|Fig. 1. Factors contributing to stress corrosion cracking|
Components fail for a variety of reasons, which includes a corrosion phenomena characterized by the fact that stress (and/or deformation) is present to provide a trigger that leads to sudden crack formation, propagation and failure. Let’s learn more.
Stress corrosion cracking (SCC) is the type of failure mechanism caused by a combination of environmental, material and stress conditions (Fig. 1). It is generally considered the most complex of the failure modes since it can attack soft or hard parts; ferrous or nonferrous materials; ferritic or austenitic structures; and materials in the unalloyed or alloyed state. Cracks may propagate in a transgranular or intergranular fashion or in a combination of the two. The stress, however, must be in the form of tensile stress above some minimum (i.e. threshold) value, usually below the yield stress of the material and in the presence of a corrosive environment that includes sulfides, chlorides, caustics and hydrogen. Temperature is a significant environmental factor affecting crack formation, and pitting is commonly associated with SCC phenomena. In addition, catastrophic failure can occur without significant deformation or obvious (surface) deterioration of the component.
The SCC phenomena can be affected by many factors in addition to stress level, including alloy composition, microstructure, concentration of corrosive species, surface finish, micro-environmental surface effects, temperature, electrochemical potential and the like. Further complications are initiation and propagation phases and the observation that in some cases cracks initiate at the base of corrosion pits.
There is no identified single mechanism explaining SCC, but several theories have been proposed.
- Active path propagation: Localized preferential corrosion (a.k.a. dissolution) at the crack tip, along a susceptible path, with the bulk of the material remaining in a more passive state. The rate of metal dissolution can be several orders of magnitude higher when an alloy is in its active state compared to its passive condition.
- Hydrogen embrittlement: High hydrogen concentrates in highly stressed regions, such as at the crack tip or other stress concentrators, leading to localized embrittlement.
- Brittle film-induced cleavage: Cracks initiated in a brittle surface film may propagate (over a microscopic distance) into underlying, more ductile material before being arrested by ductile blunting of the crack tip. If the brittle film re-forms over the blunted crack tip (under the influence of corrosion processes), such a process can be repeated over and over again.
|Fig. 2. Fastener failure – Stress corrosion cracking|
Negating the Effects of SCC
A combination of good design, correct selection of SCC-resistant materials, environment management, maintenance and inspection can effectively control this type of corrosion. Stresses to consider include:
- Applied (tensile) stresses
Thermally induced factors
- Temperature gradients
- Differential thermal forces (expansion and contraction)
Buildup of corrosion products
- Volumetric dependent
- Poor fit up (tolerance problems)
- Press and shrink fits
- Fastener interference
- Joining method
Residual stresses (from the manufacturing processes)
- Joining (welding, brazing, soldering)
- Forging or casting
- Surface treatment (plating, mechanical cleaning, etc.)
- Heat treatment (quenching, phase changes)
- Forming and shaping
- Cutting and shearing
One of the most important considerations to negate the effects of SCC is choosing the proper alloy. It is relatively simple to choose a component with adequate strength and good (general) corrosion resistance. However, knowing the particular type of SCC issues that may be at work in the application is an important step in achieving a resistant material. In certain environments, it may be necessary to choose a material that will experience some general corrosion since general corrosion is visually evident, and, with proper preventative maintenance, general corrosion can be seen and components replaced as necessary. On the other hand, SCC is rarely visually apparent and often occurs without warning (Fig. 2). When it does, a catastrophic failure often follows.
Other methods include removing the corrosive environment or changing the manufacturing process or design to reduce the tensile stresses. A combination of good design, careful selection of stress corrosion-resistant grades (e.g., stainless steel) and effective management, including maintenance and inspection, all can effectively control corrosion. Specific steps can be taken to prevent the onset of SCC and minimize its consequences when it does occur by:
- Consideration of the potential for SCC during the design and fabrication of components
- Selection of appropriate material grades
- Maintaining a chemical balance of the environment
- Ensuring that the potential for (organic or inorganic) contamination is minimized
- Maintaining proper environmental conditions (e.g., air quality)
- Regular inspections of components for signs of corrosion and SCC
Importance of Material Selection
In many applications, austenitic stainless steel fasteners (e.g., ASTM A193 grade B8) of 304 and 316 stainless steels provide good general corrosion resistance and are commonly requested. However, in marine environments where stainless steel would seem to be the logical choice, alloy-steel fasteners are preferred due to SCC concerns. Chlorides, fluorides and other halogens are known catalysts for chloride SCC. In order to reduce their susceptibility to general corrosion, alloy-steel fasteners such as grade B7 are usually provided with some type of protective coating (e.g., zinc or cadmium plating). However, the designer must still be aware that this can lead to another form of corrosion due to environmental stress cracking in the form of liquid metal embrittlement (LME) or a related failure mode, solid metal induced embrittlement (SMIE). Therefore, appropriate cautions must be taken.
In addition to SCC, other forms of embrittlement include: (a) environmentally induced cracking due to such factors as cold work (i.e. residual stress), welding, grinding, thermal treatment or service conditions; (b) hydrogen embrittlement from plating, welding, cathodic protection and as a by-product of general corrosion; (c) corrosion fatigue; and (d) liquid-metal embrittlement.
Careful consideration of the factors discussed above as well as taking the time to understand how and where a component will be used in service can help minimize stress corrosion cracking in most applications. IH