Corrosion :-(def)

Corrosion is the process of gradual deterioration of metal from its surface due to unwanted chemical or electro chemical reaction of metal with its environment .

Classification :-

Corrosion is classified depending upon the Nature of corrodent,Mechanism of corrosion and Appearence ofCorroaded products

Nature of corrodent :- depending upon thenature of the corrodent Corrosion is classified into two types : (1) Dry Corrosion (2) WetCorrosion

Mechanism of corrosion :- depending upon the mechanism of the corrosion corrosion is classified into two types : (1) Direct chemicalAttack (2) Indirect electro chemical Attack

THEORIES OF CORROSION :

(1) Acid Theory : Carbonic acid(H2CO3) is essential corrosion

REACTION :

Fe(iron) + atm. o (atm.oxygen) + H2O + CO2 ----> Fe(HCO3)2

On OXIDATION of Fe(HCO3)2 :-

2Fe(HCO3)2 + H2O + O2 ------> 2Fe(OH)CO3 + 2H2O + 2CO2

Fe(OH)CO3 + H2O ----> Fe(OH)3 + CO2

(2) Direct Chemical Attack : (Dry corrosion)

Direct chemical attack, or pure chemical corrosion, is an attack resulting from a direct exposure of a bare surface to coustic liquid or gasous agents.

----------> Corrosion by gases : A metal gets Corroaded when they are exposed to gases like atmospheric oxygen , halogens , s02 , H2S, at higher or lower temperatures . alkali metals and alkaline earth metals at lower temperatures and all metals at higher temperatures .

Example : when silver (Ag) metal is exposed to chlorine (Cl2) gas then it gets corroaded and

AgCl is formed .

(3) Electro Chemical Theory / wet corrosion :

Metal corrosion occurs when metal is in contact with conducting liquids when 2 dissimilar metals or alloys are dipped or immersed partially in conducting liquids . It occurs mainly in existance of separate Anode and Cathode areas . At anode oxidation reaction takes place and at Cathode Reduction reaction takes place .

At cathode the electrons are exited in 3 different ways :-

(1) evolution of hydrogen

(2) absorption of oxygen

(3) electro plating

(1) Evolution of Hydrogen : A metal gets corroaded when a metal is placed in an or Exposed to an Acidic Environment by evoloving a Hydrogen atom. Here Metal acts as Anode and the acidic environment acts as Cathode

REACTIONS:

At cathode : Fe ------> Fe(2+) + 2electrons

At Anode : 2H(+) + 2 electrons -------> H2

(2) Absorption of Oxygen : A metal gets corroaded when a metal rod is dipped in dilute solutions like dil.NaCl in a container . A water line is formed in the container . The part of a Metal rod above the water line acts as Cathode because of presence of higher oxygen and the part of a metal rod below water line acts as anode due to presence of low oxygen .

REACTIONS :

Anodic Area reaction :-

oxidation : Fe -------> Fe(2+) + 2 electrons

Cathodic Area reaction :-

reduction: 1/2 O2+H2O+2electrons ---> 2oH(-)

(3) Electro Plating : It is the process of deposition of coating metal on the surface of base metal by flowing the direct current through electrolytic solution consisting soluble salts of the coating metal .

REACTIONS :

Anodic Area reaction :-

oxidation : Fe ----> Fe(2+) + 2 electrons

Cathodic Area reaction :-

reduction: 1/2 O2+H2O+2 electrons ---> 2oH(-)

Types of Wet Corrosion :

(1) Galvanic Corrosion : This corrosion occurs when dissimilar cells are connected and dipped in an electrolytic solution .

Gavanic cells : Cells used for generation of electricity from chemical reactions are called as Galvanic cells or Voltaic cells

Galvanic Couples : (1) zinc & copper (2) zinc & silver (3) iron & copper

(2) Pitting Corrosion : is a form of extremely localized corrosion that leads to the creation of small holes in the metal. The driving power for pitting corrosion is the lack of oxygen around a small area. This area becomes anodic while the area with excess of oxygen becomescathodic, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with limited diffusion of ions, further pronouncing the localized lack of oxygen. The mechanism of pitting corrosion is probably the same as crevice corrosion.

