Corrosion Inhibitors in Cooling Water Systems Alexander Chirkunov and Yurii Kuznetsov Laboratory of the Physicochemical Principles of Inhibition of Metal Corrosion, Russian Academy of Sciences A.N. Frumkin Institute of Physical Chemistry and Electrochemistry RAS, Moscow, Russia
4.1 INTRODUCTION The use of water as a coolant in cooling systems is widespread due to its availability in the industrial regions, nontoxicity, relatively high heat capacity, and thermal conductivity. On the other hand, the operation of cooling water circulation systems is faced with several challenges, one of the most signiﬁcant being corrosion of the metal equipment. The damage caused by corrosion is determined by not only the direct metal loss but also indirect factors such as l l
failure of the expensive equipment; the economic losses caused by unscheduled shutdown of the heat exchange equipment because of repair or cleaning; and corrosion, scale formation, and biofouling decreasing the heat transfer and throughput of heat exchangers and increasing the hydraulic resistance of the system.
In general, corrosion is a spontaneous destruction of materials as a result of chemical or physicochemical interaction with the environment. The main cause of corrosion of metals is their thermodynamic instability. The cooling systems equipment typically includes materials with different corrosion behavior: carbon steel and stainless steel, copper and its alloys, aluminum alloys, galvanized steel, and nonmetallic materials [1,2]. Even without direct contact between different materials their corrosion resistance can vary signiﬁcantly. For example, aluminum and its alloys can be subject to pitting corrosion even in pure water, if it contains copper ions. The source of copper ions can be heat exchangers made of copper alloys. On the contrary, the corrosion rate of brass may decrease if there are iron ions in the water, forming a protective hydroxide ﬁlm on the brass surface . Thus, the presence of different materials, cracks, sites of welding, or soldering creates signiﬁcant difﬁculties for the theoretical prediction of their corrosion behavior. An equally important factor is the corrosivity of the water, which is determined by a combination of various physicochemical parameters: chemical and biological composition, pH value, temperature, hydrodynamic regime, and electrical conductivity. Various dissolved substances may differently inﬂuence the corrosion of metals. Chloride and sulfate ions adversely affect the corrosion resistance of most metals, while soluble silicates or salts of alkaline earth metals can contribute to the formation of a protective ﬁlm, although it creates another problemdthe scale formation. Dissolved gases (CO2, O2, sulfur oxides, H2S, Cl2, O3) usually contribute to corrosion, although oxygen provides the formation of a passive ﬁlm on the metal. The pH value also largely determines the corrosion behavior of materials and mechanisms of corrosion processes. The increase in temperature, as a rule, accelerates the corrosion of metals. The velocity of the medium affects the processes of mass transfer, precipitation, and so forth, therefore its change has an ambiguous effect on the corrosion rate . Microbiological pollution of water circulation systems usually creates signiﬁcant corrosion challenges. Due to the variety of corrosion mechanisms and material failures, there are different classiﬁcations of corrosion types describing their main features. With respect to the water-cooled systems where corrosion is of an electrochemical nature, there are several major types of corrosion listed below [1,3].
Mineral Scales and Deposits. http://dx.doi.org/10.1016/B978-0-444-63228-9.00004-8 Copyright © 2015 Elsevier B.V. All rights reserved.
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Mineral Scales and Deposits
General corrosion Local corrosion and pitting Galvanic corrosion Crevice corrosion Underdeposit corrosion Dealloying Intergranular corrosion Corrosion with mechanical destruction (erosion, cavitation, fatigue failure) Stress corrosion cracking Corrosion caused by microorganisms
Extensive damage and numerous types of corrosion with its causal factors contributed to the development of a large number of methods of corrosion protection: the use of corrosion-resistant materials, protective metallic and nonmetallic coatings, cathodic protection, water treatment (improvement of water quality, chemical treatment), and so on. In particular, corrosion inhibitors are widely used. Corrosion inhibitors are chemical compounds and formulations of such which, when present in small quantities in an aggressive medium, inhibit corrosion without signiﬁcant altering of the concentration of any corrosive component. The effect of inhibitors is always associated with a change in the state of the protected surface due to the adsorption or formation of sparingly soluble compounds. Such compounds reduce the active surface area of the metal and/or increase the activation energy of corrosion. There are several types of inhibitors classiﬁcation. The simplest divides inhibitors to inorganic and organic. Taking into account the inhibition of partial electrochemical reactions, inhibitors can be divided into anodic, cathodic, and mixed type. There are also compounds that inhibit corrosion not due to the slowing down of the cathodic reaction but due to its acceleration. The basis of the action of such inhibitors is their electrochemical reduction, which creates conditions for thermodynamic stability of a thin protective oxide ﬁlm, which, in turn, slows down the anodic reaction. A classiﬁcation taking into account both the chemical nature of inhibitors and the mechanism of their action includes oxidizing, adsorption, complexing, and polymer types. Polymeric inhibitors usually possess properties of any of the above inhibitors, but the peculiarities of their chemical structure (e.g., high molecular weight, a large number of functional groups, the inter- and intramolecular interactions determining the structure, and size of macromolecules) allow us to consider them as a separate class of corrosion inhibitors. As with the other methods of corrosion control, the inhibitors have certain advantages and disadvantages. Disadvantages are usually associated with the need for careful development of treatment programs and control systems. The selection of corrosion inhibitors depends on the type of water cooling system: once-through, open recirculating, or closed. Moreover, the tightening of environmental regulations limits the scope of compounds to use and also their maximum concentrations. Beneﬁts of application of corrosion inhibitors are related to their efﬁciency, ease of use, and high performance. They may also be multifunctional, i.e., simultaneously prevent corrosion, scale formation, and bacterial growth. Corrosion inhibitors can be used in conjunction with other methods for corrosion control, and sometimes use of chemical reagents is the only available way to reduce the corrosion losses.
4.2 INORGANIC CORROSION INHIBITORS The most well-known natural nontoxic inhibitor is calcium carbonate. In spite of the problems it can cause because of scaling on the heat exchanging equipment, the deposition of CaCO3 can slow down the corrosion rate of steels. The “stabilizing” water treatment is based on this very effect . Water is described as stable at a given temperature when it does not leave behind insoluble deposits of calcium carbonate and does not act aggressively on materials used in construction. However, this is not a very precise description, particularly with respect to the condition of equilibrium between the free carbon dioxide and the calcium ions in the water, which governs these effects. More properly, the stability of water is assessed from its saturation index, usually that of Langelier: SIL ¼ pH pHs ;
where pH and pHs are indices relating, respectively, to the pH value of the actual water and to that of water with the same composition but containing the equilibrium concentration of carbonic acid compounds. If pH < pHs, that is, if the
Corrosion Inhibitors in Cooling Water Systems Chapter | 4
concentration of free CO2 in the water is above the equilibrium value, then a carbonate ﬁlm will not be deposited on the walls of pipes or equipment. If pH > pHs, the calcium carbonate will be precipitated from solution and can reduce, or completely eliminate, corrosion. The object of a water stabilizing treatment is, therefore, the establishment of a layer of CaCO3 on pipe walls by the introduction into the water of sufﬁcient alkali to maintain SIL ¼ 0.5e0.7 (any stronger alkalization could lead to turbidity of the water). In addition to the saturation index, the Ryznar stability index can be used. SIR ¼ 2pHs pH
6.5 < SIR < 7dthe water is stable; SIR > 8drisk of corrosion; SIR < 6.5drisk of scale formation. However, the role of stabilizing treatment should not be overestimated. Even with SIL > 0, porous and friable deposits with weak protective properties form in the presence of 100 mg/L sulfate ions. The buffer capacity and ﬂow rate of the water, and the presence of cations of other metals (including magnesium) also have a marked effect on the quality of the calcium carbonate ﬁlm. The nature of the metal to be protected is also extremely important. In open recirculating systems cooled by evaporation, CO2 can be lost and the water will become alkaline. Furthermore, evaporation (in the cooling tower or in the pond) will increase the concentration of low-volatility impurities. All this will facilitate the formation of CaCO3, but its deposition can be retarded by the establishment of supersaturated solutions. Such supersaturation can be stabilized by the presence in the water of certain organic substances, such as polyphosphates, which are adsorbed on the faces of crystal nuclei, hence inhibiting their further growth. Depending on the nature and concentration of such substances, the degree of supersaturation of the water can vary over a wide range, but with continued evaporation it will, sooner or later, become unstable and the carbonate deposits will then adversely affect the heat exchange process as well as increase the hydraulic resistance in the system. To avoid the formation of deposits on cooling surfaces, part of the water is rejected (blowdown) and the loss is compensated by “makeup” water containing lower concentrations of Sa2þ and SP2 3 . Clearly, this is a compromise and cannot solve all the economical and ecological problems involved. With these in mind, developments have been made to aid in curtailing the blowdown by a water treatment that would allow an increase in the concentration factor of the water. This could be achieved by breaking down the bicarbonate by acidiﬁcation with sulfuric acid to a pH of 6.0e6.5: 2 2HCO 3 þ H2 SO4 ¼ 2CO2 þ 2H2 O þ SO4
