Page 407 - Corrosion Engineering Principles and Practice

376 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 377

FIGURE 9.48 The Budd BB-1 Pioneer built in 1931 and mounted on a pylon
outside the Franklin Institute Science Museum in Philadelphia. The lower
wing and empennage fabric has been lost to the weather.

atmospheres for periods of up to 20 years. From data accumulated in
these tests and the calibrations of relative corrosivity of the test sites, a
fairly clear picture can be obtained of the corrosion behavior of copper.
In addition to the corrosion penetration rates, one must be mindful
of dezincification of brasses and selective attack on some bronzes as
well as SCC of yellow brass or bronze (Fig. 6.46 in Chap. 6). These
types of corrosion contribute to the failure of the material in mechanical
respects without significant weight changes or losses in thickness.
Where copper is used as flashing on roofs, corrosion has been
encountered at the edge of the shingles as a continuous groove. This
effect is more pronounced when the atmosphere contains both chlorides
and sulfides, and with wood shingles as compared with roofs of other
composition. Tests indicate that all-copper roofs 0.5 mm or more in
thickness would last several centuries in urban atmospheres.
If the green patina on copper alloys is desired for aesthetic reasons,
pretreatment of the surface with appropriate passivating solutions is
recommended. If oxidation by sulfur compounds precedes the desired
reaction, the surface will present only a dark brown color for many years.
The behavior of copper and copper alloys in three typical atmospheres is
summarized in Table 9.6.

9.7.3 Nickel and Nickel Alloys
Electrodeposited nickel and electroless nickel are widely used as a
protective coating for atmospheric exposure, and some nickel alloys, while,
selected for other reasons, are also exposed to atmospheric corrosion.