Galvanized Iron Sheets
Galvanized Iron Sheets
Galvanized iron (GI) sheets are basically steel sheets which have been coated with zinc. These sheets include a range of hot dip galvanized and electro-galvanized steel sheet. The zinc coating provides a continuous barrier which does not allow moisture and oxygen to reach the steel. It reacts with the atmosphere to provide the base steel a protection.
By galvanizing, the protection of the steel sheets against corrosion resistance is improved by several degrees of magnitude. The corrosion protection achieved by zinc-based coatings is as a result of the galvanic effect, since zinc is anodic to iron which makes it act a sacrificial metal in an aqueous or humid environment. However, resistance to surface degeneration can be further enhanced by the incorporation of top coat on the galvanized surface. Galvanized steel sheets are also known for excellent shiny surface and formability attributes besides satisfactory peel-off resistance and fusing of the coating completely to the steel surface.
Zinc weathers at a very slow rate, so the coating normally has a long life. Zinc has a greater electro-negativity than iron and hence provides cathodic (or sacrificial) protection to the steel. This results in the zinc corroding in preference to the steel if the coating is chipped or damaged to expose the base metal, besides acting as galvanic protector. The term ‘galvanizing’ is derived from the name of Italian scientist Luigi Galvani.
The applications of steel sheets in daily life are constantly increasing. Coated steel products such as galvanized steel sheets have enjoyed particularly strong growth in those areas where corrosion resistance and cost reduction by process omission are important. Galvanized steel sheets offer superior economy in construction and automotive applications, electric appliances, and electronic equipment.
Galvanizing is the process of applying a protective zinc coating to steel. Galvanized steel sheet is one of the most used sheets for the roofs in homes, industries, and commercial premises. The roofs are made up of mild steel coated with zinc. Galvanized steel roofing sheet usually comes in different form of long run roofing such as trapezoidal, trough section / concealed fix, and corrugated. It also sometimes comes in the form of flat sheets. The galvanized steel sheets are normally fixed in place using lead-headed nails in order to prevent it from rusting.
The other functions of the zinc layer on the galvanized steel sheets are (i) to retain the steel intact with its full initial strength, (ii) to provide the surface a more pleasing appearance, (iii) to increase the life of any suitable organic finishing system applied over it, and (iv) to protect the steel from corrosive attack in most environments, acting as a continuous and lasting shield between steel and the environment.
The thickness or gauge is the most relevant criteria for the resistance of the galvanized steel sheet to the ‘pulling’ forces such as suction under strong wind. The gauge together with the type of corrugation is also the most relevant for stability and resistance to other loads. The thickness of the coating is proportional to the coating mass. However, the length and width can be chosen base on the required specification and purpose.
The galvanized coating on the steel sheet consists of the steel core, with an inter-metallic alloy layer and outer zinc layer on both surfaces. The cross-section of the zinc coating is given in Fig 1. Besides outer layer of zinc (eta layer), it contains gamma, delta and zeta layers. The hardness of gamma, delta and zeta layers of the zinc coating, as expressed in DPN (diamond pyramid number), is higher than the underlying steel. Due to this higher hardness these layers provide excellent protection against coating damage through abrasion. The thickness of hot dip galvanized coatings is determined by the thickness of the zinc-iron alloy layers which form when the steel reacts with the zinc. The alloy layer is normally 95 % and 5 % iron. Thicker galvanized coatings provide enhanced durability. The tensile strength of the zinc coated layer increases with increase in thickness.
Fig 1 Cross-section of the zinc coating
The eta layer of the coating being low in hardness is quite ductile and provides the coating some impact resistance. The zeta, delta and gamma zinc-iron alloy layers are actually harder than the base steel, resulting in galvanizing steel’s outstanding resistance to abrasion and mechanical damage. Abrasive or heavy loading conditions in service can remove the relatively soft eta layer of zinc from a galvanized surface, but the very hard zeta alloy layer is then exposed to resist further abrasion and heavy loading. The alloy layer is vital for achieving a good bond between steel and zinc.
