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1
Zinc Coating Life Predictor
1. Scope
1.1 This guideline provides information
and instructions for using the Zinc Coating
Life Predictor (ZCLP) to accurately
calculate corrosion rates of a hot-dip
galvanized coating in atmospheric
environments.
2.2 The ZCLP uses five different
environmental parameters. The data for
these parameters may be difficult to find.
This guideline gives direction for locating
this information and obtaining an accurate
corrosion rate prediction for zinc coatings in
an atmospheric environment.
2. Referenced Documents
2.1 Comparative Climatic Data for The
United States Through 20121
2.2 National Trends in Sulfur Dioxide2
2.3 Time-to-First-Maintenance Chart3
3. The Zinc Coating Life Predictor
1 Available from National Oceanic and Atmospheric
Administration, http://www1.ncdc.noaa.gov/pub/data/ccd-data/CCD-2012.pdf 2 Available from Environmental Protection Agency,
http://epa.gov/airtrends/sulfur.html 3 Available from American Galvanizers Association,
http://www.galvanizeit.org/hot-dip-galvanizing/how-long-does-hdg-last/in-the-atmosphere/time-to-first-maintenance
3.1 The American Galvanizers
Association’s Time to First Maintenance
Chart approximates the usable coating
protection of hot-dip galvanizing on steel in
atmospheric applications. The easy-to-use
chart offers a visual representation of time to
first maintenance for hot-dip galvanized
steel in atmospheric exposure based on
environmental conditions and zinc coating
thickness. Environmental data from 25
different cities in conjunction with Gregory
Zhang’s internet based Zinc Coating Life
Predictor (ZCLP) were used to create the
time to first maintenance vs. coating
thickness lines seen on the chart. However,
the chart should only be used as a tool for
quick approximations and a visual
interpretation of hot-dip galvanized steel’s
time to first maintenance. For a more
accurate prediction of time to first
maintenance, actual data for a project’s
specific location and climate should be
collected and entered into the ZCLP’s
model. This model will either give a
recommended zinc coating thickness based
on a desired time to first maintenance, or
will give an accurate estimation of time to
first maintenance based on a given zinc
coating thickness. This Galvanizing
Guideline gives direction on using the ZCLP
as an accurate approximation tool to predict
the time to first maintenance of hot-dip
galvanized steel in atmospheric applications.
2
3.2 The ZCLP, an internet based tool,
uses the environmental data of temperature,
airborne salinity, sulfur dioxide
concentration, relative humidity, rainfall,
and sheltering condition, to accurately
predict a time to first maintenance of hot-dip
galvanized steel using a given zinc coating
thickness. It is also capable of giving a
coating thickness recommendation based on
a desired time to first maintenance. Time to
first maintenance is defined as the time
between initial exposure to the atmosphere
and the loss of galvanized coating until 5%
of the exposed surface area shows red
rusting. The software uses statistical
methods, neural network technology and an
extensive worldwide corrosion database to
estimate corrosion rates from the six
parameters mentioned above.
4. Finding Data
4.1 Using the Zinc Coating Life Predictor
requires one to acquire the environmental
data for the six cited parameters for the
particular region or project in question, and
input the information into the ZCLP. From
there, one can ask for a time to first
maintenance based on a given coating
thickness, or ask for a coating thickness
based on a desired time to first maintenance.
Working through a step-by-step example
here, will best explain how to use this
valuable tool. We will use the specific
example of the atmospherically exposed hot-
dip galvanized steel of the Tappan Zee
bridge spanning the Hudson River in New
York.
4.2 Visit the AGA’s website,
www.galvanizeit.org and click on the tab at
the top of the page labeled “Hot-Dip
Galvanizing.” Then, on the left-hand side of
the page, click the link labeled “How Long
Does HDG Last?” A new list will appear
under this link; from the new list, click on
“Zinc Coating Life Predictor.” Near the
beginning of the short article about the
online software, there is a link for the
website containing the Zinc Coating Life
Predictor. Clicking on this link will direct
you to the desired page hosted by the Galv
Info Center. Clicking on the “Prediction”
Figure 1: Accessing the Zinc Coating Life Predictor
http://www.galvanizeit.org/
3
link and then accepting the terms and
conditions will lead you to the model.
Figure 1 shows a screenshot of this short
article on the AGA’s website.
4.3 The National Oceanic and
Atmospheric Administration, or NOAA,
compiled years of climatic data from several
different weather stations nationwide, into
the Comparative Climatic Data (CCD) chart
found here:
http://www1.ncdc.noaa.gov/pub/data/ccd-
data/CCD-2012.pdf. The chart gives us a
number of the parameters needed to obtain
an accurate prediction for the time to first
maintenance of hot-dip galvanized steel. The
internet allows us to quickly access and use
the publicly available data.
