Chromium problems

1Materials Technology

Ole Øystein Knudsen, Astrid BjørgumSINTEF Materials 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


� 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


� 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


Why …. and why not?


� 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


� 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


� 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


� 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


� 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


� 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)


� 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


What options do we have?


� 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


� 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


� 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


� Molybdenium– Expensive– Moderate corrosion protection– Poor adhesion properties

� Manganese+ Good adhesion+ Yellow color

– More hydrophilic thanchromium - not as good barriercoating

– Poor corrosion resistance


Molybdenium Chromium


Manganese Chromium


� Phosphating� Titanium / Zirconium based

processes� Cerium (and other rare earth

metals)� Trivalent chromium


� 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


� 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


� 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


� 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


� 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


+ 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


� 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


� 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


+ 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


� 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-


� 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


+ 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


� Self Assembling Molecules (SAM)� Silanes


Basic principle:� Organic molecules with two functional groups

� One binds to the surface oxide� The other to the organic coating


� 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


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


� 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


+ 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


� DC anodizing� Hot AC anodizing


� 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


� 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


� 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


� 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


+ 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


� 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


� 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?

Chromium problems

Technology

Transcript of Chromium problems