The 10 Types of Corrosion: An Engineer’s Guide to How Metals Fail

You see a rusty bolt and think, “corrosion.” But in the world of engineering and manufacturing, that’s like looking at a hospital full of patients and saying, “they’re all sick.” Rust is just one symptom of one type of corrosion. The reality is a complex family of destructive processes, each with its own unique cause, appearance, and prevention method.

So, what are the main types of corrosion?

The 10 most critical types of corrosion that engineers and manufacturers deal with are: 1) Uniform Attack, 2) Galvanic, 3) Pitting, 4) Crevice, 5) Intergranular, 6) Stress Corrosion Cracking (SCC), 7) Erosion, 8) Fretting, 9) Filiform, and 10) High-Temperature Corrosion.

Understanding the difference between these types is not academic—it’s the key to building safe, reliable products that last. A misunderstanding can lead to catastrophic failures, from a collapsed bridge to a compromised medical implant.

This guide will walk you through each of the 10 types of corrosion. We won’t just define them; we’ll show you what they look like, explain the hidden mechanisms that cause them, and provide the prevention strategies we use at RM (Rapid Manufacturing) to protect the critical parts we build every day.

What is Corrosion, Really? The Electrochemical Engine

Before we can classify the different types, we must understand that nearly all corrosion in a water-based environment is an electrochemical process. It’s not just a simple chemical reaction; it’s a tiny, unwanted battery. For it to occur, four things must be present:

1. Anode: The part of the metal that corrodes. It gives up electrons (oxidation) and turns into metal ions (e.g., rust).
2. Cathode: A part of the metal (or a different metal) that does not corrode. It accepts the electrons.
3. Electrolyte: A conductive liquid (like water, especially saltwater) that allows the ions to move between the anode and cathode.
4. Metallic Path: A connection that allows the electrons to flow from the anode to the cathode. The metal part itself serves as this path.

When these four elements are present, the circuit is complete, and the anode begins to dissolve. Every type of corrosion we will discuss is simply a different way of creating this destructive circuit.

Category 1: Uniform Attack Corrosion (The Obvious One)

This is the most common and recognizable form of corrosion. As the name implies, it proceeds uniformly across the entire exposed surface of a material. It’s predictable, measurable, and rarely the cause of unexpected catastrophic failure because you can see it happening and plan for it.

Appearance: A consistent, widespread rusting or tarnishing. Think of a sheet of plain carbon steel left out in the rain—the entire surface develops a layer of reddish-brown iron oxide (rust).

Mechanism: On a microscopic level, the anode and cathode sites are constantly shifting and moving around, leading to an even loss of material across the surface. This happens when a metal is in a corrosive environment, like an acidic solution or simply exposed to oxygen and moisture.

Common Example: The gradual thinning of the steel hull of a ship or the rusting of an old, unpainted metal fence.

Prevention:

• Coatings: The simplest method. Paint, powder coating, or plating creates a barrier between the metal and the electrolyte.
• Material Selection: Choose a more corrosion-resistant material. Using stainless steel instead of carbon steel is a common upgrade.
• Corrosion Inhibitors: Chemicals added to the electrolyte that slow down the reaction, often by forming a protective film on the metal surface.

Category 2: Galvanic Corrosion (The Dissimilar Metals Trap)

Galvanic corrosion is one of the most frequently encountered—and misunderstood—types of corrosion. It occurs when two different metals are in physical contact with each other and are immersed in a common electrolyte.

Mechanism: This is the “battery” concept in its purest form. Every metal has a different natural tendency to give up its electrons, a property called its electrode potential. When two different metals are connected, the one with the more negative potential becomes the anode and corrodes rapidly, while the one with the more positive potential becomes the cathode and is protected.

Engineers use a Galvanic Series chart to predict which metal will corrode. Metals at the top (like magnesium and zinc) are “less noble” and will act as the anode. Metals at the bottom (like gold and platinum) are “more noble” and will act as the cathode. The farther apart two metals are on the chart, the faster the anode will corrode.

