Why Things Corrode: The Basics

Why this matters

Corrosion is the slow chemical destruction of metal, and it is behind a huge share of field failures: rusted fasteners, eaten-through pipe, green terminals, pitted heat exchangers, seized hardware. If you understand the handful of mechanisms that drive it, you can spot where it will happen next, choose materials and protection that last, and stop blaming "old age" for failures you could have prevented. Corrosion is not random; it follows rules.

The basic mechanism: metal wants to go back

Most metals we use are refined from ores, which means we spent energy pulling them out of a stable, oxidized state. Corrosion is the metal slowly returning to that stable state. It is an electrochemical reaction: metal atoms give up electrons (oxidize) when there is somewhere for those electrons to go and an electrolyte (any conductive moisture) to carry the reaction.

That gives you the recipe for corrosion, and the recipe for stopping it. You need three things present at once:

• A metal that can oxidize (an anode).
• A place for the reaction to complete (a cathode).
• An electrolyte connecting them (water, especially salty or dirty water).

Remove any one and corrosion slows or stops. That is why a dry, clean, isolated metal part lasts, and a wet, dirty, dissimilar-metal junction rots.

Common types you will actually see

General (uniform) corrosion. The whole surface rusts evenly. Bare steel in damp air is the classic case. It is predictable and you can plan for it with coatings and thickness allowance.

Galvanic corrosion. When two different metals touch in the presence of moisture, the more reactive one corrodes faster and the less reactive one is protected. This is why a steel screw in an aluminum part rots, or copper and steel joined directly at a fitting eat one another. The bigger the difference between the two metals, the worse it gets.

Pitting. Localized attack that drills deep, narrow holes while most of the surface looks fine. Pitting is dangerous because it penetrates fast and hides; a coil or tank can leak from a pinhole while looking healthy.

Crevice corrosion. Attack that concentrates in tight gaps, under gaskets, washers, deposits, or lap joints, where stagnant moisture and low oxygen create an aggressive pocket. Anywhere two surfaces almost touch is a crevice risk.

Stress-related corrosion. Metal under tension, plus a corrosive environment, can crack in ways neither stress nor corrosion would alone. Bent, over-torqued, or constantly loaded parts in a wet, salty, or chemically harsh spot are candidates.

What makes it faster

A few conditions reliably accelerate every type:

• Moisture, and worse, salt or contaminants in the moisture. Salt makes water far more conductive, so coastal and road-salt environments corrode dramatically faster.
• Heat. Most chemical reactions speed up with temperature, so hot, wet components corrode faster than cool ones.
• Dissimilar metals in contact. The galvanic couple is one of the most common avoidable causes.
• Stagnant water and trapped debris. Standing water and dirt hold moisture against the metal and create crevices.
• Stray electrical current. A small leakage current through a damp path can drive corrosion electrically, eating metal where the current exits.

How to slow it down

You do not need exotic measures; you need to break the recipe.

• Keep it dry and draining. Slope so water runs off, drill weep paths, and do not let debris dam moisture against metal.
• Isolate dissimilar metals. Use a non-conductive separator, a compatible transition fitting, or matched materials so you never directly couple a reactive metal to a noble one in the wet.
• Coat the surface. Paint, plating, galvanizing, and grease all work by keeping the electrolyte off the metal. A coating only protects while it is intact; a scratch concentrates the attack.
• Use sacrificial protection where appropriate. A more reactive metal deliberately added to a system corrodes first and protects the rest. This is standard practice in many tanks and buried or submerged systems.
• Pick the right material for the environment. In wet, salty, or chemical settings, corrosion-resistant alloys or non-metals earn their higher cost by lasting.

Reading corrosion as a diagnostic clue

Where corrosion shows up tells you what went wrong. Green or white powder at an electrical terminal means moisture got in and the connection is now high-resistance. Rust streaks below a joint mean water is sitting there. A corroded fastener of one metal in a part of another metal is a galvanic couple someone should not have built. A pinhole leak in otherwise-clean metal is pitting, often from water chemistry or trapped deposits. Treat corrosion as evidence, not just damage, and it points you at the moisture path or material mismatch that caused it.

References

• Trade-standard materials and corrosion-control practice
• Manufacturer material-compatibility and installation documentation
• See related: Why Electrical Connections Degrade; Why Seals Leak: The Physics
• See related: Why Things Overheat: The Basics