Corrosion Lecture Notes PDF

Three types of oxides may form, depending on the volume ratio between the metal and the oxide: a. Magnesium produces a porous oxide film, b. Aluminum forms a protective, adherent, nonporous oxide film, c. Iron forms an oxide film that 2. Corrosion b...

Three types of oxides may form, depending on the volume ratio between the metal and the oxide: a. Magnesium produces a porous oxide film, b. Aluminum forms a protective, adherent, nonporous oxide film, c. Iron forms an oxide film that 2. Corrosion by gases The gases such as SO2, CO2, H2S, Cl2, F2 etc., when come in direct contact with metal surface corrosion occurs. The extent of corrosion depends on the chemical affinity between the metal and the gas concerned. The prevention of metal corrosion can be known from the nature of corrosion product. Whether the layer of corrosion product is protecting or non-protecting in nature. If the formed corrosion product is protecting (or) non porous metal is prevented. Ex: AgCl layer on metallic silver by the action of Cl2 gas. If the formed corrosion product is non protecting (or) porous , the corrosion of metals occurs non stop. Ex: H S gas attacks 3. Liquid metal corrosion The chemical action of the flowing liquid metal at high temperature, on a solid metal or alloy produces liquid metal corrosion. There are two reasons for this corrosion – Dissolution of the solid metal by liquid metal Internal penetration of the liquid metal into the solid phase, weakening the solid metal. Example: liquid Na used as a coolant in a nuclear plants, these causes cadmium corrosion. Theories of corrosion Pilling-Bedworth Ratio The Pilling-Bedworth ratio (P-B ratio), in corrosion of metals, is the ratio of the volume of the elementary cell of a metal oxide to the volume of the elementary cell of the corresponding metal (from which the oxide is created). where, RPB : is the Pilling-Bedworth ratio, V : the molar volume ρ : density, M : the atomic or molecular mass, n : number of atoms of metal per one molecule of the oxide Pilling-Bedworth rule The Pilling-Bedworth rule  Three types of oxides may form, depending on the volume ratio between the metal and the oxide: a. Magnesium produces a porous oxide film, b. Aluminum forms a protective, adherent, nonporous oxide film, and c. Iron forms an oxide film that spills off the surface and provides poor protection. Pilling-Bedworth rule The Pilling-Bedworth rule  Pilling-Bedworth Ratio The Pilling-Bedworth ratio (P-B ratio), in corrosion of metals, is the ratio of the volume of the elementary cell of a metal oxide to the volume of the elementary cell of the corresponding metal (from which the oxide is created). On the basis of the P-B ratio, it can be judged if the metal is likely to passivate in dry air by creation of a protective oxide layer. Pilling-Bedworth Ratio On the basis of measurements, the following connection can be shown: the oxide coating layer is too thin, likely broken and provides no protective effect (for example magnesium) the oxide coating provides protective effect However, the exceptions to the above P-B ratio rules are numerous. Many of the exceptions can be attributed to the mechanism of the oxide growth: the underlying assumption in the P-B ratio is that oxygen needs to diffuse through the oxide layer to the metal surface; in reality, it is often the metal ion that diffuses to the air-oxide interface. Calculation of Pilling-Bedworth Ratio V Oxide M Oxide × ρ Metal 𝐑 𝐏𝐁= = V Metal n× M Metal × ρ Oxide Oxide Oxide Atomic Metal Molecular P-B Ratio Density weight Density Weight of (g/cc) of Metal (g/cc) Oxide MgO 3.65 24.3 1.74 40.3 0.79 CaO 3.40 40.08 1.55 56.1 0.04 Al2O3 3.70 27.00 2.70 102.0 1.37 Calculation of Pilling-Bedworth Ratio V Oxide M Oxide × ρ Metal 𝐑 𝐏𝐁= = V Metal n× M Metal × ρ Oxide Oxide Oxide Atomic Metal Molecul P-B Density Weight Density ar Ratio (g/cc) of Metal (g/cc) Weight of Oxide Cr2O3 5.22 52 7.19 152.0 ? Na2O 2.27 23 0.97 62.0 ? NiO 6.67 58.69 8.91 74.7 ? K2 O 2.13 39.09 0.86 94.2 ? Calculation of Pilling-Bedworth Ratio V Oxide M Oxide × ρ Metal 𝐑 𝐏𝐁= = V Metal n× M Metal × ρ Oxide Oxide Oxide Atomic Metal Molecul P-B Density Weight Density ar Ratio (g/cc) of Metal (g/cc) Weight of Oxide Cr2O3 5.22 52 7.19 152.0 2.01 Na2O 2.27 23 0.97 62.0 0.57 NiO 6.67 58.69 8.91 74.7 1.70 K2 O 2.13 39.09 0.86 94.2 0.48 Mechanism of wet corrosion 1. The formation of anodic and cathodic areas or parts in contact with each other. 2. Presence of a conducting medium. 3. Corrosions of anodic areas only. 4. Formation of corrosion product somewhere between anodic and cathodic areas.  Electrochemical corrosion The metal surface undergoes an electrochemical reaction with the moisture and oxygen in the atmosphere. This theory is known as electrochemical theory of corrosion. Mechanism : Anodic reaction : Oxidation of metal (corrosion) Cathodic reaction : Consumption of electrons Corrosion always occurs at the anode  1) Mechanism of wet corrosion Wet corrosion takes place by the following two ways based on the nature of medium: 1) Anodic reaction: Fe (s)  Fe2+ (aq) + 2e- 2) Cathodic reaction: a) Evolution of H2 (acidic medium) in absence of oxygen 2 H+ (aq) + 2e-  H2 Displacement of hydrogen ions from the acidic solution by metal ions)  b) Absorption of O2 : Neutral medium Cathode: ½ O2(g) + H2O (aq) +2 e-  2OH- If the solution is aerated and almost neutral, O2 is reduced in presence of water to OH- c. If the solution is deaerated and almost neutral, H 2 is liberated along with OH- Cathode: H2O (aq) +2 e-  H2 + 2OH- Final Step Salt speeds up the process Wate r Fe2+ Rust e- Iron Dissolves- Fe ® Fe+2 Fe2+ + 4 OH- 2Fe(OH)2 2Fe(OH)2 + ½ O2 + (x-2) H2O Fe2O3. xH2O If limited oxygen is supplied, black rust is formed as follows 3Fe(OH)2 + ½ O2 -----------Fe3O4. 3H2O Black Rust Theories of corrosion Dry or chemical Wet or electrochemical corrosion: corrosion: This occurs at dry This occurs at wet conditions conditions (electrolytic medium) Corrosion is uniform It is a slow process Corrosion Is not uniform It involves direct It is a rapid process chemical attack It involves formation of Explained by electrochemical cells absorption mechanism Explained by mechanism of electrochemical reactions