Russian Journal of Applied Chemistry, 2009, Vol. 82, No. 10, pp. 1835−1839.
Pleiades Publishing, Ltd., 2009.
Original Russian Text
V. Burokas, A. Martushene, G. Bikul’chyus, A. Ruchinskene, 2009, published in Zhurnal Prikladnoi Khimii, 2009, Vol. 82, No. 10,
INORGANIC SYNTHESIS AND INDUSTRIAL
Aluminum Alloy Etching in Phosphoric Acid Solutions
V. Burokas, A. Martushene, G. Bikul’chyus, and A. Ruchinskene
Institute of Chemistry, Vilnius, Lithuania
Received August 13, 2008
Abstract—The rate of dissolution of aluminum and qualitative properties (gloss and uniformity) of its surface
were examined as influenced by the concentration of H
and fluorine-containing compounds and their nature,
as well as by the temperature, pH, and various additions (acids, surfactants, inhibitors).
An essential aspect in extensively applied powder
painting technologies is thorough pretreatment of the
metal surface. The adhesion and the total corrosion
resistance of the metal surface are increased by
phosphating after degreasing. For improving the quality
of phosphate coatings it is recommended to deoxidize the
aluminum surface before phosphating.
Aluminum is typically etched in a hot sodium
hydroxide solution which is followed by brightening in
a dilute nitric acid solution . In the case of silicon-
containing aluminum alloys, ﬂ uoride ions are added to the
brightener solution . In this procedure the aluminum
ions are accumulated in the alkali solution, thereby
making the latter unsuitable for further use.
This study is concerned with aluminum etching, in
which phosphoric acid is one of the basic components.
Phosphoric acid solutions are used for steel etching,
since speciﬁ cally in this solution rust is removed more
efﬁ ciently than in sulfuric or hydrochloric acid solutions
. Moreover, the iron phosphate crystals formed on the
steel surface improve paint adhesion.
Under natural conditions, aluminum and its alloys are
coated with a thin (thousandth fractions of micrometer-
thick) layer of γ-AlOOH and bionite Al
are stable in aqueous phosphate solutions at pH 4.5–8.5.
Etching of nonferrous metals has a dual purpose of
removing oxides from the surface and producing an active
layer for improving the adhesion of coatings.
We carried out amorphous phosphating in weakly acid
(pH 4.5–5.0) phosphate solutions. This requires thorough
surface pretreatment in acid solutions (hydrochloric,
sulfuric, phosphoric). Etching in phosphoric acid is
When aluminum workpieces are dipped into the
etchant bath (based on H
), the oxide ﬁ lm is removed,
and a part of Al passes into solution by the reaction
2Al + 6H
Rausch  studied steel etching in phosphoric acid and
established an empirical relationship between the metal
dissolution rate, on the one hand, and the reaction time,
acid concentration, and temperature, on the other:
where r is the rate of metal dissolution in phosphoric acid;
t, reaction time; c, acid concentration; and T, absolute
Another problem generated by aluminum etching
consists in accumulation of aluminum ions in etching
and phosphating solutions. This problem is eliminated by
introducing ﬂ uoride ions which, in the presence of Nа
convert aluminum ions into insoluble Nа
precipitated as the slurry by the reaction 
+ 3Na+ + 6H
The uniformity of Al dissolution is increased by intro-
ducing additions (surfactants, complexes, inhibitors) .
There are published data on aluminum etching in
phosphoric acid-based solutions with salt and inhibitor