galvanised steel

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Pressed Steel Tanks Guardrail, Highway Guardrails Playground Equipment "J" or "L" Bolts & Nuts Products that are Hot Dip Galvanized such as the Pressed Steel Tank for water storage and steel towers, Guardrails,Highway Guardrails or Highway Safety Crash Barriers. Playground Equipment and Garden Furniture are normally Hot Dip Galvanized and Duplex Powder Coated for its colorful appeal...for more information on these products contact us PRESSED STEEL WATER TANKS - These are pressed steel panels bolted together for water storage typically for fire protection systems, domestic water and air conditioning systems in buildings. These water storage tanks are normally supplied complete with covers, internal and external ladders, water level indicators and are mostly hot dip galvanized for corrosion protection. The pressed steel water storage tanks are bolted together from the outside with sealastrip and bitumum compound for water tightness. These pressed steel tank panels are held together with hot dip galvanized internal trusses and anchorage systems PRESSED STEEL WATER TANKS - These water storage tanks are constructed from standard galvanized pressed steel tank panels of 1.2 m x 1.2 m and of various steel thickness from 3 mm, 4.5mm and 6 mm thickness. A typical standard tank size of 1.2 m x 1.2 m x 1.2 m (ht) would hold a maximum of 1728 liters of water. Bigger tanks sizes are in multiples of the standard unit and may be constructed to hold up to 100,000 liters (3.6 m x 6 m x 4.8 m (ht)) or larger volumes.

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Page 1: Galvanised Steel

Pressed Steel  Tanks

Guardrail, Highway Guardrails

Playground Equipment  

"J" or "L" Bolts & Nuts

Products that are Hot Dip Galvanized such as the Pressed Steel Tank for water storage and steel towers, Guardrails,Highway Guardrails or Highway Safety Crash Barriers. Playground Equipment and Garden Furniture are normally Hot Dip Galvanized and Duplex Powder Coated for its colorful appeal...for more information on these products contact us

PRESSED STEEL WATER TANKS - These are pressed steel panels bolted together for water storage typically for fire protection systems, domestic water and air conditioning systems in buildings. These water storage tanks are normally supplied complete with covers, internal and external ladders, water level indicators and are mostly hot dip galvanized for corrosion protection. The pressed steel water storage tanks are bolted together from the outside with sealastrip and bitumum compound for water tightness. These

pressed steel tank panels are held together with hot dip galvanized internal trusses and anchorage systems    

PRESSED STEEL WATER TANKS - These water storage tanks are constructed from standard galvanized pressed steel tank panels of 1.2 m x 1.2 m and of various steel thickness from 3 mm, 4.5mm and 6 mm thickness. A typical standard tank size of 1.2 m x 1.2 m x 1.2 m (ht) would hold a maximum of 1728 liters of water. Bigger tanks sizes are in multiples of the standard unit and may be constructed to hold up to 100,000 liters (3.6 m x 6 m  x 4.8 m (ht)) or larger volumes.   

DAILY APPLICATIONS OF HOT DIP GALVANIZING  

The selection of a steel coating system is an integral part of all engineering design. The main consideration for the engineer in the selection of the most suitable corrosion protection system would be the performance of a steel coating and  the economics of the application of the steel coating system. There is no other corrosion protection system that could match the performance and economics of Hot dip galvanizing

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APPLICATIONS OF HOT DIP GALVANIZED STEEL IN THE FOLLOWING SECTORS OF OUR ECONOMY

1) POWER GENERATION & TRANSMISSION 2) INFRASTRUCTURE  DEVELOPMENT

1) POWER GENERATION AND POWER TRANSMISSION

In the building of a power station for power generation, steel is a major construction material. Galvanized steel is used in platforms, equipment buildings, stairs and handrails. In the area of fuel supply to the main power generating plants galvanized steel conveyor systems are common in a coal fired power station. Cooling water, water reticulation and fire protections systems consume huge amounts of galvanized steel in the form of piping and it fittings. 

As for power transmission every piece of steel in a transmission tower is completely galvanized from the main steel frame, every piece of bolt and nut used to fasten the angles together to the cable support systems are completely galvanized. The are now many transmission towers that are also painted with special paint system for identification purposes.   