REACTIONS :

Oxidation : M(metal) -------> M(2+)+2electrons

Note: here the metal taken is IRON(Fe)

Reduction : 1/ 2 O2+ H2O+2electrons ---> 2oH(-)

Total Reaction : Fe(2+) + 2oH(-) --->Fe(OH2) ---> Fe(OH3)

(3) Crevice Corrosion : Crevice corrosion is a localized form of corrosion occurring in spaces to which the access of the working fluid from the environment is limited and a concentration cell, areas with different oxygen concentration, will take place with consequent high corrosion rate . These spaces are generally called crevices. Examples of crevices are gaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled with deposits and under sludge piles.

(4) Differential Aeration Corrosion :

(or)

Concentration cell Corrosion :

This type of corrosion is caused by anelectrochemical corrosion cell. The potential difference (electromotive force) is causedby a difference in concentration of some component in the electrolyte. Any difference inthe electrolyte contacting the metal forms discrete anode and cathode regions in themetal. Any metal exposed to an electrolyte exhibits a measurable potential or voltage.The same metal has a different electrical potential in different electrolytes, orelectrolytes with different concentrations of any component. This potential differenceforces the metal to develop anodic and cathodic regions. When there is also anelectrolyte and a metallic path, the circuit is complete, current flows, andelectrochemical corrosion will occur.

(OR)

Concentration cell corrosion is corrosion that is accelerated by differences in environment between separated areas on a single metal.

(5) Water Line corrosion : Waterline Corrosion is a form of galvanic corrosion taking place at the surface of a liquid in which the stainless steel is partially immersed.

REACTIONS :

Oxidation : M(metal) -----> M(2+) + 2electrons

Reduction : 1/2 O2+H2O+2electrons ---> 2oH(-)

(6) Underground / Soil Corrosion : This type of corrosion generally occurs in the metal pipes which are buried under the soils . Due to the change in the types of soil the corrosion occurs in the buried pipe . Generally this corrosion occurs in Iron pipes and some other metal rod pipes .

Example: consider a buried Iron pipe as shown in below fig., Here the part of pipe which is under clay soil acts as anode due to the presence of less oxygen (coz the distance b/w the molecules of clay soil is too less) and the part which is under the sand soil acts as cathode due to presence of more oxygen . The corrosion occurs in the anode part

Reactions:

oxidation : Fe ----> Fe(2+) + 2 electrons
reduction: 1/2 O2+H2O+2 electrons ---> 2oH(-)

(7) Passivity/Passivation : It is a phenomenon by which metals or alloys shows highly resistance to corrosion because of formation of protective layer on the surface of metals . Here As there is a protective layer formed it fights or resists with corrosion and help the metal from corrosion . Some metals which show passivity are Nickel(Ni) , Titanium(Ti) , Chromium(Cr) , Aluminum (Al) etc.., these are some of the metals which shows the phenomenon of Passivity .

(8) Stress corrosion/Cracking : The combined effect of static tensil stress and corrosive environment on metal causes Stress corrosion or stress cracking .

Stress corrosive agents are highly specific and selective , Those are :-

1) Caustic alkalies and strong Nitrate solutiuon for mild steel .

2) Traces of Ammonia for Brasses .

3) Acid chloride solution for Stainless steel

Exapmles:

A) Season Cracking : Season crackin is common in copper alloys mainly brasses . When brasses are exposed to Ammonia solution . Both copper and Zinc are electro chemically very active in ammonia solution due to formation of stable complex ions like copper ammonia , zinc ammonia . this is cause of dissolution of brass which ultimately results cracks in presence of high tensile stress .

