COMPLETE CONTROL OF
PHOSPHATING PROCESSES
FOR SUPERIOR CORROSION PREVENTION
ith an estimated worldwide
turnover of more than
500 million US dollars,
phosphating is the most
important metal pre-
treatment process. The phosphating
process produces a hard, electrically non-
conducting surface coating that adheres
tightly to the underlying metal. This
layer protects the metal from corrosion
and improves the adhesion of paints
and organic finishes to be subsequently
applied.
W
The basic phosphating process consists
of the etching reaction and the formation
of the surface coating. After the thorough
degreasing and rinsing of the metal
workpieces, the phosphoric acid removes
interfering surface-bound metal oxides
and increases the surface roughness.
Subsequently the alkali phosphates react
with the previously generated metal
ions at the surface of the workpiece
forming a layer of insoluble tertiary metal
phosphates. With a coating thickness
smaller than 1 µm, iron phosphate
coatings provide a basic corrosion
protection and are intended for interior
use under controlled environments.
In contrast, the addition of metal cations
such as Zn2+, Mn2+, and Ca2+ to the
phosphating bath results in the formation
of very resistant zinc phosphates with a
coating thickness between 7 and 15 µm.
Due to a modified crystal structure these
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layers are perfectly suited for outdoor use
under hostile environments. Both iron
and zinc phosphating occur by the same
mechanism. However, the latter process
is characterized by the preferential
incorporation of zinc cations.
The iron cations at the metal surface
hardly contribute to the metal
phosphate formation. A first-class
corrosion protection is achieved by
the supplementary addition of further
cations (e.g. Ni2+) to the phosphating
bath, resulting in even better surface
properties.
In an at-line system, the analytical unit
is positioned directly at the process line,
allowing for on-site determination of the
parameters required. Sampling occurs
manually and if necessary at various
sampling points of the production line.
All bath-relevant parameters of the
phosphating process can be efficiently
determined with Metrohm’s modular
ADI 2045PL ProcessLab at-line analysis
system. The analytical modules are
accommodated in a single housing that is
impervious to dust and splashes.
The user-friendly operation software and
the operating unit (TFT screen with built-
in membrane keyboard and touch pad)
offer straightforward and easy handling.
Additionally, a barcode reader guarantees
unambiguous sample identification and
makes time-consuming and error-prone
manual data entry obsolete.
Automation of analytical tasks by means
of a sample changer allows to easily
manage high sample throughputs, at the
same time enhancing repeatability.
The ADI 2045PL ProcessLab reliably
records, controls and documents all
the important process parameters. The
analytical data set is stored in a database
and can be processed internally or
transferred to a process control system.
For more information on Metrohm
Process Analytics visit
www.metrohm.co.uk or email us at
[email protected]