Stress-corrosion cracking (SCC) is one of the most prevalent—and one of the most studied—forms of metallic materials failure. This phenomenon is responsible for significant economic losses in many industries and operations, including chemical and petroleum processing, nuclear and fossil fuel power generation, pulp and paper production, underground pipelines and commercial and military aircraft. Materials subject to SCC include mild and low alloy steels, stainless steels and alloys based on nickel, copper, aluminum, titanium or magnesium. (As discussed later in this article, a related process is hydrogen embrittlement [HE].)
Although SCC has received more fundamental study than any other form of corrosion-related attack, many questions still remain. This article is a brief introduction to the topic. It will review a few of the important characteristics of SCC, as well as some related cracking processes in common materials, and show how these results can be used to help minimize loses. References provided at the end of the article list some sources for more in-depth inquiries.
Keep in mind that guaranteed SCC prevention is impossible. Unexpected combinations of the governing factors commonly occur and control measures are not 100% effective. Certain actions, however, can be taken to reduce the frequency of failures. As in many equipment reliability improvement areas, the keys to significant initial progress are first to gain awareness and then to apply known but often under-utilized information.
The general nature of SCC
SCC is a synergistic failure process that occurs due to the combined actions of corrosion and mechanical cracking. The three vital interacting factors are the alloy, local stress level and local environment. One of its unique features is that a specific combination of a particular corrosive and a particular alloy is required for SCC to occur. Table I lists several of the combinations of materials and corrosive media where the process has occurred. Note that certain corrosive media cause SCC on specific classes of materials, but they have no similar effects on other alloys.
Cracks produced by SCC are caused by tensile stresses— not by compressive stresses. Compressive surface stresses retard SCC and shot peening; imparting these helpful stresses is one control method that can help.
The minimum level of tensile stress necessary for SCC is below the yield strength of a susceptible material. There have been reports of SCC in materials where the stress level was as little as 10% of the yield. This illustrates the effect of time. Depending on the relative severity of the corrosion-affecting parameters and other factors, SCC can occur at very low stress levels if given enough time.
It is well known that SCC occurs in metallic materials. However, cracking and related degradation processes also can occur in polymers and ceramic engineering materials. These materials fail by different mechanisms and are not discussed here.
Factors important to SCC incidence
Effect of residual tensile stresses…
This is a major cause of problems. Applied tensile stresses can and do produce SCC in susceptible materials in specific environments. Unrelieved residual stresses often are overlooked, yet frequently produce the stress that causes SCC.
Residual stresses frequently are associated with the heat-affected zone (HAZ) of welds. As weld metal solidifies after welding, it contracts. This means that residual tensile stresses are generated in the adjacent HAZ. The situation is made worse if the weld joint is constrained. Residual stresses also can be generated due to several other manufacturing processes where plastic deformation is created.
If done correctly, stress-relief heat treatment can be effective in reducing residual stresses to low levels. Other problems can be created if the stress relief is done incorrectly. This process should always be considered as a practical measure to control SCC.
Effect of temperature…
The corrosion process itself is, obviously, an integral component of any SCC mechanism. This electrochemical process occurs at significantly faster rates at higher environmental temperatures. The rates of most chemical reactions occur about twice as fast for each 18 degree F (10 degree C) rise in temperature. Table I provides examples where SCC is a recognized problem at the noted higher temperature but not at lower temperatures. As will be subsequently discussed, corrosion fatigue is related to SCC in several ways—and it is similarly affected by temperature.
A related alloy cracking process (covered later in this article) involves hydrogen. There, the effect of hydrogen in the metal possibly may combine with the SCC process in some situations and be independent of it in others. Defining when one or the other result is occurring often is unclear. Thus, the effect of temperature can be unclear.
Effect of concentrating aggressive ions…
SCC is a localized form of attack. Consequently, physical features that increase the number of aggressive ions in one area on a stressed metal are more likely to produce SCC in those specific areas. A classic and costly example entails chloride or other aggressive ions that are leached out of thermal insulation and then concentrated on hot, austenitic stainless steel piping or vessels covered by that insulation. This can happen when there is a gap in the rain shielding over insulation, and rain (or other moisture sources) leaches out the aggressive ions and deposits them on the metal.
Another example involves crevices created in fabricated equipment, e.g., at bolted flanges or at lapped plates where skip (or tack) welds are used instead of continuous welds. Crevices create areas that are partially—but not fully— closed to the environment. Aggressive ions in solution will tend to concentrate at the openings of and inside crevices. If tensile stresses exist, SCC likely will occur near these areas.
Effect of sensitized alloys…
Some materials become susceptible to intergranular corrosion—called IGA— when metallurgical changes result in segregation of certain corrosion-resisting alloying elements in the metal. The classic and most common example occurs in certain 300 series austenitic stainless steels. When these alloys are exposed to a temperature range of approximately 800 to 1600 degrees F for an extended period, carbon in the alloys preferentially joins with chromium and precipitates out of solid solution to the grain boundaries of the metal. This frequently takes place during cooling immediately following welding. The materials are then made susceptible—or sensitized—to IGA if they are then exposed to certain (but not all) corrosive media. Accelerated corrosion may occur in areas immediately adjacent to grain boundaries because those areas are deficient in chromium.