B) Caustic Embrittilment : It is the phenomenon during which the boiler material become brittle due to the accumulation of caustic substances . This type of boiler corrosion is caused by the use of alkaline water in high pressure boilers sodium carbonate produce caustic substances .

Na2co3+ H2O ------> 2NaOH + CO2

This caustic water close into the minute hair cracks present in the inner side of boiler by capillary action of evaporation of water . The dissolved caustic soda concentration increases progressively which attacks the surrounding area there by dissolving iron of boiler as Sodium ferrate

2NaOH + Fe ----> Na2FeO2+H2

The sodium ferrate decomposes a short distance aware from this point of formation and NaOH is regenerated magnetite is precipitated there by enhancing further dissolution of iron .

3Na2FeO2+4H2O -----> 6NaOH+Fe3O4+H2

This causes embrittlement of boiler walls . More particularly stressed parts like bends . joints , rivets etc . causing failure of the boiler .

Preventive Measures :

1) Caustic embrittlement can be prevented by using sodium phosphate as softening reagent instead of sodium carbonate .

2) Adding tanning or libnin to boiler water which block the hair cracks in the boiler box .

Factors Influencing Corrosion :

A) Nature of the Metal :

1) Position in Galvanic series : When 2 metals or alloys are electrically connected and these are exposed to electrolyte the more active metal suffers from corrosion .

2) Over Voltage : Reduction in over voltage of the metal or alloy acceleratesthe corrosion rate .

Example : Zn occupies a high position in galvanic series is placed in one normal sulfuric acid . Hydrogen evolution takes place rate of corrosion is very slow because of over voltage of zinc(0.70 volts) . However if a few drops of cuso4 are added the corrosion rate of the metal is accelerated because of reduction in over voltage of zinc . i.e., 0.33volts .

3) purity of metal : Lesser is the percentage of purity of metal faster is the rate of corrosion .

4) Relative areas of Anodic & Cathodic parts : The rate of corrosion of metal is less when the area of the cathode is smaller . Then cathodic area is smaller . Then demand for electrons will be less and this results in the decreased rate of dissolution of metal at anodic region .

5) Physical state of the metal : It means orientation of crystals , grain size etc ., . The grain size of the metal or alloy are larger its solubility and corrosion rate is lesser .

6) Nature of the surface field : If the metal is covered by protective film the metal protects the corrosion . If the metal is covered by non protective or volatile field the metal cannot protect the corrosion .

7) Specific volume ratio : It is the ratio of the volume of the metal oxides to the metal . greater the specific volume ratio lesser the oxidation corrosion rate .

8) Passive character of metal : Metals like titanium , aluminium , chromium , magnesium , nickel etc., are passive and they exhibit much higher corrosion resistance because of position in galvanic series and formation of protective layer on the surface of the layer .

9) Solubility of corrosion product : If solubility of corrosion product is more the rate of reaction is also more .

B) Nature of corrosive environment :

1) Temperature : As the temperature increases the rate of corrosion increases .

2) Humidity : With increse of HUmidity the rate of corrosion increases .

3) PH : Corrosion of those metals which are attacked by acids can be reduced by increase in the pH of the specific environment .

4) Presence of impurities in atmosphere : There are some corrosive gases in atmosphere which causes corrosion . the gases like H2SO4 , SO2 , CO2 , Halogens & cubes of sulfuric acid etc ., causes corrosion in metals .

5) Presence of suspended particles in atmosphere : If the metals are exposed to active particles like ammonium sulphate enhance the rate of corrosion . If these are exposed to inactive particles like charcoal it slowly enhances the rate of corrosion .

6) Formation of oxygen concentration cell : Anodic part having lesser oxygen concentration suffers corrosion and corrosion doesnot takes place at cathodic part due to the presence of high oxygen concentration .

Corrosion control methods :

1) Cathodic protection : Let us consider the corrosion of a metal in acidic environment , Metal undergoes corrosion by its oxidation . This reaction is simultaneously accompanied by reduction reaction .

