Stress corrosion cracking of a low alloy steel in hot caustic aluminate solutions

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Scripta Metallurgicaet Materialia, Vol. 31, No. 4, pp. 42%432, 1994 Copyright©1994ElsevierScienceLtd Pnnted m the USA.All fightsreserved 0956-716X/94 $6 00 + 00

STRESS C O R R O S I O N C R A C K I N G OF A LOW ALLOY STEEL IN H O T CAUSTIC A L U M I N A T E SOLUTIONS Liu Su--e Zhu ZiYong Ke Wei Corrosion science laboratory, Institute of Corrosion and Protection of Metals, Academia sinica, Shenyang, 110015, China

(Received September 3, 1993) (Revised April 21, 1994) 1 Introduction Stress corrosion cracking (SCC) of steels in hot caustic solutions has been the subject of numerous studies[I-4]. Type 16MnR low alloy steel is exposed to hot caustic aluminate solutions in the Bayer process for purification of alumina from hydrated oxide ores (e.g., bauxite). It is the common material of construction for the welded reaction vessels (e.g., digesters) and frequently suffers from SCC during service. The effect of the aluminate species, AlOe, is of considerable industrial significance in relation to the Bayer process. However, little work has been done on the effect of the presence of A10~ species on the SCC susceptibility of steels in hot caustic solutions. Sriram et al [5] reported that the presence of dissolved A10~ species promoted cracking in hot(92"C) caustic aluminate solutions in the passive region at potentials where SCC was not observed in the simple NaOH solution. Despite this, there is no report on the effect of the presence of AIO~ species on the SCC susceptibility of steels in high temperature, high pressure, caustic aluminate solutions. The present study attempts to investigate the effect mentioned above by using the slow strain rate testing technique, cyclic voltammetry techniques, and X - r a y diffraction analysis. 2 Experimental Procedure SCC tests were conducted on a hot-rolled pressure-vessel quality steel plate, GB 6654-86 Grade 16MnR, having a composition (weight percentage) of 0.16C,0.47Si,l.53Mn,0.014P,0.018S and 0.055Cu. The plate was 50.0mm in thickness, and cylindrical tensile test specimens (25ram gauge length and 5ram diameter) were cut from the steel parallel to the rolling direction. The specimens were polished with a 1000 grit emery paper, then cleaned with alcohol and acetone before testing, The test solution was prepared from distilled water and analytical grade chemicals, using NaOH pellets and alumina trihydrate(A120 3 • 3H20). The slow strain rate tests(SSRT) were conducted on the type SERT-5000DP-9L machine with a strain rate of 3.3 × 10-~s-tat 260"C. At the start of each test, the specimens were initially loaded to a value of 1000N and then to the required strain rate value. The 1.9 liter Hastelloy C-276 alloy lined autoclave was pressurized with nitrogen before testing.

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The final diameter(dr) of the specimen at the location was measured with a travelling microscope and the percentage reduction in the cross--sectional area(% RA) was obtained from a comparison with the original specimen diameter(d0) % RA = 100(d20-d~)/d~. Cylindrical specimens, 2.6ram diameter × 10mm long, were used for polarization experiments. The specimens were polished with 800 grit SiC metallographic paper. They were point-welded with cabofi steel rods which passed out of the autoclave head through pressure fittings. The carbon steel rods were coated with shrinkable Teflon tubing for electrical insulation. A high purity nickel wire (inserted through a high pressure fitting) served as an internal reference electrode which behaved like a hydrogen reference electrode in a solution with an atmosphere of 5 percent hydrogen (H2)-95 percent nitrogen (N 2) cover gas [6]. A fiat platinum sheet was used as a counter electrode. Polarization and cyclic voltammetry studies were also conducted at 26012 . Anodic potentiodynamic polarization studies were conducted with a model HA-301 potentiostat, a model HBI04 function generator, a model HBI02 log converter and a model 3084 A4 recorder. Cyclic voltammetry studies were conducted at scan rates of 50mVs-~and 500mVs-~respectively. The specimens were cathodically polarized at -300mV(Ni) for 15 minutes to reduce any surface oxide films before initiating scanning in the anodic direction. Immersion tests were also conducted in caustic solutions at 26012 and X - r a y diffraction analysis was used to study the film on the surface of specimens. 3 Results and Discussion

SSRT results are shown in Figure 1. It is obvious that when the free OH- concentration is held constant, the percentage reduction in an area for the caustic aluminate solution is larger than that for the simple caustic solution. On the other hand, when the AlOe" concentration remains the same, the percentage reduction in area decreases with increasing free OH- concentration. These results indicate that OH- promotes the SCC susceptibility, while AIO~ shows an inhibition effect on caustic SCC at 260"(2. NaOll AI20 ~ 31120

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ly involves in cycles of film rupture, dissolution, and repassivation. Therefore, the nature of the passive films is believed to play an important role in SCC [1,2,9]. Representative cyclic voltammograms of potential(E) versus current density(i) are shown in Figure 3, which demonstrates the behavior of the steel in 4,78M N a O H and 7.42M N a O H + l . 3 2 M A1203 3H20 solutions at a scan rate of 50 mVs -t. Evidently the polarization curve in the simple caustic solution is similar to that in the caustic aluminate solution in the forward direction (active to noble). On the other hand, on the reverse scan, the major reduction peak(l] ) shows a quite large anodie current density in the simple caustic solution and a small cathodic density in the caustic aluminate solution, respectively. This means that the film on the surface of specimens in the latter solution is more stable than that in the former solution.

