What is Corrosion? – Definition and Prevention

Electrochemical processes have the capacity to either decompose compounds into their basic elements or synthesize entirely new substances. While these reactions are often harnessed for practical and beneficial applications, they can just as easily lead to unwanted outcomes.

A familiar example is corrosion, which arises from electrochemical interactions between materials and their surrounding environment. This phenomenon, unfortunately, is a routine challenge in many settings.

What is Corrosion?

Corrosion refers to the gradual transformation of pure metals into less desirable compounds as a result of reactions with substances like water or air. This process typically begins at areas of the metal exposed to the environment and, over time, can extend throughout the entire material, ultimately compromising its integrity.

In most cases, corrosion is regarded as problematic because it undermines the valuable properties of metals. Take iron, for instance, its notable tensile strength and rigidity, especially when alloyed, make it highly useful in various applications. Yet, once iron is exposed to conditions that promote rusting, it loses these advantages. The material can become brittle, flaky, and lose its structural reliability.

It’s important to recognize that corrosion is primarily a surface phenomenon, governed by diffusion. For this reason, efforts often focus on decreasing the reactivity of exposed metal surfaces and improving their resistance to corrosion.

Techniques such as passivation and chromate conversion are commonly applied for this purpose. Still, not every corrosion mechanism is obvious at first glance, and some can be quite unpredictable in how they develop.

Additionally, corrosion is often categorized as an electrochemical process, since it involves redox reactions between metals and agents like water, oxygen, or sulfur dioxide. Understanding this aspect provides insight into both the mechanisms at play and the approaches that might help mitigate corrosion’s effects.

How & Why Does Corrosion Occur?

Corrosion is a process that takes place when atoms across the entire surface of a metal interact with their surroundings and become oxidized. As a result, metals that have a higher tendency to oxidize will generally corrode more quickly than those that are more resistant—this widespread deterioration is often referred to as general corrosion.

In contrast, there is also localized corrosion, which is limited to a specific area on the metal’s surface. This type can be particularly concerning, as it may eventually lead to the formation of tiny holes or “pits” in the material.

There are other forms to consider as well. For instance, galvanic corrosion arises when two dissimilar metals come into contact in the presence of an electrolyte, such as a liquid solution. In these conditions, it is usually just one of the metals that undergoes corrosion, as the movement of molecules from one metal towards the other initiates the process.

Ultimately, the likelihood and speed of corrosion are influenced by a range of environmental factors the types of gases present, surrounding elements, and even other nearby metals. Given these influences, it is possible to take practical steps to slow down, or in some cases, prevent corrosion from occurring altogether.

Chemistry of corrosion

Corrosion, from a chemical perspective, involves a surprisingly intricate set of reactions and is best described as an electrochemical process. If we look closely at a corroding iron surface, we find that oxidation takes place at a specific site—this area essentially acts as the anode.

What happens next is interesting: the electrons generated at this anodic region don’t just stay put. Instead, they travel through the metal itself, eventually reaching another part of the surface. At this second site, oxygen is reduced, but this reaction relies on the presence of hydrogen ions.

These ions usually come from carbonic acid, which forms when carbon dioxide dissolves in water, a common occurrence when moisture is present in the air.

Of course, there are other possible sources of hydrogen ions as well. Acidic oxides carried in the atmosphere can also dissolve in water, contributing additional hydrogen ions. The area where oxygen reduction occurs effectively serves as the cathode in this electrochemical setup.

Causes of Corrosion

Metals are susceptible to corrosion when they interact with various substances, including oxygen, hydrogen, electrical currents, and sometimes even contaminants like soil or microorganisms. This process isn’t limited to chemical reactions alone; physical stress plays a role as well.

For example, materials such as steel may begin to crack if they are subjected to excessive pressure, which in turn can accelerate corrosion.

