Corrosion Performance of Ni and Ni-P Coatings via Electrodeposition in an Emulsified Supercritical CO₂ bath

　　Sung-Ting Chung, Yan-Chi Chuang, Cheng-Yu Li, Shih-Yi Chiu, Wen-Ta Tsai*

　　* E-mail : wttsai@mail.ncku.edu.tw

　　Department of Materials Science and Engineering National Cheng Kung University, Tainan, Taiwan
 

　　 报告人:蔡文达

　　个人简介：

　　台湾台南市成功大学特聘教授 ，历年来执行数十项腐蚀相关研究课题，研究成果总计已有215篇学术期刊论文发表，其中有SCI论文166篇;而国内外学术研讨会发表的论文超过285篇，已获得国内外核准的专利有10件，其它技术报告已超过120件。

　　曾任两届防蚀工程学会理事长，主办第九届亚太腐蚀控制会议，参与海峡两岸材料腐蚀防护研讨会的举办，为推动海峡两岸腐蚀专业的交流与合作做出巨大贡献。
 

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Background information
Properties and applications of nickel (Ni) deposition

Properties: Electrical conductivity, Corrosion resistance, …etc.

Applications: Electronic packaging, MEMS, advanced integrated circuit (IC), Anti-corrosion, Anti-wear, Decoration…etc.

Methods of nickel (Ni) deposition

Dry Process

1. PVD
2. CVD
3. Sputter…etc.

Wet Process

1. Electroplating
2. Electroless plating…etc.

Advantages and Disadvantages of Wet Process

Advantages :
Cheap
Easy to operate
High deposition rate…etc

Disadvantages:
The formation of hydrogen may create several pin-holes on the film.
Wastewater treatment (toxic species)…etc

Possible solutions:
Non-aqueous or green electrolytes:
ionic liquids and supercritical CO₂…etc
#p#副标题#e#

Supercritical Fluids

　　Characteristics of CO₂

　　CO₂  Phase Diagram

 

　　Critical properties of various solvents

 

　　Characteristic of CO₂  : Gas, Supercritical Fluid and Liquid

　　Gas-like Diffusivity

　　Liquid-like Density

　　Low Surface Tension
 

New Technology for Electrodeposition
 

　　Supercritical Nano-Plating (SNP)

　　Sone and colleagues have demonstrated the electrodeposition of nickel from an emulsion of an aqueous nickel plating solution in sc-CO₂ bath.

　　Nonionic surfactant : PEO-PPO (6 wt.%)

　　sc-CO₂ volume fraction vs. Current efficiency & Resistance

　　The result indicates that the plating electrolyte can be reduced at 60% compared to that of currently industrially by used plating bath.#p#副标题#e#

　　The plated films using this method have a uniformity of surface and the grain size is smaller than that formed by using conventional electroplating methods.
 

　　Experimental Conditions for Electrodeposition

　　Substrate

　　   • Brass substrate cut in2.4 × 1 cm²×2

　　   • Polish in a slurry containing 0.3 μm Al₂O₃ powder

　　* Modified Watt’s bath including NiSO₄·6H₂O, NiCl₂·6H₂O and H₃BO₃

　　Plating condition

　　 • Current density： 5 A/dm² for Ni and Ni-Al₂O₃ deposits

　　   2 A/dm² for Ni-P alloy coatings

　  　• Temperature : 50^oC

　  　• Total charge : 108 coul.

　　  • Aqueous electrolyte : 0.1 MPa (air), 10 MPa (Ar)

　　  • Electrolyte containing CO₂  fluid： 5 MPa, 10 MPa and 15 MPa

　　Heat-treatment

　　The heat-treatment of Ni-P alloy coatings was performed from room temperature to heat-treated temperature for 1 hr. (350, 400, 450, 500, and 550^oC)

　　

  Experimental Apparatus for Electrodeposition
 

　　Materials Characterization for Electrodeposits

Experimental (I)   Pure Ni deposits
 

　　Material Characterization (Ni Electrodeposit)

Crystal structure

 

　　Current efficiency

　　

Surface roughness

　　Chemical composition(effect of sc-CO₂ fluid)#p#副标题#e#

　　Chemical shift

• C_(graphite) : 284.6 eV
• C dissolved in Ni lattice

 C_(Ni-C) : 282.8 eV

Possible reactions responsible for the formation of Ni-C deposit:
Ni²⁺ + 2e^- à Ni
2H⁺ + 2e^- à H₂
HCO₃^- + 5H⁺ + 4e^- → C + 3H₂O
 

