Temperature Influence on Inhibitory Efficiency of Three Phosphate Inhibitors by Mass Loss

The effect of temperature on steel samples immersed in concrete pore solutions contaminated by chlorides incorporating three inhibitors based on phosphate (Na 3 PO 4 , K 2 HPO 4 , and Na 2 PO 3 F) was studied by gravimetric measurements at several ranges: 298, 308, and 318 K. The results obtained for the use of these three products show that the inhibitory efficacy is lower at 318 K than that detected at 308 and 298 K of temperature. Also, we find that the best inhibitory efficiency at 298 K was detected for Na 2 PO 3 F (75.80% at 0.05 mol/l of concentration) followed by K 2 HPO 4 (65.05% at 2.5 10 − 3 mol/l) and then Na 3 PO 4 (61.48% at 7.5 10 − 3 mol/l).


Introduction
Corrosion of reinforcement in concrete is one of the most dangerous pathologies that attack reinforced concrete structures; the means of protection against corrosion are varied and expensive. During this last decade, a new alternative has been adapted which is the application of corrosion inhibitors either as an adjunct to the mass of fresh concrete or by impregnation on the facing of hardened concrete. Several families of corrosion inhibitor products have been developed to prove their protective effect against steel reinforcement corrosion initiated by the penetration of chlorides through the pores of concrete. The best known are phosphates, borates, silicates and carbonates. One of the peculiarities of these ions is that their hydrolysis releases hydroxide ions which will have the effect of increasing the pH of the medium and thus passivating the steel. Moreover, in the presence of oxygen, the anions will form with the metal cation a very insoluble iron III phosphate which will clog the anodic surface and displace the cathodic reduction reaction [1].
The required concentration of passivative inhibitor, often of the order of 10 −4 to 10 −5 mol/l [2], it depends in fact on many factors such as temperature, pH, the presence of depassivating ions such as chlorides or reducing agents such as sulfur S 2 [3].
Temperature is one of the factors that can alter the behavior of a material in a corrosive environment. It can modify the metal-inhibitory interaction in a medium [4].
The variation of temperature affects the rate of corrosion. According to Liu and Weyer [5], an increase in temperature increases the rate of corrosion. This result was confirmed in carbonated concrete and also that subject to aggressive environments like chloride ions penetration.
The objective of this research is based on the analysis of the evolution of the inhibitory efficiencies of three phosphate inhibitors (Na 3 PO 4 , K 2 HPO 4 and Na 2 PO 3 F) as a function of the temperature variation: 298, 303 and 313 K.

Methods and measurements
In this section, gravimetric tests were performed to characterize the influence of temperature on inhibition efficiency for the three phosphate inhibitors used in this study.

Gravimetric measurements
These measurements consist in determining the weight loss of a steel sample subjected to specified conditions of temperature and relative humidity; they are calculated on the basis of three tests to determine the average. The steel sample is polished with abrasive paper ranging from 120 up to 1000 grades using a polisher at a speed of 500 rpm, then rinsed in distilled water, dried with an electric dryer then we weigh the mass M1.
The steel samples are introduced into beakers containing 50 ml of electrolytic solution in an inclined position as shown in Figure 1, hermetically closed, then they are placed in a thermostatic bath while adjusting the desired temperature, after 24 h, the samples are removed from beakers then, rinsed in distilled water, degreasing is carried out with acetone and then dried with the electric dryer, after that we weigh the mass M2.

Study medium
The medium of this study is a concrete synthetic medium which simulates concrete pores contaminated by 3% of chlorides given in Table 1.

Steel preparation
The steel used is circular shaped with a diameter of 27 ± 1 and 2 ± 2 mm of thickness, and the procedure of gravimetric tests is detailed in [9]. The corrosion rate is determined by the following formula: Hence, ΔM represents the difference between the initial mass M1 and the final mass M2 after a time "t" equal to immersion time by hours. "S" is the surface of the metal exposed to the electrolytic solution.
This value of the corrosion rate is the average of three tests carried out under the same conditions for an optimal concentration at a definite time. The value of the inhibitory efficiency is given by the following formula:

Tested inhibitors
This study describes the corrosion behavior of steel immersed in synthetic concrete pore solutions contaminated by chlorides for three phosphateinhibitors (Na 3 PO 4 , K 2 HPO 4 and Na 2 PO 3 F), their molecular structure is given in Figure 2. The optimal concentration which provides maximum efficiencies for the three products cited was extracted from a previous study [10] (see Table 2). Table 3 records the mass loss results, relating to the evolution of corrosion rates as well as the inhibitory efficiencies as a function of the temperature variation: 298, 303 and 318 K for the three inhibitors.

Results and discussions
It can be seen from the results shown in Table 3 that the corrosion rates decrease in the presence of the corrosion inhibitor, it reached the maximum at the optimal concentration, for the first inhibitor sodium phosphate Na 3 PO 4 the maximum efficiency 69.28% was detected at a concentration of 7.5 × 10 −3 mol/l at 298 K, we can see clearly that the inhibitory efficiency slightly decrease as a function of temperature increase. Likewise for K 2 HPO 4 , the best efficiency 67.44% was detected at 298 K for a concentration of 2.5 × 10 −3 mol/l, also, the increase of temperature affects the inhibitory efficiency which decrease following temperature increasing, the same remark was recorded for Na 2 PO 3 F the maximal efficiency 75.8% was detected at 298 K. This phenomenon can be explained by the fact that the anodic processes (oxidation components of steel) and cathodic (proton reduction in acidic medium) are thermally activated.
This results in a current of exchange which increases the corrosion rate. Hunkeler [11] has shown in his studies that the influence of temperature on the rate of corrosion is greater than that the resistivity of the concrete. Figure 3 shows the evolution of inhibitory efficiencies as a function of temperature variation for different concentrations of tested inhibitors.  Figure 3 illustrates the influence of temperature variation on the inhibitory efficacy of the three phosphate-based inhibitors. Certainly, temperature is one of the factors that can alter the behavior of a material in a corrosive environment. It can modify the metal-inhibitory interaction in a given environment [4].
The increase in temperature causes the instability of inhibitory molecules and also reduces the inhibitory efficacies which was detected in previous researches [5].
It can be seen from Figure 4 that inhibitory efficiencies are highest in the optimum concentration for all the studied temperature ranges, although they decrease slightly as a function of temperature increase. As a result, the maximum inhibitory efficacy at T 298, 303 and 318 K deduced using gravimetric measurements was confirmed by sodium monofluorophosphate (Na 2 PO 3 F), followed by potassium monohydrogenphosphate (K 2 HPO 4 ) and thirdly sodium phosphate (Na 3 PO 4 ).
These results are in good agreement with previous research that used the same inhibitory products [10].
Indeed, sodium monofluorophosphate has been the subject of several studies [12][13][14], and it has proven remarkable inhibitory properties especially in the case of its use in zinc phosphate baths [15][16][17].
The variation of the temperature influences the rate of corrosion and consequently the mechanism of the inhibition [18]. According to Liu and Weyer [5], an increase in temperature increases the rate of corrosion.

Conclusions
Direct measurements of both corrosion rates and inhibitory efficiencies as a function of inhibitor concentrations, have confirmed that sodium monofluorophosphate (Na 2 PO 3 F) offers the best corrosion protection under study conditions (temperature 298, 303 and 313 K); its inhibitory efficiency has exceeded 70% for these temperatures.
We can also conclude that increase of temperature affects inhibitory efficiencies, which is in good concordance with literature. For inhibitors based of phosphate, the increase of temperature has a slight influence on the inhibitory efficiency for the study temperatures, moreover, at higher temperatures, the molecular activation will be greater, which leads to an increase in corrosion rates.