By Werner Braun, AVVIO
Cr(VI) and chromium >
How the screw and plating industry has cleaned up:
The substitutes for Cr(VI) and what the future may hold
Chromium, or more precisely Cr(VI), is a health hazard and environmental nuisance. AVVIO’s Werner Braun looks at how the screw industry has cleaned up its act to curb Cr(VI), examines the main substitutes and looks at the future for chromium too...
Hollywood’s movie industry already knew it when Julia Roberts, aka Erin Brockovich in the film of the same name, contributed to a worldwide inglorious fame of Cr(VI) in 2000, as the film is based on the spectacular conviction of a major American energy supplier in connection with the use of Cr(VI) in 1996.
13 years later, another film revealed the massive effects of wastewater containing Cr(VI). This time the film did not come from Hollywood, but from talented Taiwanese filmmakers [*1]. Within one and a half hours the highly motivated crew used drones to show the wonderful beauty of the famous Formosa Island to its cinema audience, but also that pollution is a serious problem in Taiwan.
It is one scene after around 57 minutes that laid bare the great impact of the screw industry in the country. In it a deceptively beautiful red river can be seen, which flows with its dangerous contents into another river (see picture above). It turned out that the red colour derived from a semiconductor engineering plant dumping wastewater containing Cr(VI) in to the river. For anyone who bought screws in Taiwan around 2014 its impact on the screw industry is well known. Many electroplating plants were closed by the government. 80% of plants had been unavailable to process fasteners. The inevitable short term result was substantial increases in lead times for plated fasteners and plating costs increased up to 30% [*2].
// For anyone who bought screws in Taiwan around 2014 its impact on the industry is well known. Many electroplating plants were closed by the government. Availability dropped significantly. //
1. Legal situation around hexavalent chromium (Cr(VI); chromium trioxid or CrO3)?
So what is this Cr(VI)? Although chromium also occurs naturally in the environment, it is primarily in the bonded form of the mineral chromite (chromic iron ore)[*3]. Only through degradation and the use in a variety of industrial processes it is that chromium is released into the environment in its pure form. Whether this represents an immediate threat, however, furthermore depends on the valence of the chromium. While trivalent chromium Cr(III) is currently largely considered harmless, it is now scientifically proven that Cr(VI) is a massive health hazard.
It was therefore only a logical step that Cr(VI) was classified as carcinogenic and mutagenic under CLP Regulation (EC) 1272/2008 (CLP - Classification, Labelling and Packing) in 2009 and finally included in Annex XIV of the REACh Regulation (EC) 1907/2006.
REACh stands for Registration, Evaluation, Authorization of Chemicals and the aim of the REACh regulation is to protect human health and the environment from so-called “Substances of Very High Concern” (SVHC) [*4]. For this purpose, the EU authority ECHA (European Chemicals Agency) set up a central register with data on chemical substances marketed in the European Union. Based on this register, the REACh Regulation provides two ways to protect humans and the environment from the use of SVHC. Firstly,
the so-called “Restriction”, in which the use for certain applications is restricted or completely prohibited (see Annex XVII of the REACh regulation. For Cr(VI) the usage for example is restricted for leather products and cement products). The second way is the so-called “Authorisation”, meaning that specific uses of certain substances require an authorisation prior to use (for a list of these substances see Annex XIV of the REACh regulation; Cr(VI) is listed there as EC No. 215-607-8 or CAS No. 1333-82-0 [*5]).
To achieve such an authorisation there is a complex application and evaluation process. For each application the applicant must demonstrate that the substance is handled safely and that no alternative substance is available, or a socio-economic study must be submitted showing that the benefit of the procedure outweighs the effort involved.
Numerous authorisations for a variety of applications have been applied for and granted in regards of Cr(VI), allowing the usage of Cr(VI) also beyond the sunset date, for Cr(VI) the 21 September 2017. The reason is, that Cr(VI) is, from an industrial usage perspective, clearly one of the most important substances ever taken into the Annex XIV and the industry was not ready to completely substitute Cr(VI) with anything else. But it is nevertheless very uncertain whether the authorisations, which are always granted for a limited period of time only, will continue to be valid for all applications concerned also in the future.
2. Are there alternatives to Cr(VI) in zinc plating and chromating applications?
Knowing the risk of further restrictions in the future, many electroplating companies started to search for alternatives to the heavy metal for different applications, since the inclusion of Cr(VI) in Annex XIV of the REACh Regulation.
