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Why metal rusts, and how to protect it
Metal deteriorates over time when exposed to oxygen, moisture and chemical attack. Understanding corrosion and being able to protect against it is an important part of the metal fabricator's trade because it determines how long a structure lasts.
Corrosion is an electrochemical process where metal reacts with its surroundings and breaks down. On ordinary steel it appears as rust. Moisture and oxygen are the typical drivers and salt and pollution accelerate the process. Different metals corrode differently — and when two different metals are in contact in a moist environment one can accelerate the breakdown of the other.
A widespread strategy is to place a protective layer between the metal and the surroundings. Galvanizing coats steel with zinc that both forms a barrier and sacrifices itself for the steel. Paint and powder coating form a barrier against moisture and oxygen. The choice depends on environment, appearance and longevity requirements.
Another way is to choose a material that in itself resists corrosion. Stainless steel forms a self-healing protective layer, and aluminium is protected by its oxide layer. Here the protection lies in the material and does not require a subsequent coating — but the materials are typically more expensive.
Regardless of method, preparation is crucial. A coating adheres well only to a clean surface. Rust, mill scale, grease and dirt must be removed by, for example, grinding, sandblasting or degreasing before painting or coating. A poorly prepared surface results in a coating that quickly fails.
Corrosion protection is not something that happens only at the end. The structure's design — whether water can run off, whether there are pockets where moisture accumulates and how metals are combined — contributes to how exposed it is. The good metal fabricator thinks durability into the design.