ABOUT GALVANIZING• Galvanization is the process of applying a protective zinc coating to steel or iron, in order to prevent rusting.• The protection is two fold: • Physical protection from moisture and air • Electrochemical cathodic protection.• Patina - a layer of zinc oxides, salts, and other compounds formed when Zn reacts with salts and moisture(protective and impermeable in nature)• Spangle – snow flake like glittering crystallites on the surface• Degree of galvanizing expressed in weight of Zn per unit surface area.
GALVANIZING METHODS• Hot dipping in molten Zn. (HDGI)-thick coat• Continuous sheet galvanizing(run a sheet/wire through molten Zn or Zn alloy ) –thin coat done before forging and can withstand bending and reshaping.(done without bracing)• Zn paint or Zn spray – thin coat, weak bonding with steel surface.• Electro-galvanizing – thin coat , strong bonding better finish
STEPS IN HDGI PROCESSi. The material generally used is Aluminum and silicon( and phophorous) killed steel so that no oxidation occurs during solidification.ii. Abrasive blasting : the process of using a forceful stream of particles, available in varying hardness, to remove residue and contaminants from steel surfaces to prepare for galvanizing.iii. Caustic cleaning to remove organic impuritiesiv. Passing the metal through a molten bath of zinc at a temperature of around 460 °C).v. Further reactions: The pure zinc (Zn) reacts with oxygen (O2) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO2) to form zinc carbonate (ZnCO3), a usually dull grey, fairly strong material that stops further corrosion in many circumstances, protecting the steel.
HDGI PROCESS• In case of HR, Si should be low and in case of CR P should be low. Else it will get irregular coating with low adherence.• Zn grades that can be used: Z1 to Z5 (Z1 is purest)• Lead is often added to the molten zinc bath to improve the fluidity(but not eco friendly), generally required in Z1 grade Zinc.• Aluminium added for shining• Chromate quenching (a layer of tri and hexavalent chromium compound) is done to prevent reaction of zinc with concrete/H2O , H2 evolution and wet storage stains.• Phosphating is done prior to any painting to increase adhesion of paints.
DUPLEX COATING• Duplex coating is liquid or powder paint over galvanized surface done to provide for synergistic protection.• Is 10 times more abrasion resistant• Is 8 times more chip resistant• Offers 20 times more edge protection• Very good long-term corrosion protection• Attractive appearance, unlimited color variety• High level of mechanical resilience.• High level of cost effectiveness due to its long service life• Environmentally-sound process.
LAYERS OF Zn COATING• The outer Eta layer that is composed of pure zinc.• The Zeta layer composed of an alloy that is 94% zinc and 6% iron• Delta layer : The second layer of zinc-iron alloy growth from the base steel formed during the galvanizing process; • The Delta layer’s chemical composition is approximately 90% zinc and 10% iron • Delta layer is 60% harder than the base steel , it protects from abrasion and corrosion• The thin Gamma layer composed of an alloy that is 75% zinc and 25% iron
ADVANTAGES AND DISADVANTAGES OF HDGI• Advantages:• Smooth and uniform coating.• Sharp edges and corners are protected optimally.• Stress release (internal)• The coating is very robust and can withstand minor bumps and bruises in connection with the mounting, transportation etc.Disadvantages :• Limitation of the product size.• Risk of distortion due stress release esp. in forged products.
APPLICATIONS OF HDGI STEEL• Roofing and walling• Safety barriers• Handrails,• Consumer appliances• Automotive body parts• Metal pails in most heating and cooling duct systems in buildings .• Cannot be used in places of extreme temperature conditions , peeling starts at 200˚C• According to the naming conventions - G90 finish would have 0.90 oz. of zinc per square foot.
ABOUT ELECTRO-GALVANIZED STEEL• Electrolytic deposition of Zn on the surface.• often used in automotive manufacturing to enhance the corrosion performance• This coating cannot stand high temperatures.• Zinc coatings, whether electrogalvanized or hot dip usually require chromate conversion coatings to deter white rust.• For fabrications that need to operate at high temperatures sulfamate nickel plating is used.• electrogalvanized coatings are susceptible to zinc whiskers• zinc coating in electro galvanization is mentioned in terms of thickness of the layer.
DIFFERENCE BETWEEN HOT DIPPED AND ELECTROPLATED ZINC :• Hot dip galvanized coatings are far thicker than electro- galvanized (usually up to 10x as thick)• Electroplated zinc coatings are smooth and shiny, and preferable for aesthetic reasons, whereas galvanizing may be spangled, or gray and drippy.• Zinc electroplating is thin and usually does not cause any problems with fasteners, whereas galvanized coatings are heavy and will interfere with fastener threads unless they are specially dimensioned to take the coating thickness into account.• Electro-galvanized coatings are susceptible to zinc whiskers• Zinc coating in electro galvanization is mentioned in terms of thickness of the layer.In HDGI it is mentioned in terms of weight of deposit.
GALVANNEALING• Annealing is a process of heating metals to above the critical temperature, maintaining a suitable temperature, and then cooling .• Annealing is used to induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties hardness and ductility.• Galvannealing is a process of galvanizing and annealing to produce specialized sheets of steel with an iron alloy layer.
GALVANNEALED STEEL• Galvannealed steel sheets have medium-light gray, smooth appearance.• They offer good paintability, weldability,• These are more corrosion resistant, and formable.• Galvannealing is commonly referred by its trade name of Jet-Kote.• Painting of galvannealed steel is important because it shows white to dark grey marks if it comes in contact with water.• Application: It is extensively used in the automotive, signage, electric equipment, and other industries requiring a metal with good paintability and long reliable service life.
GALVALUME• Trademark for a special type of hot-dip galvanized steel sheet with a coating consisting of 55% aluminum, 43.4% zinc and 1.6% silicon• Applications :• Galvalume sheets are used where it’s unique blend of corrosion resistance and high heat reflectivity is beneficial• In roofing and siding applications as well as unexposed automotive parts, appliances, furniture, outdoor cabinetry, computer cases, gutters, pipe, and in other non-construction applications
ADVANTAGES OF GALVANIZNG STEEL• Zinc is extremely durable. Esp. in hard water where it forms a passive layer of carbonates.• Galvanized surface is scratch resistant.• Satiny appearance that give aesthetic values to the product.
ALUMINIZED STEEL• Hot dipping of steel in either : • Al –Si alloy (5% to 11% silicon) : Type I • Pure Al : Type II• Silicon improves adherence.• Type I : Common in industrial products ,better adherence , intended principally for high temperature corrosion resistance.• Possible end uses: mufflers, furnaces, ovens, ranges, heaters, water heaters, fireplaces, and baking pans.• Type II : It is intended principally for applications requiring atmospheric corrosion resistance.(not high temperatures)• Possible end uses :corrugated roofing and siding, grain bins, air conditioner housings and drying ovens.
ALUMINIZED STEEL• The basic structure of aluminized steel is a thin aluminum oxide layer outside.(Al is always oxidised)• An intermetallic layer that is a mix of aluminum, silicon, and steel,• Finally a steel core.Aluminized Steel is highly resistant to corrosion andshows high levels of heat reflectivity.(mainly due tothe intermediate layer)Difference between Al and Zinc protection is thataluminium provides just the physical protection. So ,ifaluminium layer breaks/cracks corrosion will start, butnot in case of zinc.