Mechanism of Corrosion Inhibitors

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). 

Commercial inhibitors are frequently blends of several compounds, thus the mechanistic action can be multiple and difficult to identify. This will lead to major difficulties in the independent evaluation of corrosion inhibitors that are proposed in commercially available concrete repair systems. 

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The consensus is that organic compounds inhibit corrosion by adsorbing at the metal/solu-tion interface. Three possible types of adsorption are associated with organic inhibitors n-bond orbital adsorption, electrostatic adsorption, and chemisorptions. 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. During the past decade, the primary improvements in inhibitor technology have been the refinement of formulations and the development of improved methods of applying inhibitors (Totlani and Athavale 2000 Farquhar et al. 1994). 

The present work confirmed the layer by layer assembly of ZMP nanocontainers and the release mechanism of corrosion inhibitor. FTIR and TEM study confirms the successful formation of ZMP nanocontainer as a layer by layer system with the aid of ultrasonic irradiation. Zeta potential and particle size analysis also shows the formation of layers and shows appropriate change in the surface charge, which could be responsible for the release mechanism initiated by the change in pH. Release study and corrosion results from Tafel plot and corrosion rate analysis showed significant improvement in the anticorrosion properties of coatings due to the optimum loading of the ZMP nanocontainers. 

Another more informative solution test is to perform cyclic polarization in the saturated calcium hydroxide solutions mentioned above. This quickly demonstrates whether the corrosion mechanism is one of pitting and determines the primary mechanisms of corrosion inhibitors [7-9]. A modified version of ASTM G 61, Test Method for Conducting... 

From the point of view of the mechanism of corrosion inhibitors it should first be established whether uniform corrosion (e.g. iron in acidic or neutral media) or pitting corrosion prevails. Inhibitors for general corrosion can be classified into (Trabanelli, 1986 Nurnberger, 1996) ... 

J. Sinko Considerations on the Chemistry and Action Mechanism of Corrosion Inhibitor Pigments in Organic Coatings in 6th Biennial Conf. Organic Coatings, New Paltz, NY, Institute of Material Science, 2000. 

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

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. ... 

THE MECHANISM OF CORROSION PREVENTION BY INHIBITORS Effects of Inhibitors on Corrosion Processes... 

Recent developments in the mechanisms of corrosion inhibition have been discussed in reviews dealing with acid solutions " and neutral solu-tions - . Novel and improved experimental techniques, e.g. surface enhanced Raman spectroscopy , infrared spectroscopy. Auger electron spectroscopyX-ray photoelectron spectroscopyand a.c. impedance analysis have been used to study the adsorption, interaction and reaction of inhibitors at metal surfaces. 

JJ..L . Co itioniagaheAimosphere4o Redue AS j 5 .t Qtrssionjafaifaitkm >Erinciples nd. jactiee -, n.3 The Mechanism of Corrosion Prevention by Inhibitors 17.4 Boiler and Feed-water Treatment... 

Although glycol-water formulations are not prone to foaming, mechanical and chemical factors may cause foaming in the system. The use of corrosion inhibitors and the presence of contaminants may enhance the tendency to form foams. For these reasons, antifoaming agents, such as silicones, polyglycols, or oils, are sometimes added. 

T. P. Hoar, who was co-discoverer with U. R. Evans (in 1936) of the basic facts about the electrochemical mechanism of corrosion that led to Wagner and Traud s seminal theory, told about an episode from his early days as a corrosion consultant. Approached by an automotive concern for an inhibitor to stop the distressing breakdowns of its 1930s car radiators, he busied himself in his lab over a weekend and created (stumbled upon ) a potent organic inhibitor for the system concerned. The client wanted to pay a handsome fee, but Hoar wisely tempered his enthusiasm and humbly asked for just a few cents for every time the inhibitor was used. His decision, he says, provided him with a significant income for more than a decade. 

Recently, the mechanism of corrosion protection by soluble chromate inhibitors has been the subject of active research, which has attempted to understand and replicate its inhibiting functions with less toxic chemical substances. In this section, some recent findings on chromate corrosion inhibition are reviewed, particularly as they pertain to corrosion of light metals, with a focus on the use of techniques that are useful for studying mechanisms of inhibition. 

The use of corrosion inhibitors in appropriate concentrations could improve the corrosion resistance through different mechanisms of adsorption, barrier film, etc. 

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