chromium problems
DESCRIPTION
Chromium problems on health and technologyTRANSCRIPT
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1Materials Technology
Ole Øystein Knudsen, Astrid BjørgumSINTEF Materials Technology
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2Materials Technology
� Part of a wider ongoing research project financed by theResearch Council and Norwegian light metals industry:
� Participating companies� Hydro Aluminium� Elektro-Vakuum� Noral Lighting� Norsk Industrilakkering� Profil-Lakkering� DuPont Powder Coatings� Jotun Powder Coatings
Light Metal Surface Science
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3Materials Technology
� Chromium - why …. and why not?� Properties and problems� Regulation - EU and Norwegian
� Chromium free pre-treatments� Redox reactions with precipitation� pH controlled precipitation on the aluminium surface� Molecules that deposit on the aluminium surface� Strengthening the aluminium oxide
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4Materials Technology
� Focus in this presentation:� Treatments for aluminium� Commercially available processes
� Emphasis for each process:� Basic principles for the formation of the coating� Process / production friendliness� Experiences
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5Materials Technology
Why …. and why not?
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6Materials Technology
� First patented in 1923� Used extensively since for pre-
treatment before coating,adhesive bonding and surfacefinishing
� 43 000 tons of chromium wasused in metal finishingoperations in 1993
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7Materials Technology
� The chemicals contain:� Hexavalent chromium (CrO3 or CrO4
2- or Cr2O72-)
� Hydrofluoric acid (HF)� The hydrofluoric acid removes the oxide film on the
surface� The hexavalent chromium reacts with the exposed
aluminium metal and a trivalent chromium oxideprecipitates
Cr2O72- + 2 Al + 2 H+ � Cr2O3·H2O + Al2O3
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8Materials Technology
� Chromium oxide stabile inalkaline solutions up to pH 15
� Chromium oxide is waterrepellant (hydrophobic) andmay act as a barrier coatingtowards water
� Self healing effect: hexavalentchromium present in theconversion coating that mayreact at mechanical damages
� Passivating both the aluminiummatrix and the intermetallicparticles
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9Materials Technology
� Highly effective� Preventing corrosion� Adhesion promoter for organic coatings and adhesives
� Resilient: The process has low sensitivity towardsvariation in process conditions
� Effective on most/all aluminium alloys� Quality control: Skilled workers can tell the amount of
chromium on the surface by the color of the conversioncoating
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10Materials Technology
� Toxic� Classified as human carcinogen� Workers at the production line are concerned about their
health - liability for claims of workspace exposure� Consumers are concerned about hexavalent chromium
present in products� Concern about hexavalent chromium in the environment,
e.g. drinking water� Treatment of waste
� Stringent disposal limits� Increased costs for tracking inventories, monitoring, reporting� Disposal of wastes containing chromium
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11Materials Technology
� Pacific Gas & Electric� Deposits of hexavalent
chromium in the ground� Hexavalent chromium leached
into the groundwater� Cancer and other diseases
increased dramatically� In 1993 PG&E settled for $333
million, the largest settlementever in a direct action lawsuit.
� California 2001: Law that limitsthe level of hexavalentchromium in drinking water
Julia Roberts as Erin Brockovich (2000)
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12Materials Technology
� EU directive 2000/53/EC: End-of life vehicle� Every year 8-9 million tons of waste are produced from end-of life
vehicles� Aim: to harmonize the treatment of this waste in order to reduce
environmental impact� Increase recycling and reuse of materials� Reduce and control the use of hazardous substances in vehicles� Vehicles put on the market after 1 July 2003 shall not contain:
� lead, mercury, cadmium and hexavalent chromium
� Norway: Chromium on the B-list of hazardous substances� Aim: Significant decrease in the use of chromium by 2010
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13Materials Technology
What options do we have?
