ECOLOGICAL PREPAINT TREATMENT OF ALUMINIUM ... - Aluplanet

ECOLOGICAL PREPAINT TREATMENT OF ALUMINIUM ALLOYS
By Mr Paul Wynn, Atotech GMF Worldwide
Presented by the author at alumotive conference,
Garda Exhibition Centre, Montichiari Brescia Italy 2 April 2009
The preparation of Aluminium alloys prior to painting is fundamental to achieving good coating
adhesion and superior corrosion resistance. A number of well defined surface treatment
techniques have been acknowledged as accepted practise throughout the global industry.
Chemical based treatment methods have included the use of proprietary alkaline cleaners,
acidic phosphates and hexavalent chromium based chromate solutions. Many of these have
become objectionable due to the increased regulation of hazardous materials, growing
restrictions on waste discharge and the need to remain competitive through process cost
reduction.
New technologies are now entering the market as ecological alternatives to these traditional
chemical treatments. These include the adoption of bioremediated cleaners, the application of
organic dry in place coatings and the use of chromium-free passivate films.
Despite the current global economic difficulties, it is imperative that industry continues to
develop its ‘green’ credentials because applicator experience has shown the adoption of
latest technologies produce tangible benefits such as waste minimization and improved
process efficiency, at a reduced cost whilst achieving similar coating performance.
For more information contact:
Mr Paul Wynn
Business Development Manager & R&D Group Leader
Worldwide Business Technology Team
Atotech UK Limited
William Street
West Bromwich
United Kingdom
B70 OBE
Tel: +44 121 606 7109
Email: paul.wynn@atotech.com

Introduction
All metallic surfaces require preparation prior to the application of performance enhancing
surface coatings. The key to successful aluminium processing is to understand the type of
alloy to be processed, the final performance requirements and then to use a suitable
pretreatment process.
Aluminium is extremely versatile due to the wide variety of alloys available, with differing
mechanical and protection properties, supported by a number of suitable surface coatings.
Three of the main properties on which applications of aluminium are based are its low density,
high mechanical strength when alloyed and the relatively high corrosion resistance of the pure
metal. The pure metal displays the highest corrosion resistance, but as alloying elements are
added, this resistance decreases. In particular the development of high strength alloys
containing heavy metals such as copper and zinc, has increased the need for protective
surface treatments. Copper lowers resistance more than other elements, whilst magnesium
has the least effect.
There are two abundant forms of commercial alloy, as outlined on slide 1. Wrought alloys,
which are cast as ingots or billets, then hot and cold worked mechanically into extrusions,
forgings, sheet, foil, wire or tube. Common types include 2000 series for aircraft plus 5000
and 6000 in automotive. Cast alloys are directly cast into the required form by sand, gravity or
pressure die casting techniques. Although there are a large number of castings alloys, most
of these derive their properties from the addition of magnesium, silicon and copper. Examples
include AA380 used for automotive wheels and ADC12 to make carburettors.
Aluminium Types
Wrought Alloys
Alloy
Series
Ty pi cal Applic ations
Al 99.00%
min
1000
food, chemical, sheet
Copper
2000
aircraft
Magnesium
Magnesium &
Silicon
5000
6000
cans, automobile, facades, transportation
automobile, architectural, transportation
Cast Alloys
Seri es
All oy
Ty pi cal Applic ations
AA356
AA380
ADC12
Si 7.5%
Si 8.5%, Cu 3. 5%
Si 10%, Cu 3%
Zn 2%
pressure castings for vehicles
(carburetors, wheels, suspension,
transmission housings), aircraft, electrical
appliances, cookware
Slide 1: examples of Aluminium types
The alloy that should be used in any particular application will depend upon factors such as
the mechanical and physical properties required, the material cost and the service
environment involved. If a surface treatment is to be applied, then the suitability of an alloy for
producing the particular finish will be an additional consideration.
Aluminium Finishing
Pure aluminium displays excellent corrosion resistance, largely due to its affinity for oxygen.
This results in the production of a very thin but tenacious oxide film which covers the surface.
Aluminium can provide satisfactory performance without finishing treatments in many
applications, but potential problems from corrosion are usually reduced or eliminated by the
use of a coating.
The choice of finish and substrate alloy will depend upon a number of considerations, such as
the nature of the service environment, the final finish appearance required, processing cost
and others such as formability and electrical resistance. In addition to retaining its original

