How to achieve the best corrosion protection for steel

Overview:

• Can steel be anodized?
• Anti-corrosion coating of steel by burnishing
• What are the disadvantages of burnishing steel?
• Optimal corrosion protection for steel materials
• Corrosion protection techniques with self-healing
• Corrosion classes according to DIN EN ISO 12944-2

The straight answer is: No, you cannot anodize steel or stainless steel. The term "anodizing" refers to the anodizing process used for the electrolytic oxidation of aluminium. This process can only be used for aluminium.

For corrosion protection or in the case of anodizing, a corrosion-resistant layer is created on the surface of the aluminium by means of an electrochemical process - using the part to be coated as an anode in acidic electrolytes. This is not a coating, rather a continuous transition region. The anodized layer itself is transparent. It can be coloured by printing processes or dye baths without losing its characteristic appearance. Resistance to most solvents, weak acids and alkalis can be achieved through the anodized layer.

For steel, there is a process that has a comparable effect to the anodizing of aluminium described above.This process is called "burnishing". Here, similarly to the anodizing process, a thin, protective oxide layer is formed. However, instead of an acidic electrolyte solution, a hot, alkaline solution is used for this corrosion protection coating.

Burnishing gives the surface of the steel a dark brown to black colour. In addition, burnished layers are very porous. Both the dark colouring and the porous surface are unfortunately undesirable and unsightly side effects of this process and are therefore not an option for an anti-corrosion coating in many areas of application. This is especially the case when the steel elements are used where they can be seen and touched and accordingly have to meet exacting visual and haptic requirements.

Here at Zitt, we have developed an innovative electrochemical process for advanced steel materials: FERRANO. The high effectiveness of the corrosion protection achieved with FERRANO has been proven by electrochemical and technical testing (salt spray test) as well as by means of a Kelvin probe. According to DIN EN ISO 12944-2, the effect extends to corrosivity category C4. This corresponds to heavy strain in outdoor and industrial areas as well as in coastal areas with moderate exposure to salt. Category C4 for indoor areas applies to intense strain, e.g. in chemical plants or swimming pools. A list of the corrosivity categories according to DIN EN ISO 12944-2 can be found at the end of this page.

Detailed test results show that Zitt's FERRANO corrosion protection process is not only highly effective, but also exceptionally fast, inexpensive and sustainable (low energy requirements and resource consumption). Another advantage of this technique is that, in addition to optimum corrosion resistance, the FERRANO process also gives the steel material an anodized-layer-like appearance and thus a very attractive look and feel. Furthermore, it is even possible to give a steel material treated with FERRANO the desired colouring. So far, it is the only and the most effective process to protect high-performance materials, such as steel, from corrosion in a safe, fast, inexpensive and sustainable manner.

FERRANO by Zitt is comparable in its effectiveness to processes for active corrosion protection. This means that even if the FERRANO layer is damaged (e.g. scratches, stone chips, etc.), the corrosion protection is still in place. Even if the metal surface (e.g. steel or stainless steel) comes into contact with the corrosive atmosphere at the location of the damaged corrosion protection layer, no corrosion will occur there. By definition, FERRANO does not create a true self-healing coating. However, the radius in which the FERRANO coating develops its corrosion protection is so large that it can also act on neighbouring damaged areas. The undamaged layer can therefore virtually take over the work for the damaged layer. FERRANO is therefore very similar to processes with active corrosion protection and self-healing or can achieve comparable results.

FERRANO by Zitt is a sustainable, green technology (low energy demand and resource consumption during production). It is based on electrochemical processes, such as those used in anodizing with anodic current. The innovation – the high and reliable efficiency of the corrosion protection in this process – developed by Zitt lies primarily in the special composition and the interaction of chemical components.

FERRANO by Zitt offers protection up to and including corrosivity category C4 (heavy loads) according to DIN EN ISO 12944-2. These and other categories will be described in more detail in the following paragraph

The assignment of steel components to a certain corrosivity category according to DIN EN ISO 12944-2 shows how strongly corrosive loads act on a component and how these corrosion-relevant influences arise. The corrosion class is the basis for deciding which corrosion protection measures are to be taken for a corresponding component.

Source: bauwion.de (as per: 07.12.2021)

• C1 (very low impact): Heated indoor spaces without increased condensation, e.g. offices, schools, shops..
• C2 (low impact): Outdoor: (predominantly rural) environments with low air pollution. Indoor: Unheated indoor spaces with increased condensation, e.g. storage buildings, sports halls.
• C3 (middle impact):  Outdoor area: Urban and industrial atmosphere with moderate air pollution. Indoor: Production rooms with high humidity and low air pollution, e.g. breweries, dairies, laundries.C4 (high impact): Outdoor: Industrial areas and coastal areas with moderate exposure to salt. Indoor areas: Chemical plants, swimming pools
• C5-I (very high impactdue to industrial conditions): Outdoor: Industrial areas with high humidity and aggressive atmosphere. Indoor: Buildings with almost permanent condensation and high levels of air pollution.
• C5-M (very high impact due to marine influences): Outdoor area: coasts and off-shore with high salt load in the air. Indoor: Buildings with almost permanent condensation and high air pollution.

• Im1 Structural elements in freshwater, e.g. structures in rivers and lakes.
• Im2 Structural elements in sea or brackish water, e.g. lock gates, structures in harbour areas and off-shore.
• Im3 Structural elements in the ground, e.g. sheet piling, tanks and pipes made of steel.