Somewhat similar results also can occur and produce sensitized conditions in precipitation-hardening stainless steels, in some nickel-based alloys and some aluminum alloys.
Besides IGA, sensitized alloys are much more susceptible to SCC. The resulting cracking that occurs is intergranular in nature—that means any generated cracks follow along the grain boundaries. For particular corrosive media and sensitized alloys, intergranular SCC may occur when sufficient stress exists and IGA can occur when there is insufficient stress.
Comparison with other cracking processes
SCC shares some characteristics with associated mechanisms—corrosion fatigue (CF) and hydrogen embrittlement (HE). But, it also has differences.
CF, just like SCC, begins on the surface of an exposed and susceptible metal. Tensile stresses start and propagate the crack into the metal by both processes. Physical features on the metal surface that concentrate the tensile stresses are most frequently the initiation points for both types of attack. These include, for example, sharp radii at changes in cross-section or at corrosion pits. In general, traditional corrosion control measures that apply to SCC also apply to CF.
Unlike the static stresses that act in SCC, the primary stresses in CF are cyclic—as in “pure” fatigue where there is no corrosion involved. Another difference is that CF (as well as hydrogen-related cracking) produces cracks that typically follow one central path with little or no branching. The cracks due to SCC usually will have branching with many very small extensions— somewhat like a tree’s underground root system.Apart from these differences, CF might be considered as a special case of SCC.
Hydrogen-related cracking occurs when atomic hydrogen (as opposed to the molecular form) enters a susceptible metal, diffuses through it and causes embrittlement. Much of the metal’s ductility is then lost so that it can be fractured at a lower stress compared to its original condition. The embrittlement can take place throughout the metal or the effects can be localized around stress concentration areas, e.g., at the tips of cracks created by SCC or at stress concentration points on the metal surface. There are several possible ways for charging hydrogen into susceptible alloys.
Cracking due to HE can occur independently, when no active corrosion is proceeding, OR it may interact and contribute to the mechanical cracking portion of an SCC process. There’s still uncertainty about the specific role of hydrogen in many SCC processes. This is probably because no one mechanism has been found to be a valid description for all of the many possible cases of SCC.
The negative effects of HE occur primarily in high-strength alloy steels, high-strength martensitic or participation-hardened stainless steels, certain high-strength aluminum alloys and certain titanium alloys. The obvious key is “high-strength”— the higher the strength, the greater the probability of cracking due to hydrogen acting alone or in conjunction with SCC.
Traditional corrosion control measures don’t apply when cracking due to HE is acting alone—that’s because no corrosion is taking place. In those incidents, the best controls are to prevent hydrogen from entering the metal and use alloys with less than maximum strength and hardness. There are technical standards that provide guidelines for implementing the latter precautions.
SCC control methods
If stress-corrosion cracking is an issue in your operations, you have a number of control methods to consider. These include, but are not limited to, the following:
• Avoid the specific material/aggressive media combinations in Table I. Choose alternative alloys.
• Reduce applied tensile stresses in service and always consider residual stresses. Use stress-relief heat treatments after welding or after manufacturing processes that included plastic deformation of the alloy. Assure that the stress-relief heat treatment is done correctly.
• Minimize conditions that concentrate aggressive ions, e.g., chloride and other halide ions leached from thermal insulation or any damaging species concentrated by crevices.
• Avoid sensitizing susceptible alloys during cooling from welding and/or extended exposure to damaging temperatures in other heat treatments or in service. SCC or IGA can result without attention to this potentially problematic effect.
• Determine if shot peening (to impart helpful compressive stresses) on metal surfaces susceptible to SCC is practical in the given application.
• Consider if any of the traditional corrosion-control techniques besides materials selection, i.e., adding a coating, using a chemical corrosion inhibitor or cathodic protection (CP), are practical for the particular SCC-susceptible application. If CP is feasible, assure that the potential (voltage) level used can provide protection but is not too low, i.e., too cathodic, so as to produce hydrogen problems in susceptible alloys.
• Plan preventive maintenance inspections and proactive actions with SCC control in mind. For example, pay special attention to the material/ corrosive media combinations in Table I; look at welds, around crevices and at plastically deformed areas for evidence of SCC. Look for gaps in thermal insulation where moisture can enter and create conditions for SCC on the insulated pipe or vessel. Always expect more severe fatigue problems in equipment subject to cyclic stresses plus an aggressive, corrosion environment due to CF, e.g., in rotating equipment and due to flow-induced vibrations in shell & tube heat exchangers. Anticipate and plan for corrective actions that may be used if problems are found. MT