Corrosion reduction reactions

For example, for iron in aqueous electrolytes, tlie tliennodynamic warning of tlie likelihood of corrosion is given by comparing tlie standard electrode potential of tlie metal oxidation, witli tlie potential of possible reduction reactions. 

The passive state of a metal can, under certain circumstances, be prone to localized instabilities. Most investigated is the case of localized dissolution events on oxide-passivated surfaces [51, 106, 107, 108, 109, 110, ill, 112, 113, 114, 115, 116, 117 and 118]. The essence of localized corrosion is that distinct anodic sites on the surface can be identified where the metal oxidation reaction (e.g. Fe —> Fe + 2e ) dominates, surrounded by a cathodic zone where the reduction reaction takes place (e.g. 2Fi + 2e —> Fi2). The result is the fonnation of an active pit in the metal, an example of which is illustrated in figure C2.8.6(a) and (b). 

Corrosion occurs at the anode, where metal dissolves. Often, this is separated by a physical distance from the cathode, where a reduction reaction takes place. An electrical potential difference exists between these sites, and current flows through the solution from the anode to the cathode. This is accompanied by the flow of electrons from the anode to the cathode through the metal (Fig. 8). 

The production of hydroxide ions creates a localized high pH at the cathode, approximately 1—2 pH units above bulk water pH. Dissolved oxygen reaches the surface by diffusion, as indicated by the wavy lines in Figure 8. The oxygen reduction reaction controls the rate of corrosion in cooling systems the rate of oxygen diffusion is usually the limiting factor. 

Many thousands of miles of steel pipeline have been laid under, or in contact with, the ground for the long-distance transport of oil, natural gas, etc. Obviously corrosion is a problem if the ground is at all damp, as it usually will be, and if the depth of soil is not so great that oxygen is effectively excluded. Then the oxygen reduction reaction... 

A sheet of steel of thickness 0.50 mm is tinplated on both sides and subjected to a corrosive environment. During service, the tinplate becomes scratched, so that steel is exposed over 0.5% of the area of the sheet. Under these conditions it is estimated that the current consumed at the tinned surface by the oxygen-reduction reaction is 2 X 10 A m -. Will the sheet rust through within 5 years in the scratched condition The density of steel is 7.87Mg m . Assume that the steel corrodes to give Fe " ions. The atomic weight of iron is 55.9. 

The hydrogen evolution reaction (h.e.r.) and the oxygen reduction reaction (equations 1.11 and 1.12) are the two most important cathodic processes in the corrosion of metals, and this is due to the fact that hydrogen ions and water molecules are invariably present in aqueous solution, and since most aqueous solutions are in contact with the atmosphere, dissolved oxygen molecules will normally be present. 

A striking example of the interaction of solution velocity and concentration is given by Zembura who found that for copper in aerated 0-1 N H2SO4, the controlling process was the oxygen reduction reaction and that up to 50°C, the slow step is the activation process for that reaction. At 75 C the process is now controlled by diffiision, and increasing solution velocity has a large effect on the corrosion rate (Fig. 2.5), but little effect at temperatures below 50 C. This study shows how unwise it is to separate these various... 

For the corresponding equations in alkaline solutions, see Chapter 9. The metal surface attains a mixed potential corrosion potential, such that the anodic current of the metal dissolution is exactly balanced by the cathodic current of one or more reduction reactions. The corrosion potential is given by Eq. (11.41), and the corrosion current density by Eq. (11.42). 

Oxidation-reduction reactions, commonly called redox reactions, are an extremely important category of reaction. Redox reactions include combustion, corrosion, respiration, photosynthesis, and the reactions occurring in batteries. 

Oxidation—reduction reactions, commonly called redox reactions, are an extremely important category of reaction. Redox reactions include combustion, corrosion, respiration, photosynthesis, and the reactions involved in electrochemical cells (batteries). The driving force involved in redox reactions is the exchange of electrons from a more active species to a less active one. You can predict the relative activities from a table of activities or a halfreaction table. Chapter 16 goes into depth about electrochemistry and redox reactions. 

There existed oxidation-reduction reactions with the same reaction speed on the sulphide mineral surface in water. One is the self-corrosion of sulphide mineral. Another is the reduction of oxygen. If the equilibrium potential for the anodic reaction and the cathodic reaction are, respectively, E and, and the mineral electrode potential is E, the relationship among them is as follows ... 

The mechanism of steel corrosion in acid solutions, however, is different from that in neutral solutions in that the reduction reaction is the formation of hydrogen from hydrogen ion ... 

They found that a Cu electrode, pretreated by immersing it in a 0.1M BTA solution for 15 seconds, inhibited the 0 reduction reaction initially and that on subsequent cycles the currents Increased to that of bare Cu in a short time. A similar effect was observed when a Cu electrode was cycled in a ImM solution of BTA. They discovered that a solution of 0.1M BTA produced a lasting effect, indicating that a reservoir of BTA is necessary for continuous protection of the copper against corrosion. We found that bare Cu gives the same voltammogram in the 0 reduction region in both acetate buffer and phosphate buffer therefore, McCrory-Joy et. al. s results can be directly compared to the results reported here. 

An electrochemical model for the process of electroless metal deposition was suggested by Paunovic (10) and Saito (8) on the basis of the Wagner-Traud (1) mixed-potential theory of corrosion processes. According to the mixed-potential theory of electroless deposition, the overall reaction given by Eq. (8.2) can be decomposed into one reduction reaction, the cathodic partial reaction. 

The corrosion inhibitor can be a complexing agent that stops the metal redeposition reaction (reduction) by eliminating free metal ions from the solution. Theoretically, this would consecutively stop the associated oxidation reaction. Due to parasitic reduction reactions, however, the metal oxidation can continue, even enhanced by the complexing agent effect. 

In me previous chapter we discussed acid-base reactions, which are chemical reactions involving the transfer of pro Lons from one reactant to another. In this chapter, we explored oxidation-reduction reactions, which involve the transfer of one or more electrons from one reactant to another. Oxidation-reduction reactions have many applications, such as in photography, batteries, fuel cells, the manufacture and corrosion of metals, and the combustion of non-metallic materials such as wood. 

A The corrosion evident on this anchor from a sunken ship occurs by a typical oxidation-reduction reaction of the kind discussed in this chapter. 

Experiments with sodium nitrate showed rapid reduction by Fe°, with nitrite as an intermediate and ammonia as final product. Iron acts as an electron donor and the reduction is coupled with metal corrosion (Equation 13.9). The reduction reaction in the model system was found to proceed in two sequential steps (Equation 13.30 and Equation 13.31), and the overall... 

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