Metallurgy of galvanizing

When the cleaned and fluxed steel surface contacts the molten zinc of the galvanizing bath the protective flux layer is removed leaving a clean steel surface which is immediately wetted by the zinc.

 This results in reaction between zinc and steel with the formation of zinc-iron alloy layers.

The photomicrograph below shows a section of typical galvanized coating which consists of a progression of zinc-iron alloy layers bonded metallurgically to the base steel, with the relatively pure outer zinc layer. Depending on the article being galvanized, the structure of the coating can vary and create different appearances. See the section on Appearance for more details.

The structure of the galvanized coating and the relative thickness of its zinc-iron alloy layers have little or no effect on the protective life of the coating. Protective life depends on total coating thickness.

Galvanized coatings differ from paint coatings in that sharp edges are always fully coated, due to the way the zinc and steel react in the galvanizing bath.

The toughness of a galvanized coating makes it particularly suitable in situations where abrasion could be a problem either in assembly or in use.

Abrasion resistance of galvanized coatings

The photomicrograph (Figure 5) shows that the zinc-iron alloy layers are harder than the base steel, resulting in galvanizing’s outstanding resistance to abrasion and mechanical damage. Abrasive or heavy loading conditions in service may remove the relatively soft eta layer of zinc from a galvanized surface, but the very hard zeta alloy layer is then exposed to resist further abrasion and heavy loading.

Abrasion tests show that if abrasion resistance for epoxy zinc-rich primers and most other conventional paints is taken as unity, polystyrene zinc-rich primers are 5 times better, zinc silicate primers 50 times better and hot dip galvanized steel 400 times better. This comparison was made by comparing the weight of silicon carbide (in grams) to erase 100µm of coating using a Taber Abraser, with results shown below.

Table 2

The toughness of a galvanized coating makes it particularly suitable in situations where abrasion could be a problem either in assembly or in use. Conveyor systems, including buckets for quarried material and hoppers for coal wagons, are well suited. Galvanizing also limits the damage when spanners or other tools slip or when nuts turn on a galvanized surface during tightening. While the toughness of the galvanized coating does greatly simplify the handling of large, heavy sections and reduces potential remedial work, even when the base steel is exposed, the sacrificial properties of the galvanizing coating will protect any small, exposed areas and help prevent unsightly and damaging rust.

Galvanized coating thickness

During the first minute of immersion in the galvanizing bath, zinc-iron alloy layers grow rapidly on the surface of the steels which are most commonly galvanized. The rate of alloy layer growth then diminishes and is finally very slow. When the work is withdrawn from the bath an outer layer of relatively pure zinc is also carried out. The total zinc coating mass applied depends mainly on the mass and thickness of the steel being galvanized.

Table 3 Requirements for coating thickness and mass for articles that are not centrifuged

Table 4 Requirements for coating thickness and mass for articles that are centrifuged

AS/NZS 4680 specifies the minimum average coating thickness.

Table 5 Requirements for coating thickness and mass for threaded fasteners (bolts & nut) and washers

Factors influencing coating thickness.

The thickness, alloy structure and finish of galvanized coatings are influenced by:

  1. Steel thickness
  2. Surface condition of the steel
  3. Composition of the steel

Increasing the period of immersion in the galvanizing bath in the normal galvanizing temperature range will generally not increase coating thickness by any significant amount.

Thickness of the steel: The easiest way to increase the coating thickness is to increase the thickness of the steel, as shown in Table 3. Very thick steels usually produce coating thicknesses more than the minimum required in AS/NZS 4680 which provides additional durability benefits.

Surface condition of steel: Galvanizing is unique in that the coating appearance largely reflects the starting surface condition, Steel that is heavily rusted (pitted) or contains roll marks will usually show these marks after galvanizing. Similarly, the quality of the weld, especially porosity, will show after galvanizing.

Abrasive blasting steel before galvanizing roughens and hardens the surface and increases its surface area, producing thicker coatings, though sometimes at the expense of a rougher surface after galvanizing. Application of abrasive blasting to achieve thicker coatings is generally limited by practical and economic considerations. Where increased service life or reduced maintenance is required the use of a duplex (galvanizing-plus-paint) systems is a preferable alternative. Abrasive blasting of steel to achieve thicker coatings is covered in GAA Advisory Note AN 02.

Composition of steel: The content level of both silicon and phosphorous in the steel have effects on the structure, appearance, and properties of galvanized coatings. In extreme cases, coatings can be excessively thick, brittle, and easily damaged, especially on the edges. The aluminium content of certain low silicon steels can also affect the coating thickness. Table 6 and Figure 7 show typical galvanized coating characteristics based on steel composition. This relationship, known as the Sandelin Curve, is based on decades of extensive research from around the world and is published also in AS/NZS 2312.2.

