17.6 Corrosion

Corrosion and Its Prevention

Corrosion is an electrochemical process where metals gradually lose electrons and break down. It's the reason iron rusts, bridges need repainting, and ships require constant maintenance. Since corrosion is electrochemistry in action, the same principles you've learned about oxidation, reduction, and electrochemical cells apply directly here.

Electrochemical Process of Corrosion

Corrosion happens when a metal is oxidized, meaning it loses electrons. The most familiar example is iron rusting. Here's how that process works step by step:

1. At the anode (the spot on the metal surface where corrosion starts), iron is oxidized: $Fe \rightarrow Fe^{2+} + 2e^-$

2. At the cathode (a nearby spot on the same surface), oxygen is reduced: $O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$

3. The overall reaction produces iron(II) hydroxide: $2Fe + O_2 + 2H_2O \rightarrow 2Fe(OH)_2$

4. That $Fe(OH)_2$ reacts further with oxygen to form hydrated iron(III) oxide, $Fe_2O_3 \cdot xH_2O$, which is the flaky reddish-brown substance we call rust.

Notice that the anode and cathode can exist on the same piece of metal. A scratch, impurity, or difference in oxygen exposure can create distinct anodic and cathodic regions on one surface.

For corrosion to occur, you need all four of these components:

• Anode — the site where the metal is oxidized and metal ions are released
• Cathode — the site where a substance (usually $O_2$) is reduced
• Electrolyte — a solution (like water with dissolved salts) that allows ions to move between the anode and cathode
• Electrical connection — a path for electrons to flow from anode to cathode (often just the metal itself)

Remove any one of these four, and corrosion stops. That's the whole basis for prevention strategies.

Methods of Corrosion Prevention

Protective coatings work by placing a physical barrier between the metal and its environment.

• Common coatings include paint, plastic, and ceramic layers.
• They block oxygen and moisture from reaching the metal surface, which removes the electrolyte from the equation.
• The weakness: if the coating gets scratched or chipped, the exposed metal corrodes rapidly because moisture can now reach it.

Alloying means mixing a metal with other elements to change its chemical properties.

• Stainless steel is iron alloyed with chromium (and often nickel). The chromium reacts with oxygen to form a thin, invisible oxide layer that protects the iron underneath.
• Brass is copper alloyed with zinc, which resists corrosion better than pure copper in many environments.
• Alloying provides built-in protection that doesn't chip or wear off, but alloyed metals tend to cost more and the added elements can change properties like strength and ductility.

Cathodic protection forces the metal you want to protect to act as the cathode, so it gains electrons instead of losing them. There are two types:

• Sacrificial anode method — A more reactive metal (like zinc or magnesium) is physically connected to the metal being protected. The more reactive metal corrodes instead, "sacrificing" itself. This is why zinc blocks are bolted to ship hulls.
• Impressed current method — An external power source pushes electrons onto the protected metal, forcing it to be the cathode. This is common for buried pipelines and large storage tanks.

The choice of sacrificial anode is based on the galvanic series, which ranks metals by their tendency to be oxidized. You pick a metal that's more easily oxidized than the one you're protecting.

Effectiveness of Prevention Techniques

Each method has trade-offs depending on the situation:

• Protective coatings are versatile and work on everything from small tools to bridges. They're relatively cheap but require regular maintenance. Once the coating fails, corrosion begins quickly.
• Alloying gives corrosion resistance that's part of the metal itself, making it ideal for harsh environments where coatings would break down. The downside is higher material cost.
• Cathodic protection is best for large structures like pipelines, ship hulls, and storage tanks.
  • Sacrificial anodes are simple and need no external power, but they corrode away and must be replaced periodically.
  • Impressed current systems are more complex and need a constant power supply, but they offer adjustable, long-lasting protection.
  • Both types work best in highly conductive environments (like saltwater). They're less effective in freshwater or dry soil where ions can't move easily.

Types of Corrosion

Not all corrosion looks the same. Recognizing the type helps you choose the right prevention method.

• Uniform corrosion — Even degradation spread across the entire metal surface. This is the most common and most predictable type.
• Localized corrosion — Concentrated damage in specific areas, which can be more dangerous because it's harder to detect:
  • Pitting — Small holes or cavities form on the surface. These can penetrate deep into the metal while the surrounding area looks fine.
  • Stress corrosion cracking — Occurs when tensile stress (pulling force) and a corrosive environment combine to produce cracks. This can cause sudden, catastrophic failure.
• Passivation — Some metals, like aluminum and chromium, naturally form a thin, stable oxide layer on their surface that actually protects them from further corrosion. This is why aluminum doesn't rust away like iron, even though aluminum is technically more reactive.