2) INFRASTRUCTURE DEVELOPMENT 

Government of many countries now invest and spent huge part of their budgets in improving the infrastructure such as road highways and expressways, railways, Light Rail Transport system (LRT) and Mass rapid Transportation system (MRT) , Port terminals and airport facilities. These projects consume huge amounts of exposed steel and as such hot dip galvanizing is the preferred corrosion protection system.

As for other developments such as schools, hospitals, community halls and other public places galvanized steel are mainly in galvanized products such as galvanized water tanks for fire protections systems, street lights, safety barriers and road and drain covers. 

Hot dip galvanized reinforcement steel was only used  in critical construction areas such as coastal or marine concrete structures. In the last decade the use of  hot dip galvanized reinforcement steel increased with the rapid expansion of the road, highways and expressways. Steel rods and strips for reinforced earth (RE) walls and soil nails are always hot dip galvanized. Guardrails, crash cushions, decorative street lights, high masts, pedestrian overhead bridges, noise barriers, parapet handrails are some of the other products that are corrosion protected with hot dip galvanizing    

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3) TELECOMMUNICATION TOWERS

Telecommunication steel towers are difficult structures to maintain considering its location which normally are difficult to access since it is situated on hills slopes and on top of mountains. For easy installation these Steel Towers are fabricated from Steel Tubes in different sections and steel Angles of various sizes and lengths, Hot Dip Galvanized and fastened with Centrifuged Hot Dip Galvanized Bolts, Nuts and Washers. These 3 Leg Telecommunication Towers are 100% Hot Dip Galvanized and Duplex coated for long term corrosion protection and aerial Identification.

4) BUILDING AND CONSTRUCTION

Twin Towers KL Tower

The Petronas Twin Tower in Kuala Lumpur and the Kuala Lumpur Tower are prestigious projects in Malaysia. For durability most steel are Hot Dip Galvanized and Duplex coated. Commonly found in the open areas are Forged Welded Gratings hot dip Galvanized, Garden Lighting Poles Hot Dip Galvanized and Children Play Stations all Hot Dip Galvanized and Duplex Coated.

5) MINING

6) OIL AND GAS PRODUCTION

7) AGRICULTURE AND ANIMAL HUSBANDRY

8) FARMING

Real Benefits of Galvanized SteelThe use of galvanizing for structural steel protection gives you ten major, measurable benefits.

1. Lowest first cost. Galvanizing is lower in first cost than many other commonly specified protective coatings for steel. (The application cost of labour intensive coatings

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such as painting has risen far more than the cost of factory operations such as galvanizing.)

2. Less maintenance/Lowest long term cost. Even in cases where the initial cost of galvanizing is higher than alternative coatings, galvanizing is almost invariably cheapest in the long term (because it lasts longer and needs less maintenance).  And, maintenance causes problems and adds to costs when structures are located in remote areas, and when plant shutdown or disruption to production is involved.

3. Long life. The life expectancy of galvanized coatings on typical structural members is far in excess of 50 years in most rural environments, and 20 to 25 years plus, even in severe urban and coastal exposure.

4. Reliability. Galvanizing is carried out to Australian / New Zealand Standard 4680, and standard, minimum coating thicknesses are applied. Coating life and performance are reliable and predictable.

5. Toughest coating. A galvanized coating has a unique metallurgical structure which gives outstanding resistance to mechanical damage in transport, erection and service.

6. Automatic protection for damaged areas. Galvanized coatings corrode preferentially to steel, providing cathodic or sacrificial protection to small areas of steel exposed through damage. Unlike organic coatings, small damaged areas need no touch up.

7. Complete protection. Every part of a galvanized article is protected, even recesses, sharp corners and inaccessible areas. No coating applied to a structure or fabrication after completion can provide the same protection.

8. Ease of inspection. Galvanized coatings are assessed readily by eye, and simple non-destructive thickness testing methods can be used. The galvanizing process is such that if coatings appear sound and continuous, they are sound and continuous.

