Outline Curriculum (5 lectures) Each lecture  45 minutes Lecture 1: An introduction in electrochemical coating Lecture 2: Electrodeposition of coating Lecture 3: Anodizing of valve metal Lecture 4: Electroless deposition of coating Lecture 5: Revision in electrochemical coating

Lecture 1 of 5 An Introduction In Electrochemical Coating

Electrochemical Surface Engineering (Electrochemical Coating) Is it about the deposition a coating onto surface, via electrochemical reactions. The coating can be (a) metallic, (b) metal oxide or (c) conductive polymer. Metallic coating: Electroplating Metal oxide, conductive polymer: Anodizing Electroless deposition

Electrochemical Surface Engineering An electro-chemical reaction Cathode: Metals/alloys coating Anode: Metal oxides Conductive solution: ionic species Transfer of electrons

Electroplating of copper

Anodizing An electrolytic passivation process. To form a thick oxide layer on a metal. Metal oxide forms on the anode.

Electroless deposition Electroplating: consisting of two electrodes, electrolyte, and external source of current. Electroless deposition: this process uses only one electrode and no external source of electric current. Electroless deposition: the solution needs to contain a reducing agent so that the reaction can proceed: Metal ion + Reduction solution  Metal solid + oxidation solution Catalytic surface

Definition: Electron transfer reactions Oxidizing agent + n e- = Reducing agent Oxidizing agents get reduced Reducing agents get oxidized Oxidation is a loss of electrons (OIL) Reduction is a gain of electrons (RIG) OILRIG

Industrial scale anodizing of Aluminium

Example of anodizing

Brush electroplating of gold onto stainless steel substrate

Tin-Zinc coating onto steel substrate Benefits of electroplated metallic surfaces: Improved corrosion resistance. Improved wear resistance. Longer lifetime. Aesthetic surface finish.

Optical micrograph of 21 mm PEO coating on Mg alloy:

Optical micrograph of 12 mm PEO coating on Mg alloy:

Porosity in electroless Ni-P deposits (<5 mm) on mild steel

Log-log Porosity vs. thickness for electroless Ni-P deposits on steel

Electrochemical anodizing Transformation of Ti foil to TiO2 nanotubes Anodizing e.g. 10-100 V Electrochemical formation of oxide Ti + 2H2O → TiO2 + 4H+ + 4e- Chemical dissolution of oxide TiO2 + 6F- + 4H+ → TiF62- + 2H2O Competing reactions for the formation of TiO2 nanotubes

Green electrolyte, CH3SO3H Anodizing of TiO2 nanotubes from Ti foil 100 nm 100 nm 200 nm 200 nm

Surface microstructure Nanotubes Au-TiO2 vertically aligned array 100 nm 100 nm

Reflective nanocrystalline PbO2 Application: Solar heat absorber The picture on the right shows a polycrystalline coating which is non-reflective vs. a nanocrystalline coating which is highly reflective. The reflectivity of this coating was controlled via the surface roughness and grain size of the coating. In a nanocrystalline coating, the root mean square was less than 10 nm while a polycrystalline coating showed several hundreds of nm. The figure on the left shows the measurement of optical reflectance. The x-axis shows the wavelength in the visible spectrum and The y-axis shows the recorded % of optical reflectance. A carbon substrate is used as a reference line. This curve shows that a highly reflective coating can be 200 to 600 % more reflective than the non-reflective coating. 20

Rotating Cylinder Reactor High throughput electrodeposition Cu-Sn alloys The picture of the left shows a Rotating Cylinder Hull cell. This is an apparatus for a high-throughput electrodeposition of coatings. In this design, a rotating cylinder is the working electrode and a stationary metal mesh is the counter electrode. This design provides a controlled flow condition and a controlled non-uniform current distribution along the length of the cylinder. The picture on the left shows a series of tin-copper alloys deposited along the length of the cylinder. The local current density varies along the length of the cylinder. A high local current density at the bottom of the cylinder and a low current density at the top of the cylinder. By changing the local current density, a controlled alloy composition can be deposited. A high tin content at the bottom of the cylinder and a high copper content at the top of the cylinder. Many of these tin-copper alloys have important applications. For example, in surface protection of engine bearings, tribological coatings and as current collectors in lithium battery.

