Ole Øystein Knudsen, Astrid Bjørgum
SINTEF Materials Technology
Materials Technology 1

n Part of a wider ongoing research project financed by the
Research Council and Norwegian light metals industry:
Light Metal Surface Science
n Participating companies
n Hydro Aluminium
n Elektro-Vakuum
n Noral Lighting
n Norsk Industrilakkering
n Profil-Lakkering
n DuPont Powder Coatings
n Jotun Powder Coatings
Materials Technology 2

n Chromium - why …. and why not?
n Properties and problems
n Regulation - EU and Norwegian
n Chromium free pre-treatments
n Redox reactions with precipitation
n pH controlled precipitation on the aluminium surface
n Molecules that deposit on the aluminium surface
n Strengthening the aluminium oxide
Materials Technology 3

n Focus in this presentation:
n Treatments for aluminium
n Commercially available processes
n Emphasis for each process:
n Basic principles for the formation of the coating
n Process / production friendliness
n Experiences
Materials Technology 4

Why …. and why not?
Materials Technology 5

n First patented in 1923
n Used extensively since for pre-
treatment before coating,
adhesive bonding and surface
finishing
n 43 000 tons of chromium was
used in metal finishing
operations in 1993
Materials Technology 6

n The chemicals contain:
n Hexavalent chromium (CrO3 or CrO42- or Cr2O72-)
n Hydrofluoric acid (HF)
n The hydrofluoric acid removes the oxide film on the
surface
n The hexavalent chromium reacts with the exposed
aluminium metal and a trivalent chromium oxide
precipitates
Cr2O72- + 2 Al + 2 H+ ® Cr2O3·H2O + Al2O3
Materials Technology 7

n Chromium oxide stabile in
alkaline solutions up to pH 15
n Chromium oxide is water
repellant (hydrophobic) and
may act as a barrier coating
towards water
n Self healing effect: hexavalent
chromium present in the
conversion coating that may
react at mechanical damages
n Passivating both the aluminium
matrix and the intermetallic
particles
Materials Technology 8

n Highly effective
n Preventing corrosion
n Adhesion promoter for organic coatings and adhesives
n Resilient: The process has low sensitivity towards
variation in process conditions
n Effective on most/all aluminium alloys
n Quality control: Skilled workers can tell the amount of
chromium on the surface by the color of the conversion
coating
Materials Technology 9

n Toxic
n Classified as human carcinogen
n Workers at the production line are concerned about their
health - liability for claims of workspace exposure
n Consumers are concerned about hexavalent chromium
present in products
n Concern about hexavalent chromium in the environment,
e.g. drinking water
n Treatment of waste
n Stringent disposal limits
n Increased costs for tracking inventories, monitoring, reporting
n Disposal of wastes containing chromium
Materials Technology 10

n Pacific Gas & Electric
n Deposits of hexavalent
chromium in the ground
n Hexavalent chromium leached
into the groundwater
n Cancer and other diseases
increased dramatically
n In 1993 PG&E settled for $333
million, the largest settlement
ever in a direct action lawsuit.
n California 2001: Law that limits
Julia Roberts as Erin Brockovich (2000)
the level of hexavalent
chromium in drinking water
Materials Technology 11

n EU directive 2000/53/EC: End-of life vehicle
n Every year 8-9 million tons of waste are produced from end-of life
vehicles
n Aim: to harmonize the treatment of this waste in order to reduce
environmental impact
n Increase recycling and reuse of materials
n Reduce and control the use of hazardous substances in vehicles
n Vehicles put on the market after 1 July 2003 shall not contain:
n lead, mercury, cadmium and hexavalent chromium
n Norway: Chromium on the B-list of hazardous substances
n Aim: Significant decrease in the use of chromium by 2010
Materials Technology 12

What options do we have?
Materials Technology 13

n Redox reactions with precipitation
n Molybdenum
n Manganese
n pH controlled precipitation
n Phosphate
n Titanium / zirconium based processes
n Cerium
n (Trivalent chromium)
n Coupling agents between aluminium oxide and binder
n Silanisation
n Self Assembling Molecules (SAM)
n Strengthening the aluminium oxide
n DC and AC anodizing
Materials Technology 14

n Molybdates Basic prinsiple
n Permanganates n Same formation mechanism as for
chromium
n Group 6B / 7B metal in high oxidation
state reacts with aluminium in redox
reaction:
n The 6B / 7B metal is reduced and
forms an insoluble oxide
n Aluminium is oxidized
n The insoluble oxide precipitate on the
aluminium surface and forms a
protective film
Materials Technology 15

