Adsorption of inhibitors

The very new techniques of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) have yet to establish themselves in the field of corrosion science. These techniques are capable of revealing surface structure to atomic resolution, and are totally undamaging to the surface. They can be used in principle in any environment in situ, even under polarization within an electrolyte. Their application to date has been chiefly to clean metal surfaces and surfaces carrying single monolayers of adsorbed material, rendering examination of the adsorption of inhibitors possible. They will indubitably find use in passive film analysis. 

Measurements of the adsorption of inhibitors on corroding metals are best carried out using the direct methods of radio-tracer detection and solution depletion measurements . These methods provide unambiguous information on uptake, whereas the corrosion reactions may interfere with the indirect methods of adsorption determination, such as double layer capacity measurements", coulometry", ellipsometry and reflectivity Nevertheless, double layer capacity measurements have been widely used for the determination of inhibitor adsorption on corroding metals, with apparently consistent results, though the interpretation may not be straightforward in some cases. 

Direct measurements on metals such as iron, nickel and stainless steel have shown that adsorption occurs from acid solutions of inhibitors such as iodide ions, carbon monoxide and organic compounds such as amines , thioureas , sulphoxides , sulphidesand mer-captans. These studies have shown that the efficiency of inhibition (expressed as the relative reduction in corrosion rate) can be qualitatively related to the amount of adsorbed inhibitor on the metal surface. However, no detailed quantitative correlation has yet been achieved between these parameters. There is some evidence that adsorption of inhibitor species at low surface coverage d (for complete surface coverage 0=1) may be more effective in producing inhibition than adsorption at high surface coverage. In particular, the adsorption of polyvinyl pyridine on iron in hydrochloric acid at 0 < 0 -1 monolayer has been found to produce an 80% reduction in corrosion rate . 

Interaction of the inhibitor with water molecules Due to the electrostatic and co-ordinate bond interactions described under the previous two headings, the surfaces of metals in aqueous solutions are covered with adsorbed water molecules. Adsorption of inhibitor molecules is a displacement... 

BSi, (2) blocking of reactive surfece sites on BSi through adsorption of inhibitors, such as trace metals like Al, (3) loss of reactive sites on BSi as a result of partial dissolution and reprecipitation, and (4) differential dissolution rates across a given siliceous particle due to shell geometry. 

Double layer paint provides additional protection since such coatings would be less porous than single layer paint. It Is also noted that In all specimens that are not rinsed there Is a tendency to show Inductive loops In the impedance plot. It is not clear If this Is due to the adsorption of Inhibitor on steel surface or due to the formation of oxides or due to Increased porosity (28). 

The inhibition-acceleration mechanism. Moffat et al. (37) proposed the inhibition-acceleration mechanism to explain the experimentally observed comer rounding (inversion of curvature. Fig. 19 in Ref. 37) and general shape evolution in superconformal electrodeposition of copper in vias and trenches of nanometer dimensions (37,38). These authors also smdied a three-additive system composed of two inhibitors and one accelerator. They concluded that superconformal deposition and comer rounding may be attributed to competitive adsorption of inhibitor and accelerator. This model is based on the assumption of curvature (in vias and trenches) -enhanced accelerator coverage. 

Adsorption of inhibitors can be due to either physisorption or chemisorption. Inhibitors involved in physisorbtion can be desorbed with ease while inhibitors involved in chemisorption are difficult to desorb. Adsorption isotherms are shown to reflect the performance of inhibitors. The adsorption isotherms,50 53 which describe the surface coverage and performance of the inhibitors are given in Table 1.21. 

In systems in which surface diffusion is rate-limiting, the adsorption of inhibitor molecules onto terraces must reduce the catchment area of the step so that k = k0(l - 0) [62], If 0 is given by the Langmuir equation, then... 

Having established that the inhibitor for LiF dissolution acts by slowing down the motion of steps, it is of interest to know whether general adsorption of inhibitor ions along the steps stops the nucleation of kinks, or whether the inhibitor acts by adsorbing at kinks and thereby slows down their motion. Present evidence shows the latter process to be the important one because only very specific ions act as inhibitors. Out of some 30 ions tried as inhibitors for LiF> only Fe and Al ... 

