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 for Zinc Coating Thickness
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
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
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
Source: Galvanizers Association of Australia
For more information on this source please visit Galvanizers Association of Australia