Below are some of the most common and widely recognized factors that contribute to corrosion:

• Corrosion often arises when metal surfaces are subjected to excessive humidity or when water vapor condenses upon them.
• Exposure to corrosive gases, including chlorine, hydrogen oxides, ammonia, and sulfur oxides, can lead to the deterioration of electronic components and similar parts. In addition, contact with hydrogen and oxygen is another pathway through which corrosion may develop.
• Steel is vulnerable to corrosion if it is placed under significant stress, especially when cracks begin to form within the material.
• Metals that remain in contact with electrical currents over extended periods can undergo what is known as electronic corrosion.
• Finally, when metals come into contact with dirt and bacteria, this too can promote the onset of corrosion.

Corrosion of Iron

Iron corrosion refers to the gradual degradation of materials containing iron (Fe), resulting from chemical interactions with oxygen (O₂) and water present in the environment. This process leads to the formation of a reddish iron oxide layer, commonly recognized as rust. Notably, rusting is not limited to pure iron; iron alloys like steel are also susceptible.

Interestingly, iron can undergo corrosion through different pathways. For instance, in environments where oxygen is scarce, iron may react with chloride ions, resulting in corrosion even without direct oxygen involvement. There is also what’s known as “green rust,” a less familiar form, which can emerge either from metallic iron itself or from iron hydroxide.

The hallmark of iron corrosion is typically the appearance of rust, which arises from an electrochemical process. This process requires water, oxygen, and often a salt solution acting as an electrolyte. It’s for this reason that addressing one or more of these contributing factors, water, oxygen, or salts, can significantly slow down the corrosion rate in iron-containing materials.

On a chemical level, the initial reaction of iron with water and oxygen produces iron(II) hydroxide. This compound then continues to interact with additional water and oxygen, ultimately yielding hydrated iron(III) oxide, which is what most people recognize as rust.

Do All Metals Corrode?

Most metals corrode. Some like iron corrode quickly, while others naturally corrode more slowly (e.g. zinc) or corrode slowly because they are an alloy of various metals (e.g. stainless steel).

A small group of metals, called the Noble Metals, are much less reactive than others. As a result, they rarely corrode. They are, in fact, the only metals that can be found in nature in their pure form. The Noble Metals, not surprisingly, are often very valuable. They include palladium, silver, platinum, and gold.

Types of Corrosion

Many different types of corrosion are visible to the naked eye:

• Uniform Corrosion
• Localized Corrosion
• Galvanic Corrosion
• Environmental Cracking
• Flow-Assisted and Intergranular Corrosion
• Fretting Corrosion
• High-Temperature Corrosion
• Soil Corrosion

These are the most common types of corrosion, let’s explain the underlying mechanism of each.

#1. Uniform Corrosion.

Uniform corrosion stands out as the most prevalent form of corrosion observed in practice. It typically arises when carbon steel undergoes a chemical or electrochemical interaction with its environment, resulting in a gradual breakdown that affects the entire exposed surface at a consistent rate.

While this type of corrosion is widespread, it is also relatively predictable. Fortunately, by selecting suitable preventive strategies, uniform corrosion can often be effectively controlled.

#2. Localized Corrosion.

When we talk about localized corrosion, we’re actually referring to a range of specific phenomena, such as pitting, crevice corrosion, and filiform corrosion. Each of these has its own unique characteristics, but all fall under the same general category.

1). Pitting corrosion.

Pitting corrosion, sometimes simply called pitting, is a particularly localized form of corrosion that affects metal surfaces. It often appears as tiny cavities or pinholes scattered across the surface, while most of the surrounding metal remains visibly unaffected.

What makes pitting corrosion especially hazardous is its unpredictable nature and the potential for it to cause abrupt, significant damage, even if most of the surface looks perfectly intact.

This type of corrosion tends to start in spots where the metal’s protective passive film is inconsistent. Such inconsistencies can come from something as simple as minor damage to the film, uneven coating during application, or even just the presence of small foreign particles resting on the metal.

Wherever the passive layer is compromised, those exposed areas effectively become the anode, while the rest of the metal takes on the role of the cathode. If moisture is present, this difference sets up a tiny electrochemical cell: the unprotected anode areas start to corrode, while the cathodic regions remain untouched. Because this process is so focused on very small zones, the resulting pits can eat deeply into the metal’s thickness, sometimes with little warning before a major failure occurs.

