Corrosion of Materials in the Working Medium

An important factor determining the operation conditions of HRAs is the corrosion resistance of materials in hydrogen with methane added [1, 2].

The extent of carbides interacting with gases is defined by gas activity with carbide components and gas partial pressures in a mix [3-5]. The maximum activity is the oxygen, which interact even at low partial pressures (10-3-10-1vol%) harder above than hydrogen; therefore, the interaction with oxygen will be observed first. The mechanism of ZrC interacting with oxygen can be presented by following equations (Table 6.1). The detailed oxidation mechanism description of the NbC and solid solutions ZrC-NbC is complicated by two reasons: first, there is a row of the flying oxides in the system Nb-O which is sublimated at high temperatures; second, the oxidation of these carbides was studied in rather narrow temperature intervals.

Kinetic characteristics of oxidation of ZrC and NbC are resulted in Figs.6.1 and 6.2.

Determination of kinetic regularity in an intermediate interval of temperatures 1000-1300K is complicated because of an oxide film fall and an unknown surface of oxidation. The resulted data on oxidation kinetics refer to the materials practically with a stoichiometric composition. Decrease of C/Me to 0.9-0.8 in all carbide mate­rials reduces reaction rate constant. High-temperature oxidation (2,100-2,500K) in hydrogen (H2 + 0.07 ^ 0.1%O2) leads to decrease in strength of ZrC approxi­mately two times for the first 1,000 s of interacting (Fig.6.3). At higher tempera­tures (>2,600K) in the specified gas mix ZrC samples are fractured after 500 s of interaction. A source of rupture is a formation of metal zirconium on reaction 5 (Table 6.1). Metallographic analysis of a fractured places after the sample exposure at T = 2,700 K showed the segregations of metal zirconium in an oxide film.

The interaction of carbides with hydrogen at high temperatures is accompanied by structural changes, the formation of hydrocarbons, and a decrease in the carbide stochiometric according to the reaction

MeC + y/2 H2 = MeC + CxHy.

A. Lanin, Nuclear Rocket Engine Reactor, Springer Series in Materials Science 170, DOI: 10.1007/978-3-642-32430-7_6, © Springer-Verlag Berlin Heidelberg 2013

Fig. 6.1 Oxidation of zirconium carbide in air. (Z1C0.95, Ctotal = 11-4%. Cfree = 0.3; O + N = 0.10%;

= 3-5%; L = 8 ц) [4]. 1—T = 850 K, 2—T =

900 K, 3—T = 950 K,

4—T = 1,000 K, 5—T = 1,050 K

Change of lattice parameters of ZrC after an exposure time in hydrogen allows sizing up the change of relationship C/Zr on cross-section of the sample at the expense of a carbon removal (Fig. 6.4). Calculated estimations are coordinated qualitatively with the chemical analysis data.

The type of the interaction of UC-ZrC, UC-NbC, and UC-ZrC-NbC systems with hydrogen is similar. Only the degrees of decarbonation, the carbon concentration gra­dients over a sample cross-section, and recrystallization temperatures are different. This suggests that all the systems under study have the same mechanism of interac­tion with hydrogen. On the surface of samples, the reaction of carbon with hydrogen proceeds with the formation of CH4 and C2H2 and the evaporation of metal atoms, while the diffusion of carbon from the center to the surface and recrystallization occur in the bulk. Calculating the interaction for the scheme of the process under study requires solving nonstationary diffusion problems with the boundary condi­tions simulating a mass exchange with the working substance in the presence of heterogeneous chemical reactions (Fig. 6.5).

The total rate of a material removal for ZrC and NbC is maximum at first moments of an exposure time and the temperature dependence is expressed by the Eqs. (6.7) and (6.8):

The total entrainment rates for double ZrC-UC and triple ZrC-NbC-UC solid solutions in hydrogen-methane media with 0.656 volume% of CH4 at 3,150 K are approximately the same (0.94-10-6gcm-2 s-1). High temperatures in experiments lead not only to a change in the chemical composition of materials but also to a change in the density of carbides and to an increase in the grain size. The appearance of a carbon concentration gradient over the sample cross-section at temperatures up to 2,500 K for 1,000 s leads to the formation of compressing stresses up to 500MPa and to an increase in the strength due to a change in the lattice parameter.

