Chemistry 6.1 & 6.2: Corrosion & Electrochemical Reactions PDF

2:45 Tue, 14 Jan toi ll 25% CHEMISTRY X 9 History 9New tab X +...

2:45 Tue, 14 Jan toi ll 25% CHEMISTRY X 9 History 9New tab X + e-books.helwan. edu.eg/storage/29828/i E 6.1. What is corrosion? 189 Chapter 6: Introduction to Corrosion 6.1. What is corrosion? Corrosion is an irreversible damage of metals by reaction with its surrounding environment.Corrosion of metals typically produces oxide(s) or salt(s) of the original metal. B A Figure 1.Corrosion of steel in concrete. EXTRACTION ORE METALS (metal oxides in minerals) Fe,0, +3CO 2Fe +3CO, 2AI,0, +3C 4A1 +3CO, CORROSION Chapters Media Notebook Search Settings 2:45 Tue, 14 Jan toi 25% all CHEMISTRY X 9 History X 9New tab X + e-books.helwan. edu.eg/storage/29828/i E6.2. Steps of electrochemicalreaction 190 6.2.Steps of electrochemical reaction 1. Mass transfer step: this involves the transfer of ions from the bulk of solution to the electrode surface through the diffusion layer (transfer of Cu* from the bulk solution to the cathode surface). 2. Charge transfer step: this involves redox reaction between ions and electrons at the electrode surface. 3. Mass transfer step: this involves the transfer of ions from the electrode surface to the bulk of solution through the diffusion layer (transfer of Zn* from the anode surface to the bulk of solution). e Anode Cathode M Electrolyte Chapters Media Notebook Search Settings 2:45 Tue, 14 Jan t ll 25% CHEMISTRY X 9 History 9New tab X + e-books.helwan. edu.eg/storage/29828/i E6.2. Steps of electrochemicalreaction 191 A redox reaction results when an oxidation and a reduction half-reaction are combined to complete a transfer of electrons as in the following example: + Zn(s) Cu(aq) Zn(aq) +Cu(s) The electrons are not shown because they are neither reactants nor products but have simply been transferred from one species to another (from Zn to Cu in this case). In this redox reaction, the Zn(s) is referred to as the reducing agent because it causes the Cu* to be reduced toCu. The Cu* is called the oxidizing agent because it causes the Zn(s) to be oxidized to Zn. A measure of the tendency for a reduction to occur is its reduction potential, E, measured in units of volts. At standard conditions, 25 °C and Chapters Media Notebook Search Settings 2:45 Tue, 14 Jan t ll 25% CHEMISTRY X 9History X 9New tab X + e-books.helwan. edu.eg/storage/29828/i E6.2. Steps of electrochemicalreaction 192 concentrations of 1.0 M for the aqueous ions, the measured voltage of the reduction half reaction is defined as the standard reduction potential, E°. The standard reduction potentials are -0.76 V for zinc and +0.34 V for copper. The more positive (or less negative) the reduction potential, the greater is the tendency for the reduction to occur. So Cu has a greater tendency to be reduced than Zn2*. Furthermore, Zn has a greater tendency to be oxidized than Cu. The values of E° for the oxidation half reactions are opposite in sign to the reduction potentials: +0.76 V for Zn and -0.34 V for Cu. Chapters Media Notebook Search Settings 2:45 Tue, 14 Jan t ll 25% CHEMISTRY X 9 History 9New tab X + e-books.helwan. edu.eg/storage/29828/i E6.2. Steps of electrochemicalreaction 193 Table 1. Electromotive force (emf) series. Electrode reaction e(V) Li Lit + e -3.05 Al AP+ +3e -l.66 Ti +2e -1.63 Anodic Ti2+ Zn Znt +2e -0.76 CrCt 3e + -0.74 Fe e?t + 2e -0.44 Ni Nt + 2e -0.25 Sn Sn2+ +2e -0.14 Pb Pb2+ + 2e -0.13 H, 2Ht + 2e 0.00 Cu Cu?