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 3 of 5

Anodizing of Valve Metal

Anodizing• An electrolytic passivation process.

• An anodic oxidation of metal at the anode, e.g. Al, Ti, Mg and is usually accompanied by hydrogen evolution at an inert electrode (the cathode), e.g. Ti, stainless steel.

• Typical cell voltage 5 to 100 V.

Anodising: What and Why?

• Deliberately producing a stable oxide coating

– by anodic treatment of a metal surface

• Coating is usually non-conducting

– thermally and electrically insulating

• Oxide is usually protective

– against corrosion or wear or heat

• Anodised film can be post-treated

– with dye, polymer, lubricant…

Types of Surface Contamination - ‘Dirt’: Sources?

• Oils, greases and waxes• Metal oxide (or sulphide or chloride) films• Metal particles• ‘Flowed’ surface layers may be ‘glassy’• Metallurgical defects• Chemicals (including sweat)

Special Pre-treatments• Chemical polishing

– to give a bright finish• Electrolytic polishing

– capable of a mirror finish • Electrograining

– used to allow aluminium to pick up ink• Microetching

– deliberate micro-roughening of, e.g., silicon• Plating strikes

– e.g., Wood’s nickel on stainless before Watts nickel

Anodizing

At excessively high current density, a secondary anodic reaction, O2 evolution takes place at a significant rate.

This may give rise to problems such as pitting of the anodised layer and an increased possibility of hazardous O2/H2 mixture.

What does an anodised film look like ?

Depending on conditions, oxide film thickness can be, e.g., 1-30 micron

Transmission electron micrograph(TEM) cross section of an anodised film on Al150 V for 70 minutes in 0.5M phosphoric acid

Barrier layer

Porous layer

Aluminium substrate

Anodising of aluminium:state of a substance (s),(l),(g)

2Al(s) + 3H2O(aq) - 6e- = Al2O3(s) + 6H+(aq)

Aluminium Water Electrons Aluminium oxide

Protons

Reactants Products

Note the different phases and the phase changes during reaction.

Anodising of aluminium:reacting quantities

2Al + 3H2O - 6e- = Al2O3 + 6H+

2 atom 3 molecules 6 molecules 1 molecule 6 ions2 mol 3 mol 6 mol 1 mol 6 mol2 x 26.98 g 3 x 18.016 g 6F 1 x 101.96 g 6 x 1.008 g

Reactants Products

Reactions during anodising of aluminium: note phase changes

• Anode (aluminium)

2Al(s) + 3H2O(l) - 6e- = Al2O3(s) + 6H+(l)

• Cathode (e.g., stainless steel)

6H+(l)

+ 6e- = 3H2 (g)

• Cell (overall process)

2Al(s) + 3H2O(l) + 6H+(l)

= Al2O3(s)+ 6H+(l)+ 3H2(g)

Applications of Anodising?• Heat sinks

– Thermal, oxidation resistant

• Pots and pans– Decorative

• Architectural panels– Decorative and corrosion resistant

• Engineering– Wear resistant, corrosion resistant

What are the Control Variables?

• Process– Pre- and post-treatment, temperature, time

• Electrolyte– Composition, temperature…

• Metal– Type of alloy, surface finish…

Types of Anodising• Decorative

– d.c in sulfuric acid• Hard

– d.c in chromic or phosphoric acid• Plasma

– a.c. in near neutral salts

Plasma electrolytic oxidationAnodic oxidation of metal to form metal oxide.Uses higher voltage than anodizing, e.g. 100 to 1000 V.Metal oxide forms at the anode.Thicker oxide layer than anodizing, e.g. 100 to 500 m.

Post-treatment following anodising (how and why)

• Seal (Boiling water)• Dye (Organics)• Impregnate

– PTFE (Anti-stick) – MoS2 (Self-lubricating)

Conclusions

• Anodising is important in surface finishing.

• Uses: decorative and engineering applications.

• Capacitors to architectural panels are involved.

• Pre-treatment is important.

• Good process control is essential.

• So is adequate post-treatment.

• Use d.c., a.c. and plasma electrolysis techniques.