Compatibility of "Steri-7" Sterilising Solution with Brass Grade CW614N

BACKGROUND

Sentinel International produce a sterilising solution product “Steri-7”. End-users have queried whether the product is suitable for use with brass, specifically valves and other fittings used to handle pressurised gases. The principal concern is stress-corrosion cracking: brasses are known to be susceptible to stress-corrosion in the presence of ammonia.

Sentinel instructed Capcis to perform tests to investigate the behaviour of commercial brass in contact with the “Steri-7” product, initially both neat and at the recommended dilution for use.

TEST PROCEDURE

The test procedure was be in general accordance with BS EN ISO 7539-7, Corrosion of metals and alloys – stress corrosion testing: Part 7. Slow strain rate testing. The slow strain rate method involves slowly pulling a sample of the material to failure. Tests are performed in the environment and also in an inert or safe environment as a baseline. Advantages of this test method include: o the test is relatively fast compared with static stress-corrosion tests; the test covers the whole range stress – strain range, useful when the service stress cannot be defined (as in the present case).

TEST MATERIALS

National supplied sachets of sterilising solution, marked as making 5L when fully diluted. Test pieces were machined from a sample of commercial grade CW614N brass provided by Capcis. This is a leaded free-cutting grade of brass, and is the grade typically used for gas cylinder regulators, valves and associated fittings. A notch was included to produce a stress concentration, similar to the thread forms on valves and fittings. The test-piece gauge length was 25.4mm, the diameter 6.35mm and notch minor diameter 4.76mm, with a 0.4mm radius at the notch tip.