The large quantities of steel used in architecture, bridge construction, and ship building are expected to be long-lasting and retain strength and safety qualities. For this reason, steel plates and girders used must have extensive and durable protection against corrosion caused by oxygen in the air, water vapor, and salts. Applying anticorrosion coating layers with zinc-phosphate particles is a common method for preventing corrosive substances from penetrating into the material. Now, research scientists at Leibniz Institute for New Materials (INM) (Saarbrücken, Saarland, Germany) have developed a more corrosion resistant zinc-phosphate particle that is flake-like in shape with a length that is ten times longer than its width.
When compared to spherical particles, initial experiments with these new flake-shaped particles indicated better solubility because of their anisotropic properties—their properties are not the same in all directions, but are directionally dependent (i.e., they have different properties in different directions). This enables more phosphate ions to be dissolved in a solution, and repassivation of the bare metal surface is better and faster, says chemist Carsten Becker-Willinger , who is head of the institute’s program division Nanomers†.
“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,’” Becker-Willinger explains. The result is a slower corrosion process than with coatings with spheroidal particles where the gas molecules can find their way through the protective coating and reach the metal much more quickly.
The scientists were able to validate the effectiveness of the new particles in an additional series of tests. Steel plates treated with coatings containing either spheroidal or flake-type zinc-phosphate particles were immersed in electrolyte solutions. After just a few hours in the electrolytes, the steel plates with spheroidal-particle coatings were showing signs of corrosion, whereas the steel plates with flake-type-particle coatings were still in perfect condition, even after three days.
The flake-shaped zinc-phosphate particles are synthesized in a controlled precipitation process developed at INM. The researchers created their particles using standard, commercially available zinc salts, phosphoric acid (H3PO4), and an organic acid as a complexing agent. The more complexing agent they added, the more anisotropic the nanoparticles became.
†Trade name.