ASTM G67-24
Standard Test Method for Determining the Susceptibility to Intergranular Corrosion of 5XXX Series Aluminum Alloys by Mass Loss After Exposure to Nitric Acid (NAMLT Test)

Standard No.

ASTM G67-24

Release Date

2024

Published By

American Society for Testing and Materials (ASTM)  US  /  ASTM

Status

Be replaced

Replace By

ASTM G67-24a

Latest

ASTM G67-24a

Introduction

Technical Background and Evolution of the Standard

ASTM G67 was originally published in 1980, and the 2024 edition marks its sixth revision. This revision primarily adjusts the nitric acid solution concentration range to 69-70 w/w% and strengthens the temperature control requirement (±0.1°C). This standard, originating from the Aluminum Association's research on exfoliation corrosion in 5086/5456 alloys in the 1970s, has become an internationally recognized method for evaluating grain boundary sensitivity in Al-Mg alloys.

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Test Principle and Scientific Basis

This method quantitatively determines the weight loss caused by the preferential dissolution of βAl-Mg intermetallic compounds along the grain boundaries by immersion in concentrated nitric acid at 30°C for 24 hours:

Material State	Typical Weight Loss Range (mg/cm²)	Grain Boundary Precipitation Characteristics
Intergranular Corrosion Resistant Materials	1-15	Random Distribution
Sensitive Materials	25-75	Continuous Network Distribution

Application Case: Testing of 5083-H116 alloy plates for a certain ship showed a weight loss of 32mg/cm². Metallographic analysis confirmed that the continuity of the β phase at the grain boundaries reached 85%, which was positively correlated with the degree of exfoliation corrosion in actual exposure tests in seawater environments.

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Key equipment and material requirements

The standard strictly stipulates the following core elements:

• Non-metallic container: 100mL ribless beaker with watch glass is recommended, and the sample is tilted to avoid full contact
• Constant temperature water bath: Temperature control accuracy is ±0.1°C, and a continuous recording thermometer is recommended
• Ultrasonic cleaner: 40kHz frequency, power 15-50W/L
• Nitric acid reagent: 69-70 w/w% analytical grade, solution volume ratio ≥30L/m²

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Specimen preparation specifications

The standard specimen size is 50×6×t mm (longitudinal parallel to the rolling direction), when the thickness is greater than 25 mm, it needs to be machined to 25 mm and retain an original surface.

1. All cutting surfaces must be finely machined to Ra≤3.2μm
2. It is allowed to drill ≤4mm small holes to assist cleaning (not included in the surface area)
3. At least 2 parallel specimens for each group of samples

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Test Process Summary

Steps	Parameters	Tolerance
Alkaline Cleaning	5% NaOH/80°C/1min	±2°C
Decontamination	Concentrated HNO₃/30s	-
Main Test	70% HNO₃/30°C/24h	±0.1°C
Ultrasonic cleaning	DI water/1-2min	±15s

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Quality Assurance System

The standard recommends a double verification mechanism:

Verification sample:Use standard 5083 material (RR:G01-1028) with a known weight loss of 21-30mg/cm² for laboratory proficiency testing

Process control:Each batch of test must include an internal standard sample to establish X±2σ control limits (based on ≥10 historical data)

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Implementation Recommendations and Precautions

1. Safety Protection: The experiment must be conducted in a fume hood equipped with acid-resistant PPE and emergency flushing equipment
2. Data Validity: When the control sample exceeds the historical fluctuation range, the entire batch of data should be invalidated and key factors such as temperature control and solution concentration should be checked
3. Result Interpretation: For intermediate values of weight loss of 15-25 mg/cm², metallographic analysis must be used to confirm the corrosion morphology
4. Method Extension: Can be used in conjunction with ASTM G34 (EXCO test) to comprehensively evaluate the corrosion resistance of materials

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Precision and Bias Explanation

Based on the combined test data of RR:G01-1028:

• Repeatability Coefficient of Variation:4% (same laboratory)
• Reproducibility coefficient of variation: 10-26% (inter-laboratory)
• Bias statement: This method is a self-consistent system and is not suitable for bias evaluation

Note: The current accuracy data is based on the acid concentration range of the old version. The accuracy verification under the new version of 69-70% conditions is to be supplemented