Appearance: Severe corrosion located right at the point of contact between the two different metals. The more noble metal will look pristine, while the less noble metal will be heavily damaged.

Common Example:

• A classic mistake is using steel screws to fasten a brass plate in a marine environment. The steel is less noble than the brass, so it becomes the anode and corrodes away at an accelerated rate, while the brass remains untouched.
• This principle is also used for protection. Galvanized steel is simply carbon steel coated in zinc. If the coating is scratched, the zinc (less noble) sacrificially corrodes to protect the exposed steel (more noble).

Prevention:

• Avoid Dissimilar Metal Contact: The best method is to design the product using a single metal.
• Electrical Isolation: If you must use two different metals, isolate them with a non-conductive barrier like a plastic or rubber gasket and washer. This breaks the metallic path.
• Choose Metals Close on the Galvanic Series: If you must connect two metals, choosing ones that are close together on the chart (e.g., two different series of stainless steel) will minimize the rate of corrosion.
• Sacrificial Anodes: Intentionally attach a block of a much less noble metal (like zinc or aluminum) to the structure you want to protect. This “sacrificial anode” will corrode away, protecting the main structure. This is used on ship hulls and pipelines.

We’ve now covered the most visible type of corrosion and the most common “dissimilar metal” trap. These are bad, but often predictable. In the next part, we will dive into the hidden killers: the forms of localized corrosion that can cause a part to fail suddenly and without obvious warning. We’ll explore Pitting, Crevice, and Intergranular corrosion—the types that keep engineers up at night.

Category 3: Pitting Corrosion (The Hidden Puncture)

Pitting is one of the most destructive and treacherous forms of corrosion. It is a highly localized attack that creates small, deep holes (or “pits”) in the surface of a material. A component can look almost perfect on the surface but be riddled with pits that act as stress concentrators, leading to sudden, catastrophic fracture.

Appearance: Tiny holes on the surface, which are often covered and hidden by a cap of corrosion products. Wiping away the surface rust might reveal a deep cavity underneath. The vast majority of the metal surface remains unaffected.

Mechanism: Pitting initiates at a small weak point in a metal’s passive protective layer (like the chromium oxide layer on stainless steel). This is often triggered by the presence of specific ions, with chloride (Cl⁻) being the most common culprit. Once the layer is breached, an aggressive “autocatalytic” process begins:

1. The small, active pit becomes the anode, and the large, passive surface around it becomes the cathode.
2. Metal ions concentrate inside the pit, attracting negative ions like chloride to maintain charge neutrality.
3. This forms aggressive metal chlorides (e.g., ferric chloride) which hydrolyze with water, creating a highly acidic and corrosive micro-environment inside the pit.
4. The process becomes self-sustaining and accelerates, drilling a hole deep into the material.

Common Example: Pits forming on 304 stainless steel piping or tanks used in coastal areas or in chemical plants that handle chloride-containing solutions.

Prevention:

• Alloy Selection: Use materials with higher resistance to pitting. Adding molybdenum to stainless steel (as in the 316L grade) significantly increases its resistance. For even harsher environments, duplex stainless steels or nickel-based alloys are required.
• Environmental Control: Reduce the concentration of chlorides, lower the temperature, or decrease the acidity of the electrolyte.
• Maintain Clean Surfaces: Pitting often initiates under small deposits or surface contaminants. Keeping surfaces clean and smooth can prevent initiation.

Category 4: Crevice Corrosion (The Attack in the Gaps)

Crevice corrosion is mechanistically very similar to pitting but is initiated by a specific geometry rather than a random flaw in the passive layer. It is an intense, localized corrosion that occurs within shielded gaps or crevices on a metal’s surface where the electrolyte is stagnant.

Appearance: Severe corrosion damage that is entirely hidden within a gap. You won’t see it until you disassemble the parts. Common locations are under bolt heads, beneath washers and gaskets, in lap joints, and between tubes and tubesheets.

