Exploring the Different Types of Corrosion

Corrosion is a pervasive issue that can considerably undermine the structural integrity and functionality of various materials, particularly metals. Understanding how and why corrosion occurs, along with being aware of the different types of corrosion, can go a long way in diagnosing problems early and choosing effective prevention treatments. This article delves into the theory of corrosion, the scientific processes involved, and the many forms it can take—from uniform attack corrosion to the less common but equally damaging fretting corrosion.

Understanding the Nature of Corrosion

Corrosion, a natural process that can impact materials over time, is often seen as an enemy of structural integrity, especially metals. The different types of corrosion—uniform, galvanic, intergranular, crevice, and more—can turn sturdy stainless steel into a fragile frame hence the need to understand the nature of corrosion is paramount when seeking to combat against it. But what is the science behind it?

Put simply, corrosion is the reaction of a metal with its environment which leads to the degradation of the material properties. This could be due to an electrochemical reaction, or in the case of some materials like carbon steel, a simple chemical reaction.

Corrosion presents itself in various forms, but it all comes down to the same basic principles—the metal returns to their natural state, i.e, ore, and the external environment plays a significant role in this degradation, whether it’s oxygen concentration, a corrosive environment or the use of different metals together (bimetallic corrosion).

The Science Behind Corrosion

At its essence, the science behind corrosion is quite fascinating and it boils down to basic chemistry. When a refined metal, like stainless steel, is exposed to the atmosphere, it’s inherently unstable. It attempts to return to its more stable state or ‘ore’ state. This transformation is brought about through chemical reactions which are influenced by specific environmental factors. Let’s delve deeper into this.

Metals like stainless steel are composed of countless microscopic grains which meet at grain boundaries. When exposed to a certain corrosive environment, these boundaries can get attacked leading to intergranular corrosion. Essentially, the stainless steel’s passive film gets disrupted and the underlying metal begins to corrode.

Another phenomenon involves the formation of concentration cells on the surface, usually a pit or crevice, where there is a difference in oxygen concentration. This kind of localized corrosion, called crevice corrosion, is especially damaging as it can often go unnoticed.

In other cases, certain elements in an alloy may turn out to be more reactive and get preferentially corroded leaving behind a porous, weakened structure. This is called selective corrosion or selective leaching. Galvanic or bimetallic corrosion, on the other hand, occurs when dissimilar metals are in contact with each other in a corrosive environment. One metal (the anode) corrodes faster than the other (the cathode) causing the former to deteriorate at a much higher rate.

The final method we’ll discuss is stress corrosion cracking, where the combined effect of tensile stress and a corrosive environment leads to cracks and ultimate failure in equipment and other fixtures.

Now you might wonder, if corrosion is an inevitable natural process, can it be prevented or at least significantly delayed? The good news is, yes—through various methods such as cathodic protection, usage of inhibitors, controlling the environment with maintenance solutions etc. But before we dive into these methods, let’s first understand each type of corrosion in detail in the upcoming sections.

The Different Types of Corrosion

Metals, like carbon steel and stainless steel, are susceptible to corrosion. This phenomenon occurs due to chemical reactions with the environment. But not all corrosion is the same. There are several different types, each with its unique characteristics, formation process, and effects. Let’s dive into the different types of corrosion to have a better understanding.

Uniform Attack Corrosion

Uniform attack or uniform corrosion, as the name suggests, is a form where corrosion occurs uniformly over the entire exposed surface. It’s the most common type of all. It’s caused when the entire metal surface comes in contact with a corrosive environment. Although it’s easy to spot with the naked eye, it can cause severe damage if left unnoticed.

Galvanic or Bi-Metallic Corrosion

Ever heard of galvanic or bimetallic corrosion? This occurs when two different types of metals are physically or electrically in contact and are immersed in an electrolyte. The more active metal (the anode) corrodes at a boosted rate, while the less active one (the cathode) is protected. Choosing the right materials during construction or designing can prevent this type of corrosion.

Pitting Corrosion

Quite a menace, pitting corrosion is a type of localized corrosion that leads to small, yet deep holes on metal surfaces. This makes it one of the most dangerous types of corrosion, as it’s difficult to detect and can lead to big problems if ignored. Stainless steel often falls victim to concentration cell pitting corrosion.

Crevice Corrosion

Just as harmful as pitting is crevice corrosion. This snag transpires in shielded areas, like spaces under flanges or washers. The oxygen concentration in these confined places is lower than the surrounding environment, creating a concentration cell and culminating in corrosion.

Worried About the Impact of Corrosion in Your Facility?Reach out to the team of certified corrosion-control experts at A&K today.

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Intergranular Corrosion

Intergranular corrosion, which happens along the grain boundaries of an alloy, is an insidious form of corrosion. It’s often invisible to the naked eye and is particularly threatening to metals like stainless steel, where grain boundaries tend to be more chemically reactive than the grains themselves.

Selective Leaching or De-Alloying

Selective leaching, also known as de-alloying, is a unique form of corrosion. In this type, one element is preferentially leached from the metal alloy. A classic example is dezincification of brass, where zinc is selectively leached leaving behind a porous and weak copper-rich residue. Careful control of composition and regular inspections can deter this type of corrosion.

Erosion Corrosion

If you’ve been thinking that only chemical reactions cause corrosion, erosion corrosion might surprise you. This happens due to physical action, such as the high-speed flow of liquid over a surface. Carbon steel piping systems are often victims of this type of corrosion.

Fretting Corrosion

Fretting corrosion is another unique type of corrosion. It results from repeated wearing, weight-loading, or vibration in conjunction with a small amount of relative motion. Potential locations for fretting can be bolted joints or gear teeth where it can often be mistaken as wear.

Eliminate and Prevent Corrosion With A&K Facility Maintenance Solutions

Worried about the impact of corrosion? A&K Facility Maintenance Solutions can come to your rescue. With A&K’s proactive approach, quality commitment, and resources at the resource center, you can prevent these different types of corrosion from wreaking havoc on your structures. Through both passive and active preventive methods, like comprehensive maintenance painting plans, A&K can keep your facility protected.