Page 399 - Corrosion Engineering Principles and Practice

368 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 369

FIGURE 9.40 Aluminum and magnesium test specimens equipped with
crevice spacers and mounted on a rack to be exposed at the previous test
station. (Courtesy of DSTO Australia)

In addition to the atmospheric corrosivity conditions at the test
site the following factors are important for the design and interpretation
of atmospheric corrosion tests:

• Shape of the specimen
• Direction it faces
• Amount of shelter, drip, or runoff from other specimens
• Elevation
• Shading
• Unusual contamination

Panel specimens are typically placed in racks at a 30-degree angle
to the horizontal, facing the source of corrosive elements. They are
electrically insulated from the racks on which they are mounted and
are arranged so that drip from neighboring panels does not contaminate
them (Fig. 9.39). Cylindrical specimens are mounted horizontally,
facing the same direction as the panels. They may be exposed fully to
the weather or be partially sheltered, depending upon the requirements
of the test. There is a great variety of test fixtures and shape of specimens
used for environmental cracking tests. Figures 9.41 and 9.42 show
respectively a full exposure and a semi-sheltered test stations equipped
with TOW galvanic sensors, temperature, and SO level recorded with
2
a data logger.
In most exposure tests, enough specimens are used so that
removals may be made after predetermined periods of 1 to 20 years.
Very short-term tests usually can be misleading in that the condition