Metallurgical Aspects of Steel Galvanization
Metallurgical Aspects of Steel Galvanization
Steel objects, which are not protected on the surface, can get serious damages because of various environmental conditions such as rain, snow, wind, and extreme temperatures. These adverse environmental conditions convert iron into iron oxide and corrode steel with consequent increase in volume and decrease in strength. To avoid environment conditions acting on the steel surface, various protective surface coatings are used. Out of the different types of surface coatings, galvanizing is a very popular and reliable surface coating.
Hot dip galvanized coatings are applied to steel to improve the anti-corrosion performance of the steel so as to ensure that it lasts as long as possible with a minimum of maintenance. The generation of zinc and zinc alloy coatings on steel is one of the commercially most important processing technologies used to protect steel objects exposed to corrosive environments. From a technological standpoint, the principles of galvanizing have remained unchanged, since this coating came into use more than over 200 years ago.
Hot-dip galvanizing normally is frequently used on products where the surface is exposed to wear, for example thresholds in vehicles, transportation wagons, steps, handrails and gratings.
Galvanizing forms a metallurgical bond between the underlying steel and the zinc coating thus creating a barrier that is part of steel itself. Galvanized coatings are adherent to the underlying steel at least ten times more than any other coatings. During the process of galvanizing there is a reaction between the molten zinc and the iron of steel and a series of Zn-iron alloy layers are formed as shown in the Fig 2. The figure shows a typical microstructure of the cross section of a galvanized steel coating consisting of three alloy layers and a layer of pure metallic zinc.
Zinc coatings are predominantly used to improve the aqueous corrosion of steel by two methods namely (i) barrier protection, and (ii) galvanic protection. In barrier protection, the zinc coating, which separates the steel from the corrosion environment, corrode first before the corrosive environment reaches the steel. In galvanic protection, an electrolytic cell is formed since zinc is less noble or anodic to iron at ambient conditions, and hence sacrificially corrode to protect the substrate steel, even if some of the steel is exposed as cut edges or scratches in the coating. Fig 1 shows an electrolytic cell and galvanic series of metals.
Fig 1 Electrolytic cell and galvanic series of metals
Cathodic protection of galvanized coating
Metallic zinc is anodic to steel. In the presence of an electrolyte, the anodic zinc coating on the galvanized steel corrodes preferentially to the cathodic steel base thus preventing corrosion of small areas which can get exposed through accidental damage to the coating (Fig 1). This cathodic protection continues as long as there is zinc coating. The mechanism of cathodic protection by zinc is as given below.
When zinc and steel are in contact in an electrolyte, differences in electrical potential develop and an electrolytic cell is formed. Since zinc is more electrochemically active than steel, it becomes anode for all the steel, preventing the formation of small cathodic and anodic areas on the steel surface. As a result of the differences in the electrical potential within the cell, negatively charged electrons flow from the zinc anode to the steel cathode and the atoms in the anode are converted into positively charged zinc ions. At the cathode surface, negatively charged electrons attract and react with positively charged hydrogen ions from the electrolyte, liberating hydrogen gas. There is no chemical reaction between the steel cathode and the electrolyte. This phenomenon, which is known as cathodic protection, prevents corrosion of the steel cathode. The positive charged zinc ions on the anode surface react with negatively charged hydroxyl ions from the electrolyte and zinc gets slowly consumed providing sacrificial protection for the steel. When discontinuity or damage in the zinc coating exposes the underlying steel then the cathodic protection which zinc provides for the steel ensures that the exposed steel does not corrode.
Exposure tests by The American Society for Testing and Materials (ASTM) show that panel weight loss, a measure of the rate of corrosion, is much lower for zinc than for steel in a wide range of exposures. Galvanized coatings are consumed at rates between one seventeenth and one eightieth that of steel, so that even in aggressive environments, hot dip galvanizing provides long life.
