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
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