ARCCA Expert explains: What Is Corrosion And Why Does It Happen?
Ask An Expert – Corrosion
An ARCCA expert explains corrosion, how it causes failures, and how an expert investigates corrosion cases.
Recent headlines have described “Extensive Corrosion” found in the remains of the Miami-era residential building that collapsed in June. But what is corrosion, and why would it be important in this high-profile building collapse?
Corrosion is a subject that rarely gets attention from the general public. However, corrosion costs 3-4% of the world’s GDP worth of damage every year to plumbing, pipelines, transportation infrastructure, vehicles, and more. It is important to understand what corrosion is, why it occurs, and how an expert investigates corrosion-related failures.
What Is Corrosion And Why Does It Happen?
Corrosion is the destructive chemical attack of a material by its environment, often damage to a metal exposed to moisture. In the case of metals, corrosion is an electrochemical process. When the atoms in a metal lose some of their electrons, they can then dissolve in water or react with other substances to form new compounds. Corrosion of a metal often involves exposure to water, because the water facilitates both the electrical and chemical steps in the corrosion process.
Figure 1: A steel bin full of rusted metal scraps.
Most people are familiar with corrosion in the form of rusted steel. However, many other materials other than steel can corrode, and the presence of rust is just one possible symptom.
Corrosion takes many forms depending on the materials involved and the environment around them. General corrosion involves removal of large patches of material. Pitting corrosion involves the creation of narrow, deep pits. Corrosion can attack narrow gaps between parts. It can follow the flow patterns of fluid in a pipe. It can even selectively leach certain metals from an alloy, leaving holes behind. Whether something corrodes, and how fast it happens, depends on a wide range of factors. Substances in the environment, temperature, pressure, flow rates, and much more can be important when analyzing a corrosion failure.
Corrosion Failures
Selective corrosion of the zinc in a brass fitting can lead to leaks in plumbing. Mixing different metals when assembling a railing can cause Galvanic corrosion, like in this case where several people were injured at a stadium. Corrosion of electrical components can cause short circuits and other failures. Normally corrosion-resistant stainless steel parts might corrode and crack near a pool where the air is warm, humid, and contains chlorine.
Analyzing a corrosion failure involves accounting for how the damaged material interacted with its environment. Consider a steel pipe-based fire sprinkler system (Figure 2). Indoors, where it was dry, there was no visible corrosion. Outdoors under an awning where it was
protected from the rain but not condensation or humidity, only the fittings corroded. In a more exposed area, both the pipe and fittings showed patches of rust. And that was just the exterior of the pipe. The interior, which was exposed to a different environment, would exhibit different corrosion if inspected.
Figure 2: Whether or not a material corrodes, how quickly it corrodes, and to what extent depend on both the material and the environment
 |  |  |
|---|---|---|
| Indoors | Outdoors Under Cover | Outdoors Exposed |
Not Just Metals
Polymers (plastics) and ceramics may not corrode in the same way as metals, but they still age, degrade, and ultimately fail over time. Polymer pipes can degrade when exposed to certain wavelengths of light or with continued exposure to disinfectants used in drinking water, ultimately leading to failure. Many polymers that are stable in air can degrade rapidly in oxygen-rich environments, like a medical oxygen supply, cutting/welding torch, or a space vehicle. Even ceramics like glasses, dental ceramics, and carbides and nitrides used as protective coatings can be attacked by certain substances.
Not every material shares the same vulnerabilities. Just as with metal corrosion, failure analysis of corroded polymers or ceramics involves considering the material, how it failed, and the environment it was exposed to.
Talk to a Corrosion Expert
This post is just a brief introduction to corrosion, but it is a topic that affects every industry. Determining the root cause of a failure involving corrosion requires knowledge of the materials involved, their properties and vulnerabilities, and the environment that they were exposed to.
Contact us to talk to an expert today.
