Ask Peter: About Galvanizing

By October 13, 2016Uncategorized

Guard-All Building Solutions Ask Peter logoIn this paper we discuss and compare conventional (Black Steel) Hot Dipped Galvanization of structural hollow steel sections (HSS) to the New Triple coated Inline Galvanized HSS manufactured primarily by Allied Tube and Conduit in Harvey, Illinois. There has been much controversy in the Tensioned Membrane Manufacturing industry as to which product is more suitable for use in highly corrosive environments such as Salt storage, Composting and Waste handling applications. Also, which product best addresses the needs for longevity, appearance, functionality, strength, cost and environmental friendliness?

About Galvanizing

“Every year, steel corrosion costs the economy around $2.2 Million. Corrosion, also known as rust, is a natural process that occurs when steel is exposed to the environment. It can be slowed, however, with a protection technique called galvanizing, which is the coating of steel with a layer of zinc to slow the corrosion process. Zinc is used to protect steel because the zinc layer rusts more easily than steel but also rusts more slowly than steel. Therefore, the underlying steel remains safe from rust for many years.”

Comparison

New Technology Versus Old Technology

“Both galvanizing processes require a complete immersion of the steel surface in molten zinc metal yet their surface aesthetics are widely different and their functional abilities can vary greatly depending on the specific application.”

“So what makes Allied’s triple-coated, in-line galvanized products (Zinc Flo-Coat® and Gatorshield®), a decidedly better alternative to batch hot-dipped galvanized (HDG) pipe for so many applications?”

Surface Appearance

“In general, the surface of HDG pipe has a “frost-flower” or “spangled” pattern. The surface quickly develops a “gray patina” and patches of white rust are frequently noticed on HDG pipe after short-term, outside storage. A range of localized surface defects is often visible on HDG pipe including: bare spots, dross protrusions, flux, ash inclusions and general roughness.” Appearance notwithstanding the roughness of the coating can impact the integrity of tensioned membranes which are pulled over the surface protrusions during the installation process, causing serious damage.

“Continuously inline galvanized “triple-coated” tube exhibits an excellent combination of smoothness, brightness and surface gloss that is maintained for a long period.” This intrinsic property makes for ideal conditions when installing tensioned membranes. “Allied’s inline galvanizing process uses 99.99% pure zinc at relatively high line speeds (300–900 ft/min). This high-speed process applies a chromate conversion coating over the zinc surface almost immediately after the galvanizing operation. This prevents the highly reactive and freshly galvanized surface from any appreciable oxidation prior to conversion coating. Application of the third coat, a clear organic topcoat, and curing of the same film at high speed, ensures proper sealing of the conversion coating.”

Corrosion Protection

“One of the major benefits of the triple-coated product is the synergistic performance of the chromate and clear coating together in protecting the zinc surface. Some of the coatings are formulated with special corrosion inhibiting chemical additives to delay white rust growth beyond their basic barrier film properties. Because of the substantial slow down of initial white rust formation due to this synergistic performance, the triple- coated products can last for the same period or for even a longer period of exposure compared to standard HDG pipe before they exhibit the same level of corrosion failures. [Numerous independent salt spray studies have shown have demonstrated this result].”

coatings-illustration-web

“The corrosion rate of a galvanized surface in an outdoor environment is greatly influenced by humidity and temperature. However, zinc corrosion in industrial and coastal areas is highly accelerated due to the presence of sulfur dioxide, sulfate compounds and also by the chloride compounds present in the air, soil and water. Condensed moisture or rain can often accelerate such a chemical based corrosion. Due to the presence of a clear barrier coating on the surface, this attack is substantially slowed down by triple-coated products.”

Other points to note are that depending on the reliability of the Hot Dip Galvanizer there is no guarantee that the structural component material will be adequately dipped (submersed) or as to the quality of the zinc and foreign matter in the dip tanks, all of which can have a profound effect on the corrosion rate. Inline galvanization with its electrolytic and triple coat application process eliminates these variables.

