Formation of Spangles in HDGI -

Formation of Spangles in HDGI

Introduction

For many years, galvanized articles made by hot-dip coating techniques were identified by a characteristic spangle appearance. In some cases, this is still true today.

However, because of changes in the zinc refining process, in the galvanizing process, in the demands of the marketplace, and health concerns, relatively little hot-dip galvanized steel sheet made today has a visible spangle.

What is a Spangle?

The dictionary defines “spangle” as a glittering object. When the word spangle is used to describe the surface appearance of galvanized steel sheet, it means the typical snowflake-like or six-fold star pattern that is visible to the unaided eye.

The spangle features encompass a number of quite complex metallurgical phenomena. The spangles develop during the Solidification Process when the molten zinc adhering to the steel sheet is cooled below the melting point of zinc, which is approximately 787 °F [419 °C]. At this temperature, the randomly arranged atoms in the liquid zinc begin to position themselves into a very ordered arrangement. This occurs at many random locations within the molten zinc coating. Transformation from a disordered arrangement of atoms (liquid state) into an ordered crystal arrangement is known as solidification or crystallization. The small solidifying regions within the molten zinc are defined as “grains”.

As individual atoms in the molten zinc attach themselves to a solidifying grain (causing grain growth), they do so in an ordered fashion and form into a distinct array, or crystal. In the case of zinc, the crystals form with hexagonal (sixfold) symmetry. As the solid zinc grains grow larger, individual atoms of zinc arrange themselves into the often-visible hexagonal symmetry of the final spangle. When the coating is completely solidified, individual spangles define individual grains of zinc.

“Nucleation” is the transformation of randomly arranged atoms of molten metal into a small, organized array of atoms in the “seed” crystals at the initial stage of solidification. A high rate of nucleation during the freezing process tends to cause the formation of numerous small grains in the final solidified structure, while a low rate tends to favour the growth of large grains.

Dendritic Growth There is another aspect of the solidification process that leads to the snowflake pattern in galvanize coatings, viz., “dendritic” (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

D) Key Quality Control Areas for Major ApplThere are secondary dendrite arms that grow laterally away from the “primary” dendrite arms. Dendritic grain growth during the solidification of metals is very common. The reason that the dendrites are readily visible in a galvanize coating is that we are seeing a two-dimensional version of a large, ascast, dendritic, solidified grain pattern. The coating is typically less than 0.001 in (25 µm) 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 [230 mm] thick slab of steel reduced in thickness to as low as 0.050-inch [1.3 mm] 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 galvanize 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 the case of a galvanized coating with a well-defined large spangle pattern, the rate of dendrite growth dominates the solidification process leading to a small number of large spangles. One characteristic of such spangles is that they are thickest at their centers and thinnest at their edges, or grain boundaries. The grain boundaries can be said to be “depressed” and are difficult to smooth by subsequent temper (skin) passing. Galvanize coatings with small spangles generally have less depressed grain boundaries, and can be smoothed more easily by skin passing.

The nature and rate of dendritic growth during the solidification process is affected by the presence of other metallic elements in the molten metal. These can be either intentionally added alloying elements or impurities. In the case of galvanize coatings on steel sheet; the most common reason for the well-defined 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.

How Dendritic growth pattern are affected by the presence of lead in the coating bath?

Presence of lead decreases the solid/liquid interfacial energy in the solidifying coating. Lowering of the interfacial energy raises the energy barrier to heterogeneous nucleation. Lead, by inhibiting nucleation, effectively increases the spaces between neighboring nuclei, allowing larger spangles to form. It is also postulated that the presence of lead results in an increased dendrite growth velocity. Lead, being insoluble in solid zinc, is rejected during solidification and precipitates at spangle boundaries and at the coating surface.

The varying distribution of lead particles across the surface, define the optical appearance (dull vs. shiny spangles. The higher concentration of lead and other elements rejected from the freezing zinc to its surface contribute to early selective spangle darkening.

Why is spangles associated with Hot Dipped Galvanized?

In years past, 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, so a spangled appearance was common. Galvanize coatings on steel became identified by the characteristic spangle. Essentially, all hot-dip galvanized coatings had a spangle appearance. If the spangle wasn’t visible, the users “knew” that the steel had not been galvanized. The first galvanize coatings contained as much as 1% lead.

Increasingly over the past 50 years, the presence of such high lead levels has been less and less common in galvanized steel sheet, at least in North America, Europe, and Japan. Typical concentrations of lead (where it was intentionally used) in most galvanized sheet made up to about 25 years ago was less than 0.15%, sometimes as low as 0.03 to 0.05%. Even this lower amount of lead is still sufficient to develop the dendritic growth behaviour during coating solidification that results in a spangle.

Is spangle formation in Hot Dipped Gavalnized possible without the addition of lead?

As there is now much more concern about the health hazards of lead, some galvanized sheet manufacturers that wish to market a spangled product have established practices that use lead-free zinc, but add a small amount of antimony to the zinc coating bath. Antimony influences spangle formation in a similar fashion to lead. The final result is a relatively smooth, visibly spangled coating. Typically, the amount of antimony in the coating bath is about 0.03 to 0.10%.

How to supress the spangles?

Years ago, to obtain smoother coatings with lead-bearing zinc, spangles were suppressed by rapidly cooling the still molten coating as the sheet exited the zinc bath. Cooling was achieved by the use of a spangle “minimizing” device above the zinc bath. These devices directed steam or zinc dust at the surface to rapidly nucleate and freeze the zinc, essentially resulting in no spangle. The product was known as “minimized spangle” galvanized sheet and could be easily skin passed to produce an extra smooth product.

What is Spangle Free coatings and its advantages?

The production of zinc from zinc-containing ores was changed to an electrolytic recovery process. In this method of zinc production, the refined zinc is very pure, with lead being excluded. This change was in place at the time many users of galvanized sheet, especially those desiring a high-quality finish after painting, such as the automotive and appliance industries, needed a spangle-free coating. Removing the lead gave them the product they desired.

The lead level in zinc used to produce spanglefree product is a maximum of 0.007% (70 ppm), and often less than 0.005% (50 ppm). Lead-free coatings still have a grain pattern that is, at best, barely 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 magnification.

However, the grains no longer grow by a dendritic mode but by a cellular growth mode. Essentially, the zinc grains nucleate heterogeneously on the steel surface, and grow outward toward the free surface. The absence of lead takes away the strong driving force for growth in the plane of the sheet, preventing the formation of large spangles. As spangles cannot grow in size, the result is the coating appears uniformly shiny. Grain boundary depressions, for all intents and purposes, do not exist in these coatings

When combined with temper rolling by the galvanized sheet producer, can very easily be made extra smooth. The large grain boundary depressions and surface relief of a spangled coating are not present. The coating can then be painted to give a very smooth finish.

However Lead levels of even 100 ppm in zinc coatings can result in an increased rate of spangle boundary corrosion in humid, warm environments which can create a problem known as `delayed adhesion failure of the coating. Essentially, bimetallic corrosion cells are created between lead and zinc, which progressively undermines zinc adherence.

Using lead-free zinc avoids this issue. The absence of lead can only enhance corrosion resistance. The very small spangles of lead-free coatings have an as-coated shiny metallic and very uniform appearance, unlike that of large spangle, lead-bearing zinc coatings, where the luster of each spangle differs, giving the sheet a non-uniform appearance.