The drawback of this method is that OSP3 is replaced by the corrosive SO4 , which, at concentrations above 500 mg/L, can increase the attack of concrete, as in cooling towers, and lead to the deposition of calcium sulfate. SiO2 (silica) is also a natural inhibitor. At SiO2 concentrations of 15e20 mg/L ﬂowing water systems may not require additional protection, particularly at high ratio of SiO2/Na2O. In continuous operation, satisfactory protection of steel is maintained even at lower SiO2 concentrations, 4e8 mg/L . Disadvantages of silicates are their tendency to form sparingly soluble deposits in the presence of calcium and iron ions, and their high sensitivity to the depassivating action of aggressive ions, especially sulfates. Among inorganic inhibitors of the oxidizing type, chromates are well known. They have a high protective effect for many metallic materials in a wide range of pH. The reduction of chromate ions plays an important role in the mechanism of inhibition action, for example, by the following reaction: 2
CrO2 4 þ 4H2 O þ 3e /CrðOHÞ3 þ 5OH
However, chromate is reduced only at the initial point of contact of the metal with solution, and after the formation of sparingly soluble hydroxides of chromium and iron the reaction (4.4) is terminated and an equilibrium is set up between the compounds of Cr3þ and adsorbed chromate ions . When the passivation ﬁlm breaks down, the chromate migrates to the defect points and heals the ﬁlm. Because of this, chromates are still in use in conversion coatings and paints (“self-healing coatings”). In aqueous media, the use of chromates is limited due to the high toxicity and danger to the environment and human health [3,4]. Another inhibitor of oxidizing type is sodium nitrite. It can be quite effective for closed systems and is able to enhance the protective effect of other inhibitors, for example, molybdate. The disadvantages of sodium nitrite include toxicity, the possibility of oxidation to nitrate, its inﬂuence by microorganisms, and its incompatibility with oxidizing biocides . These factors limit its applicability for the corrosion control in cooling systems, particularly of the open type.
Mineral Scales and Deposits
Molybdates can be used as less toxic substitutes of chromate inhibitors, however, their oxidizing properties are inferior to those of chromates. At the potentials of active dissolution of iron in a weakly acidic solution (pH < 6), molybdate forms a sparingly soluble salt : þ Fe þ HMoO 4 ¼ FeMoO4 þ H þ 2e
In neutral solutions, the oxidizing properties of molybdate are absent for steel and the presence of another oxidant is required for effective protection . An MoO2 black ﬁlm is formed at pH < 5, which indicates the reduction of a molybdate ion, rather than isopolymolybdate compounds. Oxidizing action of molybdates may occur in neutral solutions for metals with more negative corrosion potentials than that of steel. Molybdates can prevent pitting of various steels, including stainless steel. In this case, MoO 4 does not act as an oxidizing agent but as an inhibitor of the adsorption type because the initiation and growth of pitting occurs at potentials of the passive region . Molybdates, tungstates, and similar salts are able to form heteropoly compounds, many of which are stronger oxidants than molybdate. These compounds are suitable for the protection of metals in highly aggressive media . For example, in acidic brine (65% LiBr), phosphomolybdic acid (PMA) is an effective inhibitor of steel corrosion only at temperatures above 120 C. Addition of PMA into the brine accelerates the cathodic reaction on steel causing an increase of Ecor of 0.1e0.15 V. In weakly alkaline solutions, PMA is able to reduce the corrosion rate of steel in a wide range of temperatures. The effect of PMA on corrosion activation energy (DGcor) depends on the pH. At pH 5.0, the value DGcor decreases and at pH 8.5, it increases, which indicates a change in the mechanism of protection due to breakdown of the acid and introduction of PMA fragments into oxide ﬁlm. Thus, the mechanism of protection of steel is not limited to increasing the oxidizing ability of solutions containing PMA. Largely, it is associated with changing the composition of the oxide ﬁlm resulting in the improvement of corrosion resistance. The drawbacks of such inhibitors can be attributed to the relatively high cost and the narrow range of pH of aqueous solutions where they are stable. Polyphosphates obtained by thermal dehydration of NaH2PO4, in particular sodium hexametaphosphate ((NaPO3)6; HMP) and metatripolyphosphate (Na5P3O10), have long been known as inhibitors of scale formation and corrosion, and are widely used for water treatment in circulation systems for protection from corrosion and calcium salt deposits . Polyphosphates have certain advantages over many inhibitorsdthey are nontoxic, they are not expensive, and they may inhibit corrosion of the steel at low concentrations. Their main disadvantages include the possibility to enhance the corrosion at high concentrations due to the formation of soluble complexes with metal cations and the activation of the local corrosion of steel, particularly in alkaline aqueous solutions . An additional risk is related to the tendency of polyphosphates to undergo hydrolysis at pH > 7.5 resulting in their conversion to orthophosphate, which, in turn, can stimulate the formation of deposits. However, according to Ref. , in distilled water at room temperature, only 10% of the polyphosphate was found to be converted into phosphate after 30 days. At 85 C and pH 7e9, the protective effect of polyphosphates persists for 1 day. The stability of polyphosphates decreases with increasing pH and in the presence of calcium ions. Despite this, they perform a dual functiondto protect the steel from corrosion and to prevent the deposition of calcium carbonate. Thus, HMP forms soluble complexes with certain metal ions, including calcium, and keeps them soluble. This prevents formation of carbonate deposits, which could affect the heat transfer. However, with excess of Ca2þ ions the insoluble compound Ca5(P3O10)2 is formed , so it is necessary to strictly observe the concentration ratio of polyphosphate and Ca2þ. For preferential formation of soluble complexes, some excess polyphosphate is required, and the pH should be maintained at ﬁve to seven, as the more alkaline environment increases the probability of localized corrosion. For effective formation of a protective ﬁlm of calcium polyphosphate, a sufﬁcient concentration of calcium in the water is required, wherein the minimum value depends on the type of polyphosphate and falls into the range 50e100 mg/L based on CaCO3 . Furthermore, it is necessary to have a certain concentration of oxygen in the water. In the absence of calcium the content of oxygen in the water must be 1 mL/L, but in the presence of calcium it can be reduced to 0.15 mL/L . At the same time, according to Ref. , even in the presence of Ca2þ and oxygen the inhibition effect is provided by the hydrolysis products (HPO2 4 ) of polyphosphates, which causes the deposition of calcium orthophosphate on metal. The antiscaling action of polyphosphates is associated with their adsorption on crystalline planes, suppressing the growth of these crystals . Polyphosphates are classiﬁed as inhibitors of the mixed anodeecathode type. They promote the formation of a protective ﬁlm on the steel surface, consisting essentially of g-Fe2O3 and FePO4. Cations of zinc, calcium, or nickel facilitate the protection of steel by polyphosphates, while steel corrosion is reduced primarily by the inhibition of the cathodic reaction. The anticorrosion efﬁciency of polyphosphates increases with the increase in phosphate chain polymerization degree; however, with increasing molecular weight their solubility in water decreases. Formulation of HMP and calcium inhibits the corrosion of steel, and the formed ﬁlm provides the “after-effect” protection. The main ways of developing
Corrosion Inhibitors in Cooling Water Systems Chapter | 4
polyphosphate inhibitors are associated with the creation of inhibition formulations (with zinc salts, phosphonates, and water-soluble anionic polymers). Currently, there is a tendency to limit the use of phosphates. In 1980 in Japan, the restrictions on the content of phosphorus in wastewater were imposed and the ﬁrst “nonphosphorous” programs of water treatment were developed . However, by reviewing these programs, the authors of Ref.  concluded that despite some progress, the widespread use of “nonphosphorous” treatments is still difﬁcult. Water-soluble zinc [1,2] and aluminum  salts are also well-known inorganic inhibitors in neutral aqueous media. Their action is due to the deposition of the sparingly soluble metal hydroxides on the metal surface to be protected. These metal hydroxides are formed by the reaction with the hydroxide ions generated by the cathodic reaction of oxygen reduction. In the pH region 4.1e4.5, aluminum cations may be effective inhibitors of iron corrosion under both ﬂow and stagnant conditions . They may be present in water in very small amounts: an Al3þ concentration of 0.6 mg/L provides protection of 60% in water with total mineralization of 200 mg/L . The presence of Al3þ cations leads to their incorporation into the composition of magnetite by the partial substitution of Fe3þ and formation on the steel surface of g-FeOOH, a-FeOOH, and Fe3O4 (both stoichiometric and nonstoichiometric magnetite). In the absence of Al3þ, the a-FeOOH and Fe2þ hydroxides are formed predominantly. Corrosion products in both cases comprise the carbonates. Zinc salts are generally ineffective, but they are capable to provide the synergism of protection with other corrosion inhibitors, mainly inorganic and organic anions (chromates, polyphosphates, phosphonates, water-soluble polymers, etc.). However, despite the high efﬁciency of these compositions, there is a tendency to reduce the use of zinc-containing inhibitors. This is not only due to environmental requirements but because the risk of even a short-time appearance of biogenic hydrogen sulﬁde in the circulating water or pollution of water by sulﬁde-containing products leads to a sharp drop in the concentration of Zn2þ and hence to the possibility of weakening the inhibitor protection. Rare earth metal cations, in particular cerium, deserve some attention because they are considered more acceptable from an environmental point of view and are able to protect aluminum alloys [14,15]. It should be noted that the range of inorganic compounds able to protect metals from corrosion in aqueous solutions is relatively narrow. It is also limited by environmental requirements, which exclude the application of chromate-containing inhibitors, despite their high efﬁciency, and restrict the use of phosphorus- and zinc-containing compounds. In this regard, the organic inhibitors are more widespread, as well as various related formulations, which may include both organic and inorganic components.
