Stress Corrosion Cracking Testing of Aluminium Alloys: What ASTM G47 and G44 Mean for Aerospace Manufacturers in India

India's aerospace forging sector is growing fast. The India aerospace forging market generated a revenue of USD 1,180 million in 2024, with aluminium being the largest revenue-generating material segment. Companies like SQuAD Forging (Aequs/Aubert & Duval), Bharat Forge, HAL, and Dynamatic Technologies are manufacturing aero-structural parts, engine components, and landing gear assemblies from high-strength aluminium alloys, primarily the 7xxx and 2xxx series.

But here is the question every forging house and OEM procurement team eventually faces: how do you prove that your 7075-T6 or 7175 forgings will not crack in service when exposed to a corrosive environment under sustained stress?

The answer is stress corrosion cracking (SCC) testing, and the governing standards are ASTM G47 and ASTM G44.

Why SCC Is a Non-Negotiable for Aerospace Aluminium

Stress corrosion cracking is one of the most dangerous failure modes in aluminium alloys. It requires three conditions to occur simultaneously: a susceptible material, a corrosive environment (even mild humidity or salt air qualifies), and sustained tensile stress. The crack initiates at the grain boundaries and propagates without any visible warning. By the time it shows up in a visual inspection, the component may already be compromised.

High-strength 7xxx series alloys (7075, 7175, 7050) are particularly vulnerable in the short-transverse grain direction. This is the orientation most relevant to forged aerospace parts where the grain flow follows the die shape. A wing spar forging, a landing gear strut, or a fuselage bulkhead, all of these components carry sustained loads in exactly the orientation where SCC susceptibility is highest.

This is not a theoretical concern. Aerospace primes and NADCAP auditors require SCC test data as part of material qualification and first article inspection. Without it, your forging does not fly.

What ASTM G47 Actually Tests

ASTM G47 is the standard test method specifically designed for evaluating SCC susceptibility of 2xxx and 7xxx series aluminium alloy wrought products. It uses the alternate immersion exposure method defined in ASTM G44, but with specimen types and durations tailored to aluminium aerospace alloys.

Here is how the test works in practice:

The test specimen (typically a cylindrical tension specimen of 3.18 mm diameter, per ASTM G49) is loaded to a constant stress using a stressing ring or proof ring assembly. The loaded specimen is then placed in an alternate immersion apparatus that cycles between 10 minutes of immersion in 3.5% NaCl solution and 50 minutes of drying in ambient air. This one-hour cycle runs continuously for 30 days (some specifications extend to 40 days or longer).

The NaCl solution simulates marine and humid environments. The alternate immersion cycle is more aggressive than full immersion because the repeated wetting and drying concentrates chloride ions at the metal surface, accelerating crack initiation.

After the exposure period, each specimen is examined for cracking. If cracking occurs in any specimen, that heat or lot is flagged as susceptible.

The short-transverse direction is the critical orientation. ASTM G47 specifically calls out that specimens should be stressed in the short-transverse direction relative to the grain structure, because this is where 7xxx alloys are most vulnerable.

What Labs Actually Need to Get Right

Running a G47 test sounds straightforward on paper. In practice, several things go wrong at labs that are not experienced with aluminium SCC testing.

Specimen preparation matters enormously. The 3.18 mm diameter tension specimen per ASTM G49 has tight dimensional tolerances and surface finish requirements. Any machining damage, residual stress from cutting, or grain disturbance at the specimen surface will produce misleading results. At TCR, the machine shop follows strict specimen preparation protocols with documented geometry verification against the standard drawing before testing begins.

Solution control is not optional. The 3.5% NaCl solution must maintain a pH between 6.4 and 7.2 throughout the test duration. pH drift outside this range changes the electrochemical conditions and invalidates the test. The solution must be refreshed periodically. Many labs treat this as a minor detail. It is not.

