Corrosion Notes - Applied Chemistry PDF

# Applied Chemistry ## Corrosion ### 8.17 Nature of the Corroding Metal - **Physical state:** The rate of corrosion is influenced by the physical state of the metal. The smaller the grain size of the metal or alloy, the greater will be its solubility, and hence greater will be its corrosion. -...

# Applied Chemistry ## Corrosion ### 8.17 Nature of the Corroding Metal - **Physical state:** The rate of corrosion is influenced by the physical state of the metal. The smaller the grain size of the metal or alloy, the greater will be its solubility, and hence greater will be its corrosion. - **Purity of metal:** Impurities in a metal cause heterogeneity and form minute electrochemical cells. The 100 per cent pure metal will not undergo any type of corrosion. - **Over voltage:** The over voltage of a metal in a corrosive environment is inversely proportional to corrosion rate. - **Nature of surface film:** In an aerated atmosphere, practically all metals get covered with a thin surface film of metal oxide. The ratio of the volumes of the metal oxide to the metal is known as a specific volume ratio. Greater the specific volume ratio, lesser is the oxidation corrosion rate. - **Relative areas of the anodic and cathodic parts:** Corrosion is more rapid and severe, and highly localized, if the anodic area is small. - **Position in galvanic series:** The extent of corrosion depends upon the position of the metal in the galvanic series. - **Passive character of metal** - **Solubility of corrosion products:** In electrochemical corrosion, the solubility of the corrosion products in the corroding medium is an important factor in deciding the extent and the rate of corrosion. - **Volatility of corrosion products:** If corrosive product is volatile in nature, they volatile as soon as they are formed. Hence, the underlying metal surface is exposed for further attack, resulting rapid and continuous corrosion. ### 8.18 Nature of the Corroding Environment - **Temperature:** The rate of corrosion is directly proportional to the temperature. - **Humidity of air:** The rate of corrosion will be more when the relative humidity of the environment is high. - **Presence of impurities in atmosphere:** Atmosphere in industrial areas contains corrosive gases like CO2, H2S, SO2 and fumes of HCl, H2SO4. - **Presence of suspended particles in atmosphere:** If the suspended particles are chemically active in nature (like NaCl, Ammonium sulphate), they absorb moisture and act as strong electrolytes, thereby causing enhanced corrosion. - **Influence of pH:** Generally acidic media (i.e. pH < 7) are more corrosive than alkaline and neutral media. - **Nature of ions present** - **Conductance of the corroding medium** - **Formation of oxygen concentration cell:** The region where oxygen concentration is lesser becomes anodic and oxygen- concentration rich portion becomes cathodic. The anodic portion suffers corrosion. - **Flow velocity of process stream** - **Polarization of electrodes** ## 8.19 Protection from Corrosion (Preventive Measures for Corrosion Control) - **Material selection:** - The chosen metal should be as pure as possible because the presence of impurities enhances the rate of corrosion. - The choice of noble metals is preferable because they are highly resistant to corrosion. - Avoid the contact of dissimilar metals in the presence of a corroding environment. - If two dissimilar metals in contact have to be used, they should be as close as possible to each other in the electrochemical series. -** Proper designing:** - When anodic and cathodic materials are used together, then the area of anodic material should be large. - The anodic part should not be painted or coated because any damage in coating would cause rapid localized corrosion. - Whenever the direct joining of dissimilar metals is unavoidable, an insulating fitting may be applied in-between them to avoid direct metal-metal electrical contact. - Angles, corners, edges, etc., should be avoided in construction. For this reason L, T and U shaped structures should be avoided as far as possible some better shapes of L, T and U structures are given below: - Poor - L - Better - L - Poor - T - Better - T - Poor - U - Better - U - The material should not have sharp corners and recesses because they help in accumulation of impurities. It should be avoided by proper designing as shown in the figure. - Sharp corners (poor design) - Recesses (poor design) - Smooth bend (best design) - Always prevent the occurrence of homogeneities in metal and in