Inhibition iron corrosion

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

Dissimilar metals in the same system Because of the specific action of many inhibitors towards particular metals, problems arise in systems containing more than one metal. In the majority of cases these problems can be overcome by the choice of a formulation incorporating inhibitors for the protection of each of the metals involved. With this procedure it is necessary not only to maintain an adequate concentration of each of the inhibitors but also to ensure that they are present in the correct proportion. This is because of two effects firstly, failure to inhibit the corrosion of one metal may intensify the attack on the other metal the best example of this is with aluminium and copper in the same system, and failure to inhibit copper corrosion — usually achieved with sodium mercaptobenzothiazole or benzotriazole—can lead to increased corrosion of the aluminium as a result of deposition of copper from copper ions in solution on to the aluminium surface. Secondly, an inhibitor of the corrosion of one metal may actually intensify the corrosion of another metal. Thus, benzoate is usually used to prevent the corrosion of soldered joints by nitrite inhibitor added to protect cast iron in the same system. A benzoate nitrite ratio of greater than 7 1 is necessary in these cases. 

Concentration Inhibition of iron corrosion in distilled water occurs only... 

Nature of the metal surface The critical concentration of an anion required to inhibit the corrosion of iron may increase with increasing surface roughness. Thus, Brasher and Mercer" showed that the minimum concentration of benzoate required to protect a grit-blasted steel surface was about 100 times greater than that required to protect an abraded surface. However, surface preparation had little effect on the critical inhibitive concentrations for chromate" or nitrite " The time of exposure of the iron surface to air after preparation and before immersion may also affect the ease of inhibition by anions. There is evidence """ that the inhibition by anions occurs more readily as the time of pre-exposure to air increases. Similarly, if an iron specimen is immersed for some time in a protective solution of an inhibitive anion, it may then be transferred without loss of inhibition to a solution of the anion containing much less than the critical inhibitive concentration . ... 

W. W. Frenier and F. B. Growcock. Process and composition for inhibiting iron and steel corrosion. Patent EP 289665, 1988. 

Corrosion is a mixed-electrode process in which parts of the surface act as cathodes, reducing oxygen to water, and other parts act as anodes, with metal dissolution the main reaction. As is well known, iron and ferrous alloys do not dissolve readily even though thermodynamically they would be expected to, The reason is that in the range of mixed potentials normally encountered, iron in neutral or slightly acidic or basic solutions passivates, that is it forms a layer of oxide or oxyhydroxide that inhibits further corrosion. 

The reactivity of the iron with the contaminants determines the feasibility and the design of the site. Many factors contribute to the reaction rate. For example, the oxide layer that forms on the iron affects the surface of the iron and inhibits further corrosion, so this layer needs to be minimized. Fortunately, iron possesses a "porous and incoherent" nature with oxide film, and iron usually exhibits satisfactory degradation rates over a long period of time (Tratnyek, 1996). 

Ferrous corrosion products do not have a biocidal effect however, iron corrosion tends to form a much more extensive and encapsulating matrix. This corrosion matrix, which is principally formed of iron oxides, hydroxides, and soil minerals, will encapsulate any organic materials. This usually takes the form of a negative cast (Keepax 1975), provided that the primary layer of corrosion product has been laid down prior to any extensive degradation of the organic material, then fine surface detail will be preserved in the corrosion cast. Because the iron corrosion will not inhibit degradation... 

Corrosion and Ionisation.—Iron will remain untarnished for indefinite periods in the presence of concentrated solutions of the carbonates of the alkali metals, even in the presence of small quantities of other salts. If, however, the alkali carbonate is very dilute, it cannot entirely inhibit corrosion. Now, the minimum quantities of alkali carbonate required to inhibit the corrosive actions of a given concentration of various other salts of the same alkali metal have been determined.1 The results show that, if the added salts are arranged in order according to the amount of alkali carbonate required to inhibit corrosion, they are also not merely in the order of the relative strengths of their acid radicles, but the relative quantities of carbonate bear a general relationship to the numerical values found for the strengths of the acids by electrical conductivity methods. This is well illustrated in the following table —... 

The effectiveness of this anticorrosive is attributed to the ability of phosphite ions to inhibit anodic corrosion reactions by formation of iron phosphites and phosphates. 

Implantations of rare gases did produce slightly higher corrosion rates than unimplanted iron. It was noteworthy that the reproducibility of the measurements of implanted samples was worse than of unimplanted ones. Oxide layers did not play the important role in this study which they did in the work of Ashworth et al. because in H2SO4 solution immediate cathodic reduction of the oxide takes place. Au implantation enhanced the corrosion rate of iron by a factor of 10 lead implantation inhibited the corrosion rate by the same factor. Copper, finally, did not have much influence, a behavior that is similar to conventional iron-copper alloys with a copper content of less than 10 %. These findings proved that ion implantation can vary the corrosion resistance of iron not only in neutral but also in acid solutions. 

