Transcript Outline Curriculum (5 lectures) Each lecture 45 minutes

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 ?
Transmission electron
micrograph
(TEM) cross section
of an anodised film on Al
150 V for 70 minutes
in 0.5M phosphoric acid
Porous layer
Barrier layer
Aluminium substrate
Depending on conditions,
oxide film thickness can be, e.g., 1-30 micron
Anodising of aluminium:
state of a substance (s),(l),(g)
Reactants
Products
2Al(s) + 3H2O(aq) - 6e- = Al2O3(s) + 6H+(aq)
Aluminium
Water
Electrons
Aluminium
oxide
Protons
Note the different phases and the phase changes during reaction.
Anodising of aluminium:
reacting quantities
Products
Reactants
2Al + 3H2O - 6e- = Al2O3 + 6H+
2 atom
3 molecules
6 molecules
2 mol
3 mol
6 mol
2 x 26.98 g 3 x 18.016 g
6F
1 molecule
6 ions
1 mol
6 mol
1 x 101.96 g 6 x 1.008 g
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 oxidation
Anodic 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.