The basic specification for hot dip galvanized coatings on iron and steel articles is defined by a single standard, EN ISO 1461 ‘Hot dip galvanized coatings on iron and steel articles – specifications and test methods’. However, there are some exceptions to this standard (see thicker coatings below).
What is the thickness of the hot dip galvanizing coating?
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.
Guidance on the design and performance of hot dip galvanizing is contained in EN ISO 14713 Part 1 and 2.
UK National Building Specification (NBS)
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 is important, therefore, that the user’s requirements for galvanizing are made clear to the fabricator and that all communications concerning galvanizing are channelled through the fabricator.
Thicker coatings than those set out in EN ISO 1461 can give additional protection for use in particularly aggressive environments and can be specified in conjunction with EN ISO 1461. It should however, be emphasised that for most applications, thicker coatings are rarely necessary.
Grit blasting prior to galvanizing is usually the most appropriate method and a requirement for a nominal coating thickness of 1000 g/m² (140 µm) has been successfully specified for steel of 6 mm section thickness. 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.