Rusting reaction

We now examine three real corrosion problems the protection of pipelines, the selection of a material for a factory roof, and materials for car exhaust systems. The rusting of iron appears in all three case studies, but the best way of overcoming it differs in each. Sometimes the best thing is to change to a new material which does not rust but often economics prevent this, and ways must be found to slow down or stop the rusting reaction. 

The emf of a concentration cell (in this case, on the surface of the steel where the rusting reaction actually occurs) is given by... 

Let us consider the rusting reaction once more. Suppose that 50.0 g of iron and 40.0 g of oxygen react. Calculate the number of grams iron(III) oxide that could be formed. 

You have learned how oxidation and reduction always occur simultaneously. Think about the chemistry of corrosion you studied in Chapter 16. When iron metal reacts with oxygen, a redox reaction creates rust, iron oxide. Electrons are always transferred when a redox reaction occurs. In the rust reaction, electrons are transferred from the reducing agent, iron, to the oxidizing agent, oxygen. 

From their two measurements for the length of the column of air - before and after rusting takes place - students should be able to calculate the percentage of the air which has been removed by the rusting reaction. This should be about 20 %, which is approximately the percentage of oxygen in the air. 

Nitrogenation of calcium carbide to produce cyanamide Rusting reaction in iron Absorption of SO2 by dry limestone injection... 

Percent rust Reaction time, hr Percent polymer formed... 

This is the most important reaction of iron from an economic point of view essentially, rusting is the formation of hydrated iron(III) oxide in the presence of oxygen and water. The process is essentialh... 

Despite its electrode potential (p. 98), very pure zinc has little or no reaction with dilute acids. If impurities are present, local electrochemical cells are set up (cf the rusting of iron. p. 398) and the zinc reacts readily evolving hydrogen. Amalgamation of zinc with mercury reduces the reactivity by giving uniformity to the surface. Very pure zinc reacts readily with dilute acids if previously coated with copper by adding copper(II) sulphate ... 

A SMILES code [22], MDL Molfile [50], or JME s own compact format (one-line representation of a molecule or reaction including the 2D coordinates) of created molecules may be generated. The created SMILES is independent of the way the molecule was drawn (unique SMILES see Section 2.3.3). Extensions to JME developed in cooperation with H. Rzepa and P. Murray-Rust also allow output of molecules in the CML format [60]. 

Oxygen concentration is held almost constant by water flow outside the crevice. Thus, a differential oxygen concentration cell is created. The oxygenated water allows Reaction 2.2 to continue outside the crevice. Regions outside the crevice become cathodic, and metal dissolution ceases there. Within the crevice. Reaction 2.1 continues (Fig. 2.3). Metal ions migrating out of the crevice react with the dissolved oxygen and water to form metal hydroxides (in the case of steel, rust is formed) as in Reactions 2.3 and 2.4 ... 

Corrosion products are almost always absent in stainless steel crevices. Areas just outside stainless crevices are stained brown and orange with oxides (Figs. 2.20 and 2.21). Metal ions migrate out of the crevice. Precipitation occurs by reactions similar to Reactions 2.3 and 2.4. Crevice interiors remain relatively free of rust (Figs. 2.16 and 2.17). 

When a clean steel coupon is placed in oxygenated water, a rust layer will form quickly. Corrosion rates are initially high and decrease rapidly while the rust layer is forming. Once the oxide forms, rusting slows and the accumulated oxide retards diffusion. Thus, Reaction 5.2 slows. Eventually, nearly steady-state corrosion is achieved (Fig. 5.2). Hence, a minimum exposure period, empirically determined by the following equation, must be satisfied to obtain consistent corrosion-rate data for coupons exposed in cooling water systems (Figs. 5.2 and 5.3) ... 

A sheet of steel of thickness 0.50 mm is tinplated on both sides and subjected to a corrosive environment. During service, the tinplate becomes scratched, so that steel is exposed over 0.5% of the area of the sheet. Under these conditions it is estimated that the current consumed at the tinned surface by the oxygen-reduction reaction is 2 X 10 A m -. Will the sheet rust through within 5 years in the scratched condition The density of steel is 7.87Mg m . Assume that the steel corrodes to give Fe " ions. The atomic weight of iron is 55.9. 

Both reactions indicate that the pH at the cathode is high and at the anode low as a result of the ion migration. In principle, the aeration cell is a concentration cell of H ions, so that the anode remains free of surface films and the cathode is covered with oxide. The J U curves in Fig. 2-6 for anode and cathode are kept apart. Further oxidation of the corrosion product formed according to Eq. (4-4) occurs at a distance from the metal surface and results in a rust pustule that covers the anodic area. Figure 4-2 shows the steps in the aeration cell. The current circuit is completed on the metal side by the electron current, and on the medium side by ion migration. 

