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GALVANISING AND CURRENT

TRENDS

A

PRESENTATION

ON

GALVANISING

Presented by

K L Rohitas

Dy. General Manager

GALVANISING

In India every year steel worth of Rs.1200 to Rs.1300 crores is being lost due to deteriorating effect of corrosion. To overcome the corrosion effect, additional expenditure is involved on account of maintenance, shut down, repairs, replacement of steel infrastructures. In light of this it becomes very important to save the steel from the menace of corrosion.

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Factors responsible for corrosion

Corrosion is basically an electro-chemical process occurring because of difference in electrical potential which exist between dis-similar metals in contact or between small size within metal surface in presence of an electrolyte. Difference in the potential of metal are caused by

i) Variation in composition

ii) Presence of impurity

iii) Uneven internal strains and

iv) Non-uniform environment

.

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The environment like damp atmosphere, surface moisture or liquid in which the metal is immersed are actually responsible for the corrosion process. All serve as electrolytes, allowing formation of scale at the metal surface which is corrosion.

Steel generally corrodes at between 0.05mm to 0.125mm per year when freely exposed to the air. The corrosion rate vary in different regions depending upon the atmospheric conditions prevailing.

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Economical method for protection against corrosion There are many methods to prevent the corrosion of steel. An

economic decision is to be taken in selecting the methods. The factors which will affect low cost protection needing regular periodical maintenance or slightly expensive initial protection with no maintenance. When an economic decision is taken it is found that the more effective, practical and economic protection is achieved by metallic zinc coating by hot dip galvanizing process. Zinc is relatively highly resistant to the corrosive action of normal environment compared to Iron and thereby provides a long term protection.

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There are five methods of applying zinc

coating to steel surface to prevent

corrosion.

Hot Dip Galvanizing

Zinc Spraying

Zinc Plating

Sheradizing

Zinc Dust Paints

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Hot Dip Galvanizing

Plain steel (mild steel) duly cleaned & free from surface contamination is immersed in molten zinc kept at a tempreture of440-460 at which reaction takes place to form a chemically bonded metal zinc alloy along with a super layer of zinc on the surface.

Zinc Spraying

Molten Zinc is sprayed from a special gun fed by zinc powder.

Zinc Plating

Zinc Salt solutions are used in electrolytical depositing a layer of zinc on clean steel surface.

Sheradizing

Suitably prepared small steel parts are tumbled in zinc dust heated to just below melting point of zinc.

Zinc Dust Paints

Steel Surfaces are coated using paints which contains sufficient amount of zinc powder.

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ADVANTAGE OF HOT DIP GALVANISING PROCESS

Since in Hot Dip Galvanizing process, the reaction between iron and zinc takes place to form an alloy it becomes more effective than any of the methods stated above. We can now define the galvanizing as process of forming zinc and iron alloy at certain temperature. Zinc prevents corrosion by

i) Providing a tough, durable barrier coating of metallic zinc which completely seals the steel from corrosive environments.

ii) The Hot Dip Galvanizing process is creating a protective coating composed of several layers of iron zinc alloy varying from 100% iron of metal base to 100% zinc on outer surface of coating. Therefore adhesion of coating is much stronger than surface protection by other process like painting, zinc spraying etc.

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Advantage of Hot-dip Galvanising Process The protective coating done by ordinary methods are not

long lasting. Once it breaks, corrosion take place and it will penetrate deeply inside the iron base and can damage metal also underneath unbroken portion of protective layer thus completely destroying the iron.

On the other hand, the Hot-dip galvanized coating exposed to corrosive atmosphere, itself, reacts with and form a film of stable compound which does not allow any further penetration of corrosion. Besides this if zinc coating is damaged the metal-zinc alloy layer will still protect the iron even on those points where iron is exposed to the corrosive atmosphere

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HOT DIP GALVANISING PROCESS

The whole galvanizing process is

divided into three stages:-

Pre-treatment

Galvanizing

Post treatment

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Pre-treatment contains following sub-processes:-

Degreasing:

The process of degreasing removes oil, grease etc. from the metal surface to be galvanised. This is done by Caustic Soda solution heated to a temperature of 60oC to 70oC.

Rinsing:

Rinsing is being done to remove the Caustic soda which will be sticking to the metal surface. It is done by circulating water at a temperature of 60oC to 70oC.

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Pre-treatment contains of following sub-processes:-

Pickling:

The purpose of pickling is to remove rust from the surface to be galvanized. Pickling is done either by Sulphuric Acid or Hydro Chloric Acid (HCL).

