The Science of Forming


 

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Zinc-Coated Steels--Part 1: Electrogalvanizing, Hot-Dip Galvanizing and Galvannealing

By: Daniel J. Schaeffler, Ph.D.

Danny Schaeffler, with 30 years of materials and applications experience, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solutions (EQS). EQS provides product-applications assistance to materials and manufacturing companies; 4M teaches fundamentals and practical details of material properties, forming technologies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bachelor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA. Danny Schaeffler Tel. 248/66-STEEL E-mail ds@eqsgroup.com: or Danny@learning4m.com

Friday, November 1, 2019
 

Steel parts rust unless steps are taken to prevent or slow down the corrosion reaction. One such approach, galvanizing, involves applying a zinc coating on the steel surface. Batch galvanizing involves dipping fully formed parts into a molten zinc bath. A wide variety of parts and sizes, from small fasteners to long beams and poles, undergo galvanizing in this manner. This approach works best on thicker parts that will not distort from the heat of the zinc bath, and on parts where the costs of the extra manufacturing step and associated handling are justified.

Galvanizing becomes more economical when performed at the steel mill and incorporated into a standard work flow. Here, base metal and coating properties are more uniform, and the galvanizing process can be adjusted to optimize specific properties. Mill galvanizing comprises two approaches: continuous hot-dip galvanizing and electrogalvanizing, with each having different variants.

Electrogalvanizing


Zinc grains, or spangle, are seen on hot-dip galvanized surfaces but not on steel with electrogalvanized or galvannealed coatings.
Mills produce electrogalvanized (EG) steel using a process very similar to the electroplating experiments you may have performed in high school. The starting feedstock is a clean, fully annealed cold-rolled steel coil. Electrogalvanizing occurs at room temperature, so property changes from heating or cooling should not occur. Electrogalvanizing is a continuous process, with the tail end of one coil welded to the head end of the next coil.

Each plating cell adds only a small amount of zinc to the surface, leading to very tight coating-weight control. A 60-G (60 g/m2) zinc coating measures about 8 microns thick. For a process using 16 plating cells, each cell applies only 0.5 microns of zinc. Cells can be turned on or off, depending on the ordered coating weight.

Electrolytic application of zinc-nickel (Zn-Ni) and zinc-iron (Zn-Fe or EGA) alloys is possible, but pure zinc represents the most common electrogalvanized coating.

The electrogalvanizing process consumes a lot of electricity, but produces an excellent surface finish. Several auto-makers use electrogalvanized steels as their products of choice for skin panels.

Hot-Dip Galvanizing

A clean full-hard coil exiting a cold-rolling mill comprises the feedstock for a continuous hot-dip galvanizing line (CGL). The CGL combines inline annealing, hot-dip galvanizing and temper passing. As seen in the electrogalvanizing process, hot-dip galvanized (GI) coils are welded together at the ends to create a continuous feed through the line.

Exiting the annealing furnace, the steel coil then passes through the galvanizing bath. The liquid metal, primarily zinc, includes small amounts of other elements, such as aluminum, that control the alloying reaction between the zinc and steel substrate. Unlike the batch process for full parts, only a small portion of the coil is in the bath at any given moment.

The coil passes through the molten zinc at speeds to 600 ft./min. Zinc coating weight is controlled by air or nitrogen knives blowing off excess liquid zinc as the coil emerges from the bath. Zinc remaining on the surface solidifies, just like water vapor crystalizes on a window during a cold winter’s day. These zinc crystals are called spangle, and give galvanized buckets and grain silos their characteristic look (see the figure).

Molten zinc chemistry and cooling practices used at the galvanizing line control spangle size. Spangle shows through on a painted surface. For surface-critical applications, order a minimum-spangle surface and confirm no print-though of the zinc coating. This may mean selecting a different type of galvanized coating should the surface appearance not be suitable for the application.

Galvannealing

A furnace may be installed after the zinc pot in hot-dip galvanizing lines. This inline furnace causes the steel and zinc to inter-diffuse, creating a hot-dip galvanneal (GA) coating. Unlike hot-dip galvanized surfaces, galvannealed steels can be produced for surface-critical applications.

Coating composition averages approximately 90-percent zinc and 10-percent iron, but is not uniform through the coating thickness. The greatest iron concentration occurs at the former interface between the steel and coating, and decreases closer to the surface, where the zinc concentration is the greatest. Different zinc-iron phases are produced, depending on the relative concentrations at each distance from the surface. These phases have different properties, including hardness, friction and powdering resistance.

All metal alloys corrode, zinc coatings included. Whereas the corrosion product of electrogalvanized and hot-dip galvanized coatings is white, iron in galvanneal coating makes the corrosion product appear red. This discoloration does not necessarily indicate rusting of the underlying steel.

For topics you’d like to see addressed in future columns, email me at scienceofforming@eqsgroup.com. MF

 

See also: 4M Partners, LLC, Engineering Quality Solutions, Inc.

Related Enterprise Zones: Materials/Coatings


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