9. Faster erection time. As galvanized steel members are received they are ready for use. No time is lost on-site in surface preparation, painting and inspection. When assembly of the structure is complete, it is immediately ready for use, or for the next construction stage.

10. A full protective coating can be applied in minutes; The galvanizing process is not dependent on weather conditions

Spectitle:Galvanized Iron And Steel: Characteristics, Uses And Problems

Procedure code: 0501009S

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Source: Developed For Hspg (Nps - Sero)

Division: Metals

Section: Metal Materials

Last Modified: 01/07/2014

Details: Galvanized Iron And Steel: Characteristics, Uses And Problems

GALVANIZED IRON AND STEEL:  CHARACTERISTICS, USES AND PROBLEMS

This standard includes general information on the characteristicsand common uses of galvanized iron and steel and identifies typicalproblems associated with these materials along with common causesof its deterioration.

INTRODUCTION

Galvanizing is a process of coating iron or steel with zinc inorder to provide greater protection against corrosion for the ironor steel base.  The process of galvanizing sheet iron was developedsimultaneously in France and England in 1837.  Both of thesemethods employed a "hot dipping" process to coat sheet iron withzinc.  Like tinplate, early galvanized metals were hand dipped.Today almost all galvanized iron and steel is electroplated.

The following are the most common methods for applying protectivecoatings of zinc to iron and steel:

1.   Hot-dip Galvanizing:  The immersion of iron or steel in molten    zinc, after the surface of the base metal has been properly    cleaned.  

    a.   This process gives a relatively thick coating of zinc         that freezes into a crystalline surface pattern known as         spangles.  

    b.   During the process, a multiple layered structure of iron-         or steel-zinc alloys is formed between the inner surface         of the zinc coating and the iron or steel.  These middle         layers tend to be hard and brittle and may peel or flake         if the iron or steel element is bent.

2.   Electrogalvanizing:  The immersion of iron or steel in an    electrolyte, a solution of zinc sulfate or cyanide.    Electrolytic action deposits a coating of pure zinc on the

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    surface of the iron or steel.  

    Advantages:

    a.   The thickness of the coating can be accurately controlled         using this process.  

    Limitations:

    a.   The thick coatings provided by the hot-dip galvanizing         process are not usually possible with this method.

3.   Sherardizing:  The placing of a thoroughly cleaned iron or    steel element in an air-free enclosure where it is surrounded    by metallic zinc dust.  The architectural element is then    heated and a thin, zinc alloy coating is produced.

    Advantages:

    a.   The coating will conform to the configurations of the         element.  

    Limitations:  

    a.   This process is usually limited to relatively small         objects.

4.   Metallic Spraying:  The application of a fine spray of molten    zinc to a clean iron or steel element.  The coating can then    be heated and fused with the surface of the iron or steel to    produce an alloy.  

    Advantages:

    a.   Coating is less brittle than those produced by some of         the other processes.

    b.   Coating will not peel or flake on bending.  

    Limitations:

    a.   The coating is more porous and becomes impermeable with         time as products of corrosion fill in the pores.

5.   Painting:  Paint containing zinc dust pigments may be applied    as a protective coating to galvanized iron and steel.

    Advantages:  

    a.   The paint may be applied in situ.

    Limitations:

    a.   This is a less effective method of zinc coating than the         others listed above.

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    b.   Paint does not adhere well to pure zinc, nor to         galvanized iron or steel.

    c.   When paint peels from galvanized iron and steel, it         usually comes off completely along with the primer,         exposing a clean metal surface.

    d.   If sheetmetal features are well-painted, it is difficult         to identify whether they are zinc or galvanized iron or         steel.

         1)   If the metal is galvanized, it will have a spangled              appearance and may show some rust or rust stains              from the iron or steel base metal.  Both galvanized              iron and steel are magnetic

         2)   If the metal is cast or pressed zinc, it will have              a grayish-white appearance.  Pure zinc is not              magnetic so a magnet will not stick.

         3)   A magnet test will also reveal whether a painted              sheetmetal feature is zinc or galvanized iron or              steel.  Both galvanized iron and steel are              magnetic, pure zinc is not.

