Process intergranular corrosion solid metals in a liquid-metal medium has not been specifically studied. Some probable mechanisms of this process are outlined below, the existence of which is confirmed by indirect experimental observations.
1. One of the causes of intergranular corrosion is over high level potential energy of atoms located in intercrystalline zones, compared with atoms inside crystallites. Therefore, the activation energy of dissolution for these atoms is less than for the rest. Accordingly, the probability of their release into the solution ωt increases. It was previously shown that the dissolution rate constant for the process controlled by the first stage is equal to α=ωтρ"/n∞. dissolution of neighboring ones. As a result, the corrosion front under conditions of isothermal and non-isothermal dissolution will deepen along the grain boundaries of the metal, i.e., intergranular corrosion will occur. In some cases, the advance of dissolution along the grain boundaries is so large that it causes separation of whole grains from the matrix. intercrystalline fracture is the corrosion of nickel in liquid lithium at 1000 ° C. The microstructure of the metal characteristic of this case is shown in Fig. 42.

Let us estimate the depth of intergranular corrosion of a metal caused by the difference in the dissolution rates of atoms from the grain body and intergranular zones. For isothermal dissolution in this case, the number of atoms passing into the liquid metal solution per unit time is determined by a dependence similar to equation (1):

where the index "z" means that the corresponding characteristic refers to the dissolution from the surface of the grain, and the index "g" - to the dissolution from the intergranular zone. Taking into account, as before, that N-nVzh, we obtain the differential equation

Solving equation (95) and using the initial condition: t=0, n=0 and the saturation ratio dn/dt = 0, we find the kinetic equation for isothermal dissolution, taking into account the influence of intergranular zones in the following form:

To determine the depth of general (lz) and intergranular (lg) corrosion, we write the differential equation

where ρz and ρg are the bulk density of crystallites and intercrystalline zones, respectively. The solution of this equation is obtained using equality (96) and the initial condition t=0, lz=0, lg=0:

In order to obtain the ratio of the depth of intergranular and general corrosion, we write the following approximate expression for the latter:

where the symbol a denotes, as before, the dissolution rate constant equal to ωt * ρ "/n∞, and ng∞ is that part of the solution concentration that is achieved due to the dissolution of the metal of the intercrystallite zones. Subtracting (99) from (98), we find :

Let us divide equation (100) by (99), neglecting the second term on the right side of equation (99), which is obviously much less than the first. Then we obtain the required relation in the form

It follows from equation (101) that intergranular corrosion increases with time.
Of greatest interest is the maximum value of the ratio lg/lz, which is reached by the end of the dissolution process. From (101) we find that when the solution is saturated, i.e. at t→∞, this ratio is equal to

The value (lg/lz)max can be estimated as follows. In the first approximation, we can consider ng∞/n∞≈αgSg/αgSz; further, taking into account that α=ω*ρ"/n∞ and ρ"=α*ρ, where α is the interatomic distance, we obtain (log/lз)max≈ωг/ωз. The probability of transition of atoms from the grain surface into the solution is expressed by the dependence ωz= v exp (-Qр/RT). Due to the fact that the activation energy of dissolution from intergranular zones is less than from the body of crystallites by the amount of their excess energy ΔQg, the probability of transition of atoms from these areas into the solution will be ωg=v*exp [-(Qp-ΔQg)/RT] Using these expressions, we obtain (lg/lz)max≈exp (ΔQg/RT).
Let us numerically estimate the ratio of the depth of intergranular corrosion to the total one for γ-iron. The average value of the free energy of the grain boundaries in it, according to the work, is 8040 cal/g * atom. Taking this into account, we obtain that at a temperature of 800 ° C, the ratio (lg / lz)max is approximately equal to 40. Therefore, the depth of intergranular corrosion in iron can be almost 40 times greater than the depth of general corrosion. However, it should be noted that with a significant deepening of the corrosion front along the grain boundaries, the dissolution process will be retarded by the diffusion of dissolved atoms through the liquid metal in the resulting narrow and long channel, which can be considered as an increase in the thickness of the boundary film in this area. Such a process will naturally limit the depth of intergranular corrosion.
The ratio of the depth of intergranular corrosion to the depth of general corrosion under thermal mass transfer conditions can be obtained using the mass transfer equation in the form

where Δt is the time of passage of the hot zone by the flow of liquid metal. Since the weight of the metal transferred to the cold zone during the time t is equal to ΔP=S*Rpm*t, where S is the surface from which the dissolution occurs in the hot zone, then on the basis of equality (103) we obtain

