6.5. Corrosion of metals

Chemical corrosion of metals is observed in dry gases and nonelectrolytes. In the first case the metals collapse, reacting with gases and vapour at a temperature over 100°C (oxidization of metal during heating, corrosion of the furnace accessories, blades of steam and gas turbines). In the second case - corrosion takes place in petroleum, petrol, oils etc.

The metals mostly collapse in result of electrochemical corrosion, i.e. under the action of electrolytes - water solutions of salts, acids and alkalis. This type of corrosion takes a place in atmospheric conditions, at the action of sea, river, underground and other types of water.

In the iron-carbon alloys mostly, ferrite is an anode, and the cementite or nonmetallics are cathodes (Fig. 6.11).

Especially intensively electrochemical corrosion flows at periodic influence of electrolyte on a metal, for example, in the case of the previous moistening and drying of metal constructions of hydrotechnical structures.

Corrosion of metals is accelerated by the action of electric current. Electrocorrosion of metals takes place during the connecting of construction to the source of current, for example, if the pipeline is located near-by tram-car rails. The carbon and sulphur dioxides, chloride of hydrogen and chlorous salts are instrumental in the active passing of corrosion process.

The corrosion resistance of different metals is compared with the unique scale (Table 6.4).

Таble 6.4

Scale of corrosion resistance of metals

Group of resistance	The corrosion rate, mm per year	Points	Group of resistance	Corrosion rate, mm per year	Points
High resistant	Less than 0.001	1	Reduced resistant	0.1-0.5	6
Enough resistant	0.001-0.005	2		0.5-1	7
Resistant	0.005-0.01	3	Low resistant	1-5	8
	0.01-0.05	4		5-10	9
	0.05-0.1	5	Nonresistant	More than 10	10

The corrosion of the metal products control is a very important task. Annually, about 10% of melted metal are lost in result of corrosion in the whole world.

Different methods are applied for the metal protection against corrosion. The additions of nickel, chrome, phosphorus and especially copper effectively promote the corrosion resistance of steels; manganese influences negatively. The widespread method of increasing of the structural steels corrosion resistance is their alloying with 0.2-0.4% Cu, at that the corrosion resistance grows on 20-30%.

Metals are protected from the corrosion by applying paints and varnishes, inorganic non-metal and metal coatings. Effective varnish coatings are developed on the basis of synthetic polymers. The most various protective films belong inorganic coatings of metal: oxide, phosphate, cement, asbestos-cement, enamel and others.

Metal coatings differ by the mechanism of protective action (anodal and cathode), and also by the method of application (galvanic, metallizative, sprayed from the fusion, etc.). Anodal coating is characterized by more negative electrode potential, than in a metal which is protected. In the case of damage, the coating collapses with greater speed, carrying out a protector role in relation to the basic metal. The zinc and aluminum are appointed mainly for anodal coatings of steel; the copper, tin, nickel and other-for cathode. If the anodal coatings are damaged, their protective action disappears.

Service life of coatings is 25-50 years. They enable to provide replacement of high-alloy steels with the low-alloy, to defense the steel in atmospheric conditions and at a high temperature, in salt water, etc.

Aluminum is the most perspective among the nonferrous metals for coatings. The protective films from aluminum and its alloys are coated by arc spraying pistols, by evaporation in a vacuum, electrolytic deposition, plasma and laser spraying.

T he active method of corrosion protection is an electrochemical method. It is based on the change of electrochemical potential of construction due to polarization of them by a direct current from an external source or from a protector. At protector defense (Fig. 6.12) to the metal, which is protected from corrosion, connect zinc anodes which enable to defend a valuable construction from corrosion. The cathode defense is use for the corrosion protection of marine hydraulic structures, main pipelines and other responsible constructions. In this case, the protected structure is connected to the negative pole of permanent current source, so the steel acts as cathode. The mechanical scrap, connected to the positive pole is an anode.

The corrosion rate of metals is reduced by additions of the inhibitors and passivators. The salts of alkaline and alkaline-earth metals (chromites, nitrates and others) as the strong oxidants – successfully decelerate the corrosive process in neutral and alkaline environments. The using of additions (in particular calcium nitrite-nitrate) is an effective at the protection against corrosion of steel reinforcement in the concrete.