hydro)oxides, formed by a solid-state mechanism and the outer layer of iron phosphate complexes mainly as FeHPO 4 , Fe 3 (PO 4 ) 2 and even Fe(PO4), formed via a dissolution–precipitation mechanism [49]. The impact of phosphate corrosion inhibitors on the stability of the passive film depends on the [Cl − ]/[OH − ] ratio. It can be explained by the fact that the high concentration of hydroxyl groups relative to chloride ions causes a predominant effect to form a robust passive film, which in turn stops the pitting corrosion without the aid of phosphate. This corrosion protection mechanism is associated with a continuous increase in the resistance of the passive film by adsorption of phosphate species in the weak points of the passive film, which block the anodic sites [51]. Studies performed under applied mechanical stress revealed that in the presence of phosphate corrosion inhibitors, the critical [Cl − ]/[OH − ] ratio increased from 0.4 to 5 for strained electrodes under stress conditions (80% UTS) [52]. The addition of phosphate corrosion inhibitors led to a decrease in chloride binding. This is mainly because phosphates hold a higher priority over chloride during ion exchange in Al 2 O 3 –Fe 2 O 3 -mono (AFm) and –tri (AFt) phases in the Ca–Al–S–O–H system of the concrete matrix. Moreover, phosphates exerted a significant influence on the chemical binding but a negligible effect on the physical binding [53]. More recently, the use of alternative pentasodium triphosphate compounds Na 5 P 3 O 10 , also known as sodium tripolyphospate, has shown an increased inhibition efficiency of around 80% for 480 days exposure in 3.5% NaCl, which is attributed to the development of a protective film barrier of PST on the steel rebar surface. Potentiodynamic polarization results revealed that PST affects the anodic and cathodic sites uniformly, thus presenting a mixed-type corrosion-inhibitor protection mechanism [ 54 ]. The use of inorganic corrosioninhibitor mixtures containing hexametaphosphate compounds (NaPO 3 ) 6 , used in 3.5% NaCl contained SCPS, have been proven to reduce the corrosion rate by 8.60 and 25.52 times for 3 and 5% inhibitor addition, respectively [55]. Disodium hydrogen phosphate (Na 2 HPO 4 , DHP) in simulated concrete pore solution (SCPS) and in mortar acts as an anodic corrosion inhibitor [ 43 , 46 ]. Overall, phosphates require oxygen to be effective, as they are nonoxidizing anodic inhibitors [ 38 ]. Trisodium phosphate (Na 3 PO 4· H 2 O, TSP) in mortar acts as a mixed corrosion inhibitor [ 56 ], as well as in chloride environments containing a [PO 43− ]/[Cl − ] ratio higher than 0.6 [ 41 , 57 ] and even as a cathodic corrosion inhibitor, with a [PO 43− ]/[Cl − ] ratio less than 0.6 [ 58 ]. Impedance techniques, such as electrochemical impedance spectroscopy (EIS), can be used to measure the corrosion improvement of inhibitors [ 30 , 45 , 56 , 59 , 60 ]. Yohai et al. showed in their work with mortars great corrosion enhancement of the TSP (Mix C), even with the chloride addition, over the blank (Mix A) and the chloride contaminated specimen (Mix B) (see Figure 4a), clearly seen in the Bode plot where Mix C is of one order of magnitude higher than the other two mixes (see Figure 4b) [ 56 ]. Similarly, Chaussadent et al. found an improvement in the corrosion protection by sodium monofluorophosphate as a corrosion inhibitor using the EIS results of mortar samples after carbonation (Figure 5) [ 61 ]. Another example of the use of EIS is the work from Etteyeb et al., in which the specimen containing inhibitors (see Figure 6a) increased its impedance by two orders of magnitude compared to the blank (see Figure 6b) [45]. Materials 2021,14, 6168 6 of 34 25 6.0 (a) (b) 20 5.5 N E N E (.) C: 15 0 -"" 10mHz � Ol � 5.0 E 10 .9 4.5 5 0 4.0 0 5 10 15 20 25 -4 -3 -2 -1 0 1 2 3 4 5 Zreal / kn cm 2 log (f / Hz) -60 (c) -50 -40 QJ QJ -30 -- a:, -20 -10 