June 16th 2023
Corrosion can be a persistent problem in a variety of commonly used metals. If not treated properly, corrosion can degrade metals to the point that they become unusable, unfit for purpose, or even create a dangerous health hazard when structural materials such as iron or steel become corroded - particularly when these metals are used to create buildings, bridges, or oil pipes.
Understanding how to prevent metal corrosion and rust is vital to ensuring the longevity of materials such as iron, steel, aluminium, and titanium. In this blog post, William Rowland discusses the best practice for protecting your metals from corrosion.
Metal corrosion occurs when manufactured metals come into contact with oxidising substances, usually oxygen, causing atoms within the metal to lose electrons to the oxidising substance. As more atoms lose their electrons during this process, corrosion can be seen to spread across the surface of the metal, eventually causing the integrity, strength, and appearance of the metal to degrade over time.
Pitting corrosion occurs on the surface of manufactured metals, and is characterised by the formation of small pits or cavities which can penetrate deep into the metal, causing significant damage over time.
Pitting corrosion typically occurs when a metal is exposed to an aggressive environment, such as a corrosive liquid or a moist atmosphere. The corrosion process starts with the formation of small electrochemical cells on the metal surface, where certain areas become anodic (corroding) and others cathodic (protected).
The anodic areas become susceptible to dissolution, leading to the formation of small pits. These pits act as localised corrosion sites, where the metal is attacked more aggressively than the surrounding areas. Pitting corrosion can occur in various metals, including steel, stainless steel, aluminium, and copper alloys.
The factors that contribute to pitting corrosion include:
Pitting corrosion is particularly concerning because the pits can penetrate deeply into the metal, leading to localised loss of material and weakening of the structure. In some cases, the pits can grow and coalesce, forming larger craters or perforations.
Preventing pitting corrosion involves the use of appropriate corrosion-resistant materials, protective coatings, and regular inspections to identify and address potential corrosion sites. Additionally, controlling the exposure to corrosive environments and maintaining proper maintenance practices can help minimise the risk of pitting corrosion.
Crevice corrosion occurs within crevices or gaps between two surfaces in contact with each other. It is similar to pitting corrosion but is specifically associated with confined spaces or crevices.
Crevice corrosion typically occurs in areas where there is a stagnant or restricted flow of a corrosive environment, such as gaps, joints, seams, or under deposits on metal surfaces. Examples of crevices include gaps between metal components, threaded connections, under gaskets, or behind surface deposits like scale or biofilms.
The mechanism of crevice corrosion is similar to that of pitting corrosion. Within the crevice, differences in the local environment, such as oxygen concentration, pH levels, or ion concentration, can create electrochemical cells. These cells establish anodic (corroding) and cathodic (protected) regions, leading to the corrosion process.
The confined nature of the crevice restricts the diffusion of oxygen and ions, resulting in a more aggressive corrosion environment within the crevice compared to the surrounding area. This localised attack can lead to the formation of corrosion products, accumulation of debris, and the degradation of the metal surface.
The factors that contribute to crevice corrosion include:
Crevice corrosion can be particularly problematic because it often remains undetected until significant damage has occurred. The corrosion products and debris accumulated within the crevice can hinder visual inspection, making it difficult to identify the corrosion process.
To prevent crevice corrosion, design considerations should be taken into account to minimise the formation of crevices and ensure proper drainage and ventilation. The use of corrosion-resistant materials, coatings, or sealants can provide protection in crevice-prone areas. Regular inspection and maintenance to identify and address crevices or deposits are also crucial in preventing or mitigating crevice corrosion.
Intergranular corrosion (IGC) is a type of corrosion that occurs along the grain boundaries of a metal. It is characterised by the preferential attack and degradation of the grain boundaries while the grains themselves remain relatively unaffected.
IGC typically occurs in metals, particularly alloys, where the grain boundaries have different chemical compositions or microstructures compared to the grains. These differences can arise due to variations in impurity content, segregation of alloying elements, or changes in the crystal structure near the grain boundaries.
The presence of an aggressive environment, such as a corrosive solution or elevated temperatures, can initiate intergranular corrosion. The attack is often localised and progresses along the grain boundaries, resulting in the loss of material, reduced mechanical strength, and potential structural failure.
The exact mechanisms of intergranular corrosion depend on the specific metal and the corrosive environment. However, two common modes of intergranular corrosion are:
Preventing intergranular corrosion involves various strategies, including:
Metal powder coatings provide one of the best ways to prevent galvanic corrosion and rust on a variety of metals. Coatings such as zinc powder are heated to form a protective film across the surface of manufactured metals. This type of coating is highly resistant to corrosion from environmental factors such as oxidation and moisture in the air, with other metal powders such as Tungsten providing high resistance against attacks from acid and alkalis.
William Rowland are key suppliers of metal powders in the UK, providing high quality coatings and materials to industries such as aerospace, defence, automotive, and more.
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