Stressing ring calibration and load verification are critical. The constant load applied to the specimen must be accurate and maintained for the full 30-day period. Creep relaxation in the loading fixture, temperature fluctuations affecting spring constants, or improper load calibration all compromise the result.

Metallographic examination is often required at the end of the test, particularly when the result is borderline. Cross-sectioning the specimen and examining under the microscope for intergranular crack paths confirms whether the failure mode is genuine SCC or something else entirely.

"SCC testing of aerospace aluminium is one of those areas where the details determine everything," says Avinash Tambewagh, Technical Head at TCR Engineering. "The specimen orientation, the solution chemistry, the loading accuracy, and the post-test metallography all need to be right. A lab can have the equipment and still produce unreliable data if the execution discipline is missing."

TCR's NABL-Accredited SCC Testing Capability

TCR Engineering's corrosion testing laboratory in Navi Mumbai holds NABL accreditation (Certificate NABLT0726MH18640, ISO/IEC 17025:2017) for both ASTM G44 (alternate immersion SCC) and ASTM G47 (SCC of aluminium alloys). This is not a general claim. Both standards are explicitly listed in the NABL schedule under the metallography and corrosion section.

This matters for aerospace customers. When an OEM or prime contractor asks for test certificates, they want to see the NABL mark on the report. Non-accredited test data may be technically sound but will not pass a NADCAP or AS9100 supplier audit without additional justification.

TCR's corrosion lab also holds accreditation for a range of related standards that aerospace aluminium manufacturers frequently need:

ASTM G36 (chloride SCC in boiling MgCl2 for stainless steels)

ASTM G48 (pitting and crevice corrosion)

ASTM A262 (intergranular corrosion, all practices)

NACE TM0177 and TM0284 (for sour service, when your aluminium supplier also produces steel for oil and gas)

The corrosion team also routinely performs metallographic evaluation on post-test specimens, including grain structure assessment per ASTM E112 and inclusion analysis per ASTM E45, both under NABL scope.

Room Temperature Tensile Testing: The Other Half of the Requirement

Aerospace aluminium forging qualification does not stop at SCC testing. Mechanical property verification is equally mandatory. For 7075 and 7175 alloys, room temperature tensile testing per the applicable specification (EN 2002-001 for European aerospace, ASTM B557 and ASTM E8 for US specifications) is required on every heat lot.

TCR's mechanical testing laboratory is NABL-accredited for tensile testing of aluminium alloys per ASTM E8/E8M, ASTM B557, and ISO 6892-1. The lab routinely tests UTS, 0.2% yield, elongation, and reduction in area on machined cylindrical specimens. For elevated temperature applications, the lab is also accredited for tensile testing up to 300°C for aluminium alloys per ASTM E21.

This means a single lab can handle both the corrosion qualification (ASTM G47) and the mechanical qualification (tensile per EN 2002-001 or ASTM B557) for the same heat lot, with NABL-accredited results on both. That is a significant logistical advantage for forging houses that otherwise have to split testing across multiple facilities.

Why This Matters for India's Aerospace Supply Chain

India's aerospace component market is projected to nearly double over the coming decade. The India aircraft components market reached USD 16.22 billion in 2024 and is expected to reach USD 29.50 billion by 2033. Companies like SQuAD Forging have recently achieved NADCAP accreditation for forging, positioning themselves among a select few worldwide.

As Indian forging houses move up the aerospace value chain, from subcontract machining to full forging and heat treatment of structural parts, the testing requirements become more stringent. SCC testing per ASTM G47 is not a once-in-a-while requirement. It is part of ongoing production qualification, first article inspection, and periodic lot testing for every heat of 7xxx series material destined for flight hardware.

The availability of NABL-accredited SCC testing within India eliminates the need to ship specimens to overseas labs, saving weeks of turnaround time and avoiding customs complications around corrosion test specimens.

