1. INTRODUCTION
2 PROTECT
2.1 Metallic Coatings
2.1.1 Electroplating
2.1.2 Electroless Plating
2.1.3 Thermal Spray Coatings
2.1.4 Solder Sealing
2.1.5 Hot–Dip Coating
2.1.6 Immersion Coatings
2.1.7 Vapor – Deposited Coatings
2.1.8 Mechanical Plating
2.2 Conversion Coatings
2.2.1. Phosphate Coatings
2.2.2 Chromate Coatings
2.2.3 Anodic Coatings
3 GALVANIZING PROCESS
3.1 History of Galvanizing Process
3.2 Iron and Steel for Galvanizing Process
3.3 What is Galvanizing Process
3.3.1 Surface Preparation
3.3.1.1 Cleaning
3.3.1.2 Acid Pickling
3.3.1.3 Fluxing
3.3.2. Galvanizing
3.3.3 Inspection
4 HOT – DIP GALVAN
4.1. def
4.2 Effect of Hot Galvanizing on
Material Properties
4.3 High Temperature Hot-Dip Galvanizing
5 ELECTROGALVAN
5.1 Definition of Electrogalvanizing Process
5.2 Bath Types of Electrogalvanizing
5.2.1 Acid Zinc Baths
5.2.1.1 Bath Preparation
5.2.1.2 Operation and control
5.2.1.3 Characteristics Of The Bath
5.2.1.4 Operating Parameters
5.2.2. Zinc Cyanide Baths
5.2.2.1. Bath Properation
5.2.2.2 Operation and Control
5.2.2.3 Function of Components
5.2.2.4 Operating Parameters
5.3 Electrogalvanizing Equipments
5.4 Operation of Electrogalvanizing Process
5.5 Properties of Electrogalvanized Coatings
5.6 Benefits of Electrogalvanizing
5.7 Testing of Electrogalvanizing
5.7.1 Thickness Tests
5.7.2 Corrosion Tests
5.7.3 Inspection
5.8 Certain Advantages of Electrogalvanizing over
Hot-Dip Galvanizing
6 PERFORMANCE OF GALVAN
6.1 Factors Affecting Galvanizing Quality and
Service
6.2 Service Life of Galvanized Coating
6.3 Corrosion Performance of Galvanized Coating in
Atmospheric Environment
6.3.1 Industrial & Urban Environments
6.3.2 Rural and Suburban Environments
6.3.3 Marine Environment
7 THE ECONOM
7.1 Initial Cost of Coating
7.2 Coating Life to First Maintenance
7.3 Cost of Maintenance
7.4 Hidden Costs
1. INTRODUCTION
Steel is the most vital material in our life. Its grate strength, formability, low cost and availability have made possible our world’s great industrial complex. However, the increased use of steel has highlighted their major enemy, corrosion. Corrosion is a international economic disaster area. Only in the U.S., the annual cost of corrosion and protection against corrosion is estimated at 70 billion dollars.
Corrosion may be defined in several ways: destruction or deterioration of a material because of reaction with environment; destruction of materials by means other than straight mechanical; and extractive metallurgy in words. It is convenient to classify corrosion by the forms in which it manifests itself, the basis for this classification being the appearance of the corroded metal. Each form can be identified by mere visual observation. In most cases the naked eye is sufficient, but sometimes magnification is helpful or required. Some of the eight forms of corrosion are unique but more of them are more or less interrelated. The eight forms are: (1) uniform or general attack, (2) galvanic or two-metal corrosion, (3) crevice corrosion, (4) pitting, (5) intergranular corrosion, and (8) stress corrosion. This listing is arbitrary but covers practically all corrosion failures and problems. [1]
In practice, corrosion is an insidious process, which is often difficult to recognized until deterioration is well advanced; it may have the following interrelated results:
Damage to process plant, structural assemblies another equipment.
Consequent shutdowns for repair or replacement work.
Risk of injury to personal due, for example, leakage or mechanical fractures.
Contamination of process products
Loss of product
Loss of operation efficiency
The need to redesign or to over-engineer
Unfavorable publicity.
Environmental contamination.
Customer alienation.
Whatever the results of corrosion, remedial work may be costly in terms of finance, time and manpower; extensive redesign, fabrication, materials replacement and treatment or increased maintenance may be required.
Over the past two decades, increasing attention has been given both the understanding of corrosion process and the development of methods of prevention. There are several possible reasons for this, including the following:
The extent and diversity of the use of metals has increased; new always continue to be developed.
Increasingly specialized applications of metals in particularly aggressive media as in the fields of aerospace, offshore oil and atomic energy.
The existence of more corrosive environments due to increasing air and water pollution.
Economic incentives sometimes result in the reduction of metal structures to slimmer dimensions; the impact of corrosion is thus made more rapid and/or severe.
The increasing trend towards longer periods between maintenance, faster remedial work and the desire to monitor processes automatically.
The cost of corrosion to industry (and, hence, to society) is very large. A sizeable fraction of these costs could be saved by a wider appreciation of known techniques for corrosion prevention, coupled with the development of more effective methods of protection. [2]
2 PROTECTION OF STEEL FROM CORROSION
Corrosion protection is often essential consideration in selection of carbon or alloy steel for a given structural application. Over-all economists, environmental conditions, degree of protection needed for the projected life the part, consequences of unexpected service failure, and importance of appearance are the chief factors that determine not only whether a steel part needs to be protected against corrosion, but the most effective and economic method of achieving that protection as well. Protection against corrosion can be done by two main techniques. (1) Metallic Coatings and done by two main techniques. (2) Conversion Coatings. [3]
2.1
Metallic CoatingsMetallic coatings may be applied by electroplating, hot immersion (galvanizing), chemical deposition, or spraying of molten metal (metallizing)…etc. They are used to provide protection against corrosion and resistance to wear; they serve as a base for painting to provide a reflectant surface. Some of these are briefly explain below. [4]
2.1.1 Electroplating
Electroplating is the process of passing current through the material to be deposited, the solution or bath, and the part on which the material is to be plated.
The base metal is made the cathode in an aqueous solution of a salt of the circuit and replenish the solution. Also, in order to increase its conductivity, other chemicals that will ionize strongly are added (e.g. sulfuric acid is added to an acid copper plating bath.). Plating solutions attack metals and containers, and plating equipment is expensive to maintain. The cost of plating is small compared to cost of surface preparation, cleaning, and handling of the parts. Polishing also adds considerably to the cost of plating; therefore, the advantages of improving appearance and corrosion resistance must be balanced against the increased cost. The properties of the coating will vary with the composition of the plating solution, current density, agitation, solution pH, and solution temperature. Generally, the three properties usually sought when specifying an electroplate are hardness, resistance to corrosion, and appearance.
2.1.2 Electroless Plating
Electroless plating is a chemical plating process. Nickel and copper are the metals that are most often used in this process; most other metal-plating systems are not available as electroless plating. This technique necessitates strict control of both the temperature and composition of the chemical bath, resulting in costs that frequently exceed electroplating. An important advantage of electroless plating is that it does not produce hydrogen embrittlement. Therefore, it is often used to plate case-hardened hardware for which embrittlement in service would be intolerable.
2.1.3 Thermal Spray Coatings
This method of depositing metallic and sometimes non-metallic coatings frequently referred to as metallizing. The coatings can be sprayed from rod or wire stock or form powered material. There are three systems that are used: Thermal spray coatings are used extensively for improving wearing surfaces, cloth and paper are coated for use in electrical condensers.
2.1.4 Solder Sealing
Solder sealing of metal parts to glass or ceramic parts presupposes that the metal and non-metal parts will have approximately the same coefficient of expansion during the wide variation of temperature necessary in the sealing process or in the operation of the apparatus. The procedure followed is to chemically deposit a metal on the glass or ceramic. The metallic compounds make the surface of the ceramic. The metallic compounds make the surface of the ceramic or glass chemically pure and deposit the metal at the same time. This simultaneous action enables the metal to adhere firmly to the glass or ceramic. A soft solder is added to this metal deposit. The metal container receiving the glass or ceramic is also prepared for soldering and the two are joined by the usual soldering operations.
