An excellent technique for evaluating the influence of time on
corrosion of metals and variations in the corrosive environment with time is
called the "Planned Interval Test" (PIT). Corrosion coupons generally
provide an average corrosion rate normalized over the period of exposure. The
need for the PIT procedure arises due to variations in the general and
localized corrosion rates with time as the test progresses. This effect usually
results from either the formation or breakdown of protective films on the
surface of the material during the period of the test. Variations in corrosion
rate can also occur as a result of changes in the concentration of corrosive
agents in the environment.
A schematic representation of a typical PIT test procedure is
shown in the table below. It involves the exposure of multiple coupons for at
least three durations. These durations of exposure are selected to obtain data
for the period of initial exposure, after prolonged exposure and for a short
period at the end of the longer exposure period. Differences in the corrosion
rates among these three periods will help assess:
- The differences in corrosion
rates for short and long term exposures.
- The differences in corrosion
rates for similar periods at the beginning and end of the test period.
A typical application of the PIT test sequence for laboratory
testing would be to conduct tests in the corrosive environment for a total
duration of 30 days and also have two short duration exposure schedule to be
run during the first seven days and for the last seven days of the 30 day
period. Alternately, for field use, the durations are typically much longer
with a 90 day test duration common with short duration exposures of 14 to 30
days at the beginning and end of the 90 day period.
An example where the PIT procedure can provide
benefit is where there is a decrease in corrosion rate with time, such as where
sulfide films form on steel or where lead is exposed to sulfuric acid. In these
situations, short term tests will produce higher than expected corrosion rate
data that will be completely misleading. Additionally, active-passive materials
such as stainless steels can also provide misleading data since pitting often
does not initiate immediately upon exposure but requires a period of incubation.
In this latter case, pitting rates determined from corrosion coupons can be
non-conservative since pitting rates can increase with time due to their
autocatalytic nature. Generally, longer term tests provide data that are more
realistic those obtained from short term tests.
Table 1: Planned Interval Test Method
Conditions: identical specimens are placed in the same corrosive liquid;
imposed conditions of test are constant for the entire time (t + 1); A
1,
A
t, A
t+1, and B represent the corrosion damage experienced
by each test specimen; A
2, is a calculated value obtained by subtracting
A
t from A
t+1.
| Corrosivity | Observed Weight
Changes During
Corrosion Testing | Criteria |
| Liquid corrosiveness | Unchanged
Decreased
Increased | A1 = B
B < A1
A1 < B |
| Metal corrodibility | Unchanged
Decreased
Increased | A2 = B
A1 < B
B < A2 |
| Combinations of Situations |
| Liquid Corrosiveness | Metal Corrodibility | Criteria |
| Unchanged | Unchanged | A1 = A2 = B |
| Unchanged | Decreased | A2 < A1 = B |
| Unchanged | Increased | A1 = B < A2 |
| Decreased | Unchanged | A2 = B < A1 |
| Decreased | Decreased | A2 < B < A1 |
| Decreased | Increased | A1 > B < A2 |
| Increased | Unchanged | A1 < A2 = B |
| Increased | Decreased | A1 < B > A2 |
| Increased | Increased | A1 < B < A2 |
|
Table 1: Planned Interval Test Method