REACTIONS:

Oxidation : M(metal) ----> M⁺²+2e^-(electrons)

Redution : 2H⁺+2e^-(electrons) --->H₂

As its clear from the above reaction we can prevent the dissoultion of metal by forcing the metal to behave like a cathode . This is the principle of Cathodic protection .

Types of Cathodic protection :

A) Sacrificial Anodic protection : The metal structure can be safe from corrosion by connecting it with a copper(Cu) wire to a more active metal so that all the corrosion is concentrated at the more active metal . as the more active metal is sacrificed in the process of saving metal from corrosion . Hence it is known as sacrificial anode .

Metals commonly used as sacrificial anodes are Magnesium , Zinc and thier alloys .

Zinc is used as sacrificial anode in good electrolytes such as sea water .

Magnesium is used as a sacrificial anode in high resistivity electrolytes such as soil .

Applications :

1) Protection of underground cables and pipe lines from soil corrosion .

2) Protection of ships and boats from marine corrosion sheets of Magnesium/Zinc are hung around the SHIP HULL . This sheets being anodic to iron they get corroded and then its consumed completely , those are replaced by the fresh ones

3)The formation of rusty water or water corrosion is prevented by insertion of Magnesium/Zinc sheets or rods into the domestic water tank .

B) Impressed current cathodic protection : In this method current from an external source is impressed (applied in the opposite direction) to nullify the corrosion current . This is done to convert corroding metal from anode to cathode . Once the metal become cathodic it is protected from corrosion . The anode may be either inert material or active material . The anodic materials are Platinum , Graphite , Stailess steel , Scrap iron . The anode is buried in Backfill such as Gypsum to increase the electrical contact between itself and surrounding soil .

Applications :

1)This protection method is useful when electrolyte resisitivity and current requirements are high . It is well suited for large structures and long term obligations such as buried pipe lines, tanks , marine pipes etc.,

2) Surface Coatings : Iron and steel are the most commonly used construction materials due to thier low cost , easy availability , ease of fabrication into desired structures . Iron and steel structures can be protected from corrosion by covering thier surfaces with metallic coatings . The metallic coatings often used are Zn , Al , Tin , Cu , Cd , Cr etc ., these coatings separate the base metal from corrosive environment and also functions as an effective barrier for the protection of base metal .

Types of Surface coatings :

A) Anodic coatings : In this coatings the metal which is used for coating is more anodic than the metal which is to be protected . Anodic coatings protect the underlying base metal sacrificially .

Example : coating of zinc on steel surface .

Let us discuss how galavanised steel is protected from corrosion . here Zn is coating metal and steel is base metal . If any cracks occurs in Zn films . Zn undergoes corrosion because of Zn occupies higher position than steel .

B) Cathode coatings : These coatings protects the underlying base metal due to thier noble character and high corrosion resistance , In such coatings effective protection is possible only when they are completely continuous and free from force , breaks . If the force , breaks are discontinuous occur in such coatings the corrosion of the base metals is speeded up . This is due to the fact that exposed metal acts as anode and coatings become cathode . A galavanic cell is setup and an intense localised at small exposed part occurs . This results in severe pitting and perforation of the base metal .

Methods for applications of Metallic coatings :

1) Hot dipping : It is the process of coating of low melting point metals on the surface of high melting pont materials like iron , steel . This is of two types :-

a) Galvanizing : It is the process of coating of Zinc on the surface of iron or steel . In this process iron or steel is first cleaned by acid picking with dil.H2SO4 for 15-20 minutes and 60-90° c . The article is then washed with water and dries in a dry chamber . It is then dipped in bath of molten Zinc at 425-430°c . The surface of the bath is kept covered with NH3Cl flux to prevent oxide formation . When the article is taken out , the article is coated with a thin layer of Zinc . It is then passed through a pair of hard rollers . This process removes any excess of Zinc on the articles

