BASICS OF CORROSION. Dr. Ramazan Kahraman

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1 BASICS OF CORROSION Reading Material: Chapter 1 in Principles and Prevention of Corrosion, Denny Jones, Prentice-Hall, Dr. Ramazan Kahraman Chemical Engineering Department King Fahd University of Petroleum & Minerals Dhahran, Saudi Arabia 1

2 What is Corrosion? Reaction of a metal with its environment Aqueous corrosion reaction with water (usually containing dissolved ions) High temperature oxidation reaction with oxygen at high temperature High temperature corrosion reaction with other gases 2

3 Examples of Corrosion Rusting of steel corrosion product (rust) is solid but not protective Reaction of aluminium with water corrosion product is insoluble in water, so may be protective Burning of magnesium in air high temperature oxidation Rust on an iron surface 3

4 Corrosion Science and Engineering Corrosion Science Study of the chemical and metallurgical processes that occur during corrosion. Corrosion Engineering Design and application of methods to prevent corrosion. 4

5 Why is Corrosion Happening? Because metals want to go back to their stable states. For Example, Fe is stable when it reacts with oxygen. So, in the presence of a corrosive environment, Fe tends to separate (decompose) from steel and reacts with oxygen 5

6 Nature of Corrosion Formation of cell is essential for corrosion Corrosion cell comprises of the following Anode (supplies e - - oxidation reaction) Cathode (consumes e - - reduction reaction) Electrolyte Conductor (electron path) 6

7 Electrodes Electrodes are pieces of metal on which an electrochemical reaction is occurring An anode is an electrode on which an anodic or oxidation reaction is occurring A cathode is an electrode on which a cathodic or reduction reaction is occurring 7

8 Electrochemical Cell electrons A C ELECTRON PATH HCl CATHODE ANODE ELECTROLYTE 8

9 Electrochemical Cell (cont.) 9

10 Corrosion of a Metal in Acid Metal M e - e - H + H + M +n H + H + H 2 HCl solution H + Cl - Cl - Anodic Rxn M M +n + n e - Cathodic Rxn nh + +n e - n/2 H 2 10

11 Corrosion of a Metal in Aerated Water or Aerated Basic Solutions Metal M e - H 2 O O 2 M +n OH - Aerated H 2 O or Basic Solution Anodic Rxn M M +n + n e - Cathodic Rxn (n/2)h 2 O + (n/4)o 2 + ne - n OH - 11

12 Acids and Bases An acid is a substance that produces excess hydrogen ions (H + ) when dissolved in water examples are HCl, H 2 SO 4 A base (alkali) is a substance that produces excess hydroxyl ions (OH - ) when dissolved in water examples are NaOH, KOH 12

13 Acids and Bases (cont.) Note that H + and OH - are in equilibrium in water: H 2 O H + + OH - The product of [H + ] times [OH - ] is 10-14, so in pure water both [H + ] and [OH - ] are This leads to the concept of ph, which is defined as -log[h + ] Hence ph = 0 is strong acid, 7 is neutral, and 14 is strong alkali 13

14 Corrosion of Zinc in Acid Zinc known as a base or active metal Zinc dissolves with hydrogen evolution Zn + 2HCl ZnCl 2 + H 2 But we can separate metal dissolution and hydrogen evolution Zn Zn e - 2H + + 2e - H 2 These are known as electrochemical reactions 14

15 Corrosion of Platinum in Acid Platinum does not react with acids Platinum is known as a noble metal 15

16 Zinc and Platinum in Acid Not Connected Zinc and platinum not connected, no reaction on platinum Zn Pt HCl Zn + 2HCl ZnCl 2 + H 2 metal + acid salt + hydrogen 16

17 Connection of Platinum to Zinc (This is galvanic corrosion which will be studied in detail later) Zn A electrons HCl Pt C Zinc and platinum connected, current flows and hydrogen is evolved on platinum Zn Zn e - metal metal ions + electrons (negligible cathodic rxn on Zn relative to that on Pt) 2H + + 2e - H 2 hydrogen ions + electrons hydrogen gas 17

18 External Current Applied to Platinum in Acid Hydrogen evolved on negative electrode 2H + + 2e - H Oxygen evolved on positive electrode 2H 2 O O 2 + 4H + + 4e - Pt Pt HCl Overall reaction 2H 2 O 2H 2 + O 2 18

