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  • 1. Engineering Materials Corrosion  It is important to study corrosion as its effect is primarily damaging to materials.  Corrosion may be defined as – Destruction of material by chemical, electrochemical or metallurgical interaction between the environment and material. Types of corrosion:- 1) Direct corrosion. 2) Electrochemical Corrosion.(Wet corrosion) 3) Corrosion of metal by liquids- Galvanic Corrosion. 4) Corrosion of a metal by gas.(Dry corrosion) 5) Special corrosion types. 1. Direct corrosion  It is ordinary chemical attack by corrosive solution on a metal.  Acid Pickling used to clean steel surfaces is an example of direct corrosion.  The reaction below shows direct corrosion- Fe + 2H+ =Fe++ + H2 (gas) The reaction describes the direct attack of iron by hydrogen ions in the acid pickling (eg 5-10% H2SO4) of steel  In acid solution, metal surface dissolves uniformly.  Rate of direct corrosion is relatively high as compared with the other types of corrosion.  Direct corrosion can be controlled by suitable addition of inhibitors (chemical compounds) to the corroding medium. 2nd year diploma unaided Page 1
  • 2. Engineering Materials 2. Electrochemical Corrosion:-  When corrosion takes the form of chemical reaction in conjunction with electrolysis.  Factors governing electrochemical corrosion- 1. Existing potential difference between different parts of same metal (because of difference in microstructure or composition). 2. Presence of electrolyte. Electrolyte is any solution that contains ions. Ions are electrically charged atoms (group of atoms) Electrolyte can be plain water, salt water, and alkaline solutions. 3. Completion of closed circuit (there must be two electrodes-an anode and a cathode which may be two different kinds of metals say Fe, Cu. Eg, action of hydrochloric acid solution on piece of iron  Numerous tiny anode(+) and cathode(-) area get formed on surface of iron, owing to surface imperfections, localized stresses, grain orientations, inclusions in metal, variations in environment.  At anode, +ve charged iron atoms detach themselves from solid surface and enter electrolyte solution as positive ions(Fe++ is shown) while –ve charged 2nd year diploma unaided Page 2
  • 3. Engineering Materials electrons(e) that are released pass round the external circuit to cathode, constituting current Fe Fe++ + 2electrons (e)  Iron passes and thus dissolves into solution whereas free electrons on reaching cathode, meet and neutralize some positively charged hydrogen ions become neutral atoms and these atoms combine to form (molecular) hydrogen gas (H2) 2H+ + 2e H2 (form bubbles at cathode surface)  Corrosion continues till layer on hydrogen on cathode surface slows down electrochemical reaction.  Also oxygen in electrolyte can react with accumulated hydrogen to form water and thus permitting corrosion to proceed. 3. Galvanic corrosion:-  Anode metal is made to dissolve or corrode continuously.  Two dissimilar metals are in electrical contact with each other and exposed to electrolyte.  Zn (anode) and Cu (Cathode).  Both are dipped in the electrolyte.  As two electrodes are joined by conductor, electronic current flows from anode (Zn) through conductor to Cu (Cathode).  At anode excess of electrons are removed, so more metal atoms to be oxidized and go into solution.  At cathode more electrons are added that are intercepted by positive ions deposited there.  Current flows at expense of anode metal which is corroded continuously. 2nd year diploma unaided Page 3
  • 4. Engineering Materials 4. Corrosion by gas:-  Usually occurs when the metals are exposed to gaseous environments such as oxygen, hydrogen, nitrogen, halogen, etc.  The gas molecules are absorbed on the surface of the metal and they react with surface atoms of the metal.  The products of corrosion form the thin layer or film on the metal surface.  The metal gas corrosion increases rapidly at high temperatures.  The oxidation of iron and steel is a good example of high temperature corrosion. 2Fe++ + O2 2FeO 5. Special corrosion types- Some specific corrosion types are as below:- I).Uniform corrosion:-  When entire surface of metal is attacked to same degree, it is uniform corrosion.  Uniform corrosion is unusual in metals because their structure is homogeneous and so the surface will not corrode uniformly.  It is likely to take place in impure metals or alloyed metals.  Eg of uniform corrosion is Uniform Dezincification 2nd year diploma unaided Page 4