0 -5 -4 -3 -2 -1 0 1 2 3 4 5 log (f / Hz) Figure 4. EIS spectra for mix designs: Mix A ( ), Mix B ( ) and Mix C ( ), registered after 720 days of exposure. Points represent the experimental EIS data and lines show the fitting results: ( a ) Nyquist plot, ( b ) and ( c ) Bode plots [ 56 ]. Reproduced with permission from Yohai, L. et al., Electrochim. Acta; published by Elsevier, 2016. Figure 5. EIS diagrams for carbonated mortars at 48 h after application of aqueous solutions (MFP or water) [ 61 ]. Reproduced with permission from Chaussadent, T. et al., Cem. Conc. Res.; published by Elsevier, 2006. Materials 2021,14, 6168 7 of 34 Figure 6. Impedance diagrams (Nyquist representation) of ( a ) steel electrode/S 1 solution, ( b ) steel electrode/S 2 solution and ( c ) pretreated steel electrode/S 1 solution; for two periods of immersion: 2 and 72 h and for two electrode rotation rate values: 100 and 500 rpm [ 45 ]. Reproduced with permission from Etteyeb, N. et al., Electrochim. Acta; published by Elsevier, 2007. The corrosion inhibitor effect on the anodic and cathodic polarization curves can be seen in the Evans diagrams shown in Figure 7. This is in agreement with the findings of Yohai et al., in which [PO 43− ]/[Cl − ] = 1 and phosphate behaves as a mixed inhibitor [ 60 ] and phosphate ions promote ferrous phosphate precipitation due to the higher solubility of ferric phosphate (pK sp = 26) than ferrous phosphate (pK sp = 32). The cyclic voltammograms in PSS, PSS + Cl − and PSS + Cl − + PO 43− showed the presence of a single negative and positive peak, attributed to the accumulation of magnetite on the steel surface, which is not fully reduced (see Figure 8a) [ 60 ]. However, for the PO 43− ions, which promote Fe 3 (PO 4 ) 2 precipitation, the difference was not substantial and is due to lack of formation of Fe 3+ compound, hence not getting reduced in the following cycles (see Figure 8b). By the impedance fitting, the protectiveness of the PO 43− ions was also seen, showing more ideal capacitors, which were related to the presence of a protective passive layer, however a small decrease in the impedance was associated with the change in the film composition, influencing the electronic properties (see Figure 9). Materials 2021,14, 6168 8 of 34 Figure 7. Evans diagrams showing the effect of a corrosion inhibitor, ( a ) on the anodic branch (anodic inhibitor), ( b ) on the cathodic branch (cathodic inhibitor), and ( c ) on both the anodic and cathodic branches (mixed inhibitor). Figure 8. Cyclic voltammograms for steel: ( a ) (tenth cycle) in PSS (—), PSS + Cl − (–  –), PSS + Cl − + PO 43− (– # –), and ( b ) cycles 1–10th in PSS + Cl − . Scan rate: 10 mV s −1 [ 60 ]. Reproduced with permission from Yohai, L. et al., Electrochim. Acta; Published by Elsevier, 2013. Materials 2021,14, 6168 9 of 34 Figure 9. Impedance spectra recorded on steel electrodes aged over 24 h at E corr in SSP + Cl − with and without inhibitor. The symbols represent the data and the lines the fitting results. ( a ) Nyquist representation, ( b ) and ( c ) Bode representation, and ( d ) imaginary part of impedance as function of frequency, in logarithmic scale. PSS + Cl − (–  –), PSS + Cl − + PO 43− (–#–) [60]. Reproduced with permission from Yohai, L. et al., Electrochim. Acta; Published by Elsevier, 2013. It has been also observed that TSP behaves as an anodic corrosion inhibitor, which penetrates through the pores and necessitates the use of high concentrations [ 45 , 62 ], which can cause rheological changes to the mortar. TSP increases the critical concentration threshold of the ratio [Cl−]/[OH−] for steel in SCPS and in mortar [63]. Corrosion begins in active sites of the reinforcement when the concentration of chloride at the steel/concrete interface exceeds a critical value [ 64 ]. The corrosion rates for various measurements are presented in Table 1as an example [65]. Table 