TYPICAL USES

Typical historical uses for galvanized iron and steel included:

-    Cornices and other wall ornaments

-    Door and window hoods

-    Decorative formed shingles and pantiles designed to imitate    other materials

-    Roof ornaments such as crestings and finials

Typical uses today include:

-    Sheetmetal for flashing, and gutters and downspouts.

-    Hot-dipped galvanized steel nails.

PROBLEMS AND DETERIORATION

Problems may be classified into two broad categories:  1) Naturalor inherent problems based on the characteristics of the materialand the conditions of the exposure, and 2) Vandalism and human-induced problems.    

Although there is some overlap between the two categories, the

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inherent material deterioration problems generally occur graduallyover long periods of time, at predictable rates and requireappropriate routine or preventive  maintenance to control.Conversely, many human induced problems, (especially vandalism),are random in occurrence; can produce catastrophic results; aredifficult to prevent, and require emergency action to mitigate.Some human induced problems, however, are predictable and occurroutinely.  

NATURAL OR INHERENT PROBLEMS

CORROSION:

Galvanized iron and steel's resistance to corrosion depends largelyon the type and thickness of the protective zinc coating and thetype of corrosive environment.

The zinc coating on galvanized iron and steel may be corroded by:Acids, strong alkalis, and is particularly vulnerable to corrosionby sulfur acids produced by hydrogen sulfide and sulfur dioxidepollution in urban atmospheres.  

1.   Natural Corrosion:

    a.   The zinc coating on galvanized iron and steel develops a         natural carbonate on its surface by exposure to the         atmosphere and by the action of rainwater.  This coating,         however, is usually not thick enough to protect the metal         from further corrosion.

    b.   The carbonate can become brittle and crusty and         eventually split, exposing fresh zinc for corrosion.         Since the zinc coating on the iron or steel is very thin,         it can corrode up to the base metal exposing the base to         the atmosphere as well.

    c.   In industrial atmospheres, the zinc carbonate coating can         be broken down by the same acids that attack zinc.  These         acids convert the carbonate to zinc sulfate, which is         water soluble and washes away with rainwater, often         staining the adjacent building elements.

2.   Chemical Corrosion:

    a.   Galvanized iron and steel have good corrosion resistance         to:  Concrete, mortar, lead, tin, zinc and aluminum.

    b.   Galvanized iron and steel have poor corrosion resistance         to:  Plasters and cements (especially Portland cements)         containing chlorides and sulfates, acidic rainwater run-        off from roofs with wood shingles (redwood, cedar, oak,         and sweet chestnut), moss, or lichen, condensation on the         underside of zinc plates and ponded water on the exterior         surfaces of the zinc features

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3.   Galvanic (Electrochemical) Corrosion:  This type of corrosion    is an electrolytic reaction between the zinc coating and    dissimilar metals when in the presence of an electrolyte such    as rain, dew, fog or condensation.

    a.   To prevent the corrosion of the zinc coating due to         galvanic action, contact between galvanized items and         copper or pure iron or steel should be avoided.

    b.   Galvanized iron and steel are corrosive to all metals         except lead, tin, zinc and aluminum.

    c.   Applying a protective coating such as paint to galvanized         iron and steel will alleviate the problems caused by         corrosion of the protective zinc coating.  

VANDALISM OR HUMAN-INDUCED PROBLEMS

Mechanical or Physical Deterioration:

1.   Abrasion:  Causes removal of the protective metal surface.    The soft zinc coating on galvanized iron and steel make it    vulnerable to abrasion damage, especially at roof valleys and    gutters where the coating can be worn paper-thin by the    scouring of rainwater.

2.   Fatigue:  A type of deterioration caused by cyclical expansion    and contraction of sheet metal features, especially roofs,    without adequate provisions for this movement.  

    a.   Zinc is very vulnerable to fatigue failure because it has         a relatively high coefficient of thermal expansion.  

    b.   Fatigue failure may also occur when the metal sheets are         too thin to resist buckling and sagging.  It results in         the bulging and tearing of the zinc coating and resembles         a cut or a crack.

