Temperature hydrogen overvoltage

Temperature the hydrogen overvoltage decreases with increasing temperature. It can be zero at higher temperatures. For example, the hydrogen overvoltage on nickel is zero at 90 °C. 

In an alkali-chlorine cell a saturated (about 6 N) solution of sodium chloride is electrolyzed at ordinary temperatures, between a steel cathode (hydrogen overvoltage 0.2) and a graphite anode (oxygen overvoltage 0.6 volt chlorine overvoltage negligible). The nature of the electrode process. Explained ... 

The decomposition of alkali metal amalgams by water proceeds very slowly owing to the high hydrogen overvoltage on mercury. The rate of this chemical decomposition increases with temperature which reduces the hydrogen overvoltage, but this is still insufficient for technical practice. 

A black phosphorus electrode [7, 30] has a low hydrogen overvoltage, and electrolysis in aqueous solution is accompanied by reduction of the cathode material with formation of phosphane. The largest PH3 yield of 27.5% (calculated from the loss of the electrode weight) at 12.5% current efficiency was obtained in 1M K2HPO4 (pH 7.5) at a current density of 0.07 to 0.09 A/cm2 and at 20 C. Altering the pH to either the acidic or basic side reduces the PH3 yield. An increase in catholyte temperature from 20 to 55 0 lowers the PH3 yield by a factor of 5 to 6. Decreasing the current density also lowers the current efficiency [30]. 

This reaction proceeds rapidly in acids, but only slowly in alkaline or neutral aqueous media. The corrosion rate of iron in deaerated neutral water at room temperature, for example, is less than 5 p,m/year. The rate of hydrogen evolution at a specific pH depends on the presence or absence of low-hydrogen overvoltage impurities in the metal. For pure iron, the metal surface itself provides sites for H2 evolution hence, high-purity iron continues to corrode in acids, but at a measurably lower rate than does commercial iron. 

The reaction rate of sulfites with oxygen at low temperature is slow, so a catalyst is usually added, the best being cobalt, manganese, and copper salts. Cobalt gives the greatest increase in reaction rate. Copper should not be added to water that contacts steel or aluminum because it lowers the hydrogen overvoltage and increases the corrosion rate. 

High temperatures always increase the rate of attack of metals in acids because the driving force for the anodic and cathodic reactions is greater and hydrogen overvoltage is decreased. Other factors such as the greater solubility of corrosion products and the higher rate of solution of metal oxides... 

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