CW509L | CuZn40 Stress corrosion cracking in alpha brass

In this case, fractures occurred in CuZn40 fasteners.

For stress corrosion cracking (SpRk) to occur, three conditions usually have to be present:

• High stresses (internal stresses such as hardness and/or component stresses due to external influence such as bolting or welding)
• Corrosion medium
• sensitive material

The corrosion mechanism is intergranular stress corrosion cracking, although the “sensitive material” condition is not present here.
The pure alpha alloys are insensitive to SpRk compared to the alpha-beta brass. The hardness of 109 HV0.2 is just below the critical limit of 110-115 HV. Intergranular fracture was present over the entire fracture cross-section. Nitrogen was detected as the presumed corrosion medium at several points on the fracture surface. Therefore, ammonium or other nitrogen compounds in combination with high tensile stresses due to the bolting are probably responsible for the damage.

A laboratory comparison fracture produced a ductile honeycomb fracture. The microstructure consists of pure alpha with typical recrystallisation twins.