Tafel slopes inhibitor effects

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

Blocking of reaction sites The interaction of adsorbed inhibitors with surface metal atoms may prevent these metal atoms from participating in either the anodic or cathodic reactions of corrosion. This simple blocking effect decreases the number of surface metal atoms at which these reactions can occur, and hence the rates of these reactions, in proportion to the extent of adsorption. The mechanisms of the reactions are not affected and the Tafel slopes of the polarisation curves remain unchanged. Behaviour of this type has been observed for iron in sulphuric acid solutions containing 2,6-dimethyl quinoline, /3-naphthoquinoline , or aliphatic sulphides . 

This last effect may be an indication of adsorption of a small impurity in the electrolyte. The inhibited corrosion rates decrease with time and become essentially constant after about two hours. These slopes are not dependent on scan rate or on corrosion rate. The most interesting effect is observed when the inhibited hydrochloric acid solution is aerated the anodic Tafel slope increases while the cathodic Tafel slope decreases dramatically. As would have been expected from the resistance probe measurement the corrosion rate in the aerated inhibitor solution increases. 

Adsorbed species may also accelerate the rate of anodic dissolution of metals, as indicated by a decrease in Tafel slope for the reaction. Thus the presence of hydrogen sulphide in acid solutions stimulates the corrosion of iron, and decreases the Tafel slope - - . The reaction path through (Fe-HS-W has been postulated to lead to easier anodic dissolution than that through (Fe-OH) s.. This effect of hydrogen sulphide is thought to be responsible for the acceleration of corrosion of iron observed with some inhibitive sulphur compounds, e.g. thioureas , at low concentrations, since hydrogen sulphide has been identified as a reduction product. However, the effects of hydrogen sulphide are complex, since in the presence of inhibitors such as amines , quaternary ammonium cations , thioureas . ... 

The corresponding corrosion potential (Ecorr), corrosion current density (icon), anodic Tafel slope (ba), cathodic Tafel slope (be) and CR for uninhibited and inhibited systems from PP measurement are listed in Table 3. The data demonstrates that the Ecorr values shift to more positive values as the concentration of added studied inhibitors are increased. On the other hand, the corrosion current densities are markedly declined upon addition of the studied corrosion inhibitors. The extent of its decline increases with increasing of the corrosion inhibitor concentration. Moreover, the numerical values of both anodic and cathodic Tafel slopes decreased as the concentration of inhibitors were increased. This means that the three natural products have significant effects on retarding the anodic dissolution of aluminium alloy and inhibiting the cathodic hydrogen evolution reaction. 

Fig. 10 24 Modes of inhibition, shown by the effect an polirizeittofi for charge-transfer-comrolted eicctrode reactions, (a) Decrease in the apparent anodic Tafet slope, (b) Decrease in the anodic exchange current (in the case of anodic inhibitors, passivation may also occur), (c) Decrease in the apparent cathodic Tafel slope, (d) Decrease in the cathodic exchange current.

Inhibitors may also retard the rate of hydrogen evolution on metals by affecting the mechanism of the reaction, as indicated by increases in the Tafel slopes of cathodic polarization curves. This effect has been observed on iron in the presence of inhibitors such as phenyl-thiourea, acetylenic hydrocarbons, aniline derivatives, benzaldehyde derivatives, and pyrihum salts. 

---