The galvanizer can play a major role in preventing the formation of undesirably thick hot dip galvanized coatings.

True or False?

Thick hot dip galvanized coatings are desirable in corrosive environments, provided that coating adhesion properties are to the required standard. A zinc coating can be described as a wasting protector in that its thickness is reduced over a period of time while the rate at which the coating thickness is reduced, is determined by the degree of corrosion attack pertaining in specific environments.

Coating growth during the hot dip galvanizing process is largely influenced by the chemical reaction which occurs when uncontaminated steel surfaces are exposed to molten zinc. The process results in the formation of a series of iron / zinc alloy layers which are normally overcoated by relatively pure zinc. The degree to which these alloys develop, will determine the overall thickness of the coating.

It is important to be aware that the iron / zinc alloys present in the coating, provide barrier protection from corrosion up to 30% greater than that available from pure zinc in most environments while they also provide, but to a lesser degree than pure zinc, ongoing cathodic protection. This is in contrast to paints containing zinc where the cathodic protective properties of the zinc content diminish over a period of time.

The ultimate coating thickness achieved by the hot dip galvanizer is determined mainly by the steel composition, with silicon and phosphorus playing the major roles. Other factors which have an influence are steel thickness and surface roughness.

In the case of moderately reactive steels, coating growth is parabolic with the time of immersion while for steels containing levels of silicon and or phosphorus which render them to be reactive, coating growth is linear with time. This can result in excessively thick and brittle coatings which are unacceptable.

The galvanizer can take certain measures to minimise excessive coating growth but this is only possible if he is in possession of the steel analysis, which is invariably not made available. In any case, the precautions that a galvanizer can take are only moderately effective, particularly in cases where, of necessity, the immersion cycle in the zinc is extended, due for example to the configuration of the structure, and the inadequacy of fill and venting holes in a tubular structure that is hot dip galvanized.

The influence of silicon and or phosphorus content on hot dip galvanized coatings of steels is an interesting phenomenon.

Silicon:

This is added to the steel melt in small quantities as a deoxidising agent which is essential to avoid gas inclusions that may be present, due to the continuous casting process adopted by modern steel producers. Such steels are described at “silicon killed”. An alternative deoxidising agent is aluminium. When aluminium additions are used, the steel is “aluminium killed”. While the aluminium content of steel has no influence on the rate of Fe/Zn alloy coating growth during hot dip galvanizing, this cannot be said of silicon.

An interesting feature is that the actual silicon content has a variable influence on coating growth when steel is in contact with molten zinc. To illustrate, at silicon levels below 0.03%, the effect is minimal. From 0.03% upwards, the reaction increases progressively, peaking at about 0.08%, after when it declines. At levels of about 0.1% to 0.3% silicon coating which are well in excess of specified minimum thickness are produced which normally provide good adhesion properties. Once a silicon content of 0.3% is exceeded, alloy layer formation is accelerated during hot dip galvanizing, thus excessively thick and brittle coatings are once more produced. This effect is referred to as the Sandelin Curve (See fig 33 – SPG). Provided that the immersion cycle in molten zinc is not excessive, steels containing between 0.1% and 0.3% silicon will provide coatings which are thicker but which also possess acceptable adhesion properties. These coatings frequently display a surface appearance which can vary from a uniform silver colour to dull grey or even patches of both. Following ageing of the coating the patches of silver and dull grey are likely to change to dull grey and black. (See pic in Journal No 17). Since such hot dip galvanized coatings are thicker than the minimum conventionally specified, corrosion resistance and hence, maintenance free life is extended.

For aesthetic reasons an attractive surface finish is essential, the less reactive steels (aluminium killed) are recommended. (See journal no. 17). When the coating on steels containing reactive levels of silicon is damaged, the Delta alloy layer invariably remains intact i.e. about 25% of the original coating is present at damaged surfaces. This is of benefit when handling damage is repaired on site.

Phosphorus:

The influence on hot dip galvanized coatings of phosphorus is somewhat different. At levels up to 0.02%, this element has little effect on the final structure and appearance of the coating. Above 0.03%P, the reaction between molten zinc and steel becomes progressively more severe to the extent that above 0.04%P it is impossible to achieve an acceptable coating at normal immersion times. The coating that is produced when steel has an inordinately high level of phosphorus displays a dull grey colour often with a surface roughness commonly referred to as the “Tree Bark effect” (See SC 26). This is because phosphorus interferes with the stable formation of the Delta alloy layer thus liquid zinc continues to react unabated with the underlying steel throughout the entire immersion cycle.

Unlike the brittle coatings associated with undesirable silicon levels, when a high phosphorus content is the cause, the entire coating, including the alloy layers, are removed on damaged surfaces. (See SC 15 in Practical Guidelines)

When specifying or ordering steel which requires to be hot dip galvanized it is important to avoid the reactive levels of both silicon and phosphorus. The silicon content of most material currently supplied is normally in the non-reactive or moderately reactive range. As far as phosphorus is concerned there is a distinct trend towards the supply of material containing inordinately high levels of this element, to the extent that 0.03% up to as high as 0.07% phosphorus content is frequently encountered. The problem would seem to be mainly confined to flat products produced from aluminium killed steel.

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