Ease of Fabrication

“Unlike HDG pipe, the inline galvanized product has a very thin iron-zinc alloy layer that acts as a metallurgical bond between the outer layer of pure zinc and the base steel. This thin alloy layer is carefully maintained by controlling a number of variables in the production process. Because of its composition and controlled thickness, the finished pipe can take severe fabrication without flaking any zinc coating.”

“The HDG process typically has three distinct alloy layers between the base steel and the zinc coating on the external surface. The inherent alloy compositions and configurations of these alloy layers make them hard and brittle. As a result, severe fractures can occur during fabrication of HDG pipe. The zinc coating may even flake off from the product surface during simple fabrication such as bending.”

Product Strength

“In the continuous inline galvanizing process, after the pipe has been formed, welded, cleaned, galvanized and quenched, it goes through a sizing operation which cold works the pipe and the zinc coating. This cold working substantially increases the yield and tensile strength of the finished product – typically 10% to 20% over that of the base steel, depending on the degree of cold reduction ratio and other factors.”

“For this reason, the thinner wall, lighter weight, triple-coated pipe can easily support a much higher load as opposed to standard heavy wall HDG pipe. This quality of the triple-coated product offers an economic advantage (more finished pieces per ton) along with structural superiority.”

Clear Topcoat and Ease of Painting

“Coating formulation and its application technologies have been greatly improved over the past ten years. Inline tube producers are now using high performance polymeric coatings with low or negligible VOC content and with additives to enhance flexibility for easier fabrication. Conversely, HDG pipe is not generally coated, and if they are, the application consistency is never achieved in this off-line process. Heavy film thickness and unevenness of the topcoat creates aesthetic problems along with fabrication and painting problems.”

“The clear topcoat does not need special surface preparation and can be easily painted with most standard industrial and decorative paints including powder coating.”

“The batch HDG pipes are not easily painted due to a number of surface imperfections like excessive roughness, lumpiness, dross protrusions, blisters, flux and ash inclusions etc. Extensive surface preparation work is needed prior to any paint job over HDG pipes. Application of pigmented decorative coatings over HDG products will not produce an acceptable finish without extensive secondary processes.”

Process Control and Consistent Quality

“The continuous inline galvanizing production method requires over 30 distinct processes to work in tandem and synchronous harmony. Due to the inter-dependency of these successive operational stages, a stringent process control system is implemented for a continuous triple-coated product line. The inline high-speed operation (up to 900 feet per minute) requires that the process conditions be carefully monitored and adjusted (if necessary).”

“The finished tube undergoes the identical process condition for its entire length because of extremely short duration in any tube mill section. This consistency of product from one end to the other is guaranteed due to process stability and speed.”

“Conversely, the batch hot-dip process cannot precisely maintain the exact conditions for each of the lifted bundles from one section to the other. The process control window is relatively wide as often evidenced by the surface appearance and non-uniformity of the coating and the variations noted from piece to piece. Although these products have a greater zinc coating thickness, it cannot be controlled in a stringent fashion.”

Zinc-Rich Interior Coating

“For outdoor applications, the inside surface is always coated with a true zinc-rich coating bearing 87% to 93% zinc dust. This type of formulation provides a high level of corrosion protection for the inside tube surface.”
Environmental Friendliness

“Prime Western grade zinc, with a lead content of up to 1000 PPM (parts per million), is typically used in the batch hot-dip galvanizing process.”

“The inline galvanized process uses 99.99% pure electrolytic zinc that is virtually free of all lead and cadmium.”

