Aluminum is an ideal material for a wide variety of industries. However, because it tends to corrode, efficient passivation is crucial to ensure consistently high quality of the products produced. Due to the EU-wide restrictions on the use of chromium (VI) compounds, chromium (III) offers the best alternative among several options.

In 2013, chromium (VI) has been included in the list of substances subject to authorization of the European Chemicals Regulation REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) due to its hazardous substance properties and its broad scope of application. The background here is the classification in category 1 (carcinogenic) and in category 2 (mutagenic) according to the CLP regulation. Since a so-called “sunset date” in September 2017, there is (still) no general ban on hexavalent chromium compounds, but further use of chromium (VI) -containing substances is only possible after a previous application for approval.

With its low density, good electrical conductivity and the ability to process it in many ways and to alloy it with numerous elements, aluminum can be recommended as an extremely advantageous material. But since it is also prone to corrosion, aluminum must be sustainably protected. Passivation produces thin conversion layers (non-metallic, mostly inorganic adherent coatings) traditionally with chromium (VI) -containing solutions in the form of transparent, yellow or green chromate coating.

The resulting layer on the one hand ensures good resistance to corrosive media. On the other hand, it consists essentially of (aluminum) mixed oxides which are stable and inert. The clear limitation of chromium (VI) challenges surface engineering to look for alternatives for the passivation of aluminum parts.

In demand: An effective alternative for the best corrosion protection.

For coaters, it is therefore important to find an efficient solution for their customers, which ensures both a thorough pre-treatment and optimum corrosion protection. In fact, are suppliers providing a number of options for it, such as completely chromium-free systems based on complex zirconium or titanium filaments or titanium-silane-based solutions.

Last but not least, there are chromium (III) -containing products for a rinse or no-rinse application, because – that is the good news – the restriction of chromium (VI) does not make it necessary to completely dispense with the usage of chromium. Although REACH regulates the use of chromium (VI) compounds to a great extent, chromium (VI) -free does not necessarily mean chromium-free. For example are appropriate chromium (III) compounds, appropriate process solutions and surfaces treated with chromium (III) by no means prohibited.

Indispensable: powerful systems and consideration of customer requirements

In order to find the right alternative to chromium (VI), some aspects of the electroplating service provider have to be put to the test in advance. For example, the existing pre-treatment plant plays an important role. Depending on how it is constructed, it is already predetermined whether it is worked with a dipping or spraying process or a rinse or no-rinse process is performed. Decisive factors are the wishes of the customers and end customers of the galvanizer in regards to technical performance – keyword: corrosion protection.

The adhesive strength in the subsequent coating is an essential factor. In addition, the application-related performance in regards to the stability of the process, the process reliability and the quality control measures are essential. 

Tested: Optimal results in the comparison test

Chromium, in its most consistent oxidation state and with a non-toxic oxide, offers all the positive properties of conventionally used chromium (VI), while providing excellent corrosion protection. It is also not subject to labeling. Moreover, chromium (III) achieves very good results in lacquer adhesion in comparison to chromium-free compounds, and there are no differences in the filiform test (FFC) either.

Results of an acetic acid salt spray test (AASS test) proved that the Qualicoat specifications for the use of chromium (III) are observed even after 2016 hours.

In concrete use, chromium (III) is therefore ideal for the long-lasting coating of contact parts as well as parts that are subsequently painted. In this context, in addition to their property as an ideal adhesion promoter, the method also benefits from their good conductivity and low contact resistance. There is therefore no reason for coaters to dispense with the use of chromium (III).

Used: Chromium (III) passivation with C.Jentner

With this in mind, the plating specialist C.Jentner has installed a new process for the chrome (III) -based passivation of aluminum parts at its headquarter in Pforzheim.

Used: Chromium (III) passivation with C.Jentner

Against this background, the plating specialist C.Jentner has installed a new process for the chrome (III) -based passivation of aluminum parts at its headquarters in Pforzheim. The own plant, which is expected to be available by the middle of 2018, holds 2,500 liters. Through the use of chromium (III) -containing bath approaches, different aluminum workpieces can be reliably provided with effective corrosion protection and a high adhesive strength for subsequent layers. The internal laboratory of C.Jentner supervises the complete passivation process; Afterwards, in each case a thorough corrosion test is carried out.

In this way customers of C.Jentner from the aerospace, electronics and automotive industries as well as in the manufacture of housings receive optimally coated workpieces.

Author: Sabine Sturm, ars publicandi GmbH, Rodalben