CORROSION

 

Corrosion is defined as “the destruction or deterioration and consequent loss of metals or alloys through chemical or electrochemical attack by the surrounding environment”.

In simple corrosion and metal extraction can be regarded as

                            Metal                     Metal ore

 

The primary factors that initiate corrosion on metals are atmospheric air, water and also conducting surface of the metal.
Egs: Rusting of iron, green scales are formed on copper vessels

Corrosion of metal occurs either by direct chemical attacks or by electrochemical attack on the metal by the corrosive environment.
If the corrosion takes place due to direct chemical attack (in the absence of moisture ) that type of corrosion is known as dry corrosion.
If the corrosion of metal takes place due to electrochemical attack in presence of moisture or a conducting medium such corrosion is known as wet corrosion or electrochemical corrosion.

Electrochemical theory of corrosion:

Most of the corrosion takes place on the basis of electrochemical reactions on the surface of metal such a type of corrosion is known as wet corrosion.

Electrochemical theory of corrosion can be taking iron as an example.
When a metal like iron is exposed to the environment according to electrochemical theory corrosion of metal takes place due to the formation of anodic and cathodic regions on the same metal surface or when the two metals are in contact with each other in a corrosive medium.

These anodes and cathodes are formed due to the heterogeneities at the interfaces of the metal and environment. The heterogeneities on a metal surface could develop due to several factors like
1. On a metal surface if the concentration of the oxygen is different (if in the metal the area which is exposed to more oxygen acts as cathode, the area which is exposed to less oxygen concentration acts as anode).
2. Due to contact of two different metals (egs: if copper and iron are in contact with each other, then Fe acts as anode and copper acts as cathode due to change in electrode potential).
3. If metal surface subjected to stress (area under stress acts as anode).

Thus anodic and cathodic area are formed, in presence of corrosion medium ( like moisture etc.)
At anode oxidation takes place so that metal is converted into metal ions with the liberation of electrons.

M                   Mn+ + ne-

Egs: Fe             Fe2+ + 2e-

 

At the cathodic regions, reduction takes place since the metal at cathodic region cannot be reduced further, so some constituents of the corrosive medium take part in the cathodic reaction. Since in the cathodic reaction as the constituents of the corrosion medium are involved, they are more complicated and dependent on the nature of environment. Most common type of cathodic reaction are 1. Liberation of hydrogen 2. Absorption of oxygen.

Cathodic reactions:

1. Liberation of Hydrogen type
2. If the medium is acidic and in the absence of O2

2H+ +2e-                  H2

1. If the medium is neutral or alkaline in the absence of O2.

 

2H2O + 2e-           2OH- + H2

1. Absorption of oxygen type

         

1. If the medium is acidic and in the presence of O.

4H+ + O2 + 4e-                   2H2O

1. If the medium is neutral or alkaline and in the presence of O2.

2H2O + O2 + 4e-              4OH-

The metal ions (Fe2+) liberated at anode and some anions (OH-) formed at cathode diffuse towards each other through the conducting medium and form a corrosion product some where between the anode and cathode as

2Fe2+ + 4OH-           2Fe(OH)2

In an oxidizing environment, the insoluble Fe(OH)2 oxidised to ferric oxide as following reaction.

                       4Fe (OH)2 + O2 + 2H2O                   2(Fe2O3 . 3H2O) + H2 
yellow rust     

if the concentration of oxygen is limited then  Fe(OH)2 is converted into magnetic oxide of Fe and is known as black rust.

3Fe(OH)2 + ½ O2                    Fe3O4.3H2O
BLACK RUST

 

 

Types of Corrosion: Corrosion on the metals taking place depending on the nature of metals and depending on the types of environment by different mechanisms, giving different types of corrosion.

1. Galvanic corrosion or differential metal corrosion:

 

This occurs when two dissimilar metals are in contact with each other in a corrosive conductive medium; a potential difference is set up resulting in a galvanic current. The two metals differ in their tendencies to undergo oxidation. The metal with lower electrode potential or more active metal acts as anode and the metal with higher electrode potential acts as cathode. The potential difference is main factor for corrosion to take place. The anodic metal undergoes corrosion where as cathodic metal gets un -attacked.

