Corrosion inhibitor

A corrosion inhibitor is a chemical compound that is added to the environment in order to reduce the rate of metal corrosion. The inhibition efficiency, Ri, indicates to what extent the corrosion rate is slowed down by the presence of the inhibitor  

Vq and v designate the rates of corrosion in the absence and in the presence of the inhibitor, respectively. The inhibition efficiency depends on a number of parameters, an important one being the inhibitor concentration. 

In practice, inhibitors are often defined according to their field of application. In aqueous environments, inhibitors for acid environments are typically used to minimize metal corrosion during pickling of steel, an operation that removes oxide scales by dissolution in an acid. In the petroleum industry, large quantities of inhibitors for acid environments are used to avoid corrosion of drilling equipment. Inhibitors for neutral environments are used above all for the protection of cooling-water circuits. Inhibitors not only reduce the rate of uniform corrosion, but they also serve to protect metals from localized corrosion and stress corrosion cracking [18]. 

Corrosion inhibitors are used also in organic fluids such as lubricating oils and gasoline. These liquids often contain traces of water and ions that can cause corrosion. Corrosion inhibitors for paints, include both inorganic pigments and organic molecules such as for example derivatives of tannic acid (Section 12.3). 

The presence of a corrosion inhibitor reduces the rate of corrosion by affecting the rate the partial electrochemical reactions involved. Depending on which partial reaction is mostly slowed down one distinguishes three types of inhibitors  

Corrosion inhibitors for steel in concrete are of great interest to the concrete repair community. In this section we will discuss what they are, how they work and what work is going on to assess their effectiveness. 

All of these inhibitors have been widely used by corrosion engineers for many years to protect steel and other metals such as electronic components. In some cases their chemistry is well understood, in other cases less so. 

1 Corrosion inhibitors for admixing into fresh concrete 

Specifically for steel in concrete there are several types of inhibitors. There are those that are admixed into new concrete and those that are applied to hardened concrete as part of a rehabilitation programme of a corrosion 

Vapour phase inhibitors or migrating corrosion inhibitors have been used to impregnate packaging, greases and waxes for many years to protect steel machinery and components, particularly before use. An American company 

By definition, a corrosion inhibitor is a chemical substance that, when added in small concentration to an environment, effectively decreases the corrosion rate. The efficiency of an inhibitor can be expressed by a measure of this improvement as indicated in Eq. (8.24)  

In general, the efficiency of an inhibitor increases with an increase in inhibitor concentration (e.g., a typically good inhibitor would give 95 percent inhibition at a concentration of0.008 percent and 90 percent at a concentration of 0.004 jjercent). There is often some positive synergism between different inhibitors and therefore mixtures are commonly chosen in commercial formulations. The scientific and technical corrosion literature has descriptions and lists of numerous chemical compounds that exhibit inhibitive properties. Of these, only very few are actually used in practice. This is partly because the desirable properties of an inhibitor usually extend beyond those simply related to metal protection. Considerations of cost, toxicity, availability, and environmental friendliness are of considerable importance. 

The use of chemical inhibitors to decrease the rate of corrosion processes is quite varied. In the oil extraction and processing industries, for example, corrosion inhibitors have always been considered to be the first line of defense against corrosion. Table 8.14 presents some inhibitors that have been used with success in typical water environments to protect the metallic elements of industrial systems. 

Commercial formulations generally consist of one or more inhibitor compounds with other additives such as surfactants, film enhancers, de-emulsifiers, oxygen scavengers, and so forth. The inhibitor solvent package used can be critical in respect to the solubility/dispersibility characteristics and hence the application and performance of the products. 

Corrosion inhibition in recirculated cooling water systems historically depended on the oxidizing inhibitors such as chromates and nitrites. These days are gone since nitrites have been found to be highly conducive to organic growths and chromates have been phased out for environmental toxicity reasons. 

The history of corrosion inhibitors and neutralizers and their invention, development, and application in the petroleum industry is documented by a review of Fisher [605]. Early corrosion inhibitor applications in each of the various segments of the industry, including oil wells, natural gas plants, refineries, and product pipelines, are reviewed. 

Corrosion and scale deposition are the two most costly problems in oil industries. Corrodible surfaces are found throughout production, transport, and refining equipment. The Corrosion and Scale Handbook gives an overview of corrosion problems and methods of corrosion prevention [159]. 

