The presence of zinc corrosion products known as wet storage stain formed if the coating becomes damp and is unable to fully dry out is also not grounds for rejection provided the coating meets the local average coating thickness requirements see Table 3 and 4.
Coating thickness requirements are given in terms of Local average coating thickness requirements relating to single reference areas and Average coating thickness relating to the whole article. In effect this means no areas of an article should have an average coating thickness below the Local average coating thickness requirement while the average thickness over the whole of the article should meet the Average coating thickness.
Details of coating thicknesses requirements are given in Tables 3 and 4. The standard permits repair on uncoated areas up to 10 cm 2 in size with the total areas for repair not exceeding 0. If larger areas are present, the article should be regalvanized unless a repair is agreed within the supply chain. The standard states that at present no suitable International adhesion test exist for hot dip galvanizing and so where testing is required this should be agreed within the supply chain and reaffirms that such testing is not normally necessary as good adhesion, or more correctly, cohesion, is a characteristics of the coating.
The coating should withstand, without peeling or flaking, handling consistent with the nature and thickness of the coating and normal use of the article although it should be appreciated that thicker coatings may require more careful handling and that bending after galvanizing is not considered normal handling.
If the control ample meets the surface finish and coating thickness requirements of I. EN ISO : then a lot from which the control sample is taken is accepted in total. If a control sample fails to meet coating thickness requirements, then double the original control sample size is taken for testing and if this meets standard requirements then the lot as a whole is accepted. If this second control sample fails to meet standard requirement, then the lot is rejected. BS was the old British galvanizing standard for hot dip galvanizing.
When hot dip galvanizing is specified, the surface of the steel is completely covered with a uniform coating whose thickness is determined principally by the thickness of the steel being galvanized see Graph 1 below. This is an important advantage of the galvanizing process; a standard coating thickness is applied almost automatically.
The actual thickness of galvanized coating achieved varies with steel section size, surface profile and surface composition. Actual coating weights are often much more than the minimum specified in the standard. As coating life expectancy figures quoted are based on the minimum coating thickness they are therefore usually very conservative. There are many references to specifying hot dip galvanizing throughout the National Building Specification NBS system, the main location being G10 — Structural steel framing.
General guidance on corrosion protection is also given. In the unlikely event of being unable to identify the correct clause for a particular galvanizing application, please contact Galvanizers Association for specific advice. The galvanizer acts as a specialist subcontractor to a steel fabricator and, as such, his contractual relationship is with the fabricator, not with the ultimate user.
It should however, be emphasised that for most applications, thicker coatings are rarely necessary. For structural steelwork, it is advisable to ascertain whether thicker coatings could be achieved through their greater section thickness and without grit blasting. Achieving thicker coatings through specification of a reactive steel is normally only appropriate for specific applications. Specification of thicker coatings must only be made following consultation with the galvanizer concerning viability and the means by which they will be achieved.
HDG Datasheet 4a.
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