Corrosion
...es are apart, the larger the potential difference between them. A metal is said to be less noble than those which stand to its right in the electrochemical series. In the case of electrochemical corrosion it is always the less noble metal which is removed. Table 1. Electrochemical Potential Series, Volts. K Ca Mg Al Zn Cr Fe Ni Sn Pb H Cu Ag Au -2.92 -2.84 -2.38 -1.66 -0.76 -0.71 -0.44 -0.24 -0.14 -0.13 0.00 0.34 0.80 1.42 not noble -----------------------------------------------------------------> noble Likelihood of passing into solution decreases from left to right. The potential difference does not, however, always fully correspond with the corrosion phenomena experienced in practice. The reason is that oxide and other metal compounds have differing electrochemical potentials. Chemical corrosion reactions Metals have a tendency to combine with oxygen to form oxides and this is one of the chemical reactions. This tendency is the stronger the less noble the metal. The layers of oxide on the metal surface which are formed even in dry air may be insoluble and stable against an aqueous medium in contact with them. Therefore, if the oxide layers are dense and adhere well to the metal, they prevent further attack and act as a corrosion prevention layer. An example of this is aluminum oxide. However, iron differs in that, although it does form a surface oxide layer, this layer is loose and enables oxidation to proceed into the depth of the metal. Chemical corrosion also takes place by the action of acids and alkalis on metals. Hydrochloric acid, for example, reacts with iron, and sodium hydroxide with aluminum (Figure 1). If soluble reaction products are formed, the reaction only ends when either the aggressive medium, or the metal are used up; if salts are formed which are sparingly soluble they can form protective layers. Figure 1. Chemical corrosion as shown by acid attack.. The chemical reaction of forming the hydroxide or oxide layers is the cause of producing the ““rust””. Rusting Rusting refers to the corrosion of iron. As irons is the most widely used metal in the world, rusting is the most common type of corrosion. The Chemistry of Rusting Rusting of the iron is due to the electrochemical reaction. It requires the presence of both air (i.e. the oxygen) and water. Like the electrochemical cell, electrons given up from anode of an iron atom flow (through metal) to cathode. Consider an iron sheet exposed to open air: Figure 2. The electrochemical process of rusting on a flat iece of iron The metal gets wet due to moisture in air. The thin water film dissolves oxygen from air. In the initial stage of rusting, some iron atoms lose electrons to become Fe2+ (aq) ions: Fe (s) ®® Fe2+ + 2e- (oxidation at anodic area) The electrons are accepted by the dissolved oxygen and water to form OH- (aq) ion: O2 (g) + 2H2O (l) + 4e- ®® 4OH- (aq) (reduction at cathodic area) Thus a simple electrochemical cell is set up. One part of the iron piece acts as anode; another part acts as cathode. The cathodic area is usually the region around the outer edges of the water film, where the concentration of dissolved oxygen is higher. The Fe2+ (aq) and OH- ions in the water film react to form iron (II) hydroxide. Fe2+ (aq) + 2OH- ®® Fe(OH)2 (s) The precipitate is rapidly oxidized by dissolved oxygen to form iron (III) hydroxide. 4Fe(OH)2 (s) + O2 (aq) + 2H2O (l) ®® 4Fe(OH)3 (s) On standing, this changes to rust, a reddish brown solid. Rust is in fact hydrated iron (III) oxide with variable composition (Fe2O3 ·nH2O). Factors that speed up rusting 1) Presence of electrolytes Acid solutions make rusting go faster. In industrial areas where air is seriously polluted, there are high concentrations of carbon dioxide, sulfur dioxide and nitrogen dioxide. These gases dissolve in rain water to give ““acid rain””, which makes iron objects rust faster. Sodium chloride also makes iron rust more quickly. For example, the iron objects by seashore, ““where seawater has a high salt content, amounting to about 3.6% in the Atlantic and Pacific Oceans.”” The thin water film on iron surface contains dissolved sodium chloride from sea spray. This greatly increases conductivity of the solution, due to a higher concentration of ions. As a result, a special ““seawater rust”” is formed which is actually very strongly corrosion inducing. One would expect corrosion rates of about 0.1mm/year. Presence of soluble salts other than sodium chloride may also assist rusting. 2) Heat An increase in temperature always increases rate of chemical reactions, including rusting. 3) Humidity ““Corrosion starts when the relative humidity of the air exceeds around 65%. Many areas has a higher humidity in winter (80-95%) than in summer (60-80%)”” . In consequence, iron rusts five times faster in winter as it does in summer. However, the relative air humidity in enclosed spaces often differs from that existing in the open air; in winter, in heated room it is lower, while in summer it can be higher in cool cellars. On the whole the danger of corrosion in inside rooms is less than in the open air. ““Many water have lime and carbonic acid in equilibrium. This is called equilibrium water, where there is sufficient carbon dioxide in solution to stabilize the carbonate. The equilibrium can be expressed as follow: CaCO3 + H2CO3 Ca(HCO3)2 Provided the minimum hardness is about 2.2 milliequivalents/liter, these water form layers of mixed lime and rust that safeguard the steel piping against further corrosion.”” If the water contains an excess of carbonic acid, which prevents the formation of protective layers, there is a danger of corrosion of unprotected steel in the presence of oxygen. 4) Contact with a less reactive metal Consider iron and copper plates joined together and put in water containing dissolved oxygen. Iron loses electrons more readily than copper. Hence iron forms the anode and copper the cathode of an electrochemical cell. In this case, iron rusts even more quickly than when there was no copper. 5) Other factors Other factors that speed up rusting include the presence of sharply pointed regions in the iron piece, or a high concentration of dissolved oxygen in water. Protection From Rusting Iron is such a useful metal yet it rusts. Rusting is a serious problem. A very sum of many is spent every year to protect iron objects and replace rusted articles. Several methods can be used to prevent rusting or to slow it down. Applying a Protective Layer Both air and water a...