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). 

Protection from the effect of alkaline solutions (e.g. washing agents) can be provided for glass surface by adsorption of inhibitors introduced in the solution (salts of Be, Sn, Zn, Al, etc.). 

Indirect effects of carbon are sometimes confused with direct catalytic activity, e.g., some reactions are accelerated by activated carbon as a result of the adsorption of inhibitors. An example is found in the recovery of iodine from iodides present in petroleum salt brines. Nitrous acid is employed to oxidize the iodides to iodine —a reaction that may be retarded by the presence of inhibitors in the brine. Treatment of the brine with an activated carbon removes the inhibitors and enables the oxidation reaction to proceed. 

In the elucidation of the kinetics of the cracking of cumene on silica-alumina catalyst, the actions of inhibitors (poisons) on the reaction were studied. These inhibitors compete with cumene for cracking sites. Theoretical analysis leads to an expression from which the equilibrium constant for adsorption of inhibitors on cracking sites can be calculated. 

A film produced by the adsorption of inhibitors, such as chromates... 

Depth progression depends on the size and activity of the cathode surface. Pitting in cases of adsorption of inhibitors and in the salt layers formed under woiking conditions in corrosive media is due to an incomplete protective layer on the material surface. The intensity of corrosion depends on the area ratio between anode, uncoated surface, and cathode, coated surface. 

There is an increasing number of ab initio PBC works devoted to the adsorption of inhibitor molecules on metal or oxidized metal surfaces. They are summarized in Table 5.3 for different metals, Al, Cu, Fe, Zn, and their oxides. This hst is not exhaustive, but provides a good view of the emerging possibilities of ab initio modeling. The molecules considered as potential corrosion inhibitors are azoles, carboxylates, and amines. For each metal, the studies are presented in the order of increasing complexity of the interface works presenting adsorption at low coverage are presented, then results on fiiU layer formation. Finally, the sohd/liquid interface and adsorption at this interface is presented. 

Several reasons maybe responsible for the first inequality potential-dependent adsorption of inhibitor or desorption of catalyst, electrostatic effect at low ionic strength, (e.g., a reduction of anions at negatively charged surface), or the case when the available electrode surface decreases with an increase in polarization (inhibition or passivation). 

Adsorption of corrosion inhibitors onto metals. The inhibitive efficiency is usually proportional to the fraction of the surface 0 covered with adsorbed inhibitor. However, at low surface coverage (0inhibitor species in retarding the corrosion reactions may be greater than at high surface coverage. In other cases, adsorption of inhibitors, such as thiourea and amines, from diluted solutions, may stimulate corrosion. 

The typical increase in corrosion inhibition with inhibitor concentration, suggests that inhibition is the result of adsorption of inhibitor on the metal surface. The film formed by adsorption of soluble organic inhibitors is only a few molecules thick and is invisible. 

Typical examples of losses of an inhibitor due to these factors are precipitation of phosphates by the calcium ion, reaction of chromates with sulfides or organics, adsorption of inhibitors on suspended solids, and injection of a poorly soluble inhibitor without an adequate dispersing agent. 

The effectiveness of film-forming inhibitors, as already stated, depends upon strong adsorption of inhibitor molecules on the metal surface to be protected. Clean up, consequently, is very important in the control of corrosion. Some corrosion inhibitors have the ability to clean by nature of then-makeup or with the aid of added surfactants. These surfactants actually remove oil-coated corrosion products, allowing the inhibitor to attach itself to the clean metal. It should always be borne in mind that without proper clean-up, control of corrosion is generally unsuccessful. 

Quite often, the individual test method, while stimulating field conditions accurately, will not give reproducible results because of variables such as surface preparation, velocity and adsorption of inhibitors on solid particles, and other factors, some of which are indeterminate. Several investigators have reported the use of statistical methods in the evaluation of test results. 

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