ASTM G28 is often written on a nickel-alloy purchase order as if it were a single pass/fail test. It is not. The standard contains two boiling corrosion methods with different solutions and intended use. A workable order must identify the method, specimen condition, exposure, calculation and acceptance limit.
Important: ASTM lists G28-24 as the current edition at the time of writing. Purchase the current standard and use the product specification or purchaser agreement for acceptance. This article does not reproduce the copyrighted procedure.
The Short Answer
ASTM G28 Method A uses boiling ferric sulfate–50% sulfuric acid to evaluate susceptibility to intergranular attack in certain nickel-rich chromium-bearing alloys. Method B uses a more aggressive mixed-acid/oxidizing-salt environment for specified highly corrosion-resistant nickel alloys. The test method alone does not establish a universal acceptable corrosion rate.
Method A vs Method B
| Decision factor | ASTM G28 Method A | ASTM G28 Method B |
|---|---|---|
| General solution | Ferric sulfate–50% sulfuric acid | Mixed acid with oxidizing salts |
| Primary purpose | Detect susceptibility to intergranular attack in applicable nickel-rich alloys | Evaluate applicable highly corrosion-resistant alloy conditions in a more aggressive environment |
| Severity | Aggressive boiling acid test | Generally more aggressive/specialized |
| Result | Mass-loss corrosion rate plus visual evidence as required | Mass-loss corrosion rate plus visual evidence as required |
| Selection basis | Product/alloy specification or purchaser requirement | Product/alloy specification or purchaser requirement |
| Pass/fail | Must be specified separately | Must be specified separately |
What the Test Detects
Improper thermal exposure can precipitate carbides or intermetallic phases and deplete critical alloying elements near grain boundaries. A solution-annealed plate may be resistant, while a slowly cooled, improperly heat-treated or welded region may be susceptible.
G28 accelerates attack so manufacturing differences can be detected in a practical laboratory time. It is a quality-control and material-condition test, not a direct simulation of every plant fluid. Passing G28 does not guarantee immunity to pitting, crevice corrosion, stress-corrosion cracking or erosion in service.
How to Choose the Method
Start with the governing material or fabrication specification. Certain alloy product standards name a G28 method, sensitization treatment and maximum rate. If the standard is silent, the purchaser and supplier should agree based on alloy family, fabrication history and failure mechanism.
Do not select Method B merely because it sounds more conservative. An overly aggressive or inapplicable method can reject suitable material without improving service reliability. Conversely, an easy method may fail to reveal the microstructural condition relevant to the application.
For alloy-specific corrosion context, see Hastelloy C-276 Corrosion Resistance.
Specimen Location Can Change the Answer
The order should state whether specimens represent:
- base metal at the surface or mid-thickness;
- longitudinal or transverse orientation;
- a forging hub, rim or prolongation;
- weld metal only;
- heat-affected zone and adjacent base metal;
- as-supplied solution anneal;
- a simulated postweld heat treatment; or
- the final fabricated and repaired condition.
A coupon cut far from a production weld cannot demonstrate HAZ resistance. Similarly, a small laboratory heat treatment may not reproduce cooling in a thick forging. Sampling should follow the risk and the governing specification.
Corrosion-Rate Reporting
Mass loss is converted to a corrosion rate using specimen area, density and exposure time. The report should state units, density used, test duration, specimen preparation and any excluded surface. Because edge area and scale removal affect the result, preparation must be consistent.
Visual inspection remains important. A low average mass loss can conceal localized grain-boundary attack, while general surface etching may produce a higher mass loss without the same morphology. Where required, add microscopy and photographs.
Why “ASTM G28 Pass” Is Incomplete
ASTM test methods explain how to test; they do not necessarily define the purchaser’s acceptable limit. A complete purchase clause might specify:
- ASTM G28-24 Method A or B.
- As-received or sensitized specimen condition.
- Sample location and orientation.
- Number of specimens per heat/lot.
- Maximum corrosion rate and units.
- Visual or microscopic acceptance.
- Retest rules and lot disposition.
- Laboratory accreditation and report content.
For broader receiving controls, read How to Inspect Nickel Alloy Materials and EN 10204 3.1 vs 3.2 Certificates.
Test-Order Checklist
- Alloy, UNS number, heat and product form.
- ASTM G28 revision and Method A or B.
- Governing product/fabrication specification.
- Specimen location, orientation and dimensions.
- Surface finish and edge preparation.
- Required sensitization or simulated thermal cycle.
- Exposure duration and solution-renewal rules.
- Density and corrosion-rate units.
- Maximum acceptable rate.
- Visual/microscopic acceptance.
- Photos, raw weights, calibration and full report.
- Retest and rejection procedure.
Frequently Asked Questions
Does ASTM G28 specify a pass/fail limit?
The test method does not provide one universal limit for every alloy and product. Use the applicable product standard or purchaser-supplier agreement.
Is Method B always stricter than Method A?
It is generally the more aggressive environment, but “stricter” is not the same as “more appropriate.” Alloy applicability and acceptance basis matter.
Can G28 test a weld?
Yes, if the specimen is designed to represent weld metal, HAZ and relevant base metal. The sampling and evaluation method must be defined.
Does passing G28 prove service corrosion resistance?
No. It indicates resistance to the test’s accelerated intergranular-corrosion mechanism. Service may involve different mechanisms.
Should every nickel alloy order require G28?
No. Require it when the product specification, thermal history or service risk justifies it. Unnecessary testing adds cost and can create ambiguous rejection disputes.
Final Recommendation
Write G28 as a complete quality requirement: method, revision, specimen condition, sampling and acceptance. Send J&A Alloy the alloy, form, drawing, weld/heat-treatment route, service environment and requested laboratory clause so the correct test can be quoted.