The basic properties of galvanized steel sheets are corrosion resistance, good surface appearance, good formability for meeting th deep drawing requirements, good paintability provided proper pre-treatment is performed, and good weldability. Hardness, ductility, and adherence combine to provide the galvanized coating with very good protection against damage during rough handling. The thickness of the coating is proportional to the coating mass. The thickness of hot dip galvanized coatings is determined by the thickness of the zinc-iron alloy layers which form when the steel reacts with the zinc. Higher coating thickness of galvanized steel results in better corrosion resistance and provides enhanced durability. However, it can lead to low formability of the steel. The tensile strength of the zinc coated layer increases with increase in thickness. Further, galvanized coatings are slightly thicker at the corners and the edges which is an important advantage over most of the organic coatings which thin out in these critical areas. The texture of galvanized steel sheet is shown in Fig 2.
Fig 2 Texture of galvanized iron sheet
Process of galvanizing
The process of galvanizing steel sheet was developed simultaneously in France and England in 1837. Both of these methods employed a ‘hot dipping’ process to coat steel sheet with zinc.
Galvanizing process is broadly divided into hot dip process and electro-galvanizing process. The hot dip process is more suitable for heavy coating weights, and electro-galvanizing for lighter coatings. For reasons of efficiency, galvanizing of cold rolled strip is more common than galvanizing of cold rolled sheets. The galvanized steel sheets are produced both in annealed and full hard conditions. Fig 3 shows the two processes of galvanizing steel strip.
Fig 3 Process of galvanizing steel strip
Hot dip galvanizing – The principle of the process consists of the immersion of steel strips in molten zinc. The zinc used for galvanizing is high grade with a zinc content of minimum of 99.95 %. During the production process, the molten zinc is applied to the surface of the steel substrate in a continuous process as shown in Fig 3. The coating of the steel is achieved by passing it through a bath of molten zinc at speeds of around 180 meters per minute in the form of a continuous strip.
Normally, the continuous hot dip coating process starts with cleaning the steel with the use of alkaline liquid and is then followed by rinsing and drying. The cleaned steel is the conveyed into the annealing or heating furnace to make it softer and also impart formability and desired strength. In the heating furnace, steel is operated under a low gas atmosphere, consisting of nitrogen and hydrogen so as to enable the removal of traces of oxide which can be on the surface of the steel. Vacuum chamber known as ‘snout’ is connected to the exit end of the furnace and to the molten zinc coating bath to avoid the re-oxidation of the heated steel product by air.
In the zinc coating bath, the steel product is conveyed round about a submerge roll and reacts with the molten zinc metal so as to form the adhesive coating, and then withdrawn in a vertical direction. After the removal of the coated steel from the bath, any excess molten zinc is removed using high-pressure air to obtain a coating thickness which can be closely controlled. Finally, the steel is allowed to cool to enable the zinc coating solidify onto its surface. Solidification of the molten metal before it makes contact with another roll is important to avoid damaging or deformation of the zinc coating.
Production of galvanized steel sheets can be classified into six distinct stages. They are (i) welding process to form one continuous sheet by welding the steel sheet ends which has been sheared to the preceding sheets, (ii) a straightening process where a high-performance tension leveler is normally employed to straighten the steel sheet for getting good flatness, (iii) cleaning process in which initially the steel sheet is cleaned in an alkali bath and brushed for initial degreasing so as to reduce surface oxides for achieving complete adhesion of coating and then the sheet is dipped in an acidic bath to activate, (iv) continuous annealing process which involves the reduction of the oxidized film on the surface of the steel sheet by passing it continuously through a reduction furnace, (v) galvanizing process in which, the sheet is left in a reduction atmosphere and immersed directly into a bath of molten zinc and the steel sheet is then rolled around a roller which is dipped into the coating bath and is removed from the bath vertically by pulling, (vi) removal of excess molten zinc using high-pressure air which is carried out to achieve a coating thickness that can be closely controlled, and (vii) chemical treatment process in which the coated zinc sheet is exposed to different chemical treatments in accordance to the desired use. The treatment includes phosphate treatment for excellent paintability, and chromate free special treatment for good corrosion resistance.
