Some metals corrode rapidly only until a thin reaction-product film forms. If that film strongly suppresses further dissolution, the surface is passive.

Aluminum, chromium-containing stainless steels and titanium rely heavily on passive oxide films. Passivity is dynamic rather than absolute: the film can dissolve and reform, and its stability depends on environment and electrochemical potential.

A passive alloy can therefore have excellent general corrosion resistance yet remain vulnerable to localized corrosion.

• Pitting begins when the passive film breaks down at a small site. The pit environment can become chemically different from the bulk solution, allowing rapid local attack while most of the surface remains passive.
• Crevice corrosion develops in shielded gaps where transport is restricted and local chemistry diverges from the surroundings.
• Stress-corrosion cracking couples a susceptible material, tensile stress and a particular corrosive environment to produce crack growth that neither stress nor environment would cause as rapidly alone.

Localized attack is dangerous because average mass loss can be small while penetration becomes deep. Material selection therefore requires asking not only 'how fast does the surface corrode on average?' but also whether the alloy/environment combination permits passive-film breakdown and self-sustaining local chemistry.