The use of Hydro Chloric Acid (HCL) has been found to be more suitable than the Sulphuric Acid. Moreover it is more safe and less injurious compared to Sulphuric Acid.

Depending upon the concentration of the pickling solution ,pickling operation take 15-30 minutes. Initialy the concentration of pickling soln. is 15% which is gradually reduced.

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Rinsing:-

After material has been properly pickled i.e. free from rust, it is washed in the water. The washing is done in a double chambered tank in which water enters from tank overflows to the second tank and it is discharged creating the counter-current force thus more effective cleaning.

Pre-fluxing:

After rinsing pre-fluxing is done in the solution of Zncl2 and Ammonium Chloride heated to the temperature of 70oC to avoid any further oxidation before final process of Galvanization.

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DRYING

The pre-flux material is dried in the Drier at a temperature of 120 to 125oC. Above this temperature, the flux burns off. A hot air generator produces hot air circulated in the Drier.

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The Galvanising kettle containing molten zinc is continuously heated by Oil Fired Furnace where LDO is used as fuel in burners.This keeps the zinc always in molten state at required temperature.

After material have been fully dried it is taken to the Molten Zinc Bath where it is dipped in Molten Zinc kept at a temperature of 440 to 460oC. When the material is dipped in the molten zinc, the chemical reaction between zinc and iron takes place and alloy is formed with an additional coating of zinc on the surface. Galvanising process take about a minute and galvanised material is then removed from the galvanised bath.

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Quenching:

The material after galvanization is taken out and cooled down in the Quenching Tank containing water.

Passivation:

After galvanizing & quenching, the material is taken to the Passivation Tank where Sodium Dichromate solution at a temperature of 70oC to 80oC is kept. Material is dipped into this Tank and chromate layer is formed above the galvanizing layer.

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Advantages of Galvanizing:-

Toughness:

Galvanized coating has a unique structure which gives it excellent resistance to transit and erection damages.

Long Life:

The standard 610 gm. Per M Sq. coating has a typical maintenance free life of more than 30 years in Urban and Coastal environment.

Clean appearance

Low Cost: Galvanized coating is competitive as the same is long lasting as compared to other coatings having much shorter life.

Reliability: Because the coating is formed by chemical reaction, it is very reliable.

Ease of inspection

Time saving

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Factors affecting Galvanizing:

Thickness of the material

Surface condition of material

Pre-treatment of the material

Temperature of the Kettle

Dipping Rate

Dipping time

Rate of withdrawal

Quenching

Angle of withdrawal

Wiping efficiency

Time lag between completion of galvanization and withdrawal

Composition of Steel

Composition of Zinc Bath

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FACTORS AFFECTING THE GALVANISATION:-

(a)Thickness of the material:

As Galvanizing is the chemical reaction between iron and zinc the reaction time like any other reaction would vary according to the thickness and size of the material. If material on different thickness are being galvanized in one bath then thinner material will get galvanized early but they will have to be unnecessary in the Galvanizing Bath till the time thicker material gets galvanized thus over coating on surface will take place in thinner material.

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(b)Surface condition Surface condition of material also effect final

galvanization. If the surface of material is pitted then thick coating will be obtained as zinc will be trapped into the rough areas (pitted due to pickling).

In case rusty and relatively fresh material is being feed for pickling together in one batch then the fresh material will get pickled early as compared to the rusty material. The rusty material will take larger time than fresh material, thereby, the fresh material will get over pickled i.e. rough surface/pitting will occur resulting into over coating and rough galvanized surface.

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(c) Pre-treatment of the Material:

Degreasing

In case the material is having oil and grease it should be degreased properly otherwise oil and grease will remains sticking to the material and finally will get burnt off in the molten zinc kettle thereby resulting black spot.

Pickling

In case material has been over pickled the excess zinc consumption i.e. thick coating will occur. It is is under pickled in black spit will occur as during galvanization rust will get galvanized and leave the surface excess dross will also be formed.

Pre-fluxing

The pre-fluxing solution should not be acidic and should not have iron content in it. In case iron content are present in the pre-fluxing solution it will stick to the material thereby resulting into formation of black spots.

Dross Formation

The iron particles are free to move when dipped in the Galv. Bath these particles gets galvanized and will move out of the surface resulting into the black spot in that area and excess dross formation.