Silicon. Certain levels of silicon content will result in thicker or thinner galvanized coatings relative to the underlying steel thickness. Thicker coatings result from the increased reactivity of the steel with molten zinc and rapid growth of zinc-iron alloy layers on the steel surface. When the silicon content is less than 0.04%, the appearance will typically be bright and shiny and aesthetically attractive. When the silicon content is less than 0.01% the coating will also be aesthetically attractive although it may be lower in thickness than required in the Standard (see Aluminium, below). If the silicon content is between 0.04% and 0.14%, excessively thick grey coatings can develop, which can lead to instability in the coating adhesion and cohesion.

When the silicon content exceeds about 0.25%, the coating often tends to a grey colour and is always thicker than Standard and this means it is more susceptible to transport damage at the edges. However, in most cases, a silicon content above 0.25% will be acceptable due to the increased durability benefits to the user. In some applications when a very long durability is desired, such as bridges, a higher silicon content is desirable.

Phosphorous. When present in combination with silicon, phosphorous can have a disproportionate effect, producing excessively thick galvanized coatings. The effect of phosphorous varies depending on the silicon content and reference should be made to the Table 4 as a general guide to the effect.

Aluminium. Steels that are deoxidised with aluminium usually have very low levels of silicon (< 0.01%) and an aluminium content >0.035%. These steels will produce galvanized coatings that are thinner than the typical galvanized coating for the same thickness of steel. The galvanized coatings produced are typically shiny and smooth but exhibit reduced corrosion resistance due to the thinner coating. These galvanized coatings are usually seen in ‘laser plate’ type flat sections and some tubular steels. In Australia, BlueScope Steel’s Xlerplate Lasercut plate is usually produced with very low silicon levels, leading to thinner galvanized coatings.

Galvanized coatings on reactive steels can be dull grey or patchy grey in colour with a rough finish and may be brittle if there is excessive zinc-iron alloy layer growth. Coating service life is proportional to the increased thickness and is unaffected by appearance, provided the coating is sound and continuous. The thickness, adherence, and appearance of galvanized coatings on silicon and phosphorous steels are outside the control of the galvanizer. (See also GAA Advisory Note AN 35).

Table 6 Typical coating characteristics related to steel composition
Note: For category A steels this applies for hot rolled sections. When galvanizing cold rolled steels in this category, the formula Si + 2.5P ≤ 0.04% should be applied.

Note: For category A steels this applies for hot rolled sections. When galvanizing cold rolled steels in this category, the formula Si + 2.5P ≤ 0.04% should be applied.

Appearance

A galvanized coating is normally smooth, continuous, and free from gross surface imperfections and inclusions. However, it cannot be compared with the smooth surface of continuously galvanized sheet steel or wire since these are produced by processes which permit close control of coating thickness and appearance. Over 30 different types of typical surface conditions that can occur on batch hot dip galvanized articles are discussed in detail in the Hot Dip Galvanizing Inspector Program, which has been run by the ACA since 2016. These surface conditions include everything from ash deposits to zinc splatter, but by far the most common point of contention where aesthetics is concerned is the initial appearance or colour of the galvanizing. Irrespective of the initial lustre and colour of galvanized coatings, it does not affect the corrosion protection offered by the coating and over time the appearance of the galvanized coating will change as it naturally weathers.

Initial Appearance

From the batch galvanizing process, there are four different initial appearances a coating can develop based on its formation; shiny, spangled, dull and mottled. Some galvanized articles may even develop more than one of these appearances across their surface.

Galvanized steel with a shiny appearance is the most commonly seen and has become what people expect to see when looking at newly galvanized steel (Figure 8). This shiny appearance is created by the solidification of unreacted zinc on top of zinc-iron alloy layers when it is withdrawn from the zinc bath (Figure 5).

A spangled appearance (Figure 9) has the same general coating structure and lustre as the shiny appearance, with the only difference being how the zinc solidifies. For spangle to form, certain types of additional elements must be present in the zinc to allow the crystalline pattern to form. The concentration of these elements and the cooling rate of the article influence the size and shape of the crystal formation.

Dull-grey coatings on newly galvanized steel are seen by some as an inferior coating, but this is an inaccurate assumption. A dull-grey colouring as an initial appearance is due the coating structure being entirely made of zinc-iron alloy layers, with no top layer of zinc. While there is always zinc pulled up on top of the zinc-iron alloys when an article is withdrawn from the bath, it does not always just solidify in place as with shiny and spangled appearances. Sometimes, the zinc will continue to react with the base iron and be totally converted, leaving only zinc-iron alloy at the coating’s surface (Figure 10 and Figure 11). These coatings are usually thicker than their shiny or spangled counterparts and in-turn will have a longer service life when placed in the same environment.

mottled appearance consists of a dull-grey circular type pattern around areas with a shiny finish (Figure 12).  It is also described as a cellular, web (spider web), or mechanical scale pattern and is often mistaken to be the result of cracks in the coating. This appearance occurs due to a partial presence of zinc-iron alloy layers at the surface of the galvanized coating (Figure 13), with the pattern believed to be due to the alloys being created at grain boundaries only and the remaining zinc solidifying without reacting. The appearance may occur in a localised area or extend over the entire surface of an article.

