FAQ’s

Galvanizing is the practice of coating clean, oxide-free iron or steel with a thin layer of zinc or zinc alloy to protect the surface against corrosion.

The zinc coating provides protection to the iron or steel in two ways:

(1) It shields the base metal from the atmosphere and

(2) Because it is more electronegative than iron or steel, the zinc gives cathodic or sacrificial protection. Even if the surface becomes scratched and the base metal is exposed, the zinc is slowly consumed while the iron or steel remains protected from corrosion.

The hot dip process is adaptable to coating nearly all types of fabricated and non-fabricated products such as structural assemblies, hardware, chain, hollow ware, wire, pipes and tubes, fittings, tanks, sheets, strip, and wire cloth.

Clean, oxide-free iron or steel is galvanized by coating it with a thin layer of zinc. This protects the iron or steel by shielding it from the atmosphere, as well as providing cathodic or sacrificial protection. Even if the zinc coating is scratched and the base metal is exposed, the more electronegative zinc is slowly consumed while the iron or steel base is protected.

The most important galvanizing method used is the hot dip process which is adaptable to the galvanizing of nearly all types of fabricated and non-fabricated products such as wire, tanks, sheets, strip, pipes and tubes, fittings, hardware, wire cloth, hollow ware, and structural assemblies.

Hot dip galvanizing consists of these fundamental steps:

1. Surface Preparation — The surface is cleaned, rinsed, pickled and rinsed to remove dirt, grease, rust and scale.
2. Prefluxing — The work is immersed in a 15 to 25º Baumé solution of GALVAGARD® galvanizing flux. It is coated with a layer of flux to dissolve light oxides that may have formed since pickling and also to protect against any further oxidation.
3. Galvanizing — Clean, oxide-free work is usually immersed through a molten layer of GALVAGARD galvanizing flux into the molten zinc. A thin coating of zinc is formed on the base metal.
4. Finishing — Excess zinc is removed; the piece is quenched and inspected.

In some industries mechanical cleaning is possible and the material is finished and kept. However  there are times when they get rusted and then again starts, chemical cleaning from pickling.  Mechanical cleaning would reduce the degreasing need.

Acidic degreasers are non – Hydrochloric acid or Sulphuric Acid based acidic degreasers and cleaners . It is employed to remove light oxides, shop soils, oil, grease and other contaminants from the work.  The distinction between acid cleaning and acid pickling is a matter of degree .  In general, acid pickling refers to a more severe treatment for the removal of heavy layers of scale and oxide which is normally done using Hydrochloric Acid and Sulphuric Acid.  Acidic degreasers also have special emulsifiers to dissolve oil and grease , which normally acid cleaning wont have or fail to work on.

We offer GALVAGARD range of acidic degreasers.

Ferrous metals have a surface coating of metallic oxide or scale which must be removed before the zinc coating can be applied successfully.  Pickling involves immersing the work in an acid to dissolve the scale or oxide film. M

It is generally done with Hydrochloric Acid , however Sulphuric acid is also used by few .

Hydrochloric acid, acts by dissolving each layer of scale from the outside in. It is normally used at ambient of 70-100o F (21-38o C) temperature at 15% dilution.

Sulfuric Acid acts on scale or oxide by penetrating the oxide layer and dissolving the oxidized region between the base metal and the outer oxide layers, which also generates hydrogen gas. The pressure of the gas combined with this dissolution loosens the scale and “blows it off”. It is to be used in dilute and is normally heated  to 150-180o F (66-82o C),, it works faster than Hydrochloric acid when heated. Sulphuric acid produces less fumes , but surface finish is somewhat rougher as compared to  Hydrochloric acid. However , this can be controlled by using pickling inhibitor. It is normally diluted to 10% , and discarded when acid conc reaches 3-4%

When the iron concentration reaches 7 to 10%, the pickling rate is usually too low to be economical regardless of acid concentration with either acid. Also for good cleaning agitation is recommended.

It is necessary to rinse with water to ensure no carryover to preflux tank. Drag-out of either sulfuric or hydrochloric acid pickles will contain soluble iron, which will contaminate the preflux if not rinsed away iron from pickling

Drag-out from the pickling operation should be carefully rinsed off the work prior to prefluxing.  If sulfate is carried into the preflux tank and allowed to buildup, it will cause an increase in top flux viscosity. Sulfuric acid pickle films are somewhat more difficult to rinse off.

Any drag would impact the pH of preflux tanking, making it acidic and disturbing the control parameters of the preflux operations. Also any iron carryover would severely impact the color and iron content of pre-flux.

