Simulated Pore Solution Research Articles

Development of targeted chloride-responsive Ag/Ca-MoO4-LDH for synergistic corrosion resistance

Layered double hydroxides (LDHs), as novel corrosion inhibitor in reinforced concrete, face instability in chloride consolidation due to sensitivity to environmental anions. This study explored the use of nano-Ag loaded on MoO4-intercalated CaAl-LDH (Ag/Ca-MoO4-LDH), based on isomorphic substitutes for cement hydration products (AFm), to target and stabilize chloride ion fixation in alkaline environments, thereby enhancing steel corrosion resistance. The mechanism of targeting and the enhancement of corrosion resistance were explored through comprehensive experimental and theoretical analyses. Results showed that Ag/Ca-MoO4-LDH enhanced chloride ion consolidation stability, increased the threshold of chlorides needed to trigger corrosion in simulated chloride-contaminated concrete pore (SCCP) solutions, and boosted steel’s anti-corrosion effectiveness. Unlike direct application methods, Ag/Ca-MoO4-LDH released MoO42− in response to chloride presence, providing superior corrosion prevention. The study elaborated on the stability of Ag+ and Cl− interactions in SCCP solutions, the spontaneous exchange reactions between LDH-MoO4 and both LDH-OH and LDH-Cl, and various anti-corrosion mechanisms, including the formation of protective oxide films on steel surfaces and the precipitation of CaMoO4.

Corrosion Protection of Steel by NO3− and NO2− Intercalated Mg-Al Layered Double Hydroxides in Simulated Pore Solutions of Alkali-Activated Slag

This paper aims to perform a comparative investigation on the corrosion protection of steel in the simulated pore solutions of alkali-activated slag (SH) by NO3− and NO2− intercalated Mg-Al layered double hydroxides (MAL) which were fabricated by the calcination rehydration method. The corrosion potential, electrochemical impedance spectroscopy, potentiodynamic polarization and corrosion condition of steel were measured. Furthermore, changes in the microstructures of NO3− intercalated MAL (MAL-N3) and NO2− intercalated MAL (MAL-N2) before and after the adsorption of chloride ion were observed by X-ray diffraction and Fourier transform infrared spectroscopy. The results show that compared to the simulated concrete pore solution (OPCH), MAL-N3 and MAL-N2 exhibit lower chloride adsorption capacities and better corrosion inhibition effects in SH. The chloride adsorption capacity of MAL-N2 is lower compared with that of MAL-N3 due to the different volumes of intercalated anions. In contrast, MAL-N2 presents superior corrosion inhibition than MAL-N3. Furthermore, the decreases in [OH−] in SH due to the additions of MAL-N3 and MAL-N2 are more prominent than those in OPCH. The different synergistic effects due to the competitive anion-exchanges in the interlayers of NO3− and NO2− intercalated MAL in the two solutions contribute to the above effects.

Electrochemical Investigation of Chloride Ion-Induced Breakdown of Passive Film on P110 Casing Steel Surface in Simulated Pore Solution: Behavior and Critical Value Determination

In the petroleum industry, the casing steel is fixed with a cement sheath to ensure reliable service in demanding conditions characterized by high temperature, high pressure, and exposure to multiple types of media. After the hydration of the cement, a porous material is produced with a highly alkaline solution filling the pores, commonly referred to as the pore solution. The casing will form a protective passive film when in contact with a highly alkaline pore solution. Nevertheless, once the cement sheath cracks, chloride ions in the stratum will pass through the cement sheath to the surface of the casing. When chloride ions accumulate to a certain concentration, the passive film will be destroyed, without exerting a protective influence on the substrate. After chloride ions come into direct contact with the casing, the casing is prone to severe failure due to corrosion perforation. The casing failure can cause a blowout outside the casing and even scrapping of the oil well. Controlling casing corrosion and ensuring casing integrity relies on understanding the critical chloride ion concentration that can cause the degradation of the passive film. Therefore, to assess the electrochemical properties and analyze the damage process of the passive film under varying chloride ion concentrations, several characterization techniques were employed. These included potential–time curves (E-t), polarization curves, electrochemical impedance spectroscopy (EIS), and Mott–Schottky curves. In addition, the composition of the passive film on the surface of the P110 casing steel was qualitatively analyzed using X-ray photoelectron spectroscopy (XPS). To further understand the surface morphology of the P110 casing steel, scanning electron microscopy (SEM) was used.