4.3 ORGANIC CORROSION INHIBITORS Among the most signiﬁcant and widely used organic corrosion inhibitors for cooling water systems are phosphonic acids (Figure 4.1). These include 1-hydroxyethane-1,1-diphosphonic acid (HEDP), amino-tris(methylenephosphonic acid) (AMP), ethylenediamine-N,N,N0 ,N0 -tetrakis(methylenephosphonic acid) (EDTP), hexamethylenediamine-N,N,N0 ,N0 -tetrakis(methylenephosphonic acid) (HMDTP), 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), hydroxyphosphonoacetic acid (HPA), as well as their complexes with metal cations. The high performance of phosphonic acids in combination with their relatively low toxicity and commercial availability allows for successful use for water treatment to prevent scaling and corrosion. The results of numerous investigations of phosphonic acids and phosphonates as corrosion inhibitors, summarized in Refs. [2,4,16], have shown that their protective effect can be related mainly to chelation processes. The mechanism of corrosion inhibition by phosphonic acids is based on the possibility of two opposite processes occurring in the near-surface layer of aqueous solution. On the one hand, the interaction of the complex-forming reagents with the steel surface can cause the formation of soluble complexes MeeL (where Me is the metal cation and L is the phosphonate ligand) according to the following scheme: Meaþ þ Lb ¼ MeLðbaÞ
In this case, the probability of acceleration of metal dissolution by ligand increases along with the stability of formed soluble compounds. On the other hand, with the deﬁciency of ligand molecules or excess of metal cations, the sparingly soluble complexes are formed: nMeaþ þ mLb ¼ Men LmðmbnaÞ ;
where always n > m. Compensation of the negative charge of the ligand during complexation with the metal cation increases hydrophobicity and surface activity of the compound. If the formed complex is sparingly soluble, its accumulation
Mineral Scales and Deposits
FIGURE 4.1 Phosphonic acids. HEDP, 1-hydroxyethane-1,1-diphosphonic acid; AMP, amino-tris(methylenephosphonic acid); HPA, hydroxyphosphonoacetic acid; EDTP, ethylenediamine-N,N,N0 ,N0 -tetrakis(methylenephosphonic acid); PBTC, 2-phosphonobutane-1,2,4-tricarboxylic acid; HMDTP, hexamethylenediamine-N,N,N0 ,N0 -tetrakis(methylenephosphonic acid).
at the interface results in blocking of the metal surface and inhibition of corrosion. It should be noted that “the concept of chelation” does not exclude a positive role of the adsorption in the inhibition of iron and steel corrosion. However, taking into account the features of chemical structure of phosphorus-containing or other complex-forming agents, the concept highlights the need to consider their participation in the complexation and properties of the resulting products. Thus, the inﬂuence of phosphonic acids on the corrosion of metals can be divided into two types: 1. Ligands are hydrophilic and the complexes formed by them are easily soluble in the corrosive medium. 2. Ligands are rather hydrophobic but provide complexes that are sparingly soluble or almost completely insoluble. Complexing agents of the ﬁrst type are very soluble in water, so they often accelerate corrosion and initiate pitting on the oxidized metal surface. However, at high metal-to-ligand concentrations (N ¼ [Nfaþ]/[Lb]), the phosphonic acids are capable of forming sparingly soluble polynuclear complexes and act as corrosion inhibitors. Formation of such complexes on the metal surface is quite possible during active dissolution of iron and at low concentrations of chelating agent, when the value of N should be large in the near-surface layer. Unfortunately, in this case, corrosion inhibition occurs in a narrow concentration range of the complexing agent because if its concentration increases in solution, the value of N decreases and the formed complexes become soluble. Solution temperature greatly affects the protective ability of phosphonic acids. When temperature changes, the mechanism of action of the chelating agents described above does not change. However, the dependence of the corrosion rate of carbon steel on the phosphonic acid concentration is determined by the temperature of the solution. Increase in temperature causes quick passivation of the metal surface due to increased rate of the cathodic reaction of O2 reduction and accumulation of OH. Unfortunately, the passivation of steel in the presence of hydrophilic chelating agents, such as HEDP, leads to localization of corrosion. The complexing agent although can prevent general corrosion, it does not completely suppress metal pitting. However, the less hydrophilic complex-forming reagents, for example, HMDTP, are capable to prevent depassivation of iron by chloride ions. The most effective inhibitors are complexes of phosphonic acids with cations of various metals. In this case the mechanism of protective action is more complicated. Metal cations neutralize (partially or completely) the negative charge
Corrosion Inhibitors in Cooling Water Systems Chapter | 4
of the complexing agent, thus reducing its hydrophilicity and increasing the adsorption. In addition, substitution of the complexed cation by cations of the dissolving metal is also possible. When iron or steel immersed in aqueous solution, Fe2þ ions are produced from the anodic reaction and are partially precipitated in the form of hydroxide. These can react with phosphonates via metal (Me2þ) substitution as follows: Fe2þ þ ½MeHk La ¼ Me2þ þ ½FeHk La
The thermodynamic driving force of the reaction (4.8) is due to, as a rule, the higher value of the stability constants (KS) of phosphonate complexes of Fe2þ. The conjugate reaction of oxygen reduction generates hydroxyl ions and alkalizes the solution layer near the metal surface, which facilitates the deposition of sparingly soluble hydroxidedMe(OH)2: O2 þ 4e þ 2H2 O ¼ 4OH ;
Me2þ þ 2OH ¼ MeðOHÞ2 ;
which along with the adsorption of the inhibitor provides protective action on steel. An important role is played by the solubility of Me(OH)2. Protection efﬁciency increases with increasing KS of phosphonate inhibitors, but up to a certain limit. Above this critical value, the kinetic difﬁculties seem to appear for electrophilic substitution of complexing cation in the complex by iron ions and the inhibiting efﬁciency is reduced (Figure 4.2), i.e., to protect steel requires higher concentration of corrosion inhibitor. When the nature of complexing cation changes, while the chelating agent remains the same, there is dramatic dependence of inhibitor performance on hydroxide solubility that is formed by electrophilic substitution. Protective properties of phosphonates of divalent metals improve with decreasing the solubility of the hydroxides and with increasing the stability of complexes until their KS values exceed the values of the analogous iron phosphonate. The protection mechanism of more stable phosphonates is also associated with electrophilic substitution, but due to the formation of Fe(III) complexes characterized by a higher value of KS. In this context, rare earth metal cations are often recommended for use as inhibitors due to environmental restrictions. Their complexes are more stable than the corresponding Fe(II) phosphonates. However, in the neutral solutions Fe2þ may be oxidized to Fe3þ, thus facilitating the electrophilic substitution reaction presented in the following scheme: Fe3þ þ ½MeHn Lb ¼ Me3þ þ ½FeHn Lb
FIGURE 4.2 Effect of the stability constant, KS, of mono (1, 2) and binuclear (3) complexes of Amino-tris-(methylenephosphonic) acid (1) and 1-hydroxyethane-1,1-diphosphonic acid (2, 3) on the concentration for reliable protection of mild steel in water containing NaCl 30 mg/L þ Na2SO4 70 mg/L at pH 7.0.
