Page 384 - Corrosion Engineering Principles and Practice

352 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 353

The roof protected the surface from direct rain but the filter papers
were still exposed to deposition of the airborne chlorides and possibly
some rain. Filter papers were removed every month for chemical
analysis. A wet candle with its own roof cover was installed on each
box (Fig. 9.25) to provide a measure of the level of chlorides outside
the boxes.

9.4.4 Atmospheric Corrosivity
The simplest form of direct atmospheric corrosion measurement is by
coupon exposures. Subsequent to their exposure, coupons can be subjected
to weight-loss measurements, pit density and depth measurements
and to other types of examination. The main drawback associated with
conventional coupon measurements is that extremely long exposure times
are usually required to obtain meaningful data, even on a relative scale. It
is not uncommon for such programs to run for 20 years or longer.
Some variations of the basic coupon specimens can provide rapid
material corrosivity evaluations. The helical coil adopted in the ISO
9226 methodology is a high surface area/weight ratio coupon that
gives a higher sensitivity than panel coupons of the same material.
The use of bimetallic specimens in which a helical A91100 aluminum
wire is wrapped around a coarsely threaded bolt may provide
additional sensitivity and forms the basis of the Classify Industrial
and Marine Atmospheres (CLIMAT) coupon [11;18].
The mass loss of the aluminum wire of a CLIMAT coupon after
90 days of exposure is considered to be a relative measure of atmospheric
corrosivity. However, the results vary greatly between the various
combinations of materials suggested in the ASTM standard [11]. The
aluminum wire on copper bolts has been found by many to be the most
sensitive of the three proposed arrangements in the ASTM standard.
The use of triplicate coupons on a single holder additionally provides
an indication of the reproducibility of the measurements and the use of
vertical rods can reveal directional information on the corrosive agents
as will be illustrated in the following examples.
A CLIMAT coupon with three copper rods installed at the NASA
Kennedy Space Center (KSC) beach corrosion test site (Fig. 9.27) is
shown immediately after it had been installed [Fig. 9.28(a)], after 30 days
[Fig. 9.28(b)], and after 60 days [Fig. 9.28(c)]. KSC having the highest
corrosivity of any test site in the continental United States [19], the
mass loss recorded even after a shorter exposure than usual can be very
high. In the present example it was already 16 percent of the original
aluminum wire after 60 days. The base support of these CLIMATs
having been purposefully installed parallel to the sea coast the directional
effect of the marine salts may be illustrated by comparing the front and
back of the exposed CLIMATs [Fig. 9.29(a) and (b)].
In another study focused on the shielding effects of buildings in a
marine environment, the directional impact of marine aerosols was
revealed by comparing the level of patina on CLIMAT’s copper rod