The stainless steel sulfuric acid-copper sulfate corrosion test, widely known as the Strauss test, is the industry-standard method for evaluating the susceptibility of stainless steels to intergranular corrosion caused by sensitization. This test boils specimens in a copper sulfate-sulfuric acid solution in contact with metallic copper, followed by a bend test to reveal chromium-depleted grain boundaries. In this article, we explain the test principle, sample requirements, solution preparation, step-by-step procedure, acceptance criteria, and typical industrial applications based on ASTM A262, GB/T 4334.5, and ISO 3651-2 standards.
Technical Overview
The sulfuric acid-copper sulfate corrosion test is a standardized accelerated immersion test used to detect the susceptibility of stainless steels to intergranular attack (IGA), commonly referred to as intergranular corrosion. The phenomenon originates from sensitization: when austenitic stainless steel is heated in the range of approximately 425 to 850 degrees Celsius, chromium carbides (M23C6) precipitate at grain boundaries, depleting the adjacent zones of chromium to below the ~12 percent level required for passivation. These chromium-depleted boundaries then corrode preferentially in specific electrochemical environments.
In this test, specimens are exposed to a boiling solution of 16 percent sulfuric acid and 6 percent copper sulfate (on an anhydrous basis) while in contact with metallic copper chips or shot. The redox potential of this solution lies between the active-passive transition of the chromium-depleted grain boundary material and that of the fully alloyed matrix, so the boundaries dissolve while the grains remain passive. The deposited copper acts as a large cathodic surface, accelerating the reaction. After exposure, the specimen is bent 180 degrees; intergranular attack is revealed by fissuring or cracking on the bent surface.
The test is specified in several major standards, including ASTM A262 Practice E (and the longer 120-hour Practice F), GB/T 4334.5, ISO 3651-2, and JIS G 0575. It is the most widely used acceptance test for austenitic stainless steel mill products and welded fabrications in the pressure vessel, chemical processing, and power generation industries.
Testing Samples
Austenitic Stainless Steels
The test is primarily applied to austenitic grades such as 304, 304L, 316, 316L, 317L, 321, 347, and super-austenitic alloys. Both as-delivered material and intentionally laboratory-sensitized samples (for example, 650 degrees Celsius for 2 hours) can be evaluated, although acceptance testing is normally performed on the material in its supplied condition.
Ferritic, Duplex, and Specialty Grades
Ferritic grades such as 409, 430, and 444, as well as duplex grades such as 2205 and 2507, may also be assessed for chromium carbide or nitride precipitation at grain boundaries. For these materials, test duration and evaluation criteria should be agreed between the purchaser and supplier, since sensitization mechanisms differ from those of austenitic steels.
Product Forms and Specimen Preparation
Acceptable sample forms include plate, sheet, strip, seamless and welded tube, pipe, bar, forgings, castings, and welded joints. General preparation requirements are as follows:
- Specimens are typically 70 to 100 mm long, 10 to 20 mm wide, and up to 5 mm thick; for pipe and tube, the full wall thickness should be included.
- Weld test specimens must contain base metal, heat-affected zone, and weld metal, with the weld approximately centered.
- Scale, oxide, and embedded debris shall be removed by mechanical means such as grinding or pickling, taking care not to overheat the surface.
- Specimens are degreased with acetone or alcohol, dried, identified with a permanent marker, and weighed if weight-loss data is required.
- Sheared or cut edges should be dressed by filing or grinding to remove cold-worked material that could distort the bend test results.
Testing Items
The test program typically covers the following evaluation items:
- Susceptibility to intergranular corrosion of base metal, weld metal, and heat-affected zone (pass or fail determination).
- Verification of the effectiveness of solution annealing, quenching, and stabilization heat treatments.
- Acceptance of low-carbon (L grades) and titanium- or niobium-stabilized grades for service in corrosive environments.
- Qualification of welding procedures with respect to sensitization of the heat-affected zone.
- Post-exposure 180-degree bend testing and fissure or crack inspection of the tension surface.
- Supplementary metallographic examination of cross sections to measure the depth of intergranular penetration in questionable cases.
- Optional weight-loss measurement as supporting evidence of corrosion resistance.
Testing Methods
Test Solution Preparation
Dissolve 100 g of copper sulfate pentahydrate (CuSO4·5H2O) in approximately 700 mL of distilled or deionized water. Slowly add 100 mL of concentrated sulfuric acid (density approximately 1.84 g/cm3) with continuous stirring and external cooling, then dilute to 1000 mL. The resulting solution contains approximately 16 percent sulfuric acid and 6 percent copper sulfate on an anhydrous basis. High-purity copper chips or shot shall be placed on the bottom of the test flask in sufficient quantity to contact all specimens and prevent specimen-to-specimen contact.
Exposure Procedure
The exposure procedure follows these key steps:
- Place the copper chips in a glass flask fitted with a reflux condenser, add the test solution, and bring it to a rolling boil (slightly above 100 degrees Celsius).
- Position the specimens so that each is fully immersed and in contact with the metallic copper; maintain a solution volume of at least 8 mL per square centimeter of specimen surface area.
- Boil for the specified duration: 16 hours according to GB/T 4334.5 and equivalent ISO requirements, approximately 15 to 16 hours for ASTM A262 Practice E, and 120 hours for ASTM A262 Practice F, which is used for certain alloys or more severe acceptance requirements.
- Use the reflux condenser to prevent solution concentration drift; if needed, add only deionized water to restore volume.
- After the test period, remove the specimens, rinse thoroughly with water, scrub off any deposited copper, then rinse with alcohol and dry.
Bend Test and Evaluation
Each exposed specimen is bent 180 degrees at room temperature over a mandrel with a diameter equal to the specimen thickness (a 1t bend radius). The surface that was in contact with the corrodent, or the surface specified by the applicable product specification, is placed on the outside of the bend. The tension surface is then examined visually and at low magnification (typically 10x to 20x) for cracks, fissures, or crazing. Fine crazing or microcracks that are difficult to interpret should be distinguished from true intergranular fissuring by metallographic examination of a cross section taken perpendicular to the bend.
Acceptance Criteria
A specimen passes the test if no cracks or fissures attributable to intergranular attack are visible after bending. If the result is ambiguous, a metallographic cross section is prepared and the maximum depth of intergranular penetration is measured; acceptance is then based on the limits defined in the product specification or the agreement between purchaser and supplier. Most standards permit retesting of duplicate specimens when the first result is borderline, and the oxalic acid etch test (ASTM A262 Practice A or GB/T 4334.1) is frequently used as a rapid screening method to reduce the number of full Strauss tests required.
Safety Precautions
The test involves boiling dilute-to-moderate sulfuric acid. Operations must be conducted in a chemical fume hood or well-ventilated enclosure, with acid-resistant gloves, face protection, and a properly secured reflux setup. Solution preparation must always follow the rule of adding acid to water, never water to acid, and waste solution containing copper and acid shall be disposed of according to local environmental regulations.
Applications
The sulfuric acid-copper sulfate corrosion test is routinely applied across many industries and scenarios:
- Mill acceptance testing and material certification of austenitic stainless steel plate, pipe, tube, and fittings.
- Quality control for pressure vessels, boilers, heat exchangers, and process piping in chemical, petrochemical, and refinery plants.
- Welding procedure qualification and welder performance testing, where sensitization of the heat-affected zone is a primary concern.
- Verification of post-fabrication heat treatments, including solution annealing and stabilization treatments for grades 321 and 347.
- Procurement and incoming inspection for nuclear, pharmaceutical, food and beverage, and offshore oil and gas equipment.
- Research and development applications such as time-temperature-sensitization (TTS) diagram construction, alloy development, and evaluation of stabilizer additions or low-carbon practice.
FAQ
What does the copper sulfate-sulfuric acid test actually detect?
It detects susceptibility to intergranular corrosion caused by chromium carbide precipitation at grain boundaries, that is, sensitization. It is not designed to evaluate pitting corrosion, general corrosion, or attack caused by sigma phase, for which other tests such as the Huey nitric acid test or ferric sulfate test are more appropriate.
Why is metallic copper added to the test solution?
Metallic copper provides a large cathodic surface that depolarizes the reaction and shifts the corrosion potential of the specimen into the range where chromium-depleted grain boundaries dissolve rapidly while the matrix remains passive. Copper also deposits onto the specimen surface, effectively creating a galvanic couple that accelerates the test.
How long does the test take?
The standard exposure is 16 hours at boiling temperature for GB/T 4334.5, ISO 3651-2, and ASTM A262 Practice E, and 120 hours for ASTM A262 Practice F. Including solution preparation, specimen preparation, the 180-degree bend, and reporting, a routine test is typically completed within two working days.
How is this test different from the Huey and ferric sulfate tests?
The Huey test (boiling 65 percent nitric acid) measures weight loss over five 48-hour cycles and is sensitive to both carbide precipitation and sigma phase. The ferric sulfate-sulfuric acid test (Streicher test) is a 120-hour immersion also based on weight loss. The copper sulfate test is faster, uses a pass/fail bend criterion, and is the most common acceptance test specifically for chromium carbide sensitization.
What should be done if cracks appear after bending?
Cracks or fissures indicate probable intergranular attack, but confirmation requires metallographic examination of a cross section to verify that the failure is intergranular rather than from mechanical defects or surface crazing. Most specifications allow duplicate retesting when results are questionable.
Can the test be applied to welded components directly?
Yes. Welded specimens containing base metal, heat-affected zone, and weld metal are tested as a unit, and the bend is oriented so that all zones are stressed. This makes the test an effective tool for verifying that welding parameters and post-weld treatments have not sensitized the heat-affected zone.