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Decorative Finish | Sweep Blasting | Thicker Galvanized Coatings | Galvanized Steel in contact with Timber | Steel Detailing | Safe Buildings | Imported Steel Products | Advantages of After Fabrication Galvanizing | Standards Update | Materials Handling & Conveyors | Lintels and Shelf Angles | Zinc for Human Health | Atmospheric Corrosion Resistance | Paint over Galvanizing | Abrasion Resistance | Use of Galvanized Reinforcement | Repair of Field Welds | Wet Storage Stain | Distortion | Thickness on Flame-Cut Edges | Coastal Construction
Thicker Galvanized
Coatings for Specialised Applications
Advisory Note
GEN/2/1
August 2001
In certain specialized applications, thicker galvanized coatings are desirable
where the service environment is most aggressive or where access for future maintenance
is difficult i.e. roof support steelwork, internal ladders in water storage reservoirs
or in remote or isolated locations.
Factors, which determine zinc coating thickness, are:
- steel composition, i.e. surface reactivity
- surface profile of steel being galvanized
- time of immersion in the zinc bath during the dipping process of reactive steels
For most of the structural steel produced locally, increasing the surface
profile by abrasive blast cleaning and increasing the time of immersion during
the galvanizing dipping process can result in a coating thickness increase
of up to 50%.
Tests to develop operating procedures concluded that for optimum results the
steel should be:
- abrasive blast cleaned to Class SA.3, approximate profile height 60 um, using chilled cast iron grit or ilmenite sand
- flash pickled after abrasive blast cleaning
- allowed to remain in the galvanizing bath after 'boil off' ie the time taken to reach the galvanizing bath operating temperature. The length of time to be determined after testing has been undertaken in conjunction with a galvanized company
Cautionary Note
Certain reactive steels containing silicon, or a combination of both silicon
and phosphorous can develop coatings, which in extreme cases can be dull, excessively
thick, brittle and easily damaged by mechanical handling during transportation,
and / or on erection.
As a guide to the suitability of silicon and phosphorous containing steels
for galvanizing, the following criteria should be applied
% Si <0.04%
and % Si + (2.5 x % p) < 0.09
References
1. Sulplide Corporation Pty Ltd, TSR 11/83
Development of Thicker Galvanized Coatings in Specialised Applications
2. St Jo Zinc Company, 1978
Dependence of Zinc Pickup on Surface Preparation
3. Sulplide Corporation Pty Ltd, 1969
Abrasive Cleaning as a Method of Preparing Steel for Galvanizing
4. Galvanizers Association of Australia, 1999
After-Fabrication Hot Dip Galvanizing – A Practical Reference
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