Egs: When iron contact with copper iron has lower electrode potential acts as anode and undergo oxidation as,

           Fe               Fe2+ + 2e-  

Where as copper which is having higher electrode potential acts as cathode gets unaffected. The rate of galvanic corrosion depends upon potential difference between anodic and cathodic metals, ratio of anodic and cathodic area and environmental factors and tendency of the metal to undergo passivity etc.

Other egs: When Fe contact with Sn then Fe acts as anode and Sn acts as cathode but when Fe contact with Zn, Fe acts as cathode where as Zn acts as anode.

1. Differential aeration corrosion:

                     Differential aeration corrosion occur when metal surface is exposed to the differential air or oxygen concentration, that develops galvanic cell and initiate the corrosion. The part of the metal exposed to lower oxygen concentration acts as anode and the part of the metal exposed to higher concentration acts as cathode, so that poorly oxygenated region undergoes corrosion.
When a metal strip of iron, partially immersed in an aerated solution of NaCl the concentration of O2 is higher at the surface than inside the solution. Since cathodic reaction requires oxygen, hence cathodic area tends to concentrate near the water line so that bottom portion of the specimen acts as anode where corrosion starts.
At anode: Fe                    Fe2+ + 2e-
At cathode (near water line): O2 + 2H20 + 4e-                4OH-

Water line corrosion: This is a case of differential aeration corrosion commonly observed in steel water tanks, ocean going ships etc. in which portion of the metal is always under water
The part of the metal below the water line is exposed to only to dissolved oxygen while the part above the water is exposed to higher concentration of oxygen.
Thus the metal part below the water line acts as anode where as above the waterline acts as cathode and process of corrosion starts. The metal just below the water line is more anodic and the creep (meniscus) is the one which is more oxygenated acts as cathode and unaffected. Amount of creep of the water determines the rate of corrosion, but mass intense corrosion occurring at the water line; there a brown line is also formed due to the deposition of corrosion products. This type of corrosion is commonly observed in ships floating in seawater for a long period of time.

PITTING CORROISON: Pitting corrosion is a localized and accelerated corrosion. When a small particles of dust or water etc are get deposited on a metal (like steel). The portion covered by the dust will not be well-aerated area compared to the exposed surface hence the covered surface becomes anodic with respect to the surface exposed. In presence of an conducting medium (moisture) corrosion starts below the dust part and forming a pit. Once pit is formed the ratio of corrosion increases, because of the formation of smaller anodic and larger cathodic area intense corrosion takes place.
Pitting corrosion is one of the most destructive forms of corrosion. It causes equipment to fail because of perforation with only a small percent weight loss of the entire structure. Because of the small sizes of the pits it is highly difficult to identify the pitting corrosion. Pitting corrosion is an autocatalytic process, and once the corrosion products arte formed, it further provides the condition for differential aeration below the corrosion product and surrounding metal parts. The pit grows and ultimately may cause failure of the metal.

STRESS CORROISON: Stress corrosion of the metal formed by the combined effect of a tensile stress and a specific corrosive environment on the metal., during stress corrosion, the metal or alloy is virtually unaffected over most of its surface, while fine cracks progress through it normal to the direction of tensile stress.
The stress on the metal may be internal or external and these stress is due to some mechanical or service conditions. The metal atoms under stress are always at higher energy level so acts as anode and stress free parts of metal acts as cathode under specific corrosive environmental conditions corrosion process starts.
Egs: Brass undergoes corrosion in the presence of ammonia.
Stainless steel in the presence of Cl- and caustics.
But best example for stress corrosion is caustic embrittlement.

Caustic embrittlement: It is a form of stress corrosion takes place in boilers operating at high temperature and pressure. Caustic embrittlement focus at stressed part of boilers such as cracks, rivets, bents, joints etc.
The boiler fed water usually contains some residual sodium carbonate (used for softening process). At high temperature and pressure it undergoes hydrolysis to form sodium hydroxide.

                Na2CO3 + H2O             2NaOH + CO2                  

The alkali water sweeps through the minute cracks, crevices between the rivets and joints by capillary action. Inside the cracks water gets evaporated leaving behind NaOH. The concentrations of the NaOH gradually increase on these sites due to poor circulation of water. When concentrations of the NaOH reaches a value of 10% it attacks the metal at the stressed region dissolving it in the form of sodium ferroate ( Na2FeO2). Sodium ferroate undergoes hydrolysis-depositing magnetite as follows

     3Na2FeO2 + 4H2O              6NaOH + Fe3O4 + H2

       6Na2FeO2 + 6H2O + O2           12NaOH + 2Fe3O4

So NaOH is regenerated in the process and its concentration is keep on increasing maintaining a required environment. Thus corrosion process develops cracks and making the metal brittle by the deposition of the product.
The corrosion cell can be represented as

 Fe (under stress) / conc. NaOH / dil. NaOH/ Fe (stress free)
Anode                                                           Cathode

Caustic embrittlement can be prevented by the addition of compounds like sodium sulphite, tannin, lignin, phosphates etc. which blocks the cracks thereby preventing the infiltration of alkali.