Figure 2: CCD Table of Contents
4.3.1 Rain or Precipitation is the first
parameter needed by the ZCLP. Page seven
of the CCD chart gives guidance on the page
of the chart that indicates precipitation
information; shown in Figure 2. Here we
are interested in the “Normal Precipitation”
information. Once we scroll down to the
correct page, we find tables of data arranged
by meteorological elements, with weather
stations listed alphabetically by state. New
York State has nine weather stations to
choose from, but a quick look at a map will
tell you the “New York C.Park NY” is the
closest listed station to the Tappan Zee
Bridge (about 25 miles). The Precipitation
chart within the CCD arranges precipitation
data for each station by rainfall per month,
per year, and the number of years
represented by the data. Here, we are only
interested in the total annual average and
find it is 49.9 inches for the Tappan Zee
Bridge’s climate. This number is entered
into our model and inch/year is selected on
the drop down menu next to rainfall to give
the correct units. 4.3.1.1 For Canadian cities, rainfall
information can be found by downloading a
CSV file from
www.climate.weather.gc.ca/climate_normal
s
4.3.2 Relative Humidity is the fourth
parameter needed by the ZCLP. Once again,
we look at page seven of the CCD PDF and
find the listing for “Average Relative
Humidity.” Again, for the Tappan Zee
Bridge we use the C.Park NY weather
station. The Relative Humidity chart within
the CCD gives values for average relative
humidity in both the morning and the
afternoon for every month of the year and an
annual average of this data. For the purpose
of the ZCLP, we use an average of the
morning and afternoon annual data. 64% is
input as the relative humidity parameter for
the ZCLP.
4.3.2.1 Humidity data for Canadian cities
can be found from Environment Canada’s
website at http://www.ec.gc.ca/indicateurs-
indicators/default.asp?lang=en&n=307CCE
5B-1
4.3.3 Temperature is another important
factor for the corrosion rate of hot-dip
galvanized steel in atmospheric conditions.
In this case, we look for the daily mean
temperature of the climate in question. Once
again, on the seventh page of the CCD PDF,
we see the listing for “Normal Daily Mean
Temperature.” Similar to the precipitation
and relative humidity data, we are looking
for the annual average. For the Tappan Zee
Bridge (closest weather station is again New
York C.Park NY), the annual average for
normal daily mean temperature is 55 degrees
Fahrenheit. This is input into our model
http://www1.ncdc.noaa.gov/pub/data/ccd-data/CCD-2012.pdfhttp://www1.ncdc.noaa.gov/pub/data/ccd-data/CCD-2012.pdfwww.climate.weather.gc.ca/climate_normalswww.climate.weather.gc.ca/climate_normalshttp://www.ec.gc.ca/indicateurs-indicators/default.asp?lang=en&n=307CCE5B-1http://www.ec.gc.ca/indicateurs-indicators/default.asp?lang=en&n=307CCE5B-1http://www.ec.gc.ca/indicateurs-indicators/default.asp?lang=en&n=307CCE5B-1
4
making sure the correct units are selected
before moving on.
4.3.3.1 For Canadian cities, temperature
information is obtained by downloading a
CSV file from
www.climate.weather.gc.ca/climate_normal
s
Figure 3: Sulfer Dioxide Locations
4.3.4 Sulfur Dioxide can either be
measured by the deposition rate onto
surfaces, or by the concentration of the
pollutant in the air. For our purposes we will
use the air concentration value. This data
can be found in a different location than the
other parameters we have found, but is also
easily accessible. The Environmental
Protection Agency (EPA) monitors air
quality, including sulfur dioxide content, at
several stations around the United States.
Their website helps one locate the station
nearest to the Tappan Zee Bridge, provides
S02 levels for a number of years, and shows
the trend in SO2 levels to help make
predictions for future corrosion rates. This
data is organized into the form of a map
similar to the one shown in Figure 3. To
find these maps, take the following steps:
1. Visit www.epa.gov/airtrends/ 2. About half way down the page under
Air Quality Trends by Pollutant click
on “Sulfur Dioxide (SO2)
3. This brings up a map containing several sites where SO2 data is taken.
Zooming in on the map to the Tappan
Zee Bridge will show locations,
marked by a pin, where SO2 data is
taken.
4. Clicking on the pin nearest to the Tappan Zee Bridge will bring up a
graph similar to Figure 4 with the
most recent sulfur dioxide numbers in
parts per billion (ppb). There is also an
option to download the data from this
graph and view it on an Excel
spreadsheet.
5. Parts per billion must be converted to micrograms per cubic meter by using
the conversion factor:
Figure 4: Plot of Sulfer Dioxide Data
These steps were followed for the Tappan
Zee Bridge area and it was found that the
SO2 levels for this area are about 29.7 ppb or
11.3 micrograms per cubic meter. This value
is entered into our model, making sure we
change the units in the right drop down
menu to μg/m3.