+ + 2e +0.34 Ag Agt +e +0.80 Pt Pttt + 4e +1.20 Cathodic Au Aut + 3e +1.50 Fe-Pbcouple Fe-Zn couple cORROSION PROTECTION Fe-Fe +2e anodic ZnZr +2e anodic Pb +2e- Pb cathodic Fe +2- Fe cathodic U=9.13-(049) =031V U=0.44-(076) =032V Gnge 2:45 Tue, 14 Jan t ll 25% CHEMISTRY X 9 History X 9New tab X + e-books.helwan. edu.eg/storage/29828/i E 6.3. Costs of corrosion 194 6.3.Costs of corrosion (1)Direct economic losses 1. Replacement of corroded structure 2. Extra cost of using corrosion inhibitors (2) Indirect costs 1. Loss of products by leaks, such as oils, gas or water 2. Loss of production until the corroded part is replaced 3. Loss of efficiency, due to the deposition of corrosion products 4. Product contamination Chapters Media Notebook Search Settings 2:46 Tue, 14 Jan toi.l 25% CHEMISTRY X 9 History X 9New tab X + e-books.helwan. edu.eg/storage/29828/i E6.4. Polarization 195 6.4. Polarization Not all oxidation processes result in deterioration of material! Polarization: is decreasing the efficiency of an electrochemical reaction by disturbing its equilibrium conditions (e.g., lowering corrosion rates by adding a corrosion inhibitor to increase the polarization of either the cathode or anode). We have two types of polarization: concentration and activation polarization. Polarization is inversely proportional to the corrosion rate. Thus, increasing polarization decreases corrosion. Passivity: Some metals and alloys under particular environmentalcondition loss their chemical reactivity and become inert (passive). Passivity is the formation of a thin protective film (1-10 nm thickness) on a metal surface due to its reaction with the surrounding Chapters Media Notebook Search Settings 2:46 Tue, 14 Jan toi ll 25% CHEMISTRY X 9 History X 9 New tab X + e-books.helwan. edu.eg/storage/29828/i E 6.5. Types of corrosion 196 environment under oxidizing conditions. For the formed passive layer to be protective it should be: (1) adherent (2) non-porous,dense and compact)., e.g., the formation of oxide layers of Al,0,,TiO,, Cr,03. This is why Cr is added to Fe to prepare stainless steel. 6.5. Types of corrosion Corrosions are of two main types, general and localized corrosion 1. General (Uniform) Corrosion: This type of corrosion involves a uniform rate of metal damaging over the entire surface area that is exposed to the corrosive environment.For example, dissolution of Zn and Al metal in acids or iron rusting: steel exposed to environment. Uniform Chapters Media Notebook Search Settings 2:46 Tue, 14 Jan toi ll 25% CHEMISTRY X 9 History 9New tab X + e-books.helwan. edu.eg/storage/29828/i 6.5. Types of corrosion 197 corrosion can be caused by atmospheric attack or galvanic corrosion. a. Atmospheric corrosion It takes place due to the direct contact between a metal and its surrounding atmosphere. It involves formation of metal oxide layer, e.g., rusting of iron. The corrosion rate is influenced by several factors, such as type of metal, type of atmosphere (dust, Chapters Media Notebook Search Settings 2:46 Tue, 14 Jan toi al 25% CHEMISTRY X 9 History X 9New tab X + e-books.helwan. edu.eg/storage/29828/i 6.5. Types of corrosion 198 humidity, pollutant. Water vapor, gases, etc.), temperature (the corrosion rate increases with increasing temp.), time of exposure (the extent of corrosion increases with increased exposure time), and metal surface conditions (more roughness, more corrosion). b. Galvanic Corrosion It is a form of corrosion that happens when two dissimilar metals are physically connected in the presence of an electrolyte. The factors affecting galvanic corrosion are potential difference between anode and cathode, distance between anode and cathode and cathode/anode (CIA)area ratio. Chapters Media Notebook Search Settings  199 2. Localized corrosion Localized corrosion takes place within localized crevices or holes within the metal surface. There are two main forms, pitting and erosion corrosions. a. Pitting Corrosion This form is extremely localized corrosion that leads to the creation of small holes in the metal surfaces. This type of corrosion is highly destructive.The rate of pitting corrosion increases when there is lack of Oxygen around a small area; this area becomes anodic while the area with excess of oxygen becomes cathodic. Moreover, stagnant solutions accelerate the pitting corrosion rate. In order to prevent or minimize < Chapters Media Notebook Search Settings