Corrosion Testing for Valve Materials: ASTM A262 and G48

Two Different Questions, Two Different Tests

Corrosion testing for valve materials often gets lumped into one line item on a datasheet, but ASTM A262 and ASTM G48 answer entirely different questions.

ASTM A262 asks: is this stainless steel sensitized? Sensitization is the chromium-carbide precipitation at grain boundaries that leaves adjacent metal depleted of chromium and vulnerable to intergranular attack. The test probes a metallurgical condition.

ASTM G48 asks: how does this alloy stand up to localized attack — pitting and crevice corrosion — in an aggressive chloride environment? It probes a performance behavior under a standardized, deliberately harsh exposure.

Both show up frequently in valve specifications — for the bodies and trim of ball valves and every other pressure-retaining component in stainless or duplex materials destined for chloride-bearing process streams. Knowing which question your service is actually asking determines which test is meaningful — and prevents paying for the wrong assurance.

ASTM A262: The Intergranular Family

ASTM A262 contains several practices, and they are not interchangeable. Selecting the right one is a specification decision, not a laboratory preference:

• Practice A — oxalic acid etch screening. A quick electrochemical etch of a polished surface, examined under the microscope. The etched structure is classified; certain classifications qualify the material as acceptable without further testing, while others require follow-up testing under Practices B, C, or E. It is fast and cheap, which is why it is widely used as the gatekeeper.
• Practice B — ferric sulfate–sulfuric acid. A boiling-acid exposure for a defined period, with weight loss measured as a corrosion rate. It is aggressive toward sigma phase as well as carbides, making it relevant for certain higher-alloy grades.
• Practice C — nitric acid (Huey test). Boiling 65% nitric acid over several periods. This is the most severe of the practices and is sensitive not only to carbide precipitation but also to sigma phase and other intermetallics. It appears frequently in specifications for nitric acid service and for nuclear-adjacent work.
• Practice E — copper–copper sulfate–16% sulfuric acid (Strauss test). Boiling exposure followed by a bend, with the bent surface examined for cracks. It targets chromium-carbide sensitization specifically and remains common in European-influenced specifications.
• Practice F — a ferric sulfate variant for highly alloyed, molybdenum-bearing grades.

The practical takeaway: for CF8M and comparable cast austenitic valve components, buyers commonly pair Practice A screening with a follow-up practice when screening is inconclusive, and demand the testing be performed on sensitized-susceptible product — material in the condition it will actually ship, not a laboratory-idealized blank.

ASTM G48: Pitting and Crevice Attack

ASTM G48 exposes coupons to a ferric chloride (FeCl3) solution at a specified temperature. Ferric chloride is brutally aggressive — far more than most real process streams — which is why the test is best understood as a comparative screening tool rather than a service-life prediction.

• Method A measures pitting: weight loss, plus examination and measurement of pit depth on the exposed faces and edges.
• Method C measures both pitting and crevice corrosion, using the standard crevice fixture to create the oxygen-starved, chloride-concentrated gaps where alloys perform worst in the field. Crevice performance, not pitting performance, is usually the discriminator between merely good and genuinely superior alloys.

Typical acceptance takes the form of a maximum corrosion rate, no visible pitting at a stated magnification, or a maximum measured pit depth. Some specifications use G48 in a temperature-stepped form — finding the critical pitting or crevice temperature at which attack begins — which gives a useful ranking index for alloy selection.

Duplex and Super Stainless Applications

The materials where G48 earns its keep are the ones sitting between ordinary austenitic stainless and the exotic nickel alloys: 22Cr duplex, 25Cr super duplex, 6Mo super austenitics. These alloys exist precisely because 316L is not enough for seawater-adjacent, high-chloride service, and G48 crevice testing at defined temperatures is the standard yardstick by which their added performance is demonstrated.

For duplex materials there is an important interaction with heat treatment and welding. Improper solution annealing or an uncontrolled weld thermal cycle alters the ferrite-austenite balance and can precipitate intermetallics such as sigma phase — which simultaneously degrades toughness, corrosion resistance, and G48 results. This is why a G48 requirement on duplex valve components usually travels with microstructure examination requirements, not as a standalone stamp on a certificate. On chloride service duties, ball valves in CF8M, duplex, and super duplex trims from an experienced industrial valve maker are routinely specified with this testing combination.

Sample Preparation and Acceptance Criteria

Corrosion test results are only as trustworthy as the specimen preparation behind them. Sensible specification details to check:

• Specimens are taken from production material or production-welded test assemblies, not from separately melted laboratory stock.
• Surfaces are prepared to the specified finish — corrosion results shift with surface condition, and weld zones must be represented.
• For welded specimens, the test must include the heat-affected zone, since that is where sensitization and pitting initiation concentrate.
• Edge condition per the standard — burrs and sheared edges distort pitting behavior.
• Acceptance criteria stated numerically: a corrosion rate limit, a maximum pit depth, or a pass/fail at a defined temperature.

Vague acceptance language — "no significant attack" — is an invitation to dispute. Put the numbers in the datasheet.

When Should You Actually Require These Tests?

Not every valve needs a corrosion certificate. The tests earn their cost in recognizable situations:

• Welded assemblies and repaired castings, where the thermal cycle may have sensitized an otherwise resistant material — A262 on the weld zone is the check.
• Unstabilized austenitic stainless steels (CF8M, 316 types) destined for service in the sensitizing temperature range, or exposed to media known to attack grain boundaries — sulfuric acid, certain organics, nitric acid.
• Chloride-rich duties — seawater cooling, brine, produced water, offshore and coastal environments — where pitting and crevice initiation govern component life, and where G48 methods A and C provide a comparative ranking for trim selection.
• Duplex and super stainless purchases, where the alloy premium is justified only if the microstructure and corrosion performance are verified, not assumed.
• Sour and mixed-service valves where a client specification stacks NACE MR0175 requirements on top of corrosion testing, and the two must be coordinated rather than doubled up.

Where a specification demands A262 or G48 and the service does not justify it, the honest engineering answer is to question the requirement — unnecessary testing adds cost and schedule without adding protection.

What the Tests Do — and Do Not — Tell You

The limitation worth internalizing: neither A262 nor G48 predicts service life in a specific process stream. A262 tells you the material's grain boundaries are (or are not) in a susceptible metallurgical condition. G48 ranks alloys under a severe, standardized chloride exposure that corresponds to no single real fluid. Field performance depends on temperature, chloride concentration, pH, oxidizing species, flow, deposits, and crevice geometry — variables no coupon fully captures.

Used correctly, the tests are filters: they screen out material that would fail early under the mechanism being probed. Used incorrectly — as a warranty of field behavior — they set up false expectations. The professional approach pairs the test with sound material selection for the actual chemistry, and reads the certificate as confirmation, not as the whole answer.

A manufacturer that produces and supplies a complete valve range directly — ball valves, control valves, gate valves, globe valves, check valves, strainers, knife gate valves, plug valves, and butterfly valves — can cover stainless, duplex, and super stainless executions for corrosive duties. Send your Valve Schedule or the process chemistry for each line to the manufacturer's applications team, and its engineers can confirm which corrosion tests genuinely apply to your service, define sensible acceptance criteria, and build the testing into the order with the documentation you need at delivery.