Materials Technology 16

n Molybdenium n Manganes
n Mo6+ reacts with Al and forms n Mn7+ reacts with Al and forms
Mon+, which precipitates as Mn4+, which precipitates as
oxides on the Al surface MnO2 on the aluminium surface
n Both processes gives a conversion coating that covers the
aluminium matrix
Materials Technology 17

n Molybdenium n Manganese
– Expensive + Good adhesion
– Moderate corrosion protection + Yellow color
– Poor adhesion properties
– More hydrophilic than
chromium - not as good barrier
coating
– Poor corrosion resistance
Materials Technology 18

Molybdenium Chromium
Materials Technology 19

Manganese Chromium
Materials Technology 20

n Phosphating
n Titanium / Zirconium based
processes
n Cerium (and other rare earth
metals)
n Trivalent chromium
Materials Technology 21

n Hydrogen evolution at cathodic
sites in the alloy (intermetallic
particles)
n pH increases near the
intermetallics
n The solubility of the oxide
decreases when the pH increases
n The film (oxide) presipitates
n The conversion coating is
therefore mainly formed on the
intermetallic particles
Materials Technology 22

n Originally developed for steel and extensively used on
steel and zinc
n Available in a number of variations where zinc, iron, nickel
or manganese are incorporated in the coating
n A sealer or passivator may be applied on the coating, e.g.
containing zirconium ions
n Problem: Al3+ ions in the bath will inhibit the coating
formation. By adding fluorides the Al3+ is bound in AlF63-
and precipitates
Materials Technology 23

n Phosphoric acid exists in four levels of protonation:
H3PO4 H2PO4- HPO42- PO43-
n pH at cathodic sites on the surface increases due to
hydrogen evolution:
2 H+ + 2 e- H2
n PO43- precipitates with aluminium or other metallic ions
present in the solution when the pH increases
Materials Technology 24

n Anodic dissolution of aluminium:
Al + 3 H2PO4- ® Al(H2PO4)3
n Secondary reactions take place:
Al(H2PO4)3 ® Al2(H2PO4)3 + 3 H3PO4
Al2(HPO4)3 ® 2 AlPO4 + H3PO4
n Net reaction:
2 Al(H2PO4)3 ® 2 AlPO4 + 4 H3PO4
Materials Technology 25

n Used in plants where both aluminium and steel
components are treated, e.g. in the automotive industry
n Perhaps the chromium free pre-treatment process that is
most frequently used on aluminium today
Materials Technology 26

+ Good adhesion
+ Coating formation rate comparable to chromium
+ Pre-treat Al in same process as steel and zinc
– Less corrosion resistant than chromium
– Colorless and invisible on aluminium
Materials Technology 27

n Titanium and zirconium (also
hafnium) conversion coatings are
formed the same way
n The metal is exposed to a
solution of H2ZrF6 , H2TiF6 or
both
n Hydrofluoric acid removes the
aluminium oxide from the surface
n Near intermetallic particles,
where the pH is higher, the
fluorides hydrolyse and TiO2 /
ZrO2 precipitates
n The oxidation number for Ti/Zr is
+4 both in solution and coating -
no redox reaction
Materials Technology 28

n The coating is not homogenus -
precipitates on intermetallic
particles
n The amount of oxide precipitated
depends on the composition of
the alloy - the more intermetallic
particles the more coating
precipitates
n Thin coatings - in the order of 10
nm or 10 mg/m2
n For some processes a polymer is
included to seal the conversion
coating
n Ti and Zr oxides are stabile at pH
3 - 12
Materials Technology 29

+ The coating forms rapidly - seconds
+ Good adhesion and corrosion resistance has been found
for some alloy/coating systems
– In other systems not - the results varies with alloy, thermo-
mechanical history of the alloy and organic coating
– The coating is invisible
Materials Technology 30

n Solution containing trivalent cerium (Ce3+) and hydrogen peroxide
(H2O2)
n Film formation takes place by process similar to the Ti/Zr mechanism
First: Later:
n The hydrogen peroxide n Dissolution of aluminium oxide
oxidizes the trivalent cerium on the rest of the surface
n Small cathodic particles
2 Ce3+ + H2O2 2 Ce4+
appears where the same
n Ce4+ precipitates at local precipitation process occurs
cathodes when pH increases n The islands grow into a
continuos film
H2O2 + 2 e- 2 OH-
Materials Technology 31

n Trivalent cerium hydroxide is
stabile at pH > 7
n Thickness
n Matrix: 100 - 200 nm
n Intermetallic particles: ~1 µm
n Precipitation on intermetallic
particles - decreases the
cathodic reaction rate
n Used in combination with
manganese which mainly builds
on the Al matrix
n The coating is colored
Materials Technology 32