1. Evaluation of corrosion rate of carbon steel in concrete [ 65 ]. Reproduced with permission from Millard, S. et al., NDT E Int.; Published by Elsevier, 2001. Corrosion Rate Rp, kΩcm2icorr,µA/cm2Mass Loss, µm/Year Very High 2.5−0.25 10−100 100−1000 High 25−2.5 1–10 10–100 Low 250−25 0.1–1 1–10 Passive >250 <0.1 <1 Table 2shows corrosion potential (E corr ) and corrosion current density (i corr ) measured for various phosphates [ 8 , 15 , 38 , 61 , 63 – 66 ]. The probability for active corrosion, according to ASTM C876 is high (~90%) for E corr < − 0.27 V vs. SCE, uncertain for −0.27 V < Ecorr <−0.12 V vs. SCE and 10% chance for corrosion for E corr > − 0.12 V vs. SCE [67]. Materials 2021,14, 6168 16 of 34 Table 4. Penetration depth for P and F in mortar specimens immersed in 5 wt.% Na 3 PO 4· H 2 O (TSP), Na2HPO4(DHP) and Na2PO3F (MFP) solution, water/cement (w/c) ratios of 0.5 and 0.6. Penetration Depth (mm) Element TSP w/c 0.5 TSP w/c 0.6 DHP w/c 0.5 DHP w/c 0.6 MFP w/c 0.5 MFP w/c 0.6 P 0.88 1.10 0.36 0.64 1.33 1.23 F – – – – 1.36 1.40 It should be noted that differences were not observed in the penetration of MFP, DHP or TSP for the two w/c ratios (0.5 and 0.6), which may be interpreted that they are too close to observe any effect. It has been reported that MFP imparts effective corrosion-inhibition protection only for reinforcing steel bars with concrete cover not thicker than 1 cm [100]. The thermogravimetric analysis (TGA) for samples A and B is shown in Figure 14 [42] . Sample A was fabricated using 500 mL of 0.4 M NaHPO 4 and 500 mL of 0.6 M Ca(NO 3 ) 2· 4H 2 O and at pH ~12.5. Sample B was prepared using 500 mL of 0.4 M NaHPO 4 and 500 mL of 0.6 M Ca(NO 3 ) 2· 4H 2 O and at pH ~8.5. Sample A showed continuous weight loss (4.50% total) within the range of 50–400 ◦ C, likely caused by occluded water evaporation. Within the ranges of 550–630 ◦ C and 700–800 ◦ C were two weight reductions, likely caused by the dehydroxylation of the hydroxyapatite (Ca 5 (PO 4 ) 3 (OH)) (HAP). Sample B showed a similar behavior to sample A. A gradual weight reduction of 6.50% was observed over the range of 25–600 ◦ C, signifying a low crystallinity. Again, two decreasing steps are shown, attributed to the dehydroxylation of the HAP in the ranges of 600–630 ◦ C and 700–800 ◦ C, corroborating the XRD results (Figure 14b). Figure 14. Phosphate samples obtained using 0.4 M NaHPO 4 and 500 mL of 0.6 M Ca(NO 3 ) 2· 4H 2 O at pH ~12.5 (sample A) and pH ~8.5 (sample B): ( a ) thermogravimetric (TGA) results, and ( b ) X-ray diffraction (XRD) patterns for samples A and B [ 42 ]. Reproduced with permission from Bastidas, D.M. et al., Constr. Build. Mater.; published by Elsevier, 2010. Materials 2021,14, 6168 17 of 34 4. Effect of Phosphate on the Steel Reinforcement MCI specimens with embedded steel rebar in mortar and immersed in 0.2 M phosphate solutions (MFP, DHP, TSP) or distilled water for the control specimen were monitored for E corr versus time and are shown in Figure 15, top [ 43 ]. Overall, the E corr values were shown to be within the level of low or uncertain risk for corrosion, for E corr < − 0.28 V vs. SCE the probability of corrosion was high (>90%); for − 0.28 V< E corr < − 0.12 V vs. SCE corrosion was uncertain; and for E corr > − 0.12 V vs. SCE there was a 10% probability of corrosion [ 67 ]. The control test alone measured E corr values that corresponded to a high risk of corrosion and the results from MFP, DHP and TSP suggest they acted as anodic inhibitors. The ACI specimens are shown in Figure 15a for embedded steel rebar in mortar with a blending of 3 wt.