Chromium problems
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Transcript of Chromium problems
1Materials Technology
Ole Øystein Knudsen, Astrid BjørgumSINTEF Materials Technology
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
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
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
5Materials Technology
Why …. and why not?
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
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
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
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
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
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)
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
13Materials Technology
What options do we have?
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
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
16Materials Technology
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
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
19Materials Technology
Molybdenium Chromium
20Materials Technology
Manganese Chromium
21Materials Technology
� Phosphating� Titanium / Zirconium based
processes� Cerium (and other rare earth
metals)� Trivalent chromium
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
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
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
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
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
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
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
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
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
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-
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
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
34Materials Technology
� Self Assembling Molecules (SAM)� Silanes
35Materials Technology
Basic principle:� Organic molecules with two functional groups
� One binds to the surface oxide� The other to the organic coating
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
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
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
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
40Materials Technology
� DC anodizing� Hot AC anodizing
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
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
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
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
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
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
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?