appearance coatings and deposits can give colour, add other decorative effects and improve
overall corrosion protection. There are a diverse range of finishes applied to aluminium and its
alloys. Principal amongst the commercially important types are:
• Anodic oxidation coatings, which are commonly known as anodizing. These are
electrochemically produced oxide coatings which provide maximum protection.
• Chemical conversion coatings. These can be characterised as thin oxide, phosphates or
chromates that are used for light service conditions and as a base prior to the application
of paint and other organic coatings.
• Electroplated deposits with suitable surface preparation. A full range of decorative and
functional electroplates can be applied onto aluminium.
• Painted finishes. These are organic based technologies such as paint, powder coatings
and lacquers enabling greater flexibility in texture, colour and functional properties.
Coated aluminium is an important material, being widely used by a number of global
industries such as consumer products, transport and construction.
Treatment Process
The tenacity of aluminium’s natural oxide film has a serious adverse effect in the production of
surface treatments, so it must be removed or modified before coatings can be successfully
applied. The main function of conversion coatings such as phosphate or chromate is to
improve adhesion and corrosion protection of paints under wet and dry conditions, whilst
improving the corrosion protection of the uncoated metal surface. They are formed as a result
of reaction of the metal surface with a wide range of different chemical solutions. They can
both be successfully applied in bulk volumes by spray or immersion. It is common to utilise
three stage, five stage or longer process lines. The actual number of treatment stages is
guided by the type of substrate being processed and the final performance requirement.
A typical conveyorized five stage spray washer prior to painting or powder coating, is shown
in slide 2. In this example, the initial surface preparation is by spray alkaline cleaning.
Following water rinsing, an iron or zinc phosphate will be applied or it can be a chromate
conversion. After further water rinsing, an acidic seal may be applied, particularly for
phosphate applications where increased corrosion protection is required. The treated
components would then be oven dried prior to painting.
Treatment Process
Treatments improve adhesion and corrosion resistance
Alkaline Clean > Phosphate or Chromate > Oven Dry
Spray Zo nes
Alkaline
clean
Water
Rinse
Phosphate
or
Chromate
Water
Rinse
Acidic
seal
Oven
Dry
Slide 2: five stage spray line

Alkaline Clean
Aqueous alkaline cleaning is the most important stage in surface preparation. Working at
elevated temperature and solution pH, the traditional powdered chemical cleaners are
designed to provide rapid and uniform surface wetting of the aluminium surface. They will
break up and disperse surface soils from the substrate.
Since aluminium is readily attacked by high pH, specialist formulations are used that operate
at comparatively low alkalinity and pH, whilst being inhibited to ensure they are non-etching.
Under normal operation, the working solution will become increasingly contaminated over
time and process efficiency is reduced, as indicated on slide 3. Whilst additions of the
formulation chemistry can be made to achieve short term performance improvements, a
saturation point will be reached and the process solution has to be disposed of and replaced.
The regular replacement of cleaning solutions increases overall process costs and in addition
to the cleaner concentrate, there will be further costs and liabilities associated with waste
disposal.
Alkaline Clean
Performance against Time
Slide 3: solution performance against time
Phosphating
Phosphates originally developed for steel have been adapted for aluminium. Formulations
contain a metal acid phosphate solution, an oxidizing agent and a complex fluoride which
accelerates the deposition process. Coatings are formed as a result of the reaction of
aluminium in phosphoric acid with fluoride as the principal reaction driver. The main
requirement of the metal in solution (iron or zinc) is to form an insoluble phosphate during
reaction. Saturation of the solution at the interface leads to the deposition of the phosphate
coating.
Amorphous iron phosphate and crystalline zinc phosphate treatments have been frequently
used prior to painting. The primary component of zinc phosphate deposits on aluminium is
hopeite Zn 2 Fe(PO 4 ) 2 .4H 2 O. Iron phosphate coatings typically consist of vivanite
Fe 3 (PO 4 ) 2 .8H 2 O and magnetite Fe 3 O 4 .
Phosphate processes are highly acidic and operate at elevated temperature. The mechanism
involved is complex, but during the precipitation reaction, insoluble phosphate compounds are
formed as by-products. These take the form of sludge and scale, which deposit and build onto
application equipment reducing process efficiency, examples of these can be seen in slide 4.
These typically cause the clogging of spray nozzles, the blocking of pipes and pumps, plus
the scaling of the washer. Sludge formation consumes approximately 20 to 40% of process
chemistry, therefore regular additions are critical to maintain efficiency and performance.