Corrosion inhibitors, which are used for the protection of oil pipelines, are often complex mixtures. The majority used in oil production systems are nitrogenous and have been classified into the following broad groupings  

Cl are chemical compounds or their blends able to retard metal corrosion even in small additions in aggressive media. Cl change the kinetics of electrochemical reactions that bring about corrosion so that the corrosion process rate retards significantly. 

Commercial Cl include more than three thousand brands differing in their purpose, physical-chemical properties and mechanisms [12,46-48]. 

By their designation Cl are subdivided into five classes inhibitors in solutions of acids, alkalis, water and neutral water media, in non-aqueous liquid media and the inhibitors of atmospheric corrosion. 

By their mechanism of influencing the velocity of electrochemical reactions Cl are divided into three groups inhibitors hampering only anodic (anodic Cl) or only cathodic (cathodic Cl) reactions and mixed Cl that abate both reactions. 

By their physical-chemical properties and protection principle there are volatile, contact and creep inhibitors of atmospheric corrosion. Contact Cl are the substances whose protecting effect is exhibited only on direct application on the metal surface. Many of them migrate via diffusion and undergo creeping. Volatile Cl with relatively high volatility can easily transfer into vapor and be adsorbed onto metal surfaces to protect them from corrosion. 

A corrosion inhibitor is a chemical substance that, upon addition to a corrosive environment, results in reduction of corrosion rate to an acceptable level. Corrosion inhibitors are generally used in small concentrations. The principles and applications of corrosion inhibitors are discussed at great length in a recent monograph.47 A corrosion inhibitor should not only mitigate the corrosion, but also be compatible with the environment in the sense that it should not cause any complications. Usually the corrosion inhibitor is rated in terms of inhibition efficiency I and is given by the relationship. 

A corrosion inhibitor can function in two ways. In some situations the added inhibitors can alter the corrosive environment into a noncorrosive or less corrosive environment through its interaction with the corrosive species. In other cases the corrosion inhibitor interacts with the metal surface and as a consequence inhibits the corrosion of the metal. Thus, based on the mode of interaction, there are two broad classes of inhibitors. 

In the case of inhibitors which adsorb on the metal surface and inhibit the corrosion there are two steps, namely (i) transport of inhibitor to the metal surface and (ii) metal -inhibitor interactions. The process is analogous to drug molecule transport transported in the body to the required site and its interaction with the site to provide relief from the ailments. The most important step involves the interaction of the metal with the inhibitor molecule. These are chemical interactions and will be dealt with later. 

Depending upon whether the cathodic reaction or the anodic reaction is suppressed by the added inhibitor, inhibitors have been further classified as follows  

Cathodic inhibitors Anodic inhibitors Mixed type inhibitors 

Protecting metallic substrates against corrosion is one of the prime functions of most coatings. Corrosion resistant coatings are typically formulated using anticorrosive pigments and suitable binders with the optimum formulation parameters. In addition, organic compounds known as corrosion inhibitors are often added at low levels, and can effectively reduce the corrosion rate of a metal or slow down the individual corrosion reactions. 

Various organic compounds are useful as corrosion inhibitors, such as acetylene derivatives, heterocyclic compounds containing nitrogen, sulfur and/or oxygen, long-chain aldehydes and ketones, amines, carboxylic acids and their derivatives, thiourea derivatives and thiophosphates. 

Based on their function, the corrosion inhibitors used in the coating industry are classified into two categories  

PAS resins tend to form corrosive gases, such as sulfur dioxide when heated to an elevated temperature, and hence involve problems that metallic portions of processing machines, molds, etc., are corroded upon their molding and processing. The usual processing machine is made of an iron-based material and hence tends to suffer from chemical corrosion when coming into contact with a PAS resin melted upon its molding. On the other hand, the molded products also become liable to color. 

The corrosion of a mold causes a great economical loss because the mold is expensive. Moreover, it is then difficult to precisely mold. In order to solve corrosive problems involved in the PAS resins, it has been proposed to blend various kinds of corrosion inhibitor. 

However, certain corrosion inhibitors, such as y-alumina, calcium carbonate, zinc oxide, sodium oxalate, etc., show insufficient corrosion inhibition. For some corrosion inhibitors, the mechanical strength of the PAS resin is deteriorated. More satisfactory corrosion inhibitors are nickel compounds, such as nickel carbonate, nickel hydroxide, and nickel cit-rate. The anticorrosive effect is already satisfactory when the corrosion inhibitor is added in amounts of 0.1 %. 

A corrosion inhibitor in general terms is a chemical substance that when added in a small amount to an environment effectively reduces the corrosion rate of a metal or alloy exposed to the corrosive environment. A more precise definition of an inhibitor is not possible because of the number of mechanistic and/or chemical considerations when classifying corrosion inhibitors. 