3.   Creep:  The permanent distortion of a soft metal which has    been stretched due to its own weight.  Thin areas of the metal    are especially prone to failure.  Creep may be prevented by    the use of properly sized individual sheets and bays, properly    designed joints, and an adequate number of fasteners.

4.   Distortion:  Permanent deformation or failure may occur when    a metal is overloaded beyond its yield point because of    increased live or dead loads, thermal stresses, or structural    modifications altering a stress regime.  

Connection Failure:

1.   Wind and thermal stress can damage a roof by pulling joints    apart and loosening fasteners.

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                         END OF SECTION

In WaterA less common environment for galvanized steel is submerged in or exposed to water. Moisture is highly corrosive to most metals including steel and zinc.  Despite the difficulty of predicting corrosion, hot-dip galvanizing steel is one of the best methods of corrosion protection for submersed applications because of its complete, uniform coverage. Similar to the zinc patina in atmospheric exposure, some waters allow the zinc coating to develop a passive film on the surface slowing the corrosion rate.

Similar to soils, the varieties of water make predicting corrosion rates difficult. Many parameters affect corrosion of metals in a water environment , including pH level, oxygen content, water temperature, agitation, the presence of inhibitors, and tide conditions. The first step in deciding whether galvanized steel is the right coating for your application is to determine what type of water will be used. Water can be divided into a number of different types; pure water (e.g., distilled water or de-ionized water), natural fresh water,   seawater , or  potable water (treated drinking water)and each has different mechanisms that determine the ultimate corrosion rate.

Pure Water

Pure water, also known as de-ionized or distilled water, is usually very corrosive to zinc coatings due to the presence of dissolved oxygen and carbon dioxide. Corrosion rates of steel increase with aeration of pure water; dissolved oxygen in pure water is five to ten times more aggressive than carbonic acid

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Natural Fresh Water

Galvanizing is successfully used to protect steel in fresh water exposure. Fresh water refers to all forms of water except sea water. Fresh water may be classified according to its origin or application. Included are hot and cold domestic, industrial, river, lake and canal waters. Corrosion of zinc in fresh water is a complex process controlled largely by impurities in the water. Even rain water contains oxygen, nitrogen, carbon dioxide and other dissolved gases, in addition to dust and smoke particles.

Ground water carries micro-organisms, eroded soil, decaying vegetation, dissolved salts of calcium, magnesium, iron, and manganese, and suspended colloidal matter. All of these substances and other factors such as pH, temperature, and motion affect the structure and composition of the corrosion products formed on the exposed zinc surface.

Relatively small differences in fresh water content or conditions can produce relatively substantial changes in corrosion products and rate. Thus, there is no simple rule governing the corrosion rate of zinc in fresh water. However, one trend data supports is hard water is much less corrosive than soft water. Under conditions of moderate or high water hardness, a natural scale of insoluble salts tends to form on the galvanized surface. These combine with zinc to form a protective barrier of calcium carbonate and basic zinc carbonate which slow the corrosion rate.

The biggest factors in the zinc corrosion rate in fresh water are dissolved gasses, hardness/mineral rate, flow rate, and other ions/chlorides.

Gasses:  More oxygen means more corrosion products on the surface of the zinc which increases the corrosion rate. For this reason, fully immersed in the water is better than partial immersion because there is less oxygen under water. Well waters tend to have lower content of dissolved oxygens, so corrosion is low, while surface waters and springs have higher rates.

Hardness: In hard water, zinc combines with carbonates and bio carbonates to form zinc carbonate, which unlike zinc oxide, is not water soluble. The zinc carbonate deposits on the surface of the zinc and creates a passive film on the galvanized part slowing corrosion. The softer the water, the lower it is in carbonate; therefore, soft water is more corrosive than hard.

Flow Rate: Higher flow rates tend to increase corrosion because it acts similar to wind in atmospheric exposure – it means increased abrasion.

Other Ions: Anion is the most aggressive ion to zinc, namely when over 50 mg/L. This tends to be more pronounced in soft waters which often have 80 mg/L whereas hard waters have 700 mg/L. The protective film deposited by carbonated in hard water stifles the effects of anions.

