Nature's Invisible Shield
The remarkable corrosion resistance of stainless steel, aluminum, and titanium comes not from the bulk metal but from an invisible oxide film just 1-10 nanometers thick. This passivation layer forms spontaneously when the metal contacts air or water, creating a barrier that reduces dissolution rates by three to six orders of magnitude. The film is self-healing — scratch it, and it reforms within milliseconds in oxidizing environments.
Chromium: The Essential Ingredient
For stainless steels, chromium is the critical alloying element. Below approximately 10.5% Cr, the surface oxide is patchy and non-protective. Above this threshold, a continuous Cr₂O₃-rich film forms that is both dense and adherent. Each additional percent of chromium widens the passive potential range and increases the breakdown potential, which is why super-austenitic stainless steels contain up to 25% Cr for severe service.
Film Growth and the High-Field Model
Passive film growth follows the high-field model: the electric field across the thin film drives metal ions through the oxide lattice. Film thickness increases logarithmically with applied potential — a result of the exponential field-dependent ion migration rate. This self-limiting growth mechanism explains why passive films remain extremely thin even at high potentials: as the film thickens, the field drops and growth slows to a virtual stop.
Breakdown and Pitting
The Achilles heel of passivation is localized breakdown. Chloride ions preferentially attack the film at microstructural weak points — MnS inclusions, grain boundaries, or mechanical defects. Once a pit initiates, the local chemistry inside becomes acidic and chloride-enriched, preventing repassivation and driving rapid local attack. This simulation lets you explore how chromium content, pH, potential, and chloride concentration compete to determine whether the film survives.