Let us now turn to fig. 43, which shows the kinetics of metal dissolution of crystallites and intergranular zones. It can be seen from the schemes that the initial (ϗ*nн∞+nк) and final (nв) concentrations of the solution in the hot zone are the same for both crystallites and intercrystalline zones, Ho, the value of Δt turns out to be different for them, which is associated with different values ​​of αg and αc . Keeping in mind that the area of ​​these sections of the structure also differs, that ΔP=ρSl, and using the ratio of the weight of the metal dissolved from the intercrystallite zones and from the crystallites, we find

Since ρз≥ρг and Δtз≥Δtг, lg/lз≥1, i.e., intergranular corrosion also occurs under conditions of thermal mass transfer. If nv is significantly less than nv∞, then you can use the approximate equality Δtg / Δtз = αз / αг (see Fig. 43). Based on the last relation, equation (105) takes the form lg/lz≈ρz/ρg*αg/αz. Consequently, the intensity of intergranular corrosion in this case is determined by the ratio of the dissolution rate constants of the metal of intergranular zones and crystallites. It is interesting to note that the intensity of metal destruction along the boundaries of crystallites during thermal mass transfer is equal to the maximum value of the lg/lz ratio during isothermal dissolution. This means that we can use the previously made assessment of this ratio, from which it followed that the depth of intergranular corrosion, for example, iron at 800 ° C, can be approximately 40 times greater than the depth of general corrosion. At the same time, it should be emphasized that if the intensity (lg/lz) of intergranular corrosion changes little over time, then the difference between the depth of intergranular and general corrosion increases continuously. Thus, the depth of general corrosion under mass transfer conditions is determined by the equation

and the depth of intergranular corrosion for the case considered above is equal to

whence it follows

Thus, in this case, the difference between the depth of intergranular and general corrosion increases with time according to a linear law. However, such development of intergranular corrosion cannot be unlimited. As already mentioned, the maximum depth of intergranular corrosion is determined by the moment of transition to the control of the dissolution process in the channels formed by neighboring crystallites by the diffusion mechanism.

The destruction of grain boundaries due to the increased energy of the atoms located here can also occur in the process of dissolution, as well as under isothermal conditions when the solution reaches saturation. In the latter case, corrosion will be carried out by energy transfer of mass. This sometimes explains the effect of liquid metals on solid ones during isothermal tests of long duration, although the solution has long since reached an equilibrium concentration.
It should be noted that the energy transfer of mass is local in nature and covers only small areas of the surface. This feature is explained by the fact that driving force transport is the energy gradient dU/dx, where x is the distance along the surface. metal. The process of energy transfer of mass is a set of processes of dissolution, diffusion in the surface film of liquid metal and crystallization. The diffusion rate in this case is determined by an equation similar to (78):

where D is the diffusion coefficient in the liquid metal; S is the surface area through which diffusion takes place; f - coefficient of proportionality. Obviously, at a sufficiently large distance between regions with different atomic energies, the energy gradient will be small, and the diffusion rate will be negligibly low. As a result, the process of transfer between these sites will not practically occur.
2. Intergranular corrosion of alloys can be associated with selective corrosion. This effect should be observed in two cases. If an easily soluble element is horophilic, then, naturally, its predominant dissolution will cause the destruction of intercrystalline zones to a greater extent than the crystallites themselves, where the initial concentration of this element is much lower. An example of such influence seems to be the selective dissolution of nickel from austenitic steels. It is known that steels of this class are usually subjected to intergranular corrosion in liquid metals, and this effect is especially pronounced when testing steels in lead and bismuth. If we take into account that nickel is a horophilic element in iron alloys, then this effect can be explained.

