Page 382 - Corrosion Engineering Principles and Practice

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

example, is relatively simple, but has the disadvantage that it also
collects particles of dry salt that might not deposit otherwise [17].
This technique uses a wet wick of a known diameter and surface area
to measure aerosol deposition (Fig. 9.24). The wick is maintained wet
using a reservoir of water or 40 percent glycol/water solution.
Particles of salt or spray are trapped by the wet wick and retained. At
intervals, a quantitative determination of the chloride collected by
the wick is made and a new wick is exposed.
In reality, the wet candle method gives an indication of the salinity
of the atmosphere rather than the contamination of exposed metal
surfaces. The technique is considered to measure the total amount of
chloride arriving to a vertical surface and its results may not be truly
significant for corrosivity estimates.
In order to understand the chloride deposition rates in a confined
spaces, such as ventilated subfloors in a sea coastal area, a special
collecting box shown in Fig. 9.25 was used in which the airborne
chlorides were collected on horizontal and vertical filter papers
positioned at different locations from the box openings (Fig. 9.26).

Roof

Test tube

Gauze

FIGURE 9.24 Schematic of a wet candle chloride apparatus.