Rotating Cylinder Reactor High throughput electrodeposition Cu-Sn alloys The picture of the left shows a Rotating Cylinder Hull cell. This is an apparatus for a high-throughput electrodeposition of coatings. In this design, a rotating cylinder is the working electrode and a stationary metal mesh is the counter electrode. This design provides a controlled flow condition and a controlled non-uniform current distribution along the length of the cylinder. The picture on the left shows a series of tin-copper alloys deposited along the length of the cylinder. The local current density varies along the length of the cylinder. A high local current density at the bottom of the cylinder and a low current density at the top of the cylinder. By changing the local current density, a controlled alloy composition can be deposited. A high tin content at the bottom of the cylinder and a high copper content at the top of the cylinder. Many of these tin-copper alloys have important applications. For example, in surface protection of engine bearings, tribological coatings and as current collectors in lithium battery.

Nanoparticles SiC in a nickel matrix Wear resistance coating Darker contrast: nanoparticle SiC 100 nm Ni-SiC coating This is a nickel coating containing silicon carbide as hard and wear resistance nanosized particles. The picture of the left shows a cross-sectional view of the coating. A large volume of nanoparticles can be deposited into the nickel coating. This coating is thick and with a good adhesion to the substrate. The picture on the right shows a higher magnification of the coating. The nanoparticles were uniformly distributed and agglomeration of the nanoparticles is minimal. This coating can be used as electroforming of metal micro-gear in MEMS. It can also be used to replace Silicon parts of MEMS components, which can be brittle. The use of metal MEMS components can provide a more resilient property, harder and longer lasting. Copper substrate 200 m

TEM image Nanotubes TiO2 in a nickel matrix 20 nm Nickel matrix 100 nm

Electrodeposition of polypyrrole Stainless steel substrate Polypyrrole 1.0 cm 25 1.0 cm

Electrocatalysts for H2O electrolysis Nanocrystalline and amorphous Ni-Co alloys 0g Co 2 g 10 g 20 g 40 g 60 g 80 g 100 g 150 g 200 g 100g Ni This picture shows a series of nickel-cobalt alloys. The sample on the left shows a pure nickel coating. The next sample shows a nickel containing 2 grams of Cobalt in the solution and the last sample is 200 grams of Cobalt in the solution. High Co content leads to a higher coating stress. And the phase structure changes from a cubic to a hexagonal phase. The challenge of this work is to reduce the coating stress via a multilayered, graded coating. 1.0 cm Co content in alloyed electrocatalyst increases More effective electrocatalyst to evolution oxygen 26

Large scale electrodeposition Thick film, multilayered Ni-Co on Fe substrate This is a picture showing a scale up version of an electroplating bath for Ni-Co alloys deposition. This demonstration unit can accommodate 6 tanks and the flow condition is controlled via a reciprocating motor. The picture on the right shows a three layer coatings. The bottom layer is a semi-bright nickel which provides adhesion to substrate. The intermediate layer is a bright nickel for corrosion protection. And the top layer is a nickel-cobalt alloy for wear protection. The coating is thick and do not show signs of coating stress or adhesion problem to the substrate. Each tank = 5 Litres 20 cm

Multilayered - and -PbO2 This picture shows a cross-sectional view of a multilayered coating of lead dioxide. The left side shows the microstructure of the coating to the substrate And the right is the front side of the coating. This coating was anodically deposited from a single bath system. The layers were alternated with a controlled phase structure, by changing the applied anodic current density. Layer 1 consists of a pure beta structure while layer 2 consists of a mixture of alpha and beta structures. An unique feature about this coating is its thickness. This coating is several mm thick, showing no signs of coating stress and cracks. 28

Thin film lead-acid battery Nanosized materials PbO2 + PbSO4 100 nm 29

Summary Electrochemical coatings range from nanoparticles of metal on nanostructured, inorganic supports through to hard <100 mm Cr coatings on steel. Applications include catalysts, fuel cell-, solar cell- and battery electrodes together with tribological/corrosion resistant coatings for electronic materials, transport and heavy engineering. Plasma electrolytic oxidation uses the application of a high a.c. voltage to produce a hard, wear resistant oxide coating on light metals (such as Mg alloys) for automotive, aerospace and leisure. Electroless Ni deposits (typically <20 mm in thickness) on steel or Al alloys are widely used in engineering applications for their corrosion and wear resistance. Thin coatings tend to have high porosity.