The chemical analysis of steel determines the extent to which the Fe / Zn alloy layers in a hot dip galvanized coating grow during immersion in the molten zinc.

It is the silicon content that determines the degree of alloy growth with all other elements playing an insignificant role. At levels up to about 0.04%, silicon has a negligible influence on the degree of coating growth which is parabolic with immersion time. At 0.08% Si, a level is reached where the Fe / Zn alloys grow profusely at a linear time rate. This can result in a dull grey coating with brittle tendencies and a rough surface finish.

Relationship between the silicon content of the steel and thickness of zinc coating for a similar dipping time at three different bath temperatures.

True or False

There are four elements in steel that influence the metallurgical reaction between molten zinc and steel i.e. silicon, phosphorus and to a lesser degree carbon and manganese.

The influence of silicon is well documented and clearly depicted on the well known Sandelin Curve (see above) which indeed confirms a high degree of Fe / Zn alloy growth at a peak level of 0.08% followed by a decline from about 0.125% Si and a substantial increase once more above 0.25%. The influence of phosphorus is somewhat different. Up to 0.02% P; the influence on coating properties is negligible. At levels above 0.03%, however, the diffusion rate increases rapidly regardless of silicon content to the extent that, when a level of 0.05% is reached, the overall coating thickness can be as much as 500µm while adhesion to the steel substrate is virtually non-existent.

Fortunately, undesirably high levels of phosphorus are not encountered in structural steels which conform strictly to the relevant specifications but this problem is encountered with some cheaper so-called commercial grade steels which may be structurally acceptable but not necessarily suitable for hot dip galvanizing.

An interesting phenomena which is not always understood is the influence of a combination of relatively low silicon and phosphorus levels at which individually these elements would have little impact. This phenomena is referred to as the silicon equivalent where 2.5P + Si = Si equivalent, e.g. (0.02P x 2.5) + 0.03Si = 0.08 silicon equivalent.

In this example, the two elements individually have only a modest influence on alloy growth whereas when both are present in steel at these levels, the effect is the same as for silicon alone at 0.08% which is the first peak in the Sandelin curve.

It must be said that thick hot dip galvanized coatings provide a greater degree of protection from corrosion provided that the adhesion properties are satisfactory which is usually the case. The downside is the dull grey and frequently rough surface finish which aesthetically is undesirable in applications such as architectural features.

Heavy uneven hot dip galvanized layer on the pipe section.

Uniform hot dip galvanized layer on the clamp section. Note the defined crystal structures of the intermediate iron/zinc alloy layers.

Smooth appearance of the hot dip galvanized layer on the clamp section.

A close up showing the difference in surface appearance between the reactive and nonreactive steel after hot dip galvanizing.

Pimpled and rough appearance of the hot dip galvanized pipe section.

The hot dip galvanized clamping pipe.

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