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METAMORPHOSIS #1005_3/ 2005
(  Satoshi Kinoshita )
| Series: | Paintings: Landscape | | Medium: | "iron rust" on non-stretched canvas | | Size (inches): | 47 x 24 | | Size (mm): | 914 x 610 | | Catalog #: | PA_088 | | Description: | Signed, titled, date, copyright in magic ink on the reverse.
Iron + oxygen + water = Iron rust.
How does rust work?
Rust is the common name for a very common compound, iron oxide. Iron oxide, the chemical Fe2O3, is common because iron combines very readily with oxygen -- so readily, in fact, that pure iron is only rarely found in nature. Iron (or steel) rusting is an example of corrosion -- an electrochemical process involving an anode (a piece of metal that readily gives up electrons), an electrolyte (a liquid that helps electrons move) and a cathode (a piece of metal that readily accepts electrons). When a piece of metal corrodes, the electrolyte helps provide oxygen to the anode. As oxygen combines with the metal, electrons are liberated. When they flow through the electrolyte to the cathode, the metal of the anode disappears, swept away by the electrical flow or converted into metal cations in a form such as rust.
For iron to become iron oxide, three things are required: iron, water and oxygen. Here's what happens when the three get together:
When a drop of water hits an iron object, two things begin to happen almost immediately. First, the water, a good electrolyte, combines with carbon dioxide in the air to form a weak carbonic acid, an even better electrolyte. As the acid is formed and the iron dissolved, some of the water will begin to break down into its component pieces -- hydrogen and oxygen. The free oxygen and dissolved iron bond into iron oxide, in the process freeing electrons. The electrons liberated from the anode portion of the iron flow to the cathode, which may be a piece of a metal less electrically reactive than iron, or another point on the piece of iron itself.
The chemical compounds found in liquids like acid rain, seawater and the salt-loaded spray from snow-belt roads make them better electrolytes than pure water, allowing their presence to speed the process of rusting on iron and other forms of corrosion on other metals.
-science.howstuffworks.com
Rust Chemistry:
Rusting of iron consists of the formation of hydrated oxide, Fe(OH)3, FeO(OH), or even Fe2O3.H2O. It is an electrochemical process which requires the presence of water, oxygen and an electrolyte. In the absence of any one of these rusting does not occur to any significant extent. In air, a relative humidity of over 50% provides the necessary amount of water and at 80% or above corrosion of bare steel is worse.
When a droplet of water containing a little dissolved oxygen falls on an steel pipe, the solid iron or Fe(s) under the droplet oxidizes:
Fe(s) --> Fe2+(aq) + 2e-
The electrons are quickly consumed by hydrogen ions from water (H2O) and dissolved oxygen or O2(aq) at the edge of the droplet to produce water:
4e- + 4H+(aq) + O2(aq) --> 2H2O(l)
More acidic water increases corrosion. If the pH is very low the hydrogen ions will consume the electrons anyway, making hydrogen gas instead of water:
2H+(aq) + 2e- --> H2(g)
But where's the rust? The equations above tell only a small part of the story.
Hydrogen ions are being consumed by the process. As the iron corrodes, the pH in the droplet rises. Hydroxide ions (OH-) appear in water as the hydrogen ion concentration falls. They react with the iron(II) ions to produce insoluble iron(II) hydroxides or green rust:
Fe2+(aq) + 2OH-(aq) --> Fe(OH)2(s)
The iron(II) ions also react with hydrogen ions and oxygen to produce iron(III) ions:
4Fe2+(aq) + 4H+(aq) + O2(aq) --> 4Fe3+(aq) + 2H2O(l)
The iron(III) ions react with hydroxide ions to produce hydrated iron(III) oxides (also known as iron(III) hydroxides):
Fe3+(aq) + 3OH-(aq) --> Fe(OH)3(s)
The loose porous rust or Fe(OH)3 can slowly transform into a crystallized form written as Fe2O3.H2O the familiar red-brown stuff that is called "rust". Since these processes involve hydrogen ions or hydroxide ions, they will be affected by changes in pH. With limited O2, magnetite is formed (Fe3O4).
If other ions like calcium or carbonate are present, they make a variety of precipitates that mix in with the iron hydroxide to produce a crusty, twisted coating which can either slow corrosion by cutting the iron off from the acid, water, and air supply or grow into convoluted shapes called tubercles. The growth of these tubercles can greatly affect the flow of water through water mains as shown here.
-www.corrosion-doctors.org
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