Intensity of Mass Transfer Processes in Concrete with Inhibitors in Chloride Corrosion

. The article presents the results of a study of the effect of alkaline and alkaline-earth metal nitrates added to concrete cement stone on the intensity of mass transfer processes occurring during corrosion in a highly aggressive liquid chloride-containing medium. The studies were carried out on samples of cement stone from Portland cement of the CEM I 42.5 N brand with W/C = 0.3 in a 2 % MgCl 2 solution. As inhibitory additives, KNO 3 , NaNO 3 , Ca(NO 3 ) 2 , Zn(NO 3 ) 2 , Mg(NO 3 ) 2 were introduced into the cement mixture in an amount of 0.5 % by weight of cement. When additives of nitrates of alkaline or alkaline-earth metals are introduced into the cement mixture at the stage of concrete production, the rate of mass transfer corrosion processes decreases by 1.5 times. It was found that inhibitors do not have a significant effect on the change in the strength of concrete cement stone during chloride corrosion. The mass transfer indicators (coefficients of mass conductivity and mass transfer, the density of the flow of the transferred components) were determined, according to which the periods of reaching the limiting concentration of chloride ions at the surface of steel reinforcement in concrete during corrosion in 2 % MgCl 2 solution were calculated. To achieve such concentration of chloride ions at the surface of steel reinforcement in concrete without an inhibitory additive, it will take 174 days, in concrete with 0.5 % KNO 3 additive – 260 days.


Introduction
Damage to reinforced concrete structures is most often associated with corrosion of reinforcement [1,2] in a practically non-aggressive or mildly aggressive environment, therefore it is necessary not only to increase the durability of concrete, but also to increase its ability to protect reinforcement for a long time.Under the influence of atmospheric conditions and various media, the permeability of concrete gradually increases [3][4][5].As a result, water, carbon dioxide and aggressive ions, such as chlorides, can penetrate into concrete and have a harmful effect on steel reinforcement [6][7][8].
After reaching a concentration of chloride ions at the surface of the reinforcement equal to 0.4 % of the mass of concrete, irreversible corrosion processes begin, causing the dissolution of steel [9][10][11].The accumulation of corrosion products at the reinforcement surface leads to an increase in internal pressure in reinforced concrete and cracking of the concrete coating [12][13][14].
To increase the duration of the period of initiation of steel reinforcement corrosion in reinforced concrete products operated in environments of high aggressiveness, special additives are introduced into the cement mortar at the stage of concrete production, which inhibit the action of aggressive particles on concrete and reinforcement.The introduced corrosion inhibitors can also improve the performance characteristics of concrete, for example, accelerate setability and hardening, increase strength [15][16][17].
In the case of exposure to chloride-containing media, additives can significantly reduce the permeability of concrete and increase its passivating ability, that is, provide passivation of steel reinforcement with a significantly higher chloride content in concrete [18][19][20].In this way, it is possible to solve the problems of long-term operation of reinforced concrete structures at chemical industry enterprises, in offshore hydraulic structures (oil and gas production on the offshore shelf), livestock complexes, etc.
Conducting a study of the effect of inhibitory additives on the intensity of the development of corrosion processes in concrete cement stone is necessary to establish the regularities and parameters of mass transfer and determine the service life of reinforced concrete.