Mechanism: The process starts with a differential aeration cell.

1. The electrolyte inside the crevice is stagnant, and the dissolved oxygen is quickly consumed by the initial corrosion reaction.
2. The oxygen cannot be easily replenished due to the tight geometry.
3. The area inside the crevice, now depleted of oxygen, becomes the active anode. The area outside the crevice, with plenty of oxygen, becomes the cathode.
4. Just like in pitting, a self-sustaining cycle begins. Metal ions and chlorides concentrate inside the crevice, the pH drops, and the corrosion rate skyrockets within the hidden gap.

Common Example: Severe corrosion of a stainless steel bolt under the head where it clamps down on a plate in a marine environment. The exterior of the bolt looks fine, but it can fail unexpectedly.

Prevention:

• Design Out Crevices: This is the most effective method. Use welded joints instead of bolted or riveted ones. Ensure complete penetration welds.
• Use Solid, Non-Absorbent Gaskets: Porous gaskets can act like sponges and create perfect conditions for crevice corrosion. PTFE gaskets are a common choice.
• Use Sealants: Calk or apply a sealant to fill gaps in lap joints.
• Ensure Proper Drainage: Design parts so that water and electrolytes cannot pool in crevices.

To clarify the difference between these two similar but distinct forms of localized corrosion, here is a direct comparison:

Category 5: Intergranular Corrosion (IGC) (The Attack on the Boundaries)

This is a particularly insidious form of corrosion because it attacks the grain boundaries of the metal, not the grains themselves. It can destroy the integrity of a material with almost no visible sign on the surface, causing it to lose strength and ductility. The part may look fine but can fracture or even crumble with very little stress.

Appearance: On the surface, it may only appear as a light etching. A microscopic examination is needed to see the attack along the grain boundaries. In severe cases, entire grains can fall out when the material is stressed, giving it a sugary or rough texture.

Mechanism: The most famous example is the “sensitization” of austenitic stainless steels (like the common 304 grade).

1. When these steels are heated into a specific temperature range (approx. 450-850°C or 850-1550°F), for example, during welding, the carbon in the steel combines with the chromium.
2. This forms chromium carbides (Cr₂₃C₆) along the grain boundaries.
3. This process steals chromium from the area immediately adjacent to the grain boundaries. Since chromium is what gives stainless steel its corrosion resistance, these depleted zones become highly susceptible to corrosion.
4. The grain boundaries now act as anodes, and the corrosion proceeds rapidly along these narrow paths, separating the grains.

Common Example: “Weld decay” in a 304 stainless steel pipe used to carry a corrosive fluid. The corrosion doesn’t occur in the weld itself but in the narrow bands on either side of it (the Heat-Affected Zone) that were held in the sensitization temperature range.

Prevention:

• Use Low-Carbon Grades: Select “L” grades like 304L or 316L. The lower carbon content (e.g., <0.03%) means there isn’t enough carbon to form significant amounts of chromium carbide. This is the most common modern solution.
• Use Stabilized Grades: Use grades like 321 (stabilized with titanium) or 347 (stabilized with niobium). These elements have a stronger affinity for carbon than chromium does, so they form harmless carbides, leaving the chromium in solution to protect the steel.
• Post-Weld Heat Treatment: For non-L grades, a high-temperature “solution anneal” can be performed to redissolve the chromium carbides and restore corrosion resistance. This is often impractical.

We have now covered the forms of corrosion that attack a material from within, based on chemistry and geometry alone. But what happens when you add mechanical forces into the equation? In the final part, we will explore the types of corrosion that are driven by stress and physical wear, including Stress Corrosion Cracking (SCC), Erosion Corrosion, and Fretting, completing our guide to the 10 critical types of corrosion.

Stress Corrosion Cracking (SCC): The Silent Catastrophe

Stress Corrosion Cracking (SCC) is one of the most insidious and dangerous failure mechanisms in engineering. It is defined as the cracking of a material produced by the combined action of a corrosive environment and a static tensile stress. Its terrifying nature comes from its ability to cause a seemingly sound part to suddenly fracture without any obvious signs of corrosion or plastic deformation.

