Fillerliqureprotective Coatings

FILLERS OR EXTENDERS: Fillers are the inert materials which improve the properties of the paint and also reduce its cost. Generally, the main functions of extenders or fillers are. a. They serve to ill the voids in the film. b. They reduce the cost of the paint. c. They improve the durability of the film. d. They reduce cracking of the paints after drying. e. They increase random arrangement of pigment particles and f. Also they act as carriers for the pigment colour. Some of the examples of commonly used extenders are silica, talc, clay, gypsum, barites, magnesium silicates, calcium sulphates etc. PHOSPHATE COATING: Phosphate coating is obtained by the chemical reaction of aqueous solution of phosphate of iron, manganese and Zinc with base metals such as Iron, Steel and Zinc. The chemical reaction produces a surface film consisting of manganese iron phosphates or Zinc-iron phosphates. For increasing the reaction rate, accelerators are also added such as nitriles, chlorates or nitrates. Application of phosphate coating on the base metal surface can be done by brushing or spraying. This coating so not offers corrosion resistance but offer excellent bases for painting, impregnation with oils etc. Iron and Manganese coatings are generally used to reduce wear caused by friction. Phosphate coating can be done on other metals such as Aluminum, Cadmium and Tin. The colour of phosphate coating is usually grey.

CHROMATING: Chromating is the process of coating a surface film with a mixture of trivalent and hexavalent chromium for protection from corrosion. Chromating is generally used for the protection.

Galvanization Galvanization is a process in which the iron article is protected from corrosion by coating it with a thin layer of zinc. In this process, at first iron or steel is cleaned by pickling with dilute sulphuric acid solution (H2SO4) at a temperature range of 60-900C for 15 to20 minutes. Therefore, it removes scale, rust and other impurities present if any and then washed well and dried. Then after, dipped in the bath molten zinc which is at 425-4500C. To prevent it from oxide formation, the surface of bath is covered with a flux (NH4Cl). When the iron piece is taken out it is coated with a thin layer of zinc. And to remove excess zinc, it is passed through a pair of hot rollers; lastly, it is annealed at a temperature of 4500C and then cooled slowly. For the protection of iron and steel products, it is most widely used in the form of wires, pipes, nails, bolts, screws, buckets, tubes, roofing sheets etc.,

CEMENTATION: Cementation is the process in which base metal is packed in the powdered coating metal and heated to the temperature just below the melting point. Resulting in the formation of layers of alloys of varying composition. Generally an inert or reducing atmosphere is usually maintained during this process. Due to the diffusion of coating metal into base metal, an alloy of two different metals is formed. It is also suitable for coating small articles of uneven surfaces and shapes such as screws, bolts, valves and gauge tools. The coating metals used in this process such as zinc, chromium and aluminum are those which can alloy even with iron.

VITREOUS COATINGS: Vitreous enamel coatings are glass like materials of a wide of compositions, which are fixed on metals to provide the required protection. They are applied to cost iron and steel equipments such as tanks, kettles, piping and tank cars. They are applied by either of two methods. _________________________________________ 1 ________________________________________ Science of Corrosion prepared By B.SRINIVAS

A. The ‘wet process’, in which a slurry of glass constituents such as mixed calcium, nickel and Aluminum slats together with ground silica is painted upon a sand-blasted steel surface at room temperature. The object is heated gently to evaporate the water and then, temperature is raised slowly to red-heat, to allow the solids to fuse. A continuous glass is formed on the surfaces of steel object, which is bounded to the steel surfaces by means of Ferrous silicate. However, coating applied in this way tends to rather irregular and some what porous. B. The ‘dry process’ involves in dusting the dusting the dry glass constituents over the red hot object. The object is then reheated in order to fuse these glass constituents, the coatings applied in this way are usually between 0.4 to 0.8 mm thickness. This process gives rather better results than the wet process, but coasts more to carry out. C. VARNISHES: Varnish is a homogenous colloidal solution of natural or synthetic resin in thinner or oil. It is mainly used for decorative protection and also corrosion resistance. There are mainly two kinds of varnishes namely spirit varnish and oil varnish. D. SPIRIT VARNISH: It is a homogenous mixture of resin and organic solvent. Spirit varnish quickly due to the evaporation of solvent alone. But the film produced is some what brittle and has a tendency to track or peel off. The ingredients of spirit varnish are resins, solvents or thinners, driers and anti-skinning agents etc. E. OIL-VARNISH: It is a homogenous colloidal solution of a resin in a drying oil and a volatile. The presence of oil gives a smooth lustrous coating. On coating the varnish, first the evaporation of solvent takes place followed by oxidation and polymerization of drying oil. Varnishes take longer period (one day or 24 hours) for drying. PAINTS: Paint may be defined as the mechanical dispersion mixture of pigments and fillers which are in a liquid medium and later becomes film forming oil. The volatile liquids such as thinners are again added to these liquids. CONSTITUENTS OF PAINT: a. Pigment: It is solid constituent present in paint which provides a decorative colour effect to protect it from ultraviolet rays. Mainly, pigments are used in paints to 1. Provide strength and capacity to paint 2. Improve the impermeability LACQUER: Lacquer is compounded substance formed from colloidal dispersion of cellulose derivatives. Solvent, resins plasticizers and diluents. They dry quickly in air by the evaporation of solvent and diluents forming transparent, hard glossy and water proof film. a. Cellulose resins: The common cellulose resins used in lacquers are cellulose dinitrate, cellulose diacetate. Ethyl cellulose acetobutyrate etc. The cellulose resins provide water proofness, hardness, glossiness and durability. b. Other resins: these include phenol-aldehyde, alkyl, copal, dammer etc. The main functions of these resins are to produce glossiness, water proofness and adhesive nature. c. Plasticizers: The common plasticizers used are tricresyl phosphate dibutylpthalate etc. It reduces brittleness, improve flexibility of film and improve adherence. d. Solvents: Ethyl acetate, butyl acetate, Methyl-ethyl ketone etc. are some of the common solvents used. Their function is to dissolve the film forming constituents i.e., cellulose resin and other resins. e. Diluents: Benzol, Toluol solvent naptha, petroleum spirit etc., are used diluents in lacquers. Their function is mainly to reduce the viscosity of the lacquer, so that it can spread out and evaporate quickly. Electro plating is the deposit of a metal, by electrolysis over the surface of metals, alloys or non-metals. OBJECTIVES: _________________________________________ 2 ________________________________________ Science of Corrosion prepared By B.SRINIVAS

A. In internal combustion engines, an electro plated chromium coating not only avoids the wear but also improves the running performance. B. Base metals such as fe, al and their alloys are electroplated with gold and Silver so as to increase commercial as well as decorative value. Electroplating of Ni on steel: In plating bath solutions, the sulphonate bath consist of Nickel sulphonate (600 gm), Nickel chloride (5 gm), basic acid (40 gram/Lit), PH = 4 and sulphate bath consist of Nickel sulphate (250 gm) list, Nickel chloride (45 gm), Boric acid (30 gm/Lit) PH = 4 to 5 Nickel plating applied on Iron.

_________________________________________ 3 ________________________________________ Science of Corrosion prepared By B.SRINIVAS