History | Why Galvanize? | Process/Applications/Properties | 10 real benefits | Improving site efficiency | Durability & Service Life | Zinc Coatings - How they work
Durability
of heavy galvanizing and service life
A GUIDE TO THE SERVICE LIFE OF GALVANIZING
IN THE AUSTRALIAN ENVIRONMENT
BACKGROUND
Steel Protection
No coating has
proven to be more serviceable and of such predictable performance
in the Australian atmosphere for protecting steel than hot
dip galvanizing. This excellent performance of galvanized
coatings in the atmosphere, and under many other exposure
conditions, is mainly due to the formation of a protective
layer of patina which consists of insoluble zinc oxides,
hydroxides and carbonates, depending on the environment.
When the protective patina has stabilized, reaction between
the coating and its environment proceeds at a greatly reduced
rate resulting in long coating life. Its good performance
in the Australian climate is also largely a result of immunity
to the destructive influence of UV and IR solar radiation
of our region. Therefore, in terms of durability alone,
such coatings are likely to far outperform organic coatings
under this influence.
Even in the more corrosive wetter and coastal climates,
the corrosion rate of galvanizing is very low compared
with that of steel, as Table 1 examples. In addition,
it is a robust coating, highly resistant to wear and
impact during transport, installation and service. Unlike
organic coatings that tend to shrink from sharp corners
or can be difficult to apply to complex shapes, galvanizing
ensures an essentially even coat over all surfaces accessed
by the molten zinc. Importantly, it protects the steel
substrate until the zinc has corroded away, unlike conventional
paints, where corrosion of the steel can progress unobserved
under the paint film.
Table 1 Comparative Corrosion Rate (mass loss) of
Steel & Zinc in 2 years (Ref. 1, Ch. 6 ,Table 6.1)
Both the Australian Standard AS/NZS 2312:20022 and the
International Standard EN ISO 147133 provide considerable
information on the corrosion rate of zinc under various
conditions of atmospheric service. In addition, over
the last few decades the CSIRO has carried out extensive
mapping to establish the corrosivity of the Australian
climate. It is important to note that, while the service
life estimates in these two standards are consistent,
they do not extrapolate beyond 25 and 20 years respectively.
However, based on extensive case histories and the fact
that the service life of galvanizing is directly related
to the zinc thickness, extrapolation well beyond 25 years
is justifiable.
In terms of the durability of galvanized, two factors
are significant.
- Extensive literature in Australia and overseas
have shown that the corrosivity of the Australian atmosphere
is very much related to the proximity of the coast and
frequency of on-shore prevailing winds.
- The impact of industrial activities on galvanized
in Australia is minor 4,5,6. This is essentially because
of the low concentration of industrial activity in most
areas of the country and the marked reduction in the
use of sulphur bearing fuels over the last 50 years,
particularly during the early 1970's when environmental
protection legislation was strengthened7. This change
mirrors those throughout the developed world due increasing
industry responsibility, particularly in the reduction
in air pollution by sulphur dioxide8,9.
Diag. 1 Sulphur Dioxide Concentration and Corrosion
Rate
for Zinc in Stockholm in Recent Years9
WHAT DETERMINES THE LIFE OF GALVANIZING?
The service life of galvanized is dictated by a range
of factors.
Zinc metal thickness
(i) Relationship Between Galvanizing Thickness and
Steel Thickness
The service life of any particular post fabrication
galvanized item is directly proportional to the thickness
of the zinc alloy coating. This in turn is a function
of the thickness of the metal; the thicker the steel,
the thicker the layer of zinc, as Table 2 shows. Indeed,
one of the great advantages of galvanizing is the predictability
of the thickness of the zinc for any given steel thickness.
This is particularly important for sharp edges and complex
shapes, where conventional paints don't always cover
well. In-line galvanizing does not follow this relationship,
because the process restricts the galvanizing thickness
to allow the steel to retain ductility, for further manufacturing.
