Corrosion & Prevention 2013 Paper 047 - Page 1 PERFORMANCE OF HOT DIP GALVANIZING IN COASTAL ENVIRONMENTS: A REVIEW A. Sheehan 1 1 Galvanizers Association of Australia, Melbourne SUMMARY: The performance of hot dip galvanizing in coastal environments continues to be questioned by some people in industry, including engineers, architects, specifiers and builders. Six years ago, a study was conducted along the South West coast of Victoria, Australia, as a direct response to some comments by corrosion professionals as to the poor performance of hot dip galvanized steel in coastal applications. This study returned to Victoria’s South West to review the performance of the hot dip galvanized steel along a stretch of approximately 400kms of the coastline. The specific sites in the study conducted six years ago were revisited for this review. The sites consisted of iconic buildings, water treatment infrastructure and some more common structures, such as bridge rails and safety barriers along the Great Ocean Road. The water treatment infrastructure and buildings are generally over 10 years old, with some items being 15 years or older. The coating thickness of various hot dip galvanized steel articles was measured at each site, along with climatic conditions such as relative humidity. The results of this review of the performance of hot dip galvanizing along Australia’s south east coastline support that with proper design and consideration of the location’s corrosivity, hot dip galvanized steel structures can provide robust and maintenance free options for the prolonged protection of steel in coastal environments. Keywords: Hot dip galvanizing, Coast, Coating thickness, Time of wetness, Design, Case studies. 1. BACKGROUND In mid-2007, a study was conducted on the performance of a number of structures, with varying ages, along approximately 400km of Victoria’s south west coast due to a growing negative perception and communication regarding hot dip galvanized steel and its performance in coastal areas[1]. Three case studies were looked at where (batch) hot dip galvanized steel was used in structures located only short distances from the coastline along with a survey of galvanized bridge rails on the coast’s Great Ocean Road. The study noted how interpretation of literature, standards and accelerated testing had supported industry belief of hot dip galvanizing’s poor performance in coastal locations, yet the empirical evidence collected from the study showed little to no sign of deficient performance (in the form of highly corroded or rusted surfaces) of the hot dip galvanizing in these areas. Instead, it demonstrated that with proper design, hot dip galvanizing provided a robust and maintenance free option for the durable use of steel in coastal environments [1]. This paper aims to build on the aforementioned study by presenting further evidence of hot dip galvanizing’s true performance in coastal environments around Australia. 2. INTRODUCTION The hot dip galvanizing process and coating has a long history with few major changes to the basic process in over 150 years. During this time, there has been much research and empirical data collected and published about the durability of galvanized steel. Some notable sources include Porter’s ‘Corrosive Resistance of Zinc and Zinc Alloys’ and Slunder and Boyd’s ‘Zinc and its Corrosion Resistance’. Standards, such as ISO 9223 and AS/NZS 2312, have taken advantage of the vast amounts of data
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Corrosion & Prevention 2013 Paper 047 - Page 1
PERFORMANCE OF HOT DIP GALVANIZING IN
COASTAL ENVIRONMENTS: A REVIEW
A. Sheehan1
1Galvanizers Association of Australia, Melbourne
SUMMARY: The performance of hot dip galvanizing in coastal environments continues to be questioned
by some people in industry, including engineers, architects, specifiers and builders. Six years ago, a study
was conducted along the South West coast of Victoria, Australia, as a direct response to some comments
by corrosion professionals as to the poor performance of hot dip galvanized steel in coastal applications.
This study returned to Victoria’s South West to review the performance of the hot dip galvanized steel
along a stretch of approximately 400kms of the coastline.
The specific sites in the study conducted six years ago were revisited for this review. The sites consisted
of iconic buildings, water treatment infrastructure and some more common structures, such as bridge rails
and safety barriers along the Great Ocean Road. The water treatment infrastructure and buildings are
generally over 10 years old, with some items being 15 years or older.
The coating thickness of various hot dip galvanized steel articles was measured at each site, along with
climatic conditions such as relative humidity. The results of this review of the performance of hot dip
galvanizing along Australia’s south east coastline support that with proper design and consideration of the
location’s corrosivity, hot dip galvanized steel structures can provide robust and maintenance free options
for the prolonged protection of steel in coastal environments.
Keywords: Hot dip galvanizing, Coast, Coating thickness, Time of wetness, Design, Case studies.
1. BACKGROUND
In mid-2007, a study was conducted on the performance of a number of structures, with varying ages, along approximately
400km of Victoria’s south west coast due to a growing negative perception and communication regarding hot dip galvanized
steel and its performance in coastal areas[1]. Three case studies were looked at where (batch) hot dip galvanized steel was used
in structures located only short distances from the coastline along with a survey of galvanized bridge rails on the coast’s Great
Ocean Road.
