What is hydrogen over voltage and what is it effect upon its rate of corrosion?

Question

please give a genuine definition no matter how long it is.

Answer 1

is not so simple but ..

overvoltage is a term to define the difference in potential of an electrode or metal or gas (hydrogen) between two situations:

when they are placed in a solution and not corrode and when they start to corrorde..

when an electrode or metal is placed in a solution, exmaple water, it assume a potential.

if this metal start to corrode the potential of the metal shift, change value.. this difference in potential value is what is called overpotential..

the same apply to hydrogen..the hyfrogen overvoltage means the difference between the equilibrium reaction of hydrogen in solution and hydrogen in solution  when it start to react with a metal and corrode...

the effect on corrosion rate is that it can control the corrosion reactions.

the overvoltage of hydrogen, the rate at which the hydrogen reacts and evolve as hydrogen gas is different for every metals so  corrosion rate are different in different metals..

if you can increase the overvoltage you can"slow down" the corrosion reaction because to have corrosion  in acidic solution you need to have this reactions.

2H+   + 2 electrons -----> H2 gas

For example some corrosion inhibitor increases the hydrogen overvoltage and reduced the dissolution current while it also retarded both the anodic and cathodic processes

this is a very simplified explanation...look at this documents where the corrosion reaction is explained with graph..

http://www.corrosionist.com/Corrosion_Fu...

http://www.npl.co.uk/upload/pdf/the_elec...

hope this help u

V

Answer 2

http://www.accessscience.com/abstract.as...

Overvoltage:

The difference between the electrical potential of an electrode or cell under the passage of current and the thermodynamic value of the electrode or cell potential under identical experimental conditions in the absence of electrolysis; it is also known as overpotential. Overvoltage is expressed in volts, often in absolute value; it is a measure of the rates of the different processes associated with an electrode reaction.

I don't have my references handy, but for steel in seawater, the potential is something like -650mV Ag/AgCl; -800mV provides cathodic protection. Aluminum or zinc anodes are typically -1100mV, and hydrogen overvoltage about -1300mV. Once you are more negative than the hydrogen overvoltage, H2 evolves, using lots of CP current. This is not a problem for passive systems, but for active CP systems, they can evolve H2, limiting the allowable voltage of the areas in contact with the main body of electrolyte (enclosed spaces are a different story).