Page 380 - Corrosion Engineering Principles and Practice

348 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 349

9.4 Measurement of Atmospheric Corrosivity Factors
Various methods have been developed for measuring many of the
factors that influence atmospheric corrosion. The quantity and
composition of pollutants in the atmosphere, the amount collected on
surfaces under a variety of conditions, and the variation of these with
time have been determined. Temperature, RH, wind direction and
velocity, solar radiation, and amount of rainfall are easily recorded.
Not so easily determined are dwelling time of wetness (TOW), and
the surface contamination by corrosive agents such as sulfur dioxide
and chlorides. However, methods for these determinations have been
developed and are in use at various test stations. By monitoring these
factors and relating them to corrosion rates, a better understanding of
atmospheric corrosion can be obtained.

9.4.1 Time of Wetness
Time of wetness is an estimated parameter based on the length of
time when the relative humidity is greater than 80 percent at a
temperature greater than 0°C. It can be expressed as the hours or days
per year or the annual percentage of time.
A method of measuring the TOW has been developed by Sereda and
correlated with the corrosion rates encountered in the atmosphere [13].
The moisture sensing elements in this sensor are manufactured by
plating and selective etching of thin films of appropriate anode
(copper) and cathode (gold) materials in an interlaced pattern on a thin
nonconductive substrate (Fig. 9.23). When moisture condenses on the
sensor it activates the cell, producing a small voltage (0 to 100 mV) across
a 10 Ω resistor.
7

FIGURE 9.23 Interlocking combs of gold and copper electrodes in a Sereda
humidity sensor.