Process of galvanization
Typical processing methods used in producing zinc coatings include hot-dip galvanizing, thermal spraying, and electro deposition. The hot dip galvanizing is a common and popular technique for the galvanizing of the steel objects. It consists of immersion of the steel object in a liquid bath of zinc or a zinc alloy, either by batch or continuous processing. The continuous process is more advantageous for coiled products such as sheet, wire, and tube, whereas the batch process is normally used for bulk products.
In general, prior to immersion in the liquid zinc bath, the steel object to be galvanized is first cleaned to eliminate any surface oxide which can react in the zinc bath. The surface of the object is to be very clean and free of surface oxides when introduced into the liquid zinc coating bath. After hot-dipping, in which the steel reacts with the bath forming the coating, the object is withdrawn, cooled, and sometimes subsequently heat treated. Fig 2 shows the process of galvanizing.
Fig 2 Process of galvanization and galvanized layer cross-section
In case of the galvanizing of the cold rolled sheet, the sheet, typically, receives an in-line annealing at temperatures above 650 deg C ahead of the coating bath, and is then cooled to around 470 deg C to 490 deg C before it enters the bath. The zinc, which melts at 419 deg C, is normally at a temperature of 465 deg C. Steel sheet has sufficient high temperature strength so that it can be pulled through both the annealing furnace and the zinc bath without tearing or deforming. During the time that the sheet is immersed in the bath (in some coating lines it is as brief as around 2 seconds), the steel and molten zinc undergo a metallurgical reaction.
During this reaction, the surface atoms of the steel, which are in the solid state, interact with the zinc atoms in the bath, which are in the molten state. This interaction is called ‘diffusion’. Zinc atoms move in the direction of the steel and iron atoms in the steel migrate towards the molten zinc. The result is the formation of a solid ‘mixed’ layer between the steel and the molten zinc. This layer contains zinc and iron atoms in specific proportions, and is called an ‘inter-metallic’ compound. The mixing of atoms of different metals is known as alloying and the diffusion zone which is formed during galvanizing is an inter-metallic alloy. It is this alloy zone, when properly formed, which provides the excellent bond between the steel and the zinc coating.
As shown in the Fig 2, the hardness of gamma, delta and zeta layers of the zinc coating, as expressed in DPN (diamond pyramid number), is higher than the underlying steel. Due to this higher hardness these layers provide excellent protection against coating damage through abrasion. The eta layer of the coating being low in hardness is quite ductile and provides the coating some impact resistance. The zeta, delta and gamma zinc-iron alloy layers are actually harder than the base steel, resulting in galvanizing steel’s outstanding resistance to abrasion and mechanical damage. Abrasive or heavy loading conditions in service can 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.
Surface tension forces cause a layer of molten zinc to adhere to the steel when it exits the bath. After excess zinc is wiped off, the remaining liquid solidifies when it cools below 419 deg C. The final product (galvanized steel) consists of the steel core, with an inter-metallic alloy layer and outer zinc layer on both surfaces. If the zinc bath is aluminum free, a cross-section of the coating can look similar to that in Fig 2.
Hence, the composition of a zinc coated steel object consists of (i) the overlay or coating alloy, (ii) an interfacial layer between the overlay and the substrate steel containing a series of inter-metallic compounds, and (iii) the substrate steel. Each of these regions can be affected by the bath time and temperature, as well as the chemistry of both the bath and the substrate steel. The inter-metallic alloy layers shown in Fig 2 are a mixture of zinc and iron atoms. They provide a high degree of bonding between the steel and the zinc outer coating. Unfortunately, these alloys have very poor ductility, i.e., they are hard and brittle. When the galvanized sheet is formed into a shape, there is a high probability of shear cracks developing in the alloys, and the zinc coating flaking off. This behaviour seriously limits the ability to form the galvanized sheet into shapes such as drawn cups, roofing panels, tight lock seams, or highly stretched automotive fenders.
The alloy layer is vital to achieving a good bond between steel and zinc. This layer is also to be continuous (over the entire surface area of the object) for the coating to be free from pores. Without interfering with the formation of an alloy bond zone, the nature of the alloy is to be changed so that the forming of the galvanized sheet into intricate shapes becomes possible.
