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FINISH FINISHPROCESSES

FINISH PROCESSE FINISH PROCESSE Surface Preparation MECHANICAL CHEMICAL

Methods: wire brushing, polishing, buffing wheels, surface conditioning abrasives, belt polishing, mass finishing, blast finishing, and impact blasting with glass beads. Blast Finishing: Abrasives (sand) are propelled by a high velocity jet of air or water. Purpose: Purpose: To remove surface contamination, take off old paints, roughen surfaces before painting, remove surface irregularities, and for surface modification (removing distortion, decreasing corrosion & increasing fatigue life) MECHANICAL SURFACE PREPARATION

· (-) The blasting machine should be isolated for operators ’ safety. · Blast finishing depends on (1) The pressure (should be optimum). (2) the blast angle (3) the distance from the nozzle to the surface. Impact Blasting with Glass Beads: This finish process uses glass beads instead of abrasives. · (+) 1. Glass beads are chemically inert (no chemical reactions with the surface). 2. Low consumption. 3. Safe process. 4.Environmentally acceptable.

Purpose: · Purpose: 1)To remove burrs and contamination from the surface. 2) To perform peening on the surface (plastic deformation). This increases fatigue life, corrosion resistance, and surface strength. PLATING PROCEDURES Plating Plating is the process of application of coatings for the base material to impart it properties not inherent in the base. Purpose Purpose: To provide properties for the surfaces (Corrosion resistance, wear resistance, conductivity, colour, and reflectability).

Methods of Plating 1-Brush Plating: Brush plating is used to impart surfaces a masking by a liquid organic coating. Nickel and copper solutions are used with these coatings. Before plating the surface must be precleaned and free from oil, grease, dirt, and dust. The process is performed by hand. 2-Metalising Non-conductors: It is the process of imparting non-conductors a metal-layer in order to increase electrical conductivity, strength, and appearance. There are TWO methods of metalising:

1- Using conductive paints: The non-conductor is coated with a conductive paint that consists of varnish or lacquer and a conductive component (graphite, copper, or silver). 2 Catalytic Deposition: {Catalyst = a substance that makes another substance change chemically without changing itself}. In this process a metal film is applied onn a non-conductors by catalytic deposition. (Example: Electroless nickeling of plastics). Electroplating: Purpose Purpose : provides surfaces with decorative and protective coatings. Deposited metals: (anode) Al, Ni, Cr, Cu, Ag, Zn, and Au.

Principle: Principle: The workpiece (cathode) is plated with a different metal (anode), while both are suspended in a bath containing an electrolyte. A d-c of density of 6 to 24 V is required. Electroplating depends on 1)current density. 2) temperature of electrolyte. 3) condition of surface. Chrome Plating: Chrome plating is used to impart wear and abrasive resistance to surfaces (dies and punches). The process consists of passing a d-c from an anode to a cathode (workpiece) through a suitable electrolyte chromium-carrying electrolyte in the presence of a catalyst.

Tin Coating (Electrotinning) It is the process of electroplating steel sheets by tin. These sheets are used in manufacture of food cans. Copper and Nickel Plating Nickel plating provides a good corrosion resistance and bright appearance for steel. But it is recommended to apply copper plating before nickel because copper provides adhesive properties for nickel. Galvanising It is a zinc coating (by spraying or electroplating) on low-carbon steels to protect them from atmospheric deterioration. A spangle effect can be obtained if tin and aluminium are added. (+) Low cost, (-) Zinc is harmful for environment. Example: Highway guard rails.

Parkerizing Most enamels and paints require a base or primer before being used. For this aim, Parkerizing is applied. Parkerizing is the process of placing a thin phosphate coating by dipping steel in a solution of manganese dihydrogen phosphate at 90  C and for 45 min. Anodising Anodising is the process of forming a coating on an aluminium (anode) surface by oxidation (not by plating). The process is the reverse of electroplating. Sulphuric or chromic acid is used as an electrolyte. (+) Provides porous surface for better painting by organic coatings.

Hard Surfacing Hard Surfacing is the process of increasing the surface resistance to wear and abrasion. Methods: Methods: 1) Fusion welding, 2) Heat treatment. 3) by contact with other materials (compression). Calorizing Calorizing is the process of protection steel from oxidation at high temperatures by forming a protecting film of aluminium oxide Al2O3 on the steel surface. Al2O3 layer is produced by defusing Al into the steel surface at a high temperature.

Determination of Mass Deposited Faraday ’ s law is used to describe electrolytic processes (electroplating): “ The mass of the metal deposited (m) is equal to the electrochemical equivalent of that metal multiplied by the current flowing and the time for which it flows ”. An efficiency factor is also considered in Faraday ’ s law. m = E x i x t x  Where: E = Electrochemical efficiency, g per A i = Current flowing, A t = Time of current flowing, s  = Deposition efficiency, decimal Design Considerations of Metal Deposition

Note: Note: E and  are constant for a certain metal, so m depends on i and t, that is why ampere – hour meter is applied (to determine i and t ). Plating Time: The time required for plating, min: F = Conversion factor to deposit one mil of electroplate thickness, A- h/ft 3. T = Plate thickness, mils CD = current density, A/ft 2. Note: Note: mil = 1/1000 in F x T x 60 CD t =

Tank capacity: If the tank is circular, so, knowing D, the graph in fig 22.7 is used to determine v/h (volume per unit height). If the tank is rectangular, graph in 22.8 is applied.

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