2.1.5 Hot–Dip Coating
The process of hot-dip coatings is quite common. Galvanizing is a hot-dip process in which the base material is immersed in a tank of molten zinc. Adhesion results from the tendency of the molten zinc to diffuse into the base metal. The protective coating is made up of several layers. The layer that is closest to the base metal is made up of iron-zinc compounds while the outermost layer is primarily zinc.
Hot dipping is a rapid, inexpensive process that allows the coating of corrosion-resistant metals onto base metals at less cost than by electroplating. However, the process is limited to shapes that will not trap the molten metal upon extraction from the dip tank. Tin, lead, and aluminum coatings may be applied to the base metals in addition to zinc. Base metals are restricted to the materials with higher melting temperatures such as cast iron, steel, and copper.
2.1.6 Immersion Coatings
Immersion coatings are applied by dipping a base metal having a higher solution potential into an aqueous solution containing ions of the coating metal. No electric current is used. Deposition occurs while the base metal is in the solution. The coast is usually quite thin and can be controlled quite closely. Nickel, tin, zinc, gold, and silver are used as immersion-coating materials.
Nickel immersion coatings are used on steel parts that are so constructed that it would be difficult to maintain a uniform electrodeposited coating, such as valves, gears, threaded parts, and other items having deep recesses. Tin immersion coatings provide a bright, and protective coating on such items as paper clips, pins and needles.
2.1.7 Vapor – Deposited Coatings
As the name implies, vapor-deposited coatings result from the condensation of a metal film on the base metal. Aluminum is the most widely applied vapor film. The aluminum is heated and vaporized in a vacuum. The aluminum vapor then condenses on the surfaces of the base metal. As would be expected, vapor deposited films are very thin. Consequently, they have had little application where extreme resistance to corrosion is required.
2.1.8 Mechanical Plating
This is a proprietary procedure where metal powders are mechanically welded to a substrate. The parts to be plated are tumbled in a medium of metal powder and glass beads. During the tumbling, the surface of the parts is activated. This process is used to plate soft materials such as tin, cadmium, and zinc on steel. [4]
However, most plating, dipping, and immersion tanks are limited in size, metallic coatings are usually applied to smaller parts. An upper constraint is approximately 2000 cubic inches in most plants. This family of coatings provides for bright metallic surfaces with close tolerance control and good resistance to abrasion. And also, they provide adequate resistance to corrosion and elevated temperatures.
2.2 Conversion Coatings
Conversion coatings are those produces when a film is deposited on the base material as a result of a chemical reaction. The most widely used conversion coatings are: (1) Phosphate Coatings, (2) Chromate Coatings, (3) Anodic Coatings. [4]
2.2.1. Phosphate Coatings
Phosphate coatings are used principally as a base for the application of paint or enamel. Also, they are used to aid in the farming of sheet and heading of bar and rod. The process itself does tend to rustproof the base material, which is usually iron, steel, aluminum, or zinc. The metal surface is treated with a dilute solution of phosphoric acid and other elements so that a mildly protective layer of insoluble crystalline phosphate is obtained. A typical phosphate coating process consists of spray cleaning, phosphatizing, water-spray rinse, and chromic acid-spray rinse. The phosphate coating is usually 0.0001 to 0.0003 in. in thickness. The process is very rapid because the capacity of the equipment rather than the cycle time limits the rate of production. This process is widely used in the automotive and electrical-appliance industries for preparing automobiles, washing machines, refrigerators, and similar products to receive an organic finish.
2.2.2 Chromate Coatings
Chromate dip coating may be applied at the mill in order to provide for corrosion protection of galvanized sheet. A chromate dip-coated galvanized product is said to be "passivated" or "stabilized". Chromate coatings are also used on nonferrous materials including aluminum, magnesium, zinc-coated materials, and cadmium-coated materials. Chromate coatings are generally quite thin, being less than 0.00002 in. thick, and are used chiefly for added resistance to corrosion and as a base for paint. Chromate conversion coatings result when the base metal is either sprayed or immersed in a solution of chromic acid, chromium salts, together with hydrofluoric acid or hydrofluoric acid salts, phosphoric acid, or other mineral acid. The resulting chemical attack produces a protective film containing chromium compounds. The process can give either of two types of films: a yellow iridescent film of a clear film.
Chromate films are less expensive to apply than anodized coatings because the process is faster and less overhead is involved. They also give greater resistance to corrosion than do anodic films; however anodic films offer superior wear-resistant qualities. Chromate coatings must not be used on galvanized steel parts that are to be resistance-welded or phosphated and painted. Chromate will cause problems in welding and will interfere with proper phosphating, thus resulting in poor paint adhesion.
2.2.3 Anodic Coatings
Anodic Coatings result when an oxide is applied to aluminum and magnesium and their alloys. Here the base metal is connected as an anode in an electrolytic immersion, and an oxide film is deposited on the base metal that increases its resistance to atmospheric as well as galvanic corrosion and offers a good foundation for painting. The process outlined, using chromic acid, will give a coating of aluminum oxide and chromium salts varying in thickness from 0.00003 to 0.0002 in. It provides excellent corrosion protection and has been used extensively by the aircraft industry. Sulfuric acid, oxalic, and boric acid bath processes also apply anodic coatings. It is possible to import excellent colored coatings by immersing the parts in warm dye solutions, and then sealing the dye in the porous oxide coatings by dipping in dilute nickel acetate.
As a family, conversion coatings are inexpensive and allow the maintenance of close tolerances. However, in view of the fact that they are very thin, they do not in themselves provide a great amount of resistance to corrosion or abrasion.