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14Materials Technology
� Redox reactions with precipitation� Molybdenum� Manganese
� pH controlled precipitation� Phosphate� Titanium / zirconium based processes� Cerium� (Trivalent chromium)
� Coupling agents between aluminium oxide and binder� Silanisation� Self Assembling Molecules (SAM)
� Strengthening the aluminium oxide� DC and AC anodizing
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15Materials Technology
� Molybdates� Permanganates
Basic prinsiple� Same formation mechanism as for
chromium� Group 6B / 7B metal in high oxidation
state reacts with aluminium in redoxreaction:� The 6B / 7B metal is reduced and
forms an insoluble oxide� Aluminium is oxidized
� The insoluble oxide precipitate on thealuminium surface and forms aprotective film
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16Materials Technology
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17Materials Technology
� Molybdenium� Mo6+ reacts with Al and forms
Mon+, which precipitates asoxides on the Al surface
� Manganes� Mn7+ reacts with Al and forms
Mn4+, which precipitates asMnO2 on the aluminium surface
� Both processes gives a conversion coating that covers thealuminium matrix
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18Materials Technology
� Molybdenium– Expensive– Moderate corrosion protection– Poor adhesion properties
� Manganese+ Good adhesion+ Yellow color
– More hydrophilic thanchromium - not as good barriercoating
– Poor corrosion resistance
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19Materials Technology
Molybdenium Chromium
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20Materials Technology
Manganese Chromium
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21Materials Technology
� Phosphating� Titanium / Zirconium based
processes� Cerium (and other rare earth
metals)� Trivalent chromium
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22Materials Technology
� Hydrogen evolution at cathodicsites in the alloy (intermetallicparticles)
� pH increases near theintermetallics
� The solubility of the oxidedecreases when the pH increases
� The film (oxide) presipitates� The conversion coating is
therefore mainly formed on theintermetallic particles
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23Materials Technology
� Originally developed for steel and extensively used onsteel and zinc
� Available in a number of variations where zinc, iron, nickelor manganese are incorporated in the coating
� A sealer or passivator may be applied on the coating, e.g.containing zirconium ions
� Problem: Al3+ ions in the bath will inhibit the coatingformation. By adding fluorides the Al3+ is bound in AlF6
3-
and precipitates
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24Materials Technology
� Phosphoric acid exists in four levels of protonation:
� pH at cathodic sites on the surface increases due tohydrogen evolution:
� PO43- precipitates with aluminium or other metallic ions
present in the solution when the pH increases
H3PO4 H2PO4- HPO4
2- PO43-
2 H+ + 2 e- H2
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25Materials Technology
� Anodic dissolution of aluminium:Al + 3 H2PO4
- � Al(H2PO4)3
� Secondary reactions take place:Al(H2PO4)3 � Al2(H2PO4)3 + 3 H3PO4
Al2(HPO4)3 � 2 AlPO4 + H3PO4
� Net reaction:2 Al(H2PO4)3 � 2 AlPO4 + 4 H3PO4
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26Materials Technology
� Used in plants where both aluminium and steelcomponents are treated, e.g. in the automotive industry
� Perhaps the chromium free pre-treatment process that ismost frequently used on aluminium today
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27Materials Technology
+ 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
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28Materials Technology
� Titanium and zirconium (alsohafnium) conversion coatings areformed the same way
� The metal is exposed to asolution of H2ZrF6 , H2TiF6 orboth
� Hydrofluoric acid removes thealuminium oxide from the surface
� Near intermetallic particles,where the pH is higher, thefluorides hydrolyse and TiO2 /ZrO2 precipitates
� The oxidation number for Ti/Zr is+4 both in solution and coating -no redox reaction
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29Materials Technology
� The coating is not homogenus -precipitates on intermetallicparticles
� The amount of oxide precipitateddepends on the composition ofthe alloy - the more intermetallicparticles the more coatingprecipitates
� Thin coatings - in the order of 10nm or 10 mg/m2
� For some processes a polymer isincluded to seal the conversioncoating
� Ti and Zr oxides are stabile at pH3 - 12
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30Materials Technology
+ 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