Table 2 Variation in Galvanizing Thickness with Steel
Thickness (Note 1)
(ii) Galvanizing Thicknesses on the Australian Market
Only specific galvanizing thicknesses are commercially
available on the Australian market, as shown in Diag.
2.
After-fabrication galvanizing produces the maximum thickness
possible relative to steel thickness, with long term
protection its only objective.
This contrasts with In-line products, which are produced
by a different process. For these In-line products, the
zinc thickness is accurately controlled, regardless of
steel thickness, so that it remains smooth and ductile
to allow for later cold forming and for varying manufacturing
and end usage . However the corrosion protection is necessarily
less because of the lower zinc thickness.
Diag. 2 Galvanizing grades in the Australian Market
Surface Wetness
The longer a galvanized surface remains wet, the more
likely it is to corrode. Therefore, high rainfall and
humidity and orientation of the surface, including slope
and the potential for crevices and laps to hold water
will tend to accelerate corrosion.
Airborne Sea Salts (and localised Industrial pollution,
notably acid or alkaline fallout).
As discussed under "Atmospheric Corrosion" below,
remote from severe coastal and industrial locales, galvanizing
is an extremely durable coating. While in areas close
to the sea or highly acidic pollution, corrosion will
increase, post fabrication galvanizing is still widely
and successfully used in these areas at high thicknesses
or when topcoated with selected paints (duplex system).
The natural cleansing action of rainwater washes any
contaminants off a galvanized surface, where they might
otherwise accelerate corrosion. However, in extremely
corrosive environments, such as severe coastal service,
corrosion in sheltered locations can be greater. Examples
of this would be steel surfaces sheltered from sun and
wind and prone to long term condensation. Within structural
steel, these are usually small areas.
Dissimilar metals10
Stainless steel and aluminium are commonly used in contact
with galvanizing, notably as fasteners and, except in
very corrosive locations, are most satisfactory. However,
copper and its alloys can accelerate the corrosion of
galvanizing in corrosive situations, when in direct electrical
contact. Corrosion products of copper and its alloys
can also accelerate the corrosion of galvanizing.
Selected Chemicals
A wide range of organic and inorganic chemicals and
solvents are compatible with galvanizing. For specific
guidance consult Slunder and Boyd11 or Porter1. However,
galvanizing should not be used in contact with strong
acids and strong alkalis, below pH 6 and above pH 12
respectively or, if it is, the galvanizing will need
to be top-coated with appropriate chemically resistant
coatings.
Additional Paint Finish
In service conditions where the life of galvanizing
may be limited, the addition of a paint finish, (a duplex
coating system) can extend the service life of the steel.
Indeed, if the paint system is maintained by appropriate
reinstatement from time to time, so as to preserve the
galvanizing, the service life of the structure should
be unlimited. Indeed, in corrosive locations, such as
severe coastal or industrial service, the duplex system
should provide a synergistic improvement over and above
the separate contributions of each coating2,3,13. The
type of paint selected and the surface preparation will
very depending upon the environment and the aesthetic
demands13.
THE LIFE OF GALVANIZING IN THE ATMOSPHERE
As discussed in the introduction, the durability of
galvanizing in the atmosphere is largely due to the corrosion
protection afforded by the insoluble corrosion products
that slowly accumulate on the surface as it weathers.
The international community has carried out extensive
studies on the corrosion of steel and zinc in the atmosphere
and define their performance relative to five "corrosion
categories"7, from C1 the most benign to C5 the
most severe. They are summarised in Table 3:
Note 1 It is important to note that this provides only
generalised guidance and other factors, such as specific
microclimates and unwashed areas, may have an influence.
Knowing the type of environment for any given galvanized
structure, the life to first maintenance can be estimated
from Diag. 3.
Diag. 3 Estimated Life to First Maintenance for Galvanized
Steel2.
THE LIFE OF GALVANIZING IN WATER
Except for some very hard waters, the corrosion of galvanizing
in water is considerably greater than most conditions
of atmospheric service, however galvanizing can be appropriate
in many water immersion situations. Typical corrosion
rates for zinc in water are shown in Table 4.
