Most steels are suitable for galvanizing and produce a typical shiny silver coating.  As galvanizing is a metallurgical coating, the reactivity of the base steel will affect the coating structure and appearance in the following manner.

Structure

Normal steels produce a coating of zinc-iron alloy layers on the steel, covered by a pure zinc layer.

High reactive steels produce a coating which is all or nearly all zinc-iron alloy and is typically 2 to 10 times thicker than normal galvanizing.  A premium may have to be charged for galvanizing reactive steels.

Adherence

The coating is more brittle when compared to the "typical" galvanized coating.  As the thickness of the coating increases, a reduction of adherence is experienced.  The galvanizer cannot be held responsible to meet the normal specifications and adherence tests.

Visual

Visually, the zinc-iron alloy coating may have a splotchy matte grey appearance due to the absence of the free zinc layer.  It is the top free zinc layer which impacts the typical bright silver galvanized coating.

On exposure, the areas of fully alloyed coating may darken significantly relative to the areas with the pure zinc top coat leaving a possible aesthetic problem due to the dark and light splotchy coating.

On further exposure, the alloy coatings may prematurely develop a reddish brown discoloration which may also create an aesthetic concern.  This premature staining is caused by the corrosion of the iron in the galvanized coating and is not a failure of the coating.

These visual effects are beyond the control and responsibility of the galvanizer.  If the designer is concerned with aesthetics, then a less reactive steel should be specified.

Corrosion Resistance

In general, galvanized coatings are specified more for their corrosion resistance than for their appearance.  The corrosion protection and service life is not affected by surface appearance, only coating thickness.  Therefore, a coating several times thicker will last several times longer.  However, the thicker coatings are also more likely to get damaged during regular handling.  This will result in minor repairs.

Steel Selection

Steels containing carbon > 0.25%, Phosphorus > 0.05%, Manganese > 1.35%, and particularly Silicon > 0.05% are reactive steels which may result in coatings with the above properties.

The two most important elements in reactive steels are Silicon and phosphorous.  A numerical value called Silicon Equivalent (S.E.) is used to measure the reactivity of steel.  Silicon Equivalent is calculated as follows:

The galvanizer should always be advised of the grade of steel selected in order that he might determine whether or not special galvanizing techniques will be required.  A sample of the steel can be tested to determine it's suitability for galvanizing if desired.

The galvanizer, has little control over the affects of highly reactive steels.  It is therefore the responsibility of the designer, fabricator or end user to weight the advantages and disadvantages of using reactive steels and determine whether the steel should be galvanized, top coated, or substituted with a less reactive steel.