The nature of the corrosive destruction of metal constructions depending on the conditions of corrosion can be very different. Together with the general (continuous) corrosive destruction, those by more or less commensurate before its intensity beyond the entire metallic surface, which contacts with the corrosive environment, are very frequent the cases, when only the separate to a greater or lesser extent localized sections of the surface of metal undergo selective destruction, whereas remaining surface (much more significant) remains almost undestroyed.

Slit, pitting, intercrystalline (International Whaling Commission) corrosion are the most common (ЩК) forms (ПК) of the local corrosion, which bring basic damage. The presence of mechanical stretching stresses conducts usually down even the more localized localized corrosion - corrosion fatigue (КУ) and corrosion cracking (КР).

The general corrosion is critical at the point of the basic volume of the corroded and corrosive- pulverized metal, whereas with the localized corrosion article can lose its operating characteristics already with the relatively low losses of mass. Local (local) corrosion more frequently is most dangerous for this reason. For example, International Whaling Commission, KR and KU frequently catastrophically reduce structural strength and is render inoperableed it with the minute quantities of corroded metal. Analogous for the sake of this PK before the chemical apparatuses, the cisterns and other capacities it is possible to lead down the loss of airtightness with the relatively small total losses of metal. To scientific research and to the development of the methods of fight with the local forms of corrosion is paid extremely considerable attention.

1. Slit corrosion

In many practical cases of operating the metal constructions is observed BY [SHCHK], i.e., the selective, intensive destruction of metal before the slot (clearance).

Slots before the apparatuses and the constructions are usually inevitable during the articulation of different components, in particular between the sealing materials and the metal. Selective corrosion can lead down the premature wear of the constructions, which are exploited also due to the atmospheric conditions. Riveted constructions, for example railroad bridges frequently suffer from this form of corrosion. The cases of destroying the structures in the places of contact with the building materials are possible.

The greatest sensitivity to [SHCHK] manifest passive metals (Ks-[stali], AI- alloys,) in the case of their possible depassivation before the slots.

Methods of protection.

As the shielding it is possible to propose the following methods.

• Packing of clearances by polymer films, by rubber, by lubricant, which must ensure the airtightness, which prevents impact of moisture on purpose.
• Rational construction, which foresees the impossibility of the entry of aggressive medium beside the clearances of different design connections.
• Selection Ks-[materialov]. On the basis the data about the resistance to [SHCHK] it is possible to see that the metals and the alloys, whose KS is caused as far as their passive properties, are very sensitive to [SHCHK], since passive state can be easily destroyed before the slot.
• [EKh]- protection. To avoid [SHCHK] it is possible to use cathode protection, i.e., the polarization of constructions from the external source, or contact with the sacrificial anodes (protectors).
• Barrier protection. Application of paints, steadfast to the operating conditions.

2. Pitting corrosion

This - one of the dangerous forms of the corrosive destruction, characteristic for the conditions, when the passive state of alloy can partially be destroyed. PK undergo the very limited sections of metal, and entire remaining surface is steady and is found before the passive state that it leads to the formation of deep defeats - point ulcers or pittings.

For the study PK and generalizing accumulated experimental data many studies are carried out. The easily passivating metals and the alloys usually undergo this form of corrosion: Fe and especially such important and widespread construction materials as Ks-[stali], and also [A]l and its alloys, Ni, Zr, Ti and other

Methods of the protection of metals from PK

• Selection Ks-[splava]. [S]r is highly stable to PK. Therefore due to the conditions of the danger PK are more preferable steels with the high content Of [s]r. Regarding especially high resistance to PK differ high-chromium ferritic steels of the increased cleanliness before the interstitial impurities, which contain 18 or 25 -29% Of [s]r, and also these steels, additionally alloyed by 1 4%.
  Due to the very aggressive conditions: with an increase in the concentration of halide anions and temperature should be used Ti or its alloys Ti - (0,15 + 0,20) % Pd.Ti - 2,5% of Ni - 2% of Zr, Tj - 2% of Ni - (1 + 2)% of Mo - most steadfast to PK from the accessible construction materials.
• [EKh]- protection and the application of inhibiters.
  For the averting PK it is possible to displace the potential of alloy or down the side of the less positive values of passive region (anodic protection), or it is more negative than the stationary potential (cathodic).
  The reliability of the application of anodic protection of steels from PK rises in the case of the presence of some inhibiters before the solution.
• Barrier protection. Application of paints, steadfast to the operating conditions.

