Study guide
3.2 Pitting Evaluation & Corroded Area Limits
API 510 Pressure Vessel Inspector Exam
Track your progress as you read
Finished sections get a check mark.
1 Chapter 1: Introduction, Scope & Certification Requirements
1.1 Exam Structure, Administration & Study Strategy 1.2 API 510 Code Scope, Exclusions & Jurisdictional Boundaries 1.3 Roles, Responsibilities & Inspector Qualification
2 Chapter 2: Inspection Planning, Data Evaluation & Remaining Life
2.1 Corrosion Rates, Remaining Life & Corrosion Allowance Calculations 2.2 Inspection Intervals, On-Stream Inspection & Interval Deferrals 2.3 Condition Monitoring Locations (CMLs) & Data Analysis
3 Chapter 3: Vessel Evaluation, Corrosion Averaging & Fitness-for-Service
3.1 Corrosion Averaging & Minimum Thickness Evaluation 3.2 Pitting Evaluation & Corroded Area Limits 3.3 Fitness-for-Service (API 579-1) & Risk-Based Inspection (API 580)
4 Chapter 4: ASME Section VIII Div 1 Design & Thickness Calculations
4.1 Internal Pressure Design & Cylindrical Shell Calculations 4.2 Ellipsoidal & Hemispherical Head Calculations 4.3 Spherical Shells, Static Head & Vessel MAWP 4.4 Openings, Nozzle Reinforcement & External Pressure Basics
5 Chapter 5: ASME Section VIII Fabrication, Welds & Joint Efficiency
5.1 Weld Joint Categories (UW-3) & Joint Efficiencies (UW-12) 5.2 Radiographic Examination (UW-11, UW-51, UW-52) 5.3 Post-Weld Heat Treatment (PWHT) & Impact Testing (UCS-56, UCS-66)
6 Chapter 6: Welding Inspection & Metallurgy (API 577 & ASME Sec IX)
6.1 Welding Procedures (WPS/PQR) & Essential Variables 6.2 Welder Performance Qualification (WPQ) & Testing 6.3 Welding Processes, Metallurgy & Weld Discontinuities (API 577)
7 Chapter 7: Nondestructive Examination (ASME Section V)
7.1 Radiographic Examination Techniques & Image Quality Indicators (Article 2) 7.2 Ultrasonic Thickness Measurement (Article 23, SE-797) 7.3 ASME Section V Articles 1, 6 & 7: General Requirements, PT & MT
8 Chapter 8: Damage Mechanisms (API RP 571)
8.1 Atmospheric Corrosion & Corrosion Under Insulation 8.2 HTHA, High-Temperature H2/H2S Corrosion & Sulfidation 8.3 Amine, Caustic & Chloride Stress Corrosion Cracking 8.4 Wet H2S Damage, Brittle Fracture & Erosion-Corrosion 8.5 Boiler-Water, Steam-Condensate & Hydrochloric Acid Corrosion 8.6 Caustic Corrosion & Naphthenic Acid Corrosion
9 Chapter 9: Pressure Vessel Inspection Practices (API RP 572 & API RP 578)
9.1 Internal & External Vessel Inspection Practices (API 572) 9.2 Structural Components, Internal Trays, Linings & Skirts 9.3 Material Verification Programs & Positive Material Identification (API 578)
10 Chapter 10: Pressure-Relieving Devices (API RP 576)
10.1 PRD Types, Construction & Operating Principles 10.2 PRD Causes of Failure, Shop Testing & Overhaul 10.3 PRD Inspection Intervals, Records & Rupture Disks
11 Chapter 11: Repairs, Alterations, Rerating & Pressure Testing (API 510 & PCC-2)
11.1 Repair vs. Alteration Authorization, Execution & Methods 11.2 PCC-2 Core Welded Repairs: Articles 101, 201, 202, 211 & 212 11.3 PCC-2 Special Welding: Articles 209, 210, 215 & 216 11.4 PCC-2 Excavation, Flange, Exchanger & Test Articles 304, 305, 312, 501 & 502 11.5 Vessel Rerating Requirements & Engineering Calculations 11.6 Pressure Testing: Hydrostatic, Pneumatic & NDE in Lieu of Testing
3.2 Pitting Evaluation & Corroded Area Limits
Key Takeaways Explain This Topic AI
Under API 510 Section 7.4.3, widely scattered pits are acceptable if the remaining wall thickness at the bottom of every pit is at least 50% of the required thickness (t_pit_rem ≥ 0.5 * t_required + CA_future).