Effects of exposure in seawater sea-sand concrete pore solution on fatigue performance of carbon FRP bars

The study aims to propose a methodology to predict the fatigue performance of carbon fiber-reinforced polymer (CFRP) bars under the seawater sea-sand concrete (SWSSC) environment. The effects of exposure in SWSSC pore solution on the fatigue damage mechanisms and fatigue performance of CFRP bars are presented. Firstly, the tensile fatigue tests of reference CFRP bars were performed under various stress levels with a constant stress ratio of 0.1. Secondly, the tensile fatigue tests of exposed CFRP bars subject to the simulated SWSSC pore solution were carried out. The tensile failure modes and fatigue damage mechanisms corresponding to different stress levels, were identified using scanning electron microscopy (SEM). Besides, the relationship between fatigue life expectancy and underlying fatigue damage mechanisms was carefully analyzed. Finally, the effective stress level method (ESLM) was proposed to evaluate the residual fatigue life of exposed CFRP bars based on the benchmark S–N curve. Combined with the degradation models (chemical etching/diffusion models) proposed in our previous research, ESLM could bridge corrosion fatigue life prediction of the exposed CFRP bars and the benchmark fatigue data of the non-exposed CFRP reinforcement. The predicted results have good correlations with the experimental data. This method can also provide valuable insight for the corrosion fatigue life prediction of other types of FRP composites.

Corrosion behavior of steel fiber reinforced concrete under ambipolar stray current interference

This paper preliminarily discussed the influence of measured stray current (SC) with ambipolar characteristics on the corrosion behavior of steel fiber in simulated pore solution and concrete environment by means of corrosion morphology observation, weight loss experiment, electrochemical test, etc. In simulated solution, the results indicated that the influence of short-term SC on steel fiber was limited, while the addition of Cl- led to the weakening or even disappearance of the self-repairing ability of passivation film and accelerated the occurrence of pitting corrosion on the surface of steel fiber. For steel fiber reinforced concrete (SFRC) samples, the circuit loop was formed through the discrete steel fiber, and its resistance depended on the steel fiber content. Although the concrete provided finite protection, the SFRC suffered from severe corrosion under the combined effect of intense SC and Cl-, and the preferential corrosion site was the end of steel fiber.

Chloride-induced corrosion patterns of reinforcements in simulated pore solutions of calcium sulfoaluminate cement concrete: An analytical study

The widespread use of calcium sulfoaluminate cement (CSA) for rapidly repairing marine structures exposed to high chloride levels has spurred limited investigation into the corrosion behavior of reinforcements in low-pH CSA systems. To address this knowledge gap, researchers conducted a comprehensive study on the impact of primary ions within CSA (OH−, SO42−, and Al(OH)4-) on steel rebars in simulated pore solutions (SPSs). The investigation employed various analytical methods such as Mott-Schottky/cyclic polarization curves, corrosion current, impedance spectrum analyses, and microstructural examinations. The results demonstrate that SO42− diminishes the corrosion resistance of reinforcements by depassivating them, consequently reducing the critical chloride concentration (Ccrit). However, this influence is mitigated with elevated OH− or Al(OH)4− concentrations. Furthermore, the presence of Al(OH)4- in the pore solution contributes to an increased Ccrit by stabilizing alkalinity via OH− hydrolysis. Based on the empirical findings, the researchers developed an empirical formula for Ccrit, offering insights into estimating the service life of CSA-based concrete structures in real marine environments.

On the corrosion parameters acquired through potentiodynamic scans of carbon steel rebar in simulated pore solution and mortar

This study developed a methodology for quantifying the corrosion parameters such as corrosion potential, corrosion current density and Tafel slopes for rebar in simulated pore solutions and mortar with realistic rebar conditions. We quantitatively investigated the impact of depassivation duration, chloride concentration, carbonation, and relative humidity on these parameters. Specifically, in partially saturated mortar, it was observed that the critical chloride content lacked a distinct threshold. The degree of depassivation, as indicated by shifts in corrosion parameters due to chloride ions and carbonation, was influenced by humidity.

