 Good morning. Today's session we are going to learn about corrosion that is determination of rate of corrosion as well as the factors influencing the rate of corrosion. At the end of this session students will be able to explain the methods for determination of rate of corrosion and the factors influencing the rate of corrosion. Corrosion. Corrosion is defined as unintentional destruction of metal due to direct chemical and electrochemical reactions starting from the surface. This process will reduce this service life of any metallic material that we are using in our day to day practices. Thus it is a very severe problem and form a separate study matter under the name that is corrosion science. Determination of rate of corrosion. Basically there are two methods. First weight loss method and the second one is electrical resistance method. First weight loss method. A test is set up in the laboratory to show relative corrosion resistance of material as measurement by loss in weight of the metal. A clean metallic standard test piece is measured and weighed. It is exposed to corroding media for a known time. Then the piece is taken out, cleaned to remove the corrosion products. Its final weight is measured. The rate of corrosion of the material piece is calculated. The calculation is done with the help of this given formula that is rate of corrosion that is equal to r it is equal to k w upon d A t where k stands for constant. W is loss in weight of metal in milligrams. D is the density of the material in grams per centimeter cube. A is surface area of test piece in square centimeter and t is time of exposure in seconds. The results are generally expressed in millimeters per year or milligrams per square decimeter per day. Second method electrical resistance method. This method is employed for materials to be used in the form of thin wire or strip and the property used is electrical resistance increases as the corrosion decreases the cross section of metallic material. Hence, periodic or continuous measurement of the resistance between the ends of a specimen can be used to monitor the corrosion. The electrical resistance has nothing to do with the electrochemistry of a corrosion reaction. Here only the bulk property is measured which depends on the cross sectional area of the material. At this juncture pause the video and answer this question. The question is the rate of corrosion depends on A density of the material in grams per centimeter cube. B surface area of test piece in square centimeter. C time of exposure in seconds and D all of this. Answer for this question is the rate of corrosion depends on D that is all of this factors influencing the rate of corrosion. There are two basic factors. First factor related to the nature of metal and the second one is factor related to the nature of environment. First factor related to the metal. The properties are as follows that we are going to learn. First position of metal in galvanic series. Second hydrogen over voltage. Third purity of metal. Fourth relative areas of anode and cathode. Fifth physical state of metal. Sixth nature of oxide film. Seventh is volatility and solubility of a corrosion product. Second nature of environment. The properties are temperature, humidity, impurity present in atmosphere, pH of the medium and oxygen concentration and oxygen concentration set. First position of metal in galvanic series. It decides the rate of corrosion. A metal having higher position undergoes corrosion when connected to another metal below it. Also more different difference in the position of the galvanic series will cause faster corrosion at anodic metal. Second factor hydrogen over voltage. When two dissimilar metals are in electrical contact certain extra voltage is needed to evolve hydrogen at a particular metallic cathode than that is needed to evolve hydrogen at a platinum electrode. Depending upon the nature of a cathodic metal evolution of hydrogen is either a slow or brisk. This evolved hydrogen form a film around the cathodic metal thereby developing back potential. Thus the extra voltage is required to overcome this effect of back potential and is known as a hydrogen over voltage. Purity of metal. Pure metal always resist the corrosion. The rate of corrosion increases due to more exposure to the impurities. Fourth, relative areas of anode and cathode. Smaller the area of anode compared to cathode will lead to the faster corrosion of anode. For small anodic area the current density will be larger at anode. Larger cathodic area will demand more electron which will be fulfilled by fast reaction at anode that is oxidation takes place rapidly that is nothing but rapid corrosion. Physical state of the metal. Small granular metal will corrode faster than the larger one. Also the type of structure formed by a metal will have effect on corrosion rate. That is a bent metal is rapidly corroded due to the stress. Sixth nature of Oxide film. If this oxide film is porous and oxygen can be diffused through it it is more corrosion is observed. Also if volume of metal oxide is more than the volume of metal leads least corrosion or no further corrosion occurs. Volatility and solubility of corrosion product. In both the cases the corrosion will be faster volatility. Oxides of molybdenum or tin chloride are volatile so faster is a rate of corrosion in case of molybdenum as well as stream whereas in case of the solubility the corrosion product gets dissolved that's why the rate of corrosion will be enhanced by water. Factors related to the nature of environment the first one is temperature. The rate of diffusion increases by rise in temperature. Hence the rate of corrosion is also increased. Humidity. In humidity gases like carbon dioxide, oxides of sulfur, oxides of nitrogen are dissolved which forms electrolyte. It will cause galvanic corrosion. Some oxides are water soluble. Humidity washes away the corrosion products and the metal surface is further gets corroded. Impurity present in an atmosphere. Politins like hydrogen sulfide, oxides of sulfur, oxide of carbon and acid vapors causes more pollution where they dissolve. Some suspended particle get dissolved in humidity and form electrolyte which helps in corrosion. Fourth pH of the medium. Rate of corrosion changes with change in pH. Naturally in acidic medium the rate of corrosion is faster. Also in basic medium submittals such as lead, zinc, aluminium etc. form complexes and hence they corrode. For instance zinc corrode minimum at pH 11 but at higher pH more than 11 it corrode faster. Oxygen concentration cell and oxygen concentration. Oxygen is one of the important element responsible for the corrosion. It forms oxide and hydroxide in presence of water on the surface of metal as corrosion product. Oxygen concentration cell is formed on the surface of metal due to difference in oxygen concentration. For example iron rod half dipped in water. The portion which is immersed in water will be anode and outer portion will be cathode. So at anode the oxidation takes place and at cathode the reduction takes place. Ultimately this anode and cathodic reaction terminates into formation of iron hydroxide that is rust itself. Reference a textbook of engineering chemistry by Jane and Jane I use for this particular section. Thank you.