Sol Gel Coatings: An Environmentally
Acceptable Alternative
to Cr-Based Coatings/Pre-Treatments
*R Akid & H Wang
Materials & Engineering Research Institute
Sheffield Hallam University
Howard Street, Sheffield, S1 1WB. U.K.
* Corresponding author (r.akid@shu.ac.uk)
July 17-18th 2006
Outline
• Challenges
• Background
– Conventional Cr-based pre-treatment
– Aim of research
– Sol gel chemistry, preparation and variant coating
systems
– Current trial applications
– Alternative Cr-replacement study
• Anti-corrosion performance studies
• Summary
Challenges
End of Vehicle life (EVL) Directive
•
Around two million vehicles reach the end of their life in the UK each
year. Currently, between 74-80% of the weight of a typical ELV is reused or recycled.
•
The EVL Directive (2000/53/EC) is concerned with cars, vans and
certain three-wheeled vehicles. It's main requirements are that:
– producers limit the use of certain hazardous substances in the
manufacture of new vehicles and automotive components and
promote the recyclability of their vehicles
– ELVs are subject to de-pollution prior to dismantling, recycling or disposal
– treatment facilities operate to higher environmental standards and have
permits if they want to deal with undepolluted ELVs
– producers pay ‘all or a significant part’ of the costs of treating negative or
nil value ELVs at treatment facilities by 2007
Challenges
Waste Electrical and Electronic Equipment (WEEE) Directive
•
The (WEEE) Directive aims to reduce the amount of WEEE being disposed of to
landfill by promoting separate collection, treatment and recycling.
•
The Directive:
–
introduces requirements to mark EEE products with a ‘crossed out wheelie
bin’ symbol
– sets targets for the amount of WEEE to be collected separately from private
households
– requires the UK to establish and maintain a register of EEE producers
– makes distributors and retailers responsible for making arrangements to
take back WEEE free of charge in convenient way for customers
– introduces recycling and recovery targets for various categories of WEEE.
Challenges
Restriction of Hazardous Substances (RoHS) in :
•
•
•
•
large and small household appliances
IT and telecommunication equipment
TVs, videos, hi-fi
lighting, electrical and electronic tools (except large stationary
industrial tools)
• toys, leisure and sports equipment
•
This Directive bans the placing on the EU market of new electrical
and electronic equipment containing more than agreed levels of:
– lead, cadmium, mercury, hexavalent chromium, polybrominated
biphenyl (PBB) and polybrominated diphenyl ether (PBDE) flame
retardants
Conventional Chromate Treatments on:
Zn, Al, Mg..
conversion coating
mixed Cr oxide
passivation
metal
subsequent
processing
Organic top coat
Component
Primer (may or may
not contain Cr(iV)
metal
Aim of research
• To develop an environmentally acceptable
(Cr-free) coating/pre-treatment for a
selection of metals, including Al, Zn, Mg
and mild steel to replace chromate and
phosphate treatments.
The Need!
•
•
•
•
Corrosion costs approx 3.5% GDP
Equivalent cost in USA is 1 Hurricane Katrina p.a.
I tonne of metal every 90 seconds
Approx £600 per person per year in the UK
The need!
Wong Tai Sin Temple – New Kokloon (July 2006)
"Doaist god of healing"
– secret for transforming Cinnabar (Hg2S) into an elixir for mortality!
Sol gel Methodology
Precursor 1
Organic and
Inorganic components
Mix and Age*
Precursor 2
Functional Additives,
e.g., corrosion inhibitors,
bio-active molecules, etc.
*Ageing time dependant upon formulation chemistry
Apply to
metal;
Dip, Spray..
Use as
functional/
barrier
coating
Cure at
selected
temperature
Apply top coat directly to sol gel
for anti-corrosion coating
Sol gel processing
• Conventional sol–gel synthesis involves, in the first
step, the formation of a colloidal suspension (sol),
usually obtained by controlled acid- or basecatalyzed hydrolysis and partial condensation of
metal alkoxide precursors in alcoholic or other nonhydrolytic organic solvents. Further condensation
leads to a viscous gel or dispersion of colloidal
particles, depending on composition, pH and
reaction conditions.
