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FLUXING IN HOT DIP GALVANIZING

Nowadays fluxing is one of the most important steps of hot dip galvanizing. Galvanizing process can be

performed by using only hydrochloric acid and molten zinc, but every extra step added to the process

guaranties an increse on the efficiendy, a decrease in the number of rejected materials and most

importantly reduction in consumption of zinc.

A good fluxing process; -reduces the coating thickness,

-increases the quality of the coating, -accelerates the process,

-reduces ash and dross formation.

Fluxing is one of the most important factors that leads to the reduction of galvanizing process costs

ensured that the application is proper and the flux solution is under constant control.

FLUX TYPES USED TODAY

There are two types of flux that are generally used in hot dip galvanizing. Double salt ZnCl2.2NH4Cl and

triple salt ZnCl2.3NH4Cl fluxes. As double salt flux contains more ZnCl2 comparet to the triple salt flux,

less need for sensitive controlling and more efficiency in working of the flux is achieved by using double

salt flux.

Triple salt flux creates excessive fume when it enters the zinc bath due to its high ammonium amount

and especially if the component needs to wait for long times before entering the zinc bath, oxidation

over the steel surface may be seen. Due to the ease of use such as the ones stated above, double salt is

a more popular flux type.

THE ROLE OF FLUXING IN HOT DIP GALVANIZING

-Prevents oxidation of the steel through the drying process. A poorly fluxed component is oxidized when

left waiting after the fluxing process or during drying. Uncoated areas may form and the oxide layer over

the steel surface oxidizes the molten zinc causing an increase in the ash formation.

-As the steel component enters the bath, the flux reacts with the ZnO layer formed over the galvanizing

bath, melts this ayer and lets the steel react directly with the molten zinc. For a poorly fluxed

component, galvanizing reaction starts after the thin ZnO layer over the bath that spreads to the

component is dissolved. This causes the component to stay in the bath for longer times. The sections

with less ZnO are coated earlier causing a less homogeneus and usually thicker coating.

-Flux prevents transportation of impurities and foreign materials such as iron and oxides, chlorides,suphides and sulpates to the zinc bath and with this action it reduces the dross formation as well as

inhibiting the effects of these impurities on the iron-zinc reaction.

FLUX CONTROL

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Iron, zinc chloride and ammonium chloride in flux, flux temperature and pH must be controlled in order

to work with a flux at optimum conditions. All the parameters can be measured by basic titration

methods.

-Temperature: Ideal temperature is between 60-70oC. If an effective drying is used after fluxing, a flux at

40oC is also workable. Having a flux temperatue of 60oC lets the component to dry quickly and even

with a poor drier, ensures that the component is dry when it enters the bath, which reduces ash

formation, zinch splashes and explosions. The flux layer formed at lower temperatures is less

homogeneous and effective. At higher temperatures, a thick flux layer is formed and causes stains over

the substate surface and increase in ash formation.

-pH: Ideal pH is between 2 - 3,5. More iron is dissolved in lower pH values and the iron carried to the

zinc bath increases. pH values higher that 5,5 may cause the zinc chloride to precipitate, making the flux

useless. The pH value should be adjusted back to 2-3,5 interval if any value other than these is observed.

-Iron amount: The iron should be kept below 3g/L to prevent dross formation caused by the iron carried

to the zinc bath after fluxing. In most of the modern flux regeneration systems, this value is seen to kept

below 1g/L. Basicly the following can be applied to control the amount of iron:

* Preventing dissolution of iron by keeping the pH under control.

* Controlling the iron amount in the pickling bath by special chemical additions thus letting less iron

into the flux tank.

* Using an effective rinsing after the pickling stage.

* Using a flux regeneration unit that precipitates iron by adding hydrogenperoxide-ammonium or using

electrolysis.

It should be remembered that every 1 g of iron carried to the zinc bath combines with 25g of zinc to

produce dross.

-Zinc chloride and ammonium chloride amounts: Ideal concentration is 550g/L. In double salt flux, zinc

chloride (ZnCl2) amount is 300g/L, ammonium chloride (NH4Cl) amount is 250g/L. If the ratio is altered,

increase in ash formation, non-homogeneous coating thickness and increase in fuming over the zinc

bath may occur.

As ammonium chloride is cheaper than zinc chloride, the proportions of each chemical must be known

when purchasing the flux salt/solution.