Rusting of iron

Sodium is soft and reacts violently with both air and water. Iron also reacts with air and water but much more slowly, forming rust. Gold, however, remains totally unchanged after many hundreds of years. Sodium is said to be more reactive than iron and, in turn, iron is said to be more reactive than gold. 

By carrying out reactions in the laboratory with other metals and with air, water and dilute acid, it is possible to produce an order of reactivity of the metals. 

If a metal reacts with dilute hydrochloric acid then hydrogen and the metal chloride are produced. 

If similar reactions are carried out using other metals with acid, an order of reactivity can be produced by measuring the rate of evolution of hydrogen. This is known as a reactivity series. 

An order of reactivity, giving the most reactive metal first, using results from experiments with dilute acid, is shown in Table 10.1. The table also shows how the metals react with air/oxygen and water/steam, and, in addition, the ease of extraction of the metal. 

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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 ... 

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. 

Rusting of iron consists of the formation of hydrated oxide, Fe(OH)3 or FeO(OH), and is evidently 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. The mechanism is complex and will depend in detail on the prevailing conditions, but may be summarized as ... 

However, in this section emphasis is placed upon damp and wet atmospheric corrosion which are characterised by the presence of a thin, invisible film of electrolyte solution on the metal surface (damp type) or by visible deposits of dew, rain, sea-spray, etc. (wet type). In these categories may be placed the rusting of iron and steel (both types involved), white rusting of zinc (wet type) and the formation of patinae on copper and its alloys (both types). 

Oxygen from the atmosphere, dissolved in the electrolyte solution provides the cathode reactant in the corrosion process. Since the electrolyte solution is in the form of thin films or droplets, diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Moreover, convection currents within these thin films of solution may play a part in further decreasing concentration polarisation of this cathodic process . Oxygen may also oxidise soluble corrosion products to less soluble ones which form more or less protective barriers to further corrosion, e.g. the oxidation of ferrous species to the less soluble ferric forms in the rusting of iron and steel. 

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... 

Fig. 3.1 Effect of relative humidity and atmospheric pollution on the rusting of iron (after...

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

Instead it turns out that in almost all cases die reverse is true. Nearly all exothermic chemical reactions are spontaneous at 25°C and 1 atm. Consider, for example, the formation of water from the elements and the rusting of iron ... 

Notice that the second law refers to the total entropy change, involving both system and surroundings. For many spontaneous processes, the entropy change for the system is a negative quantity. Consider, for example, the rusting of iron, a spontaneous process ... 

The quantity consumed or produced is conveniently expressed in partial pressure units if the substance is a gas. Concentration units are convenient if the reactant or product is in solution. The time measurement is also expressed in whatever units fit the reaction microseconds for the explosion of household gas and oxygen, seconds or minutes for the burning of a candle, days for the rusting of iron, months for the rotting of wood. 

Table 8.1 Description of the rusting of iron at each level of chemical representation of matter Level of Representation...

Other oxidations, like the rusting of iron at room temperature, proceed slowly with a slow release of heat. In the case of iron, the hnal product is iron oxide (rust). 

Redox reactions can proceed by direct transfer of electrons between chemical species. Examples include the rusting of iron and the metabolic breakdown of carbohydrates. Redox processes also can take place by indirect electron transfer from one chemical species to another via an electrical circuit. When a chemical reaction is coupled with electron flow through a circuit, the process is electrochemical. Flashlight batteries and aluminum smelters involve electrochemical processes. 

This equation tells us that 4 iron atoms react with 3 oxygen molecules to form 2 rust compounds. Note that the number of iron atoms and oxygen atoms are the same on both sides of the equation (2 x Fe203 = 4 Fe and 6 O). Let s see how we went about arriving at the balanced equation for the rusting of iron. 

As we mentioned previously, the balanced chemical equation not only indicates what chemical species are the reactants and what the products are, but it also indicates the relative ratio of reactants and products. Consider the balanced equation for the rusting of iron ... 

When the ordinary process of rusting of iron takes place, hydrogen is evolved. 

IXacing cars, such as the one shown helow, can reach speeds that are well above 200 km/h. In contrast, the maximum speed of many farm tractors is only about 25 km/h. Just as some vehicles travel more quickly than others, some chemical reactions occur more quickly than others. For example, compare the two reactions that occur in vehicles the decomposition of sodium azide in an air bag and the rusting of iron in steel. 

The rusting of iron involves the reaction of iron, oxygen, and water in a naturally occurring galvanic cell on the exposed surface of the metal. There may be many of these small cells on the surface of the same piece of iron. 

Sometimes, the rusting of iron is promoted accidentally. For example, by connecting an iron pipe to a copper pipe in a plumbing system, an inexperienced plumber could accidentally speed up tbe corrosion of tbe iron pipe. Copper is less reactive than iron. Therefore, copper acts as the cathode and iron as the anode in numerous small galvanic cells at the intersection of the two pipes. 

Why is the rusting of iron a spontaneous reaction at room temperature ... 

Still other examples are the dissolution reactions, the attack of metal chips by acids, and the rusting of iron. 

Metals develop a natural corrosion-resistant film when exposed to the environment. Examples include the rusting of iron, tarnishing of silver, and the formation of the patina on copper. These passive films help prevent further corrosion. However, films do not provide complete resistance to chemical attack and are destroyed by various corrosive agents. 

Oxidation numbers are used when writing redox equations to account for the transfer of electrons. For example, consider the simple reaction representing the rusting of iron ... 

Take two oxidation examples, the rusting of iron and the burning of a candle, the latter occurs at a much faster speed than the former. 

How do iron, cobalt, and nickel react with the oxygen of the air What is the rusting of iron What factors affect this process What will happen if we take two plates, one of galvanized iron and the other of tin-plated iron, scratch each of them with a file to remove the protective layer of metal, wet them with a 10% sodium chloride solution, and let them stay in the air ... 

Kinetics is the study of the rate of change of chemical reactions. These reactions can be very fast, i.e. instantaneous reactions such as detonation, those requiring a few minutes, i.e. dissolving sugar in water, and those requiring several weeks, i.e. the rusting of iron. In explosive reactions the rate is very fast and is dependent on the temperature and pressure of the reaction, and on the concentration of the reactants. 

Some chemical reactions, such as the rusting of iron, are slow, while others, such as the burning of gasoline, are fast. The speed of any reaction is indicated by its reaction rate, which is an indicator of how quickly the reactants transform to products. As shown in Figure 9.6, initially a flask may contain only reactant molecules. Over time, these reactants form product molecules, and as a result, the concentration of product molecules increases. The reaction rate, therefore, can be defined either as how quickly the concentration of products increases or as how quickly the concentration of reactants decreases. 

Corrosion> A process of gradual disintegration or decomposition of a chem nature. The most common example of corrosion is the rusting of iron in air or acidic water. Corrosion of metals is electrochemical in nature. Extensive info is available in the literature regarding measures that have been and can be taken to minimize or prevent various forms of corrosion which occur in industries. It is important to recognize the various forms of corrosion and to effectively apply the available info in or der to overcome corrosion problems. See the Refs given below... 

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