Overpickling occurs when the pickling solution attacks and dissolves base metal after scale removal. It usually results because the scale layers are not uniform and the thicker or more deeply embedded material requires more time for removal. Proper selection and use of inhibitors will prevent overpickling. Quality problems due to overpickling also include possible hydrogen embrittlement and excessive surface roughness.

Pickling Inhibitors are used to retard the attack of acids on steel without greatly influencing the attack on scale. Thus, they lower acid consumption, reduce hydrogen formation and subsequent absorption by the steel, decrease acid fuming, and prevent “burned” and pitted steel.  The use of an inhibitor will provide a better quality galvanized product as well as a more satisfactory and economical operation.

Inhibitors reduce the rate of attack of the acid on the steel by as much as 95%, and thereby reduce the amount of hydrogen evolution by the same amount.  Since the evolution of hydrogen contributes to the formation of a fog of dilute acid around the pickle tank, inhibitors also improve working conditions.  

Our PICKLEGARD range of  inhibitors are also available which will perform two functions

 (1) reduce the attack of the acid on the steel and

(2) form a blanket of foam on the pickle bath which aids in reducing the evolution of acid fog and steam around the pickle tank.

Regardless of whether sulfuric acid or hydrochloric acid pickling is used, poor rinsing will:

Introduce iron into the preflux solution. If iron is allowed to build up, the iron-contaminated film of preflux solution left on the work will carry iron into the galvanizing kettle, increasing dross formation. Iron carried into the kettle reacts with 24 times its own weight of zinc to produce dross.

Drag acid values into the preflux solution. An excessively acidic preflux solution will cause corrosion not only in the preflux tank itself but also after the work leaves the tank. The net result will be additional iron salts carried into the galvanizing kettle, forming costly dross. There is also the possibility of the corrosion on the surface leading to uncoated areas especially when dry galvanizing is practiced.

Inadequate rinsing after sulfuric acid pickling leads to sulfate contamination of the preflux solution. The sulfate contamination is carried into the top flux and causes increased viscosity, “gravel” formation and decreased flux activity.

GALVAGARD range of  galvanizing flux is used to dissolve surface oxides that form on the steel after the pickling operation. This flux can be put on the steel either by dipping the steel in an aqueous preflux solution of GALVAGARD , as in “dry” galvanizing, or by passing the work through a molten salt top flux which floats on thesurface of the molten zinc in “wet” galvanizing.  

Preflux  bath  treatment  will prevent formation of oxides in the time before actual galvanizing step,  allowing some delay between fluxing and dipping of work.

A top flux will also minimize oxide formation on the  molten zinc or zinc alloy and insulate the kettle surface.

When used in combination with top fluxes,  the dip in the preflux bath will cover the entire surface of the work, including small crevices, with a fresh layer of flux; this sometimes cannot be done thoroughly when all of the fluxing is done by passing the work through a molten top flux. A preflux will also provide incremental activation of the top flux with the fresh flux carried over. This means that the use of a preflux of GALVAGARD  does not increase the over-all flux consumption for top flux users, since some GALVAGARD from the preflux is carried over and becomes part of the top flux. This replaces all or part of the solid flux that would ordinarily be added to the top of the kettle. The advantage of adding flux to the top of the kettle by means of predipped coating on the work is that it continuously activates the top flux in small increments.

If not prefluxed , pickling operation and time Lage would of forming iron salts or oxides on the surface of the work. This increases dross formation when carried over into the kettle. Iron salts may also increase coating weights.

GALVAGARD fluxes  work is by removing the surface iron oxides (Fex Oy) from steel, giving a metallurgically clean surface for the iron/zinc reactions of galvanizing.

The fumes or smoke evolved from a conventional hot dip galvanizing operation include the following:

• Sublimation of flux – which is property essential for its working , as it breaks down to Hydrogen chloride gas which reacts and removes metallic oxide
• Aluminum Chloride – from reaction of aluminum in the molten zinc with ammonium chloride in the flux.
• Lubricants – from improperly cleaned work which contaminates the process solutions and ultimately gets carried into the galvanizing kettle.
• Steam – from incompletely dried preflux film or water formed in chemical reactions in the galvanizing kettle.
• Zinc Oxide – from the open surface of a dry galvanizing bath and moisture from flux or wet work.

 We have low fuming products, which can substantially reduce smoke, however it cannot completely finish the  smoke. Using high foam product blanket flux would reduce this substantially when used as blanket flux.

The term “dry” galvanizing refers to the most common fluxing practice in the industry. There is no top flux on the bath ,  instead, all of the fluxing activity comes from the preflux dip.

Successful production of high-quality work by the dry galvanizing method is dependent upon the surface preparation of the work. The cleaning and pickling must be good, and the preflux concentration should be high enough to compensate for the absence of top flux activity.

