Iron continued rusting

In some cases, the oxide-coating protects the surface from further oxide buildup. One example is that of aluminum where an oxide coating appears almost instantaneously once the pristine surface is exposed to air. Yet, there are many cases where the oxide layer continues to buildup until the metal is totally consumed (One example is that of iron and "rust"). How is this possible Wagner hypothesized that both metal and oxide ions difiosed through the metal oxide layer so as to build up the layer thickness from both sides. The following diagram is one representation of such a mechanism ... 

But there was more to the universe than its oneness. There was, for instance, the bewildering diversity of its material phenomena, its rocks and plants and animals, its processes of growth and decay, of corrosion and combustion, of reactions between some substances and the complete lack of reaction between others. Here one encountered selective cohesion as well as limited cohesion. A piece of iron will rust and continue to rust, up to a certain point. Beyond that point no further combination with the air occurs. The process is complete. Not all chemical changes follow this pattern, but sufficiently many do to attract attention. 

When aluminum metal is exposed to air, a protective layer of aluminum oxide (AI2O3) forms on its surface. This layer prevents further reaction between aluminum and oxygen, and it is the reason that aluminum beverage cans do not corrode. (In the case of iron, the rust, or iron(III) oxide, that forms is too porous to protect the iron metal underneath, so rusting continues.) Write a balanced equation for the formation of AI2O3. 

The continuing loss of surface iron as rust forms and then flakes off eventually causes structural weakness. 

Why does iron continue to corrode even after a layer of rust forms on the surface, whereas aluminum stops corroding after a corrosion layer forms ... 

Why does a chemical reaction go naturally in a particular direction To answer this question, we need to look at spontaneous processes. A spontaneous process is a physical or chemical change that occurs by itself. It requires no continuing outside agency to make it happen. A rock at the top of a hill rolls down (Figure 19.4, top). Heat flows from a hot object to a cold one. An iron object rusts in moist air (Figure 19.5). These... 

Provided there is a suitable excess of chromium over carbon in the alloy, the irons will not rust when exposed to the atmosphere in the as-cast state. Alloys which have been found to tarnish in the as-cast state because of an inadequate excess of chromium may be found to be completely stainless in the machined and polished state, presumably because a thin film is more likely to be continuous on a smooth surface than on a rough one. 

Another way to protect a metal uses an impervious metal oxide layer. This process is known as passivation, hi some cases, passivation is a natural process. Aluminum oxidizes readily in air, but the result of oxidation is a thin protective layer of AI2 O3 through which O2 cannot readily penetrate. Aluminum oxide adheres to the surface of unoxidized aluminum, protecting the metal from further reaction with O2. Passivation is not effective for iron, because iron oxide is porous and does not adhere well to the metal. Rust continually flakes off the surface of the metal, exposing fresh iron to the atmosphere. Alloying iron with nickel or chromium, whose oxides adhere well to metal surfaces, can be used to prevent corrosion. For example, stainless steel contains as much as 17% chromium and 10% nickel, whose oxides adhere to the metal surface and prevent corrosion. 

Solids are generally considered chemically inert at room temperatures and the most common-place evidence is often overlooked. That is, solids do not appear to be reactive until they are heated. However, the atoms or ions comprising solids are under constant vibratory motion with the lattice and can "diffuse" from site to site. If vacancies are present, they are continually being "fQled" and "emptied" even at room temperature. Those solids based upon Iron (Fe) undergo continuous oxidation to form a layer of "rust". Thus, solids are not completely stable and are under continuous change over time. 

The term pyrophoric has usually been applied to the ignition of very fine sizes of metal particles. Except for the noble metals, most metals when refined and exposed to air form an oxide coat. Generally this coating thickness is of the order of 25 A. If the oxide coat formed is of greater size than that of the pure metal consumed, then the coat scales and the nascent metal is prone to continuously oxidize. Iron is a case in point and is the reason pure iron rusts. 

Corrosion occurs when the metallic iron in DRI is wetted with fresh or salt water and reacts with oxygen from air to form rust, Fe(OH)3. The corrosion reactions continue as long as water is present. Because water evaporates at approximately 100°C, corrosion reactions have a low temperature limit even though the reactions are exothermic. Small amounts of hydrogen may be generated when DRI reacts with water. However, this poses no safety problem as long as proper ventilation is provided. 

Another problem associated with upholstered furniture is the use of iron upholstery tacks, which will rust and thereby cause large holes in the upholstery fabric (Plate 9). Modem upholsters find that the use of the magnetic tacking hammer hastens production and, therefore, continue... 

PLEX HT is a blend of chelating agents which has the ability to dissolve iron rust from cotton and cotton blend fabrics in strong caustic solutions such as are used in saturation of cotton fabric prior to continuous peroxide bleaching. It also has the ability to keep the chelated iron from redeposition as the pH drops during subsequent rinsing. 

PLEX HT works effectively to remove iron rust even in the peroxide bleach bath in batch processes such as bleaching prior to dyeing on becks and on jets. It also works well in the bleach bath on continuous one stage machines such as the Jemco range and the Argarthen machine. 

Once beautiful and sturdy, mighty iron can turn into a mass of rust as a result of the action of air and water. First, iron reacts with oxygen in the presence of water to form FeO. In FeO, iron s oxidation number is 2+ because it has lost Its two 4s electrons. Then, FeO continues to combine with oxygen to form the familiar orange-brown compound, Fe203. In this oxide, iron s oxidation number is 3-1-because it has lost two 4s electrons and one 3d electron. 

Pure forms of this metal are rarely found in nature because it combines easily with water and air to form rust, a hydrated oxide of iron. Rust s reddish material does not stick to the iron s surface for long. It crumbles off, continuously exposing new layers of fresh iron to the air. This weakens the iron, causing it to eventually disintegrate. 

The presence of the electrolyte is required to provide a pathway for the current and, in urban areas, this is commonly iron(II) sulfate formed as a result of attack by atmospheric SO2 but, in seaside areas, airborne particles of salt are important. Because of its electrochemical nature, rusting may continue for long periods at a more or less constant rate, in contrast to the formation of an anhydrous oxide coating which under dry conditions slows down rapidly as the coating thickens. 

Iron forms also an oxide layer (better known as rust) on the surface but on the contrary to aluminium oxide the layer does not have the same adhesion and thereby peels off Hereby new iron metal may be exposed to oxygen and water whereby the corrosion process may continue. This is the subject for the following example ... 

Chromium plated steel (steel is mostly iron) is often used in motor car bumpers. If the bumper is scratched, will the chromium continue to protect the bumper against rusting ... 

Some reactions go virtually to what we call completion—the conversion of such a large quantity of the reactants to products that what is unconverted is not noticeable and is unimportant. The combination of hydrogen and oxygen to form water is a reaction of this kind. Once a spark has gotten the first few molecules over the activation energy hill, the reaction continues rapidly and explosively until one or both reactants are used up. A slow reaction can also go to completion. As time passes, an iron nail exposed to the atmosphere continues to rust away gradually, until only the rust remains. 

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