By: Dr. Irannejad

Transcription

1 By: Dr. Irannejad 1

2 3 1. E.E. Stansbury and R.A. Buchanan, Fundamentals of Electrochemical Corrosion, First edition, P.R. Roberge, Handbook of Corrosion Engineering, McGraw-Hill, L.L. Shreir, R.A. Jarman, G.T. Burstein, Corrosion Metal/Environment Reactions, third edition, ASM Metals Handbook, vol. 13 (corrosion). 5. Roberge, P.R. Handbook of Corrosion Engineering, McGraw-Hill, Philip A. Schweitzer, Fundamentals of corrosion- Mechanisms, Causes, and preventative methods. Taylor & Francis Group,

3 7. Zaki Ahmad, Principles of Corrosion Engineering and Corrosion Control, Elsevier Science & Technology Books, E. McCafferty, Introduction to Corrosion Science, Springer Science, L L Shreir, R A Jarman, and G T Burstein. Corrosion- Metal/Environment reactions (Volume 1), L L Shreir, R A Jarman, and G T Burstein. Corrosion- Corrosion Control (Volume 2), H.M. Shalaby, A. Al-Hashem, M. Lowther and J. Al-Besharah, Industrial corrosion and corrosion control technology, Kuwait Institute for Scientific Research, مهندسی خوردگی نویسنده: مارس. ج. فونتانا مترجم: احمد ساعتچی انتشارات جهاد دانشگاهی صنعتی اصفهان

4 Introduction What is Corrosion? Why does metal undergo corrosion? Importance of Corrosion Examples of Corrosion Forms of corrosion Classification of corrosion Electrochemical potential Overview of Electrochemical Corrosion 7 8 4

5 Corrosion: Deterioration of materials by interaction with their environment. The term materials refers to those substances used in the construction of machines, process equipment, and other manufactured products. These materials include metals, polymers, and ceramics. The environments are liquids or gases, although under special circumstances certain solid-solid reactions might be included as corrosion. 9 Corrosion is extractive metallurgy in reverse. 10 5

6 Only few elements exist in free element state in natural circumstance. Corrosion of metal, i.e. conversion of element to stable compound, is thermodynamically favored. Naturally occurring copper single crystals. Naturally occurring gold. Naturally occurring copper sheet

7 The need to control corrosion almost always reduces to considerations of safety and economics. 13 Machines, equipment, and functional products may fail due to corrosion in such a manner as to result in personal injury. Aloha Incident

8 One-fifth of the iron and steel produced annually in the world is used to replace rusted metal. 15 Corroded metal often loses its structural integrity and attractiveness. 16 8

9 Motor Vehicles: $23.4 Billion per year + safety issues 17 Highways & Bridges: $8.3 Billion per year + safety issues Before After 18 9

10 19 Rusting of steel and cast iron in water Corrosion of copper, aluminum, and cast iron in automotive cooling systems Corrosion of iron-base, copper-base, nickel-base, alloys in the chemical process Corrosion of automobile exhaust systems by direct reaction of the metal with high-temperature gases Corrosion of turbine blades in gas turbines by hot combustion gases Corrosion of metallic surgical implant materials used in orthopedic Corrosion of iron-base and nickel-base alloys by liquid metals used as heat transfer agents Enhanced deterioration of structural concrete and stone by interaction with condensed moisture and acidic contaminants in the air Stress-corrosion cracking (SCC) of gold and brass by mercury SCC and pitting of stainless steel in sea water 20 10

11 Railway bridge White marble Brass sculpture Waterline corrosion 21 Corrosion at sea Corrosion of Aluminium Corrosion of plastics 22 11

12 Corrosion of Concrete

13 1) Based on materials: Corrosion of metals; Corrosion of non-metals (wood, plastic, concrete, stone, etc). 2) Based on Media: Natural corrosion; Industrial corrosion (e.g. solution containing acid, base, H 2 S, etc). 25 3) Based on uniformity: General corrosion; Local corrosion. 4) Based on mechanism: Chemical corrosion (2Fe + O 2 = 2FeO); Electrochemical corrosion; Biochemical corrosion

14 1. Direct Chemical Corrosion or Dry Corrosion This type of corrosion occurs mainly through the direct chemical action of atmospheric gases such as O 2, halogens, H 2 S, CO 2, SO 2, N 2, H 2 or liquid metals on metal surface in the absence of moisture. 2. Electrochemical Corrosion or Wet Corrosion This type of corrosion occurs when : a. A metal is in contact with a conducting liquid. b. Two dissimilar metals or alloys are immersed partially in a conducting solution. Corrosion and its Control, Dr. Priyabrat Dash, NIT Rourkela, India Corrosion by O 2 or Oxidation corrosion Dry Corrosion 2. Corrosion by H 2 and other gases 3. liquid metals corrosion 28 14

