Everything about Passivation totally explained
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 (for example 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's a few atoms thick.
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're 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
Aluminium 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 aluminium. For example, chromating is often used as a sealant to a previously-anodized surface, to increase resistance to salt-water exposure of aluminium 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 aluminium; both are trademarked processes and are now owned by Henkel Surface Technologies. (External Link
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Chem film is any generic chromate conversion coating used to passivate aluminium. One such example is
. 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 aluminium. 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 aluminium, 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. (External Link)
Rationale for passivating aluminium
Aluminium naturally forms an oxide almost immediately that protects it from further oxidation in many environments. 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 aluminium 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 aluminium surfaces that won't be affected by the saltwater atmosphere. Alodine, Iridite, and chem film coatings can provide varying amounts of protection for aluminium surfaces.
Further Information
Get more info on 'Passivation'.
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