1. electrochemical stability of the siliceous brass ЛК80-3
Thermodynamics
of chemical and
electrochemical stability
of the siliceous brass ЛК80-3
Nikolaychuk P. A., Tyurin A. G.
Chelyabinsk State University

2. Brass is the copper - zinc alloy, alloyed by other metals
Siliceous brasses are alloyed by silicon and contain Cu, Zn and Si.
Siliceous brasses are highly resistant to corrosion in air and water environments.
The thermodynamic description of their corrosion-electrochemical behaviour is quiet interesting scientific task.

3. ЛК80-3 Silicon (”Кремний”) (”Латунь”)
“Percentage by weight of the alloying component (Si)
“Alloying component
Silicon (”Кремний”)
ЛК80-3
“Brass””
(”Латунь”)
“Percentage by weight of the first basic component (Cu)
“The percentage by weight of the second basic component (Zn) is calculated by subtracting from 100 the percentages of all other components.

4. The composition of siliceous brass ЛК80-3
Component
Weight
fraction
Mole Cu
0,80
0,774 Zn
0,17
0,160 Si
0,03
0,066

5. Phase equilibria in Cu - Zn - Si system

6. The phase diagram of binary Cu - Zn system17
The phase diagram of binary Cu - Zn system

7. The phase diagram of binary Cu - Si system3
The phase diagram of binary Cu - Si system

8. The phase diagram of binary Zn - Si system
The siliceous brass ЛК80-3 is one-phase alloy, which consists of ternary solid solution with face-centered cubic lattice (α-phase)

9. The excessive Gibbs energy GE of α-phase Is determined, according to Redlich-Kister power series

10. Each of the Lij parameters is treated as function of temperature (T) and solid solution composition (xCu, xZn, xSi)

11. Each of the Lij parameters is treated as function of temperature (T) and solid solution composition (xCu, xZn, xSi)

12. Each of the Lij parameters is treated as function of temperature (T) and solid solution composition (xCu, xZn, xSi)
α-phase (fcc solid solution) is NOT stable in binary Zn - Si system

13. Calculating thermodynamic activities of the components of brass ЛК80-3
The excessive chemical potentials of the components are calculated as follows:

14. Calculating thermodynamic activities of the components of brass ЛК80-3
The partial derivatives of the excessive Gibbs energy by the mole fractions of the components are calculated in this way:

15. The activities of the components
All values of thermodynamic activities are lesser, than unity. Thus the calculations prove the fact, that brass ЛК80-3 is one-phase alloy

16. The chemical stability: equilibria with oxygen

17. The phase diagram of binary Si - O system

18. The standard electrode potential of the silicon electrode is measured experimentally and available in literature:
Using this value and according to Hess’ law the standard Gibbs energy of formation of SiO2 is calculated:

19. The phase diagram of binary Cu - O system

20. There is NO published thermodynamic data on copper dioxide CuO2
There are several evidences, that in addition to Cu2O and CuO oxides, presented on the diagram, the Cu2O3 and even CuO2 oxides can be formed in Cu - O binary system
There is NO published thermodynamic data on copper dioxide CuO2
An intermediate compound is formed between the oxides CuO and SiO2. It is copper silicate CuSiO3

21. The phase diagram of binary Zn - O system

22. There is NO published thermodynamic data on zinc dioxide ZnO2
An intermediate compound is formed between the oxides ZnO and SiO2. It is zinc silicate ZnSiO3.

23. Thermodynamic properties of compounds of one type change periodically with alteration of basic element nuclear charge

27. The pressure of oxygen in air at standard conditions is equal to 0,21 bar. This means, that all equilibria, having equilibrium oxygen pressure greater, than 0,21 bar, are not realized in air at these conditions
Despite Cu2O3 can be formed in air at standard condition, it seems, that it can not form a continuous single phase

28. As follows from calculations, the copper silicate CuSiO3 can not be formed in all.
Silicon content in brass is lower, than zinc content, so SiO2 reacts with Zn, forming ZnSiO3, rather than it can react with CuO, because oxygen equilibrium pressure of first reaction is lesser in 1083 times, than that of second reaction.
The equilibria, marked by * in previous table, are not achieved

29. The oxidation of the siliceous brass ЛК80-3 in air at standard conditions is ended by forming the oxide film, that consists of CuO and ZnSiO3

30. The electrochemical stability: equilibria in liquid environments

31. In addition to compounds, considered earlier, following ions can be formed in liquid environments and involved in chemical and electrochemical equilibria:

32. The scheme of consecutive transformation of copper (II) various forms with increasing of environment pH

33. The scheme of consecutive transformation of zinc (II) various forms with increasing of environment pH

34. The diagrams of electrochemical equilibrium (potential - pH) are used to represent possible chemical and electrochemical reactions in system and analyze its corrosion-electrochemical behaviour
The potential - pH diagrams of “brass ЛК80-3-Н2О” system are plotted at 298 K, air pressure of 1 bar and activities of all ions in solution, equal to 1 mole/l and to 10–6 mole/l

35. ai = 1 mole/l

36. ai = 10–6 mole/l

37. Basic chemical and electrochemical equilibria in system “brass ЛК80-3 - H2O”

38. Basic chemical and electro-chemical equilibria in system “brass ЛК80-3 - H2O”

39. Domain of immunity

40. Domain of active dissolution

41. Domain of passivity

42. Domain of trans passivity

43. Domain of water electrochemical stability

44. Domain of water electrochemical stability
Strongly acidic environments (pH < 4):
Acidic, neutral and alkaline environments (4 < pH < 13):
Strongly alkaline environments (pH > 13):

45. Conclusions
The passivation film, based on zinc silicate ZnSiO3, is much more resistant in chemical and electrochemical terms, than that of zinc or copper oxides
The presence of zinc and silicon determines the high corrosion resistance of the siliceous brass. Since zinc and silicon have no valuable mutual solubility and can only form mechanical mixture, copper is needed to bind them into solid solution and make the alloy one-phase
This way, the thermodynamic conclusions confirm the fact of good influence of silicon on copper-zinc alloys corrosion-electrochemical behaviour

46. Thank you
for attention!