![]() 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. 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. This is why the selection of the right material is vital for cost, availability and most importantly – safety.Bimetallic corrosion is a purely electrochemical reaction driven by the difference in electrode potentials between the two metals. It is very important to note that each corrosion resistant metal has its own properties and composition that will cope with some types of corrosion better than another. An investment of a high-performance alloy will save much of this hassle. Some prevention methods that need consistent updating and maintenance can be a hassle – both economically and practically. However, generally, the selection of the correct corrosion resistant metal to begin with can prevent needing further treatments and prevention methods. Each type of corrosion will differ in the prevention method. There are various ways to prevent the effects of corrosion. “Aqueous corrosion is an electrochemical reaction of materials due to a wet environment, resulting in the deterioration of the material and its vital properties.” – What Is Aqueous Corrosion by Corrosion Prevention ![]() Attack of an alloys tensile strengths at high temperatures is a common situation for stress-corrosion cracking. Unexpected sudden failure can occur with this type of corrosion. It is a type of localised corrosion, which means that the attack is accelerated due to exposure of an area on the metal that an otherwise protective coating or layer has broken down. Spots and pits will appear on the metal with this type of corrosion. ![]() Rust will appear in the presence of uniform corrosion. Irons and steels will corrode in ‘uniform’ when exposed to certain environments such as soils and natural waters. This type of corrosion will occur whenĪ) When metals are electrochemically dissimilar Galvanic corrosion will occur when two separate metals/alloys are in contact with each other. For example, within the marine and offshore industries, engineers will require metals that can withstand exposure to seawater. This is a huge concern for sectors of industry with environmental considerations. Metals that are attacked by corrosive substances are open to damage that can effect the durability and safety of the alloy.
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