3 GALVANIZING PROCESS
Galvanizing, basically a simple process of cleaning steel and dipping it in molten zinc, is a well-tried and widely used method of protecting steel against corrosion. Each year throughout the world twenty billions of steel are being galvanized. Transmission towers, overhead supports for railway electrification, bridge suspension systems-all depended on galvanizing for their rust resistance. Today there is a new awareness that reducing maintenance costs can make worthwhile long-term savings and thus there is an increasing demand for durable protection of steel. [5]
3.1 History of Galvanizing Process
The derivation of term "galvanizing" has absolutely nothing to do with protecting steel form corrosion. Back in the 1700’s a Italian physiologists named Lugi Galvani discovered that if you place two dissimilar metals in direct, electrical contact with each other and subject them to an electrolytic solution, ions from the noble metal go into solution, liberating electrons and causing a current flow into the more noble metal preventing it’s ions from going into solution. The process described which became known as "Galvanizing" aptly named for Mr. Galvani, employs the use of zinc as the anode, or least noble metal. The zinc slowly releases its ions causing the current to flow into the metal it’s applied to. In the formative years of electrical science, zinc was the most widely used metal for producing galvanic electricity. In 1837, French scientist Soral took out a patent in France for a process of dipping steel in molten zinc and provided the process with name "galvanizing" in honor of Galvani, who died in 1798. [6]
3.2 Iron and Steel for Galvanizing Process
The chemical composition of irons and steels, and even the form in which certain elements such as carbon and silicon determines the suitability of ferrous metals for galvanizing and determines the suitability of ferrous metals for galvanizing and may markedly influence the appearance and properties of the coating. Steels that contain appearance and properties of the coating. Steels that contain less than 0.25% carbon, less than 0.05% phosphorus, less than 1.35% manganese; and less than 0.05% silicon, individually or in combination, are generally suitable for galvanizing using conventional techniques. To avoid brittleness of the iron must be low in phosphorus and silicon; a preferred composition may contain about 0.01% phosphorus and about 0.12% silicon. Although steel containing up to 0.4% C have been galvanized on production basis without difficulty, low carbon steels that contain not more than 0.15% C generally are considered to be the most suitable for galvanizing. [7]
3.3 What is Galvanizing Process
Galvanizing is a process for rust proofing iron and steel by application of a metallic zinc coating. Another definition of galvanizing is the practice of immersion clean, oxide free iron or steel into molten zinc in order to form a zinc coating that is metallurgical bonded to the iron or steel’s surface. The galvanizing process is inherently simple. The simplicity of the process is a distinct advantage over other methods of providing corrosion protection. It is applicable to products of nearly all shapes and sizes, ranging from nails, nuts, and bolts to large structural assemblies and steel in coils and lengths. On all steel and iron parts, galvanizing provides long-lasting, economical protection against a wide variety of corrosive elements in the air, water, or soil. [3]
All galvanizing process consists of three basic elements. These are: Surface Preparation, Galvanizing and Inspection. (Figure1.) [6]
Figure 3.1 Steps of Galvanizing Process
3.3.1 Surface Preparation
Surface preparation is the most important step in the application of any coating. In most instances, where a coating fails before the end of its expected service life it is due to incorrect or inadequate surface preparation. A good quality coating can only be achieved if the surface preparation of the steel has been carried out correctly. It is important to know that the surface preparation in a galvanizing plant does not remove heavy grease, dirt, point or varnish residues, welding slang, silicones, with galvanizing, the surface preparation process contains its own built-in means of quality assurance and quality control in that zinc will simply not react with a steel surface that is not perfectly clean. Surface preparation for galvanizing typically consist of three steps: (1) Cleaning, (2) Acid Pickling, (3) Fluxing. [8]
3.3.1.1 Cleaning
All contaminations can be removed from the work surface by one or more several methods, including vapor degreasing, solvent cleaning, emulsion cleaning and alkaline cleaning. The most common of these in the after fabrication galvanizing process is the use of heated alkaline cleaning (caustic cleaning) baths. It is effective after 1-20 minute immersion. And then steel receives a rinse in cold running water. [7]
3.3.1.2 Acid Pickling
Aqueous solution of sulfuric or hydrochloric acid are used to remove mill scale and rust from parts before galvanizing. Hydrochloric acid is generally used instead of sulfuric acid in galvanizing because it acts much more quickly than sulfuric acid, and is much more effective in operations. The pickling solution consists of the acid with 5 to 15% of total weight. [9] To increase effectiveness, hydrochloric acid solutions are usually used at about room temperature. After a further rising operation, fluxing process is applied. [10]
3.3.1.3 Fluxing
Fluxing is necessary to dissolve and absorb any remaining impurities on the metal surface and to ensure that clean iron or steel contacts the molten zinc. Three main methods of fluxing are used; In the Old Dry Process, the salts from a hydrochloric acid pickle are dried on the work is rinsed in running water, immersed in a tank of flux and then dried. In the Wet Process the work is taken direct from the rising tank to the galvanizing bath, which has a blanket of molten flux floating on the zinc. The "wet" process may be modified by first pre-fluxing the work as the "dry" process. Zinc ammonium chloride is used both for pre-fluxing and for the flux blanket. The choice of fluxing process varies with the type of work, but does not affect the thickness and protective value of the final coating. [5]
3.3.2. Galvanizing
When immersed in the galvanizing bath, iron and steel are immediately wetted by the zinc and react to form iron-zinc layers. As the work is removed from the bath, some molten zinc is taken out on top of the alloy layers, the whole coating
being metallurgically bonded to the basis metal. At the normal galvanizing temperatures of 445°C – 465°C, the rate of reaction is very rapid at first, with a "boiling-off" action. The main thickness of coating forms during this initial period and it is therefore impractical for the galvanizer to form a very thin coating. Subsequently reaction slows down and the coating thickness does not increase greatly if the article is immersed in the bath for a much longer time. The normal period of immersion is a minute or two but longer for work that requires draining from internal spaces or is particularly heavy. [5]
3.3.3 Inspection
The most important method of inspection for galvanized articles is visual. A variety of simple physical and laboratory tests may be performed for; (1) Thickness, (2) Uniformity of Coating, (3) Adherence of Coating, and (4) Appearance.
Fluxing Pickling Galvanizing
Figure 3.2 Fluxing, Pickling and Galvanizing
The preparation steps consist of cleaning and pickling operations that free the surface of dirt, grease, rust and scale. The preflux step serves to dissolve any oxide that may have formed on the iron or steel surface after pickling and prevents further rust from forming. Clean oxide-free work is galvanized by immersion into molten, zinc. Finishing operations include quenching, removing excess zinc and inspection. [11]
Figure3.3 Finished materials
4 HOT – DIP GALVANIZING PROCESS
4.1. Definition Of Hot – Dip Galvanizing Process
Hot–dip galvanizing is process in which an adherent, protective coating of zinc and zinc compounds is developed on the surfaces of iron and steel products by immersing them in a bath of molten zinc. In the hot-dip galvanizing, after the material is cleaned and rising, the work piece is immersed in pickling bath. Hot-dip galvanizing is commonly associated with hydrochloric acid pickling, preflux washes and no flux blanket on the kettle. However, sulfuric acid pickling has been used in conjunction with hot-dip galvanizing. After cleaning and pickling steps, the work piece is immersed in an aqueous flux solution (zinc ammonium chloride) at about room temperature, 24 to 38°C.[10] After these steps, it is dried and then it immersed in the molten zinc bath (galvanizing step). In hot-dip galvanizing, the molten zinc bath is operated at temperatures usually in the range of 445 to 465°C. at 480°C and above, the dissolution rate of iron and steel in zinc is extremely rapid, and the effects of these temperatures on both work piece and galvanizing tank are generally harmful. [16] The coating can be achieved by diffusion of molten zinc atoms in hot-dip galvanizing. In the hot-dip galvanizing of fabricated articles, the thickness of the coating is controlled by immersion time. Although timing is to some extend depend on case of handling and must be established by trial for each design of part being coated, the duration of immersion is usually in the range of one to five minute. Finishing step of hot dipping can be archived easily after all steps are completed. [9]
4.2 Effect of Hot Galvanizing on Material Properties
The tensile strength, yield strength, elongation at rapture, and reduction of area of hot rolled steels remain virtually unchanged after hot-dip galvanizing. In welded structures, the weld stresses may be reduced by 50 to 60% as a result of hot-dip galvanizing. Increased strength levels induced by cold working or heat treatment is generally reduced by hot dipping. The degree of strength reduction depends on such factors as the amount of working, the nature of heat treatment, and base steel chemistry. [10] Impact toughness is slightly reduced, but not so much that the applicability of the steel is affected. The formability of steel is not affected. However, if the steel is sharply bent, the zinc coating may craze or crack on the tension side of the bend, depending on thickness of coating and bend radius. [9]
Fatigue strength of various types of steels is affected differently as a result of the hot-dip galvanizing process. Rimmed and aluminum-killed steels exhibit relatively little reduction of fatigue strength, whereas the fatigue strength of silicon-killed steels can be reduced considerably by hot-dip galvanizing. The reason for this difference in fatigue strength for silicon-killed is attributable to the different structure of the coating. Under the influence of fatigue stresses, cracks may from the iron-zinc layer and act as crack initiators in the steel surface.
Hydrogen embrittlement does not result from the hot-dip galvanizing of ordinary unalloyed and low-carbon steels. Any hydrogen absorbed during pickling is effectively eliminated on immersion in the zinc bath because of the relatively high temperature about 460°C. Hardened steels can become brittle because of hydrogen diffusion into the steel. Such materials should always be tested for embrittlement after picking before large are hot-dip galvanized. [7]
4.3 High Temperature Hot-Dip Galvanizing
When galvanizing process is performed at a temperature of approximately 550°C, it is called high temperature hot-dip galvanizing. The following bath conditions for high temperature hot-dip galvanizing are considered ideal: [10]
Temperature at 560°C
Iron content between 0.1 and 0.2%
Lead content about 1%
Aluminum content of 0.05%.
