Prevention and Control of Corrosion Muhamed Musthafa Pulikkal M.Tech (NDT) Roll no.-213118015

INTRODUCTION Corrosion- It is the gradual destruction of materials (usually metals) by chemical or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and stopping corrosion. There are three main components necessary for corrosion to occur: Metal (example: iron) Oxygen (usually from the atmosphere) An electrolyte (usually water)

Prevention and Control of Corrosion Corrosion prevention techniques can be generally classified into 6 groups: Environmental Modifications Metal Selection and Surface Conditions Cathodic Protection Corrosion Inhibitors Coating Plating

Environmental Modification- Methods to reduce the sulfur, chloride or oxygen content in the surrounding environment can limit the speed of metal corrosion. For example, feed water for water boilers can be treated with softeners or other chemical media to adjust the hardness, alkalinity or oxygen content in order to reduce corrosion on the interior of the unit. Metal Selection and Surface Conditions- Proper monitoring and the elimination of unnecessarily vulnerable surface conditions, along with taking steps to ensure that systems are designed to avoid reactive metal combinations and that corrosive agents are not used in the cleaning or maintenance of metal parts are all also part of effective corrosion reduction program. The development of new alloys, designed to protect against corrosion in specific environments are constantly under production. Nickel alloys and titanium alloys are all examples of alloys designed for corrosion prevention.

Cathodic Protection- This method, known as a sacrificial system, uses metal anodes, introduced to the electrolytic environment, to sacrifice themselves (corrode) in order to protect the cathode. In a sacrificial system, metallic ions move from the anode to the cathode, which leads the anode to corrode more quickly than it otherwise would. As a result, the anode must regularly be replaced. Example: Using Mg blocks in ships

Corrosion Inhibitors- Corrosion inhibitors are chemicals that react with the metal's surface or the environmental gasses causing corrosion, thereby, interrupting the chemical reaction that causes corrosion. The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime. A common mechanism for inhibiting corrosion involves formation of a coating, often a passivation layer, which prevents access of the corrosive substance to the metal Volatile amines are used in boilers to minimize the effects of acid. Corrosion inhibitors are often added to paints.

Coatings- Paints and other organic coatings are used to protect metals from the degradative effect of environmental gasses. Anti-corrosion coatings allow for added protection of metal surfaces and acts as a barrier to inhibit the contact between chemical compounds or corrosive materials. Ceramic coatings - Ceramic coatings provide corrosion, erosion and thermal protection. Polymer coatings - Polymer coatings/paints are composed of binders (major polymer constituent of the coating), solvents (water or Hydrocarbon solvents) for viscosity control, pigments and additives (eg., Corrosion inhibitors). Oil and grease coating

Plating- Metallic coatings, or plating, can be applied to inhibit corrosion as well as provide aesthetic, decorative finishes. There are four common types of metallic coatings: 1-Electroplating: A thin layer of metal - often nickel, tin, or chromium - is deposited on the substrate metal (generally steel) in an electrolytic bath. The electrolyte usually consists of a water solution containing salts of the metal to be deposited. 2-Hot dipping: When immersed in a molten bath of the protective, coating metal a thin layer adheres to the substrate metal.

3-Electroless plating: also known as chemical or auto-catalytic plating, is a non-galvanic plating method that involves several simultaneous reactions in an aqueous solution. It is mainly different from electroplating by not using external electrical power. In the manufacture of printed circuit boards, electroless plating is used to form the conductive part. The non-conductive part is treated with palladium catalyst and then made conductive by electroless copper plating

4-Mechanical plating: also known as peen plating, mechanical deposition, or impact plating It is a plating process that imparts the coating by cold welding fine metal particles to a workpiece. It is commonly used to overcome hydrogen embrittlement problems. Commonly plated workpieces include nails, screws, nuts, washers, stampings, springs, etc.

Process The process begins with a descaling and removing soil from the workpiece. After cleaning, the parts are prepared by combining them with water, medium, and a surface conditioner. The surface conditioner lightly coats the workpiece in copper, while the medium removes any residual mill scale or oxides. Finally, accelerators, promoters and metal powder are added to the mix. The accelerators and promoters provide the proper chemical environment for the plating to occur, such as the maintenance of a pH level of 1 to 2 to prevent oxidation and promote adhesion. The medium that is already in the mixture cold welds the metal powder to the workpiece through impacts that are induced by the tumbling action of the tumbler (Due to the rotation of tumbler) The surface finish can be improved with a water polish. The time required for the above process is approximately 50 minutes.

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