4.3.4.1Sulfer Dioxide levels for Canadian
cities can be found from Environment
Canada’s website at
http://www.ec.gc.ca/indicateurs-
indicators/default.asp?lang=en&n=307CCE
5B-1
4.3.5 Salinity deposition is a
measurement of the salt deposited per
surface area on a daily basis. This is the
www.climate.weather.gc.ca/climate_normalswww.climate.weather.gc.ca/climate_normalswww.climate.weather.gc.ca/climate_normalshttp://www.epa.gov/airtrends/http://www.ec.gc.ca/indicateurs-indicators/default.asp?lang=en&n=307CCE5B-1http://www.ec.gc.ca/indicateurs-indicators/default.asp?lang=en&n=307CCE5B-1http://www.ec.gc.ca/indicateurs-indicators/default.asp?lang=en&n=307CCE5B-1
5
Table 1: Salinity Based on Environment
parameter with the least readily available
information. For developing the Time to
First Maintenance Chart, the AGA used
estimations for this value based on real
world data and distance from the sea. Areas
closer to the sea have a higher salinity
deposition rate than areas further away from
a body of salt water. Table 1 shows some
average salinity rates used for the making of
the Time-to-First-Maintenance Chart based
on the type of climate. These values can be
used as an approximation.
We now determine that the Tappan Zee
Bridge is located in an Industrial
Environment with very close proximity to
the sea and use the value of 25 mg/m2*day.
This value is then input into the ZCLP.
4.3.6 Sheltering Condition is up for
interpretation for each individual project.
One is able to pick from the choices of Open
Air, Rain Sheltered, or Indoor. Rain
sheltered conditions are considered to still
be an outdoor application, but with some
protection from rain and other weather
elements. The Tappan Zee Bridge stands
over the Hudson River with no sheltering.
Therefore, for this parameter we say the
galvanized steel is in an open air condition
and choose this option from the drop down
menu to the right.
5. Using Results
5.1 Now that we have found the data
needed for the ZCLP to make a prediction, it
may all be entered into the ZCLP and a time
to first maintenance prediction can be made.
After the data has been entered and the
correct units have been selected, as shown in
Figure 5, clicking on the “Submit” button
leads us to a new screen with options for
prediction the life of the coating based on a
determined thickness, or for determining
what thickness of coating is needed to last a
determined amount of time.
5.1.1 Calculate Coating Life, located at
the top right-hand side of the page, will
predict the amount of time it will take for
zinc coating to degrade based on the factors
previously entered and the specified coating
Environment Salinity
(mg/m2*day)
Rural 5
Suburban 5-10 (Depending on
Proximity to Sea)
Temperate Marine 25
Tropical Marine 30
Industrial 5-25 (Depending on
Proximity to Sea)
Figure 5: Data Input Into ZCLP
6
thickness. ASTM A123 specifies a
minimum coating thickness of 3.9 mils (100
micrometers) for thicker structural steel
pieces like those used in the construction of
the Tappan Zee Bridge. Here, we will enter
this minimum specified thickness into the
ZCLP. Keeping in mind, however, that this
is only a minimum thickness required and
the actual measured thickness may be
greater than this value. When this coating
thickness is input into the ZCLP with the
correct units selected, and “Calculate Life”
is clicked on, a new page appears with a
readout of corrosion rate and predicted
coating life. For the Tappan Zee Bridge, the
rate of corrosion is predicted at 0.04 mil/yr
with a coating life of 109 years. These steps
are shown in Figure 6 and Figure 7.
5.1.2 Calculate Coating Thickness,
located at the bottom right-hand side of the
page will predict the coating thickness
needed to last a specified amount of time
based on the environmental factors
previously specified. As an example, we can
say the desired design life of the Tappan Zee
Bridge is 75 years. This time can be entered
into the ZCLP; also keeping in mind that
this is only the amount of time it will take
for the initial coating to corrode away. With
minor touch up and repair of the coating at
this time, the steel will be protected years
beyond this point. 75 years is input into the
ZCLP, “Calculate Coating Thickness” is
clicked on, and the model leads us to a new
screen. Again, the corrosion rate is predicted
at 0.04 mil/yr with a coating thickness of 2.7
mils needed to provide corrosion protection
the Tappan Zee Bridge for 75 years. These
steps are shown in Figure 8 and Figure 9.
Figure 6: Calculate Coating Life Based on Coating Thickness
7
Figure 7: Service Life Results
Figure 8: Calculate Coating Thickness Based on Service Life
Figure 9: Coating Thickness Results
8
Annex
A.1.1 To find the conversion factor for SO2
concentrations in ppb to
we first start with
We then divide this by the molar weight of SO2,
and multiply it with the molar volume of air at
STP and a conversion factor.
Ppb is associated with 10-9
leaving us with 0.382
ppb SO2 per 1
SO2.
A.1.2 The conversion between units of
rainfall is:
A.1.3 The conversion between temperature
units is: