Page 367 - Corrosion Engineering Principles and Practice

336 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 337

Cycling temperature may accelerate these processes and produce
severe corrosion on metal objects in unheated warehouses, and on
metal tools or other objects stored in humid environments. Since the
dew point of an atmosphere indicates the equilibrium condition of
condensation and evaporation from a surface, it is advisable to
maintain the temperature some 10 to 15°C above the dew point to
ensure that no corrosion will occur by condensation on a surface that
could be colder than the ambient environment.
As the ambient temperature drops during the evening, metallic
surfaces tend to remain warmer than the humid air surrounding them
and do not begin to collect condensation until some time after the dew
point has been reached. As the temperature begins to rise in the surrounding
air, the lagging temperature of the metal structures will tend to make them
act as condensers, maintaining a film of moisture on their surfaces.
In the following example, a complete truckload of expensive
vehicle parts were lost due to this effect. In the “Just in Time”
manufacturing era, system parts can be produced at different plants
within a few driving hours of the final assembly line. These parts will
therefore only be in transit for a few hours and usually do not require
much protection against corrosive elements.
On a spring Friday, two trailer trucks were loaded at the part-
manufacturing facility, with engine parts packed in their habitual
dunnage* for delivery at the assembly plant seven driving hours away.
The first truck left early that morning and the parts were received in
good condition at the assembly plant. The second truck was loaded later
during the day. However, a long weekend was coming and the decision
was made by the transport company to park the trailer truck near the
manufacturing area for delivery to the assembly plant the following
week. The second truck left the manufacturing area on the following
Monday afternoon to be delivered the next day to the assembly plant
where it was discovered that all the parts were surface rusted and
deemed unusable for production.
Figure 9.7 shows a plot of the ambient and dew-point temperatures
extracted from the weather data file for the three days the loaded truck
spent in the manufacturing area while parked outside without any
particular care or attention. An obvious feature in Fig. 9.7 is the very small
difference between the ambient and dew-point temperatures during the
three nights the truck was parked idle in the manufacturing area.
However, this factor alone would not be sufficient to explain the serious
corrosion experienced by all the parts stored in the second truck.
A more dramatic situation is revealed by considering that a certain
time lag existed between the ambient temperature and the temperature
of the steel parts enclosed in their donnage. The five hours lag
between these two temperatures, shown as a dotted line in Fig. 9.8, is

* Dunnage is the name for the materials used in holds and containers to protect goods
and their packaging from moisture, contamination, and mechanical damage.