DFT-details & atomic-scale MC/MD simulations and experimental exploration of the Silybum extract bio-molecules effectiveness in mitigating aggressive effects of simulated corrosive concrete pore solutions

The growing emphasis on sustainability and environmental awareness is paramount today. Corrosion control, often achieved through inhibitors, is a prominent approach, with plant extracts being favored for their accessibility and ease of extraction. Yet, their efficacy in alkaline environments, common in construction materials like cement and concrete, remains constrained. This study evaluates the potential of Silybum extract as a corrosion inhibitor within a simulated chloride-contaminated concrete setting, employing electrochemical and surface analyses to scrutinize the inhibitory characteristics and surface chemistry of the treated samples. Following the analysis of concentrations at 0.5%, 1%, and 2% Silybum extract in the simulated concrete solution, the optimal inhibition efficiency of 96.5% was obtained based on polarization measurements. The electrochemical impedance spectroscopy (EIS) confirmed that the log of impedance at low-frequencies (log|Z|10mHz) for the inhibited sample had a noticeable increment compared to the blank sample, rising from 4.60 to 5.13 after 48 h of exposure. The use of Silybum extract resulted in a remarkable 89% increase in total resistance after 5 h of immersion, as compared to the blank basic solution. Silybum exhibited a protective effect on metal substrates through three key mechanisms: forming a passive surface film via electron donation, inhibiting corrosion reactions through surface adsorption, and reducing corrosive species diffusion via physical surface coverage by heavy components. These mechanisms were supported by various surface studies which revealed the formation of a protective layer instead of corrosion products originating from Silybum components. Also, the interaction of cations in the basic solution with Silybum components and the adsorption of the obtained complex on the metal surface was demonstrated via computer modeling calculations. Theoretical studies revealed negative adsorption energies for the complexes (−1129.83 kcal/mol energy in molecular dynamics (MD) simulation for the [Ca(Silychristin)3]+2 complex), confirming their strong chemical affinity for interfacial attachment.

Enhancing the performance of lightweight high-performance concrete through an alternative pre-wetting method for lightweight aggregates using simulated cement pore solution

Lightweight aggregates (LWAs) are typically pre-wetted to prepare lightweight concrete. However, the incorporation of pre-wetted LWAs by water, may dilute the pore solutions of the cement matrix surrounding the LWAs during the process of internal curing. To this end, this paper proposes an alternative pre-wetting method for LWAs using simulated cement pore solutions (saturated Ca(OH)2 or real cement solution) instead of water. The effects of LWAs pre-wetted by various agents on the performance of lightweight high-performance concrete (LW-HPC) were investigated and compared. Experimental results confirmed that water released from LWAs diluted the pore solutions of cement in LW-HPCs. By using cement solution and Ca(OH)2 solution, the compressive strengths of LW-HPCs were respectively increased by 41.7% and 28.2% at 3d, and 11.6% and 3.2% at 28d compared with LWAs pre-wetted by water only. Among the three pre-wetting agents, the saturated cement solution was found to be the optimal one for promoting the hydration of cement, elastic modulus of the interfacial transition zone, and compressive strength.

Durability of GFRP and BFRP reinforcing bars in simulated seawater sea sand calcium sulfoaluminate cement concrete pore solution

Thanks to the much smaller carbon footprint of calcium sulfoaluminate cement (SAC) concrete and the corrosion resistance of fiber reinforced polymer (FRP) reinforcement, FRP-reinforced SAC concrete can possibly replace conventional reinforced concrete in some cases. In this study, for the first time, the durability of glass FRP (GFRP) and basalt FRP (BFRP) bars, made with epoxy resin, is investigated in simulated pore solutions of concretes made with (a) SAC and fresh water, (b) SAC and seawater and sea sand. The bars were immersed in each solution and kept at constant temperature of 30 °C, 45 °C and 60 °C for up to 180 days. Bar samples were tested for their retained tensile strength and physiochemical degradation after 30, 60, 90, and 180 days of immersion. Scanning electron microscopy (SEM), X-ray dispersive spectroscopy (EDS), and Fourier transform infrared (FTIR) spectroscopy were used to identify the underlying damage mechanisms. Results show that solution (b) is less harmful to BFRP than solution (a), while the opposite is true in the case of GFRP. Both solutions caused the dissolution of the epoxy resin and the leaching of the Fe in the basalt fiber. Overall, GFRP exhibited higher durability than BFRP. Based on image analysis, the deteriorated zone within the bar cross-section is not a uniform ring, but its area correlates with the loss of tensile strength. Inspired by the Avrami-Erofe'ev Equation and the Eyring's Transition State Theory, a novel time-temperature-dependent FRP bar degradation model is proposed. The proposed model shows close agreement between the predicted and measured tensile strengths.