The cumulative linear dimension of pitting along any straight 8.0-inch (200 mm) line drawn in any direction must not exceed 2.0 inches (50 mm), representing a 25% linear limit.
The cumulative surface area of all pits within any 100 square inch (650 cm²) area of the vessel wall must not exceed 7.0 square inches (45 cm²), representing a 7% surface area limit.
Pitting occurring directly within weld seams or heat-affected zones (HAZs) cannot be evaluated under widely scattered pitting rules due to severe stress intensification and cracking susceptibility.
Linear grooving, canal corrosion, and flow-induced erosion cannot be evaluated as widely scattered pitting and must be treated as local thin areas (LTAs) or notch-like flaws under API 579.
Last updated: August 2026
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.
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 pit, rem t_{\text{pit, rem}} t pit, rem ) cannot be less than one-half ( 50 % 50\% 50% ) of the minimum required wall thickness ( t required t_{\text{required}} t required ) , plus the corrosion allowance ( C A CA C A ) required until the next scheduled inspection: t pit, rem = t actual − d pit ≥ 0.50 ⋅ t required + C A future t_{\text{pit, rem}} = t_{\text{actual}} - d_{\text{pit}} \ge 0.50 \cdot t_{\text{required}} + CA_{\text{future}} t pit, rem = t actual − d pit ≥ 0.50 ⋅ t required + C A future
Where:
- t actual t_{\text{actual}} t actual = actual thickness of the unpitted surrounding plate (in.)
- d pit d_{\text{pit}} d pit = depth of the deepest pit measured with a pit gauge or depth micrometer (in.)
t required t_{\text{required}} t required = minimum required shell/head thickness calculated per ASME code formulas (in.)
C A future CA_{\text{future}} C A future = estimated metal loss prior to the next inspection ( C R × years until next inspection CR \times \text{years until next inspection} CR × years until next inspection )
Criterion 2: Linear Pitting Limit (The 8-Inch Rule)
An inspector draws a straight line measuring 8.0 in. 8.0\text{ in.} 8.0 in. ( 200 mm 200\text{ mm} 200 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 in. 2.0\text{ in.} 2.0 in. ( 50 mm 50\text{ mm} 50 mm ) : ∑ i = 1 n L i ≤ 2.0 in. ( along any 8.0 in. line ) \sum_{i=1}^{n} L_i \le 2.0\text{ in.} \quad (\text{along any } 8.0\text{ in. line}) i = 1 ∑ n L i ≤ 2.0 in. ( along any 8.0 in. line )
This represents a maximum linear density of 25 % 25\% 25% . If pits coalesce or form a continuous linear chain exceeding 2.0 in. 2.0\text{ in.} 2.0 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 sq. in. 100\text{ sq. in.} 100 sq. in. ( 650 cm 2 650\text{ cm}^2 650 cm 2 ) area of the shell or head (e.g., a 10 in. × 10 in. 10\text{ in.} \times 10\text{ in.} 10 in. × 10 in. square or an 11.28 in. 11.28\text{ in.} 11.28 in. diameter circle), the cumulative surface area of all pits must not exceed 7.0 sq. in. 7.0\text{ sq. in.} 7.0 sq. in. ( 45 cm 2 45\text{ cm}^2 45 cm 2 ) : ∑ i = 1 n A i ≤ 7.0 sq. in. ( in any 100 sq. in. region ) \sum_{i=1}^{n} A_i \le 7.0\text{ sq. in.} \quad (\text{in any } 100\text{ sq. in. region}) i = 1 ∑ n A i ≤ 7.0 sq. in. ( in any 100 sq. in. region )
This represents a maximum surface area density of 7 % 7\% 7% . If more than 7 % 7\% 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).
