Corrosion hydrogen evolution

When water pH is between about 4 and 10 near room temperature, iron corrosion rates are nearly constant (Fig. 5.5). Below a pH of 4, protective corrosion products are dissolved. A bare iron surface contacts water, and acid can react directly with steel. Hydrogen evolution (Reaction 5.3) becomes pronounced below a pH of 4. In conjunction with oxygen depolarization, the corrosion rate increases sharply (Fig. 5.5). 

Even in good alloys and under favorable conditions, the a value does not lie above about 0.6. In enamelled storage tanks where the current requirement is low, the a value can fall to as low as about 0.1. The cause of the high proportion of selfcorrosion is hydrogen evolution, which occurs as a parallel cathodic reaction according to Eq. (6-5b) or by free corrosion of material separated from the anode on the severely craggy surface [2-4, 19-21]. 

In oxygen-free water, the self-corrosion is practically solely due to hydrogen evolution... 

The current yield of aluminum depends on the composition of the water and the operating conditions it usually lies between a = 0.8 and 0.9 (see Section 6.2.3). Self-corrosion occurs, as with Mg, with hydrogen evolution. 

Figure 4-419 illustrates the concept of corrosion process under concentration polarization control. Considering hydrogen evolution at the cathode, reduction rate of hydrogen ions is dependent on the rate of diffusion of hydrogen ions to the metal surface. Concentration polarization therefore is a controlling factor when reducible species are in low concentrations (e.g., dilute acids). 

It follows from equation 1.45 that the corrosion rate of a metal can be evaluated from the rate of the cathodic process, since the two are faradai-cally equivalent thus either the rate of hydrogen evolution or of oxygen reduction may be used to determine the corrosion rate, providing no other cathodic process occurs. If the anodic and cathodic sites are physically separable the rate of transfer of charge (the current) from one to the other can also be used, as, for example, in evaluating the effects produced by coupling two dissimilar metals. There are a number of examples quoted in the literature where this has been achieved, and reference should be made to the early work of Evans who determined the current and the rate of anodic dissolution in a number of systems in which the anodes and cathodes were physically separable. 

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. 

Fig. 1.30 Corrosion of a metal in an acid in which both metal dissolution and hydrogen evolution are under activation control so that the .log i curves are linear, (a) Effect of pH on and I o Hi increase in pH (decrease in an + ) lowers E and decreases / o (b) Effect of...

Fig. 1.34 Corrosion and passivation of Fe-18Cr-SNi stainl s steel. Potentiosiatic anodic curve JKLM, hydrogen evolution reaction, curve Hl low concentration of dissolved oxygen, curve t> FG, high concentration of dissolved oxygen, curve AflC (Section 3...

Reference has previously been made to pH in connection with calcium carbonate, but it has also a more general significance. The pH of natural waters is, in fact, rarely outside the fairly narrow range of 4.5 to 8.5. High values, at which corrosion of steel may be suppressed, and low values, at which gaseous hydrogen evolution occurs, are not often found in natural waters. 

Much of the information available on resistance of nickel-iron alloys to corrosion by mineral acids is summarised by Marsh. In general, corrosion rates decrease sharply as the nickel content is increased from 0 to 30-40%, with little further improvement above this level. The value of the nickel addition is most pronounced in conditions where hydrogen evolution is the major cathodic reaction, i.e. under conditions of low aeration and agitation. Results reported by Hatfield show that the rates of attack of Fe-25Ni alloy in sulphuric and hydrochloric acid solutions, although much lower than those of mild steel, are still appreciable (Tables 3.35 and 3.36). In solutions of nitric acid, nickel-iron alloys show very high rates of corrosion. 

The standard electrode trotential, Ep, 2+ Pb = —Q.126V . shows that lead is thermodynamically unstable in acid solutions but stable in neutral. solutions. The exchange current for the hydrogen evolution reaction on lead is very small (-10 - 10"" Acm ), but control of corrosion is usually due to mechanical passivation of the local anodes of the corrosion cells as the majority of lead salts are insoluble and frequently form protective films or coatings. 

The corrosion of tin by nitric acid and its inhibition by n-alkylamines has been reportedThe action of perchloric acid on tin has been studied " and sulphuric acid corrosion inhibition by aniline, pyridine and their derivatives as well as sulphones, sulphoxides and sulphides described. Attack of tin by oxalic, citric and tartaric acids was found to be under the anodic control of the Sn salts in solution in oxygen free conditions . In a study of tin contaminated by up to 1200 ppm Sb, it was demonstrated that the modified surface chemistry catalysed the hydrogen evolution reaction in deaerated citric acid solution. 

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