The annealing furnace is used to apply the heat cycle needed to achieve the required mechanical properties and to activate the surface with a reducing gas. This makes it easy to coat zinc on the strip surface. The coating weight is controlled by a purge gas jet blown on both surfaces of the strip from a nozzle above the pot, to remove excessive molten zinc. This process gives a relatively thick coating of zinc that freezes into a crystalline surface pattern known as spangles. During the process, a multiple layered structure of iron-zinc alloys is formed between the inner surface of the zinc coating and the steel strip. For hot-dip galvanization, the typical coating thicknesses can range from 12 micrometers to over 160 micrometers (for both sides together).
Since the paint adhesion and weldability of the surface of the zinc layer are not necessarily good, galvannealing has been developed to improve these properties. In the basic process for galvannealed strip, the zinc-coated strip emerges from the pot and is heated in a galvannealing furnace, forming an iron-zinc alloy layer by the inter-diffusion of iron and zinc coating layer, so that the surface of the zinc layer also contains some amount of iron. The galvannealing line is normally equipped with a skin pass mill, a tension leveller, and chemical treatment equipment for chromating, following the galvannealing furnace.
Electro-galvanizing – Very thin formable zinc coatings, ideally suited for deep drawing or painting, can be produced as coated steel strip by electro-galvanizing. In this process, zinc is electrolytically deposited on the steel sheet. The coating is thin and uniform and has very good adherence. It is free of the zinc crystals present on hot-dip galvanized sheet. The coating is composed of pure zinc and has a homogeneous structure. It is also possible to produce electro-galvanize coatings of zinc-nickel and zinc-iron. Electro-galvanizing process coating is normally not as thick as that produced by hot-dip galvanizing. One advantage of electro-galvanizing is that it is done in cold condition and does not alter the mechanical properties of the steel.
The principle of the process consists of immersion of steel sheet in an electrolyte, a solution of zinc sulphate or cyanide. Electrolytic action deposits a coating of pure zinc on the surface of the steel sheet. The advantages of this process is that the thickness of the coating can be accurately controlled while the limitation of this process is that the thick coatings provided by the hot dip galvanizing process are not normally possible with this method.
In the electro-galvanizing process, electroplating cells are used to apply zinc to the steel substrate. The continuous process consists of surface preparation, plating and oiling. Surface preparation is essential to assure good coating adhesion and consists of spray alkaline cleaning, brushing, electrolytic alkaline cleaning, and sulphuric acid surface activation. In the plating section, electrical current is passed through the strip which is immersed in an aqueous solution containing zinc ions. As a result, the zinc is uniformly deposited onto the steel substrate. The thickness of the deposit is constantly monitored by an in-line x-ray coating weight gauge. After plating, a uniform application of rust preventive oil or pre-lube is electro-statically deposited on the strip.
Corrugated galvanized steel sheet
A part of the galvanized steel production is corrugated to produce galvanized corrugated steel sheets (GC sheets). Corrugated galvanized steel sheets are at present the best known, most widely available low cost roof covering material. These sheets are lightweight roofing material made of thin sheets, stiffened by corrugations. Corrugations, such as waves, considerably increase the strength and stiffness of the light-weight material. There is a large variety of qualities of corrugated steel sheets which are available in the market. These sheets are very popular. Most of the corrugated sheets are light-weight and considerably of low cost.
Corrugated galvanized steel sheets are made of thin plain galvanized steel sheets, stiffened by corrugations. The corrugations, such as waves or folds, increase the strength and stiffness of the sheeting material considerably. Without these waves, the sheets have limited load bearing capacity and are highly deformable. The sheets are normally made of mild steel, which is then galvanized to increase the resistance against corrosion and increase the durability or service life.