One of their main advantages is their effectiveness as a pretreatment for other metals,
making is possible to process mixed metals such as iron, steel, zinc and aluminium on the
same line. Therefore they have gained considerable importance in vehicle manufacturing.
Phosphating
Insoluble phosphate by-products
• Precipitate as sludge and scale
Slide 4: examples of sludge and scale residues
Chromating
Chromating is based upon the oxidation of aluminium by an acidic solution containing
hexavalent chromium and fluoride ions. The process modifies the surface characteristics of
the natural oxide layer to form a more corrosion resistant coating of chromium oxides.
Chromium containing products of the reaction are deposited on the metal surface to form the
conversion coating.
Hexavalent chromium is responsible for the well known ‘yellow’ colour. The chromate coating
is a gelatinous olation polymer of mixed chromium oxides and has a characteristic ‘mudcracked’
surface, the result of dehydration of the film under drying. It is used on all alloy types
in a multitude of applications, providing excellent adhesion and superior corrosion resistance.
Generally chromates have outperformed zinc and iron phosphates on Aluminium, due to the
superior corrosion resistant properties of hexavalent chromium in addition to the increased
depth of pitting that occurs on phosphate treated surfaces. However Hexavalent Chromium
compounds have become increasingly regulated and are already being replaced in a number
of key areas including electronic, electrical and automotive components.
Ecological Treatments
Commercial industrial treatments based upon alkaline cleaners, acidic phosphates and
chromates are under increasing scrutiny due to the regulation of hazardous materials, the
growing cost and restriction on waste disposal and the need to remain competitive through
the use of more cost efficient processes. Therefore it has become desirable to find more
beneficial and less objectionable treatments.
New ecological technologies are gaining acceptance as credible alternatives and have
already entered commercial service with applicators. These include the use of BioChemical
cleaners, Organic dry-in-place coatings and Chromium-free passivates. Bulk applied by spray
or immersion, they are readily introduced into existing process lines and are therefore drop in
replacement technologies that do not require capital investment.
A clear target for industry is the provision of sustainable products that fully comply with the
latest European Union Directives and Regulations. Industry also needs to meet the ever
increasing demands of consumers for eco-friendly products. The desire for environmentally

