Localized corrosion depth testing is the quantitative measurement and evaluation of metal loss confined to small, discrete areas of a metallic surface, such as pits, crevices, and under-deposit attack. Using contact gauges, optical microscopy, 3D surface metrology, and non-destructive evaluation in accordance with ASTM G46, ISO 11463, and GB/T 18590, this testing determines maximum pit depth, pitting rate, and pitting factor, providing critical input for fitness-for-service assessment of pipelines, storage tanks, pressure vessels, and marine structures.
Technical Overview
Localized corrosion depth testing is a specialized metallurgical evaluation used to measure and characterize metal loss that is concentrated in small, discrete regions of a component rather than distributed uniformly across its surface. Common forms of localized corrosion include pitting corrosion, crevice corrosion, under-deposit corrosion, galvanic attack, and chloride-induced penetration of passive alloys such as stainless steel and aluminum.
Localized corrosion is particularly dangerous because a component can retain an apparently acceptable average wall thickness while a single deep pit penetrates toward perforation or triggers a leak or crack initiation. For this reason, measuring only general wall thickness is insufficient for assets exposed to chloride environments, produced water, acidic condensates, or microbiologically influenced corrosion. Localized corrosion depth testing quantifies the severity of attack and feeds the results into remaining life calculations and repair decisions.
The principal internationally recognized references for this testing include ASTM G46 (Standard Guide for Examination and Evaluation of Pitting Corrosion), ISO 11463 (Corrosion of metals and alloys — Evaluation of pitting corrosion), GB/T 18590 (the Chinese national standard aligned with ISO 11463), ASTM G48 (pitting and crevice corrosion testing of stainless steels in ferric chloride solution), and ISO 8993 / ISO 8994 for rating pitting on anodized aluminum. Fitness-for-service references such as ASME B31G and API 579 use measured maximum pit depths to calculate the remaining strength of corroded piping and equipment.
Testing Samples
Field-Exposed Coupons and Retrieved Components
Corrosion coupons retrieved from process streams, pipe spools, tank floor plates, heat exchanger tube sections, flange faces, fasteners, and structural members removed from service are examined for maximum pit depth and pit density. Samples should be cleaned according to standardized chemical cleaning procedures to remove corrosion products without removing base metal, then preserved in desiccated storage before measurement.
Laboratory-Exposed Test Coupons
Coupons subjected to accelerated laboratory exposure, such as ferric chloride immersion per ASTM G48, salt spray exposure per ASTM B117, cyclic electrochemical polarization per ASTM G61, or immersion in process simulants, are measured after exposure to determine whether the material and surface condition resist localized attack in the intended service environment.
Welded Joints and Heat-Affected Zones
Weldments, clad overlays, and their heat-affected zones are frequently preferred sites for pitting and crevice attack due to compositional and microstructural segregation. Dedicated specimens allow pit depth comparison between weld metal, heat-affected zone, and base metal.
Coated and Clad Surfaces
Coated steel substrates, thermally sprayed layers, and roll-bonded clad plates are tested after holiday formation or coating breakdown to characterize under-film and under-deposit penetration depth and to verify the integrity of the remaining barrier layer.
Testing Items
Depending on the specimen type and the applicable standard, localized corrosion depth testing typically reports the following parameters:
- Maximum pit depth and average pit depth at designated measurement locations
- Pit density, expressed as the number of pits per unit area
- Pit size and shape distribution, including width-to-depth aspect ratio
- Pit morphology classification per the rating charts of ASTM G46
- Pitting rate, expressed in mm/year or mils per year (mpy)
- Pitting factor, defined as the ratio of deepest pit penetration to average general corrosion penetration
- Crevice corrosion depth at gasket surfaces, lap joints, and under deposits
- Remaining wall thickness beneath the deepest observed attack
- Statistical extrapolation of maximum pit depth over a larger surface using extreme value analysis
Testing Methods
Direct Contact Depth Measurement
Depth micrometers and needle-probe dial or digital gauges measure the vertical distance between the pit rim and the pit bottom. This method offers a typical resolution of approximately 0.01 mm and is well suited to pits with openings large enough to admit the probe. It is widely used for field surveys and coupon evaluation.
Optical Microscopy and Metallographic Cross-Sectioning
For narrow or undercut pits, a metallurgical microscope with a calibrated focus column measures depth as the difference in focal plane between the pit edge and pit floor. Where the deepest pit has been located by screening, cross-sectioning through the pit produces a direct geometric profile of depth, undercutting, and subsurface propagation, which is documented with micrographs and image analysis.
Replica (Molding) Method
Silicone-based or dental impression materials are pressed into corroded areas to capture a negative of the pit topography. The cured replica is then measured in the laboratory with a depth gauge or profilometer. This technique allows detailed pit geometry to be recorded on in-service equipment without cutting out material and preserves a physical record for follow-up inspections.