The coating that is obtained from high temperature hot-dip galvanizing has a light gray, and uniform appearance. Brighter coats may be obtained by the aluminum addition to the bath described above and by the aluminum addition to the both described above and by quenching instead of air-cooling. Coating adhesion and ductility are equivalent to coatings galvanized at conventional temperatures. [10]
Verma and Doij have investigated the variety of structural properties for coating formed at 560°C. [12] And also they have shown the differences between high temperature galvanized coating and conventional hot-dip galvanized coating. The differences are: (1) The corrosion protection offered by high temperature hot-dip galvanized coating. (2) The high temperature hot-dip galvanized coating was found to be harder than conventional coating. As a result, the coating on high temperature hot-dip galvanizing can be expected to be more wear resistant than on conventional hot-dip galvanized. (3) The microporus surface of the high temperature hot-dip galvanized coating renders it eminently suitable for painting. (4) The high temperature hot-dip galvanized coating thickness is always in the range of 60-80 mm. Excessive alloy growth, such as is frequently observed in conventional galvanizing, was never observed in high temperature hot-dip galvanized coating is excellent. [12]
5 ELECTROGALVANIZING PROCESS
5.1 Definition of Electrogalvanizing Process
Electrogalvanizing is the name applied to the process of covering steel or iron with coating of zinc by means of an electric current. It is sometimes termed cold or wet galvanizing to distinguish it from the more common method of hot-dip galvanizing. [9] In the Electrogalvanizing, the work piece is immersed in cleaning bath. All contaminants can be removed after cleaning operation. If good coating properties are desired, the alkalinity, current density and temperature of the cleaning bath should be kept as low as possible. Electrogalvanizing usually involves pickling with heated sulfuric acid in conjunction with a kettle top flux blanket. In the Electrogalvanizing, the work is not usually prefluxed after cleaning and pickling but is placed in the molten zinc bath through a top flux blanket on the kettle. However, an aqueous preflux may be used in conjunction with a top flux on the zinc bath. [10] A flux blanket on the surface of the molten zinc bath is used to remove the impurities and to keep that portion of the surface of the zinc bath through which the steel is immersed free from oxides. The flux blanket floats on the surface, and when work piece is immersed in the bath, their surfaces are wetted by the molten flux. The flux must have sufficient chemical stability to maintain a chemically activate foam at the galvanizing temperature and to perform its cleaning function at a high rate speed. [6] After these operations, rinsing must be needed. When possible, the rinse water should be slightly acid (pH, 2.5 to 3.5). And then the work piece must be entered directly into the galvanizing bath, it is dried and finally finishing stop can be completed. [13]
5.2 Bath Types of Electrogalvanizing
Two types of solutions; namely acide zinc bath, and cyanide zinc bath are generally used for electrogalvanizing step. Today acid zinc bath is possibly the fastest growing system in the field. These coatings are substantially pure, ductile, adherent and free from the brittle alloy, which is characteristic of some hot dipped coatings, although modern hot dipped coatings are nearly as ductile as electrogalvanized one. [13]
5.2.1 Acid Zinc Baths
Zinc is popular as a coating for steel because it offers good corrosion resistance at a low coating cost. Since zinc is anodic to steel, it will protect the steel from corrosion even though the deposit is porous or contains small breaks. Zinc coating by electrogalvanizing has the advantage that the thickness of the deposit can be easily controlled. Also, the electrogalvanized coating is free from brittle iron-zinc compound layers that are formed in a hot process. Because the coating is not brittle, it may be applied to a sheet that has to withstand subsequent forming operations.
Acid zinc baths are used where it is describe to have a high plating rate and low cost. The deposits are not as attractive as those from the bright cyanide baths and the throwing power of the acid bath does not compare with that of the cyanide bath. The baths are primarily used for coating wire and steel strip. A typical formula for an acid zinc bath is as follows:
| g./l. | oz./gal |
ZnSO4 7H2O | 360 | 48 |
NH4Cl | 30 | 4 |
NaC2H3O2 H2O | 15 | 2 |
Glucose | 120 | 16 |
Current Density | 10 – 30 amp./sq.ft. |
Temperature | 75 – 85 °F |
pH | 3.5 – 4.5 |
Zinc sulfate is used as a source of metal; ammonium chloride increases the conductivity; sodium acetate acts as a buffer and glucose acts as an addition agent. Various formulations are possible and are equally effective as the one given. Other chlorides may be added to the bath, aluminum salts may be used in place of the ammonium salts or in place of the acetate and also various addition agents may be used. The chemicals selected should supply metal content, act as a buffer, and increase conductivity and acts as an addition agent. The same requirements hold for other acid baths, such as nickel, but the plating range of a nickel bath is readily changed by changing the formula. In an acid zinc bath, the formula may be selected with more freedom, depending on the availability, cost and purity of chemicals. Zinc sulfate is a high-purity commercial chemical made from a cheap commercial acid: sulfuric acid. The other chemicals in the formula given are produced on a large scale and are widely in the chemical industries. [14]
5.2.1.1 Bath Preparation
All of the chemicals used are readily soluble so that the order of addition is of little importance. After the chemicals are dissolved, the pH is adjusted and any required treatment of the bath is judged by the appearance of the deposit on initial plating. If the plate is rough, pitted or of color, this may be due to suspended or soluble impurities. Suspended impurities can be removed by filtration or by allowing to settle. Soluble noble-metal impurities can be removed by low-current density electrolysis, by immersion plating on anodes or by treatment of the bath with zinc dust. [14]
5.2.1.2 Operation and control
Although the bath is normally operated at current densities of 10 to 30 amperes per square foot, much higher current densities may be used if the both are agitated. The current density is only restricted by the degree of agitation. For plating on wire, under special conditions, a current density as high as 2000 amperes per square foot may be used. If the bath is agitated during rapid plating, it is important that is kept free of insoluble material, therefore, continuous filtration is necessary. However, in still plating operations, occasional filtering or occasional removal of sludge from the bottom of the tank is sufficient.
The bath temperature should be maintained near room temperature, so that if large current are used, cooling becomes necessary. The bath is not a major problem. Control of pH is the most important factor. The pH gradually rises due to reaction of the acid in the bath with anodes. When the pH rises to the higher limit, sulfuric acid is added to restore to lower pH limit. The acid that reacts with the zinc anodes forms zinc sulfate at a rate, which approximately balances the zinc lost by drag-out.
Anodes should be used that are high in zinc content and uniform in grain size for even anode corrosion. Zinc is an active metal and most anode impurities will collect on the anode as sludge. If these are excessive, they may cause un even corrosion and they may contaminate the bath with suspended impurities. In the later case the anodes may be bugged. [14]
5.2.1.3 Characteristics Of The Bath
The anode and cathode efficiencies of the bath are very high and the anode and cathode polarizations are low. Because of these conditions, bath balance and bath control are easy, but these conditions also cause the throwing power of the bath to be very poor. Thus the bath is limited to plating simple shapes or to the use of special racking or anode arrangements obtain good metal distribution. [14]
5.2.1.4 Operating Parameters
Chemical Composition: Zinc, total chloride, pH, and boric acid, when used, are to be controlled and maintained in the recommended ranges by periodic replenishment using chemically pure materials. Too high zinc content causes poor low current density deposits, while too low concentrations cause high current density burning. High colored may cause separation of brightener, while low chloride concentrations reduce conductivity of solutions. High pH values cause the formation of precipitates and anode polarization, while too low pH values cause poor plating. Insufficient boric acid reduces the painting range. Brighteners have to be replenished by periodic additions. Since the nature of these brighteners is proprietary, the suppliers specify concentrations and control procedures.
