Effect of Shot Peening and Plasma Electrolytic Oxidation on the Intergranular Corrosion Behavior of 7A85 Aluminum Alloy

doi:10.1007/s40195-014-0104-9

Abstract

Abstract:

The effects of shot peening (SP) and plasma electrolytic oxidation (PEO) on the intergranular corrosion behavior of the novel high strength aluminum alloy 7A85 (AA 7A85) were investigated by electrochemical polarization and electrochemical impedance tests. The intergranular corrosion mechanism of SP, PEO and PEO combined with sealing-treated AA 7A85 was studied by the metallographic analysis, residual stress testing, X-ray diffractometer analysis and scanning electron microscopy. The results show that AA 7A85-T7452 is very sensitive to intergranular corrosion. SP would significantly improve its intergranular corrosion resistance. This is attributed to the combination action of residual compressive stress and grain refinement. PEO would reduce the largest corrosion depth by 41.6%. Moreover, PEO without sealing did not eliminate the intergranular corrosion due to the existence of the micropores and microcracks in the oxide coating. However, PEO combined with the SiO₂ sol–gel sealing treatment could effectively protect the AA 7A85-T7452 from intergranular corrosion because of the good corrosion resistance and barrier function of the sealed coating.

Key words: 7A85 aluminum alloy, Plasma electrolytic oxidation, Shot peening, Intergranular corrosion, Residual stress

Zuoyan Ye, Daoxin Liu, Chongyang Li, Xiaoming Zhang, Zhi Yang, Mingxia Lei. Effect of Shot Peening and Plasma Electrolytic Oxidation on the Intergranular Corrosion Behavior of 7A85 Aluminum Alloy[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(4): 705-713.

Figures/Tables 11

Fig. 1 XRD patterns of 7A85 alloy before and after SP

Fig. 2 Surface morphology of coatings: a PEO; b PEO + S

Fig. 3 Cross-sectional element distribution a & XRD pattern b of PEO coating surface

Table 1 Calculated Tafel parameters from the polarization plots

Surface	E_corr (V)	I_corr (A/cm²)	R_p (Ω/cm²)
BM	-0.765	1.20 × 10^-3	8.17 × 10³
SP	-0.758	3.26 × 10^-3	5.78 × 10³
PEO	-0.749	1.22 × 10^-4	1.32 × 10⁵
PEO + S	-0.740	2.28 × 10^-7	8.09 × 10⁷

Fig. 4 Tafel polarization plots for samples with different surface finishes after immersion in IGC solution at 35 °C for 30 min

Fig. 5 Electrochemical impedance spectrum a & equivalent circuit b for PEO coating after immersion in IGC solution at 35 °C for 10 min

Table 2 Calculated impedance parameters of PEO and PEO + S coatings

Coating	R_s (Ω/cm^-2)	R_p (Ω/cm^-2)	C_coating (F/cm²)	R_b (Ω/cm^-2)	R_coating (Ω/cm^-2)	Q(C_b) (Ω^-1/cm²/s^-n)	Q - n
PEO	18.9	411.5	1.904 × 10^-8	6.142 × 10³	–	3.479 × 10^-5	0.46
PEO + S	19.8	–	2.460 × 10^-7	–	6.851 × 10⁵	–	–

Fig. 6 Electrochemical impedance spectrum a & equivalent circuit b for PEO + S coating after immersion in IGC solution at 35 °C for 10 min

Fig. 7 Corrosion morphologies of different samples after 24-h immersion: a BM; b SP; c PEO; d PEO + S

Fig. 8 Cross-sectional morphologies of different samples after IGC tests: a BM; b SP; c PEO; d PEO + S

Table 3 Intergranular corrosion depth of samples with different surface finishes

Surface states	Average depth (μm)	Maximum depth (μm)
BM	176.4	200.2
SP	61.5	77.4
PEO	108.7	116.9
PEO + S	0	0

References 16

[1]	D.J. Chakrabarti, J. Liu, R.R. Sawtell, G.B. Venema, Mater. Forum 28, 969(2004)
[2]	M.C. Baruch, U.S. Patent 3856584(1974)
[3]	D. Wang, D.R. Ni, Z.Y. Ma, Mater. Sci. Eng. A 494, 360(2008)
[4]	S.Y. Chen, K.H. Chen, G.S. Peng, L. Jia, P.X. Dong, Mater. Des. 35, 93(2012)10.1016/j.matdes.2011.09.033
[5]	S. Curtis, E.R. de Los Rios, C.A. Rodopoulos, A. Levers, Int. J. Fatigue 25, 59(2003)10.1016/S0142-1123(02)00049-X
[6]	D.T. Asquith, A.L. Yerokhin, J.R. Yates, A. Matthews, Thin Solid Films 515, 1187(2006)10.1016/j.tsf.2006.07.123
[7]	D.T. Asquith, A.L. Yerokhin, J.R. Yates, A. Matthews, Thin Solid Films 516, 417(2007)10.1016/j.tsf.2007.06.166
[8]	T.B. Wei, F.Y. Yan, J. Tian, J. Alloys Compd. 389, 169(2005)10.1016/j.jallcom.2004.05.084
[9]	X.H. Zhang, D.X. Liu, Int. J. Fatigue 31, 889(2009)10.1016/j.ijfatigue.2008.10.004
[10]	V. Raj, M. Mubarak Ali, J. Mater. Process. Technol. 209, 5341(2009)10.1016/j.jmatprotec.2009.04.004
[11]	J.H. Liu, D. Li, P.Y. Liu, B.L. Lan, J. Mater. Eng. (2), 30 (2005)(in Chinese)
[12]	A. Venugopal, R. Panda, S. Manwatkar, K. Sreekumar, L.R. Krishna, G. Sundararajan, Trans. Nonferr. Met. Soc. 22, 700(2012)10.1016/S1003-6326(11)61234-X
[13]	X.W. Yu, C.N. Cao, Thin Solid Films 423, 252(2003)10.1016/S0040-6090(02)01038-6
[14]	J. Wloka, T. Hack, S. Virtanen, Corros. Sci. 49, 1437(2007)10.1016/j.corsci.2006.06.033
[15]	D. McNaughtan, M. Worsfold, M.J. Robinson, Corros. Sci. 45, 2377(2003)10.1016/S0010-938X(03)00050-7
[16]	X.D. Liu, G.S. Frankel, Corros. Sci. 48, 3309(2006)10.1016/j.corsci.2005.12.003