Page 235 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

PROTECTIVE COATINGS 213

80
Thickness of
zinc coating
70
0.5 mils
60 2.1 mils
10 mils
Service life, years 40
50 mils
50

30
20

10
0
Rural Tropical marine Moderate Heavy
industrial industrial
Environment
Figure 4.3 Expected service life of galvanized steel under different atmospheric conditions.
(Reproduced with permission of National Association of Corrosion Engineers from A User’s
Guide for Hot Dip Galvanizing, TPC9, Houston, TX, 1983 (9.)

nonalloyed coatings, although the protection mechanism is essentially the same. Both
alloyed and nonalloyed zinc coatings act as barriers to the corrosive environments and
as sacrificial anodes when the barrier is breached.
The extent of protection by galvanizing depends entirely on the thickness of the
galvanized layer. Galvanizing is unique in the sense that empirical data accumulated
over the years provide guides for estimating the service life of galvanized coating
under a wide range of exposure conditions. Figure 4.3 shows the service life predic-
tions for several standard environments (9). Hot dip galvanizing is the most common
process and consists of dipping the steel in a bath of molten zinc.
The galvanizing industry in the United States is divided into two classes; namely,
fabrication and sheet galvanizing. The fabrication business deals with structural com-
ponents such as piping, I-beams, poles, handrails, and other heavy duty products. The
sheet business deals with galvanized sheet metal for equipment, roofing, panels, and
other nonstructural steel applications.
In 1998, fabrication industry sales were nearly $750 million (American Galvaniz-
ers Association, Personal communication, Apr. 2000.). The profitability of galvaniz-
ing is directly tied to the price of zinc that remained fairly steady at $1.06–1.15/kg
over the past 10 years. The improved efficiencies have offset the rising material/labor
costs.
According to the American Zinc Association, sheet and strip galvanizing
accounted for 540,000 metric tons of zinc (Metallized coatings supplier, Personal
communication, 2000.).
The commodity price of zinc was $1076 per metric ton, with an additional average
premium of $60 per metric ton paid by manufacturers.