Study of the Corrosion Behavior of Nanocrystalline Ni-P Electrodeposited Coating
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INTRODUCTION
THE Ni-P alloy has received a lot of attention due to its potential as a protective coating in machinery, automotive, and aerospace industries. The Ni-P alloy with a P content greater than 8 wt pct possesses excellent mechanical properties, such as hardness, wear resistance, and corrosion resistance. This protective film can be deposited by electroplating and electroless plating onto a metallic or nonmetallic substrate to prevent attack by hostile environments.[1–4] Flis and his co-workers[2] reported that the electrolessdeposited Ni-P alloy with 2.9 to ~12.3 wt pct P can suppress the corrosion rate. Corrosion pits with internal cracks were found on the surface of the Ni-2.9 pct P alloy polarized at 0.4 V for 500 seconds in 0.1 M NaCl solution. The passivated film was considered to be Ni3 (PO4)2 8H2O. Rofagha et al.[3] studied the effects of the grain size and deposit P content on the corrosion behavior of the Ni-P alloy in 0.1 M H2SO4 solution. They found that in contrast to polycrystalline Ni, passivation was not observed for the Ni-P alloys. Pitting was found on both nanocrystalline and amorphous deposits. Moreover, the dissolution rate increased as the grain size of the Ni-P alloy decreased. Petukhov and his co-workers[4] also reported that no passivation was observed for the polarization of the Ni-P alloys (6.6 to 13.4 wt pct P) in 0.5 M H2SO4 solution. They proposed that the dissolution was controlled by the transient H.B. LEE, Ph.D. Candidate, and D.S. WUU, Professor, are with the Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan R.O.C. C.Y. LEE, Professor, is with the Department of Mechanical Engineering, National Taipei University of Technology, Taipei 106, Taiwan R.O.C. C.S. LIN, Professor, is with the Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan R.O.C. Contact e-mail: [email protected] Manuscript submitted August 22, 2009. Article published online November 17, 2009 450—VOLUME 41A, FEBRUARY 2010
volume diffusion of Ni and accompanied by the surface accumulation of P. Lu and Zangari[5] observed that the electroless-deposited Ni-P alloys exhibited partial passivation at low anodic polarization in 0.5 M H2SO4 solution. The mechanism was mainly due to the chemical effects related to the presence of P. Bai and his co-workers[6] studied the corrosion behavior of Ni-P deposits using the brine-fog test in 5 wt pct NaCl solution at 37 C for 24 hours. The weight loss decreased for the specimen with higher P contents. They also reported that after heat treatment at 300 C for 1 hour, the Ni-28P coating had further improved corrosion resistance. Bozzini et al.[7] investigated the erosion corrosion of the Ni-P coating. Severe pitting and corrosion crack occurred after extended immersion. Accordingly, the accelerating weight loss due to the erosion was observed. The potentiodynamic measurement of the electroless-deposited Ni-9.5 pct P in the solution containing 0.6 M NaCl and 0.32 M H3BO3 showed the a
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