The Mechanism: A Trifecta of Trouble

For SCC to occur, three conditions must be met simultaneously:

1. A Susceptible Material: Not all materials are prone to SCC in all environments. 300-series stainless steels, for example, are famously susceptible in environments containing chloride ions.
2. A Specific Corrosive Environment: The environment that causes SCC is specific to the material. Ammonia will cause brass to crack, while chlorides will attack stainless steel.
3. A Static Tensile Stress: This stress can be from an external load, but more often it is a residual stress left over from manufacturing processes like welding, cold forming, or improper heat treatment.

When this trio of conditions exists, the stress opens up a microscopic crack on the material’s surface. The corrosive medium then attacks the newly exposed crack tip, which is under the highest stress, causing the crack to propagate further. This creates a vicious cycle that continues until the remaining cross-section of the part can no longer support the load, leading to a sudden, brittle-like fracture.

Appearance and Detection

SCC is incredibly difficult to detect visually. The cracks are extremely fine, often microscopic, and can be filled with corrosion products that hide them. The bulk of the material surface may show very little general corrosion, giving a false sense of security. Detection almost always requires specialized non-destructive testing (NDT) methods like dye penetrant testing or ultrasonic inspection.

Erosion Corrosion: The Scouring Attack

Erosion Corrosion is an accelerated form of corrosion caused by the combined action of a corrosive fluid and the mechanical wearing effect of that same fluid’s movement. It’s the chemical equivalent of a river carving a canyon through rock.

The Mechanism: Wear and Tear on a Chemical Level

Many metals, like stainless steel and aluminum, protect themselves with a very thin, tough, and inert layer of oxide called a passive film. In a static corrosive fluid, this film is stable. However, when the fluid is moving at a high velocity—especially if it contains abrasive solid particles (like sand or slurry)—it can physically scrub this protective layer away.

The moment the passive layer is removed, the fresh, reactive metal underneath is exposed to the corrosive fluid and immediately begins to corrode. A new passive layer tries to form, but it too is immediately scoured away by the flowing fluid. This rapid cycle of stripping and re-corroding leads to a much faster rate of material loss than either erosion or corrosion would cause alone.

Appearance and Detection

Erosion corrosion leaves a very distinct directional pattern on the metal surface. It often appears as grooves, gullies, waves, or teardrop-shaped pits, all of which are aligned with the direction of the fluid flow. It is most commonly found in areas where the flow changes direction or speed, such as pipe elbows, tees, pump impellers, and valve outlets.

Fretting Corrosion: The Vibration Killer

Fretting Corrosion occurs at the interface of two tightly-pressed surfaces that are subjected to slight, repetitive back-and-forth movement, such as vibration. It’s a classic problem in bolted joints, press-fit bearings, and any other clamped mechanical assembly.

The Mechanism: Rubbing and Rusting

The process begins with the microscopic sliding motion (fretting) between the two surfaces. This motion breaks down the protective passive layer on the metal surfaces, exposing fresh, reactive metal. This exposed metal immediately oxidizes. The resulting hard oxide particles get trapped between the surfaces.

Because these oxide particles are often harder than the base metal, they act as an abrasive grit, accelerating the wear and breaking down even more of the passive layer. This creates a feedback loop where rubbing causes oxidation, and the resulting oxide particles accelerate the rubbing damage.

Appearance and Detection

Fretting is typically identified by the presence of pits or grooves on the metal surfaces, surrounded by a characteristic oxide debris. For steel parts, this debris looks like a reddish-brown “cocoa” powder. The damage is highly localized to the contact area between the two components.

Dealloying: The Alloy’s Achilles’ Heel

Dealloying, also known as selective leaching, is the preferential corrosion of one element from a solid solution alloy. This process leaves behind a porous, weak remnant of the more corrosion-resistant element.

