Immersion of polyamide 6,6 in saline solutions: understanding the impact of water and salts on the cracking mechanisms

Polyamide 6,6 (PA6,6) is a well-known and widely used polymer in the automotive industry. During its lifetime, it can come into contact with road salts such as NaCl, MgCl2 or CaCl2. In the literature, authors report an environmental stress cracking behavior due to the contact with these saline solutions. However, to the best of our knowledge, there is no consensus on the absorption and cracking mechanisms.

During simple immersion tests, we have shown that without applied stress, no significant absorption of ions is detected. The mechanical effects have therefore been related to the water activity in the saline solutions.

When applying a constant stress during the contact with road salts, different cracking behaviors have been evidenced. At room temperature, no cracking is observed under yield stress. At yield stress:

- water induces plasticization wrinkles but the diffusion path has yet to be observed

- NaCl-type salts create thin crazes with no detected absorption of salts

- CaCl2-type salts create large crazes with detected absorption of salts

The distinction between the different behavior has been attributed to the localization of plasticity and the plasticity instabilities created by the saline solutions.

Further experiments will aim at describing these instabilities considering various hypotheses: surface tension effects, chemistry, osmotic pressure in the crazes,…