Two research projects carried out by scientists from the Leibniz Institute for New Materials (INM), Saarbrücken, Germany, have yielded new anti-corrosion coatings that could have important implications for the longevity of metals used by process plants.
The first is a triple treatment for heat exchangers. Here, the scientists developed a nano-coating with an anti-corrosive, anti-adhesive and anti-microbial effect. The coating is particularly aimed at heat exchangers used for liquid processing that must remain completely free of microbe contamination.
Such heat exchangers are prone to persistent biofilm development in their many grooves and recesses. This requires regular cleaning with aggressive chemicals, which in turn increases their sensitivity to corrosion — especially if they are made of mild steel. The newly developed INM coating reduces the cleaning effort required and, therefore, the potential for corrosion.
Figure 1. The new nano-coatings have an anti-corrosive, anti-adhesive and antimicrobial effect. Source: Ollmann.
Its anti-adhesive properties come from the use of hydrophobic compounds that INM says are similar to common polytetrafluoroethylene. These inhibit the formation of any undesired biofilm and allow transporting out residues more easily before they clog up the channels of the heat exchangers.
Moreover, the coating features special, unspecified, structures as diffusion barriers. These reduce the sensitivity of the heat exchangers to the corrosion caused by aggressive substances such as cleaning agents.
Colloidal copper also is used in the coating to prevent microbes, bacteria or fungus from adhering to surfaces. Oxygen or water present in many processes that use heat exchangers stimulate the release of copper ions from the copper colloids. These migrate to the surface of the coating and, as a result of their antimicrobial effect, prevent microbes from proliferation and growth.
“In addition, we can keep the paint chemically stable. Otherwise, it would not withstand the aggressive chemicals that are required for cleaning,” explains Carsten Becker-Willinger, head of nanomers at INM.
He also points out the paint can be adapted for special mechanical loads, for example, heat exchangers that experience a lot of mechanical vibration, which in turn can lead to abrasion at points of contact. Other uses could include heat exchangers used in industrial air conditioning and equipment used in water purification plants.
The coating can be applied using standard methods such as spraying or immersion and subsequent hardening. It can be used on stainless steel, steel, titanium or aluminum. By selectively adapting individual constituents, the developers are able to respond to the particular, special requirements of different users, says Becker-Willinger.
The second research project at INM focuses on the traditional zinc-phosphate paint used widely as an anti-corrosion coating on steel used for construction. Built up steadily in layers, this paint is designed to prevent oxygen, water, salts and other potentially corrosive substances from coming in contact with the metal.
Here, the INM scientists developed a new type of zinc-phosphate particle that is flake-like in shape, being ten times as long as it is thick.
The first experiments with the new particles found that their anisotropy — directional dependence — gave them a better solubility than the traditional spherical particles.
“Now, more phosphate-ions are soluted, and repassivation of bare metal surface, for example as a consequence of a mechanical damage, is better and faster,” says Becker-Willinger.
“In first test coatings, we were also able to demonstrate that the flake-type particles are deposited in layers on top of each other thus creating a wall-like structure. This means that the penetration of gas molecules through the protective coating is longer because they have to find their way through the ‘cracks in the wall,’” he adds.
The results showed this slowed the corrosion process compared to that with spherical particles where the gas molecules can find their way through the protective coating to the metal surface much more quickly.
In further tests, the scientists validated the effectiveness of the new particles. To do this, they immersed steel plates in electrolyte solutions. They coated one with spheroidal zinc-phosphate particles and the other with the anisotropic version. After just a few hours, the steel plates in the electrolytes with spheroidal particles were showing signs of corrosion. In contrast, the steel plates with the flake-type particles remained in perfect condition, even after three days of immersion.
The new particles are synthesized in a controlled precipitation process developed by INM.
The scientists presented both new coatings in public for the first time at the Hanover Fair that ran April 24–28 in Hanover, Germany.