Passive corrosion protection
Passive corrosion protection embraces all measures that create a shield against corrosive agents. This can be achieved by giving the material a single-layer coating or by means of a coating made up of several layers. Structural measures that, like roofing, avoid accumulations of water also count as passive corrosion protection.
A general rule that applies is that the cleaner the surface, the better the coating is able to protect the material, as any foreign matter or impurities can lead to flaws. The surface must also be pre-treated to ensure that the coating adheres to it well.
Metal surfaces are normally cleaned of dirt, grease, oil and other supplies during production using an aqueous alkaline solution. Particular impurities such as rust and scale can be mechanically removed in a subsequent step using appropriate shot blasting.
If the surface of a base metal has been thoroughly cleaned, then it is extremely reactive. It therefore has to be protected against corrosive agents straightaway. There are three overlays and coatings available for this that can be differentiated chemically:
Metallic protective coats
Such protective coats can consist of tin, gold, nickel, copper, chromium, zinc or alloys of these metals. They are applied by chemical or electroplating means.
One widespread method is hot-dip galvanising. A steel component or machine part gets dipped into a hot molten mass of zinc or zinc alloy. The mass is at a temperature of 440 – 460° C depending on the part’s shape. For high-temperature galvanising of small parts even temperatures of >530° C are customary. In this process a thin zinc layer covers the steel part. This protective coat blocks corrosive agents such as oxygen from getting to the iron and additionally offers active cathodic corrosion protection.
Inorganic, non-metallic protective coats
These protective coats passivate the metal surfaces and thus protect the material against corrosion and can be applied, for instance, thermally by enamelling processes. In this case, the base enamel gets annealed onto the steel at 800° C. Another possibility is the depositing onto metallic surfaces of inorganic protective coats consisting of aqueous electrolyte solutions by means of chemical conversion. So-called conversion layers are created in this process, which significantly increase not only the corrosion protection, but also the adhesion for the subsequent coatings. One of the most common conversion layers is tri-cationic phosphating, which is used for pre-treating car bodies. Alongside this, others are in use, such as chromium-free pre-treatments based on titanium and zirconium fluoro-complexes. There is also the use of hybrid pre-treatments, which utilise silane/siloxane coatings from a sol-gel process.
Another area of application of passivated coatings is the post-treatment of zinc plating providing cathodic protection. The zinc layer gets treated here with a chromium(III) passivated coating, which makes the resistance to corrosion much longer-lasting. Dörken MKS consciously forgoes here the use of any questionable raw materials such as nickel or cobalt, despite higher quality in terms of corrosion protection and appearance, and is available under the PROZINC® branding.
It is, however, also possible for passivated coatings to arise spontaneously. One example is the reaction of aluminium with oxygen, in which aluminium oxide is produced. This oxide forms a thick coating, which prevents any further corrosion of the aluminium. Anodic treatments can accelerate and strengthen this mechanism.