Hardness, ductility, and adherence combine to provide the galvanized coating with very good protection against damage during rough handling. 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 the total coating mass.
The thickness of the coating is proportional to the coating mass. The thickness of hot dip galvanized coatings is determined by the thickness of the zinc-iron alloy layers which form when the steel reacts with the zinc. Higher coating thickness of galvanized steel results in better corrosion resistance and provides enhanced durability. However, it can lead to low formability of the steel. The tensile strength of the zinc coated layer increases with increase in thickness. Further, galvanized coatings are slightly thicker at the corners and the edges which is an important advantage over most of the organic coatings which thin out in these critical areas.
The thickness, alloy structure, and finish of galvanized coatings are influenced by (i) surface condition of steel, and (ii) composition of the steel. Increasing the period of immersion in the galvanizing bath does not increase coating thickness except in the case of silicon steels. Also, double dipping or galvanizing a second time does not increase the thickness of the galvanized coating and can adversely affect coating appearance.
Surface condition of steel – Grit blasting steel before galvanizing roughens the surface and increases its surface area, resulting in higher reactiveness to molten zinc. Greater zinc-iron alloy growth occurs during galvanizing, producing thicker coatings, though at the expense of a rougher surface and a poorer appearance. Application of this method for achieving thicker coatings is normally limited by practical and economic considerations.
Composition of steel – Both silicon and phosphorous contents can have major effects on the structure, appearance, and properties of galvanized coatings. In extreme cases, coatings can be excessively thick, brittle, and easily damaged.
Certain levels of silicon content result in excessively thick galvanized coatings. These very thick coatings result from the increased reactivity of the steel with molten zinc, and rapid growth of zinc-iron alloy layers on the steel surface. Excessive growth in coating thickness takes place on steels with silicon contents in the range 0.04 % to 0.14 %.Growth rates are less for steels containing between 0.15 % and 0.22 % silicon, and increase with increasing silicon levels above 0.22 %.
The presence of phosphorous above a threshold level of around 0.05 % produces a marked increase in reactivity of steel with molten zinc, and rapid coating growth. When present in combination with silicon, phosphorous can have a disproportionate effect, producing excessively thick galvanized coatings.
As a guide to the suitability of silicon and phosphorous containing steels for galvanizing, the criteria to be applied are (i) % Si less than 0.04 %, and (ii) % Si + (2.5 x % P) is less than 0.09 %. Galvanized coatings on silicon steels are normally dull grey or patchy grey in colour with a rough finish, and can be brittle.
Coating service life is proportional to the increased thickness and is unaffected by appearance, provided the coating is sound and continuous. In general, the thickness, adherence and appearance of galvanized coatings on silicon and phosphorous steels are outside the control the galvanizing personnel.
Mechanical properties of galvanized steels
The galvanizing process has no effect on the mechanical properties of the structural steels which are normally galvanized.
Strength and ductility – A very large number of experiments and tests have been conducted over the years in order to ascertain whether, and to what extent, hot-dip galvanizing affects the mechanical properties of low-carbon, un-alloyed and low-alloyed steels. The test results are briefly summarized below and apply both to steel galvanized at the normal (460 deg C) and high (560 deg C) temperatures.
The ultimate tensile strength, yield strength, elongation at rupture point and contraction of hot-dip galvanized steel remains virtually unchanged after hot-dip galvanizing in both the welded and not-welded states. The strength of cold-worked or heat-treated steel can be reduced during hot-dip galvanizing. The extent of the reduction depends on the degree of working or on the nature of the heat treatment. The notch toughness of hot-dip galvanized steel reduces somewhat compared with artificially aged samples, but not enough to affect the use of the steel.
The ductility of steel is not affected by hot-dip galvanizing. Excessive bending can however lead to cracking in the zinc coating itself. 1-t bends in many of the steels are embrittled by galvanizing, but galvanized 2-t and 3-t bends in all steels can be completely straightened without cracking.
