Page 381 - Corrosion Engineering Principles and Practice
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350 C h a p t e r 9 A t m o s p h e r i c C o r r o s i o n 351
Thin sensing elements are preferred in order to preclude influencing
the surface temperature to any extent. Although a sensor constructed
using a 1.5-mm thick glass-reinforced polyester base has been found to
be satisfactory on plastic surfaces, this will not be the case with the
same sensing element on a metal surface with a high thermal conductivity
[14]. For metal surfaces, the sensing element should be appreciably
thinner. Commercial epoxy sensor backing products of a thickness of
1.5 mm, or less, are suitable for this purpose.
9.4.2 Sulfur Dioxide
Sulfur dioxide is usually measured in terms of its concentration in air in
units of µg/m . Precise methods are available to monitor continuously the
3
amount of sulfur dioxide in a given volume of air. The pollution levels
may also be measured in terms of the concentration of dissolved sulfate
2−
(SO ) in rain water. However, this is only indirectly related to the effect of
4
sulfur dioxide on corrosion since only the actual amount of hydrated
sulfur dioxide or sulfur trioxide deposited on metal surfaces is important.
There are two widely used methods for determining the sulfur
dioxide (SO ) concentration in the atmosphere of interest. Both
2
employ the affinity of lead oxide to react with gaseous SO to form
2
lead sulfate. The most common method used in corrosion work is the
sulfation. These devices can be either purchased or prepared in the
laboratory. They consist of small disks of lead oxide that are exposed
facing the ground under a small shelter to prevent the reactive paste
from being removed by the elements [15].
The disk surface is thus exposed only to gaseous SO , not
2
particulates. The American Society for Testing of Materials (ASTM)
procedure suggests a 30-day exposure, followed by a standard sulfate
analysis [16]. The other method sometimes used is the peroxide
candle, similar in its function to the chloride candle, but again using
lead peroxide to capture SO .In this procedure, a lead peroxide paste
2
is applied to a paper thimble in the laboratory, and allowed to dry
thoroughly before exposure. The thimble is then exposed in an
instrument shelter to the test yard environment. In both cases, the
SO deposited results are appropriately reported in terms of deposition
2
rate on the surface in units of mg/m /day.
2
9.4.3 Airborne Chlorides
Airborne salinity refers to the content of gaseous and suspended salt
in the atmosphere. It is measured by the concentration in the air in
units of µg/m . Since it is the salt that is deposited on the metal surface
3
that affects the corrosion, it is usually reported in terms of deposition
rate in units of mg/m /day. Chloride levels can also be measured in
2
terms of the concentration of the dissolved salt in rain water.
A number of methods have been employed for determining the
contamination of the atmosphere by aerosol transported chlorides,
for example, sea salt and road deicing salts. The wet candle method, for