Thus, even if the zinc coating is scratched, the zinc will still oxidize before the iron. This suggests that this approach should work with other active metals. Another important way to protect metal is to make it the cathode in a galvanic cell. This is cathodic protection and can be used for metals other than just iron. For example, the rusting of underground iron storage tanks and pipes can be prevented or greatly reduced by connecting them to a more active metal such as zinc or magnesium Figure 3.
This is also used to protect the metal parts in water heaters. The more active metals lower reduction potential are called sacrificial anodes because as they get used up as they corrode oxidize at the anode. The metal being protected serves as the cathode, and so does not oxidize corrode. When the anodes are properly monitored and periodically replaced, the useful lifetime of the iron storage tank can be greatly extended.
Corrosion is the degradation of a metal caused by an electrochemical process. Large sums of money are spent each year repairing the effects of, or preventing, corrosion. Some metals, such as aluminum and copper, produce a protective layer when they corrode in air. Iron corrodes forms rust when exposed to water and oxygen. The rust that forms on iron metal flakes off, exposing fresh metal, which also corrodes.
One way to prevent, or slow, corrosion is by coating the metal. Coating prevents water and oxygen from contacting the metal. Paint or other coatings will slow corrosion, but they are not effective once scratched. Zinc-plated or galvanized iron exploits the fact that zinc is more likely to oxidize than iron.
As long as the coating remains, even if scratched, the zinc will oxidize before the iron. Here is the word equation for the reaction:. Iron and steel rust when they come into contact with water and oxygen — both are needed for rusting to occur.
In the experiment below, the nail does not rust when air containing oxygen or water is not present:.
The film of chromium oxide is also self-healing, and hence the corrosion protection provided by the chromium oxide is stable and durable.
Corrosion is a coupled electrochemical reaction between one anode and one or more cathodes. In carbon steel there can be two or more phases; one of the phases will act as anode and the other s act as cathode s. Corrosion as an oxidation reaction takes place at the anode.
For example, the ferrite phase forms a galvanic couple along with phases such as martensite , and corrodes in preference to martensite. In a ferritic-martensitic combination of phases, when the proportion of ferrite is increased, the corrosion current density will increase. Pure iron has better resistance to oxidation-corrosion, but has inadequate resistance against aggressive and other reactive chemicals.
Compared to wrought iron , pure iron has substantially higher corrosion resistance, which because of its homogeneous structure rusts on its outside surface, whereas the wrought iron with a laminated structure creates rust layers in between its laminations.
Iron metal of high purity can remain free from corrosion in laboratory settings for many years. However, in saline environments or polluted industrial surroundings, pure iron has poor resistance to corrosion. The corrosion resistance of pure iron in water depends upon the pH of the water and any dissolved oxygen present in it.
If the pH value is above 5 and the dissolved oxygen is negligible, then the corrosion rate is almost negligible.
If the pH is marginally lower than 5, the corrosion risk goes up. The corrosion risk also depends on the extent of immersion of the ferrous surfaces. If the surfaces are constantly and fully immersed then the corrosion rate is minimum and corrosion resistance is maximum. If the immersion is partial and cyclically variable where some parts are cyclically exposed to air, then the corrosion risk and rate of corrosion could increase. By choosing the correct combination of alloying elements, the corrosion resistance of cast iron can be optimized to specific operating environments.
Molybdenum, copper, chromium, nickel and silicon are some of the significant alloying elements. Molybdenum adds to cast iron's mechanical strength and significantly increases corrosion resistance to hydrochloric acid. A small quantity of copper added to cast iron increases its corrosion resistance to acids such as hydrochloric acid and sulfuric acid. A chromium addition in smaller percentages helps to improve saltwater corrosion resistance. Nickel , added generally to improve mechanical properties, also adds to cast iron's corrosion resistance by creating a nickel oxide film on the surface.
This is sometimes aided by alloying elements such as silicon and chromium. By improving the metal's hardness, nickel also protects against cavitation corrosion or erosion corrosion caused by entrapped solids in the fluid coming in contact with the metal.
Above this level the corrosion resistance improves tremendously, most often at the expense of ductility, mechanical strength and machinability.
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