Seawater

Galvanized coatings provide considerable protection to steel immersed in sea water and exposed to salt spray. The factors that influence the corrosion of zinc in fresh water also apply to sea water. However, the biggest determinants to galvanized steel’s performance in seawater are temperature and ion interaction. 

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Temperature: Seawater temperature varies widely from 28.4 F at the poles to 95 F near the equator. For all waters, the warmer the water, the higher the attack on zinc because reactions between oxygen and zinc happen faster at higher temperatures. This is why tropical seawater is much more corrosive than temperate seawater. Temperate seawaters have a freeze cycle, and are often less corrosive to galvanized steel than even fresh soft waters.

Ion Interaction: In moderate temperature ranges zinc forms salts with magnesium and calcium that are not water soluble. These passive compounds form on the surface preventing the zinc metal from reacting with oxygen and chlorides which slows the corrosion rate. Tropical waters tend to stay at 70 or above, making it difficult to develop these compounds, as the colder the temperature the better the formation.

Given the high level of chloride in sea water, a very high rate of zinc corrosion might be expected. However, the presence of magnesium and calcium ions in sea water has a strong inhibiting effect on zinc corrosion in this type of environment. Results from accelerated laboratory tests sometimes use a simple sodium chloride (NaCl) solution to simulate the effects of sea water exposure on galvanized steel and should be viewed skeptically. Real world results often differ significantly from accelerated laboratory tests. 

Potable Water

In the mid-1980s Congress passed the Clean Water Act, which includes the Drinking Water Standard. This standard requires that any material or coating that comes in contact with drinking water must be tested. The EPA contracted the National Sanitation Foundation (NSF) to write the test procedure, which after many drafts and public meetings, was finally published as NSF Standard 61: “Drinking Water Systems Components: Health Effects.” Therefore, only galvanizers that have submitted test coupons of their galvanized steel and have been approved by the NSF have the authority to galvanize steel for use with potable water. Despite the great lengths that a galvanizer must endure to gain this certification, hot-dip galvanized steel is a very suitable application for potable water.

Tidal Zones & Water Agitation

One of the most corrosive areas for galvanized steel is on wash zones and tide lines.  The agitation accelerates the corrosion rate of the zinc.  Often the “washing” motion removes the passive scales which are forming on the surfaces, exposing fresh zinc which tries to redevelop

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more scales/patina. This leads to rapid erosion of the zinc coating resulting in increased corrosion rates.

Zinc Coatings - How They WorkShare on emailEmail Share on printPrint Share on twitterTwitter Share on facebookFacebook

For over a century, zinc has enhanced the longevity and performance of steel. Zinc coatings provide the most effective and economical way of protecting steel against corrosion which, left unchecked, is estimated to cost an industrialized country’s economy at least 4% of GDP each year.

Zinc-coated or galvanized steel offers a unique combination of properties unmatched by any other material. These include:

high strength formability light weight corrosion resistance aesthetics recyclability low cost

For this reason, galvanized steel sheet is an ideal material for a multitude of building and manufacturing applications - from automobiles to household appliances to residential, commercial and industrial construction.

Technical Performance of Zinc CoatingsThere are many factors to consider when selecting the most appropriate zinc coating. In addition to corrosion protection, the coating’s formability, adherence, appearance and cost should also be

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considered. These factors each impact on selecting the correct zinc coating for a given application. All zinc coatings, like the steel they protect, are recyclable.

Corrosion ResistanceWhen left unprotected, steel will corrode in almost any environment. Zinc coatings protect steel by providing a physical barrier as well as cathodic protection for the underlying steel. It is important that the correct zinc coating is specified to provide optimal performance under the exposure conditions to which the coating will be subjected.

When painted zinc-coated steel is scratched, zinc protects boththe underlying steel from corrosion and the overlying paint coat from lifting.

Barrier ProtectionZinc coatings provide a continuous, impervious metallic barrier that does not allow moisture to contact the steel. Without moisture, there is no corrosion, except in certain chemical atmospheres. The effectiveness of zinc coatings in any given environment is directly proportional to coating thickness. Coating life is determined by the coating corrosion rate, itself a function of many factors such as time, composition of the atmosphere and the type of coating.

