SCC Test — ASTM G36 / ASTM G39 / NACE TM0177

What Is SCC?

The stress corrosion cracking test is a test in which the specimen is exposed to a defined corrosive environment under a defined tensile stress, determining crack formation, time to cracking, or threshold stress.

Purpose of SCC

The purpose is to determine whether the material–environment–stress combination leads to stress corrosion cracking, and to verify safe-use limits.

Which Products Is SCC Applied To?

Stainless steels and nickel alloys
Oil and gas production materials
Aluminium and brass components
Pressure-vessel and pipe materials

In Which Sectors Is SCC Used?

MetalSteelMiningMachineryOil & Gas

Standards Related to SCC

ASTM G36 (boiling magnesium chloride)
ASTM G39 (bent-beam specimen)
ASTM G30 (U-bend specimen)
NACE TM0177
ISO 7539 series

SCC Conditions

Environment: Defined in the standard; for example, boiling magnesium chloride solution in ASTM G36
Strain: Applied as constant load, constant deformation, or slow strain rate
Time: Exposure duration defined in the standard or specification
Temperature: Defined in the method

Equipment for SCC

Corrosive-environment test cell and reflux condenser
Constant-load or constant-deformation rig
Slow strain rate testing machine
Metallurgical and stereo microscope

Sample Requirements for SCC

Specimen geometry and surface preparation defined in the standard
The applied stress level must be documented
Specimen orientation (relative to the rolling direction) must be defined
Control specimens must be run alongside

Duration of SCC

Days to months, depending on the method; NACE TM0177 tests typically take 720 hours.

SCC Procedure

1The specimen is prepared and stress is applied.
2It is exposed to the defined corrosive environment.
3It is monitored for the specified duration or until cracking occurs.
4The specimen is removed and cleaned.
5The presence of cracking is assessed visually and metallographically.

SCC Acceptance Criteria

The acceptance criterion is given in the specification: no cracking under the defined duration and stress, or a threshold stress above the specified value.

Evaluation of Results

Accelerated SCC environments (e.g. boiling magnesium chloride) are for screening purposes; they cannot be directly converted to field service life. The result is valid only for the environment and stress condition used in the test.

SCC Report

Standard and method applied
Environment composition and temperature
Applied stress
Exposure duration
Crack findings and metallographic sections

Accreditation

EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.

Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.

Scope and authorisations

Related Regulations

Oil and gas material specifications (e.g. NACE MR0175 / ISO 15156)

Metal and Metallurgy Testing — What Does This Testing Field Cover?

The section below describes the field that the SCC test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.

Metal and metallurgical testing determines the load a metal or a welded joint can carry, how it fails and what its internal structure looks like. The output of this family is often not about the product itself but about the identity of the MATERIAL: is the steel really of the declared grade, does the weld seam deliver the strength the design assumes, does the casting contain an unacceptable internal defect?

The practical importance of the field comes from the fact that a metal part rarely fails gradually. A plastic part deforms, ages, cracks and usually gives warning. A weld seam or a fastener, by contrast, often breaks without warning — especially where fatigue and brittle fracture are involved. That is why in fields such as pressure equipment, lifting gear, structural steel and pipelines, material testing is a requirement of REGULATION rather than of design.

The family brings together three groups of methods, and each answers a different question. Mechanical tests quantify how the material behaves under load. Non-destructive examination looks for defects without damaging the part. Metallography and chemical analysis explain why the material behaves the way it does — through grain structure, phase distribution, inclusion content and alloy composition.

The acceptance criterion for these tests almost never comes from the test standard, and this is the key to understanding the field. The test standard says how the measurement is to be made; the acceptable values come from the product standard, the material grade or the fabrication code. ISO 6892-1 tells you how to run a tensile test; EN 10025 tells you what yield strength that steel must deliver. When reading a report, the product standard it was assessed against is essential.

Sampling is an inseparable part of the result in this field. Two specimens taken from the same heat may give different values depending on the direction and location they were taken from; in rolled products, specimens parallel and perpendicular to the rolling direction produce different results. Product standards therefore also govern sampling location and orientation, and the report must document this.

Method groups in this field

Mechanical tests
Tensile, compression, bending, notch impact (Charpy), hardness (Brinell, Vickers, Rockwell), fatigue, creep and fracture toughness. These give the behaviour of the material under load in numerical form.
Non-destructive examination
Radiography, ultrasonic examination, penetrant, magnetic particle and visual examination. It looks for surface and internal defects without damaging the part and is an indispensable step in welded fabrication.
Metallography
Specimen preparation, etching and microscopic examination. Grain size, phase distribution, inclusion content, case depth and the weld heat-affected zone are assessed.
Chemical analysis
Alloy composition is verified by optical emission spectrometry and wet chemistry; confirmation of the material grade runs through this step.
Corrosion behaviour
Salt spray, pitting and crevice corrosion and intergranular corrosion tests. In stainless steels these can indicate the quality of the heat treatment.

Commonly confused in this field

A material certificate (such as EN 10204 3.1) is not a test report; it is the manufacturer's declaration. In critical applications independent verification is carried out, and the two must not be confused.
ASTM and ISO tensile tests measure the same quantities but use different specimen geometries; the results are not treated as directly equivalent, and whichever the specification calls for is the one applied.
Conversion tables between hardness and strength are APPROXIMATE. Calculating yield strength from hardness is not sufficient evidence for material acceptance.
In non-destructive examination, "no defect found" does not mean there is no defect; defects below the detection limit of the method may be present. The report must state the method applied and the sensitivity level.
A weld seam passing does not show that the welder or the procedure is qualified; these are separate processes.

Which decisions these results support

Verifying the material certificate and accepting incoming material
Approving welded fabrication; welder and procedure qualification
Manufacturing control of pressure equipment and structural components
Failure analysis and determining the root cause of field damage
Verifying heat treatment and surface treatment processes

Related Tests

Frequently Asked Questions About SCC

Does a material that passes the SCC test not crack in the field either?

The result is valid only for the environment, temperature and stress condition tested. A different service environment gives a different result; material selection must be evaluated against the real service condition.