Mineral Scales and Deposits
Formation of Fe(OH)3 proceeds simultaneously and competitively with the above process; however, Fe(OH)3 exhibits weak protective properties. Consequently, the phosphonates of rare earth elements can yield protective capacity to the divalent metal phosphonates. At the same time, the less stable aluminum phosphonate can react (4.8) and form a more effective protective ﬁlm, free from the adverse inﬂuence of Fe(OH)3. For example, the AleAMP complex is one of the most effective corrosion inhibitors in soft water with low chloride content. Phosphonates are able to form polymolecular layers on the metal surface [17,18]. The ﬁrst layer is usually strongly bonded with the metal surface, and its structure and properties are due to the interaction of phosphonate with the metal. The formation of phosphonate layers is a spontaneous process that can be provided even by simple immersion of metal into aqueous solution of inhibitor. In this case, the initial state of the metal surface may have a signiﬁcant impact on reducing the corrosion rate. According to Kalman et al. [19,20], the presence of an iron oxide ﬁlm plays an important role in the adsorption of phosphonates. According to Ref. , an ideal phosphonate corrosion inhibitor of the “complexing type” is required to possess the following features: 1. It must be capable of generating metal phosphonate thin ﬁlms on the surface to be protected; 2. It should not form very soluble metal complexes because these will not eventually “deposit” onto the metal surface, but will remain soluble in the bulk; 3. It should not form sparingly soluble metal complexes because these may never reach the metal surface to achieve inhibition, but may generate undesirable deposits in the bulk or on other critical system surfaces; and 4. Its metal complexes generated by controlled deposition on the metal surface must create dense thin ﬁlms with robust structure. If the anticorrosion ﬁlm is nonuniform or porous, then uneven oxygen permeation may create sites for localized attack, leading to pitting of the metal surface. Thus, the mechanism of protection of metal by complexes is associated not only with the adsorption but also with the surface reactions of electrophilic substitution of complexing cations, complexing cation precipitation, and formation of hetero- and polynuclear complexes. In this regard, the dependence of their efﬁciency on the chemical composition is obvious, although it presents quite often complex character and behavior. Due to the combination of useful properties, the phosphonate inhibitors are widely used in water treatment. The most widely known are HEDP, AMP, HMDTP, PBTC, and HPA [2,21], but the list of investigated phosphonic acids and reagents on their basis is constantly growing. For example, studies of carboxymethyl phosphonic acid (CMPA) and 2-carboxyethyl phosphonic acid (2-CEPA) in the presence of Zn2þ  have revealed greater efﬁciency of 2-CEPAeZn. The authors attribute this to the following facts: 2-CEPA keeps the Zn2þ in a more soluble form and larger size of Fe2þe2-CEPA complex, which is more stable than Fe2þeCMPA complex. Propylphosphonic acid (PPA) in combination with Zn2þ may possess protective properties for carbon steel in neutral environments. It induces ﬁlm formation on the steel surface, consisting of complexes [Fe(III)/Fe(II)/Zn(II)ePPA], Zn(OH)2, iron oxides, and hydroxides . In some cases, various alkyl phosphonic acids with variable-length carbon chains have been discussed . On the one hand, such phosphonic acids are more hydrophobic and have higher surface activity; however, their applicability in water systems is limited by their tendency to form colloids that are of lower stability in the presence of water hardness salts. The advantages of phosphonate inhibitors include a high protective capacity in aqueous solutions with varying salinity and in a wide temperature range, a combination of antiscaling and anticorrosion actions. Disadvantages are associated with the possibility of localized corrosion, and reduced efﬁciency in stagnant zones and in the presence of even small quantities of hydrogen sulﬁde. Phosphonate inhibitors may also be subjected to degradation in the presence of oxidizing biocides . The relatively high cost, environmental requirements on phosphorus content, and metal ions in the water limit the use of phosphonate inhibitors. However, there is considerable research on innovative ways to improve their efﬁciency and to reduce concentration. There are three main areas : 1. changing the chemical structure of the phosphonic acid to induce lower toxicity to the phosphonate or its complexes with nontoxic metals with high protective capability; 2. creation of conditions in the near-surface solution layer, namely raising the pH of the solution, to facilitate precipitation of sparingly soluble hydroxides of the complexing cation; and 3. combined use of phosphonates with various types of nonoxidizing additives. It should be noted that the use of mixtures of complex-type inhibitors can increase anticorrosion effectiveness. Thus, in Ref.  the superiority of the inhibitor IFHAN-36, containing not only zinc but tin phosphonates as well, was
Corrosion Inhibitors in Cooling Water Systems Chapter | 4
FIGURE 4.3 Effect of sodium nitrite and sodium m-nitrobenzoate on inhibition efﬁciency of 1-hydroxyethane-1,1-diphosphonic acid (HEDP)eZn in hot water (80 S) containing 150 mg/L NaCl and 350 mg/L Na2SO4 (pH 7.0).
demonstrated over the widely known HEDPeZn in the protection of steel and aluminum in a medium with high concentration of aggressive chloride and sulfate ions. Changing the pH of the solution near the metal surface to destabilize metal phosphonates can enhance their protective capability. This method was used for development of the combined protection of steel in corrosive seawater . It includes the cathodic polarization of steel construction with the “sacriﬁcial” anode and the simultaneous introduction of small amounts of phosphonate inhibitors. Further development of this method was reported recently . Another way to increase the pH of the near-electrode layer is the use of oxidant additives , which are either reduced at a higher rate than that of oxygen (e.g., anion of nitrobenzoic acid) or when reduced, they generate more OH species per unit of transferred charge (sodium nitrite, hydrogen peroxide, etc.). It should be noted that the reduction of oxidant should not be accompanied by the formation of soluble products like in the case of chromate or molybdate. Therefore, to study the effect of oxidants on the protective properties of metal phosphonates, it is most convenient to use NaNO2 or H2O2, for example. The possibility of increasing the effectiveness of anticorrosion protection was studied in detail on the example of the mixture of metaleHEDP, primarily zinc, with NaNO2 [27,28]. Oxidant additives enhance protective properties of HEDPeZn and allow one to reduce the concentration of phosphonate in formulation (Figure 4.3). Enhancing the inhibition properties of PBTC in the presence of NaNO2 has been demonstrated in Ref. . A third way to increase the efﬁciency of phosphonates is the use of various nontoxic and available nonoxidative additives. When the latter are used in combination with water-soluble polymers, adsorption inhibitors, and complexing agents, the concentration of phosphonates in water systems can be reduced. Heterocyclic compounds that are capable of forming insoluble complexes with copper, zinc, and other metals in aqueous solutions are among the complexing-type inhibitors that can be noted. Imidazoles, triazoles, thiazoles (Figure 4.4), and others are well-known compounds among the class of azoles [1,2,30e32]. In most cases, azoles form thin ﬁlms of insoluble complexes with cations on the metal surface with these metals. The inhibitor 1,2,3-benzotriazole (BTA) and its derivatives practically have no competitor in corrosion prevention of copper and its alloys. Imidazoles are mainly known as corrosion inhibitors in acid media, but some derivatives thereof, such as benzimidazole, also exhibit protective properties in neutral and alkaline media. Benzimidazole and 2-mercaptobenzimidazole are effective for zinc and copper, whose cations form rather stable complexes, and they are also effective inhibitors of general and selective corrosion of brass in aqueous solutions. 2-mercaptobenzimidazole is effective for copper corrosion inhibition in the acidic and neutral media [30,32]. 4-methyl-1-phenyl-imidazole and 4-methyl-1-(p-tolyl)-imidazole are more effective inhibitors of copper corrosion in neutral medium rather than in acidic medium . The protective properties of benzimidazole derivatives depend on the nature of the substituent. Introduction of electrondonor substituents enhances the protection of zinc, while electron accepting shows increased effectiveness for copper. Benzimidazoles can also be used in combination with other inhibitors. These compositions are designed mainly for the protection of metals in acidic environments; however, 2-aryl- and 2-alkyl-benzimidazoles together with anionic surfactants can be recommended as inhibitors of corrosion and microbiological fouling in recirculating water systems .