The study noted how interpretation of literature, standards and accelerated testing had supported industry belief of hot dip
galvanizing’s poor performance in coastal locations, yet the empirical evidence collected from the study showed little to no sign
of deficient performance (in the form of highly corroded or rusted surfaces) of the hot dip galvanizing in these areas. Instead, it
demonstrated that with proper design, hot dip galvanizing provided a robust and maintenance free option for the durable use of
steel in coastal environments [1].
This paper aims to build on the aforementioned study by presenting further evidence of hot dip galvanizing’s true performance
in coastal environments around Australia.
2. INTRODUCTION
The hot dip galvanizing process and coating has a long history with few major changes to the basic process in over 150 years.
During this time, there has been much research and empirical data collected and published about the durability of galvanized
steel. Some notable sources include Porter’s ‘Corrosive Resistance of Zinc and Zinc Alloys’ and Slunder and Boyd’s ‘Zinc and
its Corrosion Resistance’. Standards, such as ISO 9223 and AS/NZS 2312, have taken advantage of the vast amounts of data
Corrosion & Prevention 2013 Paper 047 - Page 2
available and established guidelines for industry on the durability of corrosion protection systems, one of which is hot dip
galvanized steel.
Hot dip galvanizing has been used in numerous coastal environments to wide practical success throughout its history. Yet, in
recent times, there has been a movement towards the use of theory, with the extrapolation of practical data and testing over
short periods, for the prediction of a coating’s durability. Due to the broad range of information from various locations and
environments around the world, unsuitable data and theories can easily be used when determining potential durability if there is
a lack of understanding of the different influencing factors on such predictions; for example, the use of widely differing micro-
environments and material specifications.
The corrosivity categories from ISO 9223, by which general environments are broadly defined, are often used to help determine
corrosion rates and therefore durability of various materials in specific applications. These are a great tool when the general
environment is correctly identified. The categories take into account four major influencing factors of corrosivity in a general
environment:
1. The amount of sulphur dioxide (SO2) in the atmosphere
2. The amount of airborne salinity in the atmosphere
3. Relative humidity (RH)
4. Temperature
How these four major factors apply to the Australian climate and its different coastlines affects the application of the
corrosivity categories given in ISO 9223. Climate conditions in Australia can be unique when compared to America and
Europe, so even though the location may be similar in terms of distance from the coast and salinity, the degree of corrosivity
may vary considerably. For example, the level of SO2 in Australia’s atmosphere, for the most part, has not been a significant
factor and generally only needs to be considered in specific industries or applications [2].
Australia’s major cities are built on widely varying coastlines leading to variations in the amount of salt spray that occurs and
how far inland that airborne salt travels. However, it is likely the differences in the Australian relative humidity and
temperature (i.e. estimated time of wetness), when compared to the European and North American countries, allows hot dip
galvanizing to be successful along Australia’s coastal regions. ISO 9223 defines time of wetness as the ‘period when a metallic
surface is covered by adsorptive and/or liquid films of electrolyte to be capable of causing atmospheric corrosion’ [3]. In
Annex B of the Standard, it goes on to state the length of time during which the relative humidity is greater than 80 % (at
temperatures > 0 °C) is used to estimate the calculated time of wetness [3]. As they are next to large bodies of water, there may
be a general expectation that coastal environments would have more time in which the RH of the environment is 80% or
greater, hence they would have a greater estimated time of wetness. However, due to other climatic factors and topography, the
amount of time at which RH is 80% or greater will vary in different coastal locations.
The other factor in the corrosion of hot dip galvanizing, which is often not considered, is that the corrosion rate slows down
significantly over time. This is shown in the values of maximum corrosion attack given for extended exposures in Annex A of
ISO 9224 [4]. Zinc corrosion products (Table 1 [5]) on the surface of the hot dip galvanized coating are believed to be
responsible for the reduction in the corrosion rate, due to the formation of a insoluble barrier (patina) between the atmosphere
and the underlying coating. This patina prevents the continual corrosion of the galvanizing and effectively reduces the
corrosion rate. However, the patina is formed over an extended period of time; over many natural wet-dry cycles. The time and
environment it takes for the patina to form are two reasons why accelerated corrosion testing techniques are not able to achieve
an accurate correlation between their results and real life performance [6] of the hot dip galvanized coating.
Table 1. Zinc Corrosion Reactions in Different Atmospheres[5]