What is a Spangle on Hot dip Galvanizing(part-3) ?
For many years, galvanized articles made by hot–dip coating techniques were identified by a characteristic spangled appearance. In some cases, this is still true today. However, because of changes in zinc refining processes, in the galvanizing process, and in the demands of the marketplace, not all hot–dip galvanized steel sheet made today has a visible spangle.
What is a Spangle?
A spangle is the visible aesthetic feature of crystallites on the surface of a galvanized steel sheet. The spangle appears as either a snowflake or a six-pointed star pattern. This is produced on the steel sheet when certain alloying elements are either added to the liquid zinc or available as impurities.
The size and orientation of the zinc grain affects the corrosion and mechanical properties of the zinc alloy. This can be altered by alloying elements such as:
Spangles give the zinc coating a decorative appearance determined by orientations of the zinc crystals and the distribution of the alloying elements in the coating.
The spangle is formed when liquid zinc adhering to a steel surface is cooled to temperatures below the melting point of zinc. The zinc atoms, which are randomly arranged in the liquid form, start to position themselves in an orderly pattern at random locations within the molten zinc coating. This process of transforming from disorderly atoms in the liquid state to an orderly pattern is solidification or crystallization.
These small solidifying regions in the molten zinc are referred to as grains. Grain growth occurs when individual atoms from the molten zinc continue attaching themselves to the solidifying grain in an orderly pattern. The individual atoms of the growing solid grain arrange themselves into the often-visible hexagonal symmetry of the final spangle. When the coating solidifies completely, the individual spangles formed represent the respective zinc grains.
Dendritic growth is a different solidification process that also gives rise to spangles in a galvanized steel sheet. Spangles produced in this process have a snowflake appearance.
Factors that affect spangle size are:
- Zinc chemistry
- Cooling rate
- Smoothness of the substrate
- Alloying element
There is another aspect of the solidification process that leads to the snowflake pattern in galvanized coatings, viz., “dendritic” (meaning tree-shaped) growth. Dendritic growth causes the individual growing (solidifying) grains to grow into the melt (the molten zinc coating) with a distinct leading rounded edge.
Dendritic growth of grains during the solidifying of metals is very common. The reason that the dendrites are readily visible in a galvanized coating is that we are basically seeing a two-dimensional version of an as-cast, dendritic, solidified grain structure.
Remember, the coating is less than 0.001 inch (25 micron) thick, considerably less
than the diameter of a spangle. In other metals (for instance the steel substrate), the original as-cast, three-dimensional, dendritic structure of the grains is subsequently broken up into many smaller, more equiaxed grains.
This is due to the effects of hot rolling (for example, rolling a 9-inch thick slab of steel into a 0.050-inch thick steel sheet), cold rolling and recrystallization during the sheet annealing process.
The rate of growth of the dendrite arms during the solidification of a galvanized coating competes with the rate of nucleation of new grains within the molten zinc. This process determines the final size of the completely solidified structure. In a galvanized coating with a well-defined large spangle pattern, the rate of dendrite growth dominated the solidification process leading to a small number of large spangles.
The Effect of Zinc Bath Chemistry
Dendritic growth is not the only way in which grains can grow during the solidification process. It requires one or more special conditions to be present. One of these conditions is the presence of other elements in the molten Grains Metals, like many solids in nature, have a crystal or “grain” structure.
For example, the steel sheet beneath the galvanized coating consists of many small grains of iron-carbon alloy (steel). The individual grains of steel are very small compared with the grains of zinc in the zinc coating, and are “glued” to one another by atomic bonding forces.
Think of this as “grains of sand” in a sandstone rock. The size of the individual grains of sand may be larger than the grains in the steel sheet, but this analogy allows the concept of grain structure to be visualized. metal. These can be either intentionally added alloying elements or impurities.
In the case of galvanized coatings on steel sheet, the most common reason for the well-defined dendritic growth pattern is the presence of lead in the coating. It has long been thought that the reason lead results in large spangles is that it reduces the number of nucleation sites. In recent work 1 , it is proposed that that the presence of lead decreases the solid/liquid interfacial energy in the solidifying coating. This leads to an increase in dendrite growth velocity, resulting in large spangles.
Lead precipitates at the coating surface and the varying distribution of lead particles across the surface define the optical appearance (dull vs. shiny spangles).
Lead is a common impurity in zinc. In years gone by, the most common method of zinc metal production involved smelting, distillation and condensation. Lead is a common metal found in zinc-containing ores, and this refining process carried it through as an impurity in the zinc. In the early days of galvanizing, lead was almost always present in the zinc, and it was common to see a spangle pattern. Galvanized coatings on steel became identified by the characteristic spangle. Essentially, all hot–dip galvanized coatings had a spangled appearance. If the spangle wasn’t visible, the users “knew” that the steel had not been galvanized.
The first galvanized coatings contained as much as 1% lead. During the past 35 years, the presence of such high lead levels has not been common in galvanized coatings on steel sheet, at least not in North America,Europe, and Japan.
Typical concentrations of lead (where it is intentionally used) in most galvanized sheet made during this time has been less than 0.15%, often as low as 0.03 to 0.05%. Even this lower amount of lead is still sufficient to develop dendritic growth behaviour during the solidification process.
Today, a typical level of lead in the coating bath on lines where the product requires a well-developed spangle pattern is in the range of 0.05 to 0.10% lead.
As there are now environmental concerns about the use of lead, some galvanized sheet manufacturers have established practices on their older or low speed lines that use lead-free zinc, whereby a small amount of antimony is added to the zinc coating bath.
Antimony influences spangle formation in a similar fashion to lead. The final result is a smooth, visibly spangled coating. Typically, the amount of antimony in the coating bath is about 0.03 to 0.10%.
