Environmental Effects on the Materials There are significant impacts of environmental factors on Engineering Materials. 2
Environmental Effects on the Materials Improper selection of Engineering Materials may cause economical as well as life loss. 3
Railway line Collapsed in France in 1873 Tenerife airport disaster (1977) Most Deadly Aviation Disaster 583 Fatalities Disasters may happen due to Improper selection of Engineering Materials 4
A chimney tower stands over the sarcophagus that covers destroyed Reactor No. 4 at Ukraine's Chernobyl nuclear power plant on Saturday, April 22, 2006. This structure now required to withstand Environmental Damaging Effects for Long period of Time. 5
Study of environmental effects on the materials is important from selection of a material for specific purpose point of view. Material should be selected which can withstand environmental effects. 6
Environmental Effects on the Materials Generally, environmental degradation classified into following classes Chemical degradation electrochemical degradation Radiation-induced degradation Wear-related degradation 7
Oxidation (Atmospheric interaction): –In general, metals and alloys form stable oxide compounds with air at elevated temperature. However, oxides may be form at room temperature. –In oxidation process a thin layer of oxide compound form at the surface of material. –Frequently, four basic oxidation mechanisms are noted in metals. 8
For an un-protective surface oxygen gas is available to the metal surface at an essentially constant rate. dy/dt = C 1 which gives, y= C 1 t + C 2 (Linear growth rate law) For film growth that is limited by ionic diffusion, the growth rate diminishes as the film thickness grows. Therefore, rate of film growth is inversely proportional to film thickness. dy/dt = C 3 x 1/y which gives, y 2 = C 4 t + C 5 (Parabolic growth rate law) 9
10 Tendency of making protective coating on a material is important & is indicated by an especially simple parameter ‘Pilling- Bedworth ratio’, R. R PB =M d / n m D, If R PB 1, oxide volume is sufficient to form a protective coating. Protective oxides generally have R PB values between 1 &2. Where, M = Atomic or Mol-wt of oxide ‘M a O b d = Density of metal n = number of atoms of metal per one molecule of the oxide m = Atomic or Mol-wt of metal ‘M a O b D = Density of oxide ‘M a O b
12 What about Hydrogen, Nitrogen, Sulfur ? Aqueous Corrosion (Electrochemical Attack): –Corrosion is the dissolution of a metal into an aqueous environment. –Reaction is driven by an attempt to equilibrate the ionic concentration in both sides of the overall cell.
18 Corrosion Prevention: –Corrosion prevention is vital from economic and safety point of view. –Corrosion costs multibillion-dollar to modern society. –When complete prevention is impossible, minimization must be done.
19 Methods of Corrosion Prevention: –Proper selection of material is also important in corrosion prevention. –Protective coating is one of the way of corrosion prevention. –Design selection can also minimize damage. –Threaded joints and similar high-stress regions are to be avoided. –When galvanic couples are required, a small area anode next to a large area cathode should be avoided.
21 Other Methods of Corrosion Prevention Sacrificial Anode: It provides a sacrificial anode to corrode and save the structure. A sacrificial anode, or sacrificial rod, is a metallic anode used in cathodic protection where it is intended to be dissolved to protect other metallic components. The more active metal is more easily oxidized than the protected metal and corrodes first (hence the term "sacrificial"); it generally must oxidize nearly completely before the less active metal will corrode, thus acting as a barrier against corrosion for the protected metal.
22 A ship hull made up of steel it will corrode quickly in the saline seawater environment.
30 Chemical Degradation of Ceramics and Polymers: –Like metals ceramics and polymers degrade but to a lesser extent. Examples: –Silicates reacts with water and degrade in the environment. –Plastics degrade rapidly in harsh environment. –Polymers also reactive with several solvents
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