Sparks created by arcs in electrical switchgear, engines, motors, or by friction (e.g. lighter spark). Aluminium, magnesium, titanium and their alloys have an affinity for oxygen and in a thermite reaction with rust produce temperatures <3000°C. A thermite flash can result from the... 

Reaction with other substances Thermite reaction Rust Exothermic reactions with aluminium, aluminium alloys... 

Chemical Reactivity - Reactivity with Water No reaction Reactivity with Common Materials No reactions Stability During Transport Stable Neutralizing Agents for Acids and Caustics Not pertinent Polymerization Polymerization is accelerated by heat and exposure to oxygen, as well as the presence of contamination such as iron rust. Iron surfaces should be treated with an appropriate reducing agent such as sodium nitrate, before being placed into isoprene service Inhibitor of Polymerization Tertiary butyl catechol (0.06 %). Di-n-butylamine, phenyl-beta-naphthylamine andphenyl-alpha-naphthylamine are also recommended. 

An explosive decomposition in an ethylene oxide (EO) distillation column, similar in its results to that described in Section 7.3.2, may have been set off by polymerization of EO in a dead-end spot in the column base where rust, a polymerization catalyst, had accumulated. Such deadends should be avoided. However, it is more likely that a flange leaked the leaking gas ignited and heated an area of the column above the temperature at which spontaneous decomposition occurs. The source of ignition of the leak may have been reaction with the insulation, as described... 

Redox reactions occur in the reduction of ores (metal oxides) into pure metals and the corrosion (oxidation) of pure metals in the presence of oxygen and water. Rusting iron, 4Fe + 30, + 611,0 —> 4Fe(OH), is a good example of metal oxidation. Strong oxidizing agents can be used as antiseptics (hydrogen peroxide, Fd,0,) or bleaches (sodium hypochlorite, NaOCl). 

In an ionizing solvent, the metal ion initially goes into solution but may then undergo a secondary reaction, combining with other ions present in the environment to form an insoluble molecular species such as rust or aluminum oxide. In high-temperature oxidation, the metal ion becomes part of the lattice of the oxide formed. 

It should be noted that since the rust is formed at a position in between the anodic and cathodic sites it will not influence the kinetics of the corrosion reaction. 

Thus for non-ferrous metals, SO is consumed in the corrosion reactions whereas in the rusting of iron and steel it is believed that ferrous sulphate is hydrolysed to form oxides and that the sulphuric acid is regenerated. Sulphur dioxide thus acts as a catalyst such that one SOj" ion can catalyse the dissolution of more than 100 atoms of iron before it is removed by leaching, spalling of rust or the formation of basic sulphate. These reactions can be summarised as follows ... 

In the rusting of iron and steel, Evansconsiders that the anodic reaction of... 

This is a simplified treatment but it serves to illustrate the electrochemical nature of rusting and the essential parts played by moisture and oxygen. The kinetics of the process are influenced by a number of factors, which will be discussed later. Although the presence of oxygen is usually essential, severe corrosion may occur under anaerobic conditions in the presence of sulphate-reducing bacteria Desulphovibrio desulphuricans) which are present in soils and water. The anodic reaction is the same, i.e. the formation of ferrous ions. The cathodic reaction is complex but it results in the reduction of inorganic sulphates to sulphides and the eventual formation of rust and ferrous sulphide (FeS). 

Sulphur dioxide in the air originates from the combustion of fuel and influences rusting in a number of ways. For example, Russian workers consider that it acts as a cathodic depolariser , which is far more effective than dissolved oxygen in stimulating the corrosion rate. However, it is the series of anodic reactions culminating in the formation of ferrous sulphate that are generally considered to be of particular importance. Sulphur dioxide in the air is oxidised to sulphur trioxide, which reacts with moisture to form sulphuric acid, and this in turn reacts with the steel to form ferrous sulphate. Examination of rust Aims formed in industrial atmospheres have shown that 5% or more of the rust is present in the form of iron sulphates and FeS04 4H2 0 has been identified in shallow pits . 

Corrosion is essentially the conversion of iron into a hydrated form of iron oxide, i.e. rust. The driving force of the reaction is the tendency of iron to combine with oxygen. 

Some spontaneous reactions, notably the rusting of iron, are quite slow. Often a reaction that energy isn t,... 

---