ASTM B858 - Ammonia Vapor Test for Corrosion Cracking in Copper Alloys

5 Oct

Written By MTE

Title: A Comprehensive Guide to the ASTM B858 Test Method for Assessing Stress Corrosion Cracking in Copper Alloys

Introduction

The ASTM B858 test method, identical to ASTM Specification B858, provides a structured process for evaluating the susceptibility of copper alloys to stress corrosion cracking using an ammonia vapor test. It primarily focuses on detecting residual stresses that might lead to stress corrosion cracking when the alloys are exposed to specific environmental conditions.

Scope

This method is tailored for copper alloys known for their vulnerability to stress corrosion cracking in ammonia vapor atmospheres. It is not a universal test but is designed to simulate specific service conditions with reproducible severity, relying heavily on the pH of the corrosive solution involved.

Key Terminologies

Applied Stress: The stress due to an external load.
Residual Stress: Stresses within a body resulting from processing, including plastic deformation or casting.
Stress Corrosion Crack: The spontaneous failure of metals due to the combined effects of corrosion and stress.

Test Procedure

1. Reagents and Materials

Use reagent-grade chemicals.
Utilize water of Type IV or better quality as per Specification D1193.
Prepare ammonium chloride solution (4N), hydrogen peroxide (30–35%), and sodium hydroxide solution (300–500 g/L).

2. Apparatus

A pH meter.
A closed vessel, such as a desiccator.
Equipment for examining test pieces at 10× to 15× magnification.

3. Test Specimen Preparation

Prepare the specimen as per the material specification.
Remove any burrs to prevent acceleration of stress corrosion cracking.
Clean and degrease the specimen, following up with a rinse and air drying.

4. Test Media

Create a test solution with the appropriate pH, maintaining it at ambient temperature.
Ensure the ratio of surface area of the test solution to air volume is at least 20 cm2/L.

5. Test Procedure

Allow the dry test piece to acclimatize to the exposure temperature.
Transfer it immediately to a closed vessel containing the prepared test solution.
Ensure the ammonia vapor has free access to all surfaces of the test piece.
Maintain a constant exposure temperature between 20°C and 30°C.
Expose the test piece for 24 hours.

6. Post-Exposure Processing

Clean the test piece in pickling solution to remove corrosion products.
Examine the surface for cracks at a magnification of 10× to 15×.
Deform the test piece slightly to make fine cracks more visible.
Metallographic examination can be employed to study the nature of observed cracks.

7. Report

Include information such as sample identification, reference to the test method, pH-value used, number of replicate test pieces, test results, and any observed material features, along with the date of the test.

Conclusion

The ASTM B858 test method is instrumental in predicting the susceptibility of copper alloys to stress corrosion cracking under specific conditions. Through meticulous execution of the outlined steps, professionals can assess the reliability and durability of copper alloys, fostering informed decisions in their application and handling. Safety, precision, and adherence to the specified procedures are essential to ensure the validity and reliability of the test results.

MTE