3. Intercrystalline corrosion

International Whaling Commission - one of the most dangerous varieties of the localized corrosion of alloys, which cause the selective destruction on the grain boundaries, as a result which they are lost strength and the plasticity of alloy and prematurely are destroyed constructions. International Whaling Commission is observed before many technical Fe- alloys and in particular in Ks-[staley]: Fe -[S]r, Fe-Ni- Cr, Fe - Mn - Ni - [S]r and other, in Ni- and Al-[splavov] Ni - Mo, Ni - Cr - Mo, Al - Cu, Al-Mg- Si.

Protection from International Whaling Commission

On the basis studies of reasons International Whaling Commission are proposed the different methods of fight with it, directed down to a change in composition and structure of steels: a reduction of the content [s] down? 0,03% before the solid solution of steel with the smelting, alloying became the stabilizing elements (by such, as Ti and Nb), the heat the working of steel (austenitizing, stabilizing annealing). International Whaling Commission of steel undergo in the heat-affected zone of the weld; therefore for the averting International Whaling Commission should be subjected down stabilizing annealing or austenitizing entire weldment.

• Barrier protection. Application of paints, steadfast to the operating conditions.

4. The corrosion cracking

KR - destruction of metals and alloys under the simultaneous influence of corrosive environment and tensile mechanical stresses as a result of the accelerated formation of corrosion crackings. It is observed for many metals and alloys: the carbon and low-alloy steels, Ks-[staley], the alloys Of [s]u, [A]1, Ti, Mg and other different aspects KR intensely are studied, they are generalized before a number of works.

Protection from KR

• [EKh]- protection. Small cathodic polarization [EKH] protects steel from THE CRIT. however with an increase in the cathodic polarization time to the splitting it decreases, that already, apparently, connected for the sake of the plating brittleness they became. Cathode protection it is expedient to use earlier than are formed thin initial cracks to, i.e., superimpose cathodic polarization before the imposition of stresses or simultaneously with sinking of component beside the electrolyte.
• Ks-[splavy]. Are most steadfast to KR austenitic alloys with the high content Ni (? 45%), and also ferrite Cr- steels, which do not contain by Ni.
  Favorable influence Ti, in spite of an increase in the absorption of hydrogen as far as steel, is explained as far as the anomalously high ability of atoms Ti to seize hydrogen, to connect it beside the steady Ti - N- clusters and to prevent the diffusion Of [n]2 before the region of maximum triaxial stresses.
• The decrease of internal tensile stresses before the surface layer of alloy decreases tendency down to THE CRIT.
• Change in the composition of medium. It is indicated reaching of a substantial increase in the stability to this form of destruction before the heat-power engineering installations by the removal of oxygen from the water. The introduction of inhibiters beside the corrosive environment also is used for purposes of fight with KR of these installations.

5. The corrosion fatigue

Fatigue of metals - their brittle failure as a result of crack formation between and transcrystalline nature under the simultaneous action of corrosive environment and variable (cyclic) stresses, usually which do not exceed elastic range. Under the influence of corrosive environment the fatigue failure of metal, as a rule, occurs much more rapidly than in the vacuum or the dry atmosphere with the identical intensity of cyclic stresses.

the Ku- destruction of metals and alloys is observed with the operation of the shafts of the screw propellers of steamships, springs of automobiles, sea and mine ropes and the like K[U] of alloys and steels occurs before the fresh and sea water, before the condensates of products of combustion and the different chemical media.

6. Corrosive cavitation

[KK] - destruction of material before the rapidly moving liquid corrosive environments. To it are subjected the screw propellers of seagoing vessels, the coolant jackets of diesels, high-speed centrifugal pumps, [gidroturbinyi] of [t].[d].

With the rapid turbulent motion of liquid is formed with subsequent rapid [okhlopyvaniem] the large number of small phials. With [okhlopyvanii] of each phial appears the hydraulic impact at the nearest point of the surface of metal. Repeated continuous hydraulic impacts create conditions for the simultaneous mechanical and corrosive action.

7. Corrosive erosion

KOe either corrosion-mechanized wear - destruction of solid surface, in this case of metal, caused by the mechanical abrasive action of another solid body with the joint action of corrosive environment, or by the directly grating action of the very corrosive environment, which contains or which does not contain solid particles. In the first case this phenomenon is called also the grating corrosion or fretting. Similar structural failures are observed before the different machines and the connections of components (for example, before the ball and roller bearings, gears and the friction units even with the insignificant amplitudes of mutual displacement on the vibrating components before the process of work). Destruction from kOe also, occurs, when the weighed solid particles are located directly before the corrosive environment, for example, during pumping of pulp or with, the pipeline hydraulic conveying of solid powder-like species.