December 5, 2016
By Gary Dallin, P.Eng.
The continuous hot-dip coating process for steel sheet products—like the door on the left—is widely used and employed in all corners of the globe. Most users of continuously hot-dip coated zinc-coated steel sheet (i.e. galvanize) think of it as a product with a bright, silvery metallic sheen. However, there is a version that has a dull, matte appearance—it is called galvanneal. While both are made by the same process (Figure 1), the main difference in producing a hot-dip galvannealed coating and a hot-dip galvanized coating is an additional step in the production process.
By making a galvannealed coating, the strip, with its still liquid zinc coating, is further heated by passing it through a furnace directly above the coating bath. Heated to approximately 538 to 565 C (1000 to 1050 F), and by holding the strip at this temperature for a specific amount of time, the zinc coating alloys with iron through diffusion between the molten zinc and iron from the steel strip. The result is a coating that is an alloy of approximately 90 per cent zinc and 10 per cent iron. The iron percentage varies throughout the coating thickness, from as low as six per cent at the surface, to as high as 23 per cent at the steel interface.
Galvanneal coatings have no free-zinc present and a low-lustre matte appearance, versus the metallic sheen of galvanized coatings. The final bulk iron concentration depends mostly on the heating cycle, since the rate of diffusion is primarily a function of time and temperature. The zinc and steel chemistries can also affect the alloying behaviour, but are secondary to the heating cycle.
A galvanize coating is essentially pure zinc, with between 0.20 and 0.50 per cent bulk aluminum, which is mostly concentrated in a thin inhibition layer next to the steel. The aluminum does not affect the corrosion performance, but rather enables good coating adhesion, which is required when the coated sheet is eventually formed. Galvanneal coatings contain nine to 12 per cent bulk iron, along with the above-mentioned aluminum, which becomes more uniformly spread through the coating thickness than in the case of galvanize. However, it is not uniformly distributed—rather, it is combined with zinc in three distinct zinc-iron phases (Figure 2). It is also important for good appearance and press formability the top zeta layer contain no less than five per cent iron or there will be a risk of free-zinc on the surface. The higher bulk aluminum content through the three alloy layers is a result of its diffusion outward from the inhibition layer next to the steel where it was concentrated prior to the reheating cycle.
Galvanneal coatings are hard and brittle, so during bending and press forming, coating cracks and powdering are always present to some degree. To balance performance in forming versus coating line throughout, each coating line must develop practices to produce the optimal coating properties for the product’s particular use.
Production of galvanneal
To produce galvanneal, a moving sheet is immersed in the zinc bath and a thin, aluminum-bearing, inhibition alloy layer forms at the zinc-steel interface (Figure 3). As the strip emerges from the bath it drags excess zinc with it, which the air knives remove to obtain the desired coating weight. The strip, with the still liquid zinc coating, enters the galvanneal furnace about 3 to 4 m (10 to 15 ft) above the gas wiping knives.
Before the zinc can solidify, reheating of the strip begins. As the temperature rises, a zinc-iron diffusion reaction restarts and breaks down the aluminum-zinc-iron inhibition layer formed in the zinc pot at the steel-zinc interface. After five to seven seconds of heating, and up to about 10 seconds of soaking, enough iron diffuses into the coating to convert it to a dull matte grey appearance.
Since the early 1990s, coating lines have used induction galvanneal furnaces. Typically, they have three or more zones, which can reheat the strip from about 463 C (865 F) to as high as 565 C (1050 F) in the few seconds available. Older galvanneal coating lines use gas-fired furnaces, making it difficult to obtain a well-controlled alloying reaction. Induction galvannealing is inherently different than gas-fired convection or radiation versions because the heat for diffusion comes from within the strip for the former, not externally as with the latter.
Following the heating furnace zones, an electrically heated holding zone is used to optimize the iron content of the coating. The reactions that convert a liquid zinc coating to a solid zinc-iron coating begin at the steel interface and depend on factors, including:
These variables are not necessarily independent, and each coating line has to determine the protocol to produce a product for a particular use. For instance, a higher aluminum in the coating requires a higher reheating temperature or a longer soak time. Too high a temperature or too low an aluminum will result in high iron and excess powdering. Stabilized interstitial-free, ultra-low carbon grades react faster than plain carbon steel, and steels with higher phosphorous levels react slower in the galvannealing furnace than low phosphorous steels. (Figure 4 is a schematic representation of the galvannealing process. (Figure 5 depicts the stages the coating goes through during its conversion from zinc to zinc-iron.)
Coating weldability, paintability, formability, and adherence
The benefits of using galvanneal over galvanize are improved spot-weldability, ease of painting, and coating adhesion.
Zinc-iron alloy coatings generally have better spot-welding characteristics than pure zinc coatings. The coating’s higher electrical resistance, along with its higher hardness and melting point, allow good welds to be obtained at lower currents with longer electrode life to the extent galvannealed sheet spot-welds very much like cold-rolled sheet.
Performance of galvanneal under paint is synergistically improved because of the excellent bond formed between the paint and surface of the coating. The reason for the good bond is because the paint can mechanically lock with the zinc-iron crystals on the surface, in addition to any chemical bonding that may occur (Figure 6). This surface is why galvanneal coatings can be painted directly without the need for a primer. Compared to galvanize, galvanneal generally exhibits less undercutting corrosion beneath paint at exposed edges, scratches, or other breaches in the paint. To achieve maximum corrosion protection, galvanneal is usually treated with zinc phosphate before painting.
