Article 4, API 579-1: Fitness-For-Service (FFS) - Pitting Assessment
By Stephen J. Gliebe, P.E.
Did a recent external inspection on an exchanger reveal pitting corrosion
under the insulation? Did a unit upset result in internal pitting on the
bottom head of the reactor? If you answered yes to these questions, or if
pitting corrosion due to other factors is occurring in your plant, an
API-579-1 FFS pitting assessment may provide an alternative to costly
In this article, we will take a look at evaluating widely scattered
pitting by applying API 579-1 / ASME FFS-1 2007 Fitness-For-Service Level
1 assessment procedures.
What is Pitting?
Pitting is defined by API 579 as localized regions of metal loss
characterized by a pit diameter on the order of the plate thickness or
less. Widely scattered pitting is pitting that occurs over a significant
region of the component.
Level 1 Pitting Assessment Limitations
Level 1 pitting assessments are permitted only if certain conditions are
satisfied. The complete list of limitations in Parts 2 and 6 of API 579
should be reviewed before proceeding with a Level 1 pitting assessment.
Some of the limitations are:
- The pitting damage must be:
- located on only one surface of the component, e.g., internal
- composed of many pits.
- The component is:
- a type A component subjected to internal pressure. These are
components that have a design equation that specifically relates
pressure or liquid fill height, and other loads, to a required
wall thickness, such as pressure vessel cylindrical and conical
- not in cyclic service.
- not operating in the creep regime.
- considered to have sufficient material toughness.
Level 1 Methodology
In a Level 1 assessment, surface damage, quantified by the pitted area and
pit depth, is compared to standard pit charts to determine acceptability.
If the depth of all the pits is less than the specified corrosion
allowance, then a pitting assessment is not required.
However, if the pit depth exceeds the corrosion allowance, then a pitting
assessment should be considered. Note that for a Level 1 assessment, the
future pitting damage (FCA) is assumed to be zero, e.g., pitting corrosion
due to corrosion under insulation that has been mitigated by application
of an epoxy coating and permanent removal of the insulation. Below is a
summary of the assessment process. Details of the process, as well as the
nomenclature, can be found in API 579-1 Part 6.
- Review the limitations for a Level 1 assessment.
- Characterize the pitting damage, e.g., diameter, area, depth.
- Evaluate the component as detailed in API 579 Paragraph 6.4.2.
- Determine the uniform measured thickness trd away from the
- Calculate the MAWP using the thickness measured above.
- Locate the area of the component that has the highest density
of pitting damage.
- Obtain photographs and rubbings of the damaged areas.
- Determine the maximum pit depth wmax.
- Determine the ratio below. If Rwt < 0.2, the Level 1
assessment is not acceptable.
- Compare the surface damage from the photographs and rubbings to
the standard pit charts and select the chart that approximates the
actual damage of the component. Examples of different grades of
pitting are shown below, where black indicates pitting of the
- Determine the remaining strength factor (RSF) from the table
related to the pit chart selected. A typical RSF table is shown below.
- If RSF > RSFa, then the pitting damage is acceptable for the MAWP
calculated above. RSFa is the allowable remaining strength factor. It
is typically set at 0.90 for ASME Section VIII Division 1 equipment.
- If the pitting is unacceptable:
- Repair, rerate or replace.
- Lower the FCA.
- Conduct a Level 2 assessment.