FFS Forum: Pitting Corrosion Assessment in API 579

Looking back through my six years of FFS Forum articles, I realize that I’ve never addressed Part 6, Assessment of Pitting Corrosion (in fact, there are a few Parts that I’ve neglected, which is a problem I’ll rectify over the next several issues.). I think that might be because pitting is a fairly straightforward section and, it seems to me, not frequently used. Nonetheless, it is a handy Part and there are some interesting points and insights I can share.

I’ll also note that Inspectioneering presented an interesting and related blog post in August 2019 entitled “API 510 Pressure Vessel Inspection Code: FFS Evaluation of Widely Scattered Pits.” It discusses both API 579 and API 510. I will go into much more detail on API 579, but this blog also provides interesting observations related to API 510, which I won’t discuss in this article.

Types of Pitting

API 579 addresses (or defines) four types of pitting. These are:

• Widespread pitting
• Localized pitting
• Pitting within a locally thin area (LTA)
• LTA within a region of widespread pitting

Level 1 assessments can only be performed on widespread pitting. Level 2 and 3 assessments can be applied to any type.

Level 2 assessment of the two combined damage types (i.e., pitting within an LTA, and LTA within a pitted area) employs a “combined remaining strength factor” (RSF). This simply involves the calculation of an RSF for pitting and, separately, an RSF for the LTA. The combined RSF is the combination of these two:

Eq. 1

For more information on dealing with multiple damage mechanisms, see my FFS Forum article, “FFS Forum: Dealing with Multiple Damage Mechanisms in an FFS Assessment,” in the July/August 2020 issue of Inspectioneering Journal.

Levels 1, 2, 3: Three Different Approaches

Before we even start talking about the evaluation of pitting, let me remind you that if the pitting is entirely contained within the corrosion allowance, it does not require assessment. This is true for all the metal loss damage mechanisms (e.g., general and local metal loss and pitting), but it is NOT the case for crack-like flaws.

The three levels of assessment under Part 6 address the pitting in three entirely different ways: