ASTM G46 tells you how to examine and quantify pitting once it has happened. It is a guide to evaluation rather than a corrosion test, so it applies equally to a coupon pulled out of a laboratory exposure and to a failed component that arrived from a plant. MatX Lab evaluates pitting on stainless steel, aluminum alloys, copper alloys, and carbon steel, aligned to ASTM G46.
What Is ASTM G46?
ASTM G46 is the standard guide for examining and evaluating pitting corrosion. Pitting concentrates attack into small areas while most of the surface stays intact, which is why a mass loss figure can look reassuring on a component that is about to perforate.
This guide explains the methods for finding the pits, cleaning the surface while preserving all clues, counting and sizing the pits, and measuring their depth. The standard rating charts include ratings for pit density, size, and depth. Two individuals using the charts will provide the same description of the surface.
The pitting factor comes from the same measurements. It is the deepest pit divided by the average penetration calculated from mass loss, and it directly indicates how localized the attack was.
ASTM G46 Scope and Applications
The guide applies to metals and alloys attacked in service or in laboratory exposure, placing it alongside salt fog work, immersion testing, and electrochemical pitting studies.
Reasons this work comes in:
Failure analysis. Establishing whether perforation or leakage began as pitting, and how deep the attack reached before it broke through.
Post-exposure evaluation. Quantifying pitting on coupons that came out of a salt spray, immersion, or field exposure program.
Comparison of alloys. Ranking potential materials on the basis of pit density and maximum pit depth with the same exposure.
Life remaining evaluation. Measurement of maximum pit depth relative to wall thickness for equipment in use.
Water treatment assessment. Evaluating pitting on heat exchanger tubing and piping to judge whether chemistry control is working.
Weld and heat-affected zone assessment. Comparing pitting between weld metal, heat-affected zone, and base metal on the same specimen.
When you need to measure pitting resistance rather than evaluate it after the fact, ASTM G48 provides ferric chloride immersion tests, and electrochemical methods provide pitting potentials. G46 covers what you do with the surface afterward, regardless of which route produced it.
ASTM G46 Test Procedure
Cleaning is where evidence gets destroyed. Everything before it is documentation, and everything after it depends on it having been done carefully.
| Step | What happens |
| As-received documentation | The surface is photographed before anything touches it, with deposits, scale, and corrosion product recorded in place. |
| Deposit sampling | Where corrosion product or deposit chemistry matters, material is collected for analysis before cleaning removes it. |
| Cleaning | Corrosion product is removed by the method appropriate to the alloy, using repeated cycles with mass checks so base metal loss stays controlled. |
| Visual examination | The cleaned surface is examined and photographed, and pit locations are mapped across the specimen. |
| Density and size rating | Pit density and pit size are rated against the standard charts, and counts are made over a defined area rather than the whole part. |
| Depth measurement | Depth is measured by microscope focus, by micrometer against a reference surface, or by sectioning through selected pits. |
| Metallography | Cross sections through the deepest pits show morphology, undercutting, and whether the attack followed grain boundaries or inclusions. |
| Calculation | Maximum depth, mean depth of the deepest pits, and pitting factor are reported against the exposed area. |
Limitations: Cleaning removes not only the metal but also corrosion products, and an overly cleaned specimen may appear to have deeper pitting than actually exists; hence the cycling of the cleaning process, along with recording masses. The deepest pit found in an area is the deepest pit within that area and not the deepest pit on the specimen itself; extension to a large-scale structure requires statistical analysis rather than assumptions. Microscopic depth-measurement techniques for pits have difficulty measuring narrow pits because one cannot view the bottom; in such cases, sectioning is necessary. Sectioning is destructive, and it is rare that the deepest point is sectioned; thus, measurements obtained using this method underestimate the pit. The deposits and scales removed during cleaning contain the chemistry of the attack; therefore, sampling must be conducted beforehand to retain that information.
ASTM G46 Specimen Requirements and Test Conditions
| Parameter | Typical requirement |
| Specimen form | Coupons, cut sections, tube samples, or complete failed components |
| Condition on arrival | As removed from service or exposure, uncleaned and unbrushed |
| Packaging | Dry, wrapped without adhesive contact, with deposits left undisturbed |
| Area evaluated | Defined and recorded, since density and depth statistics depend on it |
| Reference surface | An unattacked region retained where depth is measured against original wall |
| Supporting data | Exposure history, alloy grade, service chemistry, and duration where known |
Do not clean the part before sending it. Wire brushing, solvent washing, and pickling all remove the evidence this evaluation depends on, and the deposits removed frequently contain the chloride or sulfide that caused the pitting.
Attach a brief history of the exposure along with the specimen. The alloy grade, exposure duration, temperature, and solution composition influence the meaning of the pitting pattern, and without that data, the assessment would be a damage observation rather than an explanation.
ASTM G46 Test Results and Reporting
Your report documents the surface as received and after cleaning, with photographs at both stages.
The number of pits, pit size, and pit depth are provided relative to the standard ratings and the measured values, along with the area examined. The maximum pit depth is provided along with the average pit depth and the pitting factor, determined based on weight loss.
The interpretation belongs to the data and is not supposed to replace it. If the morphology suggests that the mechanism involves chlorination underneath a deposit, crevice initiation at the joint level, or an attack related to inclusions, we provide our interpretations based on the available evidence.
ASTM G46 FAQs
Is G46 a corrosion test?
No. It is a guide for examining and quantifying pitting that has already occurred. The exposure that caused the pitting is a separate piece of work, whether that was a laboratory test or years in service.
How do you determine pit depth without sectioning the part?
Microscope focusing to a known surface determines the depth of open pits, while micrometer measuring is used for accessible geometry. Narrow and undercut pits require sectioning, and this is discussed prior to any cutting.