Factors affecting rate of corrosion: 

Several factors affecting the rate of the corrosion, which can be divided into two parts 

1. Factors affecting on metal (related to the metal)
2. Factors affecting on corrosive environment.

 

Factors affecting the metals:

1. Nature of the metal: The tendency of the metal to undergo corrosion is mainly dependent on the nature of the metal. IN general the metal with lower electrode potential have more reactive and more susceptible for corrosion and metal with high electrode potential are less reactive and less susceptible for corrosion for egs: metals like K, Na, Mg, Zn etc have low electrode potential are undergo corrosion very easily, where as noble metals like Ag, Au, Pt have higher electrode potential, their corrosion rate are negligible but there are few exception for this general trend as some metals show the property of passivity like Al, Cr, Ti, Ta etc.
2. Surface state of the metal or nature of the corrosion product (passivity):

The corrosion product is usually the oxide of the metal; the nature of the product determines the rate of further corrosion process.
If the oxide layer, which forms on the surface, is stoichiometric, highly insoluble and non-porous in nature with low ionic and electronic conductivity then that type of products layer effectively prevents further corrosion, which acts as a protective film. For egs: Al, Cr, Ti develop such a layer on their surface and become passive to corrosion and some metal like Ta, Zr and Mo not only forms such a protective layers but are capable of self repairing oxide films when it is damaged. Hence these are extremely passive metals.
If the oxide layer forms on the metal surface is non-stochiometric, soluble, unstable and porous in nature and have an appreciable conductivity, they cannot control corrosion on the metal surface for egs: oxide layer formed on metals like Zn, Fe, Mg etc.

3. Anodic and Cathodic area:

The rate of the corrosion is greatly influenced by the relative sizes of cathodic and anodic areas.
If the metal has smaller the anodic area and larger the cathodic area exposed to corrosive atmosphere, more intense and faster is the corrosion occurring at anodic area because at anode oxidation takes place and electrons are liberated. At the cathode these electrons are consumed. When anode is smaller and cathode region is larger all the liberated electrons at anode are rapidly consumed. This process makes the anodic reaction to takes place at its maximum rate thus increasing the corrosion rate. If the cathode is smaller and reverse process takes place decrease rate of corrosion.
For egs: If tin (Sn) coated on iron (Fe) and in that some area are not covered or some pin holes are left, there forms smaller anodic area and larger cathodic area because tin is cathodic with respect to iron so intense localized corrosion takes place. On the other hand if Zn coated to Fe then if there are some pin holes are there creates larger anodic area and smaller cathodic area because Fe is cathodic with respect to zinc so that rate of corrosion is very less.

4. Hydrogen over voltage:

A metal with low hydrogen over voltage on its surface is more susceptible for corrosion. When the cathodic reaction is hydrogen evolution type with low hydrogen over voltage, liberation of H2 gas is more easier so that cathodic reaction is very fast, that makes anodic reaction faster hence overall corrosion process is very fast. If the H2 over voltage is high so cathodic reaction is slow hence corrosion reaction also slower.

 

Factors related to corrosive environment:

1. pH of the medium:  Usually higher acidic nature (low pH) higher is the rate of corrosion. If the pH is greater than 10 corrosion of iron is very less due to the formation of protective coating of hydrous oxides of iron.

 If pH is between 10 and 3, then presence of oxygen is essential for corrosion   of iron. If the pH is 3 or lower than 3, severe corrosion occurs in the absence of air due to the continuous evolution of H2 at cathode. However metals like Al, Zn etc undergo fast corrosion in highly alkaline medium.

1. Temperature: On increasing the temp. Rate of corrosion process also gets increases because on increase of temp. Conductance of the aqueous medium increases hence rate of diffusion also.

In some cases on rise in temp. Decrease the passivity, which again leads to increase in the corrosion rate.

