Austenitic stainless steels are widely used as structural materials for chemical and power plants. External stress corrosion cracking (ESCC) by sea salt particles can occur in these plant component materials. ESCC susceptibility depends on environmental factors. We examined the effects of temperature and humidity on ESCC experimentally. Uniaxial tensile loading specimens were used for the ESCC test. Specimens were kept in a constant temperature and humidity chamber after loading and dropping the artificial sea water onto the gage section. Specimen surface and fracture surface were examined after the ESCC test. Crack length and depth were measured and used to evaluate ESCC susceptibility.

There was no clear dependency of the maximum crack length and crack depth on applied stress at 353 °K and relative humidity (RH) of 35 % and 55 % probably because of high corrosive power of the test environment. On the other hand, stress dependency of the ESCC was identified at 343 °K and 333 °K. It would be mainly caused by increase in the incubation time of the ESCC by decreasing temperature.

We also obtained the activation energy of the ESCC growth rate. Apparent activation energy of the average ESCC growth rate was calculated to equal to 5.6~9.4 kcal/mol in case when RH is equal to 35 and 55 % with an applied stress of 313 MPa. Activation energy for peak current density of active zone was calculated at 24kcal/mol and 12kcal/mol for artificial sea water with pH equals to1.0 and 1.3, respectively, as the result of anodic polarization measurement. Thus the pH at the crack tip of ESCC will be close to 1.3 rather than 1.0.

Subject
External stress corrosion cracking, Humidity, Crack growth, Materials, Fracture surfaces, Activation energy, Acidity, Stress, Crack depth, Crack length, Stainless steel, Chlorides, Chloride stress corrosion cracking
Keywords:
stress corrosion cracking, austenitic stainless steel, chloride ion, activation energy
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2008
Association for Materials Protection and Performance (AMPP)
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