The corrugation adds stability to the sheet or panel materials so they can span larger widths than traditional smaller sized roof covering materials like for example tiles. As rather large surfaces can be covered per sheet this can result in material and cost savings for the support structure and savings in installation time. These properties make corrugated steel sheets particularly interesting for humanitarian shelter responses, in emergencies as well as for recovery and reconstruction.
The important measurements which define corrugated galvanized steel sheets are the dimensions of the sheet (length and width after corrugation), the thickness, and the number of the corrugations. The standard dimensions of these sheets are normally covered by national standards. Thickness is normally expressed in gauge with lower the gauge, the thicker is the corrugated sheet. Since gauge size standards have been developed based on the weight per surface area of the sheet for a given steel material, hence it indicates the equivalent thickness in mm of the steel material. The number of corrugations varies with the width of the galvanized corrugated sheet.
The quality and performance of corrugated galvanized steel sheets is determined by three main criteria namely (i) for stability and resistance to loads which are its dimensions, especially its thickness and the type of corrugation, (ii) for durability (resistance to corrosion) service life and appearance which are due to the type of galvanizing or coating used, and (iii) for durability (resistance to corrosion) and service life which depend on the thickness of the galvanization / coating on the sheet surface. The weight per surface area has less direct relevance for the performance of the sheet.
The basic measurements of corrugated galvanized steel sheets are (i) corrugation pitch, (ii) corrugation depth, (iii) ray of arch, (iv) number of corrugation pitch, and (v) number of crests (corrugations). Fig 4 shows some of the dimensions of corrugated steel sheet.
Fig 4 Important dimensions of corrugated galvanized sheet
The corrugation process is carried out using the process of roll forming. This process is automated to achieve high productivity and low labour costs. In the corrugation process, sheets are pulled off by rolls and through rolling dies which form the corrugation. After the sheets pass through the rollers it is sheared off at a desired length. The standard shape of corrugated material is the round wavy style, but can be easily modified to a variety of shapes and sizes by simply changing the dies. Fig 5 shows a typical corrugating machine.
Fig 5 Corrugating machine for galvanized steel sheet
Galvanized steel sheets specification
The galvanized steel sheets are produced as plain coils / sheets (GP) or corrugated sheets (GC). These are value added steel products which are tough, sturdy, light weight, bright, corrosion resistant and easy to transport. These are normally produced in the thickness range of 0.12 mm to 2.0 mm and width upto 1,830 mm. The weight of zinc coating varies from 100 grams square meter (gsm) to 750 grams square meter. The weight of zinc coating varies with the thickness of the steel sheet and the application of the galvanized steel sheet. Estimated life of galvanized steel sheets in different atmospheric conditions is given in Tab 1.
|Tab 1 Estimated life of galvanized iron sheets in different atmospheric conditions|
|Zinc coating weight||Rural||Tropical marine||Suburban||Urban||Highly industrial|
GP and GC sheets are classified as ordinary (GP), ordinary hard (GPH), lock forming (GPL), deep drawing (GPD), extra deep drawing (GPED), corrugated ordinary (GC) and corrugated ordinary hard (GCH). For producing these different qualities of these galvanized steel sheets, the quality of raw material (cold rolled sheet) is chosen accordingly.
Mechanical properties of galvanized steel sheet describe their behaviour under the application of external environment. The important mechanical properties are given below.
Strength – It is the resistance offered by the sheet when an external load is applied. The strength of the sheet is a function of its ability to withstand external forces. The stress on the steel sheet can be compressive, tensile, compressive and tensional, or shear.
Elasticity – The galvanized steel sheet is to have the ability to return back to its initial position after undergoing deformation when the load is withdrawn. The elastic limit of the galvanized sheet is the maximum stress which it can withstand without permanent deformation.