Deterioration of basalt-based reinforcement in corrosive environments: experimental monitoring and mathematical models

The basalt-based reinforcing elements are considered as alternative to conventional steel-based reinforcing elements (rebars, wires). The motivation to use basalt-based elements (fibers, composite bars, meshes etc.) is better corrosion resistance of basalt fibers especially in sea-water environment, compared to carbon steel. Nevertheless, it does not mean that basalt fibers are 100% corrosion resistant. The basalt fibers are produced from silicate melt of proper composition, i.e. the basalt fibers are vulnerable to both acid and alkaline hydrolysis, as well as other silicates do. When basalt fibers are used as reinforcement in concrete, the alkaline hydrolysis will become an important issue. The present paper deals with experimental observation of basalt fibers in alkaline environment of Simulated Pore Solution. The fibers deterioration was monitored by their mass loss and SEM microscopy. Jander’s model was used to describe mathematically the kinetics of the fibers alkaline hydrolysis. The results revealed that a corrosion products layer is formed on the fibers surface in this environment. The composition of this layer corresponds to N-A-S-H and C-A-S-H phases known from alkali-activated aluminosilicates or hydrated Portland cement.

Mango extract behavior as a potent corrosion inhibitor against simulated chloride-contaminated concrete pore solution; coupled experimental and computer modeling studies

Plants extracts are nominated as reliable green sources among all classes of effective materials for metals' corrosion mitigation in various aggressive media. Herein, Mango extract was announced as a potential candidate for declining the corrosion rate of mild steel in the simulated concrete pore solution (CPS). With aiding polarization and EIS tests, the mechanism and anti-corrosion performance of the inhibitor were investigated. Surface studies were also performed for studying the inhibitors' impact on the metal surface morphology after immersion in CPS. Besides, Quantum chemistry and atomistic simulations are covered to clarify the mechanism of protection on an atomic scale. The electrochemical claims represented the mixed type inhibition and slight anodic inhibition dominancy. 2 wt.% Mango extract could diminish metal corrosion rate in the harsh CPS by around 98% index. Also, molecular simulations (Monte Carlo, MC and Molecular Dynamics, MD) represented the spontaneous absorption of the complexed heavy molecules (coordination compounds of extract molecules and inorganic cations) owing to the negative adsorption energy. Additionally, the electronic-scale quantum chemistry modeling illustrated that the adsorption of complexes mainly happened through their electron-rich sites.

Licorice (Glycyrrhiza glabra) as corrosion inhibitor of carbon steel reinforcing bars in mortar and its synergic effect with nitrite

Nitrites are effective corrosion inhibitors for concrete structures, but their toxicity leads to their total or partial substitution. Licorice and its synergic effect with nitrites are studied both in simulated pore solutions and in mortars with chlorides. EIS and polarization studies show that a 50% licorice + 50% Na2NO2 mix can reduce the corroding surface and the corrosion rate in solution. Moreover, the efficiency of this mix is higher than that of Na2NO2 when it is added to mortars with 0.8% Cl-. The tested inhibitors, in the amount considered (0.2%), do not affect the mechanical properties of the mortars.

Spontaneous passivation and passivity breakdown of a novel 10CrMo steel in simulated seawater concrete pore solutions

Spontaneous passivation and passivity breakdown of a novel 10CrMo steel in simulated seawater concrete pore solutions

Durability of GFRP and CFRP Bars in the Pore Solution of Calcium Sulfoaluminate Cement Concrete Made with Fresh or Seawater.