  • 5. Engineering Materials  Dezincification in corrosion means removal of any specific element from an alloy Eg zinc (a specific element) is removed from any metallic alloy. II).Atmospheric Corrosion:-  It is very frequent on ferrous materials.  Humid atmosphere is mainly responsible for this type of corrosion.  It follows oxygen absorption mechanism. III).Pitting corrosion:-  It is a non uniform corrosion.  Deep isolated holes or pits are formed on metal because of inhomogeneities in metal like inclusions, coring (difference in composition within an individual grain or from one part of casting to another part), pores, blowholes etc.  These pits accelerate corrosion process.  Pitting corrosion results from electrochemical reaction.  Pitting corrosion occurs when there is break in protecting layer Eg- chromium plate (film) on steel breaks then the point of film breakage becomes anode and surrounding unbroken film as cathode. IV).Intergranular corrosion:-  It is a non uniform corrosion.  In this type of corrosion, metal corrodes at grain boundaries.  Intergranular corrosion occurs due to existence of potential difference between grain boundaries and rest of the metal or alloy. 2nd year diploma unaided Page 5
  • 6. Engineering Materials  Consider a solid solution eg Brass.  Brass is a solid solution of Zn and Cu.  In this solid solution, a phase precipitates at the grain boundary.  When a phase precipitates, material (i.e. grains) in vicinity (nearer) to the grain boundary becomes depleted of dissolved element thus creating potential difference between grain boundaries and rest of the alloy (grains).  Being a high energy region, grain boundary becomes anodic due to low electrode potential than the grains.  Hence grain boundaries being anode, corrosion occurs at the grain boundary and then penetrates further into the body of alloy.  Zinc content in brass is higher at grain boundary and corrosion in brass will start by corroding of anode i.e. zinc.  Intergranular corrosion is prevented by avoiding the precipitation of phases at the boundaries and this is done by fast cooling of the heated alloy.  Fast cooling of the heated alloy will avoid phase precipitation at the boundaries while slow cooling of the alloy allows the phase precipitation at the boundaries giving rise to Intergranular corrosion. V) Stress corrosion:-  In cold working of steel, during plastic deformation of metal, dislocations try to move from one grain to another through grain boundary Fig.  These grain boundaries behave as an obstacle to the movement of dislocation. 2nd year diploma unaided Page 6
  • 7. Engineering Materials  Force given during plastic deformation is consumed by this dislocation movement and finally the dislocations pile up (get placed one by one nearer to each other) at the grain boundary.  Hence in cold worked stressed metals, pile up dislocations at grain boundaries and other points increases energy in those regions so that they become sufficiently anodic to the rest of the structure.  Therefore corrosion takes place in these regions of high energy and stresses locked up (in metal), give rise to formation of cracks which grow with corrosion constituent.  Cracks may be transgranular (in grains) or Intergranular (along grain boundaries) which rise and proceed stress corrosion further. VI) Corrosion Fatigue:-  It is combined action of corrosion and repeated loading (stresses) and this is much more serious than the sum of these 2 factors acting individually.  Action of corrosive medium will tend to be concentrated at any surface flaw or irregularity and behaves as a focal point for the initiation of the fatigue crack.  Once the crack has been formed, it will spread rapidly as a result of corrosive action combined with alternating loading which tends to break any film that may form on the surface. VII) Fretting corrosion:-  Fretting simply means -To produce a hole or worn spot in; corrode  Fretting corrosion is similar to the Corrosion fatigue and occurs mainly at the close fitting machine parts which are subjected to vibrational stresses and corrosive atmosphere.  This type of corrosion is mainly seen in splines, keyways, bearings etc which fit machine parts into it or with it.  These parts constantly deal with vibrational stresses and subject to corrosive atmosphere which proceeds corrosion. 2nd year diploma unaided Page 7
  • 8. Engineering Materials VIII) Erosion corrosion (or Impingement corrosion.)  Erosion corrosion refers to combined effects of – 1) Mechanical abrasion on metal surface caused by impingement (striking) of air bubbles, abrasive particles remaining in the liquid coming in contact with the metal. 2) Chemical corrosion on metallic surface  Erosion corrosion is caused by breakdown of the protective film at the spot of impingement of bubbles and particles, which ultimately forms localized pits and holes forming anodic points over there.  The place where the scale has been broken forms anode whereas the unbroken protective film forms anode and hence corrosion proceeds as same as in chemical corrosion.  Erosion corrosion takes place in pump mechanisms, turbines, condenser tubes and pipings and tubes carrying sea water. IX) Cavitation corrosion:-  When a fluid flows in a solid body it often changes pressure of its flow.  Due to such change in pressure, bubbles or cavities form and collapse.  Bubbles collapsing near a solid surface exert an impact force on the surface like that of many small hammer blows.  This action gradually removes particles of the metal surface forming pits, holes marks.  When corroding liquid comes in contact with these pits, the result is corrosion.  This type of corrosion is cavitation corrosion occurring due to cavities (bubbles). 2nd year diploma unaided Page 8