1. Polarisation at anodic and cathodic area: Polarisation of cathode or anode decreases the rate of corrosion. If anodic polarization takes place due to some reaction, then tendency of metal to undergo oxidation decreases hence dissolution of metals as metal ion decreases. This is usually due to increase in conc. Of ions of the dissolved metals in the vicinity of electrode or also due to the anodic passivity. Cathode polarization decreases the cathodic reaction hence hindering the combination of cathode reactant and electron. For the corrosion to continue both anodic and cathodic reaction should take place simultaneously if any one reaction is slower then the rate of corrosion is slower. Use of depolarizers reduces the polarization effect hence the rate of corrosion reaction increases.

CORROSION CONTROL: Corrosion of a metal is a natural spontaneous process, by which metal is converted into a more stable compound so that corrosion control is more realistic than corrosion prevention. In general preventing the formation of galvanic cells can control corrosion.
The methods used to control corrosion are as follows

1. Protective coatings:

Corrosion of metal can be controlled by isolating them from the corrosive atmosphere. This can be done by covering the metal (base metal) with a layer of another metal. This process is known as metal coating.
The Principal type of coatings applied on the metal surface are
1. Metal coating 2. Inorganic coating 3. Organic coating.     

 

Metal coating: This coating is the deposition of a protective metal over the surface of the base metal. The method can be applied by electrodeposition, flame spraying, cladding, hot dipping etc.
On the basis of coating there are two classes:

1. Anodic coating: It is produced by coating a base metal with more active metal which are more anodic with to the base metal for egs: Iron is coated with Zn, Mg, Al etc.,

One of the important properties of this type of coating are that, even if the coating is ruptured, the base metal does not undergo corrosion. The exposed part of the base metal is cathodic with respect to the coating metal and coating metal only undergone corrosion there by protecting the base metal. The protection is there as long as coating is there. Galvanisation is one of the best egs in anodic coating.

Galvanisation: It is a process of coating a base metal (iron) with zinc(Zn) metal. This process usually carried out by hot dipping method.
Process: first the base metal surface is washed properly with organic solvents to remove any organic matter (like oil, grease etc) on the surface afterwards it washed with dil. H2SO4 to remove any inorganic matter (like rust). Finally the base metal is well washed with water and air-dried. The base metal then dipped in a bath of molten zinc maintained at 425-4300c and covered with a flux of NH4Cl to prevent the oxidation of molten zinc. Then excess zinc on the surface is removed by passing through a pair of hot rollers so that a proper thin coating is obtained.
Application: Galvanized articles are mainly used in roofing sheets, fencing wire, buckets, bolts nuts, pipes and tubes etc. but galvanized articles are not used for preparing and storing food stuffs. Since zinc dissolves in dil. Acids and become toxic.

2. Cathodic coating: These are the coating produced by coating a base metal with more cathodic (noble metal) for egs: iron is coated with tin, nickel, Cr and Cu. But these coatings provides protection only when it is undamaged and absolutely free from gaps otherwise rapid corrosion of the base metal takes place as a result the formation of large cathodic and small anodic area.

Tinning is the best egs to explain cathodic coating.

Tinning: It is a process of coating base metal (iron) with tin (Sn). It can be carried out by hot dipping method. 
The iron sheet (base metal) first washed thoroughly with organic solvents to remove any organic matters. Then treated with dil. H2SO4, to remove rust and seal deposits, finally it is washed well with water and air-dried. It is then passed through ZnCl2 and NH4Cl flux (molten) so that molten tin can adheres properly on the metal surface then base metal passed through tank that contains molten tin.
Finally passed through a series of rollers immersed in palm oil. So that uniform, undamaged, continuous deposit of tin takes place. Tinning will provide complete protection against corrosion if it covers the surface completely.
Application: tinned articles are largely used in the manufacturing of containers used for storing foodstuffs, copper utensils are coated with tin so that contamination of food with copper can be prevented.