Calcium sulfoaluminate cement concrete (CSAC) reinforced by fiber-reinforced polymer (FRP) bars, termed bars for brevity, is a good alternative to steel-reinforced concrete in marine environments due to the corrosion resistance of FRP and the lower pH of CSAC. For the first time, multi-mechanical tests are conducted to compare the durability of glass FRP (GFRP) to that of carbon FRP (CFRP) after exposure to CSAC pore solution. The bars were immersed in a simulated pore solution of CSAC made with either fresh water and river sand or with seawater and sea sand. Solution temperature was held constant at 30 °C, 45 °C or 60 °C for 30, 60, 90 and 180 days of immersion. Tensile, horizontal and transverse shear tests, as well as detailed microstructural analyses, were conducted to determine the level and mechanisms of degradation for each type of bar. Sea salt increases the degradation of both bars, but it degrades GFRP more than CFRP. The bars' retained tensile strength is a reliable indicator of their durability, while their post-exposure horizontal and transverse shear strengths are found inconsistent and counter intuitive. In the GFRP, the fiber, the epoxy matrix and their interface suffered damage, but in the CFRP, the carbon fiber was not damaged. Under the test conditions in this study, the maximum reduction in the tensile strength of the GFRP was 56.9% while that of CFRP was 15.1%. Based on the relevant ASTM standard, the CFRP bar satisfies the alkaline resistance requirement of the standard in the CSAC pore solution with and without salt, whereas the GFRP bar does not meet the same requirement in the above pore solution with salt.

Application of corrosion resistance of FRP composites rebar in sea water sea sand concrete

In recent years, Fiber Reinforced Polymer(FRP) has been widely used as a reinforcement material to improve the flexural capacity of beams and slabs, and FRP has good corrosion resistance. And because of the wide variety of fiber materials, so more than the role of ordinary steel.This paper will discuss whether it can replace steel to improve building durability in alkaline environment such as seawater sea sand concrete.Due to the lack of natural river sand and water resources, people have been trying to use sea sand seawater instead in recent years. But there are many safety accidents because of the corrosion of steel bars. Due to the good corrosion resistance of FRP, this paper discusses the feasibility of its service as a steel bar in marine sand concrete, including its failure characteristics after corrosion and the problems and improvement strategies when working with Seawater-Sea Sand Concrete(SWSSC). The research object of this paper is Glass Fiber Reinforced Polymer(GFRP) steel bar and Carbon Fiber Reinforced Polymer(CFRP) steel bar. Glass fiber and carbon fiber are widely used materials in current production activities, mostly in the field of mechanical manufacturing. Although engineering materials have been used in recent years, the time is short. In this paper, it is discussed that in the simulated pore solution of concrete stirred by seawater and sea sand and exposed in the environment of seawater immersion, by increasing the temperature to accelerate the reaction rate, it is found that the corrosion resistance of the two kinds of steel bars is very different, but the corrosion mechanism is similar, both of them are caused by the dissolution of the resin matrix. The fiber is bare, and the internal fiber actually has the bearing capacity, but the ability of the material to bear the force is still reduced due to the loss of the bonding of the resin matrix. Therefore, this paper proposes that it is possible to find a more corrosion-resistant resin matrix or add a layer of medium at the interface between the fiber and the matrix to maximize the role of the fiber.

CaAl–NO2 LDH hybrid self-healing microcapsules with chloride triggering: Towards synergistic corrosion resistance

The synergistic corrosion resistance microcapsules (SCRMs) were synthesized by utilizing CaAl–NO2 LDH (NO2-LDH) as chloride trigger and high-performance corrosion inhibitor to hybridize self-healing microcapsules. The morphology, chemical structure and self-healing performance of the microcapsules were characterized. The chloride triggering and corrosion resistance property of the SCRMs were evaluated. The interaction of hybrid wall, NO2-LDH and chloride ion was calculated by first-principles. The findings revealed that the microcapsules show regular spherical with obvious core-wall structure. EDS and chemical structure measurements confirmed that NO2-LDH was successfully embedded within the microcapsule wall. The SCRMs showed excellent corrosion resistance property in simulated chloride-contaminated concrete pore solution (SCPS), and the highest inhibition efficiency was 97.85%. The microcapsule wall was hybridized by NO2-LDH species through chemical pathways. The first-principles calculation results confirmed that Ca and Al species are the main factors for chloride ion immobilization of hybrid wall. The ion exchange process of microcapsules destroys the molecular force of NO2-LDH@ethyl cellulose hybrid wall and improves the mechanical triggering efficiency of microcapsules in cement-based composites.