The Practical Workflow: From Sample to Certificate

For manufacturers planning to send specimens to TCR for ASTM G47 testing, here is what the process looks like:

Sample size: Input blanks of 15 x 15 x 55 mm (for SCC) or 15 x 15 x 50 mm (for tensile) from the heat lot are sent to TCR's lab at Mahape, Navi Mumbai.

Specimen preparation: TCR's machine shop machines the cylindrical tension specimens (3.18 mm diameter per ASTM G49) with full traceability to the incoming material. Each specimen is verified dimensionally against the standard drawing before testing.

SCC test execution: Specimens are loaded to the specified stress, placed in the alternate immersion apparatus, and cycled for 30 days. Solution pH and specimen condition are monitored and documented throughout.

Post-test evaluation: Specimens are inspected visually and, where required, subjected to metallographic examination to confirm the nature of any cracking.

Tensile testing: Parallel specimens from the same heat are tested for UTS, yield, elongation, and reduction in area per the applicable standard.

Reporting: NABL-accredited test certificates are issued for both SCC and tensile results. For international aerospace customers, reports can include the measurement uncertainty statement per ILAC G8 guidelines.

For technical queries on specimen requirements, testing durations, or standard-specific details, the technical team is reachable at +91 22 6738 0914. For commercial enquiries, write to sales@tcreng.com.

Beyond Aluminium: TCR's Broader Corrosion Testing Scope

While this article focuses on aluminium SCC testing, it is worth noting that TCR's corrosion department runs a much wider programme. The lab is one of India's most established facilities for NACE TM0177 (SSC) and TM0284 (HIC) testing for sour service qualification in oil and gas. The lab has dedicated autoclaves and H2S testing infrastructure, and is approved by PDO, ONGC, and EIL for this scope.

For manufacturers that operate across both aerospace and energy sectors (several forging houses do), the ability to qualify materials for both environments at one accredited lab is a practical advantage.

TCR also performs salt spray testing per ASTM B117 and ISO 9227, CASS testing per ASTM B368, and intergranular corrosion testing per ASTM A262, all under NABL scope.

The full list of accredited corrosion tests is available on the NABL website and in TCR's downloads section.

FAQ

What is ASTM G47 testing?

ASTM G47 is a test method for evaluating stress corrosion cracking susceptibility of 2xxx and 7xxx series aluminium alloy products. It uses alternate immersion in 3.5% NaCl solution with specimens stressed in the short-transverse direction, typically for 30 days.

What is the difference between ASTM G44 and ASTM G47?

ASTM G44 defines the general alternate immersion exposure procedure (cycle times, solution composition). ASTM G47 applies that procedure specifically to 2xxx and 7xxx aluminium alloys, with added requirements for specimen orientation, stressing direction, and evaluation criteria.

Is TCR Engineering NABL-accredited for ASTM G47 testing?

Yes. TCR holds NABL accreditation (Certificate NABLT0726MH18640) for both ASTM G44 and ASTM G47 under its corrosion and metallography scope.

Which aluminium alloys require SCC testing?

High-strength 2xxx series (2024, 2014, 2219) and 7xxx series (7075, 7175, 7050, 7475) alloys are most commonly tested. These alloys are used in aerospace structures, defence components, and marine applications where SCC risk is a primary concern.

How long does an ASTM G47 test take?

The standard exposure period is 30 days of continuous alternate immersion. Some client specifications require 40 days. Including specimen preparation, loading, and post-test evaluation, total turnaround is typically 35 to 50 days.

Can TCR perform both SCC and tensile testing on the same heat lot?

Yes. TCR's mechanical testing and corrosion testing labs operate under the same NABL accreditation, allowing both ASTM G47 SCC testing and tensile testing per ASTM E8, ASTM B557, or EN 2002-001 to be conducted and reported from a single facility.

What specimen size is needed for ASTM G47 testing?

Input blanks of approximately 15 x 15 x 55 mm are sufficient. TCR's machine shop will prepare the 3.18 mm diameter cylindrical tension specimens per ASTM G49 from the supplied material.