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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

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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

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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

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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

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Electrochemical corrosion is a process occurring between a metal and the electrolyte environment not in one electrochemical reaction and the corrosion rate depends on the electric potential on the metal surface.

If an iron strip electrode in a ferrous sulphate solution and a copper strip electrode in cupric sulphate solution taken separately (without dissolved oxygen in the solution), each of these strips will be called half-cell which are immersed in solution of its salt.

When these half-cell are linked by a salt bridge and form a closed circuit with a potentiometer to measure electric difference potential between the two half cells. 

The reversible potential for two possible reactions occurring on the metal surface are as follows:

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

$Cu^{2+} + 2e- \rightrrow Cu(s) ……………………………..E^{0} Cu2+ / Cu = + 0.337 V$$

The electrical potential reaction is less and hence iron electrode is less noble than copper when the salt bridge is built. 
The iron will oxidize or corrode.

$Fe(s) \rightarrow Fe^{2+}(aq) + 2e-$

This reaction is called anodic and corroded or oxidised electrode is called an anode. 

The iron ions retreat from anode into the solution and finally the anode corrodes. The electrons released in the reaction accumulates on the anodes surface and result in a negative charge formation on the surface.

An electric current called corrosion current appears in this process and the electrons do not flow in the solution. The copper cations $(Cu^{2+})$ in the aqueous sulphate solution move towards the copper electrode and react with the free electrons released in the anodic process that are coming from iron anode.

The combination of two dissimilar metals in a general electrolyte is called a galvanic cell, voltaic cell, electrolytic cell or simply electrochemical cell or voltaic battery.

Electrochemical corrosion occurs only in electrolyte solutions, mainly in water containing electrolyte, in soil, in humid place and many chemicals. There are no electrode potentials on metal surface in the absence of electrolytes in dry air, in dry liquid bromine or in dry organic solvents.

Types of Electrochemical Corrosion

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In most common types of corrosion is considered under two major categories. 
  • Uniform corrosion
  • Non-uniform or localised corrosion 
Uniform corrosion
Unform corrosion occurs when corrosion is quite evenly distributed over the surface leading to relatively unform thickness reduction. Metals without significant passivation tendency in actual environment are liable to this form. 

Uniform corrosion is assumed to be the most common form of corrosion and is reponsible for most of materials loss which often leaves behind a scale or deposit. Predicting the thickness reduction rate can be done by means of simple tests. 

Theoretical calculation performed to measure corrosion rate are based on the assumption that occuring corrosion is uniform corrosion and thus metals exposed to other types of corrosion reveal corrosion long before the theoretical calculations.

The uniform corrosion rate is calculated either as the unit weight loss per area per time in mg / dm2 / day. This can be also thickness loss of material per unit time which is expressed as corrosion penetration rate.

CPR = kw / ρat

Where, w is the weight loss after exposure; ρ represent density, ‘a’ as exposed area; k is a constant and the magnitude depends on system of units used CPR usually expressed in two methods (either in mm per year OR mils per year).

There is a electric current associated with electrochemical corrosion reactions and the rate of corrosion can also be expressed in terms of corrosion current or current density.

The corrosion rate is determined in units of mol per meter2

The formula is as follows: ‘r’ = i / n F

Where, n is number of electrons associated with ionization of each metal atom and F is equal to 96500 coulomb / mol

Non-uniform corrosion
For any form of atmospheric corrosion, the dusts as well as other solid particle settling down on surface are hygroscopic and attract moisture from air. For the atmospheric corrosion to occur, a thin film of aqueous material is necessary and for this the relative humidity is essential.
Water which is present as humidity bonds in the molecular form through oxygen atom with the metal surface or to layers of metals and act as a lewis base by absorbing on electron deficient sites. Water might also make bond in dissociated form in which the main reason is the formation of metal oxide or metal hydroxide bonds. 

The final products which result from water adsorption are then absorbed on the substrate surface as hydroxyl and atomic hydrogen groups.
The aqueous layer may also contain sulfur di oxide, carbon di oxide and chlorides which all accelerates the process of corrosion. The corrosion deposits on the metal surface and high relative humidity values bind to metal surface. 

SO2 also is responsible for most of the significant corrosive air pollutant in air which originates from combustion reactions of petroleum and coal, containing sulfur as main ingredient.

Salts can cause high conductivity and carbon particles can lead to a large number of small galvanic elements since they act as efficient cathodes after depositon on the surface. 

Salt content of the air increase with increasing altitude which result in more corrosion. 

At low altitudes forests and mountains slow down the wind speed resulting in less salt water content.

The atmospheric corrosion rate is the highest when the metal is first exposed to air and slowly decrease with time.

Increase in temperature increase corrosion but that also decrease the water content in atmosphere and hence there is temperature when corrosion takes place but reduces with time.

Fundamentals of Electrochemical Corrosion

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Electrochemical measurements by scientists on chemical systems differ for a variety of reasons. Either it’s for thermodynamic data about reaction, or generate an unstable intermediate like radical ion or study its rate of decay or its spectroscopic properties.

Electrochemical methods are employed as tools in the study of chemical systems and also investigations in which the electrochemical properties of the systems themselves are of primary interest and also design a new power source for electro synthesis of some product.

Corrosion is the spontaneous oxidation of a metal by oxygen, by water, by acids or most frequently by the action of oxygen in the presence of an aqueous environment. 

Each of these reactions, mentioned below, involves the oxidation of the metal and a simultaneous reduction but as the metal can conduct electrons and the aqueous environment can conduct ions. 

$M(s) + n H_{2}O (l) \rightarrow M (OH)_{2} (s) + n/2 H2 (g)$

$M(s) + n H^{+} (aq) \rightarrow M n+ (aq) + n/2 H2 (g)$ 

The dual corrosion reactions need not take place at the same site. There are various reasons which the surface of a metallic object might spontaneously develop cathodic and anodic regions.

Corrosion occurs near the junction of two metals 
  • Metals frequently contain impurity inclusions
  • Specific regions of surface are subjected to different metallurgical treatments which leads to small differences in activity
  • Metals are assemblies of small crystals and their surfaces of different crystallographic faces while are susceptible to oxidation than others
  • Different oxygen concentrations at various sites on the metal surface. This differential aeration is commonly responsible for establishing corrosion cells. Differential aeration can be insidious in fostering corrosion at sites where corrosion is most harmful.
  • Organisms living on or near a metal structure will either generate or consume oxygen and so contribute to differential aeration. In addition the biota often perturb the pH in their vicinity which help in establishing a corrosion cell
  • The accumulation of corrosion products may itself facilitate corrosion by catalytic mechanism which leads to direct oxidation of iron : $Fe (s) \rightarrow 2e^{-} + Fe^{2+} (aq)$ is a slow process even when in acidic environment. As ferrous ions accumulate, some become oxidize to ferric ions
$2 Fe^{2+} (aq) + ½ O_{2} (aq) + 2 H^{+} (aq) \rightarrow 2 Fe^{3+} (aq) + H_{2}O (l)$

$2 Fe^{3+} (aq) + Fe (s) \rightarrow 2 Fe^{2+} (aq)$
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