Agitation is mandatory in acid chloride baths to achieve practical operation current densities. Solution circulation is recommended in barrel baths to supplement barrel rotation. In rock baths, solution circulation is usually accomplished by location the intake and in charge of the filter and opposite ends of the plating tank. Cathode rod agitation is suitable for many hand operated rack lines. Air agitation is the preferred method for most installations. A low-pressure air blower should be used as a supply source.
Temperature Control
Cathode Current Efficiency.
PH Control
Iron Contamination
5.2.2. Zinc Cyanide Baths
Zinc cyanide baths are use to coat steel for protection from rusting. Thus, while they are used for the same purposes as the zinc baths. First, the throwing power is good, so that irregular pieces can be easily covered. Second, bright deposits can be obtained, and bright deposits have sales appeal, even applications where they are not required. Also, bright deposits do not stain as readily as dull deposits so that they remain attractive longer. However, it must be kept in mind that zinc is an active metal. This very property that is responsible for its good protection of steel is also a property that will cause the zinc to loss its original appearance much more rapidly than other electrodeposites.
The basic zinc cyanide bath is described as;
is a common problem in all acid chloride zinc baths. Iron is introduced into the bath from parts failing into the tank during operating, from attack by the solution on parts at current densities below the normal range, such as the inside of steel tubular parts, and from contaminated rinse waters used before plating. Iron contamination usually appears as dark deposits at high current densities and in barrel plating as stained dark spots reproducing the perforations of the plating barrel. A high iron contends turns the plating solutions brown and marky. Iron can be readily removed from acid chloride baths. By oxidizing soluble ferrous iron to insoluble ferric hydroxide. This is accomplished by adding concentrated hydrogen peroxide to the bath, usually on a daily bases. [10]of acid zinc baths is usually done on a daily basis. Electrometric methods are preferred over papers. The pH of bath is lowered with a hydrochloric acid addition and when required, the pH may be raised with a potassium or ammonium hydroxide addition. The high cathode current efficiencies exhibited by acid chloride zinc baths are one of the most important properties of these baths. The average cathode current efficiencies for these baths are approximately 95 to 98 % over the entire range of operable current densities. No other zinc plating system approaches this extremely high efficiency at higher current densities. In practice, this high efficiency can lead to productivity increases 15 to 50 % over cyanide baths. In barrel plating, barrel loads may often be doubled in comparison with cyanide baths and equivalent plating thickness achieved in half the time. is more critical in acid zinc baths than in cyanide zinc baths and auxiliary refrigeration should be provided to maintain the bath at its maximum recommended operating temperature, usually 35°C. Operating acid chloride bath above its maximum recommended temperature causes low over-all brightness, usually at low current low densities and rapidly progressing over the entry part. High temperatures may also bring the bath above the cloud point of the brighter system. As the acid bath gets hot, it reaches a point where additives start coming out of solution, giving the bath a milky or cloudy appearance. This causes total bath imbalance. Conversely, low temperatures, usually below 21°C cause many baths to crystallize out and the organic additives to separate out of solution causing roughness and, in extreme cases, a sticky globular deposit on the bath and work, which clogs filters and completely curtails operations.| | Plain Cyanide Bath |
| | g./l | oz./gal |
Zinc Cyanide | 60 | 8 |
Sodium Cyanide | 23 | 3 |
Sodium Hydroxide | 53 | 7 |
Temperature | 104-122°F |
Current Density | 10-20 mp./sq.ft. |
Cathode Efficiency | 90-95 % |
This plain zinc cyanide bath will produce good deposits for protection of steel from dusting. If bath is modified, by increasing the total concentration and by increasing the temperature, the plating rate can be increased.
It mercury salt is added to a zinc cyanide bath, an alloy deposit will be obtained that has a more pleasing appearance than a deposit from the plain zinc cyanide bath.
| | Zinc Mercury Bath |
| | g./l | oz./gal |
Zinc Cyanide | 37.5 | 8 |
Sodium Cyanide | 22.5 | 3 |
Sodium Hydroxide | 30.0 | 7 |
Mercuric Oxide | 0.25 | 0.03 |
Temperature | 86-122°F |
Current Density | 40 amp./sq.ft. |
Anodes | 0.1-1 %Mercury |
The baths is controlled in a manner similar to the control of the plain cyanide bath expect that one more constituent has to be controlled: mercury. If mercury becomes high in the deposit, spots will develop on aging. If the bath is allowed to stand idle, mercury will immersion-plate on the anodes and the bath will eventually become depleted of mercury. The zinc-mercury bath has good throwing power and good covering power and the deposit protects steel in the same manner as a plain zinc deposit. [14]
5.2.2.1. Bath Preparation
the bath is best prepared by dissolving the sodium hydroxide and sodium cyanide first. The zinc cyanide may than be added since it will be soluble in this solution. Other chemicals that are required may be added and the bath can be made up to volume. A plating test should than be made to determine the plating quality. The chemicals uses for the zinc bath are of relatively high purity so that good deposits should be obtained immediately. If impurities are present, they will be revealed by a off-color deposit in the plating test. Or, they will be revealed by shift in plating range in the bright bath, which is sensitive to impurities. Noble-metal impurities can be removed by electrolysis or more readily by treatment with zinc dust followed by filtration. [14]
5.2.2.2 Operation and Control
The zinc cyanide bath is controlled by maintaining the sodium hydroxide content within limits and by adjusting the ratio of zinc metal to sodium cyanide. The difficulty of maintaining these limits will depend on the type of bath used, the application and amount of work plated.
For higher plating rates and for barrel plating applications, higher chemical concentrations and higher temperature and if heavy currents are used, cooling will be necessary to keep the temperature from becoming too high.
The anode-current density should be held at 10 to 30 amperes per square foot to obtain proper corrosion. The anode-current density is obtained by adjusting the anode area, depending on the total tank current. Since zinc is chemically active warm alkaline solution, the anodes may go into solution faster than metal is removed at the cathode. This condition will be offset somewhat by the drag-out, if it is excessive, it can be remedied by replacing some of the zinc anodes with steel anodes. Bath balance depends on adjustment of the number and type of anodes so as to adjust the rate of solution of zinc to equal the rate of removal by plating plus drag-out.
Rapid plating conditions are the most difficult to maintain because the bath composition changes readily. However, such a bath can be maintained if frequent chemical and plating checks are made. If the sodium cyanide is high and zinc additions are required, zinc oxide may be added in place of zinc cyanide. [14]
5.2.2.3 Function of Components
Zinc. The chief function of the zinc complexes is to provide reservoir from which the zinc plated is replaced. The size of this reservoir can be varied several-fold with little effect if appropriate changes are made in operating conditions and concentrations of the other bath components. However, as the zinc content is reduced the effects of small changes in other variables are magnified and fluctuations in performance are apt to become increasingly troublesome. Thus to reach a suitable compromise the desirability of easier control and more consistent performance must be balanced against increased chemical and waste-deposal costs.
In well-run bath additions of zinc salts should not be required. The anodes will ordinary provide more than sufficient zinc to replace that plated out and lost by drag out. Steel or other inert anodes are used to prevent excessive accumulation of zinc in the bath. Under some unusual circumstances, however, substantial losses of zinc can occur as a result of the precipitation of insoluble zinc compounds.
Cyanide
The importance of these properties of cyanide has been emphasized by problems experienced when cyanide concentrations have been reduced to meet waste-disposal restrictions. Low cyanide concentrations were found to be extremely beneficial for avoiding anode polarization, retaining zinc in solution, and possibly in preventing iron contamination of plating baths.
For any given set of operating conditions, the optimum cyanide concentration will depend on concentrations of the other bath components, especially that of zinc. So much so, in fact, that the cyanide-to-zinc ratio [commonly expressed as (total NsCn/Zn)] provides a more reliable index of the bath’s capabilities than does the cyanide concentration alone.