The second case of intergranular corrosion of selectively dissolving alloys is possible with a uniform distribution of a readily soluble element in the matrix. The condition that ensures local destruction of the alloy along the grain boundaries, in this case, is more than high speed diffusion of a readily soluble element along grain boundaries than along their volume. Intergranular corrosion of chromium steels, observed in liquid bismuth, is apparently associated with the predominant boundary diffusion of chromium, since, according to the work, it is not horophilic in iron-based alloys. In some alloys, a readily soluble element can be both horophilic and have a higher coefficient of boundary diffusion, which should lead to a significant increase in intergranular corrosion of the alloy in the liquid metal.
In the case of chemical interaction of liquid metal with an alloy component or components, intense intergranular destruction can also be observed, caused by the above reasons.
3. The destruction of solid metals along grain boundaries in a liquid metal medium can occur at a certain ratio of the free surface energy of the boundary of two grains and the free energy of the interphase boundary solid - liquid metals.
Let us consider the equilibrium condition for surface tensions at the meeting point of the boundary between two grains and liquid metal (Fig. 44). Let us denote by γtt the surface tension of the boundary of two grains, and γtzh is the surface tension of the boundary of each grain with liquid metal (we will assume that γtl does not depend on the orientation of the grain). Let further θ be the dihedral angle between the contact surfaces of two neighboring grains with the liquid metal medium. Then the equilibrium condition, in accordance with the scheme in Fig. 44, will

Thus, depending on the ratio of the surface tension values, the surface relief of the solid metal at the exit point of the grain boundary will be different. If the equilibrium condition corresponds to a small acute angle, then in this case intergranular corrosion should be observed. Moreover, with a decrease in the dihedral angle, intergranular corrosion will increase. At θ=0, the medium will penetrate deep into the solid metal along the grain boundaries and dismember it into separate grains. In the other extreme case, at θ=180°, there will be no intergranular corrosion. The angle interval 90°≤θ≤180° can be considered as a case of the formation of small grooves along the grain boundaries, which are found on the polished surface of the hard metal after a short dissolution in the liquid metal. It is obvious that the lower limit (90°) is conditional, since even at smaller values ​​of the dihedral angle, intergranular corrosion is small. Apparently, a particularly dangerous range of values ​​of 0 should be considered 0. In view of the extreme complexity of the experimental determination of the free surface energy of solids and the energy of the solid-liquid metal interphase boundary, these quantities are known only for very few materials. There are also no sufficiently reliable methods for their theoretical calculation. Therefore, the above considerations cannot be applied to combinations of metals that are of interest in our case. As an illustration of the described effect, we point out the intergranular penetration of bismuth into copper and the absence of damage to grain boundaries when copper is immersed in lead. Given the almost complete wetting of copper by bismuth (the contact angle is close to zero) and poor wetting by lead, the difference in the action of these liquid metals becomes clear. By adding zinc and tin to bismuth, which increase the energy of the copper-bismuth interface, intergranular corrosion of copper at 600 ° C was eliminated.
It should be noted that in the case of simultaneous various kinds corrosion, the values ​​of the surface energies of the boundary of two grains and the interphase boundary can change significantly with time, which will cause a corresponding change in the dihedral angle. The energy of the boundary between two grains can change as a result of selective corrosion or boundary diffusion of liquid metal. The interfacial energy can change its value due to the formation of a solid solution or intermetallic compound on the metal surface, as well as due to a change in the composition of the liquid metal medium.
Intergranular corrosion, caused by a certain ratio of surface energies, can occur both in the process of dissolution and after saturation of the solution by energy transfer of mass.
4. Intense destruction of solid metals along the grain boundaries is observed in the presence of impurities in the liquid metal. The most characteristic example is the intergranular corrosion of materials in liquid sodium containing a significant admixture of oxygen. Thus, stainless chromium and chromium-nickel steels and nickel-based alloys undergo intergranular corrosion in sodium with an admixture of 0.5 wt.% oxygen at 700 ° C.
The reason for this effect of oxygen is the chemical interaction of oxygen ions or sodium oxide with alloy components occurring in intergranular zones. Due to the small volumes in which this interaction occurs and the small amount of reaction products, the processes of intergranular corrosion in liquid metals with impurities have not yet been studied.
5. Intergranular corrosion can also be observed in the interaction of alkali metals with oxides, sulfides, phosphides and carbides, located in solid metals mainly along grain boundaries. Such processes will be discussed in the next chapter.

