Passivation is the process of making a material "passive", usually by the deposition of a layer of oxide that adheres to the metal surface. In air, passivation affects the properties of almost all metals–notable examples being aluminium, zinc, titanium, and silicon (a metalloid). In the context of corrosion, passivation is the spontaneous formation of a hard non-reactive surface film that inhibits further corrosion. This layer is usually an oxide or nitride that is a few nanometers thick.
The conditions necessary for passivation are recorded in Pourbaix diagrams. Some corrosion inhibitors help the formation of a passivation layer on the surface of the metals to which they are applied. Some compounds, dissolving in solutions (chromates, molybdates) form non-reactive and low solubility films on metal surfaces.
In the area of microelectronics, the formation of a strongly adhering passivating oxide is important to the performance of silicon.
In the area of photovoltaics, a passivating surface layer such as Silicon Nitride, Silicon Dioxide or Titanium Dioxide can reduce surface recombination - a significant loss mechanism in solar cells.
Pure aluminium naturally forms a thin surface layer of aluminium oxide on contact with oxygen in the atmosphere through a process called oxidation, which creates a physical barrier to corrosion or further oxidation in most environments. Aluminium alloys, however, offer little protection against corrosion. There are three main ways to passivate these alloys: alclading, chromate conversion coating and anodizing. Alclading is the process of metallurgically bonding a thin layer of pure aluminium to the aluminium alloy. Chromate conversion coating is a common way of passivating not only aluminum, but also zinc, cadmium, copper, silver, magnesium, and tin alloys. Anodizing forms a thick oxide coating. This finish is more robust than the other processes and also provides good electrical insulation, which the other two processes do not.
For example, prior to storing hydrogen peroxide in an aluminium container, the container can be passivated by rinsing it with a dilute solution of nitric acid and peroxide alternating with deionized water. The nitric acid and peroxide oxidizes and dissolves any impurities on the inner surface of the container, and the deionized water rinses away the acid and oxidized impurities.
Ferrous materials, including steel, may be somewhat protected by promoting oxidation ("rust") and then converting the oxidation to a metalophosphate by using phosphoric acid and further protected by surface coating. As the uncoated surface is water-soluble a preferred method is to form manganese or zinc compounds by a process commonly known as Parkerizing or phosphate conversion. Older, less-effective but chemically-similar electrochemical conversion coatings included black oxiding, historically known as bluing or browning. Ordinary steel forms a passivating layer in alkali environments, as rebar does in concrete.
A typical passivation process of cleaning stainless steel tanks involves cleaning with sodium hydroxide and citric acid followed by nitric acid (up to 20% at 120 °F) and a complete water rinse. This process will restore the film, remove metal particles, dirt, and welding-generated compounds (e.g. oxides).
- ASTM A967: Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts
- Chromate conversion coating (chemical film) per MIL-DTL-5541F for aluminium and aluminium alloy parts
- A standard overview on black oxide coatings is provided in MIL-HDBK-205, Phosphate & Black Oxide Coating of Ferrous Metals. Many of the specifics of Black Oxide coatings may be found in MIL-DTL-13924 (formerly MIL-C-13924). This Mil-Spec document additionally identifies various classes of Black Oxide coatings, for use in a variety of purposes for protecting ferrous metals against rust.
- Budinski, Kenneth G. (1988), Surface Engineering for Wear Resistance, Englewood Cliffs, New Jersey: Prentice Hall.
- Brimi, Marjorie A. (1965), Electrofinishing, New York, New York: American Elsevier Publishing Company, Inc.