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31Materials Technology
� Solution containing trivalent cerium (Ce3+) and hydrogen peroxide(H2O2)
� Film formation takes place by process similar to the Ti/Zr mechanism
First:� The hydrogen peroxide
oxidizes the trivalent cerium
� Ce4+ precipitates at localcathodes when pH increases
2 Ce3+ + H2O2 2 Ce4+
Later:� Dissolution of aluminium oxide
on the rest of the surface� Small cathodic particles
appears where the sameprecipitation process occurs
� The islands grow into acontinuos film
H2O2 + 2 e- 2 OH-
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32Materials Technology
� Trivalent cerium hydroxide isstabile at pH > 7
� Thickness� Matrix: 100 - 200 nm� Intermetallic particles: ~1 µm
� Precipitation on intermetallicparticles - decreases thecathodic reaction rate
� Used in combination withmanganese which mainly buildson the Al matrix
� The coating is colored
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33Materials Technology
+ 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 processhas to be adapted to each specific alloy
– Many process steps– Building of the conversion coating takes long time– Expensive
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34Materials Technology
� Self Assembling Molecules (SAM)� Silanes
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35Materials Technology
Basic principle:� Organic molecules with two functional groups
� One binds to the surface oxide� The other to the organic coating
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36Materials Technology
� Relatively new process� few experiences available� few published results
� Less corrosion resistant than Ti/Zr based processes andchromium
� Sensitive to surface cleanliness prior to application
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37Materials Technology
Basic principle
+ 3 H2O SiO- HO- HO- H
R + 3 R-OH
R
Si
O O O
Al Al Al
R: ethyl / methyl
R: organic functional group
-C3H6-NH3 O-C3H6-O-CH-CH2
-C2H4-Si (OR)3
SiO- RO- RO- R
R
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38Materials Technology
� Applied by exposing the surfaceto a diluted solution of silanes inwater
� Homogenous� 50-100 nm thick� Cross linked via Si - O - Si
bonds into a three dimensionalnetwork
Joop Mulder, Corrosion Management, No 44, Nov./Dec 2001
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39Materials Technology
+ 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
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40Materials Technology
� DC anodizing� Hot AC anodizing
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41Materials Technology
� The aluminium alloy is polarizedanodically
� Electrolyte: Diluted acid, e.g. 15% sulfuric acid
� The aluminium oxidizes:2 Al + 3 H2O � Al2O3 + 6 H+ + 6 e-
� At the cathode hydrogen isformed:6 H+ + 6 e- � 3 H2
SubstrateCounter electrode
e-
H+
H2
V
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42Materials Technology
� The oxide gets a hexagonalstructure� A barrier layer in the bottom
~100 Å thick� A porous layer 1 - 30 µm (pre-
treatment: ~1 µm)� The thickness of the oxide layer
depends on:� Electrolyte� Temperature� Current density� Time of treatment� Alloy composition
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43Materials Technology
� Traditionally used for surfacefinishing:� Several µm thick oxide� Several minutes to build the
oxide� Pre-treatment:
� ~1 µm is sufficient� Faster
1 µm
TEM image of DC anodized AA6060
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44Materials Technology
� The aluminium is both anodeand cathode
� During cathodic hydrogenevolution, degreasing alsotakes place� No need for degreasing prior to
AC anodizing� Process temperature: 80°C
1 µm
TEM image of AC anodized AA6060
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45Materials Technology
+ Corrosion resistancecomparable to chromium
+ Easy to control metal removaland oxide thickness
Hot AC+ Few process steps
+ Anodizing + rinsing+ Rapid process - seconds
– Need for special equipment:power source
– Not possible in spray lines– Adhesion usually somewhat
lower than for chromium
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46Materials Technology
� GSB: Gutegemeinschaft fuer die Stueckbeschichtung vonBauteilen, Germany
� Qualicoat: The rest of Europe
� Both have approved chromium free processes� Ti/Zr based pre-treatments: Both� Anodizing: Qualicoat� Phosphating: GSB
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47Materials Technology
� Many of them contain hydro fluoric acid� HF is also on the B-list of hazardous chemicals� Fluorides: 10 mg/l limit in waste water� No signals from Norwegian authorities or the EU regarding
changes in the attitude to HF� New chemical compounds - do we know all their health
and environmental effects yet?