I. PURPOSE OF THE WORK

Familiarize yourself with methods for detecting intergranular corrosion

steels and ways to deal with it.

II. THEORETICAL JUSTIFICATION

In stainless steels, carbon can be contained in carbides, which in the electrolyte will be more electropositive than, for example, ferrite. Consequently, electrochemical heterogeneity takes place - one of the necessary prerequisites for the occurrence of electrochemical corrosion.

In the consumption of stainless chromium-nickel steels, the maximum

the specific gravity (about 80%) is still a universal alloy of the austenitic class of the Kh18N9 type. These alloys have medium strength characteristics ( in 700 MPa), high ductility ( 40%), good weldability, high corrosion resistance in many aggressive environments: in organic (acetic, picric) and nitric acids, sea water, humid air , solutions of many salts and alkalis.

The high corrosion resistance of austenitic chromium-nickel steels is due to light passivation, in which chromium plays the main role. The iron-chromium state diagram is shown in fig. 4.1.

Chromium narrows the  region, which closes at 12% chromium and 1000C. Carbon, on the contrary, expands the  region and binds chromium into cubic Cr 23 C 6 and trigonal Cr 7 C 3 carbides, depleting the solid solution of chromium (1% carbon binds approximately 10 ... 12% chromium).

Chromium promotes the transition of iron to a passive state, while obeying the rule of stability limits ( rule n/8 Tamman).

According to this rule, the corrosion resistance of a solid solution is not directly dependent on the composition of the alloy, but varies with jocks. A sharp change in corrosion resistance occurs when the concentration of chromium or other alloying element reaches 1/8 atomic fraction or a multiple of this number, i.e. 2/8, 3/8, 4/8 etc. The position of the stability boundary (the value of n depends on the nature of the metals and the degree of aggressiveness of the medium). For example, the Fe-Cr-C alloy in 50% HNO 3 at 90С has three (n=1, 2 and 3) stability limits (Fig. 4.2), the Fe-Cr alloy in FeSO 4 solution has one (Fig. .4.3).

The n/8 rule is of great practical importance, since it makes it possible to rationally alloy the solid solution in order to increase the corrosion resistance. Thus, a sharp increase in corrosion resistance (electrode potential), shown in Fig. 4.4 corresponds to the content in the solid solution of 1/8 atomic fraction of chromium, which is equal to 12.5% ​​(atomic) or 11.7% (by mass). A higher chromium content practically does not increase the corrosion resistance of iron (Fig. 4.1, 4.2).

WITH

A significant disadvantage of stainless steels of the austenitic class of the Kh18N9 type is their tendency, under certain conditions, to intergranular corrosion. Intergranular corrosion is one of the most dangerous types of corrosion damage, since often, without changing appearance metal structure, leads to a sharp decrease in strength and ductility.

Intergranular corrosion of austenitic chromium-nickel steels is associated with low stability of grain boundaries after slow cooling of the steel in the temperature range of 450...850С, which occurs mainly during welding.

For explanation causes of intergranular corrosion There are several theories, of which the most common and experimentally proven is the theory of depletion of grain boundaries in chromium. According to this theory, when heated in the temperature range of 450...850С, chromium-rich Cr 23 C 6 or (Cr, Fe) 23 C 6 carbides precipitate along the grain boundaries. Almost all of the carbon of the alloy is involved in the formation of these carbides, and chromium is only in the areas adjacent to the grain boundaries, which is explained by the high diffusion rate of carbon compared to the diffusion rate of chromium at the above temperatures. In connection with the formation of carbides, the boundary regions of the grains become depleted in chromium, and when the chromium content is less than 1/8 of the atomic fraction (less than 11.7% by weight), these regions lose their passive state (see Fig. 4.4).