The interaction of carbides with hydrogen at 3,100K for 1,000s produces an inhomogeneous concentration of carbon not only but also causes a change in the structure, a decrease in the density, and a reduction in the strength to 50 % (Fig. 6.6). It should be noted that exposure at a lower temperature near 2,600K may even increase strength due to the formation of residual compressive stresses. The results of model tests agree well with the HRA tests in the IVG-1 reactor.

Decarburization of NbC in hydrogen occurs more intensive. Transformation of NbC into Nb2C occurs at 2,450K after 6,000 s. The gradient of concentration of carbon on cross-section is insignificant though Nb2C grains are observed all over

cross-section. Active growth of grain is observed at a surface of the sample since 2,500K. Temperature lifting to 3,000K leads to intensive decarburization and the damage of samples (Fig. 6.7).

The general entrainment of niobium carbide is connected with carbon loss, and the metal component of carbide evaporates slightly, as opposed to ZrC when an apprecia­ble evaporation of zirconium is observed. This fact supports higher concentration of
carbon in ZrC after interacting. Interaction of solid solutions ZrC-NbC with hydro­gen at high temperatures along with decarburization, the enrichment of surface by more refractory component of niobium is observed.

Apart from the chemical action of a high rate working substance flowing around an element, a force erosion action of the flow on the surface is also possible; as a result, some weakly attached carbide particles are carried away by the flow.

Based on the dependences of the entrainment rate of the carbon component from carbides, another protection method was proposed in which hydrocarbons are added to hydrogen in the amount at which their mean concentration in the flow becomes equal to their equilibrium concentration over the surface. Then the carbon entrain­ment must be zero. But the equilibrium composition of hydrogen can be achieved not over the entire HRA length but at individual points only. In other places, either the entrainment or deposition of carbon on the washed surface occurs. Therefore, it is reasonable to regard the method of protective additions as supplementary to the main method of protection with the help of coatings.

Thermal stresses, a1, in a coating of thickness h1 originate even at uniform heating to temperature T due to a difference of coefficients of linear expansion of a coating a1 and substrate a2. Accepting the condition of equal deformation on the boundary line between coating and substrate, the stress in the coating can be expressed:

_ E1E2 a — a2) T

ai ~ IE1 (1 — 1×2) + E2(1 — 1×1)

where index 1 refers to coating and index 2 refers to substrate. E is modulus elasticity, ц is Poisson’s coefficient. Resistance of coating can be determined by the level of limiting tensile stresses.

The thermal stress destruction of a coating can occur as a result of coating cracking or at the expense of its peeling from a substrate to the subsequent destruction of a brittle thin covering. The fracture aspect is defined by extent of adhesion and parameters of a thermal loading (Fig. 6.8).

The corrosion resistance of coating made of low-density pyrographite (LDP) (1.35-1.6 g cm-3) in the HIP, their weight in the hydrogen flow at 1,570K decreases for 6,000 s by approximately 3-8 %, while the strength decreases by 30 %.

The strength of casing from ZrC composition with pyrographite after annealing essentially decreased. Some casings (No 2, 5, 6, Table6.2) were destroyed at a light touch.

The loss in the mass of cases made of PGV pyrographite with distinctly anisotropic properties (density from 1.2 to 2.25 gcm-3) in hydrogen at 2,300 K for 4,000 s reaches 45 %. Due to a low porosity, the interaction of uncoated pyrolytic graphite with hydrogen, for example, at 2,100 K is two times weaker than that for usual pressed graphite types.

Table 6.2 Density and loss of free carbon from the ZrC composition with pyrographite after exposure

No

Y (g/cm2)

Test condition

Cfree (% weight)

After test

T(K)

t (s)

Initial

After test

AC/Cinit

AY/Y

1

3.5

2,270

10,000

25.5

18.3

0.28

0.57

2

3.1

2,270

10,000

19.7

8.9

0.54

0.9

3

3.0

2,470

6,000

34.5

20.9

0.39

1.3

4

3.3

2,470

4,000

19.7

4.2

0.78

0.9

5

3.5

2,470

6,000

27.0

8.9

0.67

0.57

6

2.8

2,470

4,000

22.0

2.7

0.87

0.1

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