15 Mechanism of corrosion is the actual atomic, molecular, or ionic transport process that takes place at the interface of a material. Since electrochemical corrosion involves the release of ions to the environment and movement of electrons within the material. This mechanism can occur only if the environment can contain ions and the material can conduct electrons. 29 Anodic Reaction (oxidation): liberating electrons Zn Zn e - Fe Fe e - Al Al e - Fe 2+ Fe 3+ + e - H 2 2H + + 2e - 2H 2 O O 2 + 4H + + 4e - zinc corrosion iron corrosion aluminium corrosion ferrous ion oxidation hydrogen oxidation oxygen evolution 30 15

16 Cathodic Reaction (reduction): accept electrons 31 Anode (anodic area): The site where the metal is being oxidized. Cathode (cathodic area): The site where oxygen, water, or some other species is being reduced

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18 Mechanism of rusting of iron 35 Electrochemical Corrosion requires four elements: 1- Anode 2. Cathode 3. Electrolyte 4. Electrical Path 36 18

19 An electrolyte is a compound that ionises when dissolved in suitable ionising solvents such as water. This includes most soluble salts, acids, and bases. Strong electrolytes produce ions and conduct an electric current. Weak electrolytes produce a few ions. Nonelectrolytes do not produce ions

20 Corrosion Mechanisms Particularly under the broad definition of corrosion as the deterioration of materials by reaction with the environment, the number of mechanisms whereby deterioration occurs is large. In general, a mechanism of corrosion is the actual atomic, molecular, or ionic transport process that takes place at the interface of a material. These processes usually involve more than one definable step, and the major interest is directed toward the slowest step that essentially controls the rate of the overall reaction. When electrochemical corrosion is occurring, mechanisms may be inferred from measurements of electrical potential and current. 39 Since electrochemical corrosion involves the release of ions to the environment and movement of electrons within the material, this mechanism can occur only if the environment can contain ions and the material can conduct electrons. The most important case of electrochemical mechanisms is the simple corrosion of metals in aqueous solutions, where atoms at the surface of the metal enter the solution as metal ions and electrons migrate through the metal to a site where, to sustain the reaction, they are consumed by species in contact with the metal

21 In more complicated cases, the metal ions move into solution by forming complex ions, or they combine with other species in the solution and precipitate compounds such as hydroxides, oxides, or sulfides. At sufficiently high temperatures, metals corrode in gases, particularly oxygen to form oxides. Whereas the mechanism in this case appears to be one of direct chemical attack, the mechanism may still be electrochemical in nature, with ions and electrons moving in the oxide which acts as the electrolyte supporting the electrochemical mechanism. Polymeric and ceramic materials generally do not support electron conduction and hence corrode by either direct chemical or physical mechanisms. 41 Electrochemical Corrosion Processes and Variables Before examining in detail the theories of aqueous corrosion and quantitative calculations of corrosion rates, it will be useful to develop qualitatively the major phenomena involved. The following sections review several general types of metal/corrosive-environment combinations, the chemical reactions involved, idealized mechanisms for the transfer of metal ions to the environment, and the electrochemical processes occurring at the interface between the metal and the aqueous environment

22 Uniform Corrosion with ph as the Major Variable For metals, M, that are thermodynamically unstable in water, the simplest corrosion reactions are: Thus, the metal passes from the metallic state to ions of valence m in solution with the evolution of hydrogen. 43 Because the metallic phase is an electron conductor, it supports the electron transfer, allowing the two processes to occur at separate sites on the metal surface. In limiting cases, these processes occur within a few atom diameters on the surface with the sites constantly changing with time, thus producing uniform corrosion. Otherwise, the corrosion is nonuniform. Uniform corrosion supported by ph is represented schematically in Fig In this example, oxygen is excluded by a nitrogen gas purge and overblanket

23 45 Uniform Corrosion with ph and Dissolved Oxygen as Variables When dissolved oxygen is present in the solution, usually from contact with air, the following reactions apply in addition to those just considered. Uniform corrosion supported by dissolved oxygen and ph is represented schematically in Fig Since electrons are now consumed by two reactions, the rate of corrosion of the metal increases. In the case of iron, dissolved oxygen is more important in supporting corrosion than the presence of hydrogen ions when the ph is greater than approximately

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25 Uniform Corrosion with Corrosion Product Formation An example of corrosion product formation is the rusting of iron as illustrated in Fig When the ph is greater than approximately 4, and under aerated conditions, a layer of black Fe 3 O 4, and possibly Fe(OH) 2, forms in contact with the iron substrate. In the presence of the dissolved oxygen, an outer layer of red Fe 2 O 3 or FeOOH forms. The adherence and porosity of these layers change with time and can be influenced by other chemical species in the environment, such as chloride and sulfate ions. In any case, the formation of the corrosion product layer influences the corrosion rate by introducing a barrier through which ions and oxygen must diffuse to sustain the corrosion process