Plasticity – It is the ability of the galvanized steel sheet to deform permanently to some degree without rupture. Plastic deformation occurs when the elastic limit is exceeded. Normally, plasticity level rises with increasing temperature. Plasticity is one of the material properties to consider during the secondary forming processes. Plasticity enables the transformation of the galvanized steel sheet into different product of desired shape and sizes through the application of heat, pressure, or the combination of heat and pressure.
Ductility – It is the property which enables drawing of the steel on the application of load or force. The base metal of galvanized steel sheet is mild steel which is known to be ductile which enables drawing by extrusion or pulling through hole in a prepared die. The ductility of the steel sheet decreases with increase in temperature. Elongation and reduction are the key measuring parameters for the ductility.
Malleability – It is the flattening ability of the steel into sheet without crack propagation during the cold working and hot working processes. Malleability is a compressive property while ductility is a tensile property. Malleability of a material increases with the rise in the temperature.
Brittleness – The brittleness of the steel sheet is its ability to break without permanent distortion. The mild steel base of the galvanized sheet is less brittle and this enables it not to break after much deformation unlike cast iron.
Toughness – It is the ability of the steel sheet to resist elastic and plastic deformations. The galvanized steel sheet shows sizable degree of toughness due to the presence of some trace elements. The amount of energy the galvanized steel sheet can absorb before fracture is its toughness.
Galvanized steel sheets are subjected to bend test. These sheets are to withstand bending through 180 degrees around a mandrel of specified diameter without peeling or flaking of zinc coating. Galvanized steel sheets meant for corrugation are not required to undergo the bend test. Galvanized steel sheets are also to undergo coating test as per the specified standard. Both GP and GC sheets and the galvanized steel sheet coils are to be reasonably flat and free from bare spots, holes, tears and other harmful defects.
Application of galvanized steel sheets
Galvanized steel sheets are sheets which are extensively being used in various applications. Some of common uses of the galvanized steel sheets are (i) agriculture such as grain silos, sprayers, pans, and feeding troughs etc., (ii) automobile sector such as car, bus and truck bodies, undercarriage work, air and oil filters, fuel and oil tanks, and exhaust pipes etc., (iii) construction such as roofing, side walls, partitions, panels, valley gutters, louvers, false ceilings, partition walls and ducts, rolling shutters, highway bumpers, slotted angles and paint coated products etc., (iv) house hold things such as trunks, ice boxes, tubs, buckets, storage bins, water tanks, washing machines, and bath room doors etc., (v) electrical appliances such as air conditioners, refrigerators, freezers, electrical panels, and decorative lamps etc., (vi) furniture and fixtures such as desk, lockers, cabinets, racks, light weight chairs etc., and (vii) other general uses such as ducting, drums / barrels, containers, thermal cladding, railway coaches, sign boards, hoardings, and road signs etc.
Advantages with use of galvanized steel sheets
The use of galvanized steel sheets gives several advantages. These advantages are (i) low cost material since galvanizing has lower cost than many other normally specified protective coatings for steel, (ii) less maintenance cost since galvanized steel sheets are virtually maintenance free and last longer, (iii) long life since the life expectancy of the galvanized steel sheets is quite high in rural, urban, and coastal environments, (iv) reliability, since galvanizing is normally carried out as per standards and minimum coating thicknesses are applied and hence coating life and performance are reliable and predictable, (v) toughest coating since the galvanized coating has a unique metallurgical structure which gives it outstanding resistance to mechanical damage in transport, erection and service, (vi) automatic protection for damaged areas since galvanized coatings corrode preferentially to steel, providing the steel cathodic or sacrificial protection to small areas of steel exposed through damage and hence unlike organic coatings, and small damaged areas need no touch up, (vii) complete protection since every part of a galvanized steel sheet is protected, (viii) ease of inspection since galvanized coating is assessed readily by eye, and simple non-destructive thickness testing methods can be used, (ix) galvanizing process is not dependent on weather conditions, and (x) the surface of galvanized steel sheet is aesthetically pleasing.