Plate distribution on the workload can be used to indicate whether the cyanide-to-zinc ratio is in the proper range. Low ratios are characterized by relatively high cathode efficiencies and poor throwing power. As the ratio increased, two changes take place: Cathode efficiencies begin to decrease, with the effect being evident at higher current densities, and a greater proportion of the applied current is forced into low-current-density areas. At optimum ratios, good coverage is obtained in recessed or low-current-density range, and improved uniformity of plate thickness results from demising cathode efficiency on high-current-density edges. Further increases in ratio beyond the optimum range cause significant losses of cathode efficiency extending into the middle-and low-current-density ranges, thereby producing excessive gas evolution and giving low plating speeds.
Specific cyanide-to-zinc ratios are difficult to prescribe because they depend on both concentrations of other bath components and on operating conditions. For example, where ratio Zn/Zn=3.0 Might be preferred for zinc at 15g/l. Similarly, lower hydroxide concentrations also tend to call for slightly lower cyanide-to-zinc ratios. Preferred ratios are strongly depend on operating temperatures, which should not be allowed to vary more than about 2°C from the nominal value. Table 5.1 indicates ranges of ratios appropriate for various temperatures with approximately 30g/l zinc in typical commercial operations.
. Perhaps the chief function of cyanide is to make possible to deposition of usable zinc plate; in the absence of additives, deposits from zincate baths tend to be unattractive or nonadherent. The proportion of cyanide incorporated in the formulation gives lustrous or bright, adherent zinc plate over a broad current density range, regulates cathode efficiency-temperature-current density relationships, and promotes good anode corrosion. Cyanide also renders the bath more responsive to additives. Temperature | RATIO Total NsCn/Zn |
20-25 | 2.5-2.8 |
25-30 | 2.6-2.9 |
30-35 | 2.7-3.0 |
35-40 | 2.8-3.2 |
(Zn= 30 – 40 g/l ; NaOH=70 – 90 g/l is used) |
Table5.1 Cyanide-to-Zinc Ratios
| | Acid Bath | Cyanide Bath |
Throwing Power | Very Low | Very High |
Appearance | Dull Gray | Semibright to bright |
Basis Metal | Can plate all ferrous metals | Cannot coat malleable and cost iron |
Plating Speed | Rapid plating is possible | Limited plating rate |
Cost | Low | Low, but higher than acid baths |
Tanks | Acid-resisting | Steel |
Structure | Coarse – grained | Fine – grained |
Control | Simple | Complex |
Formula | Available | Proprietary for bright baths |
Electrode Efficiencies | High at all plating rates | High only under limited conditions |
Electrode Polarization | Low | High |
Preparation of Basis Metal | Clean and pickle but use more care than for cyanide baths | Clean and Pickle |
Table 5.2 Comparison of Acid Cyanide Zinc Baths
5.3 Electrogalvanizing Equipments
The basic unit for electroplating operations is the tank to hold the solutions. Tanks are constructed of various materials such as lead sheet, rubber, plastic, and tile to resist alkaline and acidic solutions. Large pieces are suspended individually. Small pieces may be mounted on racks but mostly are barrel plated or tumbled. A batch of pieces is put in a nonconducting perforated barrel and in touch with suitable contacts. The barrel is lowered into the plating solution and revolved several times each minute, and the pieces tumble around and are plated uniformly.
The electrogalvanizing process entails cleaning, washing, rising and other treatment in addition to actual deposition of metal. This means each workpiece or batch must be dipped into and transferred among a number of tanks, whether plated individually, on racks, or in barrels. That is commonly done manually for small lots, but labor is saved and quality controlled better by automatic and even numerically controlled plating machines for moderate-to large-quantity production. The three general types of machines are illustrated in Fig 5.3. The transfer devices for these machines raise the work from one tank, move it to the next, and then lower it into place at preset intervals. The straight line and return types may be single-or double-line machines, depending upon whether they have one or two rows of work carriers, one above the others. The rotary-type machine takes more floor space but it can handle large parts. The rotary-type machine takes more floor space but can handle large parts. These machines can be tied in conveniently with conveyor systems. In certain industries, specialized machines have been developed for continuously plating sheet metal, a strip of parts, or wire as it is run through a series of baths.
Fig. 5.3 Typical Plating Machine Layouts
High-speed plating techniques have been developed in recent years for large-quantity production. Instead of being immersed in a tank with current passing an average of about ½ m. through relatively still electrolyte as in a conventional electroplating, the piece is positioned in a fixture cavity with wall contours matching the part surface, typically with a gap of about 2.5 mm. Electrolyte is pumped through the cavity at around 100 dm3/min. The rapidly moving liquid overcomes ionic depletion in the layer adjacent to the work surface, and 20 or more times as much current can be passed effectively as in conventional plating, and metal can be deposited much faster.
Used fluids from electrogalvanizing tanks can be highly polluting. In recent years, stringent rules have been set down to control the levels of cyanide and metals in the effluent. A common remedy is to resort to chemical reactions to convert the waste to nontoxic substances. That way valuable materials may be lost, and in some cases evaporation and dialysis are used to concentrate and recover costly substances as well as eliminate pollution. [15]
5.4 Operation of Electrogalvanizing Process
The tank is nearly filled with the plating liquid, called the electrolyte, consisting of a solution, which is determined by selected bath. This solution must be continually agitated to maintain a uniform density, and it must also be kept clean and at a fairly constant and suitable temperature. There are two methods of supplying constant to the electrolyte and these distinguish the process as being a soluble anode process or an insoluble anode process. In the case of the farmer slabs of zinc are submerged in the electrolyte and are electrically attached to the positive bus bar and thus constitute soluble anodes, which supply the zinc to the solution and the plating current. In the insoluble anode process, special lead*alloy bars properly connected to positive bus bars constitute the anode by which the plating current is introduced into the bath. The zinc is supplied to the plating bath from an outside source where metallic zinc is dissolved in sulfuric acid or the zinc is obtained by direct leaching from zinc are and the solutions thus obtained are subjected to required controls for purity, concentration, etc. [9]
5.5 Properties of Electrogalvanized Coatings
Very thin uniform coating can be obtained by electrogalvanizing and also the thickness of zinc coating may be readily controlled in this process. Electrogalvanizing can be used where a fine surface finish is needed. Electrogalvanized materials can be rolled, formed, bent, and curved because the coating does not peel, flake, or crack. This property opens up many application such as chain link fence, freezers, washers, dryers, display cases, and building materials. One another important advantage of electrogalvanizing is that it can be done cold and thus do not change the mechanical properties of the work.
Electrogalvanized coatings are simpler in structure than hot-dip galvanized coatings. They are composed of pure zinc, have a homogeneous structure, and are highly adherent. Electrogalvanized coatings are not generally as thick as those produced by hot dipping. However, they do give good corrosion-free surface. Common thickness of electrogalvanized coatings is 1.6 and 3.5 mm. Because of its excellent adhesion, electrogalvanized coils of steel, sheet and wire have good working properties, and the coating remains intact after severe deformation. [3]
The quality of the electrogalvanized coating depends on the condition of the electrolyte, the cleanliness of the surface, and general working conditions. [7]
5.6 Benefits of Electrogalvanizing
Electrogalvanizing uses very thin formable zinc coatings and you get a very effective corrosion protector. The many benefits of Electrogalvanizing include: (1) produces a thin uniform coating of pure zinc with excellent adherence, (2) smoothness, readily prepared for painting by phosphating, (3) free of the characteristic spangles of other zinc coatings, (4) appearance of the coating can be varied by additives and special treatments in the plating bath, (5) good corrosion resistance [16] and also,
This is the most cost-effective and flexible process to serve the growing demand for a full complement of electrogalvanized products.
