WHAT IS METAL 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.

TYPES OF METAL CORROSION

PITTING CORROSION

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:

1. Presence of corrosive agents: Certain chemicals, such as chloride ions, can significantly accelerate pitting corrosion. Other aggressive substances, such as acids or saltwater, can also initiate or promote pitting.
2. Lack of protective barriers: Protective oxide layers or coatings on the metal surface can prevent pitting corrosion. However, if these barriers are damaged or compromised, the metal becomes more vulnerable to pitting.
3. Variations in the local environment: Differences in oxygen concentration, pH levels, or temperature across the metal surface can create favourable conditions for pitting corrosion.

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

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:

 

1. Presence of a crevice: The formation of a crevice, such as gaps, joints, or interfaces, provides a confined space for the accumulation of corrosive agents and restricts the flow of solutions, leading to the establishment of localised corrosion cells.
   
2. Differential aeration or concentration cells: Variations in oxygen concentration, pH levels, or ion concentration within the crevice can create differential aeration or concentration cells, accelerating the crevice corrosion process.
   
3. Corrosive environment: The presence of corrosive agents, such as chloride ions or acidic solutions, can significantly enhance the severity of crevice corrosion.

 

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

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:

 

1. Sensitization: Sensitization occurs in certain stainless steels and other alloys that contain chromium and carbon. When these materials are heated to temperatures within a specific range (usually between 500°C and 900°C), chromium carbides can precipitate along the grain boundaries. This depletes the adjacent regions of chromium, reducing their corrosion resistance and making them susceptible to intergranular attack when exposed to a corrosive environment.
2. Grain boundary segregation: Some metals and alloys can exhibit segregation of impurities or alloying elements along the grain boundaries during solidification or subsequent heat treatment. This segregation can create galvanic cells between the grain boundaries and the grains, leading to intergranular corrosion. In such cases, the grain boundaries act as anodic sites, while the grains act as cathodic sites.

 

Preventing intergranular corrosion involves various strategies, including:

 

1. Selection of appropriate materials: Choosing alloys that are less prone to intergranular corrosion in the intended environment is essential. For example, the use of low-carbon stainless steels or stabilised grades can minimise sensitization and intergranular corrosion susceptibility.
2. Heat treatment: Proper heat treatment processes can help eliminate sensitization and reduce segregation of alloying elements, enhancing the resistance to intergranular corrosion.
3. Corrosion-resistant coatings: Applying protective coatings, such as paints or corrosion inhibitors, can provide a barrier between the metal surface and the corrosive environment, preventing intergranular attack.
4. Corrosion testing: Conducting corrosion tests, such as the ASTM A262 practice, can help assess the susceptibility of metals to intergranular corrosion and ensure their suitability for specific applications.

WHAT IS THE BEST WAY TO PREVENT CORROSION?

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.

Visit our Metal Powders page to discover a huge range of available powders, or contact our expert team directly to discuss your supply needs today.