Hot dip galvanising for structural steelworks

Part 2

IN previous editions we started on the topic on galvanising. We covered areas such as metal coating and prevention methods, why steel must be coated, advantages of galvanising and disadvantages or drawbacks with galvanising. We aim to continue with this topic as we cover some few basic aspects about Hot Dip Galvanising (HDG)

Designing for Hot Dip

Galvanising

There are certain rules that should be followed when designing components for galvanising which include:

i.Good fabrication practice to ensure maximum protection against corrosion

ii.Designing to eliminate undrained areas which could collect water in service

iii.Hollow structures incorporating closed sections must be vented to the atmosphere by drilling holes. At galvanising temperatures, trapped moisture in closed areas is rapidly converted to superheated steam which may generate explosive forces unless vented

Joining Hot Dip Galvanised steel

Galvanised steel can be joined by welding or bolting including high strength friction grip bolts. Weld should be of high quality and fume extraction equipment should be used when welding in confined areas. Damage to the galvanised coating in the weld should be renovated with zinc metal spraying or an approved zinc rich paint applied 25 percent thicker than the specified galvanised coating. Bolting is also used but the fasteners should also be hot dip galvanised

Painting Hot Dip Galvanised

steel

In most cases , a general or heavy duty coating gives a finish that can be used as it is. In some cases however, the galvanised steel may be painted for aesthetic reasons or to extend their life in aggressive environments.

Painting may extend the sum of the life of the two coatings together by 50 percent. A galvanised surface had good sticking properties , hence a correctly applied paint system will last longer on galvanised steel than it would on black steel. To obtain satisfactory adhesion, there is need for correct choice of paints and pre-treatment. Paints give good adhesion when directly applied to freshly galvanised steel

Distortions of galvanised

structural products

Distortion is the unwanted warpage that occasionally happens during hot dip galvanising and it’s usually evident after. It doesn’t always happen but when it does occur it’s a serious concern to the fabricator and galvaniser

Hot Dip Galvanising occurs at a temperature of around 450°C which is in the stress relieving temperature range for treating steel. Therefore the galvanising process releases any inherent stresses in the steel. This may result in some dimensional change and the resulting change in shape is seen as a distortion. Internal stresses in steel can be as a result of

◆ Residual stresses induced at the mill during rolling of structural sections or plate

◆ Residual stresses created by bending or welding

◆ Lack of symmetry in simple sections such as channels or built up sections

◆ Combination of thick and thin material in the same assembly

◆ Assemblies so large that they require double dipping to be coated over their entire surface. This difference in dipping times can create problem

These internal stresses are usually in equilibrium with each other and it’s usually difficult to predict precisely when an item will distort and when It won’t.

When steel structures are being fabricated , it’s impossible to avoid the introduction of some stress especially during welding. Internal stresses due to welding play the greatest part in creating distortion

Distortion can also occur if steels of significantly different thicknesses are joined in a fabrication. Symmetrical sections have less inherent tendency to distort than asymetrical products.

Other things being equal, the lighter the Guage of steel the greater the risk of distortion

This week’s question Outline the methods of preventing distortion

◆ 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: eng.chatyk@gmail.com Facebook: AtroServe Engineering Zimbabwe

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