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 Classification Morphological Description Typical Causes Code Evaluation Standard
Widely Scattered Pitting Isolated, spherical, or conical depressions dispersed across base metal CUI, stagnant halides, low-flow oxygen attack API 510 Section 7.4.3 (50% depth, 2 in./8 in. line, 7 sq. in./100 sq. in.)
Linear Grooving / Canal Corrosion Continuous, elongated, narrow channels running along flow paths or weld toes Acid condensate run-down, vapor-line reflux, boiler wash-down API 510 Section 7.4.2 (LTA) or API 579-1 Part 5 / Part 9 (Crack-like notch)
Localized Erosion / Impingement Smooth, scalloped, horseshoe-shaped thinning opposite nozzles or inlet baffles High-velocity slurry, droplet impingement, turbulent flashing API 510 Section 7.4.2 (Corrosion Averaging) or API 579-1 Part 4/5
Weld Seam Pitting Pitting concentrated directly along weld cap, root, or HAZ boundary Galvanic difference between weld metal and base plate, preferential HAZ attack API 579-1 Part 6 / Engineering Review (Cannot use basic 7.4.3 base metal rules) +-----------------------------------------------------------------------------------------+
- | 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 | |
- +-----------------------------------------------------------------------------------------+
- 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:
High Stress Concentration ( K t K_t 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.
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 H 2 S \text{H}_2\text{S} H 2 S sulfide stress cracking, caustic cracking, or chloride stress corrosion cracking) that nucleate at pit roots.
Volumetric Weld Flaws: Pitting that intersects subsurface weld porosity or lack of fusion creates a direct leakage path or crack initiation site.
Caution
Critical Inspection Rule: The widely scattered pitting rules in API 510 Section 7.4.3 apply strictly to base metal and general vessel surfaces. If pitting is concentrated along a longitudinal or circumferential weld seam, the inspector must not use the 50% depth rule; instead, the weld must be evaluated by a pressure vessel engineer using API 579-1 / ASME FFS-1 Part 6 or repaired in accordance with API 510 Section 8.
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 nom t_{\text{nom}} t nom ): 0.750 in. 0.750\text{ in.} 0.750 in.
Calculated Required Thickness ( t required t_{\text{required}} t required ): 0.500 in. 0.500\text{ in.} 0.500 in.
Future Corrosion Allowance ( C A future CA_{\text{future}} C A future ): 0.050 in. 0.050\text{ in.} 0.050 in.
Current Unpitted Base Plate Thickness ( t actual t_{\text{actual}} t actual ): 0.700 in. 0.700\text{ in.} 0.700 in.
Inspection Findings in a 10 in. × 10 in. 10\text{ in.} \times 10\text{ in.} 10 in. × 10 in. Area ( 100 sq. in. 100\text{ sq. in.} 100 sq. in. ):
Pit depths measured with a needle-point depth gauge range from 0.150 in. 0.150\text{ in.} 0.150 in. to 0.380 in. 0.380\text{ in.} 0.380 in.
The deepest pit measures d pit, max = 0.380 in. d_{\text{pit, max}} = 0.380\text{ in.} d pit, max = 0.380 in.
The total sum of pit diameters along the worst-case 8-inch straight line is 1.65 in. 1.65\text{ in.} 1.65 in.
The total cumulative surface area of all pits in the 100 sq. in. 100\text{ sq. in.} 100 sq. in. window is 5.20 sq. in. 5.20\text{ sq. in.} 5.20 sq. in.
