Engineering and Construction Matters
With Innocent Chatikobo
Hot Dip Galvanising
Hot dip galvanising is the coating of a metal surface by an alloying reaction between the steel and molten Zinc in the galvanising bath.
In this event, all parts of a fabrication including edges, corners and recesses are coated fully.
Metal coating and prevention methods
Metal should be coated and prevented against corrosion. Corrosion is eating away of metal surfaces by rust. Rusting happens through a reaction between water and steel in the presence of oxygen. Coating eliminates contact between steel and water and thus prevents rusting. Some of the methods used to coat steel are the following:
i.Painting – covering steel with a layer of paint
ii.Oiling/greasing – covering steel with oil or grease
iii.Pickling – dipping in sulphuric acid
iv.Lamination – covering with a layer of plastic. This is normally done to steel sheets
v.Electroplating – coating steel by a layer of another usually non reactive metal e.g chrome
vi.Sacrificial protection — using a more reactive metal which reacts in preference to steel
Why steel must be coated
Steel is the most abundantly available and user friendly material. It is used for a wide variety of applications due to its physical and chemical properties. Building with steel has an advantage of the speed of construction and this has to be exploited fully through protecting the steel against corrosion. Steel when preserved can last for generations.
Galvanising
As highlighted earlier on, galvanising protects against corrosion. It’s used to protect structural steelworks for a wide range of projects especially those which have much contact with water.
Galvanising is a complete system applied in factory conditions and it can provide full protection to the steelwork during transit and erection.
The metal surface has to be prepared for the galvanising to happen effectively. Firstly the surface is cleaned of all dust, oils and dirt on the surface including rust.
The final preparation for galvanising is acid pickling where the metal components are dipped in acid for a specified period of time. Acid pickling ensures a clean surface free from soluble salts even in areas which are inaccessible to the wire brush or blasting nozzle.
The metal pieces are then dipped in a galvanising bath containing molten Zinc. The coating is formed by an alloying reaction between the steel and the molten Zinc in the bath.
All parts of a fabrication including edges, corners and recesses are coated fully. Where surface contaminants such as paint or grease are incompletely removed, molten Zinc will not “wet” the steel at these areas and thus the alloying reaction cannot take place and this leaves bare patches.
This will need immediate correction by re-processing the work.
The galvanised coating is metallurgically bonded to the steel. In cases where the coating layer is damaged by severe abrasion, drilling, cutting or welding, the zinc surrounding the damaged area gives sacrificial protection preventing the spread of rust.
The life and effectiveness of galvanising as a barrier depends directly on the thickness of the coating and on the environment. Each metal component depending with application has specific standard minimum coating thickness for example for structural steel sections, according to SABS 763, their minimum coating thickness required is 85 micrometres. Metal coatings if properly done may provide a maintenance free life of more than twenty years.
There are two main types of galvanising
Hot dip galvanising: dipping of metal in a bath with molten Zinc
Electroplating: this is electrolytic coating of steel components by use of anodic and cathodic reactions
Advantages of galvanising
Provides a highly durable surface coating
Coating is resistant to corrosion and abrasion
A galvanised coating does not require on going maintenance as is the case with most paint coatings
May offer sacrificial protection even when coating has been damaged
·It can be used to improve aesthetics of a structure
Can be used where there are adverse conditions such as high moisture and chemicals
Hot dip galvanising is a relatively “fool proof” process and the requirement for testing and inspection is not so great as for the other coating processes
Drawbacks/disadvantages with galvanising
Unlike painting it cannot be undertaken on site
Damaged areas cannot just be “touched up” as one would do with a damaged paint coating and special site repair methods are required which may cause delays in erection of steelworks
Steelwork has to be specially designed and fabricated to accommodate the galvanising process. There is need to modify fabrication methods since conventional steel designs for painting cannot be used
Is not always readily available in most places due to the investment needed hence people opt for other coating methods
Galvanising may cause some delays since an extra subcontractor is involved
In next week’s issue we will continue with the topic on galvanising where we shall touch on areas such as designing for hot dip galvanising; joining hot dip galvanised steel; understanding distortions; etc
Innocent Chatikobo is an Engineer by profession with AtroServe Engineering Zimbabwe. He has extensive knowledge and experience in Structural Engineering and Construction. For your comments, views, questions and feedback he can be contacted on the following platforms. Cell: +263 777 950 224; +263 712 376 037; +263 782 502 732
Email: [email protected] <mailto:[email protected]>
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