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Controlling Stress Corrosion Cracking (SCC)

In order for SCC to occur, we require a susceptible material, an environment that will cause cracking of that material and a high enough stress or stress intensity factor. There are, consequently, a number of approaches that we can use to prevent SCC, or at least to give an acceptable lifetime. In an ideal world a stress corrosion cracking control strategy will start operating at the design stage, and will focus on the selection of material, the limitation of stress and the control of the environment. The skill of the engineer then lies in selecting the strategy that delivers the required performance at minimum cost. In this context we should appreciate that a part of the performance requirement relates to the acceptability of failure. For the primary containment pressure vessel in a nuclear reactor we obviously require a very low risk of failure. For the pressed brass decorative trim on a light switch, the occasional stress corrosion crack is not going to be a serious problem, although frequent failures would have an undesirable impact on product returns and the corporate image. (reference)

Selection and control of material

The first line of defence in controlling stress corrosion cracking is to be aware of the possibility at the design and construction stages. By choosing a material that is not susceptible to SCC in the service environment, and by processing and fabricating it correctly, subsequent SCC problems can be avoided. Unfortunately, it is not always quite that simple. Some environments, such as high temperature water, are very aggressive, and will cause SCC of most materials. Mechanical requirements, such as a high yield strength, can be very difficult to reconcile with SCC resistance (especially where hydrogen embrittlement is involved). Finally, of course, Murphy’s Law dictates that the materials that are resistant to SCC will almost inevitably be the most expensive (and that they will be found to be susceptible to SCC in your environment as soon as you have used them!).

Control of stress

As one of the requirements for stress corrosion cracking is the presence of stress in the components, one method of control is to eliminate that stress, or at least reduce it below the threshold stress for SCC. This is not usually feasible for working stresses (the stress that the component is intended to support), but it may be possible where the stress causing cracking is a residual stress introduced during welding or forming.

Residual stresses can be relieved by stress-relief annealing, and this is widely used for carbon steels. These have the advantage of a relatively high threshold stress for most environments, consequently it is relatively easy to reduce the residual stresses to a low enough level. In contrast austenitic stainless steels have a very low threshold stress for chloride SCC. This, combined with the high annealing temperatures that are necessary to avoid other problems, such as sensitization and sigma phase embrittlement, means that stress relief is rarely successful as a method of controlling SCC for this system.

For large structures, for which full stress-relief annealing is difficult or impossible, partial stress relief around welds and other critical areas may be of value. However, this must be done in a controlled way to avoid creating new regions of high residual stress, and expert advice is advisable if this approach is adopted.

Stresses can also be relieved mechanically. For example, hydrostatic testing beyond yield will tend to ‘even-out’ the stresses and thereby reduce the peak residual stress. Similarly shot-peening or grit-blasting tend to introduce a surface compressive stress, and are beneficial for the control of SCC. The uniformity with which these processes are applied is important. If, for example, only the weld region is shot-peened, damaging tensile stresses may be created at the border of the peened area.

Control of environment

The most direct way of controlling SCC through control of the environment is to remove or replace the component of the environment that is responsible for the problem. Unfortunately, it is relatively rare for this approach to be applicable. If the active species is present in an environment over which we have some control, then it may be feasible to remove the active species, although even then it may be difficult. For example, chloride stress corrosion cracking of austenitic stainless steels has been experienced in hot-water jackets around chocolate pipes (that is to say, pipes carrying molten chocolate) in the food industry. In this situation we can’t easily change the material or the temperature, and it is virtually impossible to eliminate the residual stresses associated with welding and forming of the stainless steel. However, we can remove the chloride from the water by an ion exchange process, and, with proper control and monitoring, this approach could be successful. Of course if we were dealing with hot tomato ketchup, which has a low pH and may contain enough chloride to cause SCC, we have a far more difficult problem!

In the latter situation, where the species responsible for cracking are a required component of the environment, the environmental control options consist of adding inhibitors, modifying the electrode potential of the metal, or isolating the metal from the environment with coatings.

To take another example of chloride SCC of austenitic stainless steels, tube and shell heat exchangers are frequently constructed using stainless steel tubes (since these must be thin-walled and corrosion cannot be tolerated) with carbon steel tube plates and shell (since these can be made much thicker to provide a corrosion allowance). Chloride SCC is rarely experienced with this construction. However, it is quite common for an enthusiastic engineer to decide that the replacement heat exchanger should use an “all-stainless” construction to avoid the unsightly corrosion of the carbon steel. The result is frequently a rapid failure of the heat exchanger by SCC or pitting corrosion. This is because the carbon steel adopts a relatively low electrode potential that is well below that required to cause SCC or pitting of austenitic stainless steel, which is thereby protected. When the all-stainless construction is adopted, this unintentional electrochemical protection is lost and failure occurs.

Corrosion inhibitors are chemicals that reduce the rate of a corrosive process. They are generally rather specific to a particular alloy system, and they typically also have specific requirements in terms of the composition of the environment. Inhibitors may be effective at controlling SCC, although the requirements are rather different from those for the inhibition of general corrosion. Indeed chemicals that inhibit general corrosion may create the necessary conditions for stress corrosion cracking (e.g. hydroxides, carbonates and nitrates for carbon steel). Even when inhibitors are effective against SCC, higher concentrations may be required than for the inhibition of general corrosion.

Metallic coatings isolate the metal from the environment, and can, thereby, prevent SCC. However, the possibility of the coating being penetrated by imperfect application or by mechanical damage in service must be taken into account. For this reason zinc is a popular coating for carbon steel. The normal corrosion potential for zinc is relatively low, and if any of the underlying steel is exposed, this will be cathodically protected. However, the low electrode potential will also encourage hydrogen evolution, and this may lead to hydrogen embrittlement. Hydrogen embrittlement may also occur as a result of the hydrogen evolution during the initial electroplating operation, as noted above. Consequently, zinc plating must be used with care on strong steels. Cadmium adopts a rather more positive potential, and produces a much lower risk of hydrogen embrittlement, while still protecting the underlying steel. Unfortunately the toxicity of cadmium compounds means that it is essentially banned as a coating material.

Paints and other polymeric coatings protect the underlying metal largely by virtue of their high electrical resistance, which restricts the passage of current from the anode to the cathode (both oxygen and water diffuse relatively easily through most polymers, so paints don’t, as is often thought, work by isolating the metal from the environment). Paints may be effective at restricting SCC, particularly where they incorporate inhibitors that can inhibit any solution that does find its way to the metal. However, as with metallic coatings, it is important to think about what will happen if the coating is removed by mechanical damage.

See also:

Basics of SCC, Causes of SCC, Controlling SCC, EL AL crash, Environments & SCC, Pipeline SCC, SCC Guide, SCC definition, SCC of aircraft component, SCC Mechanism, Swiss roof collapse, Testing strategy, Williams explosions