% MFP, DHP, or TSP solid powders with OPC, water and sand. The ACI specimens were then stored in desiccators at ~95% relative humidity (RH). The DHP showed the best corrosion inhibitor performance. Figure 15. Corrosion potential (E corr ) versus time for steel in the presence of: ( a ) migrating corrosion inhibitor (MCI) and ( b ) admixture corrosion inhibitor (ACI) [ 43 ]. Reproduced with permission from Bastidas, D.M. et al., Cem. Conc. Comp.; published by Elsevier, 2013. The E corr versus time plot for MCI specimens with embedded steel rebar in mortar with 0.2 M phosphate solutions (MFP, DHP or TSP) contaminated with 3.5 wt.% NaCl is shown in Figure 16a [ 46 ]. The E corr values displayed corrosion risk levels of low, uncertain and high [ 67 ]. The MCI specimens containing MFP, DHP, TSP inhibitors showed less negative E corr values compared to the control specimen, with the exception of the TSP solution, which offered a high risk of corrosion. The E corr values over time for the ACI specimens with embedded rebar in mortar blended with 3 wt.% solid powders of MFP, DHP, or TSP with OPC, water and sand and contaminated with 3.5 wt.% NaCl are shown in Materials 2021,14, 6168 18 of 34 Figure 16a. Similar to the MCI specimens, the best corrosion inhibitor observed for the ACI specimens was the MFP with a medium and high risk of corrosion according to the E corr values. Therefore, the MFP as well as the DHP and TSP compounds may be considered to be anodic inhibitors. Figure 16. Corrosion potential (E corr ) versus time for steel in the presence of: ( a ) migrating corrosion inhibitor (MCI) and ( b ) admixture corrosion inhibitor (ACI) bottom and in contact with 3.5 wt.% NaCl [ 46 ]. Reproduced with permission from Bastidas, D.M. et al., Cem. Conc. Comp.; published by Elsevier, 2015. The corrosion current density (i corr ) measured for the MCI specimens over the test period is shown in Figure 17a. The specimens contained steel rebar embedded in mortar submerged in 0.2 M MFP, DHP, or TSP solutions, with a distilled water control specimen [43] . The measured i corr values depict the samples within low or medium risk for corrosion [ 65 ]. The dotted line located at 0.1 µ A/cm 2 depicts the estimated limit for steel passivity [ 65 , 101 ]. The MFP and DHP compounds showed the best inhibitive performance for the MCI specimens measuring i corr values below 0.1 µ A/cm 2 . Between 10 and 30 days, the TSP showed passivity, however, the i corr subsequently increased. The control sample showed active corrosion. The i corr values for the ACI specimens prepared with embedded steel rebar with a blend of 3 wt.% MFP, DHP, or TSP solid powders with OPC, water and sand are shown in Figure 17a. The ACI specimens were stored in desiccators at ~100% HR. The DHP compound was shown to be the best corrosion inhibitor for ACI specimens, showing icorr values at low or medium risk levels for corrosion. Materials 2021,14, 6168 19 of 34 Figure 17. Corrosion current density (i corr ) versus time for steel in the presence of: ( a ) migrating corrosion inhibitor (MCI) and ( b ) admixture corrosion inhibitor (ACI) bottom [ 43 ]. Reproduced with permission from Bastidas, D.M. et al., Cem. Conc. Comp.; published by Elsevier, 2013. Figure 18 shows the average and maximum corrosion weight losses for steel bars embedded in concrete for noncarbonated and carbonated conditions [ 15 ]. The measured weight losses showed a scarce reduction for Na 2 PO 3 F − treated noncarbonated specimens, while a minor increase was seen in the carbonated samples, related to the hydrolysis of PO3F−promoting the formation of F−ions. Materials 2021,14, 6168 20 of 34 Figure 18. Average and maximum corrosion weight losses for steel bars at three cover depths in various concrete specimens: ( a ) control noncarbonated, ( b ) sodium monofluorophosphate noncarbonated, ( c ) control carbonated, and ( d ) sodium monofluorophosphate carbonated [ 15 ]. Reproduced with permission from Ngala, V. et al., Corros. Sci.; published by Elsevier, 2003. The i corr values for the MCI specimens with steel rebar embedded in mortar in 0.2 M MFP, DHP, or TSP solution with 3.5 wt.