Neutral solutions, inhibitors

Anodic or cathodic inhibitors This classification is based on whether the inhibitor causes increased polarisation of the anodic reaction (metal dissolution) or of the cathodic reaction, i.e. oxygen reduction (near-neutral solutions) or hydrogen discharge (acid solutions). 

In modern practice, inhibitors are rarely used in the form of single compounds — particularly in near-neutral solutions. It is much more usual for formulations made up from two, three or more inhibitors to be employed. Three factors are responsible for this approach. Firstly, because individual inhibitors are effective with only a limited number of metals the protection of multi-metal systems requires the presence of more than one inhibitor. (Toxicity and pollution considerations frequently prevent the use of chromates as universal inhibitors.) Secondly, because of the separate advantages possessed by inhibitors of the anodic and cathodic types it is sometimes of benefit to use a formulation composed of examples from each type. This procedure often results in improved protection above that given by either type alone and makes it possible to use lower inhibitor concentrations. The third factor relates to the use of halide ions to improve the action of organic inhibitors in acid solutions. The halides are not, strictly speaking, acting as inhibitors in this sense, and their function is to assist in the adsorption of the inhibitor on to the metal surface. The second and third of these methods are often referred to as synergised treatments. 

Although halide ions are aggressive in near-neutral solutions they can be used to improve the action of inhibitors in acid corrosion (see Practice Acid Solutions). Variations exist among the halides, e.g. chloride ions favour the stress-corrosion cracking of Ti in methanol whereas iodide ions have an inhibitive action ... 

The mechanisms of corrosion inhibition will be described separately for acid and neutral solutions, since there are considerable differences in mechanisms between these two media. Definitions and classifications of inhibitors are given in Section 17.2 and by Fischer. ... 

Another class of inhibitors in near-neutral solutions act by stabilising oxide films on metals to form thin protective passivating films. Such inhibitors are the anions of weak acids, some of the most important in practice being chromate, nitrite, benzoate, silicate, phosphate and borate. Passivating... 

Environment Increase redox potential of solution Addition of anodic inhibitors Passivation of stainless steel by additions of O2, HNO3 or other oxidising species to a reducing acid Additions of chromates, nitrates, benzoates, etc. to neutral solutions in contact with Fe inhibitive primers for metals, e.g. red lead, zinc chromate, zinc phosphate... 

The polarization characteristic of a corroding metal can be controlled by various additives to the solution, called corrosion inhibitors, which adsorb on the metal and lower the rates of the cathodic and/or anodic reaction. Inhibitors are used primarily for acidic electrolyte solutions, sometimes also for neutral solutions. Various organic compounds with -OH, -SH, -NHj, -COOH, and so on, as the functional groups are used as inhibitors. The effects of an organic inhibitor, tetradecylpiperidinium... 

A group of peptide derivatives such as peptide arginals and boronic acid peptide derivatives belong to another class of reversible thrombin inhibitors. One such inhibitor is PPACK (D-Phe-Pro-Arg chloromethyl ketone), which functions as a powerful irreversible thrombin inhibitor by alkylating the histidine residue at the catalytic site of thrombin (58). It, however, is unstable in neutral solution, as it undergoes cyclization and inactivation. However, the D-methyl derivative of D-Phe-Pro-Arg-H (D-Mephe-Pro-Arg-H) called efegatran, with a molecular mass of 515 Da, is a stable selective reversible inhibitor of thrombin with a K. of approximately 100 nM. The basic amino terminus in this compound is responsible for promoting the specificity toward thrombin (63). 

Both neutralizers are injected in the fractionator overhead line in order to be present when the dew point of hydrochloric acid in solution is reached. It is important to use a quill to inject neutralizers or inhibitors because drip injection can cause dissolution of the protective scale on the inside of the pipe, which can result in corrosion and erosion in that area. Often, however, neutralization is not accomplished, and severe corrosion from hydrochloric acid still occurs at the dew point. The pH is controlled at the overhead receiver water draw because dew point pH measurement is not feasible. One method of controlling the dew point pH is to recycle water from the drum to the overhead line. This water buffers the condensate at the hydrochloric acid dew point and also provides water in which the ammonia can dissolve. 

The adsorption of inhibitor ions at kinks does not appear to be simple physical adsorption, but a chemisorption process, because the shapes of etch pits are very sensitive to the adsorbed specie. Thus Fe+ adsorbs strongly on kinks in < 001 > steps in a neutral solution, but in an acid solution it prefers kinks on < 110 > steps (Fig. 7). 

The study of the effect of pH on the reaction was not too satisfactory. As discussed previously (page 151), the oxidation is inhibited by acids, especially by hydrochloric and hydrobromic acids. It was concluded that the hydrobromic acid formed in the reaction, or added as an inhibitor, repressed the formation of the negative D-glucose ion, CaHuOd", so that the main oxidation was that of the cation CaHisOa after the initially neutral solution became acid. The increased hydrogen ion concentration drove reaction 27 to the left The cation was considered to be... 

