In our previous posts in our series on the forms of corrosion, we introduced uniform, galvanic, pitting, and crevice corrosion. In this post, we explore four additional forms of corrosion: intergranular corrosion, stress corrosion cracking, selective leaching, and erosion corrosion.
Intergranular Corrosion
Intergranular corrosion (IGC) is a form of localized corrosion that proceeds along grain boundaries or immediately adjacent to grain boundaries of polycrystalline metals and alloys. Intergranular corrosion is caused by the anodic dissolution of the grain boundaries that are depleted of alloying elements, regions with elemental segregation, or second phase precipitates. Such changes in the chemical composition alongside the grain boundaries can produce zones of reduced corrosion resistance in the immediate vicinity. The rest of the exposed metal surface (i.e., regions inside the grains) remains intact and acts as the cathode. Large cathodic to anodic areas accelerate the dissolution rate at the grain boundaries.
Intergranular Corrosion takes place in pure metals as well as alloys. The term “sensitization” is commonly used to describe microstructure sensitivities to intergranular attacks.
An important example of intergranular corrosion is the sensitization of stainless steel. During improper heat treatment of most grades of stainless steel, chromium-rich secondary phases precipitate at the grain boundaries lead to a local depletion of Cr adjacent to the precipitates, which makes these regions vulnerable to corrosive attack. Such microstructural changes, which are commonly observed during the welding process, can considerably decay the overall corrosion resistance of stainless steel. Besides stainless steel, many aluminum alloys are susceptible to intergranular corrosion. Material selection and proper heat treatment are effective measures to mitigate intergranular corrosion.
Stress Corrosion Cracking (SCC)
Stress corrosion cracking is an important form of mechanically assisted corrosion failure, which is a result of the cracking propagated from the combined and synergistic interaction of tensile stress and a corrosive environment. The tensile stress required to cause SCC is small and maybe in the form of directly applied stress or in the form of residual stress. Although stress concentration occurs at the corrosion-induced flaws (such as a corrosion pit), it does not exceed the critical value required to cause mechanical fracture of the alloy in an inert environment, while simultaneous exposure to corrosive medium and application of tensile stress will cause time-dependent crack propagation. Stainless steel, Ti alloys, Al alloys, and brass are examples of engineering alloys vulnerable to SCC.
SCC can occur in the form of intergranular or transgranular attack, depending upon the alloy/environment condition, and in general, has a brittle fracture characteristic. SCC is classified as a catastrophic form of corrosion because the detection of fine cracks can be very difficult and the damage not easily predicted. Further, a disastrous failure can happen suddenly, with minimal overall material loss.
Transgranular SCC of pre-stressed 316L stainless steel exposed in concentrated sodium hydroxide.
1985, Uster, Switzerland. 12 people were killed when the concrete roof of a swimming pool collapsed. The roof was supported by stainless steel rebar which failed due to stress corrosion cracking.
Selective Leaching (De-alloying)
Selective leaching or dealloying occurs when one element or constituent of an alloy is selectively dissolved (i.e., leaches) due to corrosion processes. The driving force for selective leaching is the difference between corrosion tendencies of alloying elements. Once a vulnerable alloy is exposed to a corrosive environment, the less noble element dissolves preferentially. The product of this dissolution is a porous structure consisting mostly of more noble elements. Dezincification of brass (Cu-Zn alloy) is a common example of dealloying, where Zn removes from the alloy, leaving a porous Cu structure with considerably impaired mechanical properties. Graphitization in the grey cast iron due to slow dissolution of the ferrite matrix is another example of selective leaching.
Dealloying is an ancient technology and was used by several civilizations to cover the surfaces of artifacts that were made of copper and/or silver with gold. Today, dealloying is a key corrosion problem in industrial alloys for its role in stress corrosion cracking.
Erosion-corrosion
Flowing media are encountered in many technical systems such as heat exchangers, and steam generators and can enhance both uniform and localized corrosion, which is known as flow-assisted corrosion. Erosion-corrosion and cavitation-corrosion are the main types of flow-assisted corrosion. Erosion-corrosion is an acceleration in the rate of corrosion attack in metal due to the flowing of corrosive fluid. Although almost all metal alloys are susceptible to erosion-corrosion, it is particularly detrimental to alloys that passivate by forming a protective surface film because the abrasive action wears away the passive film leaving exposed a bare metal surface. Typically, erosion-corrosion can be identified by surface grooves and waves having contours that are characteristic of the flow of the fluid. Materials selection, design features, and reduce the fluid velocity and promote laminar flow are general mitigation routes to minimizing erosion-corrosion damage. Filtration of abrasive particles in the fluid and corrosion inhibitors are additional measures that can be taken against erosion-corrosion attacks.
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