Inhibitive Protection of Low-Carbon Steel in Citric Acid Solutions

An effective mixture has been developed on the basis of the inhibitor IFKhAN-92 and KCNS to protect low-carbon steel in citric acid hot solutions. The proposed mixture inhibits the corrosion of steel in these solutions in a wide range of their concentrations (0.25 2.0 М) and temperatures (20 95 С). The effective inhibition of the corrosion of steel by the mixture under study is due to the strong retardation of the metal’s both electrode reactions.


Introduction
Thermal power equipment is chemically cleaned with citric acid solutions (С 6 Н 8 О 7 ) before the launch (pre-operational chemical cleaning) to remove rust, scaling and other sediments and in the process of operation (operational cleanings) to remove iron oxide and carbonate (crust) formations [1][2][3]. These solutions demonstrate low aggressiveness towards steel equipment and remove roll scale, magnetite sediments and crust formations well, including through their complexing properties. Treatment with citrate solutions is quite frequently carried out at temperatures (t) close to 100qС. The solutions of other organic acids (adipic, maleic, oxalic, succinic) are less effective in thermal power equipment cleaning than citric acid solutions. Mixtures containing pyridine toxic derivatives -Сatapin K and the inhibitor I-1-A [1], benzotriazol [4], cinnamic aldehyde [5] and various vegetal extracts [6][7][8] are recommended as additives to inhibit the corrosion of steels in citric acid solutions. In this case, benzotriazol, cinnamic aldehyde and vegetal extracts do not allow obtaining the metal protection degree of Z ! 90%. Lately, 1,2,4-triazole derivatives [9][10][11] have been gaining importance for protecting metals and alloys in mineral and organic acid solutions. The inhibitor of this class, IFKhAN-92, which we have developed, allows protecting low-carbon steel in mineral acids [12][13][14], and also in acetic and formic acid environments [15,16]. As its essential advantage, IFKhAN-92 is stable in hot acidic solutions.
It appeared expedient to develop an inhibitor based on IFKhAN-92, which is a substituted triazol, to protect low-carbon steel in citric acid hot solutions (up to 95qС).

Gravimetric measurements
The corrosion rate of low-carbon steel St3 in С 6 Н 8 О 7 solutions was measured by the mass loss of specimens (no less than three specimens per experimental point) sized 50 mm u 20 mm u 1.5 mm, using 30 ml of the acid solution per specimen. Before the experiment, the specimens surface was conditioned by grinding on an abrasive disc (ISO 9001, 60 grit) and degreased with acetone. The duration of experiments was 2 hours.
The efficiency of inhibitors was estimated from the inhibition coefficient, J = k 0 /k in , and the degree of protection Z = [(k 0 -k in )/k 0 ]·100%, where k 0 and k in are the corrosion rates in the non-inhibited solution and in the solution with the additive being studied, respectively. Coefficients of mutual influence were calculated to quantify the effect of inhibiting mixture components on the efficiency of the protection of steel: is the product of the corrosion inhibition coefficients for individual mixture components. If K m < 1, the protection coefficient of the inhibitor components is mutually reduced; at K m = 1, it shows an additive behavior; and only at K m > 1 it should be concluded that the components of the inhibitor mixture mutually enhance protection.
Citric acid monohydrate (chemically clean) and distilled water were used to prepare solutions. KCNS (chemically clean) was studied as an additive to IFKhAN-92. Due to its low solubility, IFHAN-92 was introduced into С 6 Н 8 О 7 solutions in the form of an ethanol solution, with the ethanol concentration in the pickling solution equaling 0.24 mol/ l.

Electrochemical measurements
Electrochemical measurements were carried out on a cylindrical electrode made of steel St3 (0.72 cm 2 ) in the argon-deaerated 2.0 М С 6 Н 8 О 7 solutions at t = 95qС. The steel electrode potentials were measured with reference to a silver chloride electrode and recalculated to the standard hydrogen electrode scale. The electrode that was preliminarily roughed up and degreased by acetone was held in the test solution for 30 minutes, after which the anodic and cathodic polarization curves were sequentially recorded with the help of the potentiostat IPC-PRO FM at a dynamic scan rate of 0.0005 V/s.

