The corrosion

The word corrosion comes from the Latin corrodere, which means to corrode, and corrosion is called spontaneous process of destruction of materials and products from them under the chemical influence of the environment. Stones, wood, plastics and other polymeric materials are also exposed to corrosion.

Stones, wood, plastics and other polymeric materials are also exposed to corrosion

Metals form one of the foundations of civilization on planet Earth. Among them, as a structural material, iron and its alloys are clearly distinguished, with which there is a lot of trouble associated with their rusting. Other metals corrode, but do not rust.

Corrosion of metals is often reduced to their oxidation and conversion into oxides. In particular, iron corrosion can be described by a simplified equation:

4Fe + 3O2 + 22O = 2Fe2O3*H2O

Hydrated iron oxide 2Fe2O3*H2O and is what people call rust. This is a loose powder of light brown color. It is established that the corrosion of iron contributes to the presence of sulfur in it. Modern people are struck by the resistance to corrosion of some antique items made of iron. One of the reasons for this is the low sulfur content in it. Usually in iron, it comes from coal during blast furnace smelting. In the distant past for this purpose, not charcoal but charcoal was used, which practically does not contain sulfur. Sulfur in iron is usually contained in the form of sulfides of FeS, etc. In the process of corrosion, iron sulphides decompose with the release of hydrogen sulfide H2S, which is a catalyst for the corrosion of iron.

Many metals during corrosion are covered with a dense, well bonded metal oxide film, which protects the metal from further oxidation. For example, aluminum is a very active metal and, theoretically with water, would interact in accordance with the equation:

2Al + 3H2O = Al2O3 + 32

However, its surface is covered with a dense oxide film Al2O3, which protects the metal from the action of water and oxygen. For this reason, the water in the aluminum kettle with heating boils, but does not affect the metal and therefore the kettle serves a fairly long time. However, the air often contains oxides of sulfur, nitrogen, carbon and others, and in water - dissolved gases and salts. Therefore, for example, bronze statues, corroding, are covered with a layer of green patina, whose composition corresponds to the basic sulfate of copper (2) (CuOH)2SO4.

Salts and especially chlorides are activators of corrosion and lead to accelerated destruction of metals, particularly vehicles and underground utilities. In particular, big troubles are associated with sodium chloride, scattered in the winter on roads and sidewalks to remove snow and ice.

In essence, corrosion is divided into chemical and electrochemical. Rust of iron or bronze patina coating is chemical corrosion. Electrochemical corrosion is often associated with the presence in the metal of random impurities or specially introduced alloying additives.

The essence of the electrochemical theory is that impurities in metals create micro-galvanic elements in which electrons flow from the anode to the cathode segments. Since the cathodic and anodic processes are separated on the surface, the opposite fluxes of ions, atoms and molecules are also separated. Divided streams do not interfere with each other and for this reason corrosion proceeds faster than in the absence of micro-galvanic cells.

Many inexperienced chemists at different times were puzzled by the fact that sometimes the reaction:

Zn + H2SO4 = ZnSO4 + H2,

described in all textbooks, does not go. More experienced chemists know that in such a situation, a little copper sulfate (2) (copper sulfate) should be added to the solution. In this case, copper will be released on the zinc surface:

CuSO4 + Zn = ZnSO4 + Cu

and hydrogen begins to vigorously stand out. In explaining this phenomenon in 1830, the Swiss chemist A. de la Reave created the first electrochemical theory of corrosion.

Soon after the discovery of the electrochemical phenomenon by the Italian L. Galvani, his compatriot A. Volta constructed (1800) a source of electric current (a galvanic cell). In one embodiment, the source consisted of alternating copper and zinc disks separated by a porous material impregnated with a solution of salt. In it, zinc is an anode, and copper is a cathode. Zinc in the form of an ion Zn2+ passes into a solution of sulfuric acid, and the remaining electrons from each atom flow to a more electropositive metal - copper:

Zn = Zn2+ + 2e-

Hydrogen ions approach the copper cathode, take electrons and transform into hydrogen atoms, and then into hydrogen molecules:

+ + (u) = ; 2 = 2

Thus, the streams of ion motion are separated and with an excess of acid the process proceeds until all of the zinc is dissolved.