Today, zinc is one of metals with the highest annual consumption rate in the World, following steel, aluminum and copper. As zinc is chemically active and easily alloys with other metals, it is used in the production of many alloys and compounds in the industry. Thanks to its strong electropositive characteristics, it is utilized for protection of other metals, especially iron-steel products, against corrosion. Zinc is used as the main product in galvanizing, die-casting alloys, brass and bronze alloys, zinc oxide and rolled zinc alloys.

Despite the fact that many new enterprises have been opened and capacities of some enterprises have increased, the amount of metal zinc put on the market have not increased enough due to the straits in operations. According to the data provided by International Lead and Zinc Study Groups, there are 70 countries, each having produced 1x10 3 tons/year and more in 1995. Among them, 17 countries produced zinc in an amount more than 100x10 3 tons. Total consumption of the biggest 10 countries among them constitutes 65% of the consumption in the world.

Nearly 50% of the total amount of zinc produced is used in galvanized steel, 20% in brass, 15% in casting, 5% in zinc oxide production and 7% in semi-finished products. Due to its eco-toxic effect, use of zinc is limited in some areas. Today, aluminum, magnesium and plastics are used instead of zinc, especially in the automotive sector.

1.2. International organizations in the sector

International Lead-Zinc Study Group (ILZSG) and International Zinc-Lead Research Organization (IZLRO) are among the active international organizations active serving in the sector. ILZSG has 30 member countries having 80% of metal consumption and 90% of metal production in the World.

This organization, established as an inter-governmental independent consultation institution by the United Nations, has its center in London and its activities cover the following issues:

- Compile all kinds of information regarding the production, consumption, supply-demand stock and future expectations related to lead-zinc ore and metal for the member countries,

- Create a suitable environment for inter-governmental consultations in relation to the international trade of zinc-lead,

- Carry out studies on all kinds of positions of lead and zinc in the world markets,

- Determine applicable solutions for the current and potential problems regarding lead and zinc and also the problems which cannot be solved under international trade terms.

ILZRO, which is the permanent observer member of ILZSG, carries out continuous researches on the technology of lead and zinc use and works for developing the use areas of these products.

Lead-zinc mining sector in the world is executed within an integrated system covering the stages of producing and enriching ore and obtaining molten metal. Therefore, the world's production and trade of concentrated lead-zinc is under the supervision of metal producers.

Companies in EU countries and USA, and following them, Japan, Australia and Canada have a corner in the ore production. Enterprises in these countries have the status of international companies and their partnership structure is highly complicated and nested.

2. The current situation in the world

2.1. Reserves

Zinc base reserves in the world in 1984 were calculated to be 290x10 6 tons metal zinc. Between 1984 and 1993, nearly 108.7x10 6 tons of new reserves were found. The known zinc reserves in the world are around 1.8x10 9 tons and, when the non-economic sources are taken into account, this amount reaches to 4.4x10 9 tons.

Zinc was used for the first time by the Chinese and Romans as an alloy material in the production of brass in 2000s B.C. The oldest archeological remain of zinc was found in a prehistoric Dacian settlement in Doroseh city in Transylvania, Romania. It was found as a result of the analyses on this piece of sculpture that it contained Zn in a rate of 87.5%, Pb in a rate of 11.5% and Fe in a rate of 1%.

Zinc is used in the coating sector as the basic substance of Zinc Coating applications in various cases with its perfect resistance against corrosion and also in many other sectors as minerals and byproducts that come out of alloy and zinc production to facilitate the human life and the daily life.

Zinc, which is a reactive metal containing electrochemical metals, exists in the nature in the form of mineral, like many other metals.

Zinc is used in many sectors that facilitate our daily lives. For example, zinc oxide (ZnO) is widely used with the name zinc white or snow white in the industries of glass, enamel, rubber, paint, (pigment), ceramic, plastic and paper.

Zinc chloride (ZnCl2) is used to prevent the decay of timber.

Zinc Sulphate (ZnSO4) is used in the production of reyan and fibrin, in the preparation of lithopone, in processing certain industrial waters and in pharmacy.