Mineral Scales and Deposits
FIGURE 4.4 Heterocyclic corrosion inhibitors.
Triazoles are the more widespread group of azole-type inhibitors. 1,2,3-triazole (AT) and its isomer 1,2,4-triazole in aqueous solutions exhibit anticorrosion properties for various metals and alloys; however, they are inferior to BTA. However, 3-amino-1,2,4-triazol can exceed BTA for copper corrosion protection. Alkylation of AT increases its surface activity, together with its protective properties. Among the alkyl derivatives of 3-amino-1,2,4-triazole, N-decyl-3-amino1,2,4-triazole has high barrier properties . For water treatment applications, the most common inhibitors are BTA and tolyltriazole. BTA alkyl derivatives, for example, butylbenzotriazole or pentoxybenzotriazole can be proposed for the formation of more stable ﬁlms. Chlorotolyltriazole is a halogen-resistant alternative for tolyltriazole in highly chlorinated systems . The largest number of papers concerns the study of the protective properties of BTA due to its availability, relatively high solubility in water and aqueousealcoholic media, thermal stability, and ability to form sparingly soluble compounds with several metal cations [30,35]. Most often it is used for the protection of copper and its alloys; however, BTA is also capable of forming protective ﬁlms of polymeric complex compounds on iron and other metals . Protection mechanisms of substituted BTAs may have several distinctive features. Copper corrosion inhibition efﬁciency of alkyl esters (methyl, butyl, hexyl, octyl) of carboxybenzotriazole depends on the concentration, time, and pH . At pH w 0, octyl and hexyl esters are more effective than BTA under the same conditions. With increasing pH up to eight their protective ability is reduced. For butyl and methyl esters the opposite trend was observed, i.e., the protective ability increased with pH. In acidic sulfate solution protective effects were in order octyl > hexyl > butyl > methyl, whereas at pH w 8 this order was reversed. It is assumed that the protonated substituted BTAs adsorbed on copper through the azole nitrogen, and the protective effect at low pH is determined by the attractive forces between adjacent alkyl chains, which increase with the length of the alkyl chain. Conversely, at high pH a polymeric complex is formed and steric hindrance adversely affected the long radicals. Alkylated BTA derivatives are very effective corrosion inhibitors. In sulfate solutions corrosion inhibition of copper is enhanced in the following order: BTA < 5-methyl-BTA < 5-butyl-BTA < 5-hexyl-BTA w 5-octyl-BTA . At the same time, 5-dodecyl-BTA had no signiﬁcant effect on the anodic reaction due to low solubility. In solutions containing chloride, BTA and 5-methyl-BTA showed no inhibitory properties at concentrations of 0.1 mmol/L, unlike 5-butyl-BTA and 5-hexyl-BTA. Various mercapto and thiazoles (2-mercaptobenzothiazole, MBT; 5-mercapto-1-phenyltetrazole) are mainly known as corrosion inhibitors in acid media, but can also exhibit protective properties in the neutral pH range for copper, steel, and aluminum alloys [2,4,39]. MBT is one of the most effective inhibitors of copper corrosion and was one of the ﬁrst azoles used for water treatment, but currently it is not widely used. Microorganisms can destroy it, while oxygen or halogens may oxidize the eSH group. MBT is widely used especially in the closed or once-through cooling water systems, where such problems are less pronounced . On a copper surface, MBT forms a protective complex ﬁlm of polymeric nature . Increase in the electron density on the reaction center in benzothiazole enhances inhibition of zinc anodic dissolution due to strengthening of the Ϭ-bond in protective zinc complexes. In contrast, the copper protection is enhanced by electron-acceptor substituent R (Figure 4.5). These examples show the need for all-round study of the mechanism of corrosion inhibition by heterocyclic compounds and the important place in this belongs to investigations of the “chemical
Corrosion Inhibitors in Cooling Water Systems Chapter | 4
FIGURE 4.5 The dependence of lg gan on the inductive ϬI-constants of the R in benzothiazoles at the anodic dissolution of copper and zinc in 0.5 M phosphate solution at pH 11.4. Cin ¼ 0.5 mM (Cu) and 1.0 mM (Zn); E ¼ 0.14 V (Cu); and 1.0 V (Zn). ϬI is the characteristic of substituent R and g is the coefﬁcient of corrosion retardation of the metal by the inhibitor (or gan ¼ iblank/iin in the case of the anodic dissolution at a constant electrode potential).
structure-property” function. From these considerations, it follows that MBT is a very effective inhibitor of copper corrosion in phosphate solutions. Tetrazoles are much less explored as corrosion inhibitors and are not used in circulating water systems, but the efﬁciency of the tetrazole derivatives may be quite high. Study of various tetrazoles, such as 5-mercapto-1-methyl-tetrazole, 5-mercapto-1-acetic acid-tetrazole, 5-mercapto-1-phenyl-tetrazole, 5-phenyl-tetrazole, and 5-amino-tetrazole, within a pH range from four to eight and a temperature from 40 to 80 C have shown that most of the examined compounds are capable of effectively inhibiting copper corrosion in 0.1 M NaCl. The protective capability of phenyltetrazoles was higher . Tetrazole derivatives can also act as corrosion inhibitors for iron. 5-(3-aminophenyl)-tetrazole retards general and pitting corrosion of iron in 3.5% NaCl aerated solution . Inhibition is due to the adsorption and polymerization of 5(3-aminophenyl)-tetrazole on the iron surface to form complexes with Fe2þ. 2-Propargyl-5-hydroxyphenyltetrazol was studied as an inhibitor of scaling and mild steel corrosion . Heterocyclic inhibitors are well represented in the scientiﬁc literature and are very popular objects of study. Typically, they are derivatives of 1,2,4-triazole, BTA, benzimidazole, and so forth, although there are compounds containing other heterocyclic rings. Several other corrosion inhibitors have been proposed for use for ferrous metals in cooling systems as follows: 2,5-diphenylpyrazolo[1,5-c]pyrimidine-7(6H)thione, 5-methoxyphenyl-2-phenylpyrazolo[1,5-c]pyrimidine-7(6H)thione, 5-tolyl-2-phenylpyrazolo[1,5-c]pyrimidine-7(6H)thione, 5-tolyl-2-phenylpyrazolo[1,5-c]pyrimidine-7(6H)one, 2,5-diphenyl3-iodopyrazolo[1,5-c]pyrimidine-7(6H)thione, 2,5-diphenyl-3-bromopyrazolo[1,5-c]pyrimidine-7(6H)thione, and so on . To prevent corrosion of brass in cooling water, oxadiazole derivatives (2,5-bis(n-methylphenyl)-1,3,4-oxadiazole) can be used in combination with cetyltrimethylammonium bromide as biocide . Triazole derivatives containing phosphonate groups, such as 3-benzylidene amino-1,2,4-triazole phosphonate, 3-paranitrobenzylidene amino-1,2,4-triazole phosphonate, 3-salicylalidene amino-1,2,4-triazole phosphonate, are also of interest as well as their blends with molybdates . Such inhibitors can act as multifunctional reagents for circulating water systems. Despite the numerous publications on the possibilities to use various heterocyclic compounds and their derivatives as corrosion inhibitors, the list of actually used compounds is much shorter and represented by well-known inhibitors (BTA, MBT, tolyltriazole, etc.). The reasons for this are obvious such as industrial availability, application experience, the availability of information about toxicity, and low cost. With respect to corrosion inhibition it is possible to combine heterocyclic compounds with other inhibitors, although this issue is relatively poorly studied. Typically, heterocyclic inhibitors are added with other compounds for enhancing the protection of copper alloys. However, inhibition enhancement can occur, for example, with phosphonates  or with BTA and sodium benzoate (SB) for steel passivation .
