{"id":22558,"date":"2024-01-25T13:15:03","date_gmt":"2024-01-25T11:15:03","guid":{"rendered":"https:\/\/fractory.com\/?p=22558"},"modified":"2024-09-30T19:03:16","modified_gmt":"2024-09-30T16:03:16","slug":"types-of-corrosion","status":"publish","type":"post","link":"https:\/\/fractory.com\/types-of-corrosion\/","title":{"rendered":"What Is Corrosion & the 10 Most Common Types"},"content":{"rendered":"

Corrosion is a destructive phenomenon wherein the surface of the metal deteriorates from chemical or electrochemical reactions. Its pervasive nature impacts virtually all industries – infrastructure, electronics and the automotive industry, to name a few.<\/p>\n

Let’s delve into the nature of corrosion, the types of corrosion and how to combat their effects.<\/p>\n

What Is Metal Corrosion?<\/h2>\n

Metal corrosion occurs when the metal surface reacts with a corrosive environment or is subjected to other unfavourable conditions that cause the surface to corrode. Oxidation or rust formation occurs on the exposed surface of the metal and causes the material to weaken over time, leading to structural damage. Corrosion is not only specific to metals, it can occur in other materials, such as polymers and ceramics, for example. However, for these materials, the term degradation is more common.<\/p>\n

Corrosion is a natural process that converts a refined metal to a more stable metal oxide. An electron exchange occurs between the metal and the environment, with the metal losing electrons in the process. Metals corrode naturally over time, but the type of environment that the material is subjected to can accelerate the corrosion process. Stable metals, such as gold and platinum, are less likely to corrode due to their stable chemical nature. Alloys that contain certain elements, such as iron, are more prone to rusting when exposed to air and moisture.<\/p>\n

Localised corrosion forms cracks and pits, however, corrosion may span over the entire surface of a metal. Corrosion in metal surfaces is influenced by a variety of factors, such as acidity levels in the environment, temperature, mechanical stress, and humidity.<\/strong> These factors may either induce accelerated deterioration of the metal surface or rapidly speed up the corrosion process, ultimately leading to failure.<\/p>\n

Protective coatings, galvanisation<\/a>, and heat treatment<\/a> are general methods for reducing the risk of corrosion. However, corrosion may still occur under specific conditions due to its complex nature.<\/p>\n

Types of Corrosion<\/h2>\n

There are several types of corrosion that are all influenced by a variety of mechanisms and conditions. Let’s explore each type and understand how they occur.<\/p>\n

General Corrosion<\/h3>\n

General or uniform corrosion is the most common type of corrosion<\/strong> as it occurs across the surface of a metal. This is commonly caused by the absence of a protective coating, leaving the metal exposed to corrosive agents. Electrochemical and chemical reactions occur which cause the metal to dissolve, allowing it to thin out as it forms oxides.<\/p>\n

Continuous exposure to these corroding agents will eventually dissolve the whole metal structure. Examples of metals affected by uniform corrosion are aluminium<\/a>, iron, lead, steel, and zinc. This type of corrosion is predictable and detectable to the naked eye as it is visible in the form of rust over the entire surface.<\/p>\n

Pitting Corrosion<\/h3>\n

\"Pitting<\/p>\n

Pitting corrosion is an unpredictable type of localised corrosion wherein rust pits form at the metal surface. These pits grow into cavities or holes, penetrating the surface in a downward direction. This type of corrosion is caused by structural defects, poor coating application, non-uniformities, moisture, or damage to the protective oxide layer of the metal. Early stages of pitting corrosion have pit diameters of \u226420\u2009\u00b5m.<\/p>\n

Pitting corrosion is an insidious type of corrosion since only a small amount of material on the surface is lost while the deep metal structure is damaged. Failure due to pitting can be sudden and is often devastating. Pitting can be observed in aluminium, nickel alloys, and steel. Polished metal surfaces are more resistant to pitting due to their more uniform surface area.<\/p>\n

Crevice Corrosion<\/h3>\n

Crevice corrosion is another type of local corrosion that occurs at the crevices or confined areas, usually between two metals (commonly found in assemblies), a metal and nonmetal, or from debris. These restricted areas usually allow a buildup of corrosive fluids while only having a limited supply of oxygen which prohibits shielding of the material. The imbalance in the pH level turns the fluid within the crevices acidic, which in turn breaks down the passive oxide layer, leaving the surface vulnerable to corrosion attack.<\/p>\n

The risk for crevice corrosion can be reduced through a better assembly or a joint design where gaps are eliminated between welds and joints<\/a>. Protection from moisture or fluids which promote an electrolytic environment may further reduce the risk of this form of corrosion.<\/p>\n

Galvanic Corrosion<\/h3>\n

\"Galvanic<\/p>\n

Galvanic corrosion or bimetallic corrosion is a type of corrosion wherein two dissimilar metals in physical and\/or electrical contact are subjected to an electrolytic environment. The active metal (anode) undergoes corrosion at a faster rate than the other metal (cathode) which is more stable. It can also occur in a metal that is exposed to an electrolyte with varying degrees of concentration.<\/p>\n

Galvanic corrosion is common in sea vessels where saltwater acts as an electrolyte between the two dissimilar metals. It can also occur in toxic environments, such as in handling molten metal or in chemical laboratories.<\/p>\n

A sacrificial anode<\/a> or galvanic anode that is more active than the metal workpiece is used to prevent\/lessen galvanic corrosion. These consumable anodes prevent the main metals from being oxidised by supplying their electrons, corroding the consumable anode instead.<\/p>\n

In the case of copper, when connected to a more anodic metal in the presence of an electrolyte, copper corrosion<\/a> can be significantly reduced, as copper acts as the cathode in the galvanic series.<\/p>\n

Fretting Corrosion<\/h3>\n

Fretting corrosion<\/a> occurs when two metals in contact undergo small movements caused by slips and vibrations. These oscillations lead to fretting that removes the protective oxide film and allows surface asperities on freshly exposed metals to stick to one another. This connection is subsequently broken again by the vibrations, causing the build-up of wear debris.<\/p>\n

This debris and the freshly exposed metal surfaces are prone to oxidation. Since the particles cannot escape the contact, they initiate further abrasive wear and subsequent oxidation, and the process continues with elevated wear volumes.<\/p>\n

Reactive soft metals have lower corrosion resistance since the oxide layer is easy to remove under cyclic loading and material transfer takes place.<\/p>\n

Intergranular Corrosion<\/h3>\n

Intergranular corrosion is a form of corrosion that occurs at or adjacent to the metal’s grain boundaries. Grain boundaries act as an interface between grains in the material and are considered imperfections in the material’s crystal structure. These grain boundaries result from uneven growth or impurities present as the metal alloy crystallises. Each grain can vary from 1 \u00b5m to 1 mm in size.<\/p>\n

Intergranular corrosion or intergranular attack exhibits corrosion when the grain boundaries have a difference in reactivity to the grain. It can occur when metals are subjected to high temperatures (carbide precipitation) or due to defective welding<\/a>. This lowers the material’s corrosion resistance, making it prone to damage when exposed to a corrosive agent.<\/p>\n

\n
\n Scale Your Manufacturing from Prototyping to Series<\/span>\n\n