Hot-Dip Galvanized Steel vs.

Zinc-Rich Paint
Hot-Dip Characteristic Inorganic
Galvanized Steel Zinc-Rich Paint (ZRP)
Service Life/Durabilitya
> 100yearsb Rural 27 yearsc
> 90 years Urban 17 years
> 70 years Industrial 12 years
> 50 years Marine 12 years
Coating
Exceeds ASTM A 123 minimum Thickness Variable - may not meet required minimum due
(> 3.9 mils for 1/4” thick steel) to applicator skill
Exterior & Interior Coverage Exterior surfaces only
Edge & Corner thickness ≥ flat surfaces Consistency Edge & Corner thickness < flat surfaces
Directd Indirect Life-Cycle Coste Directc Indirectf
$1.76/ft2 $0 Rural $1.27/ft2 $ 6.35 - $12.70/ft2
$1.76/ft2 $0 Urban $1.74/ft2 $ 8.70 - $17.40/ft2
$1.76/ft2 $0 Industrial $2.27/ft2 $11.35 - $22.70/ft2
$1.76/ft2 $0 Marine $2.27/ft2 $11.35- $22.70/ft2
Maybe - Zinc % may be too low for
Yes - 100% zinc metal
Cathodic Protection conductivity/Binding materials may not be
Zinc metal is anodic to steel
conductive, minimizing cathodic protection
Paint absorbed water & underfilm corrosion of
No base steel corrosion 2000 Hour Immersion Test
the substrate steel occurred
No effect on performance Ultraviolet Ray Exposure No severe degradation
Alloy layers are harder than base steel Moderate; DPN hardness approximately 33%
Abrasion Resistance
DPN hardness ranging from 179 - 250 less than HDG
-100 C to 350 C Performance Range Temperature Deterioration at 250 C
Application
Documented, timed, scientific sequence Surface Preparation Function of human expertise, intolerant of
improper cleaning/profiling
Observable completion of metallurgical Procedure Constant agitation of mixture, regulation
reaction thickness & drying time per pass
Requires specific temperature & humidity
24/7/365 - all weather Application
conditions
~ 3600 psi Bond Strength to Steel ~ 600 psi
Galvanized sign structure Corroding painted pole
90’ - 120’ long by 6’ - 8’ wide/deep Size Range of Products Unlimited

aTo 5% rust on substrate steel e 50-year life, 3% inflation, 7% interest rate, 100,000 sq. ft./typical
b ZincCoating Life Predictor, Zhang, 2001 mix of structural products
c NACE Paper 06318, Helsel, Melampy, Wissmar, 2006 f Federal Highway Administration, RD-D1-156, Vermani
d Galvanizing Industry Survey, 2006

American Galvanizers Association Phone: 720-554-0900 Fax: 720-554-0909 www.galvanizeit.org aga@galvanizeit.org

Characteristic Notes
Service Life/Durability • H
 DG typically has no maintenance costs and so the initial UV Exposure
• The primary criteria for comparison of hot-dip galvanizing cost is the total life-cycle cost. • HDG coatings are not affected by UV light.
(HDG) to zinc-rich paints (ZRP) is durability in use. • ZRP usually requires several maintenance cycles and is • UV attack on ZRP is initially not severe.
• HDG is maintenance free and commonly prevents any most commonly top-coated to deliver adequate corrosion
corrosion of the substrate steel for 50-75 years in most protection, making the life-cycle cost far greater than HDG. Abrasion Tests
atmospheric environments (industrial, urban, marine, and • HDG’s zinc-iron alloy layers are harder than the base steel
• In addition to the direct life-cycle costs, there are substantial
rural) with millions of data points to support that statement and data indicate it has three times the abrasion resistance
indirect costs, such as traffic pattern disruption, plant
(see chart lower right). of ZRP.a
shutdown, and accidents/injury. Depending on the industry
• Z
 inc-rich paint manufacturers qualify such claims only with (transportation, water/wastewater, buildings), the indirect Temperature Test
estimates of when field maintenance will be required and costs to maintain a corrosion protection system such as ZRP • In a 15 minute exposure in temperatures up to 350 C, HDG
how many times it is required in order to meet a designed ranges from 5-10 times the direct costs. coatings did not change.
service life.
Cathodic Protection (vee cuts of 10mm x 115mm) • ZRP deteriorated at 250 C and became powdery at 350 C,
• S
 alt Spray Test (simulating a marine environment): Laboratory even when cooled to room temperature.a
• HDG steel showed no signs of substrate steel corrosion after
testing of zinc corrosion systems is not appropriate to
1500 hours exposure in salt fog testing.
establish relative performance between systems. Zinc Bond Strength
corrosion mechanisms in accelerated testing are very • Z
 RP coated steel exhibited red rust on the exposed area • The bond of zinc to steel produced by the HDG process is
different from real world corrosion mechanisms. after only 24 hours, and after 550 hours red rust was evident on the order of 3600 psi, making it very difficult to damage
• However, HDG exhibited zero base steel corrosion over all of the exposed surface. the coating.
after 1500 hours of salt spray testing, at which time • T
 he bond strength of zinc-rich paints is on the order of
Immersion Test (2000 hours in corrosive mine water)
testing was stopped. 600 psi, allowing for normal coating damage caused by
• HDG exhibited no base steel corrosion, as the HDG steel
• S
 ignificant base steel attack was observed on ZRP samples formed stable surface deposits (zinc salts). rough handling and everyday wear and tear.
coated coupons after 1000 hours.a
• A
 substantial increase in nominal thickness of the ZRP Application
• D amp Sulphur Dioxide Test (simulating an industrial coating indicated swelling due to water absorption, resulting • HDG is factory controlled and can be done 24/7, 365 days
environment): in the formation of voluminous corrosion products underneath a year.
• HDG steel samples exhibited no signs of base the coating.
• T
 he proper application of ZRP requires specific temperature
steel corrosion after 40 cycles of the test. a South African Bureau of Standards Study and humidity conditions.
• Z
 RP showed base steel corrosion and severe edge
corrosion after 9 cycles. Service Life Chart for Hot-Dip Galvanizing
Coating 100
• The HDG coating generally exceeds the minimum coating 90
thickness requirement of ASTM standards; based largely on
the chemistry of the substrate steel and surface condition of 80
the steel prior to cleaning. Key
70
Service Life* (years)

Rural
• HDG applies zinc throughout a fabrication or tubular piece,
even into difficult-to-reach corners and crevices. The 60 Suburban
Temperate Marine
diffusion reaction between molten zinc and iron in steel 50 Tropical Marine
is perpendicular to all surfaces and thus edge and corner
coating thickness is the same as or greater than the coating 40 Industrial

thickness on flat surfaces. 30

• Z
 RP coating thickness is variable and based on the expertise 20
of the applicator. ZRP typically leaves difficult-to-reach
areas unprotected from corrosion. 10
Life-Cycle Cost 0
• The total direct cost of a corrosion protection system is 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
the sum of the initial cost and maintenance costs over the Average Thickness of Zinc (mils)
1 mil = 25.4µm = 0.56oz/ft2
design life of the project/facility. *Service life is defined as the time to 5% rusting of the substrate steel surface.