Corrosion in Metal: What is it and How To Prevent

Corrosion in metal is a pervasive and often underestimated force that silently degrades structures, equipment, and everyday objects around us. Have you ever noticed the reddish-brown flaking on an old iron gate, or wondered why a boat hull might slowly deteriorate over time? These are common examples of metal corrosion at work. It’s a natural process where a refined metal reacts with its environment, gradually returning to a more stable form, similar to how it was found in the earth as ore.

This process is not only about aesthetics, but the corrosion of metal can also lead to significant financial burdens, safety hazards, and environmental concerns worldwide. In this article, we will explain what corrosion is, how it occurs, different ways it appears, and, most importantly, how we can work to prevent the corrosion of metal, extending the life and usefulness of metallic materials.

What is Corrosion?

At its heart, corrosion in metal is a natural electrochemical process, a bit like a slow-motion battery. Most metals we use, like iron, start their lives deep within the earth as part of ores, often in a stable, oxidized state. To turn these ores into the shiny, strong metals we rely on, a lot of energy is put into them. Corrosion of metal is simply these refined metals trying to return to their original, lower-energy state. This “return” happens when four basic components come together:

• Anode: This is the part of the metal that actually corrodes. It’s where atoms lose electrons in a process called oxidation.
• Cathode: This is another area, often on the same metal or a different one nearby, where a substance gains those electrons (a process called reduction).
• Electrolyte: This is a liquid that can conduct electricity, like water, moisture in the air, or saltwater. It lets ions move between the anode and cathode.
• Electrical Connection: There needs to be a path for electrons to flow between the anode and cathode, usually the metal itself.

Think about common rust on an iron nail. The iron acts as the anode, giving up electrons. Oxygen dissolved in water acts as the cathode, accepting those electrons. The water itself is the electrolyte, allowing the whole reaction to proceed. Without any one of these elements, the corrosion of metal slows down or stops.

Several factors can speed up this process. The presence of water or moisture is always needed for electrochemical reactions to happen. Oxygen helps many common corrosion reactions along. Higher temperatures generally make these chemical reactions happen faster. The acidity or alkalinity (pH levels) of the environment can also increase corrosion, especially if they’re at extreme ends. The presence of salts, like road salt or sea salt, makes water a much better electrolyte, greatly speeding up corrosion in metal. Even putting two different metals in contact can cause one to corrode faster, a process called galvanic corrosion, which we’ll look at more closely.

How are Corrosion and Rust Different?

While many people use the terms interchangeably, particularly when talking about old metal, corrosion in metal and rust are not quite the same thing. Understanding the distinction helps in describing the processes more accurately.

Corrosion of metal is a broad term that describes the overall process of a metal degrading due to a chemical or electrochemical reaction with its environment. It applies to all types of metals and all forms of degradation caused by these reactions. For example, aluminum forms a dull gray oxide layer, silver tarnishes, and copper develops a green patina – all of these are forms of corrosion. Even the corrosion of metal castings made from alloys other than iron fall under the general term of corrosion.

Rust, on the other hand, is a specific type of corrosion of metal. Rust refers exclusively to the reddish-brown flaking or powdery substance that forms when iron or alloys containing iron (like steel) react with oxygen and water. It is technically hydrated iron(III) oxides. Therefore, while all rust is corrosion, not all corrosion is rust. For instance, the degradation of corrosion resistant metals like stainless steel, if it occurs, would still be called corrosion, but not rust, because rust only applies to iron.

For detailed information about the comparison between corrosion and rust, I highly recommend checking out our ‘Corrosion vs Rust: How Do They Compare?‘ article.

What Causes Corrosion?

While the basic electrochemical reaction explains how corrosion in metal occurs, several specific factors influence how quickly and severely it happens. Understanding these causes helps in anticipating and preventing degradation.

Presence of Water or Moisture

Water or even just humidity in the air is a primary ingredient for most types of corrosion of metal. It acts as the electrolyte, allowing the movement of ions that completes the electrochemical circuit. Without moisture, many corrosion reactions simply cannot proceed at a significant rate.

Oxygen

Oxygen is a common reactant in many corrosion processes. For instance, the familiar rusting of iron happens much faster in the presence of both water and oxygen. The availability of oxygen can vary in different environments, impacting the rate and even the type of corrosion in metal.

Temperature

Generally, chemical reactions, including those involved in corrosion of metal, speed up with increased temperature. A warmer environment can lead to faster degradation of metallic items.

pH Levels (Acidity/Alkalinity)

The acidity or alkalinity of the environment, measured by its pH level, plays a significant part. Highly acidic or highly alkaline conditions can greatly accelerate corrosion in metal, making some materials vulnerable where they might otherwise be stable.