The type of GALVAGARD galvanizing flux to use as a preflux depends on the temperature to which the work is to be heated prior to dipping in the molten zinc, whether a top flux is to be used, and the level of cleaning or known difficulty in galvanizing a type of work.

In most operations where there is little or no preheating, the preheating is more of a drying step with temperatures below 300oF/121oC. At these lower temperatures FLUX 100 is recommended when dry galvanizing or using a non-foaming top flux. If.

If a lower fuming system is required, GALVGARD 500 maybe us used. These fluxes will withstand preheat temperatures up to about 350o F.  Because of lower activity, very careful cleaning and pickling is necessary

For preheating work to temperatures over 400o F GALVAGARD 400

The preflux concentration and ratio have direct impact on fuming for dry kettle galvanizing. The lowest practical concentration should be used, depending on the work to be processes, how well the cleaning and pickle operations work, and similar considerations.

The ratio of the preflux is also very important; the higher the ratio, the less fume evolved.  However, higher ratios also mean lower activity of the flux, so the balance between fume and activity needed is going to be determined by each shop

The GALVAGARD 400 are a good balance between higher activity and fume generation

GALVAGARD wetting agent helps in by reducing the amount of flux carried on the work.  The ratio of the preflux is also very important; the higher the ratio, the less fume evolved.  

One reason to consider in choosing a flux, however, is the more active the flux, the higher the ash generation.

The other direct effect of ratio is in processing of material. The lower ratios, especially below 1.00, dry faster and more completely, giving less spatter, so processing can be done more quickly. If preheating is available, and good cleaning, then lower activity fluxes can be used.

Considerable difficulty often arises from the use of flux solutions which are too low in concentration. Generally, the concentration should not be allowed to fall below 12o Baumé in order to prevent galvanizing problems resulting from inadequate oxide removal, rusting after prefluxing, or poor cleaning of the work before fluxing.

The concentration of GALVAGARD flux  which will prevent rusting between the preflux step and the kettle will depend upon the time delay between the two operations, the humidity of the air to which the prefluxed work is exposed during this period, and the pH of the preflux bath. The longer the holding time and the higher the humidity, the higher the specific gravity or concentration of the preflux solution should be. The pH must be controlled

A 13o to  20o Baumé solution (measured at room temperature) of GALVAGARD® flux is often used for the preflux.

The preflux can be prepared by adding dry GALVAGARD flux to the preflux tank 75 percent full of hot water.

Add GALVAGARD® flux at many points around the tank to prevent settling in one area. At the initial make-up of a new preflux solution, the flux should be dispersed uniformly and the solution well agitated. Mechanical or air agitation may be used. After initial mixing, the balance of the water is added and the preflux mixed to a uniform solution.

The temperature of the preflux is not important to the fluxing action; the bath should be kept hot for other reasons. For example, the use of a hot flux solution will improve wetting, accelerate drying of the prefluxed work, help to maintain kettle temperatures, and reduce spattering. The optimum temperature will be determined by the thickness of the metal and the speed at which the work passes from preflux to the zinc bath; i.e., the work should be just dry enough after fluxing to prevent spattering of molten metal when immersed in the molten zinc.

The work should remain in the preflux bath only long enough to heat the work to the same temperature as the preflux bath itself. A temperature of 130o-170o F (54o-77o C) is recommended if the work has a little time after fluxing to partially dry from its own heat.

Articles to be galvanized should not be introduced wet into the molten zinc because of hazardous spattering from the instantaneous evaporation of the water. One of the advantages of the use of a preflux is that the work, protected with a continuous coating of GALVAGARD® galvanizing flux can be dried and held for a period, if necessary, without oxidation. As a general rule, the prefluxed work should not be allowed to stand any longer than necessary prior to immersion. A heavier coating of GALVAGARD®, obtained by using higher strength preflux solutions, will give additional protection from rusting or oxidation; however, the work may dry somewhat more slowly.

Preheating of the work beyond that achieved in a hot preflux tank is desirable in order to avoid the need to add excessive amounts of heat to the zinc kettle. This means that kettle capacity may be increased if preheating is done. To take maximum advantage of preheat, it is important that the work pass as rapidly as possible into the zinc without cooling.

The quality of product, efficiency of production, and waste of zinc  can be directly influenced by control of flux parameters such as density (or concentration) and acidity or pH, as well as contaminants such as iron and sulfate. It is important to remember 1 kg of iron carryover would result in 26 kgs of dross formation.

*Baueme

Baume control is essential for quality and good production. Excess Baueme would result into thicker coatings , more fume and other problems. Lower baueme would result into improper cleaning and more rejects.

