Page 389 - Corrosion Engineering Principles and Practice

358 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 359

that winds came from the 16 points of the compass during the three-
month exposure period. The dominant direction was the north to
north-east. However, the pattern of corrosion product did not
correspond to the most frequent wind direction but did correspond
to the direction with the highest wind speeds, which are in the west
to south quadrant at this particular site (Fig. 9.30).

9.5 Atmospheric Corrosivity Classification Schemes

9.5.1 Environmental Severity Index
An environmental corrosivity scale, based on atmospheric parameters,
has been developed over the years for the maintenance management of
structural aircraft systems [21]. A corrosion damage algorithm (CDA)
was proposed as a guide for anticipating the extent of corrosion damage
and planning maintenance operations. This classification scheme was
developed primarily for uncoated aluminum, steel, titanium, and
magnesium alloys exposed to the external atmosphere at ground level.
The section of the CDA algorithm presented in Fig. 9.31 considers
the distance to salt water, leading either to the very severe AA rating for
close distance to seashore or a consideration of moisture factors. Following
the moisture factors, pollutant concentrations are compared with values
of Working Environmental Corrosion Standards (WECS). The WECS
values were adopted from the 50th percentile median of a study aimed at
determining ranges of environmental parameters in the United States
and represent “average of averages.” For example, a severe A rating
would be given if any of the three pollutants considered in this scheme,
that is, sulfur dioxide, total suspended particles, and ozone levels, would
exceed the WECS values in combination with a high moisture factor.
The environmental corrosivity, predicted from the CDA algorithm,
of six marine air bases has been compared to the actual corrosion
maintenance effort expended at each base. Considering the simplicity
of the algorithms and simplifying assumptions in obtaining relevant
environmental and maintenance data the correlation obtained was
considered to be reasonable. However, subsequent attempts to enhance
the PACER LIME algorithm by using the results obtained from broad-
based corrosion testing programs have failed to provide enough
differentiation between moderately corrosive environments [22].
In order to remedy the deficiencies in the CDA scheme, Battelle
Memorial Institute has been tasked to monitor the atmospheric corro-
sivity of air force and other sites worldwide [23]. The database describ-
ing the relative corrosive severity levels of different locations and actual
corrosion rates of a variety of metals has now grown to more than
100 sites worldwide. The metals included in that study are three alumi-
num alloys (A92024, A96061, and A97075), copper, silver, and steel.
Data have been collected for metals directly exposed to the
outdoor environment in a standard sample mounting configuration