The tendency of stainless steels to intergranular corrosion is determined on samples. Tests are provided for rolled products, forgings, pipes, welds, wire, castings. Degreased and dried samples with a surface roughness class of at least 7 are tested for intergranular corrosion according to one of those given in Table. 4.1 methods.

At the end of the tests according to the AM method, the samples are removed from the flask or tank, washed, dried and bent at an angle of 90. The presence of transverse cracks on the surface of a bent sample indicates the tendency of steel to intergranular corrosion. When tested according to method B, such evidence is the presence of a continuous grid in places of anodic etching. When tested according to method D, steel is considered prone to intergranular corrosion if the steel corrosion rate after any cycle exceeds 2 mm / year or if knife corrosion is observed on welded samples, which has the form of a knife notch in the fusion zone welded joints(Fig. 4.2, 4.3).

The development of intergranular corrosion can be observed in several ways:

periodically remove samples from the solution and measure their electrical resistance: an increase in electrical resistance indicates the development of intergranular corrosion;

periodically remove samples from the solution and, throwing them on a solid slab (tile, glass, etc.), judge the development of intergranular corrosion by sound: with deep intergranular corrosion, the sample (if it is not covered with copper deposits) loses metallic ringing;

Contact corrosion can start after a long service time if the cleaning methods used are not meticulous. Oxidation and dirt in steam pipes, tools impregnated with carbon steel, and even dirty transport equipment can lead to contact of corrosive substances all the way to stainless steel containers during the cleaning period. Clean and smooth surfaces and the absence of scratches and cracks reduce the risk of contact corrosion.

The design engineer may be careful about any galvanic attack, but in turn, the personnel responsible for production, operation and Maintenance stainless steel equipment should prevent contact corrosion.

subject samples to cold bending at 180: a sample with intergranular corrosion cracks at the bend points;

examine the microsection under a microscope: the grain boundaries of steel with intergranular corrosion look wide and dark.

Fight against intergranular corrosion lead by preventing the formation of chromium carbides along the grain boundaries:

Decreased carbon content;

hardening;

Long-term heating at 860...880С;

additional doping.

Table 4.1. Test methods for intergranular corrosion

Carbon. as its content decreases, it reduces the tendency of chromium-nickel steels to intergranular corrosion. With a carbon content of less than 0.015%, these steels are practically not prone to this type of corrosion.

hardening. As a result of quenching in water from temperatures of 1050...1100С, carbon and chromium are fixed in a solid solution, which is favorable in terms of corrosion.

Long (more than two hours) heating at temperatures of 860...880С. With such heating, carbides cease to precipitate and their coagulation proceeds, and therefore the continuity of the carbide network and chromium-depleted regions along the grain boundaries is disrupted. Moreover, chromium, due to long exposure, has time to diffuse into the depleted areas, which leads to equalization of its concentration and an increase in the passivation of steel. When heated to 860...880С, internal stresses that have arisen during the formation of carbides are completely removed, and this also contributes to an increase in corrosion resistance.

Additional doping elements that bind carbon into more sparingly soluble carbides compared to chromium ones, prevents the occurrence of intergranular corrosion. Such alloying elements are Ti, Nb, Ta. For complete bonding into carbides, there must be some excess of these elements in relation to the stoichiometric composition (TiC, etc.). However, the addition of alloying elements in large quantities can lead to the formation of a ferrite component, which does not reduce, but even accelerates the development of intergranular corrosion.

Microstructures of Cr-Ni steels of the austenitic class after quenching without provoking heating and after quenching with subsequent prolonged heating at elevated temperatures.

affected by intergranular corrosion

healthy microstructure (not affected by ICC)

Table 4.2. Determination by sound of the presence or absence of intergranular corrosion in samples of steel 08X18H10T.

Conclusions on the effect of Ti on the tendency of Cr-Ni steels of the austenitic class to intergranular corrosion.

Ti is an alloying element that binds carbon into more sparingly soluble carbides compared to chromium carbides and prevents the occurrence of intergranular corrosion. For complete bonding into carbides, there must be some excess of this element in relation to the stoichiometric composition (TiC, etc.). However, the addition of alloying elements in large quantities can lead to the formation of a ferrite component, which does not reduce, but even accelerates the development of intergranular corrosion.