Page 413 - Corrosion Engineering Principles and Practice

382 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 383

The external evidence of attack may be blushing (loss of gloss),
chalking, change in color of the product, or extreme brittleness as in
Fig. 9.52. These effects are often readily observed on epoxy and
polyester polymers when they have been boldly exposed to the
environment. However, mechanical tests will usually be required to
reveal the extent of degradation of either thermoplastic or thermosetting
resins unless they have been exposed for decades to direct sunlight, as
in Fig. 9.52.
The effect of high atmospheric temperatures or heating from direct
exposure to the sun can be particularly severe on thermoplastic poly-
mers. Creep or distension of the polyvinyl chloride and polyethylene
plastics will occur readily unless provision is made to prevent over-
heating or stressing of the materials.
Polymeric materials should be thoroughly tested if they are to be
exposed freely in the atmosphere. ASTM Recommended Practice
D1435 describes the appropriate conditions for such test exposures and
suggests tests that might be used to evaluate changes in the materials.

FIGURE 9.52 Polycarbonate (Nalgene) bottle and polypropylene funnel
degraded over a few years of exposure to sun rays on a window sill.