A state in which a dense oxide film a few nanometers thick forms on a metal surface and dissolution of a metal that should thermodynamically dissolve almost stops.
Japanese name: 不動態
Passivity appears in 7 practice questions across 3 topics, including Biomaterials, Corrosion and surface treatment, Materials processing and joining.
Iron, chromium, nickel, titanium, aluminum and other metals can, under the right conditions, form dense oxide (or hydroxide) films a few nanometers thick on their surfaces, reducing the dissolution rate by several orders of magnitude. This state is passivity. Even for metals that are active in potential, the film separates the metal from the environment, so actual corrosion becomes very slight. In stainless steel a Cr-rich film plays this role, and in titanium a TiO₂ film. The film is thin and invisible, so the metallic luster is preserved.
In environments containing oxygen or oxidizers, the passive film re-forms spontaneously even when scratched. Iron, for example, becomes passive in concentrated nitric acid and hardly dissolves. On the other hand, in oxygen-poor crevices or reducing acids such as hydrochloric acid, the film re-forms with difficulty, and once broken, active dissolution proceeds. Chloride ions in particular break the film locally at weak points, so pitting and crevice corrosion are problems for stainless steel in seawater. For passive materials, localized corrosion requires more attention than uniform corrosion.
In a polarization curve, which measures the dissolution current while the metal's potential is raised step by step, the current first increases with potential, but above a certain potential (the passivation potential) it drops abruptly and stays small over a wide range of potential. This range is the passive region. Raising the potential further reveals the transpassive region, where the film breaks down and the current rises again, or, with chlorides present, the pitting potential. The higher the pitting potential, the more resistant the material is to pitting, and it is used as an index for selecting and evaluating grades. Even for the same material, the pitting potential falls as temperature and chloride concentration rise.
Answer: C. It contains about 11% or more Cr, forming a passive film a few nanometers thick that re-forms by itself
Because stainless steel contains roughly 11% Cr or more (10.5% by the standard definition), a passive film consisting mainly of Cr oxide, only a few nanometers thick, forms on the surface and prevents further dissolution of the metal. What matters is that in environments containing oxygen the film spontaneously re-forms even when scratched. Passivity is therefore readily maintained in oxidizing environments such as nitric acid, but breaks down easily in oxygen-poor environments, chloride-rich environments and reducing acids (such as hydrochloric acid). The potential of the alloy as a whole does not become as noble as gold or platinum; the main role of Ni is to stabilize the austenitic structure. A thick oxide scale several millimeters thick forms by high-temperature oxidation, cracks and spalls easily, and is entirely different from a passive film.
Answer: B. Pitting corrosion; switch to a Mo-bearing grade such as SUS316
Chloride ions locally break down the passive film at weak points and cause pitting corrosion, in which small holes are dug deeper and deeper. Inside the pit, hydrolysis of the dissolved metal ions acidifies the solution, and chloride ions collect to maintain electrical neutrality, so the corrosion becomes self-accelerating. The large surrounding passive surface acts as the cathode and the bottom of the pit as a small anode, so the pit can advance rapidly to perforation. The countermeasure is to switch to a grade richer in Mo, Cr and N, which strengthen the film (such as SUS316, equivalent to AISI 316, with about 2–3% Mo); the pitting resistance equivalent number calculated from these contents is a guide for selection. Because this is not corrosion that thins the whole surface uniformly, increasing the wall thickness is not an effective countermeasure. More carbon makes Cr carbides form more readily at grain boundaries (sensitization) and actually lowers corrosion resistance.
In stainless steel it is a very thin film of a few nanometers. It is different from the thick oxide scale that forms during high-temperature oxidation.
A treatment in which stainless steel is immersed in an oxidizing solution such as nitric acid to remove surface contamination and iron particles and to ensure that a dense passive film forms.
Chloride ions break the passive film locally, causing pitting and crevice corrosion. Grades with a stronger film, such as Mo-bearing SUS316 (AISI 316), are the countermeasure.
Because stainless steel contains roughly 11% Cr or more (10.5% by the standard definition), a passive film consisting mainly of Cr oxide, only a few nanometers thick, forms on the surface and prevents further dissolution of the metal.
What is the main reason stainless steel resists rusting in …Chloride ions locally break down the passive film at weak points and cause pitting corrosion, in which small holes are dug deeper and deeper.
When SUS304 (JIS; AISI 304) is used in chloride-rich seawat…There, Cr carbides such as Cr₂₃C₆ precipitate at grain boundaries, and the Cr concentration right next to the boundaries falls to a level that cannot sustain passivity, so corrosion proceeds along the grain boundaries.
When austenitic stainless steels such as SUS304 are welded,…The titanium surface is covered by a TiO₂ passive film a few nm thick that is stable in body fluids, so bone forms along that surface.
Which statement correctly describes osseointegration, as th…Metal ion release due to damage to the passive film is a separate problem, such as fretting corrosion.
When a metal with a Young's modulus far higher than that of…Stainless steel Pitting corrosion Sensitization Young's modulus Stress shielding Heat-affected zone Martensite Osseointegration Hydroxyapatite Bone cement