Inhibition, corrosion efficiency

The Corrosion Inhibitor, Inhibiting Corrosive Efficiency and Its Relationship with Collector Action... 

Comparing Table 7.4 and Table 7.5, it is obvious that the corrosive potential and the current of the galena electrode are lower in xanthate solution than in dithiocarbamate solution. The polarization resistance and the inhibiting corrosive efficiency in xanthate solution are higher than that in DDTC solution. It indicates that xanthate has stronger inhibiting corrosive action on galena than dithiocarbamate. 

Figure 7.41 is the polarization curves of sphalerite-carbon combination electrode in different collector solution at natural pH. The corrosive electrochemistry parameters are listed in Table 7.8. These results show that xanthate and dithiocarbamate have distinctly different effects on sphalerite. The corrosive potential and current of sphalerite electrode are, respectively, 42 mV and 0.13 pA/cm at natural pH in the absence of collector, -7 mV and 0.01 pA/cm in the presence of xanthate, and 32 mV and 0.12 pA/cm in the presence of dithiocarbamate. The corrosive potential and current decrease sharply with xanthate as a collector, indicating that the electrode surface has been totally covered by the collector film from the electrode reaction. Xanthate has big inhibiting corrosive efficiency and stronger action on sphalerite. However, the corrosive potential and current of sphalerite electrode have small change with dithiocarbamate as a collector, indicating that DDTC exhibits a weak action on sphalerite. 

The corrosive electrochemistry parameters are listed in Table 7.4. From Fig. 7.30 and Table 7.4, it can be seen that, after adding xanthate 5x 10" mol/L, the corrosive potential of galena electrode decreases from -48 to -94 mV, the corrosive current of the galena eleetrode decreases from 3.45 to 0.99 pA/cm, the polarization resistance increases to 18.7 kG and the inhibiting eorrosive efficiency increases to 28.34. Figure 7.31 shows that the EIS of galena electrode appears to have single capacitive reactance loop characteristic and the radius of the capacitive reactance loop increases with the increase of collector concentration. 

This method is, of course, a sophisticated empiricism, but it allows the corrosion efficiency of any organic to be rapidly estimated. The reservation is that there must have been some prior data on corrosion inhibition for the system concerned (e.g., Fe in acid solution) to provide the learning graph. 

Anodic passivation of steel surfaces can be efficiently achieved by metal chromates. Chromates of Intermediate solubility (e.g., zinc chromate and strontium chromate) allow a compromise between mobility in the film and leaching from the film to be achieved. Chromates inhibit corrosion in aqueous systems by formation of a passivating oxide film. The effectiveness of chromate inhibitors in aqueous systems depends on the concentration of other ionic species in solution, for example, chloride. Synthetic resin composition can also significantly influence the effectiveness of chromate pigments. The effect appears to be related to the polarity of the resin (20) chromate pigments appear to be less effective in resins of low polarity. 

Technical installations can be endangered not only by the effects of corrosion but by noxious substances, which would adversely affect the personnel in an installation, inhibiting their efficiency. Accidents may release noxious substances into the atmosphere, strongly inhibiting a person s perception in general, his ability to react, etc. Effects of this kind should be expected mainly in situations where large amounts of hazardous substances are being transported or stored in the technical installation under review. 

In a typical electrochemical assay, the efficiency of the system in inhibiting corrosion (E%) is estimated by means of Equation 15.3 ... 

At higher concentrations, e.g. 1000-10 000 ppm, the inhibition efficiency with alloys 2024 and 7075 is reduced and the inhibited corrosion rate appears to be dependent on the type of R cation. These results have not been completely explained (Hinton et al. 1984, 1985), but may be related to (a) the nature of the protective film or (b) the observation that the observed R oxide films are thicker and more defective when formed in solutions containing > 1000 ppm R cation. Also, the La and Ce films tended to be powdery and have poor adhesion. Defective films would not be expected to provide good corrosion inhibition. 

The efficiency of organic amines as corrosion inhibitors is improved when certain halogen ions are present. Halogen ions alone inhibit corrosion to some extent in acid solutions. The iodide (I ) ion is the most effective, followed by bromide (Br ) and chloride (Cl ). Fluoride (F ) does not have significant inhibitive properties. Chloride ions, for example, lower the rate of attack on steel by sulfuric acid. A combination of amine and iodide may be more inhibitive than either additive alone, i.e., the two additives are synergistic. 

The modification of silane coatings with species able to inhibit corrosion introduces a new functionality to organic coatings active corrosion protection. This can overcome the typically inert nature of the silane film, making it more efficient in combating corrosion processes. 

Pigment Dispersion. AMP is used widely as a pigment dispersant for water-based paints and paper coatings. In small amounts, it efficiently disperses pigments and improves pH stabiUty, viscosity, corrosion inhibition, and odor (13). When AMP is used in conjunction with other surfactants, enhanced performance is obtained with less of these ingredients in the dispersion. 

To meet sales specifications, gas produced at the wellheads must be free of water and hydrocarbon liquids. Twin turboexpanders are a key component in this process, providing dewpoint control with optimal efficiency. Initial processing takes place at the wellhead platforms, where methanol is injected to inhibit hydrate formation. A corrosion inhibitor is also added to prevent gas from damaging downstream equipment. 

It is in this area that most work has been carried out, particularly in relation to corrosion resistance in sulphuric acid solutionsBourelier etal. and Raicheff etal. investigated the inhibitive effect of chloride ions on corrosion in sulphuric acid. The inhibition efficiency was found to depend on the alloy composition, alloy surface and chloride concentration. The more aggressive the environment, the greater the inhibition efficiency. Yagupol skaya etal studied the effect of iodine additions to sulphuric acid on the corrosion resistance of Ni and Ni-Fe alloys. Again there was an inhibitive effect caused by the halide ion. 

The extent of inhibition afforded to metals other than mild steel depends on the metal and the inhibitor see The Nature of the Metal, and Dissimilar Metals in Contact). The cathodic type of inhibitor is perhaps less susceptible than the anodic type to the nature of the metal. However, cathodic inhibitors are usually less efficient (although performing quite satisfactorily in many systems) in terms of reduction in corrosion rate, than are anodic inhibitors. The latter, when used in adequate concentrations, can often achieve 100% protection. 

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