• Cure temperature determines degree of crystallinity
Sol gel chemistry
Precursor Si (OC2H5)4
= Si-O-R', where R' = C2H5
CH3
Si-O-R'
O
R'-O-Si-O-R'
+
H OH
Si-O-R'
O
R'-O-Si-O-R'
O
Si
0
H
O
Si- O-R'
Metal substrate
+ R'OH
Typical coating structure
functional groups as appropriate
l
l
l
l
l
O-Si-O-Si-O-Si-O-Si-O-Si-O- ..........etc
l
l
l
l
l
O O
O
O
O
l
l
l
l
l
Metal substrate
Sol gel variants
Cure regime
Component
Room Temperature +
Organic component
Inorganic component
Properties
Flexibility
Hardness
Oxidation resistance
Functionality
above 400oC
high
low
low
high
high*
low
low
high
* (long chain or alkene)
low
high
high
limited ?
Current sol gel variants being developed
Anti-corrosion coatings - Biosensors - Microbial fuel cells
Current trial applications
•Mild steel – coil coated construction steel,
•replacement of hot dipped zinc, domestic appliances
Al – automotive, aerospace
Mg – die cast
products for
automotive use
•Cr-replacement- sports goods
Stainless steel –
domestic appliances,
anti-bacterial coatings
Summary of Non-Chrome alternatives*
http://www.electronicssouth.com/ContentStore/Assets/7/17/phase_i_report%20from%20USA%20DOD.pdf
* Environmental Security Technology Certification Programme
– Naval Aviation systems
Sol gel anti-corrosion
performance trials
• Substrates
– Al alloys 2xxx, 5xxx, Mg, Mild steel, Hot dipped Zn, SS
• Coatings
– silane based, with and without inhibitors
– silane based with and without bacteria
• Environments
– typically NaCl (3.5%) or artificial seawater
• Test methods
– Linear polarisation, Impedance Spectroscopy
– Salt spray
– Scanning electron microscopy, optical microscopy
Processing
1. Clean substrate (acid/alkaline based soln)
2. Water rinse
3. Apply sol gel
4. Cure (R/T or R/T+)
5. Option – repeat steps 3 & 4 (multilayer systems)
6. Option – over-coat
Multi-layer - Ecorr results
Influence of surface roughness on Ecorr
0
1200 grit
Polished
Ecorr mV, (vs Ag/AgCl)
-100
-200
-300
-400
-500
-600
0
1
2
3
Number of Coats
4
5
Morphology of coatings
Al 2024-T3
Substrate
Sol-gel
Coating
Bakelite
Mounting
SEM
AFM
200 nm
1.00 μm
ESEM
Anti-corrosion performance trials
• Al alloys (in 3.5% NaCl)
-200
Potential, mV, vs. SCE
-400
Bare Al2024
Sol gel coated Al 2024
2024alloy
ChromateBare
on Al
5xxx
Chromate
Sol gel Al 2024
2
3
1
-600
-800
-1000
-1200
10-11
← Low
10-10
10-9
Corrosion rate
10-8
10-7
10-6
10-5
Current density (A/cm2)
high→
10-4
10-3
10-2
10-1
Anti-corrosion performance trials
• Mg (in 3.5% NaCl)
Commercial pre-treatment
sol gel coating
Anti-corrosion performance trials
g
HDZ
g
pg
2
1.00E+08
Bode Impedence (Ohms.cm )
2
Bode Impedence (Ohms.cm )
Hot dipped Zn (galvanised mild steel)
Immersed in 1.0% and pH=4 NaCl solution
First day
1.00E+07
After 24 hrs
1.00E+06
After 48 hrs
After 96 hrs
1.00E+05
1.00E+04
0.01
After 120 hrs
0.1
1
10
100
1000
10000 100000
2
Bode Impedence (Ohms.cm )
g
g
First day
1.00E+06
After 24 hrs
After 48 hrs
After 96 hrs
After 120 hrs
1.00E+05
1.00E+04
0.01
0.1
1
10
100
1000
10000 100000
Frequency (Hz)
1.00E+08
Immersed in 3.5% NaCl solution
1.00E+07
First Day
After 24 hrs
1.00E+06
HDZ + Chromate
1.00E+05
After 96 hrs
1.00E+04
1.00E+03
0.01
g
Immersed in 1.0% and pH=4 NaCl solution
Frequency (Hz)
Note: different environments
Galfan
g
1.00E+07
0.1
1
10
100
Frequency (Hz)
