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.
Austenitic stainless steels (SS) have good mechanical properties, heat resistance, weldability and general corrosion resistance. These steels are widely used in various components of chemical plants, fossil and nuclear power plants, etc. However, it is also well known that the austenitic stainless steels are susceptible to localized corrosion in environments containing chloride ions. In addition, stress corrosion cracking (SCC) can occur especially under tensile stresses. In Japan, most of plants were built in coastal area because of easiness of access to the raw materials, products transportation, etc. The sea salt particles adhere to the plants equipment, and chlorides in sea salt are condensed by dewfall and deliquesce. In this way, environment containing chloride ions are produced in the air atmosphere by ionization of the sea salt particles, i.e. NaCl and MgCl2. SCC observed in such environment is usually found at the equipments surfaces, hence such SCC is called the external SCC (ESCC). It is very important to clarify the factors governing the ESCC process as function of stress, material and environmental conditions for clarification of the ESCC behavior and for establishment of countermeasures. Temperature usually influences chloride induced SCC, and the effect can be formulated by an Arrhenius type equation(1). For example, Yajima and Arii calculated activation energy of the ESCC as 7 kcal/mol(2). Effect of humidity is also important factor for the ESCC. Chloride concentration influences initiation of the SCC and is determined by humidity.