The Mechanism: Preferential Removal

The most classic example is the dezincification of brass. Brass is an alloy of copper and zinc. In certain corrosive environments (like water with high chloride content), the more chemically active zinc is selectively corroded away, leaving behind a spongy, weak copper structure. The part may retain its original shape and dimensions, but it has lost almost all of its mechanical strength and can fail under minimal load.

Appearance and Detection

The most obvious sign of dealloying is often a color change. In the case of dezincification, the yellow brass turns to the reddish color of pure copper. While the surface may look intact, a simple scratch test will reveal the soft, porous nature of the material underneath.

High-Temperature Corrosion: Trial by Fire

The final type of corrosion is unique because it does not require a liquid electrolyte. High-Temperature Corrosion is the chemical degradation of a material resulting from direct reaction with a hot gaseous atmosphere. The most common form is oxidation.

The Mechanism: Beyond Water

At elevated temperatures (e.g., in furnaces, jet engines, or exhaust systems), metals can react directly with gases in the environment, most commonly oxygen, sulfur, or other oxidants. This reaction forms a solid layer of scale on the metal’s surface. Whether this scale is protective or destructive depends on the material and temperature. If the scale is dense and well-adhered, it can slow further corrosion. If it’s porous or flakes off easily, it exposes fresh metal to continued attack, leading to rapid material loss.

Appearance and Detection

High-temperature corrosion is usually obvious, characterized by a thick, often discolored or flaky layer of scale on the component’s surface. The engineering challenge is not detecting it, but selecting materials (like nickel-based superalloys or ceramics) that can resist it at the required operating temperatures.

Conclusion: From Recognition to Prevention

Corrosion is not a single enemy; it is a multifaceted force of nature with at least 10 distinct forms of attack. We’ve journeyed from the obvious, uniform rusting of a steel beam to the invisible, stress-driven crack that can fell an aircraft.

Understanding these 10 types is the first and most critical step in any effective reliability or failure analysis program. By accurately identifying the mechanism of attack—whether it’s galvanic, localized, or mechanically-assisted—engineers can deploy the correct preventative strategy. This could be changing a material, applying a protective coating, altering the environment, or redesigning the part to eliminate stress concentrators. At its core, fighting corrosion is about knowing your enemy.

Frequently Asked Questions (FAQ)

What are the 3 main groups of corrosion?

While there are many specific types, they can be conceptually grouped into three categories:

1. General Corrosion: Where the attack is spread more or less evenly across the surface (e.g., Uniform Corrosion).
2. Localized Corrosion: Where the attack is concentrated in specific, small areas, making it much more dangerous (e.g., Pitting, Crevice, SCC).
3. Mechanically-Assisted Corrosion: Where corrosion is accelerated by a mechanical force (e.g., Erosion Corrosion, Fretting).

What are common examples of corrosion?

• Uniform: Rust on an old car’s steel body panel.
• Galvanic: A steel screw rusting rapidly when used in a brass fixture.
• Pitting: Tiny, deep holes forming on stainless steel cookware exposed to salt.
• Crevice: Corrosion hidden under the head of a bolt on a boat trailer.
• SCC: Cracking of a brass valve stem exposed to ammonia-based cleaners.

Why are there different lists with 8 or 10 types of corrosion?

Corrosion science is complex, and experts sometimes categorize phenomena differently. A list of “8 types” is common and covers the most frequent industrial problems. A list of 10, like the one presented here, is more comprehensive and often includes more specialized but equally critical forms like Dealloying and High-Temperature Corrosion to provide a more complete engineering picture.

References

1. AMPP (formerly NACE International). (2022). Corrosion Basics. Retrieved from AMPP’s Corrosion Resources
2. American Society for Metals (ASM) International. (2005). ASM Handbook, Volume 13B: Corrosion: Materials. Retrieved from ASM International Handbooks (A definitive, peer-reviewed engineering handbook and the primary source referenced by engineers to validate alloy selection).

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