Materials and methods
Cement stone samples made of Portland cement CEM I 42.5 with W/C = 0.3 with additives of inhibitors in the amount of 0.5 % by weight of cement were placed in a liquid chloridecontaining medium of a high degree of aggressiveness.
Calcium, sodium, potassium, magnesium and zinc nitrates were selected as inhibitory additives.A highly aggressive medium for concrete cement stone was a 2 % MgCl 2 solution.
The control of the content of calcium cations in the solution of an aggressive medium was carried out by volumetric titration.The complexometric determination of calcium cations is based on direct titration of the test solution with a standard solution of complexon III with the addition of an indicator of murexide or chromogen dark blue.
The derivatographic analysis of cement stone was carried out according to the standard procedure on the Q-1500D derivatograph.
The strength was determined on samples of cement stone with a face length of 10 cm.During the compression test of the samples on the press, loading was carried out continuously with a constant rate of load increase until their destruction.At the same time, the loading time of the test sample before its destruction was at least 30 seconds.The maximum force achieved during the test was taken as a destructive load.According to the strength measurements of concrete cement stone (Table 1), it can be seen that the introduction of inhibitors into the cement mixture does not affect the strength set during the hardening of cement stone.In concrete cement stone samples with inhibitor additives, the strength loss after 150 days of exposure to a 2 % MgCl 2 solution was 26 %.In cement stone samples made without inhibitory additives, the compressive strength under the influence of this medium decreased by 34 % after 150 days.The inhibitors are in a bound state in the cement stone and they require time to enter the pore fluid, during which chloride ions penetrate into the pore solution and interact with the components of the cement stone, as evidenced by the results of the study of the kinetics of the removal of calcium ions from the cement stone, shown in Fig. 1.
Over time, getting into the pore fluid from the cement stone structure, nitrate ions interact with chloride ions and slow their flow deep into the concrete.As a consequence, the strength characteristics of cement stone concrete made with additives of inhibitors are reduced to a lesser extent [21].
Since the addition of KNO 3 had a greater effect on reducing the output of calcium from cement stone, further calculations were carried out for samples with this inhibitor.
With the help of a derivatographic analysis, the content of calcium cations in different parts of the cement stone was determined, as a result of which the concentration profiles shown in Fig. 2 were constructed.By 70 days, a state close to equilibrium is established in the system, in which mass transfer processes occur at constant parameters, which is also seen from Fig. 1 According to the concentration profiles of «free Ca(OH) 2 », the chloride ion concentration profiles by the thickness of the cement stone were calculated (Fig. 3), taking into account the stoichiometry of the chemical reaction occurring in the pore liquid: Ca(OH) 2 + MgCl 2 → Mg(OH) 2 + CaCl 2 .
where: q is mass flow density due to chemical reactions, kg/(m 2 •s); ρ 0 is density of the solid phase, kg/m 3 .
The calculation of the density of the mass flow q of the substance from the cement stone into the liquid medium is carried out according to the equation: where: ΔC liq is the mass of the substance transferred from the cement stone to the liquid medium, kg; S is the area of the corroded surface of the sample, m 2 ; τ is process time, s.To calculate the processes of substance transfer, it is convenient to enter the mass transfer coefficient β.It is defined as the ratio of the diffusion flow q to the concentration difference ∆C, kg/m 3 : where: ΔC liq is the mass of the substance transferred from the cement stone to the liquid medium, kg; S is the area of the corroded surface of the sample, m 2 ; τ is process time, s.
The calculation results are shown in Fig. 4 and Fig. 5.The values of the coefficients of mass conductivity and mass transfer vary exponentially and in the case of inhibited cement stone have lower values, which also indicates a lower rate of development and flow of corrosive mass transfer processes.
The established mass transfer indicators make it possible to predict the time to reach the concentration of chloride ions, when exceeding which corrosion processes begin to develop on the surface of steel reinforcement.0.4 % by weight of cement is traditionally taken as the threshold value of the concentration of chloride ions necessary for the onset of corrosion of reinforcing steel [9][10][11].To achieve such a concentration of chloride ions at the surface of steel reinforcement during corrosion in 2 % MgCl 2 solution in concrete without an inhibitory additive, it will take 174 days, in concrete with 0.5 % KNO 3 additive -260 days.

Fig. 1 .
Fig. 1.Kinetic curves of the concentration of calcium cations for the case of corrosion of cement stone containing inhibitory additives in an amount of 0.5 % in a 2 % MgCl 2 solution.

E3SFig. 3 .
Fig. 3. Profiles of chloride ions concentrations by the thickness of a cement stone sample without an additive (a) and with an additive of 0.5 % KNO 3 (b) in a 2 % MgCl 2 solution at τ: 1 -14 days; 2 -28 days; 3 -42 days; 4 -56 days; 5 -70 days.The concentration profiles are used to determine the values of the concentration gradients   of the transferred component used to calculate mass transfer indicators:

Table 1 .
Change in strength, MPa, of cement stone samples from Portland cement of the CEM I 42.5N grade containing inhibitory additives in an amount of 0.5 %, before and after exposure to a highly aggressive chloride-containing medium for 150 days.