friendly chemical technologies is one of the significant factors driving these new treatment
methods.
BioChemical Cleaning
A new advance in aqueous alkaline cleaning has been achieved through the combination of
latest generation inorganic and organic chemical compounds in synergy with biotechnology.
Applicators would like to reduce their energy consumption, so it would be desirable to have
lower temperature operating cleaners. This has been realised by the use and optimisation of
increased surfactant combinations with highly active dispersants. These new formulations
displace and emulsify surface soils at lower operating temperatures. At the same time there is
a need for longer life of process solutions. Through the process of bioremediation, complex
organic molecules such as oils, are converted into less complex, non-hazardous substances
such as carbon and water. This means that a wide range of oils and soils will be consumed
and eliminated from the working solution.
The latest generation BioChemical cleaners utilise microbes found in nature which have been
selected for their benefits to humans. They play a fundamental role in the transformation of
matter in various fields and are increasingly found in industrial applications. Extensively used
in the production of foods and beverages, their use has grown considerably in the chemical
and pharmaceutical industries as well as modern waste water treatment plants.
Under aerobic conditions, biodegradation of organic compounds will naturally occur and an
effective degradation of matter can be achieved, as illustrated in slide 5. The interaction of an
active biomass over time with larger organic molecules results in the formation of many
smaller molecules and increased biomass. This process known as Bioremediation enables
the continual repetition of this reaction on prolonged contact, creating smaller organic
molecules some of which will be broken down many times an be transformed into more
nicrobes, small amounts of carbon dioxide and water. This ability to continue reacting over
time is one of the unique benefits from biotechnology. This approach is now successfully used
in a number of industrial applications such as paint pretreatment, paint overspray treatment
and waste treatment.
BioChemical cleaning
Latest generation chemistry
Aerobic microbes
Bioremediation
TIME
CO 2
Water
Oil at
surface
Oil, Water
Surfactant
Emulsion
BIOMASS
Surfactant
Slide 5: illustration of the bioremediation process
The combination of latest cleaning chemistry and biotechnology ensures consistent
performance, optimum process efficiency and exceptionally long solution working life. This
avoids the need for regular solution dumps, conserving both chemistry and water, whilst
drastically reducing waste disposal needs. Further improvements can be achieved when
BioChemical cleaners are combined with dedicated filtration equipment. This synergy

etween product and equipment enables the working solution to be continuously rejuvenated
Performance improvement
BioChemical & Equipment
Synergy with equipment and chemistry
BioFilter unit for optimum performance
Consistent and optimum performance
Extended working solution life
and replenished, as referenced by slides 6 & 7.
Slide 6: BioFilter
Slide 7: optimum performance
Organic Dry In Place
Organic Dry In Place (ODIP) coatings are formed by drying a variety of aqueous chemistries
directly onto a cleaned metal surface. Operating at room temperature, the coating chemistry
is completely free of phosphates, chromium and solvents. Bulk spray or immersion applied, it
will deposit a thin coating onto a range of metallic substrates such as steel and aluminium by
chemical bonding, thereby reducing the influence of the substrate material.
ODIP means there is no need for water rinsing, just an oven dry after application. This
eliminates the need for a final water rinse, reducing energy and water consumption. Since the
system does not rely on a precipitation or conversion reaction, there are no sludges and
scales produced as by-products, which avoids extensive equipment cleaning. This new
technology is rapidly being introduced as a replacement for iron phosphate. Adhesion and
corrosion resistance is equivalent or better and is ideal if the primary performance criteria is
paint adhesion and when there are a limited number of process tanks available, for example
in a three stage spray line.
This has found numerous applications throughout the paint and powder coating sector, for a
number of substrates including aluminium. In domestic appliances it is used for the painting of
white goods like cookers and washing machines. In automotive it is gaining acceptance for
motorcycle parts made from wrought alloys and for car braking systems on cast alloys.
ODIP coatings are applied in conventional paint pretreatment lines by spray or immersion,
without the need for capital investment. The simplified bonding mechanism is shown on slide
8. Aluminium is first cleaned, this would typically be a non-etch alkaline cleaner. Following
water rinsing, the ODIP coating solution is applied by either spray or immersion. As there are
no precipitation or conversion reactions taking place, it is sufficient to fully wet the surface.
The chemistry contains active functional groups which chemically bond to the metal oxide
sites during oven drying.
Following successful pretreatment, an organic coating such as powder coat, can be directly
applied to the substrate surface. The powder contains active bonding sites which chemical
combine with the coating during the oven curing process. This is a simplified way to show
how the technology achieves highly adherent and superior bonding between metallic
substrates and paint or power type coatings.