In a study, in which the mechanical properties of 19 different structural steels from major industrial areas of the world were investigated before and after galvanizing, it has been found that the galvanizing process has no effect on the tensile, bend, or impact properties of any of the structural steels studied. Also, nor do even the highest strength versions has shown hydrogen embrittlement following a typical pretreatment in inhibited HCI, or H2SO4.
Changes in mechanical properties attributable to the galvanizing process have been detected only when the steel has been cold worked prior to galvanizing, but then only certain properties has been affected. Thus the tensile strength, proof strength, and tensile elongation of cold rolled steel are unaffected, except that the tensile elongation of 40 % cold rolled steel tends to increase by galvanizing.
Brittleness and cracking – Cold-working reduces the notch toughness of steel and increases the transition temperature for brittle fracture. Subsequent ageing at the increased temperatures strengthens this effect. Even if the steel itself is age-hardened, the effect of cold-working can be sufficient to cause the steel to have inadequate toughness to meet the applicable demands. The ageing process can in certain cases be accelerated at the elevated temperature of the galvanizing bath. However, these steels eventually become brittle whether they are hot-dip galvanized or not.
In hot-dip galvanizing, it is important to know whether or not the components have been cold-worked. Both aluminum-killed and silicon-killed steel can be affected negatively by cold deformation and ageing through galvanizing. If cold working of a susceptible steel cannot be avoided then the steel is to be stress relieved at 600 deg C to 650 deg C for 30 minutes or normalized prior to galvanizing. However, susceptible steels are relatively not very common.
Hot-dip galvanizing of ordinary unalloyed or low alloyed structural steel does not give rise to hydrogen embrittlement. Hydrogen, which can be absorbed during pickling, mostly is to be thermally expelled upon immersion in the zinc. Absorption of hydrogen can, however, lead to embrittlement of some hardened or high strength steels. Blasting instead of pickling is a possibility to avoid the problem.
Inter-crystalline cracking can in certain cases occur in hot-dip galvanizing due to the penetration of zinc into the inter-granular boundaries of steel. A pre-condition for this is that large stresses have been induced through welding or hardening of the steel. The risk of inter-crystalline cracking or fracture due to zinc penetration is negligible in the hot-dip galvanizing of ordinary structural steel. However, hardened material can be sensitive. The risk of cracking can be minimized if steel is annealed at temperatures higher than those in the zinc bath, i.e. above 460 deg C.
Embrittlement – It is quite rare for the steel to be in an embrittled condition after galvanizing. The occurrence of embrittlement depends on a combination of factors. Under certain conditions, some steels can lose their ductile properties and become embrittled. Several types of embrittlement can occur but of these types only strain-age embrittlement is aggravated by galvanizing process. The following information is for guidance in critical applications.
Susceptibility to strain-age embrittlement – Strain-age embrittlement is caused by cold working of certain steels, mainly low carbon, followed by ageing at temperatures less than 600 deg C, or by warm working steels below 600 deg C. All structural steels can become embrittled to some extent. The extent of embrittlement depends on the amount of strain, time at ageing temperature, and steel composition, particularly nitrogen content. Elements which are known to tie up nitrogen in the form of nitrides are useful in limiting the effects of strain ageing. These elements include aluminum, vanadium, titanium, niobium, and boron.
Hydrogen embrittlement – Hydrogen can be absorbed into steel during acid pickling but is expelled rapidly at galvanizing temperatures and is not a problem with components free from internal stresses. Certain steels which have been cold worked and / or stressed during pickling can be affected by hydrogen embrittlement to the extent that cracking can occur before galvanizing.
The galvanizing process involves immersion in a bath of molten zinc at about 450 deg C. The heat treatment effect of galvanizing can accelerate the onset of strain-age embrittlement in susceptible steels which have been cold worked. No other aspect of the galvanizing process is significant.
Weld stresses – In welded structures, weld stresses are partly reduced by hot-dip galvanizing. Hardening stresses in the zones affected by the heat of the weld are also reduced. This means that welded structures have a higher static strength in the hot-dip galvanized form than in the untreated condition.
