ECOLOGICAL PREPAINT TREATMENT OF ALUMINIUM ... - Aluplanet

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.