Mineral Scales and Deposits
Another class of inhibitors suitable for use in aqueous media comprises different carboxylates. These are anionic compounds containing one or more carboxylate groups, eCOO. They are rarely used in cooling water systems, but can be very effective, nontoxic, and biodegradable corrosion inhibitors. The structure of such compounds can be quite diverse and can also include other functional groups in addition to the carboxyl group such as amino acids or PBTC. Many of them are capable to inhibit the corrosion of not only ferrous but also nonferrous metals, as well as may act as multifunctional reagents having anticorrosion combined with antiscaling or biocidal activity. The most accessible and well-studied carboxylate inhibitors are the salts of aromatic and fatty carboxylic acids. A well-known corrosion inhibitor among aromatic carboxylates is SB, capable of slowing down the corrosion of iron and nonferrous metals (copper, aluminum) [2,4]. SB may promote dissolution of iron from the active state, but is capable of adsorbing on the oxidized surface, stabilizing the passive state, and slowing down the corrosion. The thickness of the adsorbed benzoate may not exceed one-tenth of the molecular layer. The introduction of additional functional groups into the benzene ring may improve the efﬁciency of aromatic carboxylates. Promising inhibitors are aminobenzoates. Their protective ability depends on the position of the amino group. Salts of m-, p-amino-, and 3-oxy-4-amino benzoic acids are also known. When a nitro group is introduced into the benzene ring, the inhibitor acquires the ability to reduce on metal surface at potentials corresponding to the active region dissolution, which results in passivation of iron. The current obtained from the reduction of nitrobenzoate on iron is signiﬁcantly higher than the oxygen diffusion current and thereby the corrosion potential shifts to the passive region [4,8,48]. The role of the chemical structure of substituted benzoates on their ability to inhibit the dissolution of iron and steel was established by means of correlation analysis methods, based on the principle of the linear free-energy relations. This approach is useful for evaluation and prediction of the protective effect of carboxylates in relation to various metals and alloys, the effect of pH, temperature of the solution, and the nature of activator . Use of this method in practical applications has enabled development of a number of new effective corrosion inhibitors. Introduction of electron-donor substituents into the ortho-position to the benzoate carboxyl group signiﬁcantly enhances its protective properties. One of the best inhibitors of this type in preventing iron depassivation is sodium anthranilate. Since the NH2 group present in its structure has electron-donor properties, its introduction to the aromatic ring of SB increases the electron density on the reaction center, which explains the beneﬁcial effect on the inhibition efﬁciency. As an example of substituted SB, sodium phenylanthranilate (PAN) has ﬁrstly demonstrated the possibility of iron passivation only due to the adsorption of the inhibitor, without the formation of the oxide layer . Interaction of PAN with iron electrode leads to the formation of an adsorption complex of the metal and amino acid. Complexes formed on the surface of the iron electrode with preliminary formed oxide layer and without it had the same nature but different bond strength: organic anions are weakly adsorbed on the oxidized surface. The introduction of polar substituents to the nucleus of PAN may lead to inhibitors that are more effective. Comparison of the adsorption and passivation action of various substituted phenylanthranilates  such as sodium salts of mefenamic acid [2,3(SO3)2S6O3NH]C6H4COOO (MEFN), N-(3-diﬂuorormethylthiophenyl)-anthranilic acid, [3-(SSOF2)S6O4NH]C6H4COOO, (DFT) and ﬂufenamic acid [3-(SF3)S6O4NH]C6H4COOO (FFN) have shown that the passivation effects of MEFN and FFN are signiﬁcantly higher than that of PAN. All of them are well adsorbed on the oxidized as well as reduced iron surface. FFN was the most effective both in the passivation of iron and in the protection of local depassivation due to its better adsorptivity. Lower aliphatic carboxylates can even stimulate the corrosion of metals or cause depassivation due to hydrophilicity of anions. However, with increasing length of the hydrocarbon chain they exhibit inhibition properties. The corrosion rate of mild steel was reduced with an increase in n from zero to eight, when salts of mono- and dicarboxylic acids of general formula (SO2)n(SPPO)2 were used in water . Complete protection from corrosion was achieved at pH 7. At lower pH values (<4.0e5.0), corrosion damage was found to be maximum. Similar results were obtained in other studies [51,52]. For example, in aerated 0.1 M solutions of monobasic carboxylic acid salts containing more than four carbon atoms in their chain (nC 4), electrode passivation was achieved due to the formation of insoluble products with iron cations. Low efﬁciency of carboxylates with nC < 4 was due to the fact that their short chains are not able to block the surface. Dicarboxylates show similar action , although their adsorption capacity is lower. It was noted that they are more effective in the presence of oxygen. Protection of low-carbon steel can be enhanced by joint use of di- and monocarboxylates or by BTA additives [53,54]. These inhibitors can protect not only the iron and steel but other metals and alloys as well, e.g., copper, as shown, for example, in cases of sodium heptanoate  or potassium sorbate . Magnesium alloy AZ31 can also be protected by salts of monocarboxylic acids (decanoic, dodecanoic, tetradecanoic) due to the formation on the surface of insoluble salts of magnesium .
Corrosion Inhibitors in Cooling Water Systems Chapter | 4
Sodium decanoate was more effective than sodium 2-ethylhexanoate or octanoate for the protection of iron in nearneutral solutions containing chloride and oxygen . Hydroxycarboxylic acids may also act as corrosion inhibitors. In particular, salts of gluconic acid (OPPS(SOPO)4SO2PO) can be recommended for cooling systems . It is evident that with hydrocarbon chain-length increase, the protective ability of the carboxylate inhibitors is enhanced due to increase of the hydrophobicity and surface activity. That is why the most effective inhibitors are higher carboxylates, including salts of carboxylic fatty acids. One of the most famous among them is sodium oleate [2,4], capable of protecting ferrous and nonferrous metals. Despite the high efﬁciency of higher carboxylates in corrosion inhibition of metals, their applicability in cooling water systems is limited by several factors such as the instability in the presence of hardness salts, the tendency for micelle formation, and the potential of foaming. Being mainly anodic inhibitors, their use poses certain risks because of the possibility of localized corrosion. Among the various carboxylates, sarcosinates deserve special attention. They have a number of advantages such as good water solubility and lower sensitivity to hardness salts. Sodium N-oleylsarcosinate (OS) is able to form passive ﬁlms on the metals in neutral chloride-containing buffer solution. Stabilization of the passive surface is associated with strong adsorption of OS due to coordination of nitrogen and oxygen atoms to metal ions and the formation of a ﬁve-membered chelate ring [60,61]. In industrial cooling water, dodecylsarcosine and tetradecyl-b-aminopropionic acid provide a high protection degree (Z > 90%) even without addition of zinc ions . OS signiﬁcantly slows down the active dissolution of iron, but its adsorption on the oxidized surface is weaker than on the reduced one. Formulation of OS and MEFN blend offers greater efﬁciency in the stabilization of the passive state. The high adsorption energy indicates that the interaction of the inhibitor with iron surface is chemisorption . Sodium N-lauroyl sarcosinate (SLS) and sodium dodecyl sulfate (CH3(CH2)11SO4Na) prevent pitting of stainless steel in chloride solutions . It was shown that SLS is a more effective inhibitor than dodecyl sulfate (DDS) in the temperature range from 25 to 85 S. DDS inhibits only pitting, while SLS inhibits general corrosion as well. DDS does not form coordination bonds with the passive ﬁlm on stainless steel. The adsorption of DDS may include only electrostatic and hydrophobic interactions, so its efﬁciency