Fatigue strength – The fatigue strength is affected differently by hot-dip galvanizing depending on steel type. There is a relatively small reduction in aluminum-killed steels, while the reduction in silicon-killed steels can be a little bit higher. The reason for this is the differing composition of the iron-zinc layer. Under fatigue conditions, cracks form in this layer which can then act as initiators for cracking in the steel surface.
In the determination of fatigue data in laboratory experiments, however, hot-dip galvanized material is compared with ‘ new’, untreated steel. But if an untreated structure is exposed outdoors, it is immediately attacked by corrosion. Pits are formed which are 5 times to 7 times deeper than the general corrosion, and the fatigue strength decreases rapidly. Conversely, the fatigue strength of hot-dip galvanized steel does not change appreciably during the exposure time, provided that the zinc coating remains on the steel surface. Under normal conditions, pitting does not occur in the zinc coating. The reduction in fatigue strength caused by hot dip galvanizing is small compared with the reduction caused by corrosion attack. It is also to be noted that abrasive blasting and, more especially, welding also reduce fatigue strength.
Practical experience shows that the fatigue strength of the steels normally galvanized is not significantly affected by galvanizing. The fatigue strength of certain steels, particularly silicon killed steels can be reduced, but any reduction is small when compared with the reductions which can occur from pitting corrosion attack on ungalvanized steels, and with the effects of welds.
For practical purposes, where design life is based on the fatigue strength of welds, the effects of galvanizing can be ignored. Fatigue strength is reduced by the presence of notches and weld beads, regardless of the effects of processes involving a heating cycle such as galvanizing. Rapid cooling of hot work can induce micro-cracking, particularly in weld zones, producing a notch effect with consequent reductions in fatigue strength.
In critical applications, specifications for the galvanizing of welded steel fabrications call for air cooling rather than water quenching after galvanizing to avoid the possibility of micro-cracking and reductions in fatigue strength.
Cold working – Cold working such as punching of holes, shearing, and bending before galvanizing can lead to embrittlement of the susceptible steels. Steels in thicknesses less than 3 mm are unlikely to be significantly affected.
Hot-dip galvanized steel and fire – Hot-dip galvanized steel does not burn, but the strength decreases when the temperature increases. Steel constructions remain stable until the critical temperature is reached. This occurs between 500 deg C and 750 deg C depending on the loading situation.
Hot-dip galvanized steel exposed to elevated temperatures – A study on how the hot-dip galvanized coatings react when exposed to elevated temperatures has indicated that the coatings can cope with temperatures upto 275 deg C, which is true if the exposure time is relatively short. For exposure times longer than a few weeks, this temperature is too high. The study results have shown that high temperature galvanized coatings (immersion at 560 deg C) respond worse to elevated temperatures than coatings formed by low temperature galvanizing.
The reason is that an elevated temperature causes solid phase transformations in the coating. The outermost pure zinc coating is transformed to an inter-metallic iron-zinc phase. When the whole coating consists of this iron-zinc phase, it becomes more brittle and cracks can form, especially during cooling (cyclic processes). High temperature galvanized coatings are especially sensitive since they consist predominantly of inter-metallic phases from the outset. Combinations of elevated temperatures and mechanical strains, for example vibrations, are to be avoided. Low temperature coatings with a large part of pure zinc have the best possibility to withstand elevated temperatures for longer periods.
Durability against wear – Pure zinc is a soft metal, but it is harder than most of the organic coatings available. However, the inter-metallic iron-zinc phases created during hot-dip galvanizing are very hard, even harder than common construction steels. Because of this, the iron-zinc phases are more resistant to wear than pure zinc. Studies have shown that the inter-metallic layer has a wear resistance which is 4 times to 5 times better than the pure zinc layer.
Atmospheric corrosion resistance – The life expectancy of a hot dip galvanized coating is roughly proportional to the thickness of the coating. Hence, in any given environment, it is possible to predict how long a coating is going to last before repair is needed. With a galvanized coating life expectancy is based upon the appearance of 5 % red rust on the steel surface.
