What is Stress Corrosion Cracking or SCC?
Stainless steel fabrications are prone to stress corrosion cracking (SCC) during their service life, when they are functioning in a corrosive environment.
Fabricators of chemical plant ought to be alive to this problem and able to foresee it, because when failure does occur on this account, it is too late to effect satisfactory repairs.
Stress corrosion, as its name suggests, results from the combined action of a tensile stress and corrosive environment. Stresses that cause cracking arise from residual cold work, welding, or thermal treatment, or they may be externally applied during service.
SCC is a brittle-type fracture in a material exposed to a corrosive environment and it should not be confused with other types of localized attack such as pitting, galvanic attack, intergranular corrosion or cavitation.
What are the factors to increase Stress Corrosion Cracking?
The fact that stress cracking can lead to the weakening of components is known from many metals. Most stainless steels, on the other hand, have relatively little tendency to stress corrosion cracking. Generally speaking, the chances of SCC occurring will increase as:
- the stress level rises,
- the attacking compounds in the electrolyte become more concentrated
- the temperature increases, and
- as exposure time increases.
Almost all metals and alloys are susceptible to SCC in specific environments. Alloys show a far greater sensitivity than pure metals.
In the case of straight chrome stainless steels having more than 12% Cr, SCC is likely to occur in the environment of steam, halides and hydrogen sulphides. Austenitic stainless steels are likely to develop SCC in the presence of chlorides and hydroxides.
Conditions for Stress corrosion cracking
Stress corrosion cracking can only occur if certain conditions are met:
- The material must be prone to stress corrosion cracking
- Temperature or certain external agents must act on the material
- There must be a tensile stress
In the case of steel, it is mainly chlorine that can cause a corresponding weakening, in rarer cases this also happens through hydrogen ions (hydrogen embrittlement).
Chromium-nickel-molybdenum steels are best protected against stress corrosion cracking, the greatest risk exists with ferritic chromium steels and austenitic steels. The frequently used lean duplex steels are only prone to stress cracking from temperatures above 50 °C.
Types of Stress Corrosion Cracking
Even though SCC is a brittle kind of failure, the cracking hardly ever propagates rapidly enough to cause a catastrophic failure, even in the case of vessels which operate under pressure.
However, the corrosion can penetrate a metal section in a matter of hours, if the section is under stress and the metal is markedly sensitive to the environment. The resulting leakage of the chemical may mean shutting down of the plant.
Metallographic examination shows that SCC propagates either through the grain boundaries (intergranular) or across the grains (intragranular or transgranular). It is very rare to find both kinds of propagation existing concurrently.
However, in both the cases, the main cracks have numerous secondary branches stemming from them.
There are two kinds of SCC:
- Intergranular and
- intragranular, as viewed metallographically.
In which areas Stress Corrosion Cracking occurs?
In austenitic stainless steels, SCC can occur in the unwelded parent material due to residual stresses from cold working or service stresses.
In the welded products where welding may add to the stresses, SCC can occur both in the parent plate and weld-metal. The most significant environment causing SCC is aqueous solutions of chlorides at elevated temperatures.
In practice, it is rare for failures to occur below 70°C. The Cl-concentration required varies with temperature, being lower as it rises. In many cases, SCC is caused by the concentration of originally small amounts of Cl in a solution, for example by evaporation heated steel surface.
on a Stainless steel weld-metals having mixed austenite/ferrite microstructure show a considerably greater resistance to SCC than the parent plate.
Hot caustic solutions may also produce transgranular SCC in these materials.
Intergranular SCC can also occur in the HAZs of unstabilized or high carbon welded stainless steels, where intercrystalline corrosion or weld decay has already set in during service.
In the petrochemical equipment, SCC of this type may occur due to the presence of polythionic acids, which are produced particularly during shutdowns, by an interaction between water, Sulphide containing scales and air.
This type of SCC is also possible in the high temperature, pure water environment of boiling water nuclear reactors.
In both these cases, the preventive measure consists of avoiding the sensitization of the stainless steel by appropriate material selection or welding procedure control. Another effective measure is of course control of the environment.
How to reduce susceptibility to SCC?
Generally speaking, susceptibility to SCC can be minimised by ensuring minimum residual stresses in the welded structure.
Since stress-relief treatment is not practicable with stainless steels, one must avoid severe cold working and adopt forming and welding procedures in such a way, that internal stresses are minimum and are evenly distributed over the whole structure.
Three different approaches may be used to avoid chloride or caustic induced SCC in stainless steels:
1) Ensure better control on environmental conditions. Consult the pamphlet issued by the Institution of Chemical Engineers, 1978 (UK) entitled “Guide notes on the safe use of stainless steel in chemical process plant”, which gives useful guidance on safe procedures for the hydrostatic testing of austenitic stainless steels, and on precautions to be taken when lagging them externally. Lagging materials sometimes contain chlorides or encourage their concentration at the metal surface.
2) When severe environmental conditions are unavoidable, use a more resistant alloy, like Incoloy or Inconel or a low Ni SSC-resistant duplex ferritic austenitic alloy or a low interstitial fully ferritic one.
3) Use a stress-relief heat treatment. As already mentioned earlier this is not easy. The minimum effective temperature is 900° C, and this introduces complications with distortion and reintroduces significant residual stresses on cooling.
Even when a perfect stress-relief has been achieved, the service stresses themselves may be adequate to promote SCC.