Corrosion inhibition is used internally with carbon steel pipes and vessels as an economic corrosion control alternative to stainless steels and alloys, coatings, or non-metallic composites. A particular advantage of corrosion inhibition is that it can be implemented or changed in situ without disruption of the process. For example, in processes that produce environments of increasing corrosivity with time, such as souring oil fields, corrosion can be effectively controlled with a suitable inhibitor. 

The major industries that use corrosion inhibitors are petroleum production and refining, chemical and heavy industrial manufacturing, and the product additive industry. The usage summary of corrosion inhibitors in various industries is given in Table 4.9. 

The data in Table 4.9 show that the largest consumption of corrosion inhibitors is in the oil industry with the single highest amount in the petroleum refining industry. 

TABLE 4.9 Consumption of Corrosion Inhibitors in the United States in 1998 

During the past 30 years, the primary improvements in inhibitor technology have been the refinement of formulations and the development of improved methods of applying inhibitors. The methods of evaluating the performance during their use have also advanced considerably. The best corrosion protection measures should be implemented on the basis of technical and economical aspects when establishing new constructions. 

The consensus is that organic compounds inhibit corrosion by adsorbing at the metal/solution interface. Three possible types of adsorption are associated with organic inhibitors r-bond orbital adsorption, electrostatic adsorption, and chemisorption. A more simplistic view of the mechanism of corrosion inhibitors can be described as controlled precipitation of the inhibitor from its environment (water and hydrocarbons) onto metal surfaces. 

In general, reinforced concrete has proved to be successful in terms of both structural performance and durabiHty if the design mles (Chapter 11) and concrete technology (Chapter 12) were adequately considered. Additional protection measures will only be necessary in very aggressive environments or when a very long service life is required. As outlined in Section 11.8, corrosion inhibitors, thus chemical compounds added as admixtures to the fresh concrete, are one possible way to improve the durabiHty of concrete structures. 

The use of corrosion inhibitors is of increasing interest as they are claimed to be useful in reinforced concrete not only as preventative measure for new structures (as addition to the mixing water) but also as surface-applied inhibitors for preventative and restorative purpose. Apphcation on the concrete surface could be a promising technique to protect existing structures from corrosion or increase the lifetime of structures that already show corrosion attack. 

The application of inhibitors on the concrete surface requires the transport of the substance to the rebar where it has to reach a sufficiently high concentration to protect the steel against corrosion or reduce the rate of the ongoing corrosion. In this context only corrosion inhibitors that prolong the service life due to chemical or electrochemical interaction with the reinforcement are considered. Any other substances that may prevent the onset of corrosion or reduce ongoing corrosion by other means, such as surface treatment (e. g. hydrophobation) or additions that reduce the porosity of the concrete (e. g. fly ash, silica fume, waterproofing admixtures, etc.), are not considered to be corrosion inhibitors and are treated in other chapters. 

In this chapter the types of inhibitors presently available and their effectiveness in reinforced-concrete stractures are outlined. For more detailed information the reader is referred to the state of the art report published recently by one of the authors [1]. 

Luca Bertolini, Bernhard Elsener, Pietro Pedeferri, Rob P. Polder Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30800-8 

It should be remembered that, even with ordinary steels, high oxygen concentrations are locally protective, so that deoxygenation is not recommended if the metal to be protected is already cathodic. With stainless steel equipment, the Eh of the system must not be taken too low, otherwise the metal may become active (Section 16.6). 

Corrosion inhibitors are solutes that blanket the electrochemically active surfaces of the corrosion-prone metal and suppress corrosion either by physically blocking the flow of ions or molecules to or from these surfaces or by altering the electrical double layer at the metal surface in such a 

Phosphates, molybdates, and (at high pH) silicates act as anodic inhibitors much as do alkalis, except that the iron oxides/hydroxides formed on anodic sites then contain some P04, Mo04 , or Si04 ( basic iron phosphates, etc.). These inhibitors require the presence of O2 to produce basic iron(III) phosphate, molybdate, or silicate films, whereas oxidizing anions such as ehromates and nitrites oxidize Fe (aq) rapidly to insoluble iron(III) oxides on anodic sites. Dianodic inhibitors combine complementary inhibition mechanisms for example, sodium triphosphate may be used with sodium chromate, or sodium molybdate with NaN02. 

