hot-dip galvanizing - Fastener World Inc.

Technology
HOT-DIP GALVANIZING
for Fasteners
by Ni-Yuan Ma, Senior Engineer of Surface Engineering
& Heat Treatment Section, Micro/Meso Mechanical
Manufacturing (M4) R&D Department, MIRDC
Chung-I Chen, Engineer of Industrial Research, MIRDC
Service Life Chart for Hot-Dip Galvanizing
300
250
200
Service Life (yrs)
294
25
50
75
100
125
3.9 mils hot-dip galvanizing layer is
the minimum thickness of 1/4＂ thick
structural steel
150
200
Rural
Suburban
Temperate Marine
150
Moderate Industry
100
Tropical Marine
50
0
175
1
2
3
4
5
6
7
8
Average Thickness of Zinc (Upper: µm, Lower: mils)
(Service life is defined as the time to 5% rusting of the steel surface)
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Technology
Iron based material is made up of 6-25%. Hot-dip galvanizing thickness
is affected by immersing time, taking-out speed from molten zinc and steel
chemical composition.
Inspection of hot-dip galvanizing fasteners, steel products and structural
pieces have been listed in details in A153/A153M (Standard Specification
for Zinc Coating (Hot-Dip) on Iron and Steel Hardware) or G164-M92 (HotDip Galvanizing of Irregularly Shaped Articles). Chart 1 indicates that the
relationship between zinc thickness and service life of hot-dip galvanizing at
different environments.
Quality of hot-dip galvanizing fasteners must conform to ASTM A153,
Class C (AASHTO M232, Class C) or ASTM F2329 specifications. Table 1
lists the hot-dip galvanizing thickness requirement of ASTM A153/AASHTO
M232/ASTM F2329.
Table 1. Minimum Weight (oz/ft2) (Thickness) of Hot-Dip Galvanizing Surface
Material Standard and Classification
Average of
Tested Samples
Any Individual
Sample
ASTM A153, Class C (AASHTO M232)
Fastener with over 3/8” Diameter &
Washer of 3/16 or 1/4 Inch Thick
1.25 oz/ft2
(0.0021”)
1.00 oz/ft2
(0.001”)
ASTM A153, Class D (AASHTO M232)
Fastener (3/8 Inch or Below) &
Washer Below 3/16” Thickness
1.00 oz/ft2
(0.0017”)
0.85 oz/ft2
(0.0014”)
ASTM F2329
Fastener Diameter Larger than 3/8”
Washer of 3/16” or 1/4” Thickness
0.0020”
0.0017”
ASTM 2329
Fastener Diameter of 3/8” & Below
Washer Thickness Smaller than 3/16”
0.0017”
0.0015”
Comparison of Hot-Dip Galvanizing
and Other Zinc Coatings
Zinc coating and hot-dip galvanizing layer can provide the same
corrosive benefit. Zinc coating can also create a barrier and cathodic
protection. Furthermore, zinc can be used as sacrificing protection for steel
base. When small area zinc coating is damaged or peeled off from steel, zinc
will sacrifice to protect exposed zinc. Thickness, density, bonding strength
and corrosion resistance of 1/4” wide zinc coating differs from hot-dip
galvanizing. Service life of hot-dip galvanizing is related to total zinc-coating
volume of steel. According to ASTM A153/A153M specification, hot-dip
galvanizing requires minimum thickness range from 1.7 to 3.4 mils while
electro-galvanizing thickness is between 0.2 and 1 mil. Electro-galvanizing
fasteners are suitable for interior application only or acceptable short-time
first maintenance. Another mechanical plating thickness is 0.2-4.3mils, its
coating density is inferior to hot-dip galvanizing. In consideration of total zinc
volume on the steel surface, coating thickness is very important. Higher zinc
density fasteners have long service life. Hot-dip galvanizing density is about
0.6 oz/ft2/mil, however mechanical plating density is 0.45 oz/ft2/mil.
300
Technology