Second Semester UNIT II CORROSION AND （第二学期第二单元和腐蚀）.pdfVIP

UNIT II CORROSION AND CORROSION CONTROL Chemical corrosion – Pilling -Bedworth rule – Electrochemical corrosion – Different types – Galvanic corrosion – Differential aeration corrosion - Factors influencing corrosion – Corrosion control – Sacrificial anode and impressed cathodic current methods – Corrosion inhibitors – Protective coatings – Paints – Constituents and functions – Metallic coatings – Electroplating (Au) and Electroless (Ni) plating. INTRODUCTION: Corrosion is an undesirable process. Due to corrosion there is limitation of progress in many areas. The cost of replacement of materials and equipments lost through corrosion is unlimited. Metals and alloys are used as fabrication or construction materials in engineering. If the metals or alloy structures are not properly maintained, they deteriorate slowly by the action of atmospheric gases, moisture and other chemicals. This phenomenon of destruction of metals and alloys is known as corrosion. Corrosion of metals is defined as the spontaneous destruction of metals in the course of their chemical, electrochemical or biochemical interactions with the environment. Thus, it is exactly the reverse of extraction of metals from ores. Example: Rusting of iron A layer of reddish scale and powder of oxide (Fe O ) is formed on the surface of 3 4 iron metal. A green film of basic carbonate [CuCO + Cu(OH) ] is formed on the surface of 3 2 copper, when it is exposed to moist-air containing carbon dioxide. CONSEQUENCES (EFFEC