Typically, metal corrosion becomes socially problematic not in harsh acidic or alkali environments but in human habitable atmospheric (humidity), temperature, ravely, rarely corrosive gases, airborne dusts, dissolved Oxyzen in water, free chlorine for disinfection, and by water dissolved elements such as Ca and Mg.

Let us think about metal corrosion in some general terms. Suppose We throw a piece of Polished metal in a corrosive environment. Depending on the metal type and the corrosive environment, the following changes would occur on the metal surface.

(1) No change on the surface

This means that no byproduct of corrosion was formed on the metal surface, or the surface deposits were in nanometer order thin, and the corrosion would not advance any further.

(2) Discolors but no further change

The corrosion byproduct has grown to become visible to the eyes, covering the surface densely but the corrosion will not progress further. The discoloration is of tens of nanometer thickness.

(3) Rough rusts are generated and the corrosion advances

The corrosion byproducts do not adhere tightly to the surface, causing the surface to be constantly exposed to the corrosion causing environment promoting the corrosion progress. The generated rust flakes off. This is the corrosion symptom for mild steel objects exposed to outdoor rain and wind. For coastal areas, salts would cause more severe rusts.

(4) No rusts evident but the corrosion advances

When the corrosion byproducts dissolve into the environment, the rust will not be visible but the corrosion will continue to progress and the metal will continue to be depleted. This is the case of metals in acids.

As seen above, there are different types of corrosion progression. Corrosion types can be categorized into:General Corrosion where the corrosion occurs on the full surface uniformly, and Local Corrosion where corrosion occurs in regional concentrations.
However, in a strict aspect, uniform corrosion cannot occur unless the Metal composition and structure are uniform and the environment is also uniformly corrosive. In reality, some localization of corrosion is occurring.

Corrosion of steel in water or high temperature high temperature gases occur almost uniformly, and are called General Corrosion, in an approximation term. The index used to indicate the magnitude of uniform corrosion are "Corrosivity" 25mg/dm²/day (25mg of corrosion per 1dm² per day, and 0.12mm/y (0.12mm thick corrosion per year).