3.2 Pitting Evaluation & Corroded Area Limits

Pitting Evaluation & Corroded Area Limits

Pitting corrosion is an extremely localized degradation mechanism characterized by cavities, pinholes, or localized depressions on the metal surface. Unlike uniform corrosion, which thins large surface areas at a steady, predictable rate, pitting can penetrate deep into a vessel wall while consuming very little overall metal volume. In refining, chemical processing, and upstream production, pitting frequently develops under insulation (CUI), beneath deposits or sludge (under-deposit corrosion), from stagnant halide solutions (chlorides), or via microbiological activity (MIC).

Because pits act as localized geometric stress risers, the inspector must evaluate whether the pitting can be classified as widely scattered pitting under API 510 Section 7.4.3 or whether it threatens the pressure-retaining integrity of the vessel.

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1. API 510 Section 7.4.3 Three-Part Acceptance Criteria

API 510 provides a straightforward, highly tested three-part screening standard for evaluating widely scattered pitting. To be accepted without requiring pressure derating or weld repair, the pitted area must satisfy all three of the following criteria simultaneously:

+-----------------------------------------------------------------------------------------+
|                   API 510 WIDELY SCATTERED PITTING ACCEPTANCE CRITERIA                  |
|                                                                                         |
|   CRITERION 1: REMAINING THICKNESS BELOW PIT BOTTOM (DEPTH LIMIT)                       |
|   - Remaining wall thickness beneath ANY pit must be AT LEAST 50% of required thickness |
|   - Formula: t_pit_rem ≥ (0.50 * t_required) + CA_future                                |
|                                                                                         |
|   CRITERION 2: LINEAR PITTING LIMIT (8-INCH LINE TEST)                                  |
|   - Total sum of pit lengths along ANY straight 8-inch (200 mm) line must NOT exceed    |
|     2.0 inches (50 mm) in ANY orientation                                               |
|   - Formula: Σ L_pit ≤ 2.0 in. (along any 8.0 in. line)  --> [Max 25% linear]           |
|                                                                                         |
|   CRITERION 3: SURFACE AREA LIMIT (100 SQUARE INCH GRID TEST)                           |
|   - Total cumulative area of all pits in ANY 100 sq. in. (650 cm²) area must NOT exceed |
|     7.0 square inches (45 cm²)                                                          |
|   - Formula: Σ A_pit ≤ 7.0 sq. in. (in any 100 sq. in. window) --> [Max 7% area]        |
+-----------------------------------------------------------------------------------------+

Criterion 1: Maximum Pit Depth / Remaining Wall Thickness

The remaining thickness of the vessel wall at the bottom of the deepest pit ($t_{\text{pit, rem}}$) cannot be less than one-half ($50\%$) of the minimum required wall thickness ($t_{\text{required}}$), plus the corrosion allowance ($CA$) required until the next scheduled inspection:

$$t_{\text{pit, rem}} = t_{\text{actual}} - d_{\text{pit}} \ge 0.50 \cdot t_{\text{required}} + CA_{\text{future}}$$

Where:

• $t_{\text{actual}}$ = actual thickness of the unpitted surrounding plate (in.)
• $d_{\text{pit}}$ = depth of the deepest pit measured with a pit gauge or depth micrometer (in.)
• $t_{\text{required}}$ = minimum required shell/head thickness calculated per ASME code formulas (in.)
• $CA_{\text{future}}$ = estimated metal loss prior to the next inspection ($CR \times \text{years until next inspection}$)

Criterion 2: Linear Pitting Limit (The 8-Inch Rule)

An inspector draws a straight line measuring $8.0\text{ in.}$ ($200\text{ mm}$) in length in any direction across the pitted region. The sum of the dimensions of all pits intersected by this 8-inch line must not exceed $2.0\text{ in.}$ ($50\text{ mm}$):

$$\sum_{i=1}^{n} L_i \le 2.0\text{ in.} \quad (\text{along any } 8.0\text{ in. line})$$

This represents a maximum linear density of $25\%$. If pits coalesce or form a continuous linear chain exceeding $2.0\text{ in.}$, stress concentration shifts from 3D localized hemispherical dispersion to a 2D planar notch, which significantly increases crack initiation risk.

Criterion 3: Total Area Limit (The 100 Square Inch Rule)

In any $100\text{ sq. in.}$ ($650\text{ cm}^2$) area of the shell or head (e.g., a $10\text{ in.} \times 10\text{ in.}$ square or an $11.28\text{ in.}$ diameter circle), the cumulative surface area of all pits must not exceed $7.0\text{ sq. in.}$ ($45\text{ cm}^2$):

$$\sum_{i=1}^{n} A_i \le 7.0\text{ sq. in.} \quad (\text{in any } 100\text{ sq. in. region})$$

This represents a maximum surface area density of $7\%$. If more than $7\%$ of the surface is pitted, the damage is no longer considered "widely scattered" and must instead be evaluated as general metal loss or a locally thinned area (LTA).

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2. Pitting vs. Grooving vs. Localized Erosion

Inspectors must distinguish between different forms of localized metal loss because the code rules governing each mechanism differ substantially.