Manufacturers using electrogalvanized steel requiring additional coatings could be eliminate in-house cleaning, chemical treatment, and coating operations-lowering their costs-along with reducing the need to comply with costly environmental requirements.
Prepainted materials facilitate just in time and continuous process manufacturing. This can result in improvements customers’ work-in-process inventory, plant utilization, and throughput.
Prepainted material users generally benefit from lower manufacturing costs and improved product quality. [17]
5.7 Testing of Electrogalvanizing
5.7.1 Thickness Tests
In as much as corrosion resistance has often been shown to be intimately related to the thickness of the deposit, the usefulness of stipulating minimum thickness in production specification is obvious. Over the years many methods of determining thickness have been suggested and many of them are in current use. Owing to the variety of electrogalvanized metals and basis metal to which they are applied, it is not surprising that a single method of thickness testing has not gained precedence over all others. Those thickness tests, which are sufficiently useful to be listed. They can be divided into seven main group: (1) Microscopic Methods, (2) Chemical Methods, (3) Electrochemical Methods, (4) Magnetic Methods, (5) Mechanical Methods, (6) X-Ray Methods, (7) Miscellaneous Tests.
5.7.2 Corrosion Tests
Corrosion testing of electrogalvanized coatings has undergone extensive study and development within the last ten years. Generally two types of corrosion tests can be applied to electrogalvanized parts. These are: (1) Tests in natural environments, and (2) accelerated tests.
5.7.3 Inspection
Of all the qualities of electrogalvanized finishes, measurable and otherwise, the one most widely recognized by the ultimate consumer is appearance. The purchaser or user of a plated object, or of a product having plated components, judges the quality of the plating primarily by its finished appearance. A high degree of luster, smooth mirror-like reflectivity, and freedom from surface defects will have a strong effect on the prospective buyer’s opinion of the quality and value of the product.
There are three classes of inspection with which we are usually concerned. Although similar in technique, they differ in their purpose.
In-process inspection. This ties in closely with manufacturing quality control
Final inspection by the producer. (Often combined with packing for shipment.)
Receiving inspection by the customer.
Third type is used when the customer is a jobber or large retailer and also when the customer purchases parts which are to be assembled into a final product.
The latter two should be closely correlated. The producer obviously should not pass any parts that customer will not accept. It is most desirable that producer and the customer come to a mutually satisfactory agreement regarding such things as significant surfaces, minimum luster, and the degrees of various defects acceptable. When possible, this should be done prior to the acceptance of a contract, since price is largely affected by the degree of finish required. [18]
5.8 Certain Advantages of Electrogalvanizing over Hot-Dip Galvanizing
Electrogalvanizing-unlike hot-dip galvanizing-forms a thin pure coating of zinc on the substrate. There are no intermediate layers composed of zinc compounds. Process temperature are lower than those of hot dipping, making electrogalvanizing is easily controlled but in hot dipping is not. Further, the heavy losses of zinc a dross in the hot-dip electrogalvanizing process, which sometimes account for one-third of zinc consumed, are completely avoided by electrogalvanizing. Any weight of coating may be applied by electrogalvanizing depending on the amount of protection desired. Moreover, on simple shapes the coatings are much more uniform in thickness. As a result, electrogalvanizing is frequently much less costly than hot-dip galvanizing. [13]
6 PERFORMANCE OF GALVANIZED COATING
6.1 Factors Affecting Galvanizing Quality and Service
There are a number of factors in the nature of steelwork presented for galvanizing that impact on the galvanizers’ ability to provide a quality product and service. These are: (1) Surface Condition of Steel (rusty, Painted, Previously Galvanized), (2) Type of Product (casting, Old Wrought Ironwork, Brazed, Soldered, or Riveted Assemblies), (3) Steel Metallurgy, (4) Surface Profile, (5) Weld Quality, (6) Dimensions. [19]
6.2 Service Life of Galvanized Coating
The life of galvanizing is dependent on both the conditions of exposure and the coating thickness, as illustrated in Figure 6.1 [3]. The life of coating is linearly related to its thickness and also the thickness is related to bath temperature, steel chemistry, cooling rate and surface condition of steel [6]. Very simply, the thicker the coating, the longer it will last.
Figure 6.1 Service Life of Galvanized Coating for Various Environment
6.3 Corrosion Performance of Galvanized Coating in Atmospheric Environment
In most environments, galvanized coating is exposed to some atmospheric contaminations. Combined with the frequency and duration of moisture (fag, dew, rain, snow) these contaminants are the primary factors determining the rate at which the zinc coating is consumed. Various atmospheric conditions are categorized as heavy industrial, moderate industrial (urban), suburban, rural and marine environment. [20]
6.3.1 Industrial & Urban Environments
This classification of atmospheric exposure encompasses general industrial emissions such as sulfurous gasses, corrosive mists and fumes inadvertently released from chemical plants, refineries and similar processing plants. The most aggressive corrosion conditions can be expected in areas of intense industrial activity where the coating frequently is exposed to rain, condensation or snow. In these areas, sulfur compounds combine with the moisture in the air to convert the normally impervious and adherent zinc oxide and carbonates into zinc sulfite and zinc sulfate. Because these zinc-sulfur compounds are water-soluble and have poor adhesion to the zinc surface, they wash away easily in rain, exposing a fresh zinc surface, starting a new corrosion cycle.
6.3.2 Rural and Suburban Environments
Compared to industrial environments, rural and suburban atmosphere settings are relatively mild, particularly if the exposures are away from seacoasts and industrial or urban activities. Corrosion is relatively slow in rural or suburban atmospheres. Because the zinc reaction films formed in the atmospheres tend to be adherent and usually are not washed off the zinc surface their retention provides outstanding protection for the steel.
6.3.3 Marine Environment
Galvanizing’s protection in marine environments is influenced by proximity to the coastline, coastal topography and prevailing winds. In marine air, chlorides from the sea spray react with the normally protective film and produce soluble zinc chlorides. These zincs salts can be removed from the surface by rain or spray, exposing a fresh zinc surface for further reaction. Under some conditions, the corrosion rate may be accelerated by wind-blown sand that can remove the zinc film from the exposed surface. [20]
7 THE ECONOMICS OF STEEL PROTECTION
Selection from the wide range of protective systems for steel normally depends on economics. Factors which determine the economics of a given coating system for a particular application include [21].; (1) Initial cost of coating, (2) Coating life to first maintenance, (3) Cost of maintenance, (4) Hidden costs
7.1 Initial Cost of Coating
Since the finished cost of quality paint systems include a labor component of 75 to 80% while the labor component of galvanizing is only about 30%, the recent steady rise in the cost of labor has caused the cost of galvanizing. Economics are moving further in favor of galvanizing as labor costs continue to rise.
The economics of large-scale factory applied protective coating processes are such that the general trend is for the cost to fall relative to those carried out on the side, or with a high labor content. This is due to improvements in equipment and techniques as well as the economics of scale inherent in factory operations.
It must be recognized that no coating applied to a structure after completion can provide the same protection as a galvanized coating, which covers the entire surface of components, including areas to which site access may be difficult or impossible. Galvanizing eliminates problems of non-site paint quality control and inspection, and preparation of steel for painting.
7.2 Coating Life to First Maintenance
For must structural steel, maintenance painting presents great difficulty and involves major expense. British Standard 5493 "code of practice for protective coating of iron and steel structures against corrosion" recognize this. And classifies protective schemes according to type and maintenance free in various situations.
The only coatings recommended in BS5493 for a life to first maintenance of more than 15 years are electrogalvanizing and metal spraying. Even in mild environments all paint schemes listed in BS5493 for a life to first maintenance less than 15 years.
At current prices these paint systems will cost about the same to apply as a galvanized coating, or a little more. In every case the maintenance free life of the galvanized coating is much longer than the paint coatings, despite similar initial costs.
