Stress Corrosion Cracking
Stress corrosion maybe defined as the combined action of sustained tensile stress and corrosion that may lead to the premature failure of materials. Certain materials are more susceptible than others. If a susceptible material is placed in a corrosive environment under tension of sufficient magnitude, and the duration of service is sufficient to permit the initiation and growth of cracks, failure will occur at a stress lower than that which the material would normally be expected to withstand. The corrosive environment need not be severe in terms of general corrosive attack. Service failures due to stress corrosion are frequently encountered in cases where the surfaces of the failed parts are not visibly corroded in a general sense. If failure is to be avoided, the total tensile strength in service must be maintained at a safe level. There is no absolute threshold stress for stress corrosion, but comparative stress-corrosion thresholds can be determined for materials subjected to controlled conditions of test. Estimates of the stress-corrosion threshold for a specific service application must be determined for each alloy and heat treatment, using a test piece, stressing procedure and corrosive environment that are appropriate for the intended service.
The stress corrosion susceptibility of alloys in this text was determined at ambient temperature by means of laboratory tests in which specimens were either sprayed with salt water or periodically immersed and withdrawn: by exposing specimens in seacoast or mild industrial environments: and by subjecting fabricated hardware to service conditions.
Most processing operations produce a flow of metal in a predominant orientation so that, microscopically, the metal is neither isotropic nor homogeneous. As a result, the properties of the metal vary according to the direction in which they are measured. The extend of directional variation depends on the property of interest. For the susceptibility to stress-corrosion cracking, the directional variation can be appreciable and must be considered in the design of the fabricated hardware. The resistance of metals to stress corrosion cracking is always less when tension is applied in a transverse direction. It is the least for the short transverse direction.
SCC table I
Materials that testing and experience have shown to possess high resistance to stress- corrosion cracking Their use is given preference.
SCC table II
Alloys and tempers listed in table II are moderately resistant to stress- corrosion cracking. They should be considered for use only in cases where a suitable alloy cannot be found in Table I.
SCC table III
Materials listed in table III have found to be highly susceptible to stress- corrosion cracking. They should be considered for use only in applications where it can be demonstrated conclusively that the probability of stress corrosion is remote because of low sustained tensile stress (whatever its origin) in critical grain directions, suitable protective measures or an innocuous environment.