signiﬁcantly decreases with increasing temperature. A similar effect was observed for aluminum alloys [65,66]. It was shown that SLS and DDS are capable of suppressing not only the general corrosion but pitting corrosion as well; however, SLS has shown higher protective ability because it is capable to chemisorb on the metal surface. Many amino acids can be recommended as nontoxic corrosion inhibitors for aqueous media. For example, the protection efﬁciency of several amino acids for aluminum pitting corrosion in 0.1 M NaCl amino acids follows the following order: arginine > histidine > glutamine > asparagine > alanine > glycine . Protective ability of L-glutamine for Al7075 alloy was low in unstirred solution, but it increases under hydrodynamic conditions . The environment friendly inhibitors cysteine, N-acetylcysteine, and methionine were proposed for the protection of copper alloys [69,70]. The possibility of application of such inhibitors largely depends on their availability and cost, which currently only slightly satisfy industry requirements. Polycarboxylic amino acids, such as the well-known chelating agents ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), are much more readily available. They were often included in different formulations (with Zn2þ, silicates, etc.). However, despite their high antiscaling properties, they are widely used in thermal power plants, heating systems, and boilers, but not in cooling systems [3,71]. Their phosphorus analogs, phosphonic acids, are much more commonly used. Water-soluble oligomers and polymers have attracted considerable attention. In most cases, they possess attributes similar to the inhibitors given above. However, peculiarities in their chemical structure, such as high molecular weight, large number of functional groups, and presence of inter- and intramolecular interactions that determine the spatial structure and size of these macromolecules, allow us to consider them as separate class of corrosion inhibitors. It is also important to note that water-soluble polymers can be multifunctional. They can act as corrosion inhibitors of various metals, scale inhibitors, and may possess sometimes biocidal activity [72,73]. The dependence of polymer properties on their chemical structure is quite complex. They depend on the ﬂexibility of the polymer chains, intramolecular interactions (i.e., nonchemical interactions of units of the macromolecule), and other factors that determine the structure of the polymer. Like the low molecular compounds, polymers can form associates and solvates in solutions, but the dimensions of these macromolecules principally determine their speciﬁc properties. Polymeric inhibitors can be classiﬁed according to the nature of functional groups as follows: sulfonate, carboxylate, anionic, and cationic. Lignosulfonates (LS) are well recognized among sulfonate polymers and are commercially available and inexpensive products. The mechanism of inhibition action of LS may be associated with the formation of protective layers on the metal
Mineral Scales and Deposits
surface due to their adsorption. Sodium lignosulfonate retards the corrosion of steel in solutions of potassium chloride, but according to the results of X-ray diffraction and X-ray photoelectron spectroscopy (XPS), it is not included into corrosion products . It appears that the inhibitor does not affect the corrosion mechanism, and only blocks a part of corroding surface because of adsorption. The LS GCL2 provides effective protection of steel from corrosion and scaling. GCL2 is modiﬁed after preoxidation by grafting and chelation. Anticorrosive effect of the LS at Cin ¼ 50 ppm reached Z ¼ 95%, and scale inhibition was even higher already at Cin ¼ 8 ppm. Inhibitor GCL2, adsorbed on the steel surface, inhibits corrosion by slowing down both anode and cathode partial reactions. It prevents scaling because of its ability to form complexes and dispersions of the formed deposits. From the different relative molecular mass (M) fractions of calcium LS, it was demonstrated that only the LS with M < 5000 exhibits anticorrosion properties. It should be noted that the carboxyl, hydroxyl, and sulfonate groups content generally decreases with increasing M . GCL2 forms an adsorption ﬁlm on the steel surface. Its adsorption was signiﬁcantly higher in stirred than in static solutions . The effect of sulfonate polymers on metallic corrosion can be associated not only with adsorption but also with the ability to form complexes with metal cations. In order to enhance the anticorrosive effect of sulfonated polymers in aqueous systems, their combination with zinc salts has been recommended [77,78]. LS in some cases can stimulate the corrosion of steel. They are also able to effectively disperse corrosion products on the metal surface. However, combination of LS zinc cations or by zinc-phosphonate inhibitor enhances the inhibition of the cathodic reaction, which allows their use as a component of the inhibition formulations . Carboxylated water-soluble polymers are also well known as, at least, one of the components of scale and corrosion inhibitors. Among these anionic polymers, the compounds based on acrylic acid and maleic acid are most widely known. Some polyelectrolytes (polyacrylic acid, polyethyleneimine, polyethyleneoxide, tannin derivatives) are capable of inhibiting corrosion of copperenickel alloys in chloride-containing solutions . However, their action cannot be limited only to physical adsorption to the metal. Electrochemical and electron microscopy studies have shown that the adsorbed polymer favorably inﬂuences the structure of the corrosion products, and is able to convert them into a passivating ﬁlm. However, these polymers do not always exhibit anticorrosive properties. For example, polyacrylic acid (M ¼ 10,000) and its quaternary polyamide (M ¼ 100,000) were shown to even accelerate the corrosion of steel . Polyacrylic acid showed weak inhibition properties only in acidic medium. This action was partly interpreted based on its complexation processes. A terpolymer of acrylic acid, 2-hydroxymethacrylate, and methylmethacrylate (M ¼ 3000) adsorbs on a calcium phosphate ﬁlm, slows down its growth, makes the ﬁlm less porous and thinner, and enhances its protective properties . Investigations of the corrosion inhibition action of certain cationic (polyethyleneimine, its derivative, polyarylamine, and polydicyanodiamide derivative) and anionic (polymaleic acid derivatives, polyacrylic acid derivatives, and polyacrylic acid) polymers have shown  that the cationic polymers do not inhibit corrosion of mild steel, while anionic ones can retard corrosion and scale formation. The efﬁciency of these polymers depends on their average molecular weight and the number of carboxyl groups. The polymerepolymer complexes [(PMAAN/PAAmM)], composed of polymethacrylic acid (PMAAN) and polyacrylamide (PAAmM) were investigated as inhibitors for corrosion of mild steel in cooling water systems . The inhibition abilities of (PMAAN/PAAmM) against corrosion and scale deposition were evaluated by corrosion tests and physicochemical methods. In a solution with low concentration of ionic species, the corrosion inhibition ability of (PMAAN/PAAmM) improved at polymer concentration >50 ppm. This effect was due to the control of polymer adsorption on steel surfaces by the formation of polymerepolymer complexes. In a solution with high concentration of ionic species, the corrosion inhibition ability of (PMAAN/PAAmM) was also effective at polymer concentration >20 ppm. This effect is attributed to control of the adsorption of the polymers on steel surfaces and the scale dispersion based on the formation of polymerepolymer complexes. Some polypeptides have been envisioned as biodegradable alternatives for polyacrylates. For example, polyaspartates of M y 1000e2000 are effective inhibitors of carbonic acid corrosion of steel  and of scaling [84,85]. In case of scale inhibition (during the deposition of barium sulfate), they are comparable to phosphorus-containing compositions, and in some cases even superior to them [86,87]. Scale and corrosion inhibitor, polyaspartic acid/3-amino-1H-1,2,4-triazole-5-carboxylic acid hydrate graft copolymer was synthesized from maleic anhydride, urea, and 3-amino-1H-1,2,4-triazole-5-carboxylic acid hydrate. The graft copolymer possesses good corrosion inhibition properties and outstanding scale inhibition performance for CaCO3, Ca3(PO4)2, and CaSO4 in simulation experiments .