It is worthwhile to mention that both the passivating film and the precipitate film, which are formed in the presence of foreign ions and molecules, usually inhibit not only the anodic metal dissolution but also the cathodic reaction of corroding metals. There are however some inhibitors, which are effective only to one of the anodic and the cathodic reactions of metallic corrosion. In the case of porous precipitate films loosely attached to the metal surface, the anodic metal dissolution may be accelerated at porous sites of the precipitate films. For instance, Zn2+ ions, Al3+ ions, Co2+ ions, and Ce3+ ions, which are hard or slightly hard Lewis acid, combine with hydroxide ions of hard base forming a porous precipitate film of metal hydroxide on metallic iron in neutral solution. The porous precipitate film thus formed effectively inhibits the cathodic oxygen reduction, but it may accelerate the anodic dissolution of metallic iron at the porous sites of precipitates [86],... 

It is shown in the next chapter that nitrites can be used as passivating inhibitors for corrosion of iron in near-neutral solutions. Since the basis for accomplishing this is related to the polarization characteristics of the reduction of the nitrite ion, brief consideration is given here to the reaction and to the form of the experimentally determined polarization curve forthis ion. The curve is shown in Fig. 3.21. Although several reactions have been proposed for the reduction of this nitrite ion, the following is considered here ... 

The presence of trehalase in the tubercle bacillus was discovered by Bloch and SuUman in 1945. Optimal hydrolysis of a,a-trehalose by the enzyme occurred in acid to neutral solutions. Since a,a-trehalose is not utilized by the bacteria so rapidly as is n-glucose, it was inferred that the bacillus only metabolizes the a, -trehalose after the trehalase has converted it into D-glucose. Trehalosamine (see p. 220) has an antimycobac-terial effect. This effect is antagonized by a,a-trehalose. One mole of trehalosamine as inhibitor removes 0.337 mole of Q ,a-trehalose from the surface of the enzyme. Total inhibition of mycobacterial trehalase is not readily produced. Cord factor was also isolated from wax D of a BCG strain of M. tuberculosis in 1959 by Nojima. A list of fatty acid esters of a, a-trehalose is given in Table I. 

Rubradirin (70) was isolated from the fermentation broth of Streptomyces achromogenes var. rubradiris [109-114]. Rubradirin (70) possesses pH indicator properties. Namely, it is red in acidic or neutral solutions and green in basic solutions, or in salt form. It is an extremely potent inhibitor of Gram-positive bacteria in vitro, and is effective in the treatment of experimental bacterial infections in mice [114]. The mechanism of the... 

Very often the long experience with chemicals operating as corrosion inhibitors, e. g. in the oil field, gas or petroleum industry, is taken as an example for the successful use of corrosion inhibitors for many decades. This undoubtedly is true and the overwhelming majority of literature on corrosion inhibitors deals with the effects of inhibitors on uniform corrosion, e. g. of steel in acidic or neutral solutions, where they can be classified into [2] a) adsorption inhibitors, acting specifically on the anodic or on the cathodic partial reaction of the corrosion process or on both reactions (mixed inhibitor), b) JUm-forming inhibitors, blocking the surface more or less completely, and c) passivators, favouring the passivation reaction of the steel (e. g. hydroxyl ions). 

Apart from a few exceptions, notably chromate, the protection ability of inhibitors depends strongly on the nature and composition of the metal or alloy and even small differences may cause substantial changes in the inhibition efficiency. A well-known example is sodium benzoate, which in neutral solution is an anodic inhibitor for mild steel but not for cast iron, although the anodic corrosion reaction is the same for both materials [5]. Similarly, the state of the surface, in particular the presence and nature of passive films and... 

Cathodic inhibitors in near-neutral solutions interfere with the oxygen reduction reaction by restricting the diffusion of dissolved oxygen to the electrode surface. These substances usually form thick surface layers with poor electronic conductivity (the latter is an important prerequisite to avoid oxygen reduction on the film surface). Examples are phosphates, polyphosphates, silicates, borates, and inorganic inhibitors, such as Zn +, which precipitate as Zn hydroxide, and Ca ", which forms calcium carbonate films in the presence of CO3 [3]. These inhibitors... 

A third classification takes into consideration the preferred environment in which the inhibitor can be optimally used, that is, an inhibitor that performs in an acid solution is an acidic inhibitor, whereas there are some for applications in neutral solutions and others yet for use in alkaline... 

Various types of inhibitors are employed in varied environments while some are good in acid solutions, they may accelerate corrosion in neutral solutions. Some environments are described in the foregoing sections. 

Neutral solutions Where ferrous materials are employed, benzoates are preferred. Benzoates are also suitable and tolerated where zinc and aluminum alloys are the materials employed. Tanins, which have variable compositions, being a natural product, are suitable for aluminum materials as well as ferrous materials. Amines, quaternary ammonium compounds, are suitable inhibitors for ferrous materials in neutral solutions as well. However, for [R4N]+X, a hypothetical quaternary amine, where R is between C12 and Cjg, inhibitor efficiency is much better if the halide, X, is iodine, I. For copper-based alloys in neutral solutions, the preferred inhibitor species are benzotriazole and mercaptobenzotriazole. 

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