Chemical Equations and Reactions Describing Chemical Reactions.
Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those...
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Transcript of Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those...
![Page 1: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/1.jpg)
Corrosion Mechanisms
Lecture#02
![Page 2: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/2.jpg)
Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are
added or removed from a chemical species
Electrochemical reactions are chemical reactions in which not only may elements may be added or removed from a chemical species but at least one of the species undergoes a change in the number of valance electron
Corrosion processes are electrochemical in nature
![Page 3: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/3.jpg)
Simplest Example: Dry Cell Battery
![Page 4: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/4.jpg)
Faraday’s Law
The mass of an element discharged at an electrode is directly proportional to the amount of electrical charge passed through the electrode
weight of metal reacting = kIt
![Page 5: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/5.jpg)
What Happens if the Battery is Not in Use? There will be some “local action current”
generated by “local action cells” because of other metallic impurities in zinc
Shelf life of an ordinary zinc-carbon rod battery is limited
![Page 6: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/6.jpg)
Local Action Cell
![Page 7: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/7.jpg)
Anode & Cathode
Anode Loss of electron in oxidation Oxidation always occurs at the anode
Cathode Gain of electron in reduction Reduction always occurs at the cathode
![Page 8: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/8.jpg)
Corrosion Cells
Galvanic cell (Dissimilar electrode cell) – dissimilar metals
Salt concentration cell – difference in composition of aqueous environment
Differential aeration cell – difference in oxygen concentration
Differential temperature cell – difference in temperature distribution over the body of the metallic material
![Page 9: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/9.jpg)
Dissimilar Electrode Cell
When a cell is produced due to two dissimilar metals it is called dissimilar electrode cell Dry cell Local action cell A brass fitting connected to a
steel pipe A bronze propeller in contact
with the steel hull of a ship
Zn anode
HCl Solution
Cu cathode
![Page 10: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/10.jpg)
Salt Concentration Cell
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Differential Aeration Cell
![Page 12: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/12.jpg)
Corrosion at the bottom of the electrical poles
![Page 13: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/13.jpg)
Differential Temperature Cell This is the type of cell when two identical electrodes
are immersed in same electrolyte, but the electrodes are immersed into solution of two different temperatures
This type of cell formation takes place in the heat exchanger equipment where temperature difference exists at the same metal component exposed to same environment
For example for CuSO4 electrolyte & Cu electrode the electrode in contact with hot solution acts as cathode.
![Page 14: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/14.jpg)
Cells responsible for corrosion may be a combination of these three types
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Forms of Corrosion
1. General corrosion or uniform attack
2. Pittingi. Impingement attack or erosion-corrosion
ii. Fretting corrosion
iii. Cavitation erosion
3. Selective Corrosion
4. Intergranular Corrosion
5. Crackingi. Corrosion fatigue
ii. Stress corrosion cracking (SCC)
![Page 16: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/16.jpg)
Uniform Corrosion
![Page 17: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/17.jpg)
Corrosion Rate and Classification of Metals mm/y – millimeters penetration per year gmd – grams per square meter per day ipy – inches penetration per year mpy – mils penetration per year (1000 mil = 1
inch) mdd – milligrams per square decimeter per
day
![Page 18: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/18.jpg)
Corrosion Rate and Classification of Metals
Mils per year (mpy) = 534W/DAT
mm/y =87.6W/DAT
W = weight loss in mg D = density of specimen material in g/cm3
A = area in cm2
T= exposure time in hours
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Classification of metallic materials according to their rate of uniform attackA. <0.005 ipy (<0.15 mm/y) – Metals in this
category have good corrosion resistance and can be used for critical parts
B. 0.005 to 0.05 ipy (0.15 mm/y to 1.5 mm/y) – Metals in this group are satisfactory if a higher rate of corrosion can be tolerated
C. >0.05 ipy (>1.5 mm/y) – Usually not satisfactory
![Page 20: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/20.jpg)
Pitting
![Page 21: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/21.jpg)
Erosion-Corrosion
![Page 22: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/22.jpg)
Fretting Corrosion
![Page 23: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/23.jpg)
Cavitation Erosion
![Page 24: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/24.jpg)
Selective Corrosion
![Page 25: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/25.jpg)
Intergranular Corrosion
![Page 26: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/26.jpg)
Corrosion Fatigue
![Page 27: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/27.jpg)
SCC
![Page 28: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/28.jpg)
Galvanic Corrosion
![Page 29: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/29.jpg)
Crevice Corrosion
![Page 30: Corrosion Mechanisms Lecture#02. Chemical vs. Electrochemical Reactions Chemical reactions are those in which elements are added or removed from a chemical.](https://reader036.fdocuments.us/reader036/viewer/2022070410/56649eab5503460f94bb0acf/html5/thumbnails/30.jpg)
Hydrogen Damage
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Failure Statistics in Germany (a) & USA (b)
a b