Presence of Salts or Pollutants

Salts, like those found in seawater or road de-icing agents, dissolve in water to create a much better electrolyte. This greatly enhances the conductivity of the water, speeding up the electrochemical reactions of corrosion of metal. Industrial pollutants in the air or water can also act as corrosive agents.

Dissimilar Metals (Galvanic Couple)

When two different types of metal are in electrical contact and exposed to an electrolyte, one metal can corrode much faster than it would alone. This is known as a galvanic couple. The more reactive metal becomes an anode and sacrifices itself to protect the less reactive one. This is a common consideration when designing structures with various corrosion resistant metals or when dealing with the corrosion of metal castings joined to different alloys.

Types of Corrosion

Not all corrosion in metal looks the same or happens in the same way. The specific conditions and the type of metal determine how corrosion manifests. Understanding these different forms helps in recognizing and addressing the problem.

Uniform (General) Corrosion

This type occurs when the corrosive attack is spread evenly across the entire exposed surface of the metal. A good example is the general rusting you might see on an old, unpainted steel fence where the entire surface slowly degrades. This type of corrosion of metal is often predictable and relatively easy to manage.

Pitting Corrosion

This involves the formation of small, deep holes or pits on the metal’s surface. These pits can be tiny, making them hard to spot, but they can penetrate deep into the material, leading to sudden and unexpected failure. This is particularly problematic in metals like stainless steel when exposed to chloride environments, such as saltwater.

Crevice Corrosion

This happens in confined spaces where oxygen access is limited. These spaces could be under bolt heads, washers, or in tight joints where moisture can get trapped. The stagnant conditions within the crevice create a localized corrosive environment, leading to accelerated corrosion of metal.

Galvanic Corrosion

This occurs when two different metals are in electrical contact and exposed to an electrolyte. One metal becomes an anode and corrodes at an accelerated rate, while the other acts as a cathode and is protected. For instance, if a steel bolt is used with a copper plate in a wet environment, the steel will corrode much faster than it would alone. This is an important consideration when working with various corrosion resistant metals.

Stress Corrosion Cracking (SCC)

This type of corrosion in metal causes cracking of a material due to the combined action of tensile stress and a specific corrosive environment. Pipelines carrying corrosive fluids, especially if under pressure, can be susceptible to stress corrosion cracking (SCC).

Intergranular Corrosion

This attacks the metal along its grain boundaries, which are the interfaces between the microscopic crystals (grains) that make up the metal structure. This weakens the metal internally without much visible surface damage. Improperly welded stainless steel can sometimes suffer from this if certain conditions are met, as the welding process can alter the grain boundaries’ susceptibility.

Corrosion of Metal Castings

When it comes to manufacturing processes, the corrosion of metal castings presents its own set of considerations. Castings, being formed by pouring molten metal into a mold, can sometimes have internal stresses, porosity, or varying grain structures that might affect how they react to corrosive environments. For example, some alloys used in castings might be more susceptible to specific types of localized corrosion if their microstructure is not uniform. The choice of alloy and post-casting treatments are often adjusted to minimize the risk of corrosion in metal castings in their intended service environment.

The Impact of Corrosion

The quiet work of corrosion in metal might seem like a slow process, but its collective effect is far from minor. The consequences of unchecked corrosion of metal reach into almost every aspect of modern life, bringing significant costs and concerns.

Economic Costs

Industries and governments around the world spend immense sums each year on dealing with corrosion of metal. This includes the money for maintenance, repair, and the replacement of corroded parts and structures. When pipes or machinery corrode, they lose efficiency, which also adds to costs for businesses. For instance, the degradation of large industrial components, including the corrosion of metal castings used in machinery or structural supports, necessitates expensive downtime and specialized repair crews, all of which contribute to the overall economic burden.

Safety Hazards

Beyond the financial aspect, corrosion in metal poses serious safety hazards. The failure of a corroded part can have catastrophic results. Think of a bridge weakened by rust, a pipeline leaking hazardous materials due to internal corrosion of metal, or a vehicle component that gives way unexpectedly. Structures relying on metal integrity, like buildings, bridges, and even aircraft, all depend on minimizing corrosion for public safety. In manufacturing, issues stemming from the corrosion of metal castings in critical applications can directly lead to equipment failure and pose risks to workers.