Baueme should always be checked in normal temperature , but cooling down the hot liquid.

*pH

In spite of careful rinsing, some acid from the pickle bath may carry over into the preflux solution. The build up of free acid in the preflux bath has an objectionable action on the steel, increases corrosion of the heating coils, and increases the amount of soluble iron in the preflux bath and thereby adds to the dross in the zinc kettle.

Acidity of the preflux bath should not be permitted to fall below 3.5 pH.  The pH can be adjusted upward by additions of GALVAGARD  900 . However sometimes GALVAGARD  910 or you can use Zinc Slab is also to be added to rebalance the composition, however both should not be used in excess and are generally suggested to be used if solution have become highly acidic.  Required corrections of solution density can still be made with GALVAGARD Flux being used.

The best practice is to rinse well after pickling so that the pH of the preflux does not require frequent attention.

Over time, the preflux solution will become contaminated with iron salts; some are soluble, giving a clear reddish- brown flux solution, but most iron salts common to a flux tank are not soluble, and show up as the “mud” found in almost all preflux tanks. This iron mud is usually a brown-black material which makes the preflux tank appear muddy and opaque.

The bath should be treated out whenever the soluble iron exceeds 0.3% (3000 ppm) for best operation.

If the sulfate or iron content regularly exceeds 0.5%, it generally indicates that rinsing after pickling is inadequate and steps should be taken to improve that phase of the operation.

Using GALVAGARD 920 you control iron in the flux .

To remove the iron, cool the solution, add the calculated amount of gALVAGARD 920 , and raise pH to 4.5-

5.5 with a GALVAGARD 900 The pH control is very important during iron removal. At a pH below

4.5 the iron may not completely precipitate and above 5.5  the whole flux would become useless.

1. Mix the solution for a 3-4 hour period to allow the precipitating reaction to reach completion.
2. Allow the precipitate to settle; use of GALVARARD 930 is frequently beneficial. Pump the clear liquid to a clean tank. When precipitate starts to be picked up in this operation, filter the remaining portion of the preflux bath.
3. Make hydrometer readings in degrees Baumé and adjust bath to preferred strength.

They are specialized formulation. (Top flux foaming agents are NOT the same as wetting agents ) Foaming agents create a long lasting foam, sustained activity, excellent fluidity and reduced smoking. The advantages include:

• Slower loss of flux by vaporization because a foaming blanket remains cooler. This means that less flux will be used and the kettle flux will remain active with lower viscosity for a longer period of time. A slower loss of flux  also means less fuming.
• Greater drying action because the work takes longer to pass through the flux blanket. This reduces chances of spattering.
• Less heat loss from the zinc because of the insulating effect of the foamed flux layer.

Glycerin has been used to supplement frothing action of the blanket. However, its action is short-lived and leads to a rapid increase in viscosity of the top flux.

The top flux blanket should be as deep and frothy as the operation will permit. While a 2 to 3 inch (5.1-7.6 cm) thick depth is effective, a greater depth is desirable. Use of a foaming agent in the flux is necessary to achieve the optimum top flux depths.

A deep, frothy flux acts as an insulator and keeps most of the flux well below kettle temperature. New flux can thus dissolve readily yet with minimum decomposition. Thin, non-frothy fluxes tend to approach kettle temperatures, and this results in high fuming and high flux consumption.

The GALVAGARD “low smoke”  flux,  the smoke emissions from such fluxes are extremely low.

Also called “Smokeless” fluxes must be used in conjunction with good metal preparation practice, because they provide a very low level of fluxing activity compared to traditional fluxes. Properly cleaned work is prefluxed in the “smokeless” preflux solution. The prefluxed work should be reasonably dry before galvanizing to avoid excessive zinc splatter. The purpose of drying is to prevent trapped steam from destroying the protective flux film or splashing zinc oxide on the work surface.

Work being galvanized with “smokeless” flux requires a slightly longer time for the preflux film to be released. This is due to a lower level of ammonium chloride gas evolution and surface agitation as compared to zinc ammonium chloride fluxing agents.

Work withdrawal should always be made through a clean, freshly skimmed zinc surface. The somewhat slower spent flux release places greater importance on the kettle operator’s skill in skimming residues away from the work as it is withdrawn from the kettle.

Even with the low fluxing activity of a “smokeless” flux, when work is carefully cleaned and pickled and the kettle flux is maintained properly, quality galvanizing can be performed.

These spots are result of oxidation of zinc , they can controlled and prevented by giving them quenching /dipping  in deoxidizing agents like GALVAGARD. They passivate the surface to prevent any further oxidation or corrosion of zinc surface.