Sleepless at the microscope: The exhaustive search for chromate replacements

Photo: Hereon/ Christian Schmid

“We are working closely with our partners at the Complutense University of Madrid, Delft University of Technology, with the project coordinator at the Spanish company CIDAUT, with AkzoNobel, Henkel and MTU Aero Engines,” explains Dr Lamaka, leader of the Hereon team. “Our task in Geesthacht is to find non-toxic coating additives that effectively prevent magnesium and mag-nesium alloys from corroding.”

Photo: Hereon/ Christian Schmid

It is precisely this aspect, that often restricts the use of magnesium in automotive and aerospace engineering. If untreated, the light metal easily reacts in salty and humid atmospheres, i.e., it corrodes. Chromates are the corrosion inhibitors currently widely used as additives for protective coatings.

The small magnesium chips were produced from the six alloys and three pure magnesium varieties. Due to their shape, they have a particularly large surface area. Bahram Vaghefinazari measures hydrogen production due to magnesium dissolution in saline solution with or without the inhibitors. Photo: Hereon/ Christian Schmid

The problem is that chromates are extremely toxic, highly carcinogenic and harmful for aquatic life, flora and fauna. New European Union legislation has therefore banned the use of chromates beginning in January 2019. Only in exceptional cases will an authorized extension be granted.

Chromate replacement would therefore be enormously important in many realms of industry. It is, however, a Herculean task: Sviatlana Lamaka, with Hereon doctoral candidates Bahram Vaghefinazari and Di Mei have tested more than 150 chemical compounds as preparatory work for the ALMAGIC project. The inhibiting effect was tested against six different alloys and three types of pure magnesium.

Samples of coated magnesium plates
are exposed to salty moist air for
several days or weeks. Photo: Hereon/ Christian Schmid

The most promising candidates, after several thousand tests, proved to be compounds binding trivalent and divalent iron (Fe³⁺/Fe²⁺). Iron, always present as a minor impurity in magnesium, tends to initiate an expanding cathodic reaction. Blocking its dissolved species results in magnesium corrosion inhibition. This concept has been previously introduced by the Hereon team at the Department of Corrosion and Surface Technology.

Sviatlana Lamaka and Bahram
Vaghefinazari check the samples. Photo: Hereon/ Christian Schmid

The search for these magnesium corrosion inhibitors took approximately two years. This work resulted in several scientific publications, but also, as Lamaka points out, “It is very satisfying for me that countless sleepless nights in front of an electron microscope contribute to solving the pressing problem of replacing carcinogenic, highly toxic chromates with environmentally friendly materials. The quest for replacement of carcinogenic chromates continues because it is one thing to find an effective inhibitor, but it is much more difficult to ensure that the inhibitor keeps performing once it is incorporated into the protective coating”.