  • 9. Engineering Materials X) Crevice corrosion:-  Crevice means- a narrow crack or opening.  Crevice corrosion means an accelerated attack at the junction of two metals exposed to corrosive environment.  If some crevices (opening or crack) is remaining between two dissimilar metals in contact with each other then corrosion is more likely to occur in such crevices which collect corrosive solutions and take long time to dry out.  Hence corrosion proceeds due to variation of oxygen concentration between the joint and outside the joint of dissimilar metals.  Crevice corrosion occurs in a low oxygen concentration area.  Fig. Corrosion Control Different corrosion control techniques are- 1) Design against corrosion:-  Proper design should permit least contact between structure and corroding agent.  Joints should be such that liquid does not get a chance to enter and stay there. 2nd year diploma unaided Page 9
  • 10. Engineering Materials  Use of dissimilar metal contacts should be avoided where presence of electrolyte may result in galvanic corrosion.  Insulation like rubber or plastic should be used to separate two dissimilar metals like Cu and Al and avoid galvanic corrosion. Fig:-  Anodic material should have as large area as possible whereas cathodic material should be of smaller area.  Structure should be stress free.  Structure should have good surface finish.  Sharp corners, pockets, slots should be avoided as they carry on corrosion process.  Avoid dirt, dust, sediments to come in contact of surface of metal 2. Use of high purity metals:-  High purity metals tend to reduce pitting corrosion as pure metals are not non-homogenous in structure and non homogeneity is the main reason for pitting corrosion.  Eg pure aluminum suffers less from pitting corrosion compared to Al alloys.  Impurities in metals reduce corrosion resistance. 3. Use of corrosion resistance alloys:-  Though pure metals are resistant to corrosion their strength is less. 2nd year diploma unaided Page 10
  • 11. Engineering Materials  To increase strength and corrosion resistance use of suitable alloying elements can be done.  Eg 10% of aluminum with iron is extremely resistant to high temperature oxidation corrosion. 4. Use of heat treatment methods:-  Heat treatment of metal leads to homogenization of solid solutions in cast alloys and hence improves corrosion resistance.  Heat treatment processes are widely used to reduce stress corrosion. 5. Cathodic protection  This method completely prevents corrosion.  No other corrosion control method except this method completely prevents corrosion.  This is accomplished by placing anode (eg Zn, Al, Mg) in contact with metal to be protected (eg steel).  Hence anode will itself get corroded and will cathodically protect the metal (steel) i.e. surface to be protected will behave as cathodic material.  This is done in underground pipelines, boilers, water heaters, elevated storage tanks. 6. Use of Inhibitors:-  Inhibitors are (chemical) compounds added to an electrolyte which protectively coat the anode or cathode and stops corrosion.  Various inhibitors are anodic inhibitors, cathodic inhibitors etc.  Anodic inhibitors suppress anodic reactions and stop metal dissolution. 2nd year diploma unaided Page 11
  • 12. Engineering Materials Eg of anodic inhibitors are all oxidizing substances eg phosphates, chromates, etc for protection of iron and steel.  Cathodic inhibitors prevent evolution of hydrogen and oxygen absorption at cathodic areas Eg of cathodic inhibitors are calcium bicarbonates, magnesium, nickel and zinc salts. 7. Environment control:-  By modifying the environment we can prevent corrosion.  This can be done by (1). Using vacuum instead of a corrosive atmosphere i.e. remove corrosive constituent (2).by using inhibitors  Slight decrease in temperature of corroding medium decreases amount of corrosion.  Changes in chemical composition of corroding medium may have great effect on corrosion behavior eg 1. Introducing inhibitors 2. Removing dissolved oxygen from corroding medium such as water.  Protective atmosphere such as an inert gas like helium, hydrogen etc have been used to stop high temperature (oxidation) corrosion of ferrous metals and alloys.  Humidity should be reduced. 8. Use of protective coatings:-  Protective coating is a layer of Inert substance which does not permit environment to penetrate into base metal which is to be protected.  Protective coatings include • Use of salt and oxide films • Metallic coatings like Zn, Cu, Cr etc 2nd year diploma unaided Page 12
  • 13. Engineering Materials • Paints(organic coatings) • Ceramic(enamel) coatings  Hence types of protective coatings are- 1. Metallic coatings 2. Inorganic coatings 3. Organic coatings  Metallic coatings are of Zn, Cu, Cr etc on metal to be protected.  Paints, varnishes, emulsions, enamel, are all organic coatings, mixture of oil and a pigment which provides protective film on base metal surface. 2nd year diploma unaided Page 13
  • 14. Engineering Materials 2nd year diploma unaided Page 14