Factors Influencing Initial Appearance

Why can’t I specify the appearance I want? A simple question with a complicated answer. The commonly seen differences in initial appearance can rarely be controlled by the galvanizer, as it is highly dependent on the metallurgical reaction of coating formation that occurs while immersed in the molten zinc. There are numerous factors that influence how a galvanized coating forms on any given piece of steel with the four main factors affecting the initial appearance being steel composition (or chemistry), the surface condition of the article, its cooling rate and its design and fabrication, including venting and draining.

1. Steel composition (chemistry)

Certain elements in the steel, in particular silicon (Si) and phosphorus (P), affect the reactivity of the iron with molten zinc. The extent of the reactivity is dependent on the concentration of each element and this relationship was first described by Sandelin in 1940 (Figure 7).

The compositions that cause a prolonged or faster rate of reaction between the iron and molten zinc while the steel is submerged are known as ‘reactive steels.’ These steels usually result in mottled or dull appearances as well as having thicker coatings.

2. Surface condition

The condition of the article’s surface contributes to the initial appearance of galvanizing. On a macro level, the surface of the galvanized coating typically matches or amplifies the article’s contours. A good example is if the steel suffered from pitting corrosion before being galvanized, the coating will follow the corroding pits. On a micro level, the surface profile affects the reactivity of the steel with the zinc. A rougher surface with a higher surface area will generally result in increased reactivity and higher potential for dull or mottled appearance, while smoother surfaces are less reactive and more likely to have a shiny appearance.

3. Cooling rate

The thickness of the steel will affect the cooling rate of the article when it is withdrawn from the molten zinc, with thicker steel retaining heat longer. This retained heat can allow the diffusion of the zinc to continue, creating more zinc-iron alloy and hence mottled or dull coatings are more likely to develop.

4. Design and fabrication, including venting and draining

The influence of design and fabrication on appearance generally relates to the adequacy of the venting and draining along with variations in section thicknesses and fabrication methods.

If vent and drain holes are too small, it will take more time for the zinc to flow around and/or into the article when being dipped as well as more time to flow out during withdrawal. This also increases the time taken to cool the article and in turn can affect the surface appearance.

Ideally, thickness variations in fabrications should be minimised to help avoid distortion but this also helps to limit variations in initial appearance due to different cooling rates.

Another factor related to design and fabrication that affects appearance is when steels of significantly different chemical composition are used in the one article (Figure 14). This can lead to completely different initial appearances between adjacent areas, despite the whole article being dipped at the same time.

Appearance Over Time

All metals oxidise in the atmosphere and create a passive film on their surface. For zinc, this natural passivation process results in a noticeable change to its appearance and generally occurs in three stages, as shown in Figure 1. The result of this process is the formation of a relatively insoluble zinc carbonate film, known as the patina, which has a matte, light grey colouring. Everyday examples of commonly galvanized objects where this light grey appearance of the patina can generally be observed include handrails, street sign posts, roadside guardrails, and light poles.

The development of the zinc patina happens over time and the speed of change will vary depending on the exposure environment. At the extremes, it can happen in as little as a couple of days or take as long as a few years, but for most common exposures in Australia and New Zealand it will develop over a few weeks or months.

Another appearance one may come across, usually on older galvanized coatings, is what’s commonly referred to as bronzing. For a typical shiny coating (whose structure is shown in Figure 5), bronzing will start after the eta layer is consumed and corrosion of the first alloy layer (zeta) starts. As there is a small percentage of iron in the alloy layers, small amounts of iron oxide (rust) are formed on the surface coating, creating a ‘bronze’ or ‘rusty’ appearance. This appearance can be confused with rusting of the base steel, however there is always a significant amount of galvanized coating remaining on top of the base steel. One method for determining the difference between bronzing and corrosion of the base steel is to take coating thickness measurements of the area.

Figure 5 Typical galvanized coating showing the pure zinc outer layer, with zinc-iron alloy layers.
Figure 6 Galvanized coatings are slightly thicker at corners and edges as shown, an important advantage over most organic coatings which thin out in these critical areas.
Figure 7 The Sandelin Curve showing the relationship with silicon and galvanizing coating thickness
Figure 8 Typical initial shiny appearance of hot dip galvanizing
Figure 9 Shiny spangled appearance
Figure 10 Hot dip galvanized coating on pipe changing to dull appearance while still being withdrawn from the molten zinc.
Figure 11 Micrograph of a dull hot dip galvanized coating with no outer zinc layer
Figure 12 Mottled appearance of a newly galvanized pipe
Figure 13 Mircograph showing part of the structure of a part dull, part shiny galvanized coating
Figure 14 Dull appearance on one section of the a fabricated handrail