1000
10000 100000
Anti-corrosion performance trials
• Mild steel
Anti-corrosion performance trials
Mild steel (with and without encapsulated corrosion inhibitors)
15 hrs no inhibitor
3.5% NaCl
528 hrs with inhibitor
Encapsulation to give functionality
water
l
l
l
l
l
O-Si-O-Si-O-Si-O-Si-O-Si-Ol
l
l
l REI l
REI
O O
O REI O
O
REI
l
l
l
l
l
REI
Metal substrate
REI Rare Earth Inhibitor
Treat encapsulated component to
provide a 'bond' to the sol gel
l
l
l
l
l
O-Si-O-Si-O-Si-O-Si-O-Si-Ol
l
l
l
l
REI
REI
REI
REI
O O
O
O
O
l
l
l
l
l
REI
Metal substrate
Modified Rare Earth Inhibitor
REI
Adhesion performance (T-bend)
T2.0
T0.5
Sol gel on HDZ (Galfan)
Sol gel on Mild steel
Functional anti-corrosion coatings
Microbiologically Influenced Corrosion (MIC)
(Bacteria & Biofilms)
Microorganisms,
especially bacteria,
colonise surfaces
to form Biofilms
}
Colonisation
of
Sulphate
Reducing
Bacteria
(SRB)
H2S formation
Biofilm formation;
up to 48hrs depending
upon temperature
Localised
Corrosion
(pitting)
‘Bacterial-active’ sol-gel coatings
Anti-'microbial induced corrosion' (MIC) coating
• Combination of anti-corrosion sol-gel coating and protective
bacteria.
• Uniform distribution of protective bacteria fixed on the surface
Viable bacterial cells
immobilised in coating
'Biocoat'
Substrate
Anti-corrosion performance trials
Anti-MIC corrosion sol gel coating
2
LPR (Ohm.cm )
SS control
1e+5
Al control
1e+4
SSbact
pfragi
SS
1
pfragi
AlAlbact
1
pfragi2
AlAlbact
2
AlAlbact
2 repeat
polymyxa
High
ESEM image
Low Corrosion rate
1e+6
1e+3
0
5
10
15
Time (days)
Paper submitted to "Advanced Functional Materials"
20
25
30
35
Coating on SS containing bacteria
Fluorescence Microscopy
Coating at day 1 stained
with EthBr. Spores not
clearly visible
Coating at day 30
(immersed in nutrient
rich artificial seawater)
is becoming colonised
by bacteria
Confirming Visual Examination
D1
D2
D4
D8
D12
Al with bacteria 1
D16
D30
Bio-active coating field trials
Sol gel treatments:
Opportunities and challenges
• Opportunities
–
–
–
–
–
–
Totally Cr & PO4 free – RoHS compliant
Non toxic
Limited VOC emissions
Reduction in process stages (reduced processing costs)
Ease of recycling (reduced total life-cycle costs)
Can be over-coated
• Challenges
– Scale up ?
– Shelf life (currently months)
– Optimisation of properties, e.g., Adhesion vs. Flexibility,
Film thickness vs. corrosion protection, Repair and recoat,
Hardness vs. cure temperature.
Summary
• Given an appropriate optimisation of the
formulation chemistry and processing; sol gel
based coating systems are a potential
alternative candidate to Cr-based and PO4
systems, offering the following:
–
–
–
–
–
–
–
–
easily prepared with simple formulation chemistry
variable cure temperature options: from R/T upwards
convenient methods of application to substrates
totally Cr-free and RoHS compliant
suitable for over-coating with paint systems
good flexibility and adhesion
excellent anti-corrosion properties
may be functionalised to give variety of properties (anti-bacterial,
hydrophobic..)
Acknowledgments
•
•
•
•
•
Dr Tom Smith (microbiologist)
Dr D Greenfield & Mr J Connell (corrosion testing)
Prof J Earthman (bacteria selection)
Sheffield Hallam University (funding/facilities)
HKMFS & HKPC for the invitation to contribute to this
conference