Effect of magnesia-to-phosphate ratio on the passivation of mild steel in magnesium potassium phosphate cement

Highlights

• Mild steel in MKPC showed comparable or even higher passivity than that in PC.

• The passivity increases with increasing magnesium-to-phosphate (M/P) ratio.

• The quality of passivation was dominated by pH of MKPC rather than phosphate ions.

• Two passivation mechanisms were proposed for low and high M/P ratios, respectively.

Abstract

The low pH of magnesium potassium phosphate (MKPC) has raised concerns over its capability to protect reinforcing steel. The passivation of mild steel in MKPC pore solutions was investigated in this study via electrochemical measurements, XPS and Raman spectroscopy. Results show that the passivity of mild steel in MKPC was comparable or even better than that in Portland cement. Although the passivity was dominated by the pH of MKPC, it also increases with increasing magnesium-to-phosphate (M/P) ratios. The formation of iron phosphate in the low pH MKPC (such as M/P 7) also made additional contributions to the passivity of steel.

Introduction

The reinforcing mild steel embedded in Portland cement concrete may remain chemically stable due to the spontaneous formation of a protective passive film on the surface of mild steel. The passivation process involves electrochemical redox reactions that occurs on the steel surface through its interaction with the alkaline pore solution of concrete [1]. The passive film formed on the mild steel in Portland cement concrete is typically composed of an outer layer of Fe(III) oxides and an inner layer of Fe(II) oxides which are thermodynamically stable in the alkaline condition of Portland cement concrete [2,3]. However, the exact composition, structure and protective capability of the passive film to be formed, which essentially determines the corrosion resistance of steel, is significantly influenced by the pore solution chemistry of concrete [4,5]. Among various factors, there is wide agreement that only at a pH value of above c.a. 10.5, a protective passive film can be formed on the surface of steel [6,7]. As long as this high pH environment is maintained, the passive film is chemically stable and, hence, can provide a good protection to steel. On the contrary, the low pH, such as a pH below 10.5, could result in an increased dissolution of Fe as well as the increased precipitation of FeOOH, leading to the formation of a more porous and less protective passive film on the surface of steel [8]. Furthermore, the presence of some inorganic species, such as phosphate ions, could also be able to improve the corrosion resistance of mild steel in corrosive conditions [[9], [10], [11]]. The corrosion inhibition mechanism of phosphate ions for mild steel can be different in different conditions, which is possibly dependent on the pH condition [9] and the concentration of phosphate ions [10,11]. For example, Mohagheghi and Arefinia [9] reported that corrosion was inhibited by the adsorption of phosphate ions in a mildly alkaline solution while precipitation of iron phosphates occurred in a highly alkaline solution. In another study, Dhouibi et al. [11] claimed that phosphate ions of high concentration can act as an anodic inhibitor, while low concentration of phosphate ions act as a cathodic inhibitor in calcium hydroxide solution. Moreover, the inhibition mechanism of phosphate ions in the chloride-contaminated alkaline solution was also found to be related to the formation of insoluble iron phosphate complexes on the steel surface [9,12] or the formation of compact iron oxides on the steel surface [11].

Magnesium phosphate cement (MPC) is attracting interest as a rapid repair material for concrete structures, primarily due to its fast setting, high early strength and good bond to the existing concrete [13]. Unlike conventional Portland cement (PC), which reacts directly with water to form a cohesive and strong monolith, MPC reacts by means of an acid-base reaction between magnesium oxide (MgO) and a soluble acid phosphate. Although both ammonium and potassium phosphate can be used as a phosphate source to formulate MPC, potassium dihydrogen phosphate, KH₂PO₄ (KDP), is often preferred. This is because, unlike ammonium phosphate, KDP does not liberate ammonia gas during hydration and is, therefore, more suitable for practical applications due to the reduced health and safety concerns [14]. This acid-base reaction between MgO and KDP (as exemplified in Equation 1 below) can also yield a hard and dense cementitious material with very low solubility, making it a possible alternative to PC for construction applications.