2). Crevice corrosion.

Crevice corrosion is a particularly aggressive form of localized corrosion, typically found in or right next to narrow gaps on a metal’s surface. These gaps, often called crevices, might appear where two materials meet, whether that’s metal to metal or metal to something else. Sometimes, even a buildup of deposits like dirt, mud, or organic material can create the kind of environment where this corrosion takes hold.

What stands out about crevice corrosion is that it attacks the metal specifically within these confined areas, while the surrounding metal often looks untouched. A critical factor here is stagnant water that gets trapped inside the crevice. Without any real movement, the oxygen within this trapped liquid quickly runs out, while positive ions start to collect.

This imbalance triggers a cascade of electrochemical changes that make the fluid inside the crevice increasingly acidic. Over time, this acidic environment strips away the metal’s protective surface layer, leaving it exposed and far more susceptible to corrosion.

3). Filiform corrosion.

Corrosion can develop beneath painted or coated surfaces when imperfections in the protective layer permit moisture to seep through. Once water gains access, it initiates corrosion under the coating itself, which gradually compromises the integrity of the underlying material.

#3. Galvanic corrosion.

Galvanic corrosion arises under quite particular circumstances. It occurs only when electrochemically different metals are in direct electrical contact and exposed to an electrolyte. In this scenario, galvanic coupling forms between the anodic and cathodic metals.

As a result, the anodic metal undergoes accelerated corrosion, while the cathodic metal tends to degrade at a much slower rate.

#4. Environmental Cracking.

Corrosion in carbon steel typically develops when certain environmental factors become unfavorable. Exposure to specific chemicals, increased stress, or fluctuations in temperature can all play a role in making the material vulnerable.

As a result, carbon steel may experience problems such as stress corrosion cracking, corrosion fatigue, liquid metal embrittlement, or even hydrogen-induced cracking.

#5. Flow-Assisted and Intergranular Corrosion.

Flow-assisted corrosion takes place when the movement of wind or water gradually wears away the protective oxide film that forms on a metal’s surface. As this layer breaks down, the underlying metal is left vulnerable, allowing corrosion to progress further into the material.

Intergranular corrosion, on the other hand, specifically targets the grain boundaries within a metal’s structure. This typically occurs because impurities, which often accumulate at these boundaries, increase their susceptibility to corrosion.

The presence of such impurities at higher concentrations along the grain boundaries makes these areas more prone to attack compared to the bulk of the metal.

#6. Fretting Corrosion.

This form of corrosion typically arises when metal surfaces are subjected to ongoing mechanical stress, such as repeated loading, vibration, or abrasion. Over time, these forces produce small pits and grooves on the metal. It is most commonly observed in moving machine components or in parts that experience continuous vibration during transport.

#7. High-temperature corrosion.

High-temperature corrosion arises through several mechanisms, including oxidation, sulfidation, and carburization, as well as from the presence of vanadium in certain fuels.

Even materials like carbon steel, which generally withstand elevated temperatures and corrosive environments, can become vulnerable when exposed to sulfates. These compounds have the potential to break down the steel’s resistance, leading to significant material degradation over time.

#8. Soil corrosion.

Soil corrosion tends to occur when carbon steel comes into contact with both moisture and oxygen present in the ground. In particular, soils that are high in moisture, have elevated electrical conductivity, greater acidity, or contain substantial amounts of dissolved salts tend to accelerate the corrosion process.

Given that carbon steel makes up about 85% of global steel production, it’s crucial to understand what makes it vulnerable. By becoming familiar with the factors that contribute to its deterioration, we put ourselves in a better position to address the significant costs and challenges that this widespread issue brings with it.

Efforts to study and manage the corrosion of carbon steel aren’t just an academic exercise—they’re essential for minimizing damage and financial loss across industries.

Effect of Corrosion

Corrosion has far-reaching consequences, including the gradual destruction of natural landmarks and historic sites, along with a heightened chance of major equipment failures. As air pollution continues to rise worldwide, the problem of corrosion has become increasingly widespread.

