Browsing by Author "Jian, Shun-Yi"

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Characteristic V-Mn conversion coating in reinforcing corrosion resistance of electroless Ni-P coated LZ91 Mg alloy
(2020-07-25)
Jian, Shun-Yi
;
Chang, Jun-Kai
;
Lin, Kung-Feng
;
Chen, Ting-Yu
;
Yuan, Jui-Hung
A V-Mn conversion coating was studied and furthermore employed to improve the corrosion resistance of electroless Ni-P coated LZ91 Mg alloy. A thin, continuous and compact V-Mn conversion coating was formed on LZ91 after the immersion of 60 s in the conversion coating solution. The results of electrochemical impedance spectroscopy (EIS) and salt spray test (SST) showed the enhanced corrosion resistance of LZ91. The impedance in EIS increased from 573 Omega.cm(2) to 11,319 Omega.cm(2), and moreover the deteriorated area ratio of the V-Mn conversion coated LZ91 decreased to < 5% after 120 h of SST exposures. Further study of a V-Mn conversion coating/electroless Ni-P composite coated LZ91 was used to investigate the improvement of the corrosion resistance in the electroless Ni-P coated LZ91. The composite coated LZ91 demonstrated better corrosion resistance of mere electroless Ni-P coated LZ91. Furthermore, the corrosion resistance of the composite coated LZ91 was greatly enhanced. The impedance of the composite coated LZ91 was 500 Omega.cm(2), larger than that of the electroless Ni-P coating of 5 Omega.cm(2). The composite coated LZ91 showed better corrosion resistance in the SST exposures in comparison to mere electroless Ni-P coated LZ91. The composite coating indicated that the positive characteristics of a V-Mn conversion coating usefully enhanced the corrosion resistance electroless Ni-P coated LZ91. Moreover, based on the above results, a mechanism for the conversion coating and corrosion resistance was proposed.
Robust corrosion resistance and self-healing characteristics of a novel Ce / Mn conversion coatings on EV31 magnesium alloys
(2020-04-30)
Jian, Shun-Yi
;
Yang, Chia-Yu
;
Chang, Jun-Kai
;
交大名義發表
;
National Chiao Tung University
Magnesium alloys have been widely employed in the aerospace and aviation applications due to the unique properties. Nevertheless, Mg alloys were sensitive to corrosion attacks. The use of hexavalent chromium has been strictly banned because of severe toxicity and harm, even though it provided useful protective coating. Many efforts have been made to find the alternative process to hexavalent chromium conversion process. In this study, the Ce / Mn conversion coating was developed on EV31 Mg alloy. A thin, homogeneous, and compact conversion coating layers was formed. The electrochemical impedance spectroscopy (EIS), potentiodynamic polarization tests and salt spray test (SST) were used to evaluate the corrosion resistance of the coatings accompanying with a cross-cut scratches. The total impedance of the Ce/ Mn conversion coating rose to 2.3 x 10(3) Omega.cm(2) compared with 3.4 x 10(2) of the bare EV31. Furthermore, the Ce / Mn conversion coating passed the 168 h of SST with less than 3% corrosion area, and meanwhile the self-healing worked effectively. Both barrier protection and characteristic self-healing effect enhanced corrosion resistance on EV31. Furthermore, a corrosion mechanism for the coating in practice was proposed to discussion.
The study of corrosion behavior of manganese-based conversion coating on LZ91 magnesium alloy: Effect of addition of pyrophosphate and cerium
(2020-07-01)
Jian, Shun-Yi
;
Tzeng, Yu-Chih
;
Ger, Ming-Der
;
Chang, Ko-Lun
;
Shi, Gia-Nan
A previous study by the authors reported that a Mn-Ce based conversion coating protects the surface of LZ91 magnesium alloy from corrosion. However, the acidic permanganate solution decomposes after several days of storage and this significantly affects the properties of the coating. To address this problem, this study presents a method to stabilize the acidic permanganate bath by adding pyrophosphate into the bath. The results show that all conversion coatings are amorphous and comprise two layers: a porous layer that contacts the substrate and a compact overlay. Coatings that are produced using a bath with Ce are more resistant to corrosion than those that are produced in a bath without Ce ions. Coatings that are stored in the Mn-Ce bath for 2 weeks are significantly less resistant to corrosion than coatings that are produced using an as-prepared Mn\\Ce bath. The salt spray test results of coatings that are produced using the as-prepared bath and stored for 2 weeks are found similar because the bath is rendered more stable by the addition of pyrophosphate. (C) 2020 The Authors. Published by Elsevier Ltd.