Zinc Coatings on Handrail Tubing: A Comparative Analysis

When applying zinc metal to steel handrail tubing for
corrosion protection, batch hot-dip galvanizing and in-line, continuous
galvanizing are the two most common methods.

Batch hot-dip galvanizing handrail tubing involves loading a
rack or overhead lifting fixture with 50 or more cut-lengths (usually 20¢
or 40¢) of tubing, thoroughly cleaning the steel and then immersing the
entire load into a molten zinc bath. This process coats all exterior and interior
surfaces. Once the newly galvanized tubing cools, it is ready for shipment to
the job site or transport to a paint contractor for paint application.

In-line, continuous galvanizing involves feeding the tubing
through a bath of molten zinc and then applying a conversion coating to prevent
the formation of naturally occurring zinc oxide and hydroxide. Usually, a
topcoat of a clear, inorganic, polymeric paint is applied over the conversion
coating. It is important to note that only the outside of the tubing goes
through this process. The tubing's inside surface receives only a coating
of zinc-rich paint.

Fundamentally, zinc metal will provide some level of
corrosion protection to steel used in a myriad of applications. However,
understanding the metallurgy, bond strength, corrosion mechanisms and testing
of these two galvanizing methods may lead to better design decisions for
particular applications.

Comparison

Zinc-coating thickness-For
standard 0.25" wall tubing, the batch hot-dip galvanizing process applies
a minimum of 3.0 mils (1.7 oz/ft2) of protective zinc. The in-line process
applies about 0.9 mils (0.5 oz/ft2). As the Service Life chart (Figure 1)
indicates, the batch-process-produced zinc coating will perform in use without
maintenance approximately three times longer than the in-line-produced zinc
coating.

Taking into consideration the organic clear-coat applied to
the handrail galvanized in the continuous process, it is important to note that
applying a barrier protection system over a galvanized coating (no matter what
process is used) results in a service 1.5 times the sum of the service life of
the two systems. While applying a barrier coating to the in-line galvanized handrail
will enhance the service-life, it still does not match that of the unpainted
batch-process galvanized handrail. In addition, since the interior of a
continuous-process handrail is not provided a metallurgically bonded zinc
coating, it is difficult to accurately compare service-life expectancies.

Bond strength-The
batch hot-dip galvanizing process generates a metallurgical reaction between
the molten zinc and the iron in the steel tubing. This reaction results in the
formation of three zinc-iron alloy layers metallurgically bonded to the base
metal, topped by an impact-resistant pure zinc outer layer. The metallurgical
bond between the zinc-iron alloy layers and the steel is measured at
approximately 3,600 psi. This bond strength is especially important for the
interior surface of batch-galvanized tubing where the coating resists corrosion
caused by trapped water and moisture. By comparison, the zinc-rich paint
applied to the inside of tubing galvanized via the in-line, continuous process
has a mechanical bond in the range of 300-500 psi. Because of this lesser
bond strength, it is possible for trapped moisture to make its way between the
zinc-rich paint and the steel tubing, causing rust formation and eventual
flaking and failure of the paint system.

Corrosion protection throughout style='font-weight:normal'>-Batch hot-dip galvanizing metallurgically
bonds zinc to all surfaces of the tubing. In fact, batch galvanized tubing and
pipe often is used in fabrications with hot-dip galvanized vessels and tanks to
store a wide range of liquids. The zinc-rich paint applied to the inside of
tubing galvanized via continuous in-line processes provides some corrosion
protection, but performs less effectively when exposed to liquids. In such
cases, moisture makes its way to the base steel of the tubing through cracks,
damaged areas or porosity in the zinc-rich paint. This allows corrosion to
begin where it is not apparent and is not easily remedied.

Inspection and testing-Salt-spray
or salt-fog tests often are used to compare the corrosion protection provided
by various coatings. Simply put, galvanized coatings, in use, require wet and
dry cycles in order to develop a naturally occurring, protective zinc-carbonate
patina that increases the coating's long-lasting corrosion protection.
Laboratory salt tests are not reflective of real-world situations. So, it is
unrealistic and ill-advised to use salt spray tests to develop a realistic
comparison between batch hot-dip galvanized tubing (without an additional
barrier protection system) to in-line galvanized tubing (that is additionally
conversion coated and top-coated with inorganic polymeric paint).

