Substitution of PFOS for use in nondecorative hard chrome plating

18
Figure 1-1-1: An industrial hard chrome plating system. The sample to be deposited with
chromium is submerged in the electrolyte (in the middle of the photo). The surface of
the electrolyte is covered with plastic balls, acting as a flexible isolation layer and
preventing evaporation of water from the heated electrolyte.
1.2.2 PFOS for control of aerosol formation
Some years ago, it was discovered that addition of polyfluorinated surfactants
(PFOS and derivatives) to the chromic acid bath would lower the surface
tension by forming a thin foamy layer on the surface of the chrome bath. That
mist suppressant layer dramatically reduced the formation of chromium-(VI)aerosols
(Cr 6+ ), which are well-known as carcinogenic, allergenic, and
dangerous for the environment. Thus, introduction of PFOS as mist
suppressant helped solving huge occupational safety problems as well as
environmental problems in the hard chrome plating industry.
The most common fluorinated surfactant used in the business today is
tetraethyl ammonium perfluorooctane sulfonate (CAS no. 56773-42-3;
Fluortensid-248), a derivative of PFOS.
The fluorinated mist suppressant is added only when necessary, i.e. when an
increased formation of aerosols in the chromic acid bath is foreseen. Some
methods (mentioned by suppliers) exist today for measuring, when additional
mist suppressant is needed, but these methods are not used in most plants. It
is merely the experience of the workers and their assessment that determines
when additionally mist suppressant is needed. This means that larger amounts
of PFOS than strongly needed may be used, even though the high price
probably prevents excessive use of PFOS.
When the chrome bath is contaminated and cannot be used anymore, the
liquid has to be disposed of as chemical waste. In Denmark, it is sent to a
chemical waste plant where the chromium is precipitated. The filter cake is
incinerated and the slag from the incineration is disposed on a landfill. The
liquid is sent to a wastewater treatment plant after removal of chromium and
other metals 1 . The residues of PFOS and degradation products will follow the
waste water and end up in sewage sludge.
1
Personal communication with customer service at Kommunekemi a/s February
2011.