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(d) Temperature of the Kettle

The control of galvanising-bath temperature is very essential if quality of the product is to be kept constant and zinc is to be used efficiently .At higher temperature metal-zinc alloy layer grows faster thereby increasing the zinc consumption. Most of the material can be satisfactorily galvanized within the temperature range of 440 to 460oC. Hence it should be seen that the temperature of Kettle is maintained within the limits of 440 to 460oC. This is important from following angles also:-

i) Safe guarding of Galv. Kettle itself.

ii) To reduce Zinc-Dross formation.

iii) To reduce Zinc Ash formation.

iv) Conserving the precious fuel.

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(e) Dipping Rate:

The material should be dipped in the Kettle as rapidly as possible. Dipping rate also influences uniformity of coating particularly with long articles/bath where the difference in dipping time between first and last part/items to enter the bath may be considerable. As the material entering first will come out last it will be having maximum coating. Here principle is full material should be dipped at one time.

(f) Dipping Time:

The dipping time is directly proportional to the zinc coating.

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(g) Angle of Withdrawal:

The angle at which material is withdrawn decides the amount of flow of free molten zinc on the material back into the bath. The maximum angle of withdrawal in the material better that flow of the zinc and coating.

(h) Rate of Withdrawal:

Rate of withdrawal determines the thickness of free zinc layer on the work. The optimum withdrawal rate for most articles is about 1.5M per minute. Here principle is that material is not withdrawn faster than the rate at which zinc drains out freely from the surface. Hence based on this principle material should be withdrawn slowly to allow free molten zinc to flow back into the Kettle.

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(i) Quenching:

Formation of iron zinc alloy layer continues even outside the Zinc Kettle after removing the material till the temperature of 250oC is reached. During this stage iron zinc alloy layer grown at the cost of pure zinc layer leaving grey coating which is undesirable. To avoid this material should be quenched in the cold as faster as possible.

(j) Wiping efficiency:

When the material is being taken out from the Bath, it should be wiped with the help of rods and bamboos as fast as possible to avoid solidification of excess zinc.

(k) Time-lag between completion of Galvanization and Withdrawal.

More the time lag between completion of galvanization (boiling off) and withdrawal more is the zinc coating/zinc consumption. It should be minimum.

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(l) Composition of Steel:

The single and most important factor influencing the galvanized coating weight is the steel chemistry. The silicon content of the steel is paramount to the weight of coating form and resulting thickness and growth of coating. Presence more silicon, phospherous and carbon produces grey coating.

(m) Composition of Zinc Bath:

The zinc bath does not have only the zinc but is composed of zinc, lead, and aluminum. In case the percentage of aluminum increases more than 0.007% of the total zinc content the alloy layer will be non-sticking. In will be peeling off. In case Lead is more in bath then dross level will rise up and dross may start coming out with the material.

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Tips for good Galvanization:

Try to get material of similar surface condition and thickness.

Ensure proper pickling

Ensure proper rinsing and proper fluxing

Ensure the material is dried and pre-heated up to a temperature of 120oC

Bath temperature should be within the limits of 440 to 460oC

The Bath should be cleaned before and after dipping of the material

Boiling of point must be monitored to avoid any excess coating

Quenching must takes place as early as possible in cold water.

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Capacity of Galv. Plants

The capacity of Galv. Plants is determined by throughout per hour i.e. quantity which can be galvanized per hour. Hourly through output of Galv. Plants is one-tenth of total zinc content in bath. But in India one-twentieth of total zinc content in bath can be achieved on hourly basis.

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Safety precautions to be taken in Galvanizing Plant

The employees working in Galvanizing Plant must wear protective clothing and use protective appliances like Hand Gloves, Gas mask, Spectacles, safety helmet etc.

While dipping the materials, the Galvanizer must stand away from Galvanizing Bath preferably in a booth having glass window for protection against zinc splash ups.

Acidic fumes coming out of the Pickling tanks are injurious to health, hence employee working in Pickling area must wear suitable mask so as to avoid inhaling of acidic fumes.

Periodical health check ups of employees is necessary to know the status of health and precautions to be taken accordingly.

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Measures to be taken for Plant Safety:

Galvanizing Plant must have arrangements for suction and treatment to acidic fumes to reduce effect on humans as well as trusses/assets of the Shed.

Since acidic fumes adversely affect the life of trusses/cranes etc., it must be regularly painted by acid resistant paints for protection against ill effects of fumes.

Galvanizing Plant must have effluent treatment Plant for proper treatment of waste acid to protect environment from pollution.

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