Passivation

Passivation is the process of making a material "passive" in relation to another material prior to using the materials together. 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. Another typical passivation process of cleaning stainless steel tanks involves cleaning with NaOH and citric acid followed by nitric acid (up to 20% at 120F) and a complete water rinse. This process will restore the film, remove metal particles, dirt, and welding generated compounds (ex. oxides).

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 atoms thick.

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Mechanisms of passivation

Under normal conditions of pH and oxygen concentration, passivation is seen in such materials as aluminium, iron, zinc, magnesium, copper, stainless steel, titanium, and silicon. Ordinary steel can form a passivating layer in alkali environments, as rebar does in concrete. 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.

Electrochemical passivation processes

Some compounds, dissolving in solutions (chromates, molybdates) form non-reactive and low solubility films on metal surfaces.

Passivation of specific materials

Aluminum may be protected from oxidation by anodizing and/or allodizing (sometimes called Alodining), or any of an assortment of similar processes. (See terminology, below.) In addition, stacked passivation techniques are often used for protecting aluminum. For example, chromating is often used as a sealant to a previously-anodized surface, to increase resistance to salt-water exposure of aluminum parts by nearly a factor of 2 versus simply relying on anodizing.

Iron based (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. Older, less-effective but chemically-similar electrochemical conversion coatings included bluing, also known as black oxide.

Nickel can be used for handling elemental fluorine, thanks to a passivation layer of nickel fluoride.

Terminology for assorted passivation processes

Bluing, also known as black oxide, and sometimes called browning when used in reference to historical processes dating from the 18th Century, is a passivation coating for the surfaces of iron and steel objects. It is one of the oldest passivation processes.

Newer, proprietary (and/or trademarked) processes for conversion coatings include Parkerized for passivating steel, dating to roughly 1912, and Alodine for passivating aluminum; both are trademarked processes and are now owned by Henkle Surface Technologies. [1],[2]

Chem film is any generic chromate conversion coating used to passivate aluminium. One such example is U.S. Patent 5,304,257 . In general, however, chromate can also mean any of several chromate conversion coatings that can be applied to a much wider range of metals and alloys than just to aluminum. In recent years, chromate coatings have become less popular due to concerns over environmental pollution from using such processes.

Iridite is another trademarked name of a whole family of proprietary conversion coatings owned by MacDermid. A competing conversion coating used on aluminum, that somewhat ameliorates the environmental pollution concerns caused by chromate coatings, it often appears as a slightly yellowish coating, of roughly the same color as a yellow highlighting pen used to mark text on paper. [3]

Rationale for passivating aluminum

Aluminum naturally forms an oxide almost immediately that protects it from further oxidation in many environments. Unfortunately, this naturally-occurring oxide provides no protection during exposure to any saltwater spray environments, such as occurs in areas near bodies of saltwater. In such coastal environments, unprotected aluminum will turn white, corrode, and largely vanish over periods of exposure as short as a few years. The only way to prevent this from occurring is to use a more robust conversion coating on aluminum surfaces that will not be affected by the saltwater atmosphere. Alodine, Iridite, and chem film coatings can provide varying amounts of protection for aluminum surfaces.

Further reading

ASTM A967 provides advice for the passivation of stainless steel parts.

Chromate Conversion Coat (Chemical Film) Per MIL-C-5541 For Aluminum and Aluminum Alloy Parts

References

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), written at Englewood Cliffs, New Jersey, , Prentice Hall
• Brimi, Marjorie A. (1965), written at New York, New York, , American Elsevier Publishing Company, Inc..