ODIP bonding mechanism
Organic Dry In Place
Chemical bond to metal oxide
Chemical bond with powder coat
Excellent paint adhesion
O-
O-
O-
R-
R-
R-
Cleaned
Aluminium
Organic
D I P
Coated
Aluminium
Powder
Coating
DIP-O-
DIP-O-
DIP-O-
DIP-
DIP-
DIP-
R-DIP-O-
R-DIP-O-
R-DIP-O-
Painted
Aluminium
Slide 8: ODIP bonding mechanism
Chromium-free Passivate
The most versatile replacement for phosphating and chromating prior to painting, are the new
Chromium-free Passivates (CFP) which are suitable for all alloy types and readily applied by
spray or immersion. Completely free of chromium and phosphates, they are generally based
on group four transition metals such as Zirconium or Titanium. Coatings display a mineral like
grain structure which means they do not exhibit ‘mud cracking’ and are more heat resistant.
Film colour is blue to iridescent, dependant upon the material being processed. Surface
characteristics are shown on slide 9, contrasted against chromate.
Surface characteristics
CHROMATE
PASSIVATE
Slide 9: characteristics of chromate against passivate
CFP has the benefit of being able to function on mixed metal lines processing metals such as
aluminium, steel and zinc thereby giving the applicator greater flexibility. Studies have shown
that passivate performance is independent of solution operating temperature when used over
steel and galvanised substrates, therefore it allows the applicator to benefit from room
temperature operation. For aluminium and magnesium, whilst room temperature operation

produces more than acceptable results and the highest levels of performance are achieved
when the solution is operated at elevated temperature around fifty centigrade.
CFP technology meets latest European Union legislation, ensuring applicators and end users
are fully compliant. It is an ideal replacement for phosphates and chromates when excellent
paint adhesion with superior corrosion resistance is required. It has been shown to exhibit
superior mechanical properties such as paint adhesion, whilst giving excellent corrosion
resistance. They are highly suitable to meet a wide variety of industrial performance
standards.
As a ‘stand alone’ passivate film, they also show good levels of corrosion protection for
everything except high copper alloys such as wrought 2000 series. The current commercially
used offerings fail to consistently meet the 168h neutral salt spray level as defined by MIL-
DTL-5541F. There are new areas of research which show improved ‘stand alone’
performance of CFP over aluminium and it has been speculated that the minimum
requirement of the standard may be achieved for high copper alloys.
CFP performance.
Evaluation studies have demonstrated that the performance of the final coating is essentially
not dependant upon the application method of the passivate. Results shown on slide 10 are of
comparative tests confirming that same performance is achieved whether CFP application is
by immersion or spray. Additional validation work has been completed to determine the
influence of substrate type. Slide 11 highlights the results of corrosion testing using neutral
salt spray, after 700h exposure for Al 6061 wrought alloy, Galvanized sheet and Cold rolled
steel. The results confirm the suitability of the technology for multi metal application, which is
important for process flexibility.
CFP spray vs immersion
CFP substrate variation
Neutral salt spray 2184 hour exposure
Polyester Powder Coat nominal 50 microns thickness
Neutral salt spray - 700 hour exposure
Type
Spray
Immersion
Alloy
Al 6061
Al 6061
Paint type
Epoxy powde r coat
Paint
thickness
Nominal 50 microns
Creep back
mm
0
0
Aluminum 6061 (Spray)
- ASTM D1654 (Rating 10)
Aluminum 6061 (Immersion)
- ASTM D1654 (Rating 10)
Rating
ASTM 1654
10
10
Aluminum 6061
ASTM D1654
Rating 10
Galvanized G90
ASTM D1654
Rating 9
Cold Rolled Steel
ASTM D1654
Rating 10
Slide 10: application method
Slide 11: substrate type
To be commercially viable as a prepaint treatment for aluminium, it is also necessary to
establish that CFP provides comparative performance with different aluminium alloy types.
The results on slide 12 show adhesion and corrosion tests for three common alloys, all were
treated through the same prepaint sequence and then coated with a polyester powder coat.
After 4000 hours neutral salt spray exposure, extruded alloys Al2024, 5052 and 6061 show
no signs of corrosion and no loss of paint adhesion.

