Mist suppression with non-PFOS (perfluorooctane sulfonate) surfactants for hard chrome plating
The Danish EPA and some Danish companies carried out a project on substitution of polyfluorinated (PFOS) surfactants for use in non-decorative hard chrome plating. Experiments on both laboratory scale and larger industrial scale have shown that it is possible to substitute PFOS as mist suppressant in the non-decorative hard chrome plating process.
Three non-chemical alternatives were tested in the study: PTFE (polytetrafluoroethylene) coated balls, a mesh or blanket placed on top of the chrome bath and prevention of air convection. For further information please see the case description.
There is an official classification for PFOS on ESIS, but it has not been possible to find a MSDS on either Fumetrol® 21 or TIB Suract CR-H. In the report from the Danish EPA there is a summary (table 4-1) of the environmental and health data for PFOS and the alternatives. There is no information available about the chemical substances in TIB Suract CR-H , because this is confidential. The Danish EPA, however, find Fumetrol® 21 to be a safer alternative to PFOS.
Hard chrome plating is a surface treatment process where a layer of chromium is electrochemically deposited on the surface of metals. The electrochemical process produces a significant amount of gases to be released from the process tank. These gases rise to the surface as bubbles. Most bubbles burst at the surface and form aerosols which are released to the atmosphere. The aerosols consist of process liquid containing chromic acid and thus may expose the environment if no mist suppressant agent is used.
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 significantly reduced the formation of chromium-VI aerosols (Cr6+), which are well-known as carcinogenic, allergenic and dangerous for the environment. Thus, it was thought taht the introduction of PFOS as mist suppressant helped solving huge occupational safety problems as well as environmental problems in the hard chrome plating industry. PFOS and similar compounds are, however, an environmental and health problem as these substances are being biomagnified through the food chain. PFOS has many adverse effects; among others it is an endocrine disrupting substance that affects the human fertility.
The main conclusions of the project are:
• A non-PFOS mist suppressant chemical alternative (Fumetrol® 21 from Atotech – based on 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctanesulphonic acid ) was found to be useful as an alternative to PFOS. The effectiveness, the durability, and the price level were similar to that of PFOS. The non-PFOS alternative is an environmental improvement as it is less persistent, less bioaccumulative, and less toxic than PFOS.
• A non-fluorinated mist suppressant chemical alternative (TIB Suract CR-H from TIB Chemicals – no information about the chemical substance is available, as this is confidential) was found and might be a promising substitute for PFOS. However, as continuous addition of the mist suppressant is needed, the usefulness of the alternative seems to be limited for automated hard chrome processes and must be assessed in each case.
Two physical alternatives were tested.
• A physical alternative – in the form of PTFE (polytetraethylen) coated balls – does not seem to lower the chromium emissions from the chroming bath. Quite the contrary, the chromium emissions seem to increase compared to when using no mist suppressant at all.
• A physical alternative – in the form of a mesh or blanket – could be suitable for large scale series plating of uniform products. However, this kind of alternative was not investigated further in this project.
The laboratory results of this project showed that the Cr6+ emission can be reduced radically by avoiding air convection. This indicates that large scale tests of the described physical methods that prevent air convection could turn out to be an alternative to PFOS. Such method would be a suitable alternative to PFOS in mass production systems. However, in plating system for frequently varying productions, it will be more difficult to establish a closed system because of the flexibility required to such systems. However, this alternative solution was not investigated further in the project, but would be a very interesting solution to investigate further.
There is an official classification for PFOS in EU. PFOS does not pass the SUBSPORT Screening Criteria (SDSC).
The ingredients of TIB Suract CR-H is confidential and not known to SUBSPORT. Therefore SUBSPORT has not been able to assess the product.
One of the substances in Fumetrol® 21 is known (the tridecafluorooctanesulphonic acid) and this substance does pass the SDSC.
Due to the fact, that the rest of the substances in the alternative products are unknown, it has not been possible for SUBSPORT to determine wether or not the alternatives pass the SDSC. In the report from the Danish EPA there is a summary (table 4-1) of the environmental and health data for PFOS and the alternatives. The conclusion in the report is that Fumetrol® 21 is a safer alternative than PFOS. SUBSPORT recommend further investigation of the contents of the alternative products.
Type of information supplier
Miljøstyrelsen (Danish EPA)
DK-1401 København K
T: +45 72 54 40 00
Freely available on www.mst.dk
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Publication or last update: 24-04-2012