To obtain smoother coatings with lead-bearing zinc, it is possible to suppress spangle growth on the sheet by rapidly cooling the coating. This is done by using spangle “minimizing” devices above the zinc bath, that in addition to increasing the cooling rate, direct steam or zinc dust at the surface to rapidly freeze the zinc. Such technology is not required in the case of lead-free zinc for the reasons explained in the next section.
In recent times, the production of zinc from zinc-containing ores has been changed to an electrolytic recovery method. In this method of zinc production, the refined zinc is very pure, with the lead being excluded.
This change occurred at a time when many users of galvanized sheet, especially those desiring a high quality finish after painting, such as the automotive and appliance industries, needed a non-spangled coating. Removing the lead gave them the product they desired. The amount of lead in the coating for lead-free coatings is less than 0.01%, and often less than 0.005%.
Lead-free coatings still have a grain pattern that is visible to the unaided eye. Typically, the spangles are about 0.5 mm in diameter and are clearly visible when seen at 5 to 10X. However, the grains no longer grow by a dendritic mode but by a cellular mode of growth. Essentially, zinc grains nucleate on the steel surface, and grow outward toward the free surface.
Why is Lead Still Used on Many Galvanizing Lines?
The manufacture of non-spangled coatings, free of lead (or antimony), is not so easily done. The reason relates to the influence of even a small amount of these additions on the viscosity of the molten zinc. Due to its lower viscosity, it is difficult to avoid small sags and ripples in the zinc coating when lead/antimony is not present. The thicker the coating, the greater the tendency to form sags and ripples during freezing. Fortunately, the automotive and appliance industries need only relatively thin coatings (typically 60 to 80 g/m 2/side) of zinc to obtain the level of corrosion resistance their customers demand. Also, the products used by these industries are made on relatively new high-speed lines, or older lines that have been refurbished to allow production at high speeds. The combination of high processing speeds and low coating weights allows producers to use lead-free
coating baths, avoid the development of spangles, and still attain a ripple-free coating. Improved gas-wiping technology and practices has also helped in producing smoother coatings.If the end user requires a heavier coating mass (100 g/m2 /side and higher), there is a tendency for the coating, when applied from a lead-free bath, to develop very visible sags and ripples.
The result is that the surface is not smooth and the coating is composed of locally thick and thin regions. This tendency for sags is exacerbated at low line speeds (<75 meters/minute). Older, low speed coating lines designed to process heavy-gauge sheet, and those that are used to make heavy coating weight products (heavier than 275 g/m2 or G90), typically still have some amount of lead in the coating bath to improve the final coating uniformity. The concentration of lead in the zinc bath is typically between 0.05 and 0.10%. Antimony additions of between 0.03 and 0.10% provide the same effect.The net result is that the final product from many lines still has a visible spangle pattern. This meets with the marketplace needs in that a number of industries, especially those that use bare (unpainted) galvanized sheet, still want the large, bright, reflective spangle pattern.
Specifying Spangle Size
Users often ask if there are specifications that govern the size (diameter) of galvanize spangles. Unfortunately there are no quantitative specifications that regulate this feature of galvanized sheet. Spangle size can be affected not only by the zinc chemistry and cooling rate, but by other factors such as the smoothness of the substrate. Consistently controlling spangle formation to a specified size, and then verifying compliance, would be an extremely difficult task. For this reason, spangle size terminology is qualitative. It is defined in ASTM A 653/A 653M, Specification for Steel Sheet, Zinc-Coated (Galvanized) as follows:
Regular spangle – zinc-coated steel sheet with a visible multifaceted zinc crystal structure. The cooling rate is uncontrolled, which produces a variable grain size.
Minimized spangle – zinc-coated steel sheet in which the grain pattern is visible to the unaided eye, and is typically smaller and less distinct than the pattern visible on regular spangle. The zinc crystal growth is arrested by special production techniques, or is inhibited by a combination of coating bath chemistry plus cooling.
Spangle-free – zinc-coated steel sheet with a uniform finish in which the surface irregularities created by spangle formation are not visible to the naked eye. The finish is produced by a combination of coating bath chemistry, or cooling, or both.
ROLE OF ANTIMONY IN SPANGLE FORMATION
Development of flowery patterns or spangles on the surface of hot dip galvanised steel sheets is a common phenomenon. While elements like lead and antimony are known to be the primary factors contributing to spangle formation, sometimes they grow uncontrollably small or big. In this study, a data mining approach has been used to find a correlation between the spangle size in galvanised sheets, and the process parameters at one of the continuous galvanising lines at Tata Steel.
All the process related data were collected from the CRM database, while the information on spangle size was generated through actual measurements. Statistical (factor analysis) and mining (neural classification mining) analyses were carried out. The most significant input variables with respect to spangle size were extracted.
The artificial neural network classification model was developed using 849 records for training with a prediction accuracy of 57%. Strip thickness appears to be most sensitive on the spangle formation; whereas lead and antimony concentration in zinc bath, and the pressure difference between the top and bottom air knives seem to be more sensitive amongst the other eight significant parameters. The classification model can be used for prediction of spangle size given the process parameters. It can also be used as an important tool to set and adjust the process parameters to produce a given spangle size.
The spangle on hot–dip galvanized steel sheet has been its primary identifying feature for many years. The demand for a very smooth product has necessitated that spangle size be reduced until it is no longer visible to the unaided eye. This was, and to some extent still is, of concern to certain segments of the marketplace,
but gradually the users of galvanized sheet are becoming accustomed to a product that does not have a large, easily seen spangle. At some point in the future there may be no demand for large spangles and producers will have no need to add lead or antimony to zinc baths.