In situations of outdoor exposure, the acidity level of rain will influence the zinc corrosion rate. With indoor exposure - ventilation ducts, floor decks and steel framing, for example - moisture may also be present. In industrial indoor situations, the atmosphere may be corrosive. Thus the type and weight of coating required depends both on the service life needed and the exposure conditions.

Corrosion resistance of coatings can also be improved by using a zinc alloy coating, such as Galfan® or Galvalume®, or by applying paint top coats. These two methods, individually or together, are recommended for exposed sheet applications where enhanced corrosion protection is required.

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Cathodic ProtectionAnother outstanding protection mechanism is zinc’s remarkable ability to galvanically protect steel. When base steel is exposed, such as at a cut edge or scratch, the steel is cathodically protected by the sacrificial corrosion of the zinc coating adjacent to the steel. In practice, this means that a zinc coating is not undercut because the steel cannot corrode adjacent to a zinc coating. This contrasts with paint and aluminum coatings where the corroding steel progressively undercuts the surrounding barrier film. The extent of this cathodic protection is determined by the type of coating, its thickness and that of the underlying steel, as well as by the area of damage.

Painted Zinc CoatingsZinc coatings are easily painted. The term "duplex coating" is used for galvanized and painted steel parts, whereas the term "coil coating" or "pre-painting" is used for continuous galvanized and painted steel sheet. Paint acts as a barrier protecting the underlying zinc coating. Zinc is an excellent substrate for paint coatings because if the paint film is broken, zinc’s high corrosion resistance prevents undercutting of the paint film. Even if the coating damage does reach the steel base, zinc’s cathodic action will prevent the steel from corroding. Zinc’s ability to extend the life of paint coatings is what makes pre-painted galvanized steel sheet such a durable product that continues to extend its market share in commercial and residential roofing and cladding applications.

Combigram of duplex systems.The zinc and steel industries have for many years conducted both research and field trials on the performance of various combinations of painted and coated sheet steel. The result of this extensive base of information is that existing performance can be predicted for a wide range of atmospheric conditions. In the example shown for a highly corrosive industrial atmosphere, a 70 µm galvanized coating plus a 100 µm layer of paint will provide a coating life of over 50 years.

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Source: Stichting Doelmatig Verzinken / Progalva

Formability and AdhesionThe formability and adhesion of continuous galvanized zinc coatings are excellent and in most cases match the formability of the underlying steel. The formability of galvanized steel - which is defined as the resistance to cracking and loss of adhesion of the zinc coating during forming - is inversely proportional to coating and steel substrate thickness. There are, however, some coatings that are more ductile than others, an important consideration for deep draw stamping applications. It is therefore necessary to balance the requirements for corrosion resistance and formability .

2.4. AssemblyAssembly refers to the technique of joining galvanized sheet steel products, mainly to themselves. In any application, the joining method should suit the metallic coating and will be determined either on the basis of its performance or the properties and characteristics of the galvanized product.

There are several effective joining methods:

Welding is the most common method of joining steel products. Resistance welding is a technique that uses resistance to the flow of electrical current to generate heat and thereby join two materials.Pre-finished galvanized steel can be spot welded if the zinc coating is not too thick to enable the welding current to pass from one electrode to the other. This technique is often used in the downstream sectors.

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Mechanical fixing methods such as screws, rivets, self-piercing rivets and lock forming can be used for a wide range of steel substrate, zinc coatings and applications.

Adhesive bonding has become more popular and can also be allied to mechanical fixing. The range of adhesive systems is wide and the selection depends on numerous variables such as surface condition, adhesion, strength and cure speed.

Surface AppearanceZinc and zinc alloy coatings can differ in appearance depending on customer needs and consumer preferences. Galvanized coating finishes can vary from extra smooth and featureless to a flowery "spangle" pattern. Galvanneal (Zn-Fe) coatings have a matt grey appearance. Electrogalvanized coatings have the smoothest finish among zinc coatings and provide the best substrate for a high quality paint finish.

There are standards covering all aspects of surface appearance - coating finishes, surface qualities (from regular to best quality) and surface treatments (chemical passivation, oiling) - all targeted to enhance further processing.