26 Electrochemical reactions involve transfer of charge. Hence, we expect that the voltage of the metal with respect to the solution will affect electrochemical reactions. Voltage of metal with respect to solution is known as the electrochemical potential. 51 Standard Hydrogen Electrode E = 0 V (by definition; arbitrarily selected) 2H + + 2e - H

27 53 Zn anode e - e - Cu cathode Salt bridge Zn 2+ Cu 2+ Zn Zn e - Cu e - Cu 54 27

28 Cu Zn

29 more anodic (active) more cathodic (inert) more anodic more cathodic metal Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K V o metal V Ex: Cd-Ni cell V < V Cd corrodes o Cd o Ni - + Cd 25 C Ni 1.0 M Cd 2 + solution 1.0 M Ni 2+ solution 57 Ranking the reactivity of metals/alloys in seawater Platinum Gold Graphite Titanium Silver 316 Stainless Steel (passive) Nickel (passive) Copper Nickel (active) Tin Lead 316 Stainless Steel (active) Iron/Steel Aluminum Alloys Cadmium Zinc Magnesium 58 29

30 59 Very Base Metals emf < -0.4V Corrode in neutral aqueous solutions, even without oxygen Includes Na, Mg, Be, Al, Ti, and Fe Base Metals emf between -0.4V and 0.0 V Corrodes in neutral aqueous solutions with oxygen Corrodes in acids to produce hydrogen, even without oxygen Includes Cd, Co, Ni, Sn, and Pb 60 30

31 Semi-Noble Metals emf between 0.0 V and +0.7V Corrodes in aqueous solutions only with the presence of oxygen Includes Cu, Hg, Ag Noble Metals emf between > +0.7V Includes Pd, Pt, Au 61 Galvanic corrosion 62 31

32 Polarization is a deviation of the electrochemical process from equilibrium due to an electric current passing through the galvanic cell. Polarization may occur either at the cathode (cathodic polarization) or at the anode (anodic polarization). Cathodic polarization is common. There are three types of polarization: 1. Activation polarization 2. Concentration polarization 3. Resistance polarization 63 Common cause of cathodic activation polarization is the reaction of Hydrogen formation and evolution at the cathode surface: First step: reduction of the hydrogen ions resulted in formation of atomic hydrogen on the cathode surface. H+ + e- = H Second step: formation of molecules of gaseous hydrogen. 2H = H 2 Third step: formation of hydrogen bubbles. H 2 + H 2 + H 2 +H = nh

33 Concentration polarization: Concentration polarization of an electrode is a result of formation of a Diffusion layer adjacent to the electrode surface where there is a gradient of the ion concentration. Diffusion of the ions through the layers controls the electrochemical reaction (corrosion, Electroplating). Resistance polarization: Resistance polarization refers to the potential drop due to either the high resistivity of the electrolyte surrounding the electrode or an insulation effect of the film on the electrode surface formed by the reaction products. 65 A protective film in oxidizing atmospheres chromium, titanium, aluminum Metal oxide layer adheres to parent metal barrier against further damage self-healing if scratched Sensitive to environmental conditions passivated metal may have high corrosion rates 66 33

34 Types of Tests ANODIC POLARIZATION CURVE this curve is usually scanned from 20mV below the Eoc upwards by scanning at a slow rate (.2mV/s) this curve can be used to identify several corrosion mechanisms shown below i p - E pp - i crit - passive current density primary passivation potential critical current density E trans - transpassive potential 67 pitting cracking zones passive active 68 34

35 Weight Loss Measurement Thickness loss Measurement Measurement of electrical current density

36 Uniform Attack Oxidation & reduction occur uniformly over surface. Selective Leaching Preferred corrosion of one element/constituent (e.g., Zn from brass (Cu-Zn)). Stress corrosion Stress & corrosion work together at crack tips. Erosion-corrosion Break down of passivating layer by erosion (pipe elbows). Pitting Downward propagation of small pits & holes. Intergranular Corrosion along grain boundaries, often where special phases exist. Galvanic Dissimilar metals are physically joined. The more anodic one Corrodes. Crevice Between two pieces of the same metal. 71 Underground corrosion Electronic components Corrosion influenced by flow (Cavitation Corrosion and Erosion Corrosion) 72 36

37 Galvanic corrosion 73 O 2 Fe Cl Fe 2+ Cl Fe 2+ Cl OH Fe 2+ Cl Fe 2+ O 2 + 2H 2 O + 4e 4OH Impurity Fe = Fe e Pitting Corrosion 74 37

38 Pitting Corrosion 75 Crevice Corrosion 76 38

39 Selective Leaching Stress Corrosion 77 Intergranular Corrosion 78 39

40 79 40