Besides other applications in the fastener industry Cr(VI) was and is still mainly used at the end of the galvanising process in the so-called chromating step. This is a procedure in which complex chromic acid salts (chromates) are formed on metallic surfaces by the action of chromic acid. During this process the base material is dissolved and the dissolved metal ions are incorporated into the chromate layer. As this chromate layer belongs to the passivation layers, the process step is often referred to as passivation. The result is an inorganic non-metallic protective layer, which is known to provide very good corrosion protection. But also the iridescent yellow colour, which can often change to reddish or greenish, is typical for this Cr(VI)-containing layer. Because of the varying colour, actually the Taiwanese name of Cr(VI) surfaces is indeed more accurate: “五彩”, which means “5-colour plating”.
And as this yellow colour is often still specifically requested by customers, the goal of the search for Cr(VI) alternatives is not only to achieve a similar corrosion resistance, but also to get optically similar results.
So for the described usage Cr(III) is used more and more as an alternative to Cr(VI). But although Cr(III) passivations can be produced in different shades, they are usually bluish or transparent. Therefore, in order to find a way to fully replace Cr(VI) with Cr(III), while achieving a similar corrosion protection and comparable colours, different methods were developed:
◼ Cr(III) passivations with addition of dyestuffs
◼ Reinforcement of the comparatively weaker Cr(III) coatings by subsequent sealing
◼ Improved galvanising process from pure zinc to a zinc alloy layer (ZnFe, ZnNi or ZnCo).
◼ Conversion from chromating to thick film passivation [*6]
At the moment, the Cr(III) passivations with addition of appropriate dyes to also achieve the desired ‘yellow optic’, is the most commonly used Cr(VI) alternative in the industry, as it has a relatively low economic impact on the full plating process, compared to other methods.
3. Now is Cr(III) really the perfect substitute for Cr(VI)?
As a matter of fact, the yellow colour of Cr(III) layers hardly ever entirely matches the well-known yellow colour of Cr(VI) chromated screws. Cr(III) layers mostly have a more orange or champagne tone. But even more important is that another weakness of Cr(III) was quickly recognised in the market. The colouring dyes used with this method are not always UV-resistant. Interested in that specific fact, AVVIO started to test a variety of screws pulled from the market that said to be Cr(VI) free and claiming to be yellow zinc plated. Starting with our tests in February/March, we couldn’t see a quick impact of the sun bleaching the colour off the screws. But when we repeated the tests in June/July, the colour of the tested screws rapidly faded. In fact after some days the Cr(III) coated screws could hardly be distinguished from similar originally blue/white galvanised screws (see Picture 1 on page 61).
The reason for the seasonal differences lies in the solar irradiance [*7], the power of the electromagnetic energy emitted by the sun. While the solar energy outside the atmosphere is constant, on Earth the angle of incidence determines the solar energy. As the angle becomes flatter, the energy is distributed over a larger earth surface and the irradiance decreases. Therefore, while the influence of the seasons in the tropics is hardly noticeable, outside the tropics there is an increasing difference between summer and winter. In Central Europe, the summer sun has a solar irradiance of about 700 W/m2 under ideal weather conditions, while it has only about 247 W/m2 in winter [*8].
To simplify things for our test, instead of measuring actual radiation energy using a pyranometer, we took the globally available UV index or UVI [*9] as a reference. Regarding our test results and the UVI table above (Picture 2), winter and early spring sunlight (UVI 1 or 2) had no strong effect on the colour of the screws in our latitudes, but in summer (UVI 6 to 8) the strength of the sun did the job in just a few days. Therefore, if you want to carry out the test yourself, we recommend to check the UVI at the test location and the time of the test!
However, even if Cr(III) is not the perfect substitute yet, it is a good first step in the right direction. And, as mentioned above, extensive research efforts are being carried out to avoid Cr(VI) in various industrial processes. Interesting alternatives such as so-called thick-layer passivation or duplex procedures are in the starting blocks to meet customer requirements with regard to corrosion protection, colouring and last but not least the EU Regulations.
Therefore, it remains exciting to see what’s coming up next and where the plating industry is heading.
About the author > Werner Braun
Werner is a true fastener aficionado and he runs AVVIO, an Austrian based technical laboratory specialised on services around fasteners. AVVIO provides a wide range of testing and consulting services in regards of ETA and CE marking and developed its own quality seal AV24. Readers are welcome to get in contact and send their feedback to firstname.lastname@example.org or visit www.avvio.at or www.av24.at
[*1] The film was directed by aerial photographer Chi Po-lin and produced by Hou Hsiao-hsien and can be found here: www.dailymotion.com/video/x2b45o9
[*4] www.reachhelpdesk.at/ and www.umweltbundesamt.at/umweltsituation/chemikalien/reach/wasistreach/
[*9] de.wikipedia.org/wiki/UV-Index, though developed as an international standard measurement of the strength of sunburn-producing ultraviolet (UV) radiation, the index should also be a good indicator of the effect of the sun on colors, since both things are related to UVA and UVB, the two spectra of UV light.
Picture > Scene from Beyond Beauty > Taiwan from Above, 2013 documentary film