By Terry Smith, Technical Marketing
Director, Hot Dip Galvanizers
Association Southern Africa
It should be reasonably common knowledge that to achieve a good quality hot dip galvanized coating thickness in terms of SANS121 of a minimum mean of about 85 microns on structural steel, the silicon content of the steel should be limited to the ideal requirements of the well known Sandelin curve (see figure 1) which basically tells us:
The two chemical constituents in steel that affect coating thickness and aesthetic appearance are silicon and phosphorous. When the silicon falls out of the two desirable ranges on the Sandelin Curve, an extremely thick and easily damaged unsightly coat of zinc is the result. However, when the phosphorous content falls into the out-of-specification range, it affects the successful metallurgical bonding of the coating (i.e. the galvanizing will delaminate in large localised areas).
Ideal steels require a silicon content to fall between 0.02 to 0.04% (the so-called ‘aluminium killed steel’) and 0.15 to 0.25% (‘silicon killed steel’).The maximum phosphorous content should be less than 0.02%. Steel that falls out of these ranges are called ‘reactive’ steel when it comes to galvanizing.
SANS 121 (ISO 1461) includes an Annex A, which addresses the essential information to be provided by the purchaser to the hot dip galvanizer. Even though the Annex is situated at the back of the Standard, it does not reduce its importance when specifying hot dip galvanizing to ensure the quality product we have come to expect from South African hot dip galvanizers. This includes information about the chemical composition of the steel.
Most general galvanizers will accept steel for hot dip galvanizing as long as the component has been reasonably designed and fabricated taking into account some simple design rules. However they cannot be aware of the potential reactivity of the steel in its black form with respect to molten zinc before hot dip galvanizing unless they have sight of material certificates which spell out the chemical composition of the steel.
For heavy duty coatings on heavy steel sections, usually required for underground mining conditions, a bit of reactivity is a good thing so that a hot dip galvanized coating thickness of in excess of 150 microns can be achieved. The galvanizer takes responsibility for hot dip galvanizing the steel but the choice of steel grade and chemical composition of the steel is the responsibility of the specifier, his fabricator and the steel supplier, particularly when the latter has been informed that the steel is to be hot dip galvanized!
As a general rule, the fabricator should take responsibility for checking the chemical composition related to its suitability for hot dip galvanizing (as well as its conformance with design requirements) when purchasing the steel. The South African steel rolling mills will take care to supply, as long as it is ordered, steel that is suitable for hot dip galvanizing.
However, when it comes to the so-called commercial quality steels, no chemical composition certificates are available. If the galvanizer is aware, that for a particular batch of steel, the chemical composition does fall into the ‘problem steels’ range he then can act accordingly. However, the methods he may use to limit coating build-up are generally insignificant in comparison to the coating buildup effect from extremely reactive steels. Should hot dip galvanizing of the steel be unsuccessful he may be able to offer his client a zinc thermal sprayed metal protective coating as an alternative.
It is also common knowledge that a substantial quantity of steel has been imported into South Africa. We are aware that sometimes the material certificates are not supplied with foreign steel and if they are supplied they are not necessarily accurate.
It has come to our notice of an incident where structural steel was hot dip galvanized where the silicon was in the ideal range but the phosphorous was as high as 0.042%! This resulted in coating cracking and significant subsequent delamination of the coating.
You have been warned to pay attention to the silicon and phosphorus content of steel you intend to galvanize.