Durability?The
three-plus mils of zinc in the zinc-iron alloy layers applied via the batch
hot-dip galvanizing process are actually harder (250 DPN) than the base steel
(159 DPN). It is extremely difficult to damage the thick zinc coating to the
extent that corrosion protection would be affected. In comparison, the
inorganic polymeric paint covering the thin layer of zinc applied in the
in-line process damages as easily as any other paint, and is particularly
susceptible to deterioration caused by the sun's ultraviolet rays. Any
damage to the paint covering exposes the thin layer of zinc, resulting in a
protection system that lasts only as long as the zinc thickness will allow, or
about a third as long as the batch-galvanized tubing. Additionally, surface
contaminants and normal wear and tear are particularly threatening to topcoat
paints.

Touchup and repair-Batch
hot-dip galvanizing is done after the tubing is cut to length and protects all
surface areas. Touchup is rarely required after transport and field
construction because the thick zinc coating is hard and bound tightly to the
steel surface. In-line continuous galvanized tubing is cut to lengths after galvanizing.
These unprotected ends are susceptible to corrosion if not touched up with zinc
solder, metalizing spray or zinc-rich paint. They also are potential sites of
structural failure.

Clearly, an architect's or engineer's decision
to use batch hot-dip galvanized tubing or in-line continuously galvanized
tubing depends on the design life desired for a handrail project and the funds
available for future maintenance. Additional factors to consider include
exposure to ultra-violet rays, moisture and durability.

American Galvanizers Association

The mission of the American Galvanizers Association (AGA) is
to preserve, enhance and protect the existence of the hot-dip galvanizing
industry throughout the world, and to fulfill the needs of the industry for
technical, marketing and other services.

The AGA provides information on the most innovative
applications and state-of-the-art technological developments in the field of
galvanizing. The AGA maintains an extensive technical library and provides
multi-media seminars in order to educate and train the specifying community.

The Association celebrates over 65 years of preserving the
past, enhancing the present and protecting the future.

The AGA provides a wide variety of interactive support
services and educational resources to members and specifiers to educate and
inform them on numerous topics.

Support Services

Technical Library-Consists
of dynamic research information on galvanizing plant operations, safety-related
issues, environmental regulations, and all other plant-related issues.

Image Library-Contains
over 5,000 visual images, including slides, videotapes and photographs (both
color and black & white) available to members. These resources are used to
market the merits of galvanizing.

Operating Assistance-The
Technical Department offers plant operation information regarding common issues
such as stormwater permits, cost reduction, productivity, compliance with
regulations of the Clean Air Act, procedures to address quality control issues,
OSHA regulations and EPA compliance.

Technical Research and Development style='font-weight:normal'>-The AGA funds research and development
programs through the International Lead Zinc Research Organization (ILZRO) and
the International Zinc Association (IZA). AGA contributes to the development of
the program plans and participates in the working groups that determine the
direction of the research.

Technical Specifications-The AGA participates in the American Society of Testing and
Materials (ASTM) committees that initiate and review materials, and process
specifications used in the galvanizing industry.

Publications-The
AGA maintains and constantly updates a wide variety of publications on topics
in the field of galvanizing. These are available to members to use in their own
marketing efforts. All are focused on educating the engineering and
architectural communities

HOT-SPEC Line-800.HOT.SPEC
(468.7732) is a service allowing specifiers, engineers, architects and other
professionals to call the AGA toll-free and have galvanizing questions answered
by our technical experts.

Specifications Assistance-Every galvanizer is confronted by a specification he/she has
never before seen. The AGA staff has the expertise and experience to provide
assistance.

American Galvanizer-The
AGA publishes a members-only newsletter that provides the latest information
about the AGA activities and developments in the galvanizing industry.

GalvanizeIt!-This
is an educational seminar designed for engineers, architects and other
specifiers. The goal is to increase awareness of the after-fabrication hot-dip
galvanizing industry. The seminars are held in major North American cities.

American Galvanizers Association

6881 S. Holly Circle, Suite 108, Englewood, CO 80112

Phone: 720-554-0900. Fax: 720-554-0909. Web:
www.galvanizeit.org

Philip G. Rahrig is the executive director for the American Galvanizers Association, Englewood, Colo.

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