With its specific characteristics, Zinc based alloys have a great importance in industry. A large part of Zinc (Zamak, Lizro and Kayem) is used in alloys. These alloys may be listed as follows:

a) Pirinç: a) Brass: A copper alloy containing 5-45% Zn. Brasses take on different names according to the zinc rate they contain. Muntz metal contains 59% Cu and 41% Zn, soldering brass contains 75% Cu and 25% Zn and red brass contains 85% Cu and 15% Zn. Brass is the most widely used alloy in industry. Various types of brass are used in architectural works, in communication, computer and television industries, in automobile industry, in condenser tubes, welding rods, valve shafts, screws, pressing and rolling processes, radiator cores and pipes, clinch nails, grids, springs, chains, various rings and tubes, cartridges, flexible hoses, electric wires, leveling, forging, spinning and forming processes and in some jewels.

b) Bronz: b) Bronze: Bronze is a copper-tin alloy. Fluidity of molten bronze is increased by adding zinc in the rate of 2-4%.

c) Die-casting Alloys: In recent years, the zinc amount used for die-casting alloys have become more than the zinc used in coating works. There are three types of standard die-casting alloys. These alloys contain 3.5-4.5% Al and 0.1-2.5 Cu. Molten alloy is casted into steel molds at 482 C and pressure up tp 4.54 kg per cm. Die-casting enables making mixed forms with very close similarities. c) Die-casting Alloys: In recent years, the zinc amount used for die-casting alloys have become more than the zinc used in coating works. There are three types of standard die-casting alloys. These alloys contain 3.5-4.5% Al and 0.1-2.5 Cu. Molten alloy is casted into steel molds at 482 C and pressure up tp 4.54 kg per cm. Die-casting enables making mixed forms with very close similarities.

Furthermore, byproducts acquired from zinc production are used in many sectors. Grade of the ore is the main factor that determines the economy of lead-zinc mining. It is stated that mining of a deposit with an average grade of 4-5% Zn-Pb will be economic. However, what make a mining of low-grade deposits economic is other metals in the ore deposit like Cadmium, Indium, Gallium, Germanium, Cobalt, Molybdenum, Selenium, Thallium, Antimony and Pyrite.

1. International Organizations In The Sector

International Lead-Zinc Study Group (ILZSG) and International Zinc-Lead Research Organization (IZLRO) are among the active international organizations active serving in the sector. ILZSG has 30 member countries having 80% of metal consumption and 90% of metal production in the World.

This organization, established as an inter-governmental independent consultation institution by the United Nations, has its center in London and its activities cover the following issues:

• Compile all kinds of information regarding the production, consumption, supply-demand stock and future expectations related to lead-zinc ore and metal for the member countries,
• Create a suitable environment for inter-governmental consultations in relation to the international trade of zinc-lead,
• Carry out studies on all kinds of positions of lead and zinc in the world markets,
• Determine applicable solutions for the current and potential problems regarding lead and zinc and also the problems which cannot be solved under international trade terms.

ILZRO, which is the permanent observer member of ILZSG, carries out continuous researches on the technology of lead and zinc use and works for developing the use areas of these products.

Lead-zinc mining sector in the world is executed within an integrated system covering the stages of producing and enriching ore and obtaining molten metal. Therefore, the world's production and trade of concentrated lead-zinc is under the supervision of metal producers.

Companies in EU countries and USA, and following them, Japan, Australia and Canada have a corner in the ore production. Enterprises in these countries have the status of international companies and their partnership structure is highly complicated and nested.

Zinc Mining In Turkeyi

More scientific studies were started to be carried out upon the estabilishment of Mineral Research and Exploration Institute in 1935. Etibank Keban Konsantre Tesisleri got into service as the first domestic concentration plant in 1952.

• Following the enhancement of current mineral deposits in Yozgat Akdağmadeni and Kayseri Zamantı in the Central Anatolia, an annual production increase of 20x103 tons for zinc and 10x103 tons for lead will be realized.
• Lead-zinc and complex base ore explorations should be encouraged in the Northern and Central Anatolia metal zone. Deposits containing approximately 5% Pb+Zn still feasibly work depending on their gold and silver content.
• Lead-zinc production of Turkey, which is still 130x103 tons/year, should be increased to the level of 250x103 tons/year. Therefore, an Imperial Smelting plant should be established around Hopa in Northern-Eastern Black Sea region. This plant is required to have a production capacity of 100x103 tons/year metal zinc and 50x103 tons/year metal lead.
• Çinkur should be put into service again.
• Çinkur should be mostly operated with the oxidized lead-zinc ores in the Central Anatolia zone and the sulfide-containing Pb-Zn ores in the Black Sea region should be utilized in the Imperial Smelting plant to be established around Hopa in Northern-Eastern Black Sea region. The Imperial Smelting plant to be established will enable the use of ores in countries like Azerbaijan, Georgia and Armenia.
• Relationships between the Caucasian Countries and Turkish Mining Institute should be developed. In addition, Pb-Zn ore mining should be encouraged in Turkic Republics and the Balkans and also the purchase and operation of smelting plants in these areas should be supported.