Improving corrosion resistance of BFRP bars by coating CNTs modified resin in simulated pore solution of seawater sea sand concrete

The corrosion of basalt fiber-reinforced polymer (BFRP) bars restricts their use in alkaline concrete structures. However, developing high-performance resin can improve the anti-alkaline corrosion of BFRP bars. This work investigates the alkaline resistance of BFRP bars by coating carbon nanotubes (CNTs) modified epoxy resin incorporating varying CNTs contents (0, 0.1, 0.3, and 0.5 wt%). The durability of untreated and treated BFRP bars with CNT-modified resin was evaluated using water uptake, transverse shear, and interlaminar shear tests. Scanning electron microscopy (SEM) and X-ray micro-computed tomography (micro-CT) were also used to observe the surface corrosion morphology and interior deterioration of BFRP bars, respectively, while Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were applied to analyze the resin deterioration of BFRP bars. It was found that CNTs modified resin can significantly improve the alkaline resistance of BFRP bars. After 180 d of immersion, the interlaminar shear strength (ILSS) retention of untreated BFRP bars is 31.8%, while it is 63.3, 75.0, 91.3, and 93.7% for BFRP bars treated with 0, 0.1, 0.3, and 0.5 wt% CNTs modified resin, respectively. The improvements in ILSS retention of BFRP bars treated with 0.5 wt% CNTs modified resin are 61.9% and 30.4% more than untreated and pure resin-treated BFRP bars, respectively. BFRP bars treated with 0.3 and 0.5 wt% CNTs modified resin exhibit insignificant resin hydrolysis and less porosity than untreated BFRP bars. The effective prevention of hydroxyl ions into the interior of BFRP bars and the enhancement of the anti-crack behavior of resin by CNTs are the leading causes of corrosion resistance improvement in BFRP bars. The outcomes of this study provide insights into using CNTs modified resin to improve the durability of BFRP bars in marine concrete structures.

Increasing the Reliability of Formation Factor-Based Transport Property Prediction for High Performance Concrete Mixtures Through Innovative Matching Pore Solution Curing

Recent specifications for concrete bulk resistivity tests (ASTM C 1876 and AASHTO TP 119) recommend a submerged standard (simulated) pore solution (SPS, solution conductivity = 78.74 mS/cm) for curing concrete specimens. The rationale for the bucket test is that immersing concrete specimens in a soak solution similar to their pore solution would eliminate the need to determine pore solution resistivity for calculating the formation factor (FF) of concrete mixtures. However, the thermodynamic modeling predictions of 91-day pore solution concentration (PSC) for eight high performance concrete (HPC) mixtures evaluated in the current study showed SPS to be a close representation only for reference ordinary Portland cement (OPC) and binary silica fume mixtures. In contrast, the average PSC of Class F and Class C fly ash mixtures was approximately 12% to 20% lower and 20% to 40% higher, respectively, than the SPS. Accordingly, an innovative matching pore solution (MPS) curing approach was developed in which mixtures are grouped based on the influence of supplementary cementitious materials (SCMs), that is, their type and replacement levels of the long-term PSC of concrete mixtures and, thereby, cured in a simulated solution matching the average PSC of a particular group. Based on experimental work in the current study, HPC mixtures under MPS demonstrated a lower coefficient of variation (COV) and more comparable (<10% difference) bulk resistivity (BR) and surface resistivity (SR) measurements compared with SPS. Moreover, the MPS improved the reliability in FF determination and FF-based transport property prediction for HPC mixtures, as verified by lower mean absolute error and improved R2 between FF-predicted diffusion coefficients and experimental measurements.

Electrochemical behavior of steel bars in magnesium phosphate cement

The electrochemical characteristics of steel bars in magnesium phosphate cement (MPC) are crucial when applying MPC to reinforced concrete. In this study, the electrochemical behaviors of steel bars in MPC were investigated via electrochemical tests combined with surface analyses. The results indicated that the steel bars in the simulated MPC pore solution could effectively passivate, forming films that mainly consisted of FePO4. In the MPC matrices, the continuous precipitation of phosphates led to the accumulation of passive films, which were responsible for the superior corrosion protection of MPC compared to traditional cement materials. MPC exhibited a self-healing corrosion feature after the depassivation of steel bars, which could be characterized by the dramatic recovery of passive film resistance and charge transfer resistance. Furthermore, the corroded steel bars in MPC with a low magnesia-to-phosphate ratio could re-passivate and stabilize for long periods of time.