the corrosive environment. - Bolts and rivets should be replaced by a butt-weld as shown in the figure. - Bolt join (poor design) - Crevices - Weld joint (good design) - Weld joint (best design) - Whenever possible, the equipment should be supported on legs to allow free circulation of air and prevent the formation of stagnant pools or damp areas. - Poor design because it prevents free circulation of air - Best design because it allows free circulation of air - Uniform flow of corrosion liquid is desirable, since both stagnant areas and highly turbulent flow and high velocities can cause accelerated corrosion. - **Cathodic protection (Electrical protection)** - The principle involved in this method is to force the metal to be protected to behave like a cathode, thereby corrosion does not occur. Cathodic protection is carried out by two methods: - **Sacrificial anodic protection (Galvanic protection)** - In this method, the metallic structure (to be protected) is connected by a wire to a more anodic metal, so that corrosion occurs at that anodic metal and metallic structure is protected. This method is generally used for the protection of underground pipes and tanks. - In this method, the more active metal like Mg is used as anode and this metal used is called as "sacrificial anode". - **Impressed current cathodic protection** - In this method, an impressed current from an external source is applied in the opposite direction to neutralize the corrosion current. - **Surface coatings:** - Protecting the surface of an object by the application of coating by different methods. - **Anodic coatings:** - In this process, the base metal (i.e. which is to be protected) is coated with more active metal (i.e. having lower electrode potential) such as Zn, Al and Cd coating on steel surface. - **Cathodic coatings:** - In this process, base metal is coated with a more noble metal (i.e. having higher electrode potential). ### 8.20 PAINTS - Paint is a term which has been used to signify a uniform dispersion of finally divided solids in a liquid called "vehicle" or "medium". The solid comprises of pigments, driers and fillers. Volatile solvent is mixed with a non-volatile forming a film on metal surface. ### 8.21 Constituents of Paints and their Function - The various constituents of paint include: - Pigment - Vehicle or drying oil ### 8.22 Methods of Application of Metal Coatings - **Hot dipping:** - In this process, metal or metal alloys such as iron, copper or steel having a high melting point is coated with a low melting metals such as tin, zinc, lead or aluminium is known as hot dipping. - Galvanizing: The process of coating a layer of zinc on iron or steel is called galvanizing. - Tinning or tin plating: The process of coating of tin on iron is called tin plating or tinning. - **Metal cladding:** - In this process, the base metal is protected from corrosion by coating of a thin uniform homogenous layer of a coating metal on the base metal. In this method, base metal sheet is sandwiched between thin sheets of corrosion resisting metals. - **Electroplating:** - In this process, noble metal is coated over more reactive metal. Most commonly used are tin plating and nickel plating. - **Electroless plating:** - In this process, we immersed the base metal article in a bath of a noble metal salt which is used for coating. The noble metal forms a layer on the base metal article by displacement of base metal by noble metal. This process is also called as ‘immersion plating' or 'displacement plating.' - **Organic surface coatings:** - Organic coatings are useful for the protection of metal surface by providing inert barrier on the surface from corrosion as well as corrosive environment. Organic coating is also helpful in decoration of metal surface. Organic coatings commonly used include paints, varnishes, lacquers and enamels. ### 8.23 Summary of Methods for Protecting Iron from Corrosion | Method | Description | |-------------------------------|-------------------------------------------------------------------------------------------------------------------------------| | Sacrificial Anode Protection | More active metal (Mg, Zn) is connected by a wire to the iron pipe, so that corrosion occurs at that anodic metal. | | The Impressed Current Cathodic Protection | An impressed current from an external source is applied in the opposite direction to neutralize the corrosion current. | | Anodic Coatings | Metal is coated with more active metal (e.g. Zn, Al, Cd) | | Cathodic Coatings | Metal is coated with more noble metal (e.g. Sn, Cu) |