Table 4 Typical Corrosion Rate for Zinc in Waters14
In freshwater, the corrosion rate depends on the ability
of the coating to develop a protective layer. The formation
of this layer is dictated by the pH, hardness, alkalinity
and total dissolved solids of the water. The pH has a
profound affect, with zinc being vulnerable outside the
pH range 6 - 12 (see Diag. 4).
Salt water at depth, with lower oxygen levels will tend
to be less corrosive, whereas at the splash zone where
the water is oxygen rich and more turbulent the corrosion
rate is much higher and therefore galvanizing alone is
not recommended.
Diag. 4 Corrosion of Zinc with Variations in pH
Water temperature is also important, increasing markedly
in corrosivity with increasing temperature up to 70°C
when it falls away (Diag. 5). For this reason it is not
satisfactory for hot soft or condensate water applications.
Diag. 5 Effect of Water Temperature on Zinc Corrosion
THE LIFE OF GALVANIZING IN SOIL
The corrosion of zinc in soil is on average considerably
greater than in the atmosphere, but can vary greatly,
even over short distances. This is essentially because
of the varying moisture content and its heterogeneity,
particularly along a vertical profile. In general terms,
course open textures are less corrosive than fine ones,
such as clays, which tend to hold water. Soil mineral
content pH and oxygen content are also important indicators.
Mineral content is easily appraised by measurement of
soil resistivity, the lower the resistivity the greater
the corrosion rate. For example, the service life of
galvanized culverts can be effectively predicted from
soil resistivity and pH15, while galvanizing is used
extensively by the rammed earth industry and, for many
years, Australian domestic water supply. However, where
long term service is required and the soil conditions
are uncertain it is often prudent to consider additional
surface protection.
REFERENCES
1. Porter F. "Zinc Handbook, Properties, Processing
and Use in Design", Marcel Dekker Inc. Y 1991.
2. AS/NZS 2312:2002 "Guide to the protection of
structural steel against atmospheric corrosion by the
use of protective coatings".
3. EN ISO 14713 "Protection against corrosion of
iron and steel in structures - Zinc and aluminium coatings
- Guidelines (ISO 14713:1999).
4. King G.A., K.G. Martin & J.F. Moresby, "A
Detailed Corrosivity Survey of Melbourne", CSIRO
DBR Aust. 1982.
5. King G.A., and Carberry B "Atmospheric Corrosivity
of the Greater Newcastle Region" CSIRO DBCE Technical
Report 92/3 1992.
6. King G.A., J. Kapetas and D. Bates - Brownsword, "Corrosivity
Mapping Used For Transmission Line Maintenance by the
Electricity Trust of South Australia, Australasian Corrosion
Association Conference 34, Paper No. 60 Adelaide, Nov.
1994.
7. D J Bartlett, Industrial Pollution and its Impact
on Corrosion and Corrosion Mitigation Practices, Aust.
Corr. Assoc. Conf. Nov 2001.
8. Indur M. Goklany, “Cleaning the Air: The Real
Story of the War on Air Pollution” Washington D.C.:
Cato Institute, 1999.
9. Landner L and L Lindeström, "Zinc in Society
and in the Environment", Miljö Forskar Gruppen
10. "After Fabrication Hot Dip Galvanizing" Galvanizers
Assoc. of Aust. 1999
11. CJ Slunder and WK Boyd, "Zinc: Its Corrosion
Resistance" Int. Lead Zinc Research Org. In. Aug.
1983.
12. ISO 9223 Corrosion of metals and alloys - Corrosivity
of atmospheres - Classification.
13. "Guide to Adopting Paint Systems for Galvanized
Steel" Galvanizers Assoc. of Aust. 2003.
14. Metals Handbook, Vol. 13 "Corrosion" 9th
Edn. ASM Int. 1987.
15. "Method for Estimating the Service Life of
Steel Culverts", Department of Transportation, California,
US., California Test 643 Nov. 1999.
|