+ Colored coating
+ Good corrosion resistance and adhesion properties have
been reported, high Cu alloys in particular (2000)
– In other alloy / coating systems poor corrosion and
adhesion properties have been reported - The process
has to be adapted to each specific alloy
– Many process steps
– Building of the conversion coating takes long time
– Expensive
Materials Technology 33

n Self Assembling Molecules (SAM)
n Silanes
Materials Technology 34

Basic principle:
n Organic molecules with two functional groups
n One binds to the surface oxide
n The other to the organic coating
Materials Technology 35

n Relatively new process
n few experiences available
n few published results
n Less corrosion resistant than Ti/Zr based processes and
chromium
n Sensitive to surface cleanliness prior to application
Materials Technology 36

Basic principle
O- R O- H
R Si O- R + 3 H2 O R Si O- H + 3 R-OH
O- R O- H
R: ethyl / methyl
R: organic functional group R
-C3H6-NH3
O Si
-C3H6-O-CH-CH2
O O O
-C2H4-Si (OR)3 Al Al Al
Materials Technology 37

n Applied by exposing the surface
to a diluted solution of silanes in
water
n Homogenous
n 50-100 nm thick
n Cross linked via Si - O - Si
bonds into a three dimensional
network
Joop Mulder, Corrosion Management,
No 44, Nov./Dec 2001
Materials Technology 38

+ Good adhesion properties, also wet adhesion
– Corrosion resistance not as good as chromium
– Emission of alcohols in the process, which may need
handling
– Sensitive to surface cleanliness
– Choice of organic functional group may be resin
dependant
Materials Technology 39

n DC anodizing
n Hot AC anodizing
Materials Technology 40

n The aluminium alloy is polarized
anodically
V
n Electrolyte: Diluted acid, e.g. 15
% sulfuric acid
e-
n The aluminium oxidizes:
2 Al + 3 H2O ® Al2O3 + 6 H+ + 6 e-
H2
n At the cathode hydrogen is
formed:
6 H+ + 6 e- ® 3 H2
H+
Counter
Substrate electrode
Materials Technology 41

n The oxide gets a hexagonal
structure
n A barrier layer in the bottom
~100 Å thick
n A porous layer 1 - 30 µm (pre-
treatment: ~1 µm)
n The thickness of the oxide layer
depends on:
n Electrolyte
n Temperature
n Current density
n Time of treatment
n Alloy composition
Materials Technology 42

n Traditionally used for surface
finishing:
n Several µm thick oxide
n Several minutes to build the
oxide
n Pre-treatment:
n ~1 µm is sufficient
n Faster
1 µm
TEM image of DC anodized AA6060
Materials Technology 43

n The aluminium is both anode
and cathode
n During cathodic hydrogen
evolution, degreasing also
takes place
n No need for degreasing prior to
AC anodizing
n Process temperature: 80°C
1 µm
TEM image of AC anodized AA6060
Materials Technology 44

+ Corrosion resistance – Need for special equipment:
comparable to chromium power source
+ Easy to control metal removal – Not possible in spray lines
and oxide thickness – Adhesion usually somewhat
lower than for chromium
Hot AC
+ Few process steps
+ Anodizing + rinsing
+ Rapid process - seconds
Materials Technology 45

n GSB: Gutegemeinschaft fuer die Stueckbeschichtung von
Bauteilen, Germany
n Qualicoat: The rest of Europe
n Both have approved chromium free processes
n Ti/Zr based pre-treatments: Both
n Anodizing: Qualicoat
n Phosphating: GSB
Materials Technology 46

n Many of them contain hydro fluoric acid
n HF is also on the B-list of hazardous chemicals
n Fluorides: 10 mg/l limit in waste water
n No signals from Norwegian authorities or the EU regarding
changes in the attitude to HF
n New chemical compounds - do we know all their health
and environmental effects yet?
Materials Technology 47