The resistance to atmospheric corrosion of hot dip galvanized steel depends upon climatic factors such as humidity, rainfall, proximity to the coast and the presence of pollutants. The latter can have a particularly detrimental effect upon the performance of the coating depending on which pollutants are present in the environment.
Corrosion rates of hot dip galvanized steel at coastal locations (generally within 1 km of the high water mark) can be high. Industry generated gases such as sulphur dioxide and nitrous oxides attack the zinc coating, as do ammonia gases. In rural areas hot dip galvanized coatings can last well in excess of 80 years. This performance can be compromised where excessive application of insecticides and fertilizers occur, which in combination with moisture, attack the coating. Interestingly, galvanized coatings weather to a paler colour in marine climates and to darker colours in rural and industrial environments.
In some instances the hot dip galvanized coating transforms to a reddish hue, often misguidedly taken for rusting. This discoloration occurs as the zinc-iron alloys react with the atmosphere to form a distinct zinc-iron alloy based protective film. In certain environments the zinc-iron alloys can even provide better corrosion control to the underlying steel than pure zinc.
Role of aluminum
Over 75 years back, it was discovered that the addition of a small amount of aluminum to the coating bath is a perfect answer to the issue of changing the nature of the alloy layer without interfering with the formation of the alloy bond zone so that the forming into intricate shapes is possible. Initially, the reason for how it works so effectively was not understood, but it was observed that having aluminum in the zinc bath had made the alloy layer very thin compared to that from an aluminum-free bath. Aluminum is an inhibitor which significantly slows down the zinc-iron reaction rate. This thinner, and hence more ductile, alloy layer allows the coated steel sheet to be formed into many complex shapes without loss of coating adhesion, since it is not prone to the development of large internal shear cracks.
Using aluminum, at a level of around 0.15 %, has become the standard for galvanizing baths in continuous galvanizing lines. The aluminum addition practice is being used even now. However there is a much better understanding now of the metallurgy of aluminum in zinc, with the result that the aluminum concentration is more closely controlled. Some producers use 0.2 % to 0.3 % aluminum, but most stay in the range of 0.15 % to 0.19 %. When making zinc-iron coatings (galvanneal), the aluminum level is lowered to the range of 0.11 % to 0.135 %.
Although the addition of such a small amount of aluminum has a pronounced effect on the ability to form galvanized sheet, it does not have much effect on the bulk corrosion behaviour. However, as the aluminum does concentrate in the alloy layer, and to some extent at the surface of the zinc, it can adversely affect issues such as spot welding, soldering, and white rust occurrence. These drawbacks are insignificant, though, in comparison with the beneficial effect that aluminum exerts on the ability to form the galvanized sheet without loss of coating adhesion.
Spangle- surface appearance of zinc coating
As the zinc coating on the steel surface solidifies, dendritic crystals grow around a core of solidified zinc, and in some cases, a flower like pattern of bright zinc crystals called ‘spangle’ forms on the surface of the galvanized steel (Fig 3). The thickness of coating is much less than the diameter of a spangle. Certain steel compositions cause the zinc iron alloy layer to grow through to the surface of the galvanized coating producing matt grey finish.
Fig 3 Spangled structure of galvanized steel
In some applications the spangle pattern is considered desirable since it gives the product an attractive appearance but in case of automotive steels, the surface roughness and difference in crystal orientation associated with spangle impair the appearance of the steel after painting hence it is not desirable in such steels. For such steels extremely fine spangle pattern is needed.
Spangles develop when the molten zinc adhering to the steel is cooled below the melting point of zinc (around 419 deg 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. This process of transformation is the process of 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 case of zinc, the crystals form with hexagonal 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.
The term ‘nucleation’ defines the process of transformation of randomly arranged atoms of molten zinc into a small organized array of atoms in the seed crystals at the initial stage of solidification. A high rate of nucleation 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.
