2.4 Forms of Corrosion: Uniform, Galvanic, Pitting & Crevice

Forms of Corrosion: Uniform, Galvanic, Pitting & Crevice

Recognizing Corrosion Types

Corrosion does not always manifest in the same way. The specific mechanism of degradation depends on the metal, the environment, the structural design, and the presence of other materials. Corrosion professionals generally categorize corrosion into several distinct forms. Recognizing the form of corrosion is the first step in diagnosing the root cause of a failure and designing an appropriate mitigation strategy.

While there are many specific forms of corrosion (such as stress corrosion cracking, erosion-corrosion, and intergranular corrosion), this section focuses on the four fundamental types most frequently encountered in general infrastructure and pipeline systems: Uniform, Galvanic, Pitting, and Crevice corrosion.

1. Uniform (General) Corrosion

Uniform corrosion, also known as general corrosion, is characterized by an electrochemical reaction that proceeds relatively evenly over the entire exposed surface of the metal.

In uniform corrosion, the anodic and cathodic sites are not fixed; they constantly shift across the surface of the metal. This results in a relatively uniform thinning of the material over time. A common example is a piece of unprotected carbon steel left exposed to the atmosphere, which will develop a relatively even layer of rust over its entire surface.

Characteristics of Uniform Corrosion:

• Predictability: Because the metal loss is even, uniform corrosion is the easiest form to predict and manage. Engineers can calculate the corrosion rate (e.g., in mils per year, mpy) and design the structure with a "corrosion allowance"—extra thickness specifically intended to be corroded away over the design life of the asset.
• Total Mass Loss: Uniform corrosion accounts for the greatest total amount of metal destroyed on a global scale.
• Mitigation: It is relatively easy to mitigate using protective coatings, cathodic protection, or by changing the environment (e.g., using inhibitors).

Despite being responsible for the most metal loss by tonnage, it is rarely the cause of sudden, catastrophic structural failures because its progress is visible and predictable.

2. Galvanic (Bimetallic) Corrosion

Galvanic corrosion occurs when two electrochemically dissimilar metals are electrically connected and exposed to the same electrolyte. As discussed in the previous section on the Galvanic Series, this creates a macro-corrosion cell.

Mechanism of Galvanic Corrosion:

• The more active metal (higher on the Galvanic Series) becomes the anode and suffers accelerated corrosion.
• The more noble metal (lower on the Galvanic Series) becomes the cathode and its corrosion rate is reduced or halted.
• The driving force is the potential difference between the two metals.

Factors Influencing Galvanic Corrosion:

• Area Ratio: This is the most critical factor. A small anode connected to a large cathode (e.g., a steel rivet in a large copper plate) will result in extremely rapid, severe corrosion of the small anode because the large cathode demands a massive amount of electrons. Conversely, a large anode connected to a small cathode (e.g., a copper rivet in a large steel plate) will result in minor, widely distributed corrosion on the anode.
• Electrolyte Conductivity: A highly conductive electrolyte (like seawater) allows the galvanic current to spread over a larger area. A low-conductivity electrolyte restricts the severe galvanic corrosion to the immediate area where the two metals meet.

3. Pitting Corrosion

Pitting is an extremely localized form of corrosion that leads to the creation of small holes or "pits" in the metal. It is one of the most destructive and insidious forms of corrosion.

Mechanism of Pitting:

Pitting often initiates at localized defects in a protective coating or at weak spots in a metal's naturally occurring passive oxide film (common in stainless steels and aluminum). Once a pit initiates, the chemistry inside the pit changes drastically compared to the bulk electrolyte outside.

• The inside of the pit becomes the anode.
• The large surrounding surface area remains the cathode.
• The environment inside the pit becomes highly acidic and concentrated with aggressive ions (like chlorides), accelerating the dissolution of metal at the base of the pit.

Characteristics of Pitting:

• Unpredictable: Pits are hard to detect because they are small, often covered by corrosion products, and grow rapidly inward.
• Dangerous: Pitting can cause a pipeline to leak or a pressure vessel to fail even when the total weight loss of the structure is virtually zero. It bypasses corrosion allowances designed for uniform corrosion.
• Autocatalytic: Once started, the conditions inside the pit drive the reaction faster and faster.

4. Crevice Corrosion

Crevice corrosion is another highly localized form of corrosion that occurs within narrow crevices or shielded areas on metal surfaces where a stagnant solution can accumulate.

Mechanism of Crevice Corrosion:

Crevices can be created by structural features (lap joints, flanges, threads, under gasket materials) or by environmental factors (deposits of sand, dirt, or bio-fouling). The mechanism is very similar to pitting.

1. Initially, uniform corrosion occurs both inside and outside the crevice.
2. Because the crevice is narrow and stagnant, the dissolved oxygen inside the crevice is quickly depleted by the cathodic reaction.
3. Oxygen remains plentiful on the surfaces outside the crevice. This creates a concentration cell.
4. The oxygen-depleted area inside the crevice becomes a fixed anode, while the oxygen-rich area outside becomes the cathode.
5. Like pitting, the chemistry inside the crevice becomes highly acidic and concentrated with aggressive ions, leading to rapid, localized metal loss.

Mitigation of Localized Corrosion

Preventing pitting and crevice corrosion involves careful material selection (e.g., using alloys resistant to specific environments), designing structures to eliminate crevices and stagnant pooling, maintaining clean surfaces, and applying effective cathodic protection, which can suppress the anodic reactions even within pits and crevices if sufficient current can reach those areas.