19 External Current Applied to Platinum in Alkali Hydrogen evolved on negative electrode 2H 2 O + 2e - H 2 + 2OH Oxygen evolved on positive electrode 4OH - O 2 + 2H 2 O + 4e - Pt Pt NaOH Overall reaction 2H 2 O 2H 2 + O 2 19

20 External Current Applied to Platinum Hydrogen evolution at one electrode 2H + + 2e - H 2 (acids) or 2H 2 O + 2e - H 2 + 2OH - (alkalis) Oxygen evolution at the other electrode 2H 2 O O 2 + 4H + + 4e - (acids) or 4OH - O 2 + 2H 2 O + 4e - (alkalis) A piece of metal in the solution 20

21 Anodic Reactions Oxidation reactions Produce electrons Examples Zn Zn e - Fe Fe e - Al Al e - Fe 2+ Fe 3+ + e - H 2 2H + + 2e - H 2 + 2OH - 2H 2 O + 2e - 2H 2 O O 2 + 4H + + 4e - 4OH - O 2 + 2H 2 O + 4e - zinc corrosion iron corrosion aluminium corrosion ferrous ion oxidation hydrogen oxidation in acids hydrogen oxidation in water or bases oxygen evolution in acids oxygen evolution in water or bases 21

22 Cathodic Reactions Reduction reactions Consume electrons Examples O 2 + 2H 2 O + 4e - 4OH - oxygen reduction in water/bases O 2 + 4H + + 4e - 2H 2 O oxygen reduction in acids 2H 2 O + 2e - H 2 + 2OH - hydrogen evolution in water/bases 2H + + 2e - H 2 hydrogen evolution in acids Cu e - Cu copper plating Fe 3+ + e - Fe 2+ ferric ion reduction Sn e - Sn 2+ 22

23 Cathodic Rxns in Acidic & Basic Solns Deaerated Acidic Solutions 2H + + 2e - H 2 Aerated Acidic Solutions 2H + + 2e - H 2 O 2 + 4H + + 4e - 2H 2 O (presence of O 2 further increases corrosion) Deaerated Neutral or Basic Solutions 2H 2 O + 2e - H 2 + 2OH - Aerated Neutral or Basic Solutions O 2 + 2H 2 O + 4e - 4OH - (this reaction causes higher corr. rate) 23

24 Corrosion Rate Simplest and most useful technique for corrosion rate determination is the Weight Loss Technique Corrosion Rate = mass / exposed surface area. time or Corrosion Rate = avg. corrosion penetration depth / time ( = mass / density. surface area. time ) Common Corrosion Rate Units gmd (grams of metal lost per square meter per day) mm/y (average millimeters penetration per year) mpy (avg. mils penetration per year, 1 mil = in) 24

25 Example A carbon steel test specimen of dimensions 2-in 3-in in with a 0.25-in hole for suspending in solution is exposed for 120 hours in an acid solution and loses 150 milligrams. Calculate the corosion rate in mpy and mm/y. 25

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27 Home Exercise Problems Prbs. 1, 4, 8, 10 and 11 of Chapter 1 in Principles and Prevention of Corrosion, Denny Jones, Prentice-Hall,

28 Faraday s Law Charge is related to mass of material reacted in an electrochemical reaction: M M n+ + ne - One metal ion n mols of electrons To produce one mol of metal ion and Reacts 28

29 Faraday s Constant One mole of metal (MW g) contains Avogadro s number ( ) of metal atoms Hence each mole of metal will produce n times that many number of electrons Charge on the electron is C (coulomb) Hence one mole of metal will produce a charge of n C C/equivalent is known as Faraday s constant (also in units of J/V equivalent) Conversions: 1 A (ampere) = 1 C/s, 1 J = 1 C V 29

30 Faraday s Law m = nf Q M where Q F n M m = = = = = charge (coulomb, C) Faraday's constant (96500 C/equivalent) number of equivalents (mols of electrons) transferred per mol of metal mass of metal oxidized (g) molecular (atomic) weight of metal (g/mole) So, if Q is known, mass loss by corrosion can be determined. The details of corrosion rate determination by electrochemical techniques will be covered later. 30

31 References Principles and Prevention of Corrosion, Denny Jones, Prentice-Hall, Web Site of Dr. R. A. (Bob) Cottis. 31