Estimates suggest that metallic corrosion costs the global economy more than $2 trillion each year. However, with effective protection strategies in place, experts believe that roughly a quarter to a third of these losses could be avoided.

When construction projects are not thoughtfully planned, structures may become compromised by corrosion and require early replacement. This not only wastes valuable resources but also runs counter to broader efforts toward sustainability.

Beyond the financial burden, corrosion can create serious safety hazards, result in injuries or fatalities, add hidden costs, and even harm an organization’s reputation.

Direct effects of corrosion may include:

• Damage to commercial airplanes or vehicle electronics
• Damage to hard disks and computers used to control complicated processes (e.g. power plants, petrochemical facilities or pulp and paper mills).
• Damage to server rooms and data centres.
• Damage to  museum artefacts
• Costs of repairing or replacing household equipment that fails

How to Prevent Corrosion

There are several cost-effective ways to prevent corrosion including:

1. Use non-corrosive metals, such as stainless steel or aluminum.
2. Make sure the metal surface stays clean and dry.
3. Use drying agents.
4. Use a coating or barrier product such as grease, oil, paint or carbon fiber coating.
5. Lay a layer of backfill, for example limestone, with underground piping.
6. Use a sacrificial anode to provide a cathodic protection system.

The rusting of iron can be prevented by greasing, painting, galvanizing, anodizing, or oiling the surface. These methods can be classified into the following categories:

1. Galvanization: Galvanization involves covering metal, typically iron or steel, with a thin layer of zinc. This protective zinc coating acts as a barrier against corrosion. When exposed to air, the zinc itself oxidizes, forming a stable layer that shields the underlying metal from further damage.
2. Alloying: To enhance the properties of metals, alloying is often used. This process blends a base metal with other metals or sometimes even nonmetals. For example, combining iron with chromium and nickel results in stainless steel, a material known for its resistance to rust. This unique combination ensures that stainless steel remains durable and untarnished even in challenging conditions.
3. Painting: One of the simplest ways to prevent iron from rusting is by painting its surface. The paint forms a barrier that keeps both air and moisture away from the metal underneath, effectively stopping the rusting process before it can even begin.
4. Greasing/Oiling: Applying grease or oil to the surface of iron objects is another practical method to ward off rust. By coating the metal, grease and oil block air and moisture, making it much harder for rust to take hold. This approach is especially common for tools and machinery that need to remain in good working order.

Natural protection

Certain metals naturally develop a passive, corrosion-resistant layer. This phenomenon happens when the metal comes into contact with oxygen in the air, leading to the formation of a thin oxide layer on the surface. That oxide film essentially acts as a shield, limiting any further chemical reaction or deterioration of the metal underneath.

You can see this process at work in the patina that appears on copper or in how some outdoor sculptures weather over time. However, this protective barrier isn’t invincible. If the film is damaged say, by the mechanical stress on a bridge, or simply from a scratch it can fail to protect the metal.

Sometimes, the material is able to “heal” itself and reform the protective layer, a process known as repassivation. But if this doesn’t occur, only the exposed sections are left vulnerable, and corrosion can become even more aggressive, since it’s focused in those specific areas.

Preventing corrosion takes several forms. For example, certain metals can be made to develop their own passive films through the use of electrical currents, even if they wouldn’t do so naturally.

Material scientists have also found that metals behave differently depending on their environment, leading to innovations like stainless steel, an alloy created to withstand specific conditions.

Another approach involves altering the metal’s structure itself; for instance, laser treatments can create a non-crystalline surface that’s less prone to corrosion.

There are also methods that rely on coatings. Galvanization, for instance, involves covering iron or steel with a more reactive metal like zinc. In this setup, the zinc sacrifices itself to protect the underlying metal, corroding first and sparing the iron or steel.

Alternatively, metals can be coated with a thin layer of a more inert or passivating metal through electroplating. Even non-metallic options, such as paints, oils, or various plastics, are commonly used to form a physical barrier against corrosive elements.

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