Galvanising; What? Why? How?
What is Galvanising or Galvanisation?
Galvanisation is a process that involves coating ferrous materials, in particular iron and steel with a thin layer of zinc in order to increase corrosion resistance. This layer of zinc acts as a protective layer shielding the substrate from harsh environments, dramatically increasing the lifespan of the component.
Most commonly used in the construction industry, galvanising steel is a cost-effective alternative compared to other methods, however can become expensive for some large-scale applications.
Advantages and Disadvantages of Galvanisation
Advantages
- Improves corrosion resistance
- Improves durability
- Provides a tough coating resistant to mechanical damage
- Greatly improves lifespan of iron/steel components
- Low cost
Disadvantages
- Although resistant to red rust, the zinc coating is not resistant to white rust when exposed to high moisture environments
Galvanisation Process
There are many two common types of galvanising processes:
Electrolytic galvanising is a form a electroplating. In the electrolytic galvanising process the workpiece is placed in a zinc solution, a current is applied which ionises the zinc solution, which in turn deposits a layer of pure zinc onto the base metal.
The most common galvanising process is hot-dip galvanising. The process stages for hot-dip galvanising are:
- Mechanical Cleaning
The first stage clean removes hard contaminants from the surface of the component. Impurities such as heavy rust, paint or welding slag are removed by a mechanical cleaning process such as blasting or grinding. - Chemical Cleaning
Once the component is free of hard contaminates chemical cleaning process to remove dirt, grease and oxides will be used. Pickling can be used to achieved this! - Flux Application
Between cleaning and immersion stages, due to the exposure of air, a thin oxide layer can form on the surface of the component. This oxide layer can interfere with the quality of the zinc coating and therefore must be remove! This oxide layer is removed by exposing the component to a chemical flux that dilutes the oxides on the surface. - Immersion
After the chemical flux is fully dried, the component is dipped into a bath of molten zinc. This immersion creates layers of zinc-iron alloys until a pure zinc coating is on the surface. Once the coating is complete, the component is removed from the bath and placed in a quenching tank to prevent exposure to air as it cools.