Evaluation Step 1: Check Remaining Thickness Below Deepest Pit
Calculate actual remaining thickness below the pit bottom: t pit, rem = t actual − d pit, max = 0.700 in. − 0.380 in. = 0.320 in. t_{\text{pit, rem}} = t_{\text{actual}} - d_{\text{pit, max}} = 0.700\text{ in.} - 0.380\text{ in.} = \mathbf{0.320\text{ in.}} t pit, rem = t actual − d pit, max = 0.700 in. − 0.380 in. = 0.320 in.
Calculate the minimum allowable thickness below the pit under API 510 Section 7.4.3: t pit, limit = ( 0.50 ⋅ t required ) + C A future = ( 0.50 ⋅ 0.500 in. ) + 0.050 in. = 0.250 + 0.050 = 0.300 in. 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.}} t pit, limit = ( 0.50 ⋅ t required ) + C A future = ( 0.50 ⋅ 0.500 in. ) + 0.050 in. = 0.250 + 0.050 = 0.300 in.
Depth Check Result: t pit, rem ( 0.320 in. ) ≥ t pit, limit ( 0.300 in. ) → P A S S E D t_{\text{pit, rem}} (0.320\text{ in.}) \ge t_{\text{pit, limit}} (0.300\text{ in.}) \quad \rightarrow \quad \mathbf{PASSED} t pit, rem ( 0.320 in. ) ≥ t pit, limit ( 0.300 in. ) → PASSED
Evaluation Step 2: Check Linear Pitting Limit Along 8-Inch Line
Measured cumulative pit length along worst-case line: L pits = 1.65 in. L_{\text{pits}} = 1.65\text{ in.} L pits = 1.65 in.
API 510 maximum allowable limit along an 8-inch line: L allowable = 2.00 in. L_{\text{allowable}} = 2.00\text{ in.} L allowable = 2.00 in.
Linear Check Result: 1.65 in. ≤ 2.00 in. → P A S S E D 1.65\text{ in.} \le 2.00\text{ in.} \quad \rightarrow \quad \mathbf{PASSED} 1.65 in. ≤ 2.00 in. → PASSED
Evaluation Step 3: Check Cumulative Surface Area in 100 sq. in.
Measured cumulative pit surface area: A pits = 5.20 sq. in. A_{\text{pits}} = 5.20\text{ sq. in.} A pits = 5.20 sq. in.
API 510 maximum allowable area in 100 sq. in. 100\text{ sq. in.} 100 sq. in. : A allowable = 7.00 sq. in. A_{\text{allowable}} = 7.00\text{ sq. in.} A allowable = 7.00 sq. in.
Area Check Result: 5.20 sq. in. ≤ 7.00 sq. in. → P A S S E D 5.20\text{ sq. in.} \le 7.00\text{ sq. in.} \quad \rightarrow \quad \mathbf{PASSED} 5.20 sq. in. ≤ 7.00 sq. in. → 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.
5. Common Exam Pitfalls & Code Traps
Exam Scenario / Question Common Trap Correct API 510 Rule
Depth Limit Base Calculating 50% of the nominal thickness Pit depth is calculated against 50% of the minimum required thickness ( t required t_{\text{required}} t required ) , not nominal.
Forgetting Future CA Leaving out the corrosion allowance in the depth check The remaining thickness must be ≥ 0.5 t req + C A future \ge 0.5 t_{\text{req}} + \mathbf{CA_{\text{future}}} ≥ 0.5 t req + C A future .
8-Inch Line vs 100 Sq. In. Mixing up 2 inches in 8 inches with 7 sq. in. in 100 sq. in. Linear limit = 2.0 in. = 2.0\text{ in.} = 2.0 in. per 8.0 in. 8.0\text{ in.} 8.0 in. ( 25 % 25\% 25% ); Area limit = 7.0 sq. in. = 7.0\text{ sq. in.} = 7.0 sq. in. per 100 sq. in. 100\text{ sq. in.} 100 sq. in. ( 7 % 7\% 7% ).
Grooving in Welds Applying pitting rules to longitudinal grooving Linear grooving is a planar notch that must be evaluated under LTA / FFS rules, never scattered pitting rules.