　　Microstructure and Micro-hardness

　　TEM microstructure

 

　　Grain size and micro-hardness

　　Electrochemical behavior

　　Potentiodynamic polarization curve

　　Test in 1 M HCl solution

EIS result

　　Electrochemical behavior (effect of crystallographical orientation)#p#副标题#e#

　　Crystal structure

 

　　Schematic diagram

 

　　
Microstructure and Micro-hardness

（effect of CO2 pressure)
 

　　TEM microstructure

Grain size and micro-hardnes

　　The dependence of micro-hardness on the grain size is also revealed

　　Hall-Petch equation  σ_{y  =}  σ_o + k_yd^-1/2
 

　　Electrochemical behavior

　　-- Potentiodyamic polarization curve

Test in 1 M HCl solution

Experimental (II)  Ni-Al2O3 composite coatings
 

　　Surface morphology and cross section micrograph

　　(electrolyte containing 10 g/L Al₂O₃)

Surface morphology

Cross section micrograph

　　Micro-hardness & Wear rate (electrolyte containing 10 g/L Al₂O₃)

　　Ni/Al₂O₃ deposits

　　Wear resistance#p#副标题#e#

　　Ni-P/Al₂O₃ deposits

　　Wear resistance

　　 Electrochemical behavior (electrolyte containing 10 g/L Al₂O₃) 

　　-- Potentiodyamic polarization curve

　　Surface morphology and cross section micrograph (electrolyte containing 10 g/L SiC)

　　Surface morphology

 

Cross section micrograph

　　Micro-hardness & Wear rate(electrolyte containing 10 g/L SiC)

　　Electrochemical behavior (electrolyte containing 10 g/L SiC)#p#副标题#e#

　　Test in 3.5 wt.% NaCl solution

　　Electrochemical behavior

　　Schematic diagrams

Experimental (III)  Ni-P alloy coatings

　　P content  effect of [H₃PO₃]

　　The co-deposition mechanism for the Ni-P alloy coatings

  Ni²⁺ + 2e^- à Ni
  6H⁺ + 6e^- à 6H_ads

Direct mechanism:
  H₃PO₃ + 3H⁺ + 3e^- à P + 3H₂O

  Indirect mechanism:
  H₃PO₃ +6H_ads à PH₃ + 3H₂O
  2PH₃ + 3Ni²⁺ à 3Ni + 2P + 6H⁺
 

　　Grain size (effect of P content)

In conventional electrolyte

In emulsified sc-CO₂ bath

　　Hardness  effect of grain size

　　Hall-Petch equation

　　Grain size > 10 nm

　　σ_{y  =}  σ_o + k_yd^-1/2

　　Inverse Hall-Petch equation#p#副标题#e#

　　Grain size < 10 nm

　　σ_{y  =}  σ_o - k_yd^-1/2
 

　　Electrochemical behavior effect of P content

　　-- Potentiodyamic polarization curve

　　Test in 1 M NaOH solution

　　(Test in 1 M HCl solution)

　　The formation of an adsorbed film of hypophosphite (H₂PO₂^-) generated by the oxidation of phosphorus present in the coating surface, which in turn blocks the water molecules from interacting with nickel, thus inhibiting nickel oxidation.
 

　　Electrochemical behavior  effect of heat treatment

　　Test in 1 M HCl solution

 Conclusions 

　　The Ni-based deposits could be successfully fabricated from an emulsified sc-CO₂ bath.

　　The surfactant addition in sc-CO₂ containing electrolyte play an important role on the surface appearance of the deposits. When electrodeposition was performed in the emulsified sc-CO₂ electrolyte, a flat and almost pinhole-free surface was observed.

　　Carburization of metal can be achieved by electrodeposition in the sc-CO₂ -containing baths. As a result, the hardness of Ni-based films deposited in the sc-CO₂  bath was enhanced by the additional solid solution strengthening mechanism.

　　In 1 M HCl solution, the Ni film deposited in sc-CO₂ bath had a lower anodic current density and a higher polarization resistance, which is mainly attributed to the crystallographical orientation.

　　The high P content in the deposit was beneficial to enhance the corrosion resistance of the Ni-P alloy coating, in 1 M HCl solution.

　　Heat treatment significantly enhances the hardness and corrosion resistance compared to the as-deposited Ni-P alloy.
 

Acknowledgements

　　The authors thank the National Science Council of the Republic of China under Contract No. NSC 98-2622-E-006-032 for partially supporting the use of instruments for surface characterization.