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Figure 4 shows the voitammograms in both solutions at a scan rate of 500mVs-~. The stabilized voltammogram in the caustic aluminate solution is also more easily reached than in the simple caustic solution. After immersion for 100h, the film on the surface of specimens in the simple caustic solution was loose and easy to be removed, meanwhile it was more dense and protective in the caustic aluminate solution. The X-ray diffractionanalysis indicated that the films for the simple caustic solution was Fc304 and for the aluminatc solution was Fe3_xAlxO4(x,~ 2) and (Fe,Mn)3_xAlxO4(x~ 2) which was not an amorphous film and differentfrom that reported by Sriram et al. for the caustic aluminate solution at 92~C [10].

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Previous studies [1,2,5,7] have shown that at anodic potentials, carbon steel was most susceptible to caustic SCC near the active-passive transition. Also the SCC susceptibility is inconsistent with a hydrogen-related mechanism of cracking, because the thermodynamically-reversible potential for hydrogen evolution, E . / . a

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tions, thereby eliminating hydrogen effects from consideration at these potentials. Figure 2 shows the potentiodynamic polarization curves for 16MnR steel in caustic solutions. Sweeping a range of potentials in the anodic direction at a relatively high scan rate(50mVs -t) indicates regions wherein intense anodie activity is likely and the currents observed relate to relatively film-free or thin-film conditions, while a s)ow scan rate(0.SmVs -I) allows time for filming to occur and indicates regions wherein relative inactivity is likely. Therefore a comparison of the two curves will indicate any ranges of potential within which high anodic activity in the film-free condition reduces to insignificant activity when the time requirements for film formation are met and this will indicate the range of potenrials within which SCC is likely. 104

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According to the boundary conditions suggested by Parkins [8], the potential range for SCC could be predicted. As shown in Figure 2, the potential range for SCC in the caustic aluminate solution [ - 8 0 m V ~ 100mV(Ni)] is narrower than that in the simple caustic solution [ - 1 2 0 m V ~ 140mV('Ni)], indicating the reduction in potential range for SCC due to the action of AlOe. This result is consistent with the results from SSRT. SCC susceptibility in hot caustic solutions is consistent with the film rupture model. The model is based on crack advance by localized electrochemical dissolution of bare metal at the crack tip and usual-

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It has been proposed [11] that during the SCC process the average crack propagation rate by an oxidation related (slip dissolution or brittle film) model is faradaically related to the oxidation charge density by V = M

npF

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strain rate at the crack tip. It is obvious that the average crack propagation decreases with increasing fracture strain of the oxide at the crack tip. As mentioned above, the film for the caustic aluminate solution is more stable and protective than that for the simple caustic solution. Therefore, we consider that the fracture strain of the oxide at the crack tip for the former solution is more than that for the latter solution, leading to a less crack propagation and less SCC susceptibility, which is consistent with results from SSRT, potentiodynamic polarization and cyclic voltammetry studies. 4 Conclusions

(1) SCC susceptibility increases with OH-concentration in caustic solution at 260"C. (2) AlOe- shows an inhibition effect on SCC in caustic solution at 260~. (3) The presence of aluminate anions in caustic solutions reduces the potential range for SCC and makes the film more stable and protective, leading to a less crack propagation during SCC process in caustic solutions at 260~. References

1. D.Singbeil, D.Tromans, J.Electrochem. Soc., VoI.128,P.2065,1981. 2. D.Singbeil, D.Tromans, Met.Trans.A, Vol.13, P.1091, 1982. 3. M.F.Maday, A.Mignone, and A.Borello, Corrosion, Vol.45, P.273,1989. 4. J.M.Sarver, J.V.Monter, and J.M.Helmey, in Proceedings of the Fourth International Symposium on Environmental Degradation of Materials in Nuclear Power System-water Reactors, Jekyll Island, Georgia, August 6-10, 1989, Ed. D.Cubicciotti, Electric Power Research Institute, P.5-107.

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5. R.Sriram and D.Tromans, Corrosion, Vol.41, P.381, 1985. 6. G.J.Thcus and J.R.Ccls, in UStress Corrosion Cracking,the Slow strain Rate TechniqueI/, ASTM STP665, G.M.Ugiansky and J.H.Payer, Eds., ASTM, 1979, P.Sl. 7. J.E.Reinochl, W.E.Bcrry, Corrosion, Vol.28, P.151, 1972. 8. R.N.Parkins, Corrosion Science, Vol.20, P.147, 1980. 9. J.E.Scully, Corrosion Science, Vol.20, P.997, 1980. 10. R.Sriram, D.Tromans, Corrosion Science, Vol.25, P.79, 1985. 11. F P.Ford, in the '~Environment-Scnsitivc Fracture: Evaluation and Comparision of Test MethodsI/, ASTM STP821, S.W.Dcan, E.N.Pugh, and G.M.Ugiansky, Eds., ASTM, Philadelphia, 1984, P.32.