+-----------------------------------------------------------------------------------------+
|                         DAMAGE MORPHOLOGY VISUAL COMPARISON                             |
|                                                                                         |
|   [WIDELY SCATTERED PITTING]            [LINEAR GROOVING]         [LOCALIZED EROSION]   |
|   +-----------------------+           +-----------------------+   +-------------------+ |
|   |   ( )       ( )       |           |      |   |            |   |   ~~~~~~~         | |
|   |         ( )       ( ) |           |      |   |            |   |  ( ~~~~~~~ )      | |
|   |   ( )          ( )    |           |      |   |            |   |   ( ~~~~~ )       | |
|   |       ( )    ( )      |           |      |   |            |   |     ~~~~          | |
|   +-----------------------+           +-----------------------+   +-------------------+ |
|   - 3D isolated cavities              - 2D stress concentration   - Smooth scallops   | |
|   - Governed by 7.4.3                 - Governed by LTA / FFS     - Governed by LTA   | |
+-----------------------------------------------------------------------------------------+

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3. Pitting in Weld Seams vs. Base Metal

Pitting in or adjacent to weld seams presents severe structural risks that do not apply to base metal pitting:

1. High Stress Concentration ($K_t$): Welds already contain residual tensile stresses from solidification and shrinkage (unless fully post-weld heat treated). A pit in the weld creates a compound stress riser.
2. Microstructural Vulnerability in the HAZ: The heat-affected zone often possesses coarse grain structures or untempered martensite, making it susceptible to environmental cracking mechanisms (e.g., wet $\text{H}_2\text{S}$ sulfide stress cracking, caustic cracking, or chloride stress corrosion cracking) that nucleate at pit roots.
3. Volumetric Weld Flaws: Pitting that intersects subsurface weld porosity or lack of fusion creates a direct leakage path or crack initiation site.

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4. Detailed Step-by-Step Pitting Evaluation Example

Let us evaluate an in-service vessel shell to determine if an area of corrosion satisfies API 510 pitting limits.

Vessel Data:

• Nominal Shell Thickness ($t_{\text{nom}}$): $0.750\text{ in.}$
• Calculated Required Thickness ($t_{\text{required}}$): $0.500\text{ in.}$
• Future Corrosion Allowance ($CA_{\text{future}}$): $0.050\text{ in.}$
• Current Unpitted Base Plate Thickness ($t_{\text{actual}}$): $0.700\text{ in.}$

Inspection Findings in a $10\text{ in.} \times 10\text{ in.}$ Area ($100\text{ sq. in.}$):

• Pit depths measured with a needle-point depth gauge range from $0.150\text{ in.}$ to $0.380\text{ in.}$
• The deepest pit measures $d_{\text{pit, max}} = 0.380\text{ in.}$
• The total sum of pit diameters along the worst-case 8-inch straight line is $1.65\text{ in.}$
• The total cumulative surface area of all pits in the $100\text{ sq. in.}$ window is $5.20\text{ sq. in.}$

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Evaluation Step 1: Check Remaining Thickness Below Deepest Pit

Calculate actual remaining thickness below the pit bottom:

$$t_{\text{pit, rem}} = t_{\text{actual}} - d_{\text{pit, max}} = 0.700\text{ in.} - 0.380\text{ in.} = \mathbf{0.320\text{ in.}}$$

Calculate the minimum allowable thickness below the pit under API 510 Section 7.4.3:

$$t_{\text{pit, limit}} = (0.50 \cdot t_{\text{required}}) + CA_{\text{future}} = (0.50 \cdot 0.500\text{ in.}) + 0.050\text{ in.} = 0.250 + 0.050 = \mathbf{0.300\text{ in.}}$$

Depth Check Result:

$$t_{\text{pit, rem}} (0.320\text{ in.}) \ge t_{\text{pit, limit}} (0.300\text{ in.}) \quad \rightarrow \quad \mathbf{PASSED}$$

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Evaluation Step 2: Check Linear Pitting Limit Along 8-Inch Line

• Measured cumulative pit length along worst-case line: $L_{\text{pits}} = 1.65\text{ in.}$
• API 510 maximum allowable limit along an 8-inch line: $L_{\text{allowable}} = 2.00\text{ in.}$

Linear Check Result:

$$1.65\text{ in.} \le 2.00\text{ in.} \quad \rightarrow \quad \mathbf{PASSED}$$

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Evaluation Step 3: Check Cumulative Surface Area in 100 sq. in.

• Measured cumulative pit surface area: $A_{\text{pits}} = 5.20\text{ sq. in.}$
• API 510 maximum allowable area in $100\text{ sq. in.}$: $A_{\text{allowable}} = 7.00\text{ sq. in.}$

Area Check Result:

$$5.20\text{ sq. in.} \le 7.00\text{ sq. in.} \quad \rightarrow \quad \mathbf{PASSED}$$

Final Determination:

Because the pitted area satisfies all three API 510 criteria, the shell is acceptable for continued operation at full design MAWP without requiring immediate repair or derating.

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5. Common Exam Pitfalls & Code Traps