% NaCl contamination are shown in Figure 19a [46] . According to the i corr values, the specimens are within low to medium corrosion risk [ 67 ]. MFP and DHP showed the best corrosion inhibition for the MCI specimens with i corr values below <0.1 µ A/cm 2 . The control sample showed active corrosion over the test period. The i corr values for the ACI specimens with embedded steel rebar prepared by blending 3 wt.% MFP, DHP, or TSP and OPC, water, sand and 3.5 wt.% NaCl contamination are shown in Figure 19a. The MFP compound showed the highest corrosion inhibitor performance and had i corr values between low or medium corrosion risk. The DHP and TSP compounds also displayed medium corrosion risk, while the control specimens had icorr between 0.3–0.9 µA/cm2, close to the TSP values. The corrosion inhibition efficiency (IE) over time for the embedded rebar in MCI and ACI is shown in Figure 20 [42]. The IE was obtained using Equation (3) [102]: IE (%)=icorr,abs −icorr,pre icorr,abs ×100 (3) where i corr,abs and i corr,pre are the steel i corr (estimated from LPR measurements) in the absence and presence of corrosion inhibitor, respectively. Materials 2021,14, 6168 21 of 34 Figure 19. Corrosion current density (i corr ) versus time for steel in 3.5 wt.% NaCl in the presence of: ( a ) migrating corrosion inhibitor (MCI), and ( b ) admixture corrosion inhibitor (ACI) [ 46 ]. Reproduced with permission from Bastidas, D.M. et al., Cem. Conc. Comp.; published by Elsevier, 2015. Overall, for the MCI specimens, the MFP compound showed the best IE. However, the MFP and DHP compounds showed the highest IE values after 50 days of exposure. The order of IE is MFP > DHP > TSP. The ACI specimens showed a similar trend to the MCI specimens (Figure 20a). As previously discussed with the enhancement of the corrosion properties seen by the EIS, by optical micrographs, it is unequivocal that the presence of PO 43− helped maintain the integrity of the steel reinforcement, seen by the absence of corrosion (see Figure 21a,c) compared to the non PO 43− , which presented clear signs of dissolution (see Figure 21b,d) [60]. Materials 2021,14, 6168 22 of 34 Figure 20. Inhibitor efficiency versus time for steel in the presence of: ( a ) migrating corrosion inhibitor (MCI), and ( b ) admixture corrosion inhibitor (ACI) [ 42 ]. Reproduced with permission from Bastidas, D.M. et al., Constr. Build. Mater.; published by Elsevier, 2010. Figure 21. Micrographs of the electrodes after having carried out anodic polarization curves: ( a ) PSS + Cl − + PO 43− , ( b ) PSS + Cl − without and with corrosion products, ( c ) micrograph magnification of PSS + Cl − + PO 43− , and ( d ) micrograph magnification of PSS + Cl − without and with corrosion products [ 60 ]. Reproduced with permission from Yohai, L. et al., Electrochim. Acta; Published by Elsevier, 2013. Materials 2021,14, 6168 23 of 34 5. Thermodynamics and Reactivity of Phosphate Corrosion Inhibitors The formation of iron phosphate compounds, Fe 3 (PO 4 ) 2 and FePO 4 [ 103 , 104 ], see Equations (4) and (5), is thermodynamically favored over the formation of iron chloride compounds, FeCl 2 and FeCl 3 [ 105 ], see Equations (6) and (7), therefore leading to the formation of a stable phosphate barrier layer that can inhibit the chloride attack. Consequently, hindering the iron acid hydrolysis reaction [106], see Equations (8) and (9). Fe2+ + 2 PO43−Fe3(PO4)2∆Gf◦Fe3(PO4)2=−2444.80 kJ/mol (4) Fe3+ + PO43−FePO4∆Gf◦FePO4=−1663.98 kJ/mol (5) Fe2+ + 2 Cl−FeCl2∆Gf◦FeCl2=−302.35 kJ/mol (6) Fe3+ + 3 Cl−FeCl3∆Gf ◦ FeCl3=−668.11 kJ/mol (7) Solutions of FeCl 2 are moderately acidic, then the hydrated Fe 2+ accepts only one hydroxyl ion from the aqueous electrolyte solution, see Equations (8) and (9) [107]. Fe2+ +2H2OFeOH++ H+∆G◦= 46.02 kJ/mol (8) Fe2+ +2H2OFe(OH)2+2H+∆G◦= 34.73 kJ/mol (9) The penetration of phosphate MCI is compromised by the low solubility and precipitation of phosphate compounds, which react with the concrete matrix forming a solid phase that no longer provides a barrier layer of protection to the carbon steel rebar. The MCI specimens showed low phosphate penetration with 1114 µ m, 380 µ m and 126 µ m for MFP, DHP and TSP, respectively, see Figure 12, due to the low porosity of the OPC paste and by the reaction of phosphate ions with portlandite (Ca(OH) 2 ) in the OPC, forming porous apatite (Ca5(PO4)3(OH,F)). This apatite formation likely blocks the inhibitor penetration. The portlandite and MFP reaction was studied by combining a 0.3 M MFP aqueous solution with 2 mL of deionized water and 0.5 g calcium oxide (CaO) in a reactor [ 108 ]. This reaction’s yields were (Ca(OH) 2 ), apatite (Ca 5 (PO 4 ) 3 (OH,F)), fluorite (CaF 2 ) and calcite (CaCO 3 ) crystalline phases and amorphous phase, which were present in a large amount (~63%). TEM images in Figures 22 and 23 show the small crystals (50 − 60 nm) of apatite and ~50 nm crystal of fluorite with 8.2 Å and 3.1 Å spacing, respectively. This corresponds to (1 0 0) reflection of the apatite and (1 1 1) reflection of the fluorite. The half-life of apatite, fluorite and amorphous was 59.7, 40.8 and 48.1 days, respectively. The first step of the reaction is proposed to be a precipitation mechanism [ 108 ]. The amorphous phases are formed by the PO 43− , F − and PO 3 F 2− ions and the dissolved Ca 2+ ions from the Ca(OH) 2 , due to the high solubility product exceeding that of the crystalline calcium phosphate. Equation (10) shows the MFP dissolution process as it reacts with the pores of the portlandite (Ca(OH)2) substrate [61]: 6Ca(OH)2+ 3PO3F2−+ 6Na+Ca5(PO4)3F + CaF2+ 6OH−+ 6Na++ 3H2O (10) the pH of 12.4 of the portlandite suspension increased to 13.5 by the addition of MFP solution. The decrease in PO 3 F 2− ion activity and the consequent increase in the amount of PO 43− ion generates the formation of hydroxyapatite (Ca5(PO4)3OH), Equation (11) [109,110]: 5Ca(OH)2+ 3PO43−+ 9Na+Ca5(PO4)3OH + 9OH−+ 9Na+∆Gf◦=−152.71 kJ/mol (11) Materials 2021,14, 6168 24 of 34 Figure 22. Lattice-fringe image showing d-spacing of 8.2 Å for an OPC mortar sample aged for 14 days [108] . Reproduced with permission from La Iglesia, A. et al., Constr. Build. Mater.; Published by Elsevier, 2012. Figure 23. TEM image of a fluorite crystal for an OPC mortar sample aged for 6 h. Selected area electron diffraction pattern showing 3.1 Å d-spacing of (111) reflection [ 108 ]. Reproduced with permission from La Iglesia, A. et al., Constr. Build. Mater.; Published by Elsevier, 2012. The equilibrium constants for Equations (10) and (11) can be calculated by using the ∆ G f◦ for the different species [ 109 , 110 ], thus the activities of the phosphate ions can be calculated for the MCI specimens, log(a PO3F2− ) = − 23.02 and log(a PO43− ) = − 10.81. These low values confirm a low mobility for the phosphate ions in the OPC paste. Calcium monofluorophosphate (CaPO 3 F) or calcium hydroxide phosphate (CaPO 3 OH), however, can precipitate as amorphous phases and present a higher activity and mobility than the Materials 2021,14, 6168 25 of 34 phosphate ions due to their increased solubility compared to the crystalline phases. The inconsistency between the diffusion for the compounds, see below (1.8 × 10 −8 cm 2 /s for MFP, 6.7 × 10 −9 cm 2 /s for DHP and 5.0 × 10 −9 cm 2 /s for TSP) and the activities can be explained by the formation of calcium monofluorophosphate dihydrate from portlandite and monofluorophosphate (PO 3 F 2− ) ion [ 111 , 112 ], thus, according to Ostwald’s rule, fluorapatite is formed [113]: Ca(OH)2+ PO3F2−+ 2Na++ 2H2OCaPO3F−+ 2H2O + 2OH−+ 2Na+(12) 3Ca(OH)2+ 3CaPO3F·2H2OCa5(PO4)3F + CaF2+ 9H2O (13) The equilibrium constant of Equation (12) and the activity of the monofluorophosphate (PO 3 F 2− ) ion can be calculated using the ∆ G f◦ value for calcium monofluorophosphate dihydrate (CaPO 3 F · 2H 2 