The most common example of this phenomenon is the selective removal of the less noble zinc element in brass alloys, also known as dezincification. This discriminatory removal of a specific element is known as selective leaching or dealloying. In specific corrosive environments, some metal alloys can experience a type of corrosion where only one element of the alloy is deteriorated and removed by corrosion. (See The Role of Chromium in Intergranular Corrosion for more on this topic.) The grain boundary is then unlikely to form an effective passive film and is now susceptible to corrosion attack. At these temperatures, chromium carbide can become precipitated at the grain boundaries, therefore reducing the local chromium concentration at the boundaries. The grain boundaries now represent a path of high corrosion vulnerability.įor example, austenitic stainless steels can be susceptible to intergranular attack if they are heated in the 500☌ to 800☌ (930☏ to 1470☏) range. Some alloys, when subjected to improper heat treatment, can have impurities segregated at the grain boundaries that can make passivation at these areas difficult.
COMMON CORROSION CROSSWORD CLEW FREE
Intergranular corrosion involves accelerated corrosion along the grain boundaries of a metal, while the bulk of the metal surface remains free from attack. Because the corrosion is confined to a localized area, pitting tends to penetrate the thickness of the material. In the presence of moisture, the anode and cathode form a corrosion cell where the anode (i.e., the areas unprotected by the passive film) corrodes. Areas where passivity has been reduced or lost now become the anode while the surrounding regions act as the cathode. These inconsistencies may be due to film damage, poor coating application or foreign deposits on the metal surface. Pitting usually originates on areas of the metal surface where inconsistencies in the protective passive film exist. This form of corrosion is also highly penetrative and is considered to be one of the most dangerous types of corrosion because it is difficult to predict and has a tendency to cause sudden and extreme failures. Pitting typically manifests itself as small diameter cavities or holes on the object's surface while the remainder of the metallic surface remains unattacked. Pitting corrosion, also known as pitting, is another localized form of corrosion that occurs on metal surfaces. The acidic liquid in the crevice breaks down the metal’s passive layer and renders it vulnerable to corrosion attack.
COMMON CORROSION CROSSWORD CLEW SERIES
This leads to a series of electrochemical reactions that alters the composition of the fluid and makes it acidic in nature. This lack of fluid movement gives rise to the depletion of dissolved oxygen and an abundance of positive ions in the crevice. One of the main criteria for the development of crevice corrosion is the presence of stagnant water within the crevice. This type of corrosion is characterized by deterioration in the area of the crevice while the surrounding areas of the metal substrate remain unaffected. These crevices can be the result of a connection between two surfaces (metal to metal or metal to non-metal), or by an accumulation of deposits (dirt, mud, biofouling, etc.).
(This topic is explored in An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion.) Crevice CorrosionĬrevice corrosion is a highly penetrative type of localized corrosion that occurs in or directly adjacent to gaps or crevices on the surface of a metal. Therefore, the further apart the metals are in the galvanic series, the higher the rate of corrosion at the anode. This type of corrosion is affected by the magnitude of the potential difference between the two metals. The movement of electrons from the anode to the cathode initiates an oxidation reaction at the anode that causes it to be dissolved, i.e., corrode. When exposed to an electrolyte, the two metals form a type of cell known as a bimetallic couple, where one metal acts as the anode and the other as the cathode. Bimetallic corrosion is a purely electrochemical reaction driven by the difference in electrode potentials between the two metals.
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