3D Optical Surface Metrology
Laser scanning confocal microscopy, white-light interferometry, focus variation microscopy, and structured-light 3D scanning generate full topographic maps of the corroded surface with micrometer-level resolution. Post-processing software automatically extracts maximum depth, pit volume, cross-sectional profiles, and depth histograms, which improves repeatability compared with single-point manual readings.
Non-Destructive Evaluation Methods
For in-service assets, pit depth is assessed without excavation or destruction using a combination of volumetric and surface techniques:
- Spot ultrasonic thickness gauging at gridded or marked pit locations to determine remaining wall thickness
- Phased array ultrasonic testing (PAUT) and time-of-flight diffraction for wall thickness mapping and pit profiling
- Pulsed eddy current (PEC) screening through coatings and insulation to detect areas of localized wall loss
- Radiographic testing (RT) for profile-based estimation of internal pit depth in pipes and small-bore components
- Magnetic flux leakage (MFL) scanning for large-area tank floor surveys
- Remote visual inspection combined with laser profilometry for accessible and confined surfaces
Data Evaluation and Statistical Analysis
Measured pit depths are evaluated per ASTM G46 using density and size rating charts, and depth-versus-area plots. Where only a limited area can be inspected, extreme value (Gumbel) statistics extrapolate the probable maximum pit depth over the full component surface, an essential step for integrity assessments performed in accordance with ASME B31G or API 579 procedures.
Applications
Localized corrosion depth testing supports material selection, integrity management, and failure analysis across all major industry sectors:
- Oil and gas gathering lines, transmission pipelines, and refinery equipment subject to carbon dioxide, hydrogen sulfide, and microbiologically influenced pitting
- Atmospheric and internal corrosion of storage tank floors and shell courses in tank integrity programs
- Pressure vessels, reactors, and columns in chemical and petrochemical plants handling chloride-bearing media
- Heat exchangers, condensers, and cooling water circuits where crevice and under-deposit attack concentrate at tube sheets and baffles
- Marine, offshore, and splash-zone structures exposed to seawater chlorides
- Power generation boilers, feedwater heaters, and condenser tubing
- Ship hulls, ballast tanks, and seawater piping systems
- Building and infrastructure components, including reinforcing steel, façade anchoring systems, and anodized aluminum architectural elements
- Failure analysis and litigation support, where documented pit depth establishes the corrosion mechanism and timeline
Accredited third-party laboratories routinely integrate localized corrosion depth measurement into corrosion testing programs, remaining life studies, and fitness-for-service evaluations for these assets.
FAQ
What is localized corrosion depth testing?
It is the measurement and evaluation of metal loss that occurs in small, discrete areas such as pits and crevices. The test determines maximum pit depth, pit density, pitting rate, and pitting factor, which are then compared with design thickness allowances and acceptance criteria.
Which standards govern the evaluation of pitting corrosion?
The most widely applied documents are ASTM G46 for examination and evaluation of pitting, ISO 11463 and GB/T 18590 for the evaluation of pitting corrosion on metals and alloys, ASTM G48 for ferric chloride pitting and crevice testing of stainless steels, and ISO 8993 / ISO 8994 for chart-based pit rating on anodized aluminum. Fitness-for-service assessment of measured metal loss is performed per ASME B31G or API 579.
Can pit depth be measured without shutting down or cutting equipment?
Yes. Ultrasonic thickness gauging, phased array ultrasonic testing, pulsed eddy current, radiographic testing, magnetic flux leakage scanning, and laser profilometry can all measure or screen localized metal loss on in-service equipment. Where direct access to the surface is possible, silicone replica molding captures pit geometry for laboratory measurement without material removal.
What is the pitting factor and why is it important?
The pitting factor is the ratio of the deepest pit penetration to the average penetration from general corrosion. A high pitting factor indicates that metal loss is highly localized, meaning average wall thickness readings significantly understate the true risk of perforation and leak.
What measurement accuracy can be expected?
Contact depth gauges typically resolve depths to approximately 0.01 mm, while ultrasonic thickness gauges achieve similar resolution on clean, parallel surfaces. Optical and 3D metrology methods can resolve features at the micrometer level, making them suitable for shallow pitting on test coupons and laboratory specimens.
How are pit depth results used to predict remaining life?
The maximum measured or statistically extrapolated pit depth is subtracted from the required minimum wall thickness, and the depth is divided by the exposure time or inspection interval to obtain a pitting rate. Combining this rate with the corrosion allowance, and applying remaining strength criteria such as ASME B31G, yields the safe remaining operating life and the date of the next inspection.
How should samples be handled before testing?
Corroded specimens should be photographed in the as-received condition, documented with location and orientation markings, then cleaned using standardized procedures that dissolve corrosion products without attacking the base metal. Specimens must be degreased, dried, and stored in a desiccator to prevent continued corrosion prior to depth measurement.