Despite advances in technology, competitive organic coatings cannot match galvanizing in performance. Simple paint systems, which cost less, to apply than galvanizing are unsatisfactory, even under moderate exposure conditions. They are very expensive in the long term.
7.3 Cost of Maintenance
A realistic estimate of the cost of maintenance is essential in the assessment of long term coating costs. This entails a careful examination of long term coating costs. This entails a careful examination of all costs involved in maintenance, including the hidden cost sand may require the application of modern accounting methods to arrive at true costs.
The maintenance cost for galvanizing is normally less than for paint coatings for the simple reason that in most environments, coating life is longer, and less maintenance is required.
7.4 Hidden Costs
The British Standard 5493 makes a number of points concerning hidden costs and constrains which improve the case for low maintenance coatings:
The removal of excessive paint film build-up after many recoats may be slow and expensive,
Steam, fune: Exhaust gases and grit may be present in industrial environments,
On many sites only manual surface preparation may be possible. And this will result in an inferior surface compared to blast cleaning.
If maintenance painting of galvanized surfaces is correctly timed. Surface preparation is easier and less critical than for deteriorated paint films.
Cyanide is consumed, as well as lost by dragout, in the normal operation of a zinc bath, and regular maintenance additions should be made often enough to prevent excessive fluctuations of the cyanide-to-zinc ratio. Estimates of dragout losses are generally not very reliable but there have been indications that perhaps half the cyanide added to a conventional bath is consumed in the plating operation. A major portion of this consumption probably results from electrolytic oxidation at inert anode surfaces, although some hydrolysis oxidation at inert anode surfaces, although some hydrolysis also occurs. The rate of hydrolysis is strongly influenced by the concentration of other bath components.
Hydroxide
Although most of the zinc in typical bath formulations is believed to exist in the hydroxy or zincate complex, in the absence of cyanide a total hydroxide concentration equivalent to about 10 parts of sodium hydroxide is required to maintain one part of zinc in solution. As little as 5 to 10 g/l of zinc that can be retained in solution but, on occasion, solubility problems have been encountered in the operation of baths so formulated. Zinc salts containing substantial proportions of hydroxide or oxide have been precipitated, necessitating maintenance additions of both zinc oxide and sodium hydroxide.
Minimum hydroxide concentrations are required to prevent anode polarization and the consequent depletion of zinc during plating. They range from about 90 g/l in dilute (10 g/l zinc) baths to about 60 g/l in stronger ones (30-35 g/l zinc). Anode current efficiency is sufficiently responsive to changes in the hydroxide concentration remains substantially unchanged during operation, increasing the sodium hydroxide by 5 to 10 g/l sodium hydroxide will cause it to drop.
Carbonate.
Reduction of the carbonate concentration can be accomplished by cooling the bath in a spare tank to cause the hydrated sodium salt to crystallize, whereupon the solution is filtered or decanted back into the plating tank. Although temperatures in the range 5 to 10°C are frequently satisfactory, preliminary testing is advisable to ensure the effectiveness of the operation or to prevent excessive zinc losses. Less commonly, excessive carbonate is removed by preparation with calcium hydroxide or, in some cases, with calcium shulphate. In many instances, the beneficial effects achieved by removing carbonate may have resulted from the simultaneous removal of other more harmful but identified constituents. [13]
5.2.2.4 Operating Parameters
Anodes.
One of the most economical forms of anode material for subsequent ball or elliptical anode casting. Although these have the disadvantage of bulky handling and the construction of specially fabricated anode baskets, their lower initial cost makes their use an important economic factor in the larger zinc plating shop.
Three grades of zinc for anodes are conventionally used for cyanide zinc plating: prime western, intermediate, and special high-grade zinc. The zinc components of these are approximately 98.5%, 99.5%, and 99.99%, respectively. The usual impurities in zinc anodes are all heavy metals, which, unless continuously treated, cause deposition problems; thus, nearly trouble-free results can consistently be obtained through the use of special high-grade zinc.
Control of Zinc Metal Content.
In a conventional new zinc cyanide installation, approximately ten spiral anode ball containers should be used for every meter of anode rod. These should be filled initially and after one or two weeks of operation adjusted to compensate for anode corrosion and dragout losses so that the metal content remains as constant as possible. During shutdown periods in excess of 48 h, most cyanide zinc platters remove anodes from the bath. In large automatics installations, this may be done by using a submerged steel anode bat sitting in yokes, which can be easily lifted by hoist mechanisms.
One of the prime causes of zinc metal buildup is the very active galvanic cell between the zinc anodes and the steel anode containers. This is evidenced by intense gassing in the area of anodes in a tank not in operation. Zinc buildup from this source can be eliminated by plating the anode containers with zinc before shutdown. This eliminates the galvanic couple.
Cathode Current Densities.
Sodium Carbonate
Sodium carbonate does not begin to affect normal bath operation until it builds up to above 75 to 105 g/L. Depending an over-all bath composition and the type of work being done, a carbonate efficiency, especially at higher current densities, decreased bath conductivity, grainier deposits, and roughness, which becomes visible when the carbonate crystallizes out of cold solutions.
The carbonate content of zinc baths build up by decomposition of sodium cyanide and absorption of carbon dioxide from the air reacting with the sodium hydroxide in the bath. Carbonates are best removed by one of the common cooling or refrigeration methods rather than chemical methods, which are simple in theory but extremely cumbersome in practice. When an operating zinc bath has reached the point where excessive carbonates presents a problem , it undoubtedly is contaminated with a great many other dragged in impurities, and dilution is often a much quicker and wiser aver-all method of treatment. [10]
is present in every cyanide and alkaline zinc solution. It enters the bath initially in one of two ways: (a) as an impurity from the makeup salts; sodium hydroxide and sodium cyanide may contain anywhere from 0.5 to 2% of sodium carbonate, or (b) deliberately added in 15 to 30 g/L additions to initial baths as previously noted. Cyanide zinc solutions operate at wide ranging cathode current densities varying from at wide ranging cathode current densities varying from extremely low, less than 0.002 A/dm
2, to above 25 A/dm2 without burning. Usable current density limits depend on bath composition, temperature, cathode film movement, and addition agents used. Zinc anodes dissolve chemically as well as electrochemically in cyanide baths, and therefore, effective anode efficiency will be above 100%, causing a buildup in zinc metal content, because cathode efficiencies are usually substantially less than 100%. There are a number of procedures, which have been developed to control this tendency. Almost every physical form of zinc anode material has been used in cyanide zinc plating, the type and prevalence varying from country to country. In the United States, cast zinc balls of approximately 50-mm diameter contained in spiral steel wire cages are by far the most common anode material. A practical variation of this is the so-called flat top anode, with a flat surface to distinguish it from cadmium ball anodes. The use of ball anodes provides maximum anode area, ease of maintenance, and practically completes dissolution of the zinc anodes with no scrap anode formation. In normal zinc plating operations, carbonate is generated by reactions such as the decomposition of cyanide and the neutralization of carbon dioxide absorbed from the air. Small concentrations (20-30 g/l) of carbonate may be slightly beneficial but additional carbonate, losses balance the rate of formation. Those equilibrium concentrations are commonly in the range 50 to 100 g/l, but substantially higher concentrations, up to 150 g/l or more have been found. also functions in several ways; aids in good anode corrosion, promotes higher plating speeds, and minimizes the hydrolytic losses of cyanide. 40 40 39 39
İCS OF STEEL PROTECTION 39 37 37 37 37 36 36
İZED COATİNG 36 35 34 34 34 34 33 33 32 30 28 24 24 23 22 20 20 19 19 18 17 17
İZİNG PROCESS 17 15 14
ınıtıon of hot-dıp galvanızıng process 14
İZİNG PROCESS 14 12 12 12 11 11 11 10 9 9 9 8 7 6 6 6 5 5 5 4 4 4 3 3
İON OF STEEL FROM CORROSION 3 1
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