Skin with high rust protection factor
In industrialized countries, corrosion guzzles up to 4 percent of economic performance annually. Substances that protect metals effectively from its ravages are often damaging to the environment or have other disadvantages. Consequently, scientists working with Martin Stratmann and Michael Rohwerderat the Max-Planck-Institut für Eisenforschung (Iron Research) in Düsseldorf are developing synthetic coatings that can protect steels and other metals from rust and heal themselves if they become damaged.
Text: Peter Hergersberg
Erin Brockovich made the world conversant with a problem that researchers at the Max-Planck-Institut für Eisenforschung now hope to solve once and for all. Brockovich’s story is one of poison, big money and the health of almost 2,000 people; and it is the story of a woman who did a great deal to bring a powerful corporation to its knees. These ingredients provided just the kind of mix that Hollywood likes to work with. The fact that the film Erin Brockovich stars Julia Roberts as the eponymous American legal clerk and environmental activist probably created a greater awareness of the problem: the carcinogenic chromium(VI).
Between 1952 and 1966, the California energy firm Pacific Gas and Electric Company allowed the substance to seep into the groundwater of the town of Hinkley. As a result, following the legal proceedings brought by Erin Brockovich, the corporation was required to pay some 200 million dollars to the inhabitants of the town, and another 133 million dollars to the lawyers.
Chromium(VI) salts are still the standard in corrosion protection, but have since been banned from use in many other applications. The search is now on for an equally effective, but environmentally sound alternative. After all, industrialized countries see 3 to 4 percent of their gross domestic product eaten away by corrosion each year; in Germany alone, this represents more than 75 billion euros. Martin Stratmann and Michael Rohwerder at the Max-Planck-Institut für Eisenforschung in Düsseldorf are studying the corrosion process, which affects primarily metal materials, and searching for ways to prevent it.
Their aim is to identify synthetic coatings that will protect various metals, but especially steel and aluminum, from corrosion, and that will heal themselves when damaged, just like the skin of living beings. Such a coating – normally underneath a colored lacquer – would constitute a bulwark against metal-guzzling rust. In most steels, a zinc coating applied directly to the sheet forms an additional barrier; in the case of aluminum, this takes the form of a dense and highly resistant oxide layer. In the event that the protective layers and even the metal underneath are damaged by scratching, it is intended that the synthetic coating will hold off any corrosion, possibly with the help of the zinc, until the gash in the material is sealed off again.
Benefits in two fields: finish first, then fabricate
Not only would this kind of overlay eliminate the problem of chromium(VI), but it could also boast other advantages. It should enable steel manufacturers, for example, to reduce at least the zinc layer, as even zinc is controversial in some ways. “The zinc coating can actually heal itself to a certain extent, because it also deposits passivating zinc oxide in gashes and holes, preventing further corrosion,” says Michael Rohwerder, head of the Molecular Structures and Surface Design Research Group. “However, it is considered problematic from an environmental perspective, and it is expensive and vaporizes easily during laser welding, contaminating the welding equipment.”
Coatings such as those being developed by the materials scientists in Düsseldorf could also minimize maintenance requirements for aircraft. Any fine scratches on their wings, for example, would be sealed off immediately by a self-healing layer. As things stand today, airlines must regularly comb their aircraft for even the slightest damage, only to repair it at considerable expense.
Finally, the self-healing skin for metals could mark a new chapter in car and machinery manufacture. Currently, car bodies can’t be treated with protective coatings until they have their final shape. It would be cheaper to treat the steel beforehand, but would do little good, as fine cracks form in the protective skin while the material is being formed. Again, a self-healing layer could solve this problem, benefitting car manufacturers and mechanical engineers, as well as steel producers. The manufacturers could cut out an expensive step that has nothing to do with their core business, while steel producers could make their products more valuable and lucrative.
Martin Stratmann and Michael Rohwerder thus have no shortage of reasons for developing self-healing polymer coatings – however, it is a task that poses many challenges. In order to understand where the difficulties lie and what tricks the Düsseldorf-based scientists have used (or plan to use) to overcome them, it is helpful to take a quick look at the corrosion process.
“Corrosion is the reversal of metallurgy,” says Martin Stratmann, Director of the Interface Chemistry and Surface Engineering Department of Düsseldorf’s Max Planck Institute. It can affect all metals except precious metals. Rust is probably the most prominent and economically relevant case, occurring when oxygen oxidizes iron. Iron corrosion destroys the work of the furnace, which transforms iron oxides and other ores into elementary metals.