A different approach to blanketing anodic areas involves dissolving organic anions with appropriate hydrophobic (i.e., water-repellent) substituents. The anionic heads of the molecules specifically seek out a positively charged anodic spot, and the hydrophobic tails serve to isolate it from the aqueous solution and so block the ionic part of the corrosion circuit or, at least, modify the electrical double layer (Fig. 16.13). Sodium benzoate and especially sodium cinnamate are particularly effective in this regard. 

Anodic inhibitors should not, however, be used on galvanized iron, in which protection of the iron depends upon the zinc coating being anodic an anodic inhibitor would coat the zinc, reverse the polarity of the corrosion cell, and cause the iron to corrode merrily away. 

When a buffer is incorporated into an ink, a metal ion derived from the buffer may contribute to the destabilization of a colorant in the ink (29). As a result, in the case where the liquid composition is used, such a high optical density as that cannot be obtained by merely using a liquid composition having a polyvalent metal ion. 

The pH of aqueous ink compositions may be adjusted by the addition of organic or inorganic acids or bases. Useful inks may have a preferred pH of from about 2 to 10, depending upon the type 

Iminodiacetic acid Iminodipropionic acid N-methyliminodiacetic acid N,N -Ethylenediaminediacetic acid Ethylenediaminetetraacetic acid 

Ethylenediamine-N,N -diacetic acid-N,N -dipropionic acid Ethylenediaminetetrapropionic acid 

2- Propylenediaminetetraacetic acid Trimethylenediaminetetraacetic acid Nitrilotriacetic acid Dihydroxyethylglycine trflns-l,2-Cyclohexanediaminetetraacetic acid Tetramethylenediaminetetraacetic acid, Pentamethylenediaminetetraacetic acid Hexamethylenediaminetetraacetic acid Octamethylenediaminetetraacetic acid 

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Cooling water systems are dosed with corrosion inhibitors, polymers to prevent solid deposition, and biocides to prevent the growth of microorganisms. 

HOCH2C = CCH2OH. White solid, m.p. 58 C, b.p. 238- C prepared by the high pressure reaction between ethyne and methanol and also from BrMgCCMgBr and methanal. Used in electroplating (Ni), as a corrosion inhibitor, and in paint and varnish removal. 

C, obtained from ethyne and propanone. It is used as a corrosion inhibitor and as a chemical intermediate. 

Corrosion inhibitors partial esters of succinic acid, fatty acids, sulfonates, phenates, amine phosphates. 

Oxygen Scavenger Corrosion Inhibitor Figure 10.23 Injection water treatment scheme... 

Beden B 1995 On the use of in situ UV-visible and infrared spectroscopic techniques for studying corrosion products and corrosion inhibitors Mater. Sc/. Forum 192-4 277... 

Collie M J (ed) 1980 Corrosion inhibitors Deveiopments since 1980 (Park Ridge, NJ Noyes) Kuznetsov Y I 1996 Organic inhibitors of Corrosion of Metais (New York Plenum)... 

Rozenfeld I L 1981 Corrosion inhibitors (New York McGraw-Hill)... 

Technetium is a silvery-gray metal that tarnishes slowly in moist air. The common oxidation states of technetium are +7, +5, and +4. Under oxidizing conditions technetium (Vll) will exist as the pertechnetate ion, TcOr-. The chemistry of technetium is said to be similar to that of rhenium. Technetium dissolves in nitric acid, aqua regia, and cone, sulfuric acid, but is not soluble in hydrochloric acid of any strength. The element is a remarkable corrosion inhibitor for steel. The metal is an excellent superconductor at IIK and below. 

The cosolvents are any one or a mixture of ethanol, propyl, and butyl alcohols. Corrosion inhibitor is also requited. 

Bronze disease necessitates immediate action to halt the process and remove the cause. For a long time, stabilization was sought by removal of the cuprous chloride by immersing the object in a solution of sodium sesquicarbonate. This process was, however, extremely time-consuming, frequentiy unsuccesshil, and often the cause of unpleasant discolorations of the patina. Objects affected by bronze disease are mostiy treated by immersion in, or surface appHcation of, 1 H-henzotriazole [95-14-7] C H N, a corrosion inhibitor for copper. A localized treatment is the excavation of cuprous chloride from the affected area until bare metal is obtained, followed by appHcation of moist, freshly precipitated silver oxide which serves to stabilize the chloride by formation of silver chloride. Subsequent storage in very dry conditions is generally recommended to prevent recurrence. 

Ethanol, C,—C alcohols, and a corrosion inhibitor. Date waived, 1988. 

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