Corrosion Corner
Polymer coatings and
laminated films as
corrosion barriers—Part II
Hello, everyone. Last month, we began a
discussion about how polymer barrier
properties can be broken down by
permeation and higher storage test temperatures.
This month, we’ll complete the discussion by reviewing
how polymer glass transition temperature
is modified by permeation and high storage test
temperatures.
Polymers have physical properties, such as tensile
strength and barrier strength, that disappear when
the polymer temperature is at or above the polymer
glass transition temperature (Tg). In other words, a
polymer loses all its physical properties, such as being
a barrier, when the temperature of the polymer
is raised above its Tg.
Example Tg magnitudes for unexposed drypolymer
are:
• Epoxies and polybutadiene: around 100°C
• Nylon 6: around 47°C
• Nylon 6,6: around 79°C
• Polypropylene: around -10°C
• PET: between 69°C and 85°C depending on the polyethylene
terephthalate (PET) grade
The Tg for a dry polymer is higher than the corresponding Tg
for a wet polymer. For example, the Tg for a dry epoxy coating is
100°C and the corresponding Tg for a wet epoxy ranges between
30°C and 60°C, depending on the chemical composition of a
formula and the extent to which formula ingredients permeate
the polymer.
It is often assumed that a higher storage temperature will accelerate
the rates of polymer and metal corrosion and thus reduce
the time needed for corrosion testing. This assumption is based
on the Arrhenius equation that states chemical reaction rates
double for every 10°C increase in temperature.
However, polymers do not follow the Arrhenius equation, as
illustrated in Figure 1. The metal corrosion rate under a polybutadiene
coating is plotted on the Y-axis and the corresponding
temperature is plotted on the X-axis. The test fluid in this case is
water.
Notice that the corrosion rate decreases from approximately
0.016mm per year to 0.005mm per year (10-1.8 to 10-2.3) as the
temperature increases from 20°C to 40°C. In other words, increasing
the temperature from 20°C to 40°C does not accelerate
the coating corrosion in this situation.
Notice also in Figure 1 that the corrosion rate increases from
approximately 0.004mm per year to 0.008mm per year when the
temperature increases from 60°–80°C. However, the corrosion
W. Stephen tait, ph.D.
Chief Science Officer & principal Consultant,
pair O Docs professionals, LLC
rustdr@pairodocspro.com
Figure 1: Coated metal corrosion rate-temperature trends
rate at 80°C should be 0.016mm per year if Arrhenius behavior is
being observed.
Consequently, some other mechanism explains the deflection
point in Figure 1. Indeed, what actually happened in this situation
is the 60°C temperature is at or above the wet polymer Tg
and the wet polymer is no longer a barrier between the water and
the underlying metal. Thus, Figure 1 illustrates that wet polymer
Tg is significantly lower than the dry polymer Tg. Temperatures
at or above Tg cause a coating to lose its barrier properties, as
evidenced by the higher metal corrosion rate.
Consequently, using higher storage temperatures to accelerate
spray package corrosion could lead to false positive results
(corrosion that does not normally occur). Corrosion test length
can be reduced with electrochemical corrosion testing when the
appropriate components are employed with this test methodology,
these being:
• Instruments;
• Measurement parameters;
• Exposure times;
• Analysis protocols;
• Sample size and
• Data analysis and interpretation protocols.
Thanks for your interest and I’ll see you in October. Spray
September 2018 Spray 33