Summary

The Benefits of New Technology Utilizing the Inline Galvanization Process

  • Reduces the amount of introduction of Lead into the environment. (Much cleaner process.) Resulting in greater sustainability versus the increasingly EPA-regulated Hot Dipping process.
  • Allows for the manufacture of higher yield strength steel products with greater structural capacity, less weight and lower costs. HDG hollow structural sections are costlier.
  • The finished product is triple galvanized and top coated with a barrier film for added corrosion resistance. Something that is rarely available in the traditional HDG method.
  • Unlike Hot dipped hollow steel sections there are no bleed holes required that allow for the infiltration of contaminants such as Water, Sulphur and Salts. Thereby contributing to the possibility of early onset corrosion.
  • Welding of Inline Galvanized components can burn away portions of the interior zinc coating during fabrication, however the absence of bleed holes greatly reduces the in migration of contaminants and thereby the possibility of corrosion.
  • Inline Galvanized material has been empirically shown to equal and outlast conventional Hot dipped Galvanized material in salt spray tests.
  • Inline Galvanized steel truss works assist in the protecting and ease of installation of tensioned membrane roof systems. HDG truss works exhibit a rough, randomly textured finish that can damage tensioned membranes during installation.

Sample Identification
G-60 Pregalvanized Tube
G-90 Pregalvanized Tube
G-115 Pregalvanized Tube
Galvanized Schedule 40 Pipe
Triple Coat Competition A
Allied Zinc Flo-Coat®
Allied Gatorshield® Square
Allied Gatorshield® Round

Hours of Exposure
192
480
576
576
744
840
1848
2016

Observations
First appearance of red rust
First appearance of red rust
First appearance of red rust
First appearance of red rust
First appearance of red rust
First appearance of red rust
First appearance of red rust
No red rust was observed

Galvanizing options by the numbers comparison:

1
Strength
Coatings
Wall Thicknesses
Custom Mill Lengths
Weld Friendliness
Zinc Thickness Control
Mechanical Suitability
I.D. Weld Seam Coating

Flo-Coat/Gatorshield
50 ksi
3 Layers
Excellent
Any Length
Excellent
Good
Excellent
Excellent

Hot-Dip (Schedule 40)
30 ksi
1 Layer
Limited
Limited
Poor
Limited
Poor
Good

Pre-Galvanized
40 ksi
2 Layers
Good
Any Length
Fair
Limited
Good
Poor

Conclusion

The triple Coated Inline Galvanization process employed by Allied Tube and Conduit meets and exceeds the requirements of the End users and Manufacturers of Tensioned Membrane structures. The concerns relating to corrosion resistance, longevity of structural components, appearance, functionality during installation, strength of steel, cost of materials and environmental friendliness have all been addressed. Furthermore, one need only examine the market place to see that the largest and most successful manufacturers of tensioned membrane structures have already tested this hypothesis and proven it to be true.


References

  • The Making, Shaping and Treating of Steel – 10th edition, published by AISE; pages: 1176 –1178
  • Zinc: Its Corrosion Resistance: 2nd edition, Jan 1986: page-96, published by ILZRO, Inc. (5% red rust exposure study in Sheffield, England — an industrial environment), page 23.
  • Galvanizing Alloy & Technology – by Goodwin and Radtke; published by ILZRO, 2nd edition, 1983, pages 6 – 17.
  • Galvanizing and Coating of Tubular Products and Shapes, Tube and Pipe Technology, Jan/Feb 2000, pages 92 – 95.
  • Research work conducted by Battelle-Columbus Laboratories and by Professor Henry Leidheiser at Lehigh University, confirmed the importance of surface homogeneity, freedom from tramp elements, and preferred zinc crystal orientation (with no zinc spangles) in providing the best substrate for good paint adhesion.
  • Batch Hot-Dip Galvanizing versus Continuous In-Line Galvanizing – Jim Krusenoski: Published: Feb 2003
  • Taking Action against Hot-Dip Galvanizing Pollution – Kelsey Behrens. Duke University, 2012 Blue Ridge Environmental Defense League.
  • Project Number 76-618D – Scientific Laboratories, Inc. Salt Spray Tests:05/05/94

Click here to download the Ask Peter: About Galvanizing white paper. If you want to know more about galvanizing, ask Peter by calling him at 1-877-397-1594.

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