CFP alloy variation
Neutral salt spray at 4000 hour exposure
Type
A1
B1
C1
Alloy
Al 6061
Al 5052
Al 2024
Paint type
Polyester
Interpon
D1036
Polyest er
Interpon
D1 036
Polyester
Interpon
D1036
Paint
thickness
Nominal 70 microns
Creep back
mm
0
0
0
Rating
ASTM 1654
10
10
10
Slide 12: alloy variation
Enhanced Process
To achieve optimum paint performance on aluminium alloys, quality assure in-service
reliability and meet the needs of demanding corrosive environments, a more extensive
prepaint treatment prior to CFP has found to be beneficial, as outlined on slide 13.
The enhanced sequence starts with a non-etch BioChemical alkaline cleaner as the preferred
choice for surface oil removal, followed by water rinsing. This is shown on the slide optimized
with the use of a BioFilter to extend performance and solution life, which is available to
applicators as an option. Etching is then required to active the surface. Experience has shown
that an acidic etch produces a more uniform substrate profile which is highly compatible with
passivation.
After etching, surface smuts may be generated which tend to be heavier the more highly
alloyed the substrate type, smuts are particularly heavy on copper based alloys. They mainly
consist of oxides and intermetallics which are readily removed by a suitably formulated acid
treatment. Following smut removal, double water rinsing is recommended prior to passivation.
Some instances have found that final finish improvements can be realized when using
deionized water in the second stage rinse, but it is always recommended to use deionized
water after passivation. This process route can be used effectively for both spray and
immersion, and for both wrought and cast aluminium alloys.
Enhanced Process
Spray Zones
BioChemical
Clean
Water Rinse
Acidic Etch
Water Rinse
Desmutt
Water Rinse
Chromium-free
passivation
Slide 13: enhanced prepaint process

Applicator Benefit
An ecological focus through the adoption of new technology makes good business sense to
remain competitive today and for long term sustainability. An increasing number of applicators
are making the technology change and realising tangible benefits, which include:
• Reduced energy consumption, by operating at lower or room temperature.
• Water conservation through extended solution life and reduced water rinses.
• Waste minimization by significantly reducing sludge waste from cleaner and coater
processes.
• Improved flexibility with the ability for process lines to handle a variety of substrates.
• Increased competitiveness by achieving a significant reduction in process related
costs.
• Using greener technology to ensure regulatory compliance.
Summary
The surface treatment industry continues to transform itself into a modern and technology
driven business. The image of low technology and shoddy practises are disappearing. In the
current economic climate, industry faces an incredible crisis from a loss of consumer
confidence, which has come from a massive and collective failure in leadership of the
financial system. The surface treatment industry must continue to invest in new technologies
and deliver innovation to ensure it retains a value added status. The adoption of ecological
prepaint treatments for aluminium alloys has been demonstrated to be commercially viable
and its use is gaining credibility in a wide range of industrial sectors.
References
1.”The Surface Treatment and Finishing of Aluminum and its alloys” by P.G. Sheasby and R.
Pinner, published by Finishing Publications Ltd sixth edition 2001
2. “Phosphating and metal pre-treatment” by D.B. Freeman, published by Woodhead-
Faulkner Ltd 1986
3. “Replacing Hexavalent Chromium in Passivations on Zinc Plated Parts” by Paul C. Wynn
and Craig V. Bishop, published in Products Finishing 2001
4. “Industrial Production Shop, Yes;Hobby Coater, No” by S. Spielman, published in Powder
Coated Tough 2007
5. MIL-DTL-5541 F Chemical Conversion Coatings on Aluninium and Aluminum alloys,
2006
6. ASTM D1654 Standard Test Method for Evaluation of Painted or Coated Specimens
subjected to Corrosive Environments, published by ASTM 2005
7. ASTM B117 – 97 Standard Practice for Operating Salt Spray (Fog) Apparatus, published
by ASTM 1997