Unexpected or premature failure of chemical process equipment constitutes a serious hazard in terms of safety of personnel, operating facilities, and the environment. By weakening reliability, such failures also adversely affect productivity and profitability. Modern industrial experience in chemical plants has been that failures due to environmental cracking are among the most serious of such problems, making up about 20 to 30% of all corrosion failures. The subject of stress corrosion cracking (SCC) is extensive, and the focus of this issue is to discuss in simple terms some of the pertinent information.
SCC has been defined as failure by cracking under the combined action of corrosion and typically tensile stress. The stress and corrosion components interact synergistically to produce cracks, which initiate on the surface exposed to the corrodent and propagate in response to the stress state.
For any given alloy-environment system, the engineering parameters of concern are as follows:
- Threshold stresses above which cracking occurs
- Metallurgical variables (heat treatment, structure, cold work) which render the alloy susceptible
- Environmental boundary conditions for cracking such as temperature, solution composition, pH, electrode potential, necessary impurities, etc.
It is important to realize that the conditions causing SCC may not occur during normal operation of equipment but also during startup, shutdown, idle periods, or system upsets. Stresses and environmental conditions under these circumstances can be quite different than those encountered during normal operation. READ MORE... (fill out the form to the right to download the full copy.)