Environmental Concerns

Environmental concerns also arise from corrosion of metal. Leaks from corroded storage tanks or pipelines can contaminate soil and water with dangerous chemicals. The constant need to produce new materials to replace corroded ones consumes vast amounts of energy and raw resources, putting a strain on the environment. While corrosion resistant metals help mitigate some of these issues, the overall cycle of degradation and replacement has a measurable environmental footprint.

Aesthetic Degradation

Lastly, the aesthetic degradation caused by widespread corrosion in metal can detract from the appearance of public spaces and personal property.

How To Prevent Corrosion

Given the widespread impact of corrosion in metal, understanding how to prevent or slow down this process is highly important. Various methods are employed, often in combination, to protect metallic items and structures from degradation.

Material Selection

One of the most direct ways to combat corrosion of metal begins before an item is even made. This involves choosing materials that naturally resist corrosive environments. Some metals are inherently less reactive. For example, using corrosion resistant metals like certain stainless steels, aluminum alloys, or titanium is a common strategy in harsh conditions.

Stainless steel castings, for instance, forms a thin, protective oxide layer that shields it from further attack. When combining different metals, it is also important to select compatible ones or separate them to avoid galvanic corrosion, where one metal sacrifices itself to protect another. This is especially true in complex assemblies, where the corrosion of metal castings made from one alloy might accelerate if directly coupled with a less noble metal.

Protective Coatings

Applying a barrier between the metal and its environment is a widely used method. Paints and various organic coatings serve this purpose, creating a physical shield against moisture and oxygen. Metallic coatings are another effective approach. Galvanizing, for example, involves applying a layer of zinc to steel. The zinc acts as a sacrificial coating, corroding itself to protect the underlying steel even if scratched. Other barrier coatings, like nickel or chromium plating, provide a physical shield. Powder coating offers a durable and attractive protective layer that can stand up to environmental exposure. These coatings are frequently applied to surfaces, including those of corrosion of metal castings, to extend their lifespan.

Environmental Modification

Changing the environment around the metal can also reduce corrosion. This might mean controlling humidity, perhaps by using drying agents in enclosed spaces to limit moisture. Removing oxygen from water systems, a process called deaeration, can significantly reduce the potential for corrosion in metal in pipes and tanks. Adjusting the acidity or alkalinity (pH) of corrosive fluids can also help. In some industrial systems, specific chemicals called inhibitors are added to the environment. These chemicals either form a protective film on the metal surface or alter the corrosive medium, reducing the rate of attack.

Electrochemical Protection

This advanced method involves using electrical currents to prevent corrosion of metal. Cathodic protection is a common technique. One form uses a sacrificial anode, a more reactive metal (like zinc or magnesium) connected to the item being protected. This sacrificial metal corrodes instead of the target metal. For example, zinc blocks are attached to boat hulls to protect the steel or aluminum. Another form of cathodic protection uses an impressed current system, where an external power source provides a protective electrical current to the metal.

Design Considerations

How a metal structure is designed can significantly affect its susceptibility to corrosion in metal. Good design seeks to avoid areas where moisture can collect, such as crevices, stagnant zones, or poorly draining surfaces. Designing for good drainage helps prevent water from sitting on surfaces. Creating designs that allow for easy inspection and maintenance also helps, as it allows for early detection of any corrosion of metal and prompt repair.

Regular Maintenance and Inspection

Finally, consistent care and oversight play a big part. Regularly cleaning metal surfaces removes dirt and corrosive agents. Repairing any damage to protective coatings promptly helps to maintain the barrier against the environment. Routine inspection allows for early identification of any signs of corrosion in metal, allowing for timely intervention before the damage becomes severe. This is particularly relevant for large pieces of equipment and structures where preventing the widespread corrosion of metal castings and other components can save considerable expense and prevent failures.

Conclusion

Corrosion in metal is a pervasive and natural process, a metal’s tendency to return to a more stable state. From general surface rust to the hidden dangers of pitting or the complex corrosion of metal castings, its forms are varied, and its impact is significant, affecting economies, safety, and the environment worldwide. However, understanding what corrosion of metal is and the factors that influence it gives us the power to act.

Through careful material selection, applying protective coatings, modifying the environment, using electrochemical protection, thoughtful design, and consistent maintenance, the effects of corrosion in metal can be greatly reduced. By applying these strategies, we can help ensure our metal structures and components remain functional and safe for much longer, making the most of these important materials.

Taiyuan SIMIS Investment Casting Co., Ltd is a professional investment castings manufacturer in China. We are equipped with advanced casting equipment and offer comprehensive in-house services, such as machining, surface treatments, heat treatments, and testing. Get in touch with our experts to get your projects started.