A galvanize coating is quite soft, and can be easily scratched. A galvanneal coating is very hard, and thus not as easily scratched when handling. The harder zinc-iron alloy powders on deformation, unlike pure zinc coatings that can gall (i.e. easily gouge) and flake.
The good frictional behaviour and ductility of zinc, combined with the excellent adhesion achieved between the coating and steel, allows galvanized sheet to be formed into many intricate shapes without any loss in coating adhesion.
Even though the galvanneal alloying reaction results in a hard, relatively brittle coating, it can be bent, stretched, and drawn when correct sheet manufacturing and part-forming procedures are used. Many parts made from galvannealed sheet require a deep drawing operation. During this process, galvannealed sheet typically exhibits some ‘powdering’ of the coating as a result of high compressive strain that can occur during the forming operation. By proper control of the steel manufacturer’s processing practices, combined with the proper setup of the drawing dies and the use of appropriate drawing lubricants, the amount of powdering can be minimized and excellent performance can be achieved.
The powdering of a galvannealed coating during forming is a function of many parameters, mostly relating to the steel manufacturing practices previously discussed. The most important characteristic of the coating affecting the powdering tendency is the coating thickness. The amount of powdering rises directly as the thickness increases. For this reason, the maximum coating weight for galvanneal is restricted to ZF180 (180 g/m2) [A60 (0.60 oz/sf)]. For many applications, an A60 coating weight is too thick to provide acceptable powdering performance, and many users specify ZF120 (120 g/m2) [A40 (0.40 oz/sf)], or less. The tendency for A60 to powder should be considered when selecting a coating weight.
The second largest influence on the adherence of galvanneal coatings is the iron percentage in the coating. The bulk iron level must be within a range of nine to 12 per cent to perform satisfactorily in most forming operations. Iron levels of less than nine per cent result in softer coatings containing free zinc at the surface. This will change the co-efficient of friction and interfere with draw-ability and stretch-ability of the sheet. Iron levels over 12 per cent result in harder coatings creating excessive powdering to the point of fouling the forming dies. Some users of galvanneal prefer iron levels near either the upper or lower end of the above range due to the nature of their forming processes. Producers of galvanneal must understand how to control coating iron levels on their equipment to provide satisfactory product for the range of their customer base.
Generally, there are no significant differences in the properties of the steel substrate, whether it is galvanized or galvanneal. Any differences in forming behaviour (e.g. splits) are usually related to the different nature of the two metallic coatings. For example, the substantial difference in coating hardness can necessitate changes to the stamping parameters (i.e. die type, die clearances, hold-down forces, and lubrication type).
The thickness of a galvanize coating has a direct influence on the corrosion performance and life of the product (i.e. the thicker the coating, the longer its life). The corrosion performance of a galvannealed coating is more complicated than its galvanized counterpart. Almost all applications of galvannealed sheet involve painting after fabricating. The primary reason is that, when unpainted, the presence of 10 per cent iron in the coating can lead to a ‘reddish-coloured’ corrosion product when the surface becomes wet. The colour is related to corrosion of the iron within the coating and does not necessarily signify that corrosion of the steel substrate is occurring.
This discolouration is purely a cosmetic effect due to the iron in the coating.
Nevertheless, many users find this staining unacceptable, thereby requiring most applications for galvanneal be painted after fabrication. For this reason, most corrosion studies of galvanneal relate to it being painted. Since paint systems have a direct influence on product life, the corrosion performance of galvanneal is generally not compared with bare (unpainted) galvanize.
The importance of the galvanneal coating thickness is often revealed at sheared edges or scratches (i.e. places where the steel and metallic coating are directly exposed to the corroding environment). At such discontinuities, a thicker coating can improve resistance to “undercutting” the paint film (i.e. a thicker galvanneal coating can slow down degradation of the paint, as evidenced by edge ‘creep-back’ corrosion and eventual total loss of paint adhesion). To maximize service life, it is therefore advisable to use as thick a galvanneal coating as potential powdering problems allow.
Considering relative corrosion rates
A pure zinc coating (galvanize) provides a high degree of galvanic protection to exposed steel such as at sheared edges and scratches. A galvannealed coating is about 10 per cent less galvanically active in most environments because it contains 10 per cent iron. Its bare corrosion rate may, in fact, be less than pure zinc, but is masked by the reddish-coloured corrosion products that can form on the surface in the presence of water. Galvanneal will maintain its original matte grey appearance if it never gets wet.
The more galvanically active galvanize coatings could be quickly consumed when acting as a galvanic protector to any exposed steel. The less galvanically active galvanneal coatings do not offer as much galvanic protection, and therefore are not as rapidly consumed during the corrosion process. It is interesting to note that automotive galvanneal coatings equivalent to about A30 have performed just as well as approximate G50 galvanize coatings with respect to the corrosion-resistance of painted outer auto body panels.
The specific needs of the application and the corrosion performance requirements dictate which coating will perform best. The other requirements for the application, such as the weldability and the specific capabilities of each product manufacturer’s paint shop need to be considered when deciding which product is best for a given situation.
Galvanneal has enjoyed immense growth since first introduced over 50 years ago as a zinc-coated steel sheet product that guarantees excellent paint adherence. The synergy of the excellent bond between a galvanneal coating and paint provides a very formable and weldable sheet metal product with superior corrosion resistance.
Gary Dallin, P.Eng., is director of the GalvInfo Center, a program of the International Zinc Association. Previously, he was employed as a metallurgist in the Canadian steel industry. Dallin has 48 years of experience with galvanized and prepainted steel sheet products, is a registered Professional Engineer in Ontario, and is active on ASTM International Committee A05 on metallic-coated steel products. Dallin can be contacted at firstname.lastname@example.org.
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