O) of –2221.29 kJ/mol [ 114 ], log(a PO3F2− ) = − 4.66, explaining the high diffusion seen in OPC. Therefore, a precipitation − diffusion mechanism is proposed, in which the precipitation of the CaPO 3 F · 2H 2 O phase occurs after the evolution of to Ca 5 (PO 4 ) 3 F as depicted in Equation (13), greatly reducing the activity of the interstitial PO3F2−ion. Apatite formation may passivate the steel by the formation of a physical barrier of hydroxyl ions (OH − ). These OH − ions may diffuse through the Feldman − Sereda porenetwork model [ 103 ], due to the lack of interaction of the PO 3 F 2− or PO 43− ions with the silicate and portlandite. It is assumed that crystalline fluorapatite (Ca 5 (PO 4 ) 3 F) is formed rather than amorphous phases. The volume variation ( ∆ V) can be calculated in Equation (10) by using the different molar volumes (V molar ): ∆ V=V molar(Ca5(PO4)3F) +V molar(CaF2)− 6V molar(Ca(OH)2) = 24.69 Å 3 , which is a loss of 7.55% from the initial volume. Similarly, the volume loss from Equation (4) showed a decrease of 4.44%. These results suggest that the porosity of the cement is increased by some components, thus promoting the penetration of MFP. The contradiction of this description and the low penetration of MFP (1114 µ m) can be explained by the deposition of fluorapatite and hydroxyapatite on portlandite, thus hindering further penetration. The hybrid system formed by a tertiary amine with inorganic phosphorus compounds as a corrosion inhibitor for reinforcing steel yielded a capillary coefficient of ~3 g/m2s1/2 for the inhibitor and ~6 g/m 2 s 1/2 for water, indicating that the inhibitor penetration was limited to the first 20 mm of concrete depth [ 29 ]. This behavior was associated with the high inhibitor viscosity compared to water and mainly a physicochemical interaction between the inhibitor mixture and the concrete pore surface leading to the formation of precipitated compounds that block the concrete porosity. Overall, the addition of MFP, DHP, or TSP showed improvements on the corrosion inhibition of steel by the E corr values. The change in E corr shifted values in the anodic direction causing the system to be at a lower risk to corrosion for both MCI and ACI specimens, see Figure 15. However, the immersion electrochemical analysis showed a high risk of corrosion by i corr values for specimens in distilled water and a low to medium risk level for MCI specimens, Figure 17, top. The ACI specimens in Figure 17b, showed that DHP presented the best inhibitive behavior, measuring i corr values in the passive range for the duration of the test. The high initial i corr values followed by a sharp decrease is suggestive of the protection provided by the inhibitor or alkaline environment or the pore network solution. The following increase in i corr may be explained by the precipitation−diffusion mechanism. DHP, MFP and TSP also show increased corrosion inhibition by E corr shifting in the cathodic direction in the presence of 3.5 wt.% NaCl, see Figure 16. The MCI specimens (Figure 16a) showed corrosion risks of high, medium and low levels, while the ACI specimens (Figure 16b) showed medium or high risks. The MFP, DHP and TSP all can be categorized as cathodic inhibitors in 0.2 M (MCI specimens) or 3 wt.% (ACI specimens). However, other studies in literature have shown conflicting mechanisms with phosphates acting as cathodic inhibitors due to film precipitation on the substrate surface [ 41 ], or Materials 2021,14, 6168 32 of 34 37. Lu, X.; Zuo, Y.; Zhao, X.; Tang, Y. The influence of aluminum tri-polyphosphate on the protective behavior of Mg-rich epoxy coating on AZ91D magnesium alloy. Electrochim. Acta 2013,93, 53–64. [CrossRef] 38. Simões, A.; Torres, J.; Picciochi, R.; Fernandes, J. Corrosion inhibition at galvanized steel cut edges by phosphate pigments. Electrochim. 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