MgO + KH₂PO₄ + 5H₂O → MgKPO₄.6H₂O

According to Eq. 1, in theory, a stoichiometric 1:1 molar ratio of MgO to KDP (M/P) should be adopted to formulate magnesium potassium phosphate cement (MKPC). However, in practice, a higher M/P is often needed in order to formulate a stable MKPC matrix. Although some researchers reported an increase in compressive strength with an increase of M/P, there is now wide agreement that the properties of MKPC are not a monotonic function of M/P. Instead, for a given water/cement ratio (W/C), an optimal M/P is determined for developing the best mechanical and durability properties. For example, it has been reported that the M/P ratio of 6 was found to be the optimal M/P for achieving the highest strength and lowest permeability at a W/C of 0.20 [15,16]. To explain this phenomenon, a three-limit theory has been developed and proposed for the mix design of MKPC based materials [17]. Additionally, it has been considered that by using a M/P higher than that based on theory, an excess of MgO is available for reaction. As a result, most of the phosphate can be consumed in the reaction, which not only can reduce the leaching of phosphate and therefore improve the integrity of the hardened MKPC, but also can improve the volume stability of the hardened MKPC [18]. On the other hand, due to the very high reaction rate of MgO and KH₂PO₄, MKPC paste hardens within several minutes, causing some operational issues. In order to slow down the early hydration reaction, dead-burnt MgO is usually used. Additionally, retarders such as boric acid or borax are also used to modulate the setting time [19,20] which is mainly attributed to the precipitation of magnesium borate compounds around the basic magnesia grains, delaying the reaction with acidic phosphate solution [20].

Furthermore, as illustrated in Eq. 1, when MgO and KH₂PO₄ are mixed with water, a neutralisation reaction occurs between MgO and phosphates, with the former providing alkalinity and the latter acidity. Therefore, unlike Portland cement, which can generate a high internal pH (usually around 13), MKPC paste normally produces a much lower pH environment, in particular, during the early stages of hydration. This can be clearly seen in Fig. 1, in which the pH values of the MKPC reported in the literature have been summarized and plotted. It is evident that the initial pH is usually lower than 7 since at the beginning of the reaction the acidic KH₂PO₄ is believed to dominate the pH of the pore solution [21]. With ongoing hydration, the pH of MKPC starts to increase, reaching the weakly alkaline range. Furthermore, it can be deduced from Fig. 1 that the pH of MKPC generally increases with increasing M/P ratio. For example, the measured pH value at M/P 10 is around 8 at one day hydration [22,23], at M/P 13 around 9.2 at one day [24], and at M/P 20 around 11 at 10 h [25]. However, most of the reported pH values in Fig. 1 are lower than 10.5. This ‘low’ pH may raise concerns over the wider application of MKPC in reinforced concrete structures since it may not favour the formation of the protective passive film on the surface of reinforcing steel in the first instance, making the steel less resistant to corrosion.

Nevertheless, some researchers have claimed, albeit based on some limited studies, that MKPC provides the reinforcing steel with comparable protection against chloride-induced corrosion when compared with PC [26,27]. This finding is, somehow, beyond normal expectations, because it is questionable whether a proper passive film can be formed initially under such a low pH environment of MKPC. However, Zhang et al. [27] has ascribed the good corrosion resistance of reinforcing steel to the good corrosion inhibition provided by the phosphate in MKPC. This hypothesis has been further corroborated by a report that a protective passive layer consisting of iron and phosphate formed when a slurry of acid phosphate with base minerals and metal oxides was sprayed on the steel surface [28]. Although the exact mechanism responsible for this improved corrosion resistance is yet to be understood, it has been hypothesised that the phosphate ions in MKPC may have played an important role. This hypothesis has been deduced from the fact that the phosphate ion, which is a good inhibitor, can promote the formation of some insoluble ferrous phosphate compounds on the steel surface, inhibiting further corrosion processes when exposed to chloride attacks. Therefore, even though the pH of MKPC would not favour the formation of passive film, it has been considered that the phosphate ions present in the MKPC pore solution could potentially benefit the corrosion resistance of rebar. While this hypothesis seems plausible, the passivation behaviour and the passivation mechanism of the steel in MKPC are still largely unknown. In particular, the influence of the M/P ratio, an important parameter for designing MKPC, on the passivation behaviour and underlying passivation mechanism is yet to be understood, even though a good understanding on this is important for designing and predicting the durability and service life of reinforced MKPC concretes in real applications.

In this paper, a systematic study was carried out to investigate the passivation behaviour of reinforcing mild steel immersed in MKPC pore solutions. MKPCs with different M/P ratios were purposely formulated to generate a range of pH and phosphate environments with an attempt to clearly identify the effects of pH and phosphate on the passivation of the reinforcing steel. The passivation behaviour of the mild steel in MKPC pore solutions was assessed using a suite of electrochemical methods which included linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The chemical composition of the passive film formed in the MKPC pore solution was then characterised by X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy. Based on the data obtained, a possible passivation mechanism of the mild steel in MKPC is then proposed and discussed in this paper.