Page 188 - CW E-Magazine (13-8-2024)

Special Report

immersion of iron or steel products in grinding of excess zinc icicles. This is using this process is very thin
molten hot zinc after the surface of followed by a final inspection(10, 11). (Figure-1).
the base metal has been properly and
adequately cleaned. This is also suited for Pros: Pros:
the galvanization of large bulky items. The thick coating formed is suitable Coating thickness can be monitored

It can be completed as a batch or a for outdoor as well as indoor appli- and controlled accurately and pro-
continuous process. cations. vides better aesthetics.
The crystallites are highly visible, It can be adopted for smaller com-

In batch galvanizing, often called and provide an aesthetic element ponents and can be cheaper com-
fabrication galvanizing, the iron or steel called a spangle. By varying the pared to a hot dip process.
product is formed into its required final cooling rate and the chemicals, the
size and shape, cleaned thoroughly and shining spangle can be calibrated to Cons:
then immersed for a specific duration be more heterogeneous. Thick coatings similar to that pro-
in the bath of molten zinc. Any shape duced by hot-dip galvanizing can’t
of ferrous metal can be galvanized in Cons: be produced using electro-galvani-
this batch processing method. Overall, During the process, a structure of zing.

this process involves three major steps: hard and brittle iron-zinc alloy is It is normally suitable only for

cleansing (caustic cleanser, pickling, formed in multiple layers between indoor applications.
rinsing) and flux coating; galvanizing; the inner surface of the zinc coating Additional painting is required for

and post-treatment such as quenching and the iron or steel. Under bending some electro-galvanized components
and grinding. loads, these brittle layers can flake used in an outdoor environment.
off.
The preparatory cleansing consists It is unsuitable for very small com- Sherardizing

of dipping the iron/steel in cleansing ponents, as the coating thickness Sheradizing is a thermal diffusion-
baths. The first bath is a caustic cleanser, cannot be accurately controlled. type of galvanizing, where a zinc coat-
which removes grease, oil and dirt. ing is formed on ferrous parts by tumb-
The second is a rinsing water bath and ling zinc powder with these parts in a
the third is a pickling bath contain- hot sealed drum, resulting in zinc alloy
ing diluted (10%) sulphuric acid or a formation on the surface. The drum is
hydrochloric acid. After the pickling heated uniformly, well below the mel-
process, which removes rust and mill ting temperature of the zinc metal. It is
scale, the product to be galvanized is a preferred method for coating small,
again rinsed in water and soaked in a complex-shaped metals.
flux solution to keep it ready for
galvanizing. Pros:
Creates a uniform coating of intri-
The galvanizing bath consists of a cate shape and complex configura-
minimum of 98% pure, molten zinc, tion.
maintained at around 455°C by conti- Coating thickness can be up to 300
nuous controlled heating. When the microns.
ferrous product reaches the bath tempe- Electrogalvanizing process
rature, the zinc reacts with the iron on Fig. 1: Electro-galvanization Process Cons:
the ferrous metal/steel surface to form Electrogalvanizing (plating) This process can’t be adopted for
a layer of coating that protects the sur- Electrogalvanizing, or plating, is an bulky items.
face. Depending on the thickness of the electrochemical process involving the It is a costly process, with a long
steel section, it may take 10-15 minutes immersion of ferrous or steel products cycle time.
for the steel to heat up and complete the in an eletrolyte of zinc metal, such as
reaction. a solution of zinc sulphate. Electrolytic Zinc spray galvanizing
action deposits a coating of pure zinc The zinc spraying process requires
The post-treatment steps can be on the surface of the product and works a thorough cleaning of the product sur-
quenching, if required, as well as the as a cathode. The coating produced face. Zinc in wire form or powder form

188 Chemical Weekly August 13, 2024

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