There is another aspect of the solidifying process which leads to the snowflake pattern in galvanize coatings viz. dendritic growth. Dendritic growth causes the individual solidifying grains to grow into the molten zinc coating with a distinct leading rounded edge. A primary dendrite arm and secondary dendrite arms which grow laterally away from the primary dendrite arm is shown in Fig 3. Dendrites are visible in a galvanize coating because we see a two dimensional version of as cast, dendritic, solidified grain structure.
The rate of growth of the dendrite arms during the solidification of the molten zinc 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 Fig 3, there is a well defined large spangle pattern which shows that the rate of dendrite growth dominated the solidification process leading to a small number of large spangles. The characteristics of such spangles is that they are thickest at the centre and thinnest at the edges (grain boundaries) and hence difficult to smoothen by temper rolling. On the other hand, zinc coatings with smaller spangles have less depressed boundaries and can be smoothen by temper rolling. The nature and rate of dendritic growth during the solidification of molten zinc is greatly influenced by other metallic elements present in the molten zinc.
The size of the spangle depends on zinc chemistry, cooling rate and other factors such as smoothness of the steel base. Fig 4 shows comparison of different spangle sizes. Spangles can be qualitatively classified in the following three categories.
Regular spangle – They are visible multifaceted zinc crystal structure on zinc coated steel. The cooling rate is uncontrolled which produces a variable grain size.
Minimized spangle – These grain patterns on zinc coated steel are visible to unaided eyes. They are smaller and less distinct than the pattern visible on regular spangle. The zinc crystal structure growth is arrested by special production techniques or inhibited by a combination of coating bath chemistry and cooling.
Spangle free – Spangle free zinc coated steels are having 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.
Fig 4 Comparison of different spangle sizes
In case of zinc coating, the most common reason for the well defined dendritic growth pattern is the presence of lead in the molten metal. Presence of lead in the molten zinc results into large spangles since it decreases the solid / liquid interfacial energy in the solidifying coating. This leads to an increase in dendrite growth velocity which results in large spangles. Lead precipitates at the coating surface, and the varying distribution of lead particles across the surface, defines the optical appearance (shiny or dull spangles).
In earlier days zinc coatings contained as much as 1 % lead. During the past 40 years, the percentage of lead has been brought down. Typical concentration of lead these days 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. Presently lead level in the range of 0.05 % to 0.10 % in the molten zinc bath is kept to achieve a well developed spangle pattern. As there are now environmental concerns about the use of lead, practices have been developed to use lead free zinc and to add a small amount of antimony in the molten zinc bath (0.03 % to 0.10 %) for achieving well developed spangle pattern.
In case of lead bearing zinc for getting smoother coating, it is possible to suppress spangle growth by rapidly cooling the coating. This is done by the use of a spangle minimizing device above the zinc bath. These devices direct steam or zinc dust at the surface to rapidly freeze the molten zinc and keep the spangle small. Such technology is not needed in case lead free zinc (lead content normally less than 0.01 % and frequently less than 0.005 %) is used. Lead free coatings give a spangle free surface which provides a high quality finish needed by the automotive and appliances industry.
Lead free coatings have a grain pattern which is barely visible to the unaided eye. Typically the spangles are 0.5 mm in diameter. In such coatings, the grains do not grow by a dendritic mode but by a cellular mode of growth. The grains nucleate on the steel surface and grow outward towards the free surface. Absence of large spangles makes the steel surface shiny and the grain boundary depressions do not exist. Non spangle coating combined with temper rolling makes the steel very smooth which can be painted to give a very smooth finish.
It is not easy to produce non spangle coatings free of lead or antimony because of their influence on the viscosity of the molten zinc. It is difficult to avoid sag and ripples in the zinc coating due to higher viscosity of molten zinc when lead or antimony is not present. The thicker is the coating; greater is the tendency to form sags and ripples. However automotive and appliances industry needs thinner coating and their products are made on high speed lines which allows producers to use lead free coating baths for avoiding spangles and still attain a ripple free coating.