Inorganic coating or surface conversion coating or chemical conversion coating:

Here a surface layer of the metal is converted into a stable compound by chemical or electrochemical reactions, which forms a barrier between the base metal and corrosion environment. These types of coating are different compared to that of metal coating because they are integral part of the metal itself and in addition to corrosion resistance they also provides increased electrical insulation and also physical appearance of the metal. In surface conversion coating two important type are

  1. Anodized coating or anodizing:
Anodizing usually  carried out in non-ferrous metals like Al, Mg, Cr, Ni, etc or their alloys by anodic oxidation process in which base metal is made as anode, in an electrolytic bath of suitable composition and by passing direct current. However commercially only Al and its alloy are anodized for corrosion resistance.
Process: The article to be anodized is degreased properly and polished. Then made as anode and copper or lead is made as cathode. The electrolyte consists of 5- 10 % chromic acid, the temperature is about 350c.  The voltage is programmed to increase from 0-50V (optimum 40v)
So as to maintain an anode current density of 10-20milli amp/cm2. In first ten minutes the potential is increased from 0-40V and at 40V continue anodizing for about 20min after wards the potential is increased from 40 V- 50V and maintained at 50vc for 5min an opaque oxide layer of 2-8micrometer thickness is obtained. If higher thickness is required 10% sulphuric acid is used as an electrolyte at 220c with around 160milliamp/cm2 of current density at 24V a colorless 25micrometer thickness layer is obtained. Afterwards the article should be dyed properly and finally treated with boil water containing cobalt or nickel sulphate or Acetate to improve corrosion resistance.

Phosphating: It is generally obtained on steel surface by converting surface metal atoms into their phosphate by chemical or electrochemical process.
The phosphating bath containing three essential components 1. Free phosphoric acid 2. Primary phosphate like Fe, Mn, or Mg phosphate. 3. Accelerator such as nitrates, nitrites hydrogen peroxide and PH usually in the range of 1.8-3.2 if the process is immersion type.  If the process is spraying type then PH is in an around 3.2 to 7.8 and temperature maintained at 35oc.

            The mechanism of phosphating involves following steps.

1. First dissolution of the metal as metal ions.
2. Metal ions reacting with phosphate ions to form a metal phosphate.
3. Deposition of the metal phosphate on the surface of the metal.

Phosphating is always given as underline (under coating) before the finishing, because phosphating along with corrosion resistance it also imparts the surface a good paint adhesion quality.

Application:  An undercoating before painting of Automotive bodies, refrigerators, washing machines etc.
One of most important application of phosphating is of galvanized iron which is otherwise difficult to paint.

ORGANIC COATING:

            Coating of inert organic materials like paints, and lacquers on metallic surface to protect the metal usually thickness of the organic coating is less than 0.4mm thickness.

     The functions of organic coating are:

1. The organic coating serves as a barrier between the metal surface and corrosive environment.
2. The pigment or drying oils present in the paint often exert and inhibitive action by electrochemical and other means.

                 There are certain requirements to have a good organic coating they are:

1. The organic coating should adhere tenaciously to the metal surface and should improve its physical appearance.
2. The film formed should be uniform, continuous and act as a barrier to air and water.
3. It should chemically increase, so that atmosphere pollution are not there.
4. Should have reasonably long life.
5. Should easily available, low cost and easy and properly applicable.

               
The performance of paint depends on the technique employed during application method.
       The application of organic coating involves following steps

1. Surface preparation:  It includes degreasing the surface and removing rust and scale from it.
2. Priming:  This is the first coat of primer like phosphate coating which must strongly adhere to the surface so that painting can properly done.
3. Filling:  Fillers like nitrocellulose epoxides etc. are applied on well-dried surface in order to improve external appeaence.
4. Sanding:  The roughness and irregularities on the surface are smoothened by means of emery paper.
5. Final finishing coat of organic paints of two or three coat.

                
Organic coating usually applied such as brushing, spraying, dipping etc. Break of organic coating causes severe corrosion.

        CORROSION INHIBITORS:

Using certain chemical substance known as inhibitors can reduce rate of corrosion.  Inhibitors slow down the anodic or cathodic reactions by forming a protective film on these regions inhibitors mostly used to provide protection to systems in which corrosion environment re-circulated or confined for a long time like IC engine, re-circulated water system etc.

      ANODIC INHIBITORS:

These include oxidizing agents like chromates, molybdates, tungstates and nitrates.  These anions react with metal ions formed at anode during oxidation reaction forming sparingly soluble respective salts.  These compound are deposited on the anodic sites forming a protective films so that further preventing the anodic reactions.
These inhibitors are found to be effective only when sufficient amount of inhibitors is added into corrosion medium.  So that entire anodic surface can covered other wise intense corrosion takes place.

ii) CATHODIC INHIBITIORS:

These can act by inhibiting the cathodic reaction, which involves liberation of hydrogen or absorption of oxygen.
If the cathodic reaction is liberation of H2 type then by adding organic inhibitors like amines, Urea, thiourea, heterocyclic compound.  They form a protective layer on cathodic region so that evolution of hydrogen gets retarded.  Evolution of H2 can also prevent by increasing hydrogen over voltage.
This can be achieved by adding oxides of arsenic or antimony.

