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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.

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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/

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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

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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

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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

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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

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Figure 7: Service Life Results

Figure 8: Calculate Coating Thickness Based on Service Life

Figure 9: Coating Thickness Results

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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: