Stainless Steel Sulfuric Acid-Copper Sulfate Test: Principles, Standards, and Procedure

The stainless steel sulfuric acid-copper sulfate test, widely known as the Strauss test, is the industry-standard acceptance method for detecting susceptibility to intergranular corrosion in stainless steels. By boiling specimens in a copper sulfate and sulfuric acid solution and then bending them, the test reveals sensitization caused by chromium carbide precipitation during welding or improper heat treatment. This guide covers the test principle, applicable standards, specimen requirements, procedure, acceptance criteria, and typical industrial applications.

Technical Overview

The sulfuric acid-copper sulfate test is a qualitative laboratory method used to evaluate the resistance of stainless steels to intergranular attack. A prepared specimen is exposed to a boiling solution of copper sulfate (CuSO4) and sulfuric acid (H2SO4), generally in contact with metallic copper chips, and is then subjected to a bend test. Cracks or fissures on the bent surface indicate that the material is sensitized and therefore susceptible to intergranular corrosion in service.

The metallurgical mechanism behind the test is well understood. When austenitic or ferritic stainless steel is held within the sensitization range of approximately 450 to 850 °C, for example during slow cooling after welding or stress relief, chromium-rich carbides precipitate at grain boundaries. The zones adjacent to those boundaries are depleted of chromium to below the roughly 10 to 12 percent level required to maintain a passive film. In the oxidizing copper sulfate-sulfuric acid electrolyte, these chromium-depleted boundaries corrode preferentially while the grain interiors remain passive. Galvanic contact with metallic copper holds the specimen at a potential where this selective attack is strongly accelerated, so a single boiling period of 16 to 24 hours is sufficient to produce a clear pass or fail result.

The test is governed by several internationally recognized standards, including:

• ASTM A262 Practice E - copper-copper sulfate-16% sulfuric acid test for detecting susceptibility to intergranular attack in austenitic stainless steels
• GB/T 4334.5 - Chinese national standard for the sulfuric acid-copper sulfate intergranular corrosion test of stainless steels
• ISO 3651-2 Method B - corrosion test in sulfuric acid-copper sulfate medium with metallic copper contact
• JIS G 0575 - Japanese standard for the sulfuric acid-copper sulfate corrosion test of stainless steels

Testing Samples

Material Grades

The test is applied principally to austenitic stainless steels such as 304 (S30400), 304L (S30403), 316 (S31600), 316L (S31603), 321 (S32100) and 347 (S34700), including extra-low-carbon and stabilized variants intended for service in corrosive environments. Ferritic grades such as 409, 430, 439 and 444 are also tested with the same chemistry where the governing specification requires it. Duplex stainless steels are more commonly assessed using practices designed for their specific sensitization behavior, although some specifications permit the copper sulfate method for screening purposes.

Product Forms and Coupon Preparation

Specimens may be taken from plate, sheet, strip, pipe and tube, fittings, forgings, castings and welded fabrications. Coupons are typically cut to dimensions on the order of 80 by 20 mm or prepared as full cross sections of thin products, with the exact size defined by the applicable product specification. All surfaces are ground to a uniform finish, usually 120 to 180 grit, sharp edges are dressed, and the coupons are degreased, rinsed, dried and identified before testing. For welded samples, the coupon is oriented so that the weld metal, heat-affected zone and base metal are all exposed to the solution and included in the subsequent bend examination.

Test Condition

Depending on the specification, specimens may be tested in the as-delivered condition, after a representative fabrication heat treatment, or after a deliberate sensitizing treatment applied in the laboratory to demonstrate intrinsic resistance. The governing purchase specification or client procedure defines which condition applies, and the condition must be reported together with the results.

Testing Items

The sulfuric acid-copper sulfate test is an acceptance test that addresses the following evaluation items:

• Susceptibility of the material to intergranular attack associated with chromium carbide precipitation
• Effectiveness of solution annealing or stabilizing heat treatment
• Soundness of welded joints and their heat-affected zones with respect to sensitization
• Compliance of delivered product with national and international material specifications
• Verification of corrective re-heat treatment after a previous failure

Weight loss is not the acceptance criterion. The pass or fail verdict is based on the absence or presence of intergranular cracks after the bend test, supplemented where necessary by metallographic examination of a cross section.

Testing Methods

Test Solution Preparation

The standard test solution is prepared by dissolving 100 g of copper sulfate pentahydrate (CuSO4·5H2O) in distilled or deionized water, adding 100 mL of concentrated sulfuric acid of specific gravity 1.84, and diluting to a total volume of 1000 mL. The resulting electrolyte contains approximately 16 percent sulfuric acid by mass. Only reagent-grade chemicals are used, and the acid must always be added slowly to water, never the reverse, to control the exothermic reaction.

Exposure Procedure

The specimen is placed in an Erlenmeyer flask fitted with a reflux condenser so that the solution composition remains constant throughout the test. Electrolytic copper chips or turnings are spread over the bottom of the flask, and the specimen rests on the copper to ensure continuous galvanic contact. The solution is brought to boiling, which occurs at approximately 106 °C at atmospheric pressure due to the elevated boiling point of the acid solution, and boiling is maintained continuously for the period required by the standard, typically 16 hours for GB/T 4334.5 and JIS G 0575 and 24 hours for ASTM A262 Practice E. The specimen must remain fully immersed at all times, and multiple specimens in one flask are separated from one another to prevent crevices and unrepresentative galvanic contacts.

Bend Test and Examination

After the exposure period, the specimen is removed, rinsed clean, dried and visually inspected for general or localized attack. It is then bent through 180 degrees over a mandrel whose diameter is related to the specimen thickness, with the surface to be evaluated placed in tension. The tensile surface is examined visually and at a magnification of 10x to 20x. Fissures, cracks or open grain boundaries on the bent surface constitute evidence of intergranular corrosion. Where the origin of a crack is uncertain, for example due to mechanical tears or pre-existing defects, a metallographic cross section is prepared perpendicular to the bend to confirm whether the attack is truly intergranular.

Acceptance and Reporting

A specimen that shows no cracks or fissures after bending is considered resistant to intergranular corrosion in this medium. Any cracking confirmed as intergranular constitutes failure. The test report identifies the material and heat number, product form and dimensions, heat treatment condition, test standard and exposure duration, solution composition and volume, bend details, examination findings, and a clear pass or fail statement.

Safety Considerations

The test involves handling concentrated sulfuric acid and boiling corrosive electrolytes for extended periods. Work must be carried out under fume extraction by trained personnel using acid-resistant apparatus, reflux condensers with adequate cooling water, and appropriate personal protective equipment. Heated flasks are allowed to cool before disassembly, and spent solutions are neutralized and disposed of according to local regulations.

Applications

Because sensitization is one of the most common causes of premature failure of stainless steel in aggressive media, the sulfuric acid-copper sulfate test is embedded in quality control and qualification workflows across many industries. Typical applications include:

• Mill certification and acceptance testing of stainless steel plate, pipe, tube, bar, forgings and fittings
• Qualification of welding procedures, welders and weld repairs where heat-affected-zone sensitization must be excluded
• Verification of solution-annealing and stabilizing heat treatments during production and after field remediation
• Pressure vessels, heat exchangers, reactors and piping in chemical, petrochemical and oil and gas plants
• Nuclear power components, pharmaceutical and food-contact equipment, and pulp and paper machinery
• Failure analysis and fitness-for-service studies of components that have experienced thermal excursions

FAQ

What does the sulfuric acid-copper sulfate test detect?

It detects susceptibility to intergranular corrosion caused by chromium carbide precipitation at grain boundaries, a condition known as sensitization. Sensitization typically results from welding, slow cooling through the 450 to 850 °C range, or inadequate solution annealing.

Which standards specify this test?

The principal standards are ASTM A262 Practice E, GB/T 4334.5, ISO 3651-2 Method B and JIS G 0575. The procedures are technically equivalent and differ mainly in exposure time and minor details of specimen handling, so the applicable standard is selected according to the purchase specification.

Why is the test called the Strauss test, and how does it relate to ASTM A262 Practice B?

The chemistry was introduced by B. Strauss in the 1920s, and the method retains his name. Note that within the ASTM A262 system the copper sulfate-sulfuric acid test with metallic copper is Practice E, while Practice B designates the separate ferric sulfate-sulfuric acid (Streicher) test; in the Chinese GB/T 4334 series the copper sulfate method is Part 5.

How long does the test take?

The boiling exposure itself lasts 16 to 24 hours depending on the standard. Including coupon preparation, solution preparation, the bend test and metallographic confirmation where required, results are normally reported within three to seven working days.

What causes a specimen to fail, and how can failure be prevented?

Failure indicates chromium depletion at grain boundaries from carbide precipitation. Prevention measures include specifying extra-low-carbon (L) grades, titanium- or niobium-stabilized grades, controlled welding with low heat input, and correct solution annealing followed by rapid cooling.

Is the test destructive?

Yes. Specimens are consumed by the boiling exposure or permanently deformed by the bend test, so testing is performed on coupons, offcuts or sacrificial samples representative of the delivered product or weld.