Gravimetric measurements
The corrosion of steel St3 in 0.25y2.0 М С 6 Н 8 О 7 solutions at a temperature of t = 20y95qC occurs at a relatively low rate of no more than 106 g/(m 2 h) ( Table 1). The corrosion of steel accelerates with the increase of the concentration С С6Н8О7 and t in the solution, which is consistent qualitatively with the data of the authors [3] on the study of the dissolution of low-carbon steels' kinetics in citric solutions. The addition of 5.0 mМ IFKhAN-92 to 0.25y2.0 М С 6 Н 8 О 7 solutions weakly reduces k of steel: Z IFKhAN-92 = 29y70%. One of the methods to enhance the inhibitory effect of IFKhAN-92 in mineral acid solutions is to add hydrophobic compounds, among which KCNS is distinguished, which we successfully used for the protection of steel in phosphoric acid media [17,18]. Such additive itself either stimulates the corrosion of steel or slightly inhibits it with the value of Z d 17%. The additive's maximum stimulating effect is observed at t = 40qC.
In the presence of 4.5 mМ IFKhAN-92 + 0.5 mМ KCNS in С 6 Н 8 О 7 solutions, the corrosion of steel was significantly slowed down: Z = 96y99%. In this case, the corrosion rate of steel in acid solutions inhibited by this mixture did not exceed 0.57 g/(m 2 h). With t ! 40qС, the J values of the mixture of inhibitors grow systematically with the increase of t.
It is important to note that the temperature peak of the inhibition efficiency of the IFKhAN-92 and KCNS mixture was not reached in the tested range of t. The increase of t by 75qС increases the value of J by 3.4y6.5 times. The mixture's protective effect at t = 20y40qС decreases with the growth of С С6Н8О7 and, on the contrary, rises at higher t.
The mixture of IFKhAN-92 and KCNS displays the synergism of components, with K m demonstrating quite high values (Table 2). It intensifies with the increase of t and declines with the increase of С С6Н8О7 . The inhibitive mixture under study retains its protective effect in citric acid solutions during at least 6 days (Table 3)  The IFKhAN-92 + KCNS mixture with fixed С KCNS = 0.5 mМ demonstrates an Sshaped dependence of Z mix on IFKhAN-92 concentration (Fig. 1). The values of Z t 90% are reached at С IFKhAN-92 t 0.1 mМ, which, based on the shape of experimental Z mix С IFKhAN-92 dependencies, corresponds to the maximum coverage of the metal's surface with the inhibitor. With С IFKhAN-92 0.01 mМ, the mixture loses its protective effect and can stimulate corrosion. We discussed the nature of the stimulating effect of low С IFKhAN-92 values on the corrosion of metal in acids earlier [19] and it is most probably related to the solution's surface tension weakened by this inhibitor, which facilitates the removal of hydrogen bubbles from the metal's surface.

Electrochemical measurements
The electromechanical measurements performed on steel in 2.0 М С 6 Н 8 О 7 (t = 95qC) have shown that the use of the 5.0 mМ KCNS additive in the solution somewhat slows down the cathodic reaction of steel and disinhibits the anodic reaction probably due to the participation of rhodanide anion in this process as a complexing reagent (Fig. 3). Addition of 5.0 mМ IFKhAN-92 refines the corrosion potential (E cor ), inhibiting mainly the anodic process. The use of the 4.5 mМ IFKhAN-92 + 0.5 mМ KCNS mixture causes a considerably stronger increase in E cor , and also the inhibition of the cathodic and especially the anodic reaction. Therefore, the corrosion of steel in С 6 Н 8 О 7 solutions is inhibited more effectively by the IFKhAN-92 + KCNS mixture due to the stronger inhibition of the metal's electrode reactions by this mixture compared to its components.