Surface coating is one of the various methods used to prevent metal corrosion. Different methods are used for surface coating. Zing coating, which is one of the metallic coating methods, is named as galvanizing. Galvanizing is generally carried out with the methods of hot dipping, electrolysis and metal spraying.

As known, galvanized materials are used in the industries of building, construction, automotive and white ware. Therefore, it is very clear that the development in galvanizing will influence the whole society. In this article, microstructures of materials made by hod-dip galvanizing and zinc galvanealing are examined. Coating structure in galvanizing generally consists of steel, G, d, z and h. The structure in zinc galvanealing contains z phase, G phase, a little amount of z phase on the surface and a large amount of d phase.

INTRODUCTION

Corrosion is the damage to metals as a result of chemical and/or electrochemical reactions by the influence of the environment. In chemical corrosion, environment where the metal takes place is dry. The gas that comes out causes an oxide layer (scale) and this layer does not transmit the electricity. In electrochemical corrosion, the environment is wet and it causes the formation of rust. For electrochemical corrosion, there should be two electrodes, one of which is anode and the other is cathode, a liquid environment and electric current between anode and cathode. The cell formed here is called corrosion cell (galvanic cell).

Figure 1. Behavior of a Zinc-Coated Steel When the Coating is Scratched.

For corrosion to be prevented, at least one of the elements forming the corrosion cell should be deactivated. Therefore, factors such as material selection, coating method, design, cathodic and anodic protection and environment control gain importance.

In this article, the microstructures of materials acquired by means of hot dip galvanizing and zinc galvannealing, which are metallic coating methods.

ZINC COATING

Within the electrochemical potential series, zinc is an element that is considerably far from inertness. In addition, when compared to steel, it is more anodic, that is subject to corrosion [3,4]. A protective layer is formed on the zinc by the influence of moisture and water and reduces the corrosion rate [2]. In order to make use of this advantage, zinc coating is applied. It is seen in the Figure 1 that zinc interacts with steel when the coating is scratched and continues to be anodic.

Figure 2. Schematic Explanation of Zinc Coating Structure by Means of Fe-Zn Equilibrium Diagram [2].

Table 1. Properties of Phases in Fe-Zn Equilibrium Diagram

Hot Dip Galvanizing

Hot dip galvanizing is the formation of a coating with zinc and/or zinc alloys by dipping the prepared steel or casted iron into the molten zinc [5].

Fe-Zn balance diagram and the phases formed are indicated in the Figure 2 [2]. Formulas and crystal structures of the phases formed in Fe-Zn alloy system are given in the Table 1 [6].

uigi Galvani Luigi Galvan, an Italian having lived between the years 1737-1798, became an anatomy professor in Bologna University in 1775.During his researches, he found out that the dead frog moved when the metal cutter came into contact with the frog while making an experiment on the frog's nervous and muscular system. At that moment, he discovered the electrochemical relation between metal and the living creatures. He focused on this idea in his studies. He compiled and published his researches on that subject in 1791.This researched caused great discussions in the world of science. Volta came up with an idea on this discovery. He thought that this was caused by a metallic arc. As a result of these discussions, the researches were initiated on the possibility of electrotherapy by giving current to the living creatures.

Alesandro Volta (1745-1827) was really impressed by the ideas and studies of Galvani. He developed the idea that there might be electric relation between different materials and energy might be produced by means of this relation. He carried out intensive studies on this subject. In conclusion of his studies, he discovered that energy was produced by the electric relation between copper and zinc and he carried out his studies to transform this energy into packable energy (battery).In 1799, Volta's galvanic based batteries were accepted by the Royal Institute.

In the light of Volta's experiences, Humphry Davy (1778-1829) studied on the reactions of different materials with different chemical properties and results of these reactions. Volta had basically focused on the energy that came out of the contact of different materials. However, a chemical reaction was created with the combination of two different metals. And this reaction caused a kind of electrocoating. As a result of his experiments, Davy produced potassium with platinum-cathode galvanic batteries. He came up with the idea that a metal might be coated with another metal.

