Slow Decay

The owner of a new Swiss industrial facility for the production of high-tech machinery was faced with a mystery: Kilometres of brand new stainless steel and aluminium pressure and cooling lines, worth several hundred thousand Swiss francs began to corrode while still under construction. What had attacked the metals so quickly? Empa experts took a close look at the entire system: Were corrosive building materials involved, were cleaning agents to blame or had the wrong materials simply been selected? Finally, they found the "culprit" in form of a small bottle on a workshop truck: Instead of using a professional leak tester, the assembly team had used a universal cleaning agent from the supermarket to locate leaks with the foam. But the supermarket agent contained acids and chlorides that corroded the metals.

New scene: The caretaker of a school in eastern Switzerland notices corrosion on the ceiling lamp fixtures in the gymnasium during cleaning work during the 2019 spring holidays. The school administration consulted the architect who had supervised the construction at the time. He informs Empa. The isolation cladding at the ceiling is dismantled. Result: The entire ceiling construction shows massive safety-relevant corrosion damage. A few years earlier, during the renovation, the workers had drilled metal hooks through insulation plates made of phenolic resin foam, being unaware about the material properties. Water condensation caused the insulation to become damp. The phenolic resin foam then developed strong acids, which caused the fixing hooks to rust throughout.

Fatally, hydrogen is not only produced intentionally – it can also be formed during corrosion and penetrate the material. Stored in minute quantities, it has the same destructive effect there:

It makes high-tech alloys brittle and fragile. In order to understand what happens and how it can be prevented, researchers have to zoom in closely on the microstructure of a material and investigate the chemical reactions in tiny areas affected by corrosion. Empa has developed its own microsensors that can analyse surfaces of less than one hundred thousandth of a square millimetre and detect less than one millionth of a percent by weight of hydrogen. Using these methods, they investigate critical zones in components, such as welded seams, which become brittle due to atomic hydrogen and could ultimately fail.

Lars Jeurgens has been leading the Empa laboratory for "Joining Technologies and Corrosion" since 2012 and, together with his team, maintains a well-balanced mix of research and industry-related services. "We have graduates from ETH Zurich and EPFL in our team and use the concentrated knowledge of these two leading engineering schools," says Jeurgens. He himself was born in the Netherlands and worked at the Max Planck Institute in Stuttgart for a long time. "Corrosion knows no boundaries – that's why we are very well connected internationally with experts from academia and industry and exchange the latest findings and methods. It is very valuable for us to share these experiences. Together, many complex problems can be solved more easily and quickly".

In addition, there is truly enough to do for the corrosion specialists. The automotive and aircraft industries, for example, are increasingly working with composite materials consisting of a wide variety of materials. Little is known about their corrosion behaviour in harsh operating environments. In many places, alloys of iron, titanium and aluminium are also used. They owe their corrosion resistance to a tiny, nanometre-thin passive oxide film on their surface, which requires special analytical methods just to detect it – only then the material surface can be tailored for its corrosion resistance.

Finally, the application of functional coatings to miniaturized electronic devices and components raises new corrosion issues. Lars Jeurgens gives the following example: "If I make a turbine with a corrosion-resistant coating and a hundredth of a millimetre of material thickness is lost every year, this is unproblematic. However, the same coating on an electronic device that is only one hundredth of a millimetre thick would be completely degraded within a year. What appears corrosion-resistant on a large scale is far from being so on a micrometre scale. We therefore need new concepts for classifying the corrosion sensitivity of a material for a given application."

Sometimes corrosion occurs even where it is least suspected: In the middle of the human body, amidst its warm, apparently harmless body fluids. Empa's experts are investigating degradation by localized corrosion on materials such as stainless steel and titanium alloys, which are often used for implants as well as silicon, which is present in numerous new implanted components. Little happens on large, smooth surfaces, but human body fluids can do a great deal in microscopically small crevices resulting from design or construction.

Recently, an Empa team was able to demonstrate the slow dissolution of a silicon-bonding layer in the laboratory. Between the titanium implant and the wear resistant coating a microscopic crevice may form. In the absence of oxygen, a very aggressive medium can develop there slowly, which can then destroy the silicon-bonding layer with the help of the body's own phosphorus compounds. Using special probes, Empa researchers are also able to elucidate the local corrosion chemistry in such fine crevices and even accelerate the corrosion processes for experimental purposes. In this way, the expected service life of an implant can be predicted quite accurately even before the operation.