Stress Corrosion Cracking
Anchor subject to stress corrosion cracking
The stress corrosion cracking is a type of localized corrosion, which develops with the formation of a crack due to the combined action of a mechanical stress and a corrosive environment to mild corrosive action. The combined action is a necessary condition for being able to define this type of phenomenon. This form of corrosion affects a wide range of metallic materials such as steel, titanium, copper alloys, etc … under the action of various corrosive environments. You can not establish general rules that can predict the stress corrosion cracking due to the variety of conditions in which it was observed this phenomenon. Sometimes, a slight variation of the system parameters can vary completely corrosion attack morphology. For these reasons it is not possible to talk of free materials to stress corrosion cracking or specific environments that promote this form of corrosion; you can only talk in terms of probabilities and formulate some general rules:
- The stress corrosion cracking occurs in the presence of an elastic stress in the metal traction;
- The speed of corrosion increases as the tensile strength;
- It occurs preferentially on alloy metals or weakly bound;
- The conditions which affect the corrosion are strictly specific for the metal and the environment;
- The presence of some specific elements may cause stress corrosion cracking or immunity;
- Special heat treatments may increase the risk of stress corrosion cracking;
- Development times of stress corrosion cracking are lower compared to a break for metal fatigue.
The stress corrosion cracking is manifested by the formation of cracks oriented perpendicular to the direction of the applied stress. Generally these cracks are transgranular type. The cracks propagate into the interior of the metal by following one or more main directions, with small ramifications.
Stress corrosion cracking on AISI 316
Crack proceeds quickly when the micro-cracks occur within the material and they are detectable by means of special techniques. The crack propagates rapidly but with a discontinuous trend and can stop when the mechanical tension is stopped. The mechanical tension tension ends or when the external load is removed or when the energy of the crack is absorbed by the metal (toughness). The resistance to stress corrosion cracking is very dependent on metal susceptibility to ignition processes: if the corrosive environment is eliminated before cracking, there will be no stress corrosion cracking, or else if it will be removed when the process is started, then the crack will propagate within the metal. The trigger of the crack is very similar to that of the pitting corrosion. The crack can starts both directly on the surface (the presence of inclusions and notches), and as a result of a localized corrosion. Compared to pitting, stress corrosion cracking is accentuated by the mechanical tension which induces the movement of dislocations (defects in metal) or by the presence of emerging phases on the surface that are inconsistent with the metal matrix. The crack propagation in time is caused primarily by a slow penetration stages, which lead to the dissolution of the metal near the apex of the crack, and a rapid penetration stages as brittle fracture processes. The dissolution of the material is effected when:
- The metal passive layer is removed (trans-passivation);
- The moment the metal form the passive layer (passivation);
- The metal is subjected to a deep corrosive activity (activities).
Instead of brittle fracture processes are manifested when there is the presence of:
- Alloying elements;
- Lattice deformation;
- Segregations at grain boundaries;
- Other crystalline defects within the metal lattice.
The most common causes can be due to:
- Internal stress caused by crystals plastically deformed or uneven crystal structures
- External stress caused by:
- Assembly difficulty of mechanical or structural parts connected to each other;
- Settling of mechanical parts;
- Temperature changes;
- Temperature gradients;
- Welding Process.
The most commonly methods used as preventions are:
- Avoid conditions that can create states of external stress, by controlling the assembly process and/or welding process;
- Create surface compression zones;
- Performing relieving heat treatments;
- Resorting to cathodic protection in such a way as to inhibit the trigger and the slow penetration stage;
- Coatings with pure metals that are less susceptible to stress corrosion cracking;
- Anodic coatings with pigments (eg. Galvanizing);
- Changing the corrosive environment in order to block the stress corrosion cracking or to transform it into a more bland corrosive phenomenon as the general corrosion.