Do You Know Galvanizing?

The galvanizing machine supplier will share this article with you.

Galvanizing is one of the most widely used to methods in metal corrosion protection. It involves applying a thin coating of zinc on a thicker base metal to help protect it from the surrounding environment. Next time you are in the car, take a look at the street signs and lamp posts you pass by. Many of them are dumb and silver color on them. That "silver" is actually the outer coating of zinc.

Why galvanize?

Very simple, galvanize the metal to make it corrosion resistant. Without a protective zinc layer, the metal is still exposed to the element and may oxidize and corrode more quickly. Galvanized steel is a cost-effective alternative material, such as austenitic stainless steel or aluminum in order to prevent corrosion.

How does it work?

There are many ways that galvanizing can protect metals. First, it creates a protective coating to protect the metal from the surrounding environment. The zinc layer prevents moisture and other elements in the air from corroding the steel below. If the zinc layer is scratched deep enough, the metal will be exposed and susceptible to corrosion.

Galvanizing can also protect metals through a process called "galvanic corrosion". When two metals with different electrochemical compositions come into contact with each other in the presence of an electrolyte (such as salt water), galvanic corrosion occurs. According to the atomic structure of these two metals, one metal is the anode and the other is the cathode. The corrosion rate of the anode is faster than itself, and the corrosion rate of the cathode is slower than itself. The reason why zinc is used for galvanizing is that it has an affinity to become an anode when it comes into contact with many different types of metals. Since the zinc coating in contact with the base metal is usually the anode, it slows the corrosion of the base metal, or the cathode.

Hot dip galvanizing process steps

Main steps within the galvanizing process include :

• cleaning cycle – decreasing and chemical clean

• fluxing – helps to wet the surface of the steel

• galvanizing – immersion of steel into bath of molten zinc

• post treatment (optional)

Cleaning cycle

The galvanizing reaction will only occur on a chemically clean surface. In common with most zinc coating processes, the secret to achieving a good quality coating lies in the preparation of the surface.
It is essential that this is free of grease, dirt and scale before galvanizing. These types of contamination are removed by a variety of processes and common practice is to degrease first using an alkaline or acidic solution into which the component is dipped. The article is then rinsed in cold water to avoid contaminating the rest of the process.
The article is then dipped in hydrochloric acid at ambient temperature to remove rust and mill scale. Welding slag, paint and heavy grease will not be removed by these cleaning steps and should be removed by the fabricator before the work is sent to the galvanizer. After further rinsing, the components will then commonly undergo a fluxing procedure.

What is the fluxing procedure in the galvanizing process?

This is normally applied by dipping in a flux solution – usually about 30% zinc ammonium chloride at around 65-80°C. Alternatively, some galvanizing plants may operate using a flux blanket on top of the galvanizing bath. The fluxing operation removes the last traces of oxide from the surface and allows the molten zinc to wet the steel.

The galvanizing process

When the clean iron or steel component is dipped into the molten zinc (which is commonly at around 450°C) a series of zinc-iron alloy layers are formed by a metallurgical reaction between the iron and zinc. The rate of reaction between the steel and the zinc is normally parabolic with time and so the initial rate of reaction is very rapid and considerable agitation can be seen in the zinc bath.
The main thickness of coating is formed during this period. Subsequently, the reaction slows down and the coating thickness is not increased significantly even if the article is in the bath for a longer period of time.
How long does the galvanizing process take?
A typical time of immersion is about four or five minutes but it can be longer for heavy articles that have high thermal inertia or where the zinc is required to penetrate internal spaces.
Upon withdrawal from the galvanizing bath, a layer of molten zinc will be taken out on top of the alloy layer. Often this cools to exhibit the bright shiny appearance associated with galvanized products.

Post treatment

Post galvanizing process treatment can include quenching into water or air cooling. Conditions in the galvanizing plant such as temperature, humidity and air quality do not affect the quality of the galvanized coating.
By contrast, these are critically important for good quality painting. No post treatment of galvanized articles is necessary and a paint or a powder coating may be applied for enhanced aesthetics or for additional protection where the environment is extremely aggressive. Chemical conversion coatings and other barrier systems may be applied to minimise the occurrence of wet storage stain.

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