Another English Chemist who carried out studies with Humphry Davy Michael Faraday observed in his experiments in 1813 that the direct current formed by the reactions of bulk metals follows a chemical electrolyzed way and set a scientific ground for this. This is known as araday's Law of Electrolysis Still, corrosion and density loss (ASTM G102) of different materials are realized in the light of this information.

The first official zinc coating experiment was performed P.J. Malouin, a French Chemist by dipping iron into the molten zinc furnace and this experiment was presented to the French Royal Academy.

Stanilaus Tranquille Modeste Sorel, another French Chemist, obtained the patent of the methods of iron coating by zinc, cleaning with 9% sulphuric acid and surface preparation with ammonium chloride in 1836.In 1837, a year later, the English Patent Institute approved this method. A galvanizing of 10.000 tons was realized in English galvanizing industry in 1850.

Galvanization was used to protect large steel structures against corrosion in its first years. Espacially the steels at unapproachable places and energy transmission lines were coated before the assembly to protect the steel against corrosion and started to be used with the advantage of long service life. As the use of steel became widespread in the world thanks to the easy workability of steel with the technological developments, the application of galvanizing increased in parallel.

The increase of steel production before the war paved the way for the development of defense industry. The constantly increasing capacity enhanced in connection with different sectors. USA and many European countries realized the industrial revolution firstly with steel.

Today, there are great examples of steel structures having remained from those years in Europe and, of course, nearly all of these steels are galvanized.

Almost for 150 years, all kinds of steels with a composition convenient for galvanizing have been galvanized in increasing rates. Galvanizing is presently essential for many sectors.

Hot Dip Galvanizing (HDG) is, in the simple sense, is carried out by dipping the steel materials with a convenient composition and design into molten zinc bath following a proper cleaning process. In connection with the sizes of bathtub where the zinc is molten, the widest galvanizing product portfolio is obtained. Especially, in the recent years, the constructions used in steel structures are widely preferred in our country for its easy assembly and long service life.

HDG has been successfully applied in the whole world for nearly 150 years. It is more affordable than other coating methods and this is most its important advantage (see Graphics).

Furthermore, the coating method to provide the longest service life is preferred due to the difficulty of maintenance of lines to be assembled with steel, like Energy Transmission Lines. This is inevitably HDG. This is also valid for industrial plants and energy production plants.

Because of the reality of earthquake in Turkey, especially after the last Marmara Earthquake, investments of industrial or domestic building are preferred to be steel based. This has had a positive influence on the development of HDG sector by virtue of the necessity of galvanizing in steel structures.

In addition, HDG is used particularly in energy sector and also many sectors like agriculture and construction. The main areas of use are as follows;

CONSTRUCTION SECTOR

• Industrial plant steel constructions (Steel components, machine park, conveyors)
• Steel Building Post and Beams
• Fittings, Bolts, Nuts etc.
• Roofing Elements
• Rain gutters
• Industrial Gratings
• Fences, environmental protection elements
• Highway railguards
• Plant technology renewal
• Fire escapes
• Water installation materials

ENERGY SECTOR

• Energy Transmission Lines
• Lighting poles
• Traverse, potentiometer, chassis
• Insulator irons
• Grounding elements
• Lightning rod
• Energy Plants (Natural Gas, Wind etc.) steel elements
• Transformer buildings

AGRICULTURE SECTOR

• Watering apparatus (Pipe etc.)
• Greenhouse constructions
• Agriculture tools
• Agriculture machines

AUTOMOTIVE SECTOR

• Automotive components, chassis, fuel pipe
• Spare tire, step, chain, wedge
• Truck chassis, bodies, canvas elements
• Railway transmission lines
• Railway car industry

OTHER

• Highway Signs
• Baffle Plates
• Urban Furniture (Park Garden constructions, stations, advertising boards)
• Playgrounds and amusement parks
• Domestic building elements (Balcony parapet, stairs, fences etc.)
• Boiler, Vessel, Tank, Water Heaters
• Garbage Containers
• Airport and Port Landing Structures
• Steel structures of coastal hotels
• Materials for water sports and mountain climbing

