To get the best deal on Tutoring, call 1-855-666-7440 (Toll Free)
Top

# Electrochemical Corrosion

Corrosion is an interface phenomenon and is dependent on the variables defining the metal, the environment and physical aspects of the interface itself. These variables give rise to electrical potential differences across the interface and drive the corrosion rate. This interface films also control the current which allows the passive type alloys with their adherent oxide films having lower corrosion rates than non-passive alloys.

Useful theories of electrode kinetics helps in understanding anodic and cathodic reaction models at the metal and atmosphere interface and also for diffusion of species to and from the interface.

The overall concept about corrosion and electrochemical process which runs in the background helps in getting a proper idea about how kinetic parameters influence the overall process and rate of interface reactions. There are factors like the metal itself, catalysts and interface quality. All these and more will be covered in this section of electrochemical process of corrosion.

## Electrochemical Corrosion Definition

Corrosion can be defined as the deterioration of materials properties due to its interaction with its immediate environment. The demands for a long term performance of engineering structures over a wide size scale continue to increase.

With the decrease in microelectronic structures decrease in size, smaller amounts of dissolution on interconnects in integrated circuits can lead to seizure of series of systems. The long term storage of the nuclear waste may become a big problem for mankind as the containers within which these are sent for safe keeping start corroding.

The use of electrochemical methods for probing corrosion process has increased to the point where they represent an indispensable set of tools.
Principles and procedure of electrochemical measurement used to investigate corrosion behaviour are something which needs to be understood and taken into proactive measures.

The parameters of electrochemical reactions and to measure corrosion potentials and current densities are also required to know how much of time and what kind of rate of corrosion is expected for the said metal and what are the steps either to enhance the catalytic process or reduce the loss.
The deterioration of materials due to reactions with their environment is basically the general idea of how corrosion works. These materials refer to substances which undergo corrosion like metals, polymers and ceramics. The immediate surroundings are mainly the atmosphere liquid or gaseous corroding agents.

• Rusting of steel and cast iron water tanks due to humid air and acidic rainfalls.
• Corrosion of copper, aluminium and cast iron in various automotive cooling systems
• Corrosion of iron or copper based alloys in chemical process industries
• Corrosion of automobiles exhaust systems by direct reaction of metal with high temperature gases and also by condensation of water and oxide absorption of either sulfur or nitrogen based aqueous acidic medium
• Corrosion of the turbine blades within gas turbines due to hot combustion gases
• Corrosion of metallic surgical implant materials used in orthopaedic and dental devices which result in metal ions to tissues as well as degrade physical properties of polymer based materials
• Corrosion of iron base and nickel base alloys by liquid metals used as heat transfer agents
• Stress corrosion cracking of gold and brass by mercury
• Stress corrosion of steel products in sea water or sea shore moist air

## Electrochemical Corrosion Testing

The definition of localised corrosion is usually restricted to specific types of attack often related to the presence of chlorides. General corrosion rates in most system areas of metal surface have been measured and designed in such a manner that metal losses during expected life of the system is enhanced which can run for more number of years.

Problems that arise when the corrosion becomes localised and the penetration rate of the metal is orders of magnitude greater than the predicted general corrosion. Localised forms of corrosion take a far greater toll than the incorrect choice of materials that give unacceptable general corrosion.
During localised corrosion, the electrochemical dissolution is well separated from the cathodic reactions. This makes an in situ study of the anodic and cathodic reactions amenable to direct measurement which is in contrast to general corrosion where reactions can take place in close proximity.
This is done in order to separate the anodic and cathodic reactions without interfering with processes taking place or altering them to an extent whereby they no longer relate to conditions during exposure.

In situ measurement like mapping of potentials in solution or physical separation of anodic and cathodic areas along with current slowing through in between is used to identify the process during corrosion. Some other methods such as weight loss and penetration rates are also used but these also require periodical removal of the metal from corroding environment.

In case of stress corrosion cracking the un-corroded area may act as cathode and when the cathodic polarisation is limiting, the interaction between the growth rates of the various cracks would begin. The identification of such effects would require a large number of samples and would be extremely difficult to extract the rate process or electrochemistry.

## Electrochemical Corrosion Process

Electrochemical reactions are chemical reactions, in which not only elements are added or removed from a chemical species but also at least one species undergo a change in the valence electron number.

The precipitation of iron hydroxide is a pure chemical reaction.

$Fe^{2+} + 2OH^{-} \rightarrow Fe (OH)_{2}$

None of the atoms involved have changed its valence and iron along with oxygen retain their divalent state. Hydrogen remains as univalent.
Ferrous ion needed in the reaction is obtained by oxidation of metallic iron.

$Fe \rightarrow Fe^{2+} + 2e-$

In order to get this reaction proceed further, the two electrons that are produced must be utilised in a reduction reaction such as reduction of dissolved oxygen.

$O_{2} + 2H_{2}O + 4e- \rightarrow 4OH-$

If the two reactions are not widely physically separated on a metal surface, the chemical reaction between the hydroxide and ferrous ions can produce a solid on the surface. This shows that chemical and electrochemical reactions can be coupled.

## Electrochemical Corrosion Mechanism

$Fe^{2+} (aq) + 2e- \rightarrow Fe(s) ………………………..E^{0} Fe2+/ Fe = -0.44 V$