Click on the picture for a better view :-)

uses : It is widely used for protection of iron and steel from atmospheric corrosion in the form of roofing sheets , wires , pipes , screws , buckets , sheets . Zinc dissolved in dil.acids to form highly tonic compounds . Hence Galvanized utensil cannot be used for storing of food stuff , especially acid ones .

b)Tinning : It is coating of Tin over the iron or steel articles . In this process first treating steel sheet in Dil.H2SO4 to remove any oxide film after this it is passed through a bath of molten tin . It is covered by a Zncl2 flux . This flux helps the molten metal to adhere on the metal sheet and helps to prevent oxidation of molten tin . Finally the metal sheets passed through a series of rollers from underneath the surface of a layer of palm oil . the palm oil protect the hard tin coated surface against oxidation . The rollers remove any excess of tin and produce a tin film of uniform thickness on the steel sheet .

click on the picture for a better view :-)

Uses :

1) Tin coated iron or steel articles or utensils are used for storing and preparing of fodd stuffs .

2) It is most widely used for protection of iron or steel from atmospheric corrosion in the form of roofing sheets , pipes , etc .,

2) Metal cladding : It is a process of coating of brass metal on dense homogeneous layer of cladding material . cladding metals can be pure metals or alloys . In this process arranging Tin sheets of coating metal and the base metal sheet in the form of sandwich which are then passed through rollers under the action of heat and pressure

Example : Duralumin is sandwiched Between two layers of 99.5% pure aluminium . This sheet is called as Alclad used in Aircraft industry .

Electroplating : It is the process of coating of base metal by a dense homogeneous layer of a coating material by passing a direct current through electrolytic solution s consists of soluble

salts of coating metal .

Organic coatings : These are paints , varnishes ,Enamels and lackers .

functions : To impart decorative and aesthetic appeal to give resistance to corrosion .

Paints :

Requirements for a good paint :

1) It should be fluid enough to be spread on the metals surface easily .

2) It should form a quite tough uniform and adherent film .

3)It should have covering or hiding power .

4) It should not crack on drying .

5)It should be corrosion resistance .

6)It should give a glassy film .

Constituents of paints :

Pigments : The most commonly used pigments are yellow pigment like zinc yellow , Brown pigments like clays having iron , green pigments like chromium oxide , red pigments like red lead , black pigments like carbon black , white pigments like Zno2

FUNCTIONS :

1) To provide a desired colour , opacity and strength to the paint .

2) To give protection by reflecting UV light to improve the impermeability of paint film to moisture .

Vehicle or dry oil : It is a film forming constituent of the paint . The liquid portion of the paint in which the pigment is disturbed it is known as Vehicle or dry oil .

FUNCTIONS :

1) They hold the pigment to the metal surface and form a protective film .

2)They give water toughness and durability to the paint .

3)They give better adhesion to the metal surface .

TYPES :

1) Non drying oils - like sunflower oil , mustard oil .

2) semi-drying oils - like fish oil , roxin oil .

3) drying oils - like dehydrated castor oil and limb seed oil .

Tinners : It reduces the viscosity of the paint . They suspend the pigments in the paint . They dissolve the film forming materials . Examples: kerosene , mineral spirits , terpentine etc .,

Dryers : The drying of the oil film is accelerated by driers . They do this by oxidation , polymerisation and condensation .

TYPES :

1) Surface driers eg: Cobalt substances

2) Bottom driers eg: Lead substances

3)Through out driers eg: Manganese substances

Fillers/Extenders : They reduce the cost of the paint . They serve to fill the voids in the paint film . They improve the durability of the film by reducing the cracking of the paints after drying . Eg: CaCO3 , CaSO4 , Gypsum , Asbestos etc .,

Plastisizers : Give elasticity to the paint film and to prevent cracking of the film eg: Tri cresyle phosphate , Tri phenyl phosphate .

Anti skinning agents : This agent prevents the jelling and skinning of paint film . eg: Poly hydroxy phenols .