Apart from all these, all kinds of steel materials used in the daily life and having a design convenient for Hot Dip Galvanizing may be galvanized. The main ones are sheets, strap pipes, tubes, wires, wire ropes, structural forms and many hardware. Galvanized materials are mostly used in construction works, agriculture, production of agriculture tools and automotive industry. Galvanized steel and iron products have very wide application areas in which they are specially used. Ceiling and wall coatings, wire and wire products, water tanks, pipes, buckets and tins, screws and bolts, nails and hinges may be given as examples. Zinc coatings are also used in the protection of structures under the sea or in other chlorite-rich solutions against corrosion. Pipelines, storage tanks, steel breakwaters, bridges and scaffolds are this kind of structures.

Galvanizing Process

Almost all kinds of iron and steel products may be galvanized with hot dip galvanizing method; however variables like the chemical composition of the material used, homogeneity and surface condition of the composition are important factors for obtaining successful results. These factors directly influence the coating thickness, structure or quality.

In Hot Dip Galvanizing process, a reaction occurs between steel and zinc when iron and steel materials contact with molten zinc and an iron-zinc alloy is formed on the steel surface. This alloy consists of iron zinc layers in different rates and amount of iron decreases and amount of zinc increases outwards. The outer layer consisting of pure zinc is formed while taking the material out of the zinc bath. Coating thickness and appearance depends on how the reaction occurs and how the outer layer hardens.

Effect of Silicon Composition

Amount of silicon in the steel to be used as galvanized has the most determining role in the reaction between iron and zinc. If aluminum is used as deoxidant in the steel production, amount of silicon in the steel will be low and short zinc-iron alloy layers will be observed in the coating and a pure zinc layer with silver color will be observed on the outer surface. However, if silicon is used as deoxidant, silicon rate in the steel and its homogeneous distribution should be taken into account. The reason is that silicon causes a very rapid reaction between iron and zinc; iron-zinc layer rapidly grows and forms a thicker alloy layer when compared to the alloy layer in the steel not containing silicon. Even sometimes the alloy layer reaches to the surface and causes a dull color darker than the pure zinc layer expected to be formed on the outer surface due to the iron in its content. This dullness may be seen in whole or in parts depending on the homogeneous distribution of silicon in the material.

In fact, the existence of silicon and color difference on the surface causes only esthetic problems. This layer on the surface does not reduce the resistance of coating to corrosion; on the contrary, it provides a longer life when compared to the bright parts, as the coating is thicker.

If a galvanized structure consisting of welded parts is to be produced and the surface appearance is important, the steels to be used should be of the same quality to prevent color differences after galvanizing.

• Material Acceptance: Material Acceptance Compliance of the products, which are delivered to our company to be galvanized, with galvanizing operations is controlled (product dimensions, painting, welding wastes etc.). Incompliances are notified to the company executive and a joint solution is found (opening holes on the materials to provide zinc flow, cleaning the paints etc.).
• Hanger of Materials: The hanger type suitable for products in the galvanizing process is determined.
• Degreasing: Grease or chemical wastes that may exist on the surface of material are cleaned by dipping the product in the alkali bath. The time of keeping in the bath depends on the cleanness of the material surface.
• Surface Cleaning in Acid: Products are dipped into HCL acid baths with a concentration of 18% and kept there until the rust on the material is cleared. Even if there is no rust on the product surface, the product should be kept in acid baths to activate the surface for galvanization.
• Flux Coating: Rinsed products are dipped into flux bath consisting of 30% NH4CI-ZN CI2 at a temperature of 50-70 C to ensure the adhesion of molten zinc to the product surface.
• Flaks Kaplama : Durulanan ürünler ergimiş çinkonun ürün yüzeyine yapışması için 50-70 oC sıcaklıkta %30 NH4CI-ZN CI2' den oluşan flaks banyosuna daldırılır.
• Galvanizing: Products with cleaned surface and coated with flux are coated by dipping into molten zinc bath at a temperature of 445-465 C. Products which are taken out of the galvanizing furnace and may be deformed are cooled in the air; other products are cooled in the water bath.
• Maintenance and Control: Coating thickness of galvanized products are measured with a digital micrometer. Products with coating defects are repaired or re-coated. Zinc leaks are cleaned with file.
• Stowing and Shipping: Products, controls and maintenances of which are completed, are properly stowed and loaded to vehicles and shipped to the companies.