If the cathodic reaction is oxygen absorption type then by adding certain oxidizing agent like N2H4 (hydrazine), Sodium sulphite, these compounds absorb oxygen as

       
        N2H4 + O2                N2 + 2H2O

      2Na2SO3 + O2                        2Na2SO4

So that cathodic reaction is retarded otherwise by adding inorganic inhibitors like sulphates of Zinc, Mg, Mn etc. They reacts with OH- ions (which are liberated at cathode) forming insoluble hydroxides which form a protective film over cathodic area and hence corrosion rate reduced. Corrosion inhibitors have certain limitations
1. They contaminate environment
2. Many inhibitors are toxic in nature.
3. It can be used only in   closed systems
4. Generally inhibitors lose their efficiencies as conc. And temperature increases.

Cathodic protection:
It is a method of protecting the metal or alloy from corrosion and no part of it is allowed to act as anode.
The technique of offering cathodic protection to a specimen (metal) against corrosion by providing electrons from an external source.
There are two methods for providing electrons for cathodic reactions there are

1. Sacrificial anode method:
The method involves the use of more active metals as sacrificial anode in contact with specimen (like iron, copper or brass). The active metals like Zn, Mg, Al, and their alloy acts as an auxiliary anode, and undergoes preferential corrosion protecting the metal structure. Here the anode metals are sacrificed to protect the metal, the method is known as sacrificial anode method, exhausted anodes are replaced by new ones as and when required.
Egs: 1. A Mg or Zn block connected to a buried oil tank
2. Mg bars are connected to ocean going ships

Sacrificial anode methods are simple with low installation cost and do not require power supply but involves recurring expenditure of replacement of consumed anodes.
2. Impressed current method:
This is another method to provide cathodic protection by supplying electrons. These can be provided by a source of direct current.
The structure to be protected should be made negative (connected to negative terminal of a D.C source). Resin bonded graphite rod, platinised Ti, Pb are used as anode and connected to the positive terminal. The main structure being cathode does not undergo corrosion, and anode being inert remains unaffected.    

.

This technique is used to protect marine structure, water storage tanks and other gas or oil pipelines. This method is simple, can protect large metal area with low maintenance cost but expensive, because it needs high current.

Anodic protection:
Certain metals like Ti and alloy like stainless steel, which readily get passivated so that cathodic protection cannot be offered in such cases corrosion process, can be slowed down by use of anodic current. BY passing anodic current in these metals an oxide layer will grow and that oxide layer will protect the metals. The potential that is required to grow the oxide film and to protect the metal can be obtained from potential-current curve.

At a predetermined range of applied potential the changes observed in the potential and the corresponding changes in current are studied. In initial stages (AB) the current increases on increasing potential indicates the dissolution of metal so corrosion takes place. When the current reaches a critical point, (Icrit ), passivation, that is due to development of oxide layer sets in and that potential  ( Ep) is called passivating potential (Ep). Above Ep along BC current flow decreases to a very small value called passivating current ( Ip). It is the minimum protective current density to maintain passivation. At this point (C ) an increase in potential will not corrode the metal since the metal is in highly passive state. The small current (Ip) is sufficient to maintain the passivity so that corrosion rate brings down.
The anodic protection to a structure is applied by using device called potentiostat. It is an electronic device that maintains a metal at a constant potential with respect to a reference auxiliary Pt electrode and a reference calomel electrode.

The metal structure kept in a suitable oxidizing atmosphere acts as the working electrode ( anode). The potential is then slightly increased to Ep (passivating potential) for the initial corrosion to start. The potential is then slightly increased till the current decreases to very small value (Tp) indicating passivation of metal.  Now the potential is kept at constant Ep and the current is maintained at Ip.

            This method has an advantage as it requires very small current, but this method can be adopted to only passive metals like Ti, Ta,Al,Cr etc., and also by using this method corrosion rate cannot be reduced to zero.

            This method usually applied in the transportation of acid.