When most people think about corrosion, they think about rust. Brown flakes on a piece of steel, spreading slowly until the part is eaten away. But here’s the truth: not all corrosion looks the same. Some types of corrosion are obvious and easy to spot. Others are silent killers that hide beneath the surface until one day a weld, pipe, or tank fails without warning.

For anyone managing welded equipment, pipelines, or structures, understanding corrosion is not optional. Welds often corrode faster than the surrounding base metal. Why? Because the weld is different — it has a unique microstructure, locked-in stresses, and sometimes even a different composition than the base material. All of this makes welds the “weak spot” where corrosion likes to start.

In this article, we’ll look at the different types of corrosion that attack welds, why they matter, and how to spot the warning signs before it’s too late.

Why Welds Corrode Faster

Imagine a welded joint as a neighborhood where three very different families live side by side:

• Base metal – the original material.
• Weld metal – the filler or melted zone.
• Heat-affected zone (HAZ) – the part of the base metal heated and altered by welding.

These three zones don’t share the same properties. The weld metal might have different chemistry. The Heat Affected Zone has been through a thermal cycle that changes hardness and toughness. Add in residual stress from the rapid heating and cooling, and you’ve got the perfect recipe for corrosion.

That’s why car bodies, for example, often rust first around the spot welds. The welds are simply more vulnerable.

The Different Types of Corrosion

Now let’s break down the main types of corrosion. Each one attacks in its own way, and each one needs a different prevention strategy.

1. General Corrosion (Uniform Attack)

This is the most familiar form. The surface rust you see on an old fence or a steel plate left outside. It spreads evenly across the surface.

• What happens: The metal reacts with oxygen and moisture, forming rust or other oxides.
• Why it matters in welds: Welds may corrode slightly faster because of stress, but generally this is predictable and easy to manage.
• Real-world example: Atmospheric rust on carbon steel beams or tanks.

Prevention: Coatings, paint, inhibitors, or simply choosing a corrosion-resistant alloy like stainless steel.

2. Galvanic Corrosion

Ever heard the saying “one metal sacrifices itself for another”? That’s galvanic corrosion.

• What happens: When two different metals touch in the presence of an electrolyte (like water), one becomes the anode and corrodes, while the other (the cathode) stays protected.
• Why it matters in welds: If the weld metal is much different in composition from the base metal, the weld may corrode faster.
• Real-world example: Magnesium blocks attached to steel ship hulls. The magnesium corrodes (on purpose), saving the steel.

Prevention: Match filler metals carefully, insulate dissimilar metals, or use sacrificial anodes when intentional protection is needed.

3. Crevice Corrosion

Think of crevice corrosion as “hidden corrosion.” It happens in tight spaces where liquid can enter but not flow out.

• What happens: The stagnant solution inside the crevice becomes more aggressive, eating away the metal.
• Why it matters in welds: Weld defects like slag inclusions or incomplete penetration can act as perfect crevices.
• Real-world example: Corrosion under gaskets, bolts, or weld toes.

Prevention: Good welding practices (no trapped slag), smooth surfaces, and designs that avoid tight gaps.

4. Pitting Corrosion

Pitting is one of the most dangerous forms of corrosion.

• What happens: Tiny holes form in the surface, growing deeper over time. On the outside, everything looks fine. Beneath the surface, the metal is being eaten away.
• Why it matters in welds: Stainless steels are especially vulnerable. Segregation of key alloys can make certain spots more prone to pits.
• Real-world example: A stainless steel tank that looks shiny and clean but fails due to a few deep pits.

Prevention: Use alloys with molybdenum or nitrogen (like 316 stainless or duplex grades), and avoid stagnant chloride environments. Nitrogen is intentionally added to duplex stainless steels to improve pitting corrosion resistance.

5. Intergranular Corrosion (Weld Decay)

This is when corrosion attacks along the grain boundaries of stainless steel.

• What happens: The heat of welding causes chromium carbides to form along grain boundaries, robbing nearby regions of chromium. Without enough chromium, the steel is no longer “stainless” because it can not properly create the chromium-oxide layer needed.
• Why it matters in welds: The heat-affected zone (HAZ) is particularly vulnerable. The corrosion often shows up as parallel “wagon tracks” running along the weld.
• Real-world example: Stainless steel piping in power plants showing linear corrosion bands near welds.

Prevention: Use low-carbon grades (304L, 316L), stabilized grades (321, 347), or control welding heat input.   With austenitic stainless steels like 304L and 316L it is necessary to have rapid cooling.  If a weld cools too slowly you risk sensitization which will lead to corrosion and cracking. 

6. Stress Corrosion Cracking (SCC)

This is where stress and corrosion team up. The result? Cracks that grow and grow until failure.

• What happens: Residual stress from welding combines with a corrosive environment (like chlorides or caustics). Cracks form and spread, often without much visible rust.
• Why it matters in welds: Welds are full of locked-in stresses (residual stresses). Add the wrong environment, and cracking happens quickly.
• Real-world example: Type 316 stainless tubes in pulp and paper mills cracking in less than a year.

Prevention: Use duplex or ferritic stainless steels in environments where austenitic steels like 304 or 316 are vulnerable. Stress relief may help — but must be done carefully.

7. Microbiologically Induced Corrosion (MIC)

Yes, bacteria can cause corrosion.

• What happens: Certain microbes create localized environments that accelerate corrosion, often forming pits covered by biological deposits.
• Why it matters in welds: MIC often attacks the ferrite phase in stainless welds, making welds more susceptible than base metal.
• Real-world example: Welded storage tanks exposed to stagnant water during construction.

Prevention: Avoid stagnant water, keep tanks clean, and design for drainage.

Why Understanding the Types Matters

Why bother learning the difference between pitting, crevice, and galvanic corrosion? Because each form requires a different prevention strategy. Paint might work for general corrosion, but it won’t stop pitting inside a tank. Using stainless steel may prevent uniform rust, but if you weld it incorrectly, intergranular attack can ruin it anyway.

If you misdiagnose the type of corrosion, you risk applying the wrong fix — and that usually means wasted money and more downtime.

Practical Steps to Prevent Corrosion in Welds

Here are some simple, practical actions:

1. Pick the right materials – Use low-carbon stainless grades, duplex alloys, or protective coatings depending on the environment.
2. Match filler metals carefully – Avoid creating galvanic couples between base and weld metal.
3. Control welding technique – Limit heat input, avoid defects, and minimize crevices.
4. Protect against the environment – Paint, cathodic protection, or inhibitors where needed.
5. Inspect regularly – Especially welds in corrosive environments like seawater, chemicals, or high-humidity conditions.

Conclusion

Corrosion isn’t just rust. It comes in many forms — uniform, galvanic, crevice, pitting, intergranular, stress-assisted, and even bacteria-driven. Welds are especially at risk because of their unique microstructure and stresses.

By understanding these corrosion types, you can make smarter choices in materials, welding procedures, and maintenance practices. And the payoff? Longer-lasting equipment, fewer surprises, and much lower costs in the long run.

Reference:

Welding Metallurgy & Weldability, John C. Lippold