It has also been found in literature [84] that iodide ions, I- bromide ions, Br and azide ions, N3, inhibit the corrosion of metallic iron in acid solution. On the other hand, the presence of hydrogen sulfide ions, HS- thiosulfate ions, S2O2 and thiocyanate ions, SCN, accelerate the corrosion of metallic iron in acidic solution [84]. 

It is interesting to see that bismuth ions, Bi3+ or BiO+, inhibit the corrosion of metallic iron, zinc, and cobalt in perchloric acid solution. The bismuth ions are reduced in the cathodic reaction of metal corrosion forming metallic precipitates of bismuth on the corroding metal surface. 

Corrosion control by pigments relies on well-known principles of corrosion inhibition. Iron and steel exposed to air are quickly covered by an oxide film aqueous electrolytes tend to break down this film, and further oxidation of the metal surface ensues. The role of anodic corrosion inhibitors is to supplement or to aid in the repair of the surface oxide film. Basic pigments may form soaps, for example, with linseed oil autoxidation of these soaps may yield soluble inhibitors in the film. Some other pigments of limited solubility act directly as inhibitors. Active metal pigments supply electrons to the iron substrate and thus lower its potential and prevent metal dissolution. 

In this lecture I will deal with the mechanisms involved in first, the dissolution or corrosion of iron and second, in the inhibition of corrosion by the formation of various types of oxide films. A few definitions will help to focus attention on the specific nature of my subject. 

Similar calculations have been carried out for the inhibition of iron corrosion in hydrochloric acid by Victoria blue which, according to Elze and Fischer (m) does not affect the Tafel slopes, but reduces both the anodic and cathodic partial reactions to a different degree. In this particular case 0q and 02 have to be adjusted by way of a trial and error calculation. 

The study of such transfer processes was one of the purposes of a fairly extensive investigation of the inhibition of corrosion of iron in hydrochloric acid by acetylenic corrosion inhibitors. The parameters of this investigation were the concentration of the acid, the concentration of the inhibitor, the flow rate and the oxygen concentration in the corrosive medium. While most of the experimental data were obtained by means of the so called resistance probe, polarization measurements were carried out in order to elucidate some of the more peculiar results. The experimental arrangement is more fully explained in (2H). 

Figure 10. Inhibition of iron corrosion by 2-butyne-l,4-diol in 6N HCl aerated...

The proof of the importance of iron sulfide in the interaction between the corrosion inhibitor and the metal surface was brought in 1970 when it was shown (3U) that corrosion inhibition was considerably enhanced and prolonged when the corrosion inhibitor was adsorbed on a presulfided specimen rather than on the nonsulfided surface. These measurements were made electrochemically and gave support to the practical well-known fact that periodic "filming" could inhibit the corrosion for relatively long periods of tdme. 

One apparent contradiction that has not been resolved is that some microorganisms in biofilms can cause locahzed corrosion and others inhibit generalized corrosion. To further complicate matters, the same organisms and mechanisms to which MIC has been attributed can also reportedly inhibit corrosion. For example, strains of Pseudomonas and Serratia are reported to increase the corrosion rate of iron and nickel compared to sterile conditions [103], but can have a protective effect on some metals under certain circumstances [104, 105]. Videla and Guiamet [106] found a protective action of S. marcescens on aluminum. Metal-binding by extracellular polymers has been reported as a mechanism for both MIC [107] and for corrosion inhibition [86]. 

N. Ohno, N. Hiroshi, K. Aramaki, Electrochemical and spectroscopic smdies on inhibition mechanism ofbenzyl thiocyanate for iron corrosion in 1 N perchloric and hydrochloric acid solutions, Corros. Sci. 36 (1994) 583-591. 

J.B. Lumsden, Z. Szklarska-Smiralowska, The properties of films formed on iron exposed to inhibitive solutions. Corrosion 34 (1978) 169-176. 

Holness, RJ, Williams, G, Worsley, DA, McMurray, HN. 2005. Polyaniline Inhibition of Corrosion-Driven Organic Cathodic Delamination of Iron. J. Electrochem. Soc., 152, B73. 

Phosphatizing. The surface of iron can be treated in such a manner that a thin layer is produced that does not fully protect the iron but inhibits the corrosion process and at the same time promotes adhesion of follow-up films. This is now a large-scale process in the car industry. 

Holness, R.J., et al. 2005. Polyaniline inhibition of corrosion-driven organic coating cathodic delamination on iron. / Electrochem Soc 152 (2) B73. 

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