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Planned Interval Tests: Field/Lab Methodology to determine duration effects


Abstract:
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.
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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:

  1. The differences in corrosion rates for short and long term exposures.

  2. 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); A1, At, At+1, and B represent the corrosion damage experienced by each test specimen; A2, is a calculated value obtained by subtracting At from At+1.


CorrosivityObserved Weight
Changes During
Corrosion Testing
Criteria
Liquid corrosivenessUnchanged
Decreased
Increased
A1 = B
B < A1
A1 < B
Metal corrodibilityUnchanged
Decreased
Increased
A2 = B
A1 < B
B < A2
Combinations of Situations
Liquid CorrosivenessMetal CorrodibilityCriteria
UnchangedUnchangedA1 = A2 = B
UnchangedDecreasedA2 < A1 = B
UnchangedIncreasedA1 = B < A2
DecreasedUnchangedA2 = B < A1
DecreasedDecreasedA2 < B < A1
DecreasedIncreasedA1 > B < A2
IncreasedUnchangedA1 < A2 = B
IncreasedDecreasedA1 < B > A2
IncreasedIncreasedA1 < B < A2
Table 1: Planned Interval Test Method



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