Corrosion and Rusting
Introduction
The most common forms of metallic corrosion are caused by electrochemical reactions, wherein two metallic phases (e.g., iron oxide and iron) react in the presence of electrolytic solution. Another mechanism of metallic corrosion is caused by chemical reaction, which explains how the protective layer of the metal is formed.
Rusting is the corrosion of iron which is the most widely used structural metal. Most of it is used in making steel. The wide range of products made from steel includes all types of vehicles, machinery, pipelines, bridges, and reinforcing rods and girders for construction purposes. Therefore, rusting causes enormous economic problem and is the reason why extensive measures of corrosion protection have had to be developed.
It is obvious that corrosion and rusting affect significantly the life of the society, so it is worthy to investigate this topic. In this essay, the cause of the corrosion and rusting and consequently the protection of the corrosion will be explored.
Electrochemical corrosion reactions
This type of corrosion takes place when two metallic phases with different electrochemical potentials are connected to each other by means of an electric conductor.
The electrochemical EMF series (Table 1) gives the electrochemical potential of metals under normal conditions with respect to hydrogen (hydrogen is 0).
The Term Paper on Reaction Alkali Metals And Alkaline Earth Metals
The rates of reaction of Alkali metals and Alkaline Earth meatals are compared in this lab. The pH of each of the resulting metal solutions are tested and the products of the reaction between calcium and water is discovered. The tested elements are sodium, lithium, potassium and calcium and each of them were placed in a beaker filled with water. The resulting solutions pH levels were tested with ...
The farther two metals in electrochemical series 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..
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 Term Paper on Corrosion Oxidation Redox Reactions
... achieved. If a metal that is lower in the electrochemical series is put together with iron, then that metal will then form the anode; being ... in a moist environment. The corrosion of iron is commonly referred to as rusting. When water drops onto the iron it often isn't pure ...
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
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
“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
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.
The Essay on Iron Steel And Metal Finishing Operations Summary
Iron, Steel and Metal Finishing Operations Summary By: Kevin Worley Southwestern Oklahoma State University Production, forming, and cleaning of steel products creates several waste-outputs. These outputs range from the benign to RCRA regulated hazardous waste. Listed below are the steps associated with a metal product from ore to finished product, detailing the various wastes. Production of Coke ...
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 are necessary for rusting to occur. Any method which can keep out one or both of them from iron will prevent rusting. The most obvious way is to apply a protective layer.
1) Coating with paint, oil or grease
Objects that are unlikely to be scratched can be coated with paint (or lacquer, or enamel).
For example, bridges, ships and car bodies are painted.
Moving parts of a machine are protected by applying oil or grease.
2) Coating with another metal
Iron can be coated with a thin layer of another metal which is resistant to corrosion. Galvanized iron is iron coated with zinc. Some roofs, buckets and dustbins are made from galvanized iron. Tin -plate is iron coated with tin.
3) Using Alloys Of Iron
Steel is produced form iron by carefully controlling the amount of carbon present (0 – 1%).
To fight against corrosion, steel can be alloyed with other metals such as chromium and nickel to produce stainless steels.
Cathodic Protection
Rusting is a redox reaction in which iron loses electrons. If iron is connected to a more reactive metal, the other metal will lose electrons in preference, preventing the formation of Fe2+ (aq) ions.
At zinc (anode) Zn (s) ® Zn2+ (aq) + 2e-
The electrons flow from zinc to iron, where the following half-reaction take place:
At iron (cathode) O2 (g) + 2H2O (l) + 4e- ® 4 OH- (aq)
Since iron is forced to accept the electrons, it cannot corrode, as corrosion involves giving off electrons. This method of preventing corrosion is called cathodic protection.
The Essay on Transition Metals Electron Configuration
Name Symbol Electron Configuration Scandium S Titanium TVanadiuChromium C Manganese MIron F Cobalt C Nickel Copper C 10 4 s 1 Zinc Z 10 4 s 2 As Chromium and Copper are in the fourth period we would expect it to follow the same pattern as the other elements in the same periods as in the 4 s sub shell is filled first rather than the 3 d sub shell. Chromium (Cr) Expected Electronic ...
Cathodic protection is also used to prevent corrosion in underground iron pipelines. Bags containing magnesium turnings are connected to the buried pipelines at intervals. (Figure 3.) The magnesium corrodes instead of the iron. The magnesium should therefore be replaced from time to time.
Figure 3. Protecting underground iron pipelines from corrosion by cathodic protection
Most ships are made of iron. To prevent rusting, zinc blocks are attached to the hull of a ship. Zinc will corrode in preference to iron.
Electrical Protection
Sometimes rusting can be prevented by using electricity. For example, the negative terminal of the car battery is always connected to the car body. This supplies electrons to the iron body, preventing it from losing electrons. In some piers, the steel structures are protected electrically by connecting them to the negative terminal of a d.c. source.
Conclusion
Corrosion is the gradual deterioration of a metal due to reaction with air, water or other substances in the surroundings. When these substances present, the metal can corrode through the process of electrochemical and chemical corrosion reaction. The most common type of corrosion is rusting which costs several billion dollors a year. To prevent such natural, spontaneous processes, method of protections and constant maintenance work are necessary; only in these ways can a steel structure be adequately protected against corrosion and its value maintained.
