Corrosion Part-A PDF

On the eve of mother’s day, Chaandilyan presented a silver ornament to his mother Ramanichandran as a token of his love and affection. The lustrous white of silver ornament became black after the few years. Identify the reason for turning of lustrous white of silver into black and als...

On the eve of mother’s day, Chaandilyan presented a silver ornament to his mother Ramanichandran as a token of his love and affection. The lustrous white of silver ornament became black after the few years. Identify the reason for turning of lustrous white of silver into black and also explicate the chemical 1. reactions.  silver ornament undergoes the dry corrosion (or) chemical corrosion  silver reacts with hydrogen sulfide to form silver sulphide which is responsible for turning silver into black. 2Ag(s) + H2S (g) → Ag2S (s) (black) + H2 What changes are observed on copper when it is exposed to an environment of moisture and carbon dioxide?  Copper undergoes electrochemical corrosion. 2.  In moist air, Copper combines with water and carbon dioxide. The product of this reaction is called Hydrated copper carbonate Cu(OH)2.CuCO3. This compound has beautiful green color, called patina.  2Cu+O2 + H2O+CO2 → Cu(OH)2.CuCO3 Raja looked the Golden Lotus in the Madurai Meenakshi Amman temple pond. The golden flower is in contact with water and air, but it remains unaffected. Identify the reason behind this.  Golden lotus flower forms an unstable metal oxide layer on its surface when it reacts with oxygen.  The metal oxide formed decomposes to form metal and oxygen. So oxidation corrosion (dry corrosion) is not possible. Gold is a noble metal, so it remains 3. unaffected. Iron corrodes faster than aluminium, even though iron is placed below aluminium in the electrochemical series. Give reason.  Oxidation corrosion occurs on iron. Since, the volume of metal oxide layer formed on the iron rod is lesser than volume of iron, the oxide layer formed is porous and 4. non-protective (VMO < VM ). So, iron corrodes faster.  Whereas, the volume of oxide layer formed on aluminum is greater than the volume of aluminum, the oxide layer formed on the aluminum is non–porous and protective (VMO > VM = protective layer). Rusting of iron is faster in saline water than in ordinary water. Why? 5.   The conductance of the saline water is high (NaCl) than the ordinary water. Identify the suitable design which aviods the corrosion in the following examples and give reason. 6. a) b) c) d)  The suitable designs are (b) and (d).  The design (b) has the smooth curvature drains the liquid completely  The design (d) was properly welded at crevices  Anodic area should be larger than cathodic area to control corrosion. Give reason. 7.  The smaller the cathodic area will demand lesser electron from a larger area of anode. So the rate of corrosion is less. How much rust will be formed when 100kg of iron have completely rusted away?  The chemical formula of rust: Fe2O3 3H2O.  (At. Wt of Fe 55.85 = 56; 2 atoms of Fe = 2 x 56 = 112 grams of iron. 8.  Oxygen = 3 × 16.00 + 3 × (2 + 16.00) = 213.74 grams.  When 100 grams of iron turned to rust the mass of the type of rust described here is (100kg) × (213.74/111.69) = 191.36 kg. Galvanized containers are not used for storage of food products, but tin coated containers are used. Give reason.  Most of the foods are acidic in nature. When it is store in galvanized containers, 9. zinc present in the galvanized containers will dissolve in the food and food will become toxic.  So, food stuffs cannot be stored in galvanized container and tin coated iron containers are used to store food. The following figure illustrates how soil stratification could be responsible for a wet corrosion on a tower anchor shaft. In this illustration, the upper soil layer is loose and somewhat gravel, bottom soil is a dense, clay type soil. Identify the type of wet corrosion occurs in the portion of shaft near the dense clay. Give reason. 10.  Differential aeration corrosion occurs in the portion of shaft near the dense clay  Upper soil layer is a loose, somewhat gravelly soil, so, more aerated, and so acts as cathode.  Below a dense clay type soil is less aerated due to fine nature of clay, so acts as anode. So the portion of the shaft near the dense clay undergoes corrosion. Why can’t we use nickel spatula to stir a solution of CuSO4? 11.  Nickel is placed above copper in electrochemical series,  Nickel acts as anode which will undergoes corrosion Dissolved oxygen is of particular interest as they are responsible for rusting of iron. Recommend a suitable chemical to remove the dissolved oxygen from the 12. corrosive medium.  Sodium sulphite is the suitable substance which absorb oxygen Nail inside the wood undergoes corrosion. Give reason.  It is due to differential aeration corrosion. 13.  The reason is nail inside the wood is poorly oxygenated and acts as anode undergoes corrosion Choose the metal which would act as a cathodic protector to iron. 14. (a) Cu (b) Mg (c) Ni (d) Zn  Zn and Mg can serve as cathodic protector to iron. Why does corrosion occur in steel pipes connected to copper plumbing? 15.  Steel is placed in higher in galvanic series than the copper. Hence, copper acts as cathode and steel acts as anode which undergoes corrosion. A sheet of steel one meter wide by three meter long has lost 0.5 g to corrosion 16. over the past 6 hrs. Calculate the rate of corrosion. Given that k = 543 and density of steel is 7.86 g/cm3  mpy Which one (A or B) gives a higher corrosion rate ? and why ? Eo(Zn2+/Zn)=-0.76V and Eo(Fe2+/Fe)=-0.44V 17.  The design –A undergoes higher corrosion. Zn acts as anode and Fe acts as cathode.  The cathodic area of design A is high whereas the anodic area is small. So the larger cathode will demand more amount of electron from the smaller area of the anode. Hence anode undergoes corrosion. Identify the alloys used to fabricate the following components in nuclear power plant reactors to mitigate the corrosion. (i) Steam generators tubes (ii) Condenser tubes (normal and sea shore power plant) (iii) Cladding for fuel rods (iv) Reactor cooling pumps S.No Article Alloy 18. 1 Steam generators tubes nickel-chromium alloys 2 Condenser tubes copper and titanium alloys Cladding for fuel rods zirconium alloyed with Sn, Fe, 3 or Nb 4 Reactor cooling pumps zirconium-niobium alloys In cathodic protection method, a plate of metal (X) is buried beside the iron pipe or tank and connected to it by wires. Here iron becomes cathode and metal (X) becomes anode. Identify X in the diagram given below and also why X is buried in backfill? 19. The X - in the diagram is sacrificial anode (Zn, Mg) and the X is buried in backfill for better conductivity of electrons. 20. Suggest any two ways to avoid electrochemical corrosion for making engineering devices.  By using the corrosion inhibitors  Protective coatings  By the Cathodic protection  Use proper design with smooth bends and properly welded crevices  Proper insulation