Tafel slope increasing

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. 

The relationship between temperature and performance is complex. An increased temperature drives increased kinetic activity for the oxygen reduction reaction (ORR) however, the reversible cell voltage decreases with increasing temperature. The Tafel slope increases in the low current density... 

Tafel-like regions in which the slope decreased with an increase in the solution pH were also found. For example, see Fig. 20, where the Tafel slope increased from 30 mV dec" for pH 2.1 to 58 mV dec" for pH 0.0. 

The influence of varying amounts of sulfate ions on the corrosion and polarization behavior of iron in acid perchlorate solutions, at constant pH, was investigated by Strom et alP The results obtained indicated an increase of the anodic current density and of the Tafel slope in the first steady-state linear range, the Tafel slope increasing from 30-40 mV dec" ... 

The polarization curves are S-shaped with a linear region at overvoltage values lower than ca, 120 mV. The linear part is narrower, the higher the chloride ion activity. Concomitantly, the steady-state Tafel slope increases from 30 mV dec" at [Cl"] < lO M to about 60 mV dec" for [Cl"] = 2M, as the product acf an increases(see Fig. 27). At higher overvoltage, the potential of unpolarizability (or the current of unpolarizability) is reached, and the current increases sharply (Fig. 28). 

It may be noted that, with a decrease in hydrogen ion activity, case III turns into case IV, which means a decrease of the steady-state Tafel slope from 40 to 24 mV dec concomitantly with an increase in Vqh" froni 1 to 2. This provides a possible explanation for the finding of Bala for an iron rotating disk electrode in acidic sulfate solutions that the Tafel slope increased from 30 to 40 mV dec with a decrease in pH from 2 to 1. 

E < ca. 1.25V vs. calomel. The Tafel slope increased very mu Langmuir-type isotherm was adopted for adsorption of Clad- When a = 0.5. 

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. 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

A qualitative measure of the corrosion rate can be obtained from the slope of the curves in Fig. 2. Z INT is given in units of s ohm" . Owing to the presence of the uncompensated ohmic resistance and lack of values for Tafel slopes [Eq. (2)], data in Fig. 2 should be viewed as indicative of significant changes in corrosion rates. Corrosion loss remained low during the first 2 months, followed by a large increase for both flushed samples and controls. The corrosion rate increased when the surface pH reached values of 1 or less. Total corrosion loss as determined from integrated Rp data was less for the control than for the flushed sample. 

Admixtures of Ir to Ru or Ru02 resulted in an increased Tafel slope for the 02 evolution reaction. The Tafel slope for the pure Ir compound was achieved for an Ir concentration below 50%. This observation can be taken as an indication that the... 

At low coverage, the Tafel slope will be 2RT/3F or c. 40 mV, as observed on high at.% Ru electrodes. As the at.% Ru decreases, the number of Ru sites decrease, resulting in more coverage of the active Ru sites by 0ac]. Hence 0ad will approach 1 and the Tafel slope will tend to reach values of 2RT/F or 120 mV, thus potentially explaining the results in Fig. 5.3. Alternatively, this change in the Tafel slope may arise from an increase in the electrical resistivity of the low at.% Ru electrodes, during the course of the chlorine evolution reaction [35]. 

Because of the different potential distributions for different sets of conditions the apparent value of Tafel slope, about 60 mV, may have contributions from the various processes. The exact value may vary due to several factors which have different effects on the current-potential relationship 1) relative potential drops in the space charge layer and the Helmholtz layer 2) increase in surface area during the course of anodization due to formation of PS 3) change of the dissolution valence with potential 4) electron injection into the conduction band and 5) potential drops in the bulk semiconductor and electrolyte. 

The Tafel curves of the galena electrode in xanthate solution under the conditions of the mechanical power and non-mechanical powers are given in Fig. 8.20. It follows from Fig. 8.20 that Ig/o of the anode action on the surface of galena raises from -6.5 to -5.9 with the increase of Tafel slope from -40 mV to -28 mV under the mechanical action. It shows that the reaction is favored in dynamics due to the grinding. 

---