Corrosion Inhibitors in Cooling Water Systems Chapter | 4
Like many other anionic inhibitors, carboxylate polymers can form complexes with metal cations, so it is recommended to use them in combination with zinc salts [72,89]. Polycarboxylate derivatives, for example, PAAmM, can also act as corrosion inhibitors, although they are usually of low efﬁciency and they are used in combination with other inhibitors such as zinc salts and phosphonates . Among the anionic polymers, there are compounds containing a phosphonate group. Polymers based on phosphonocarboxylic acids have been recommended for chemical treatment of water circulation systems [91,92]. Homopolymer of isopropenylphosphonic acid and its copolymers with acrylic acid and oxypropylacrilate may be effective ﬁlm-forming corrosion inhibitors of steel , especially with steel pretreatment with a solution of polymer and Zn2þ. Copolymers also effectively prevent scale formation. Tomin et al. reported signiﬁcant protective properties of a polymer synthesized by reaction of phosphoric acid with hexamethylenediamine and urea in the presence of glycerol . The reaction results in a water-soluble glassy polymer product which, when at Cin ¼ 50e100 mg/L and without stirring, provides a protective effect of more than 95% and contributes to reduction of the corrosion rate to values of less than 0.01 mm/year. The precise structure of the polymer has not been established, but the IR spectrum provides evidence for the presence of esters, phosphate, urea, and guanidine groups, as well as methyl and methylene fragments. A phosphinosuccinic acid oligomer has been used for water treatment as a multifunctional reagent, providing protection against corrosion and scale formation [1,94]. Polyvinylpyrrolidone (PVP) may be an efﬁcient inhibitor in aqueous media [95,96]. PVP is able to protect steel from corrosion, but when used together with zinc salts its efﬁciency decreased. Results of structural analysis of the protective ﬁlm have shown that it consists of a complex Fe2þdPVP and in the presence of Zn2þdof the same complex and zinc hydroxide. Polyamines and polyimines are able to protect stainless steel. Drastic reduction of the anodic dissolution current and suppression of pitting of stainless steels in chloride-containing aqueous solution (0.7 M NaCl) were observed when the surface was covered by 40 molecular layers of a cationic polyelectrolyte, poly(N,N-dimethyl-N,N-diallyl)ammonium chloride , used as a coagulant and ﬂocculant for water puriﬁcation from impurities. Polyethyleneimine exhibited inhibition properties with respect to stainless steel ASTM 420 in 3% NaCl . Polyguanidines (PG) are of special interest among the cationic polymers due to their excellent bactericidal properties [73,99] because in many cooling systems the microbiologically inﬂuenced corrosion and fouling is an important issue. Contamination of such systems is associated with optimal conditions for the formation and growth of bacterial communities, which include a variety of microorganisms. Microbiological fouling increases the ﬂow resistance in the system, reduces the efﬁciency of heat transfer, and contributes to the emergence of underdeposit pitting corrosion processes. PG salts are readily soluble in water and are capable of inhibiting corrosion of the equipment. The examples include polyhexamethyleneguanidine (PHMG) phosphate or chloride in which the PHMG cation has the formula HN
PHMG salts not only can be applied to prevent biofouling but also are effective ﬂocculants and have complexing properties. The application of PHMG compounds have been described in detail by Tomin et al. [73,99]. According to the research of various derivatives of PHMG (phosphate, hydrochloride, hydrate, 1-hydroxy ethane-1,1-diphosphonate, nitrate, nitrite), it was shown that the protective effect depends on the counterion of PHMG. The colloidal stability of the polyelectrolyte complexes formed depends on the counterion choice. It was shown that the use of PHMG for modifying salt coatings can signiﬁcantly improve their protective properties. Modiﬁed PHMG salt coatings in aqueous solutions interact with polyacrylates, polyphosphates, and PAAmM, which form polymer complexes with PHMG on the surface. Multivalent anions of tetraborate, molybdate, and tungstate, introduced in water, strongly interact with the coating and improve its anticorrosion properties, suppressing the growth of localized corrosion sites. Furthermore, during the treatment of water, PHMG exhibits detergency and facilitates puriﬁcation of corroding surface and removal of sludge from the system. XPS investigation of mild steel specimens exposed in the solution of PHMG phosphate has shown the formation of thin layers (Figure 4.6) of iron oxide, iron phosphate (3e4 nm), and three to ﬁve molecular layers of PHMG .
100 Mineral Scales and Deposits
FIGURE 4.6 Possible location of the PHMG chains on the surface of the iron phosphate: horizontal (left) and vertical (right).
4.4 INDUSTRIAL ASPECTS OF CORROSION INHIBITORS Thus, water-soluble polymers may be effective corrosion inhibitors of various aqueous media. In addition to anticorrosion properties, they may exhibit antiscaling and biocidal effects, allowing their use as multifunctional reagents. The mechanism of corrosion inhibition is associated with the processes of adsorption and complex formation, and depends on the nature of the metal and physicochemical properties of the polymer molecule, the nature and amount of functional groups, steric factors, and so forth . Corrosion inhibitors are rarely used as individual compounds. Usually for chemical water treatment, the compositions comprising several inhibitors are used. This may be due to the need to protect various metals in a single system, while the individual compounds may be effective only with respect to a certain metal. Another aspect is the development of multifunctional reagents (corrosion inhibitorescale inhibitorebiocide). In many cases, the components of inhibiting compositions enhance the protective effect of each other. This allows one to combine different compounds, selecting more efﬁcient and economical solutions for chemical treatment of cooling systems. The most demonstrative example is the use of anionic inhibitors (phosphonic acid polymers, water-soluble polymers) together with salts of zinc or other metal cations. In actual cooling water systems, synergistic protection of blends of phosphonates and Zn2þ is observed primarily in relation to steel, but the stability of the aluminum or copper alloys is also increased. However, for more effective protection of the latter, the addition of the well-known azole inhibitors such as BTA, MBT, benzimidazoles, and related molecules (see above) is usually recommended . Phosphonate inhibitors often form the basis of inhibitory compositions, and can also be used in conjunction with amines [102,103], carboxylates [104,105], water-soluble polymers [78,105], or oxidation-type inhibitors [27e29]. Despite numerous studies available of various corrosion inhibitors and their compositions, their practical application has dictated their use in industry based on performance, environmental, and cost requirements. An important aspect of the application of inhibitors in the industrial cooling systems is to provide control of corrosion, scale formation, and content of inhibitor in the system. Control provides reliable system operation and sound and costeffective application of inhibitors. There are several major methods of inhibitor control . One of the simplest ones is the “after-the-fact” method, which is based on direct analytical determination of the main component of the inhibitor. It can be done either in the laboratory or at a sampling site using portable equipment. A similar method is the control by means of “tracers.” This method measures a tracer that is an integral component of the inhibitor formulation. This tracer may be a colored dye, but it is more commonly an inorganic material, present at low level and is selected because of ease and accuracy of analysis and also because it is not normally found to any signiﬁcant level in natural waters. Tracers usually include phosphates, lithium, and molybdenum. Automatic analyzers, sampling periodically the system, provide semicontinuous monitoring. Analysis is usually carried out by conventional chemical or colorimetric methods. 3D TRASAR, proposed by NALCO, provides continuous real-time monitoring and automatic inhibitor dosing. It also allows one to carry out a continuous record of chemical parameters and data management. TRASAR works by incorporating a ﬂuorescent tracer in various inhibitory compounds.
Corrosion Inhibitors in Cooling Water Systems Chapter | 4
“ACUMER & OPTIDOSE Water Treatment Series” (DOW Chemical Company) also uses technology that determines concentration of labeled polymers.
4.5 CONCLUSION It can be concluded that the water treatment of industrial cooling systems is a multifaceted task, which includes not only the provision of reagents but also control over their content, condition, and system parameters. Accordingly, the development of inhibitors should take into account a large number of technological, economic, and environmental aspects. Nevertheless, the use of inhibitors in the present time is one of the most reliable and widely used methods for corrosion, scaling, and microbiological control in circulation water systems.
NOMENCLATURE AMP Amino-tris-(methylenephosphonic) acid AT 1,2,3-Triazole BTA 1,2,3-Benzotriazole 2-CEPA Carboxyethyl phosphonic acid CMPA Carboxymethyl phosphonic acid DDS Sodium dodecyl sulfate DFT Sodium N-(3-diﬂuorormethylthiophenyl)anthranilate Ecor Corrosion potential EDTA Ethylenediaminetetraacetic acid EDTP Ethylenediamine-N,N,N0 ,N0 -tetra(methylenephosphonic acid) FFN Sodium ﬂufenaminate DGcor Corrosion activation energy HEDP 1-Hydroxyethane-1,1-diphosphonic acid HMDTP Hexamethylenediamine-N,N,N0 ,N0 -tetra(methylenephosphonic acid) HMP Sodium hexametaphosphate HPA Hydroxyphosphonoacetic acid KS Stability constant LS Lignosulfonates MBT 2-Mercaptobenzothiazole MEFN Sodium mefenaminate NB Sodium m-nitrobenzoate NTA Nitrilotriacetic acid OS Sodium N-oleylsarcosinate PAAmM Polyacrylamide PAN Sodium phenylanthranilate PBTC Phosphonobutane-1,2,4-tricarboxylic acid PG Polyguanidines PHMG Polyhexamethyleneguanidine PMA Phosphomolybdic acid PMAAN Polymethacrylic acid PPA Propylphosphonic acid PVP Polyvinylpyrrolidone SB Sodium benzoate SIL Saturation index SIR Stability index SLS Sodium N-lauroyl sarcosinate
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