Why is chromium used to coat iron
Corrosion: everything important at a glance
Last updated on February 28, 2020 by Max Lehmann
Anyone who has ever removed rust is already familiar with iron corrosion. But not only ferrous metals can corrode. Therefore, every do-it-yourselfer should know how to deal with and prevent corrosion.
What is rust and what is corrosion?
The word corrosion comes from the Latin "corrodere", which can be translated as "gnaw away" or "gnaw away". Corrosion is the decomposition of various materials. The best-known example is rust. Rust is iron oxide that is created when iron or steel reacts with oxygen and water. Unfortunately, this oxide layer is very porous in iron and does not protect the underlying layers from further corrosion. In addition to electrochemical corrosion, the weathering of rocks in geology or the decomposition of tissues in medicine is also referred to as corrosion.
The ISO 8044 guideline defines corrosion as follows: “Corrosion is the reaction of a metallic material with its environment, which causes a measurable change in the material and can lead to an impairment of the function of a mechanical component or an entire system. In most cases this reaction is electrochemical in nature, but in some cases it can also be chemical or metal-physical in nature. "
Types of corrosion
- Surface corrosion
- Bimetal / contact corrosion
- Pitting / pitting corrosion
- Crevice corrosion
- Intergranular corrosion
- Special case: glass corrosion
As mentioned above, corrosion does not only occur in metallic materials. For us, however, the electrochemical corrosion of tools and materials is of particular interest. Here, too, there are different types of corrosion that must be treated differently. Everything important about the three most common types of corrosion:
Everyone has seen surface corrosion somewhere. Here the corrosion runs relatively evenly over the entire surface of the metal. The even course makes this type of corrosion relatively harmless, as it only leads to major damage if there is a significant loss of mass. With the right metal, it can even lead to a protective layer that prevents further corrosion. This is the case with chrome, for example.
Bimetal / contact corrosion
This type of electrochemical corrosion occurs when two metallic materials or electron-conducting solids touch each other. It only arises when the materials have different corrosion resistance. A stream of the less corrosion-resistant material is created and it corrodes. Contact corrosion is one of the most common types of corrosion:
- Contact corrosion occurs quickly when screws, nuts or rivets are used with a more noble component, e.g. made of stainless steel
- With different metals in water pipes and heating systems (additional risk from moisture)
- When different metal wires are clamped together (e.g. copper and aluminum)
Pitting / pitting corrosion
Pitting corrosion is the term used for small areas of corrosion or punctiform holes in passivated metals. Actually, passivated metals are assumed to have a high level of corrosion protection, and that is exactly what makes pitting corrosion so tricky. In addition, it spreads into the depths and is therefore often overlooked.
Corrosion Resistant Metals
In general, the more noble the metal, the more corrosion-resistant it is. This is also an important reason that precious metals such as gold, silver and platinum are used in the manufacture of jewelry. They remain permanently stable in natural surroundings if they are only exposed to air and water. Even if the tarnishing of silver is purely chemical corrosion, a protective layer is formed that reliably prevents further corrosion.
Because of their price, precious metals are poorly suited for the production of larger components or tools. Fortunately, there are also cheaper metals that can provide protection against corrosion. They are not immune to corrosion, but they do form a thin, protective oxide layer. This process is called Passivation. The thin layer prevents additional corrosion of the underlying material and is therefore often used during production.
Corrosion protection: how do you avoid corrosion?
There are many options for corrosion protection. Passive corrosion protection prevents the material (e.g. iron-containing tools) from coming into contact with the corrosion medium - in this case moisture. Active corrosion protection is used to protect the material even if it cannot be shielded from the corrosion medium.
The best way to avoid corrosion, of course, is to use corrosion-resistant precious metals. Unfortunately, precious metals are also correspondingly expensive. So it makes more sense to combine a cheap metal with one that offers better protection against corrosion through passivation. A well-known example is chromium, which is used in coatings and alloys. Zinc is also used in the construction industry as a protection against corrosion for steel (hot-dip galvanizing).
In the case of active corrosion protection, sacrificial anodes in particular are often used. This is a piece of base metal that is attached to more noble metals to protect them from corrosion. The sacrificial anode is "sacrificed" and corroded, but the protected, more expensive components last longer. Sacrificial anodes are particularly important in steel hot water storage tanks. They are coated with zinc on the inside, but this dissolves over time. Without the corrosion protection of the sacrificial panels, the boiler would rust through at some point. The only disadvantage: the sacrificial anode must be checked regularly and replaced if necessary.
Which metals form an oxide layer?
Chromium is the best known example of passivation. Even years later, chrome-plated shower heads or taps are still rust-free. After the first contact with air and water, a very thin layer of chromium oxide forms, which protects the lower material. Amazingly, from a purely chemical point of view, chromium is even less noble than iron. Thanks to this great property, it is also used for stainless steel. But with a mass fraction of 12 percent, the stainless steel automatically forms a passivating layer. The alloy has the advantage that the protective function is still present even if damaged.
Zinc quickly forms a protective oxide layer in the air and is therefore often used as corrosion protection in construction. Under certain conditions, zinc, like iron, also forms a corrosion product, white rust. It reduces the protection against corrosion and does not look particularly good either. As a rule, zinc is therefore not found as a pure building material but as a coating for other metals or in the form of zinc alloys, for example as titanium zinc. It is mainly used on exterior window sills, gutters, downpipes and sometimes as a roof covering.
Even with aluminum there is a spontaneous passivation, which creates an oxide layer of approx. 2-3 nm and thus protects very well against surface corrosion. For some applications, however, this thin layer is not sufficient, especially if the aluminum is exposed to strongly acidic or alkaline substances. It is particularly important in the construction industry or for automotive parts to protect the aluminum from corrosion for a long time. By means of an electrolytic treatment (anodizing process), the thickness of the oxide layer can be increased to up to 25 μm. For less demanding applications, the thin oxide layer of the spontaneous passivation is sufficient and can also be supported with a protective coating. You should also ensure that adjacent materials are compatible with the aluminum alloy in order to prevent contact and crevice corrosion.
In the case of lead, the spontaneous passivation can be observed particularly well. Its cut surfaces may have a metallic sheen at first, but a matt oxide layer quickly forms. However, you should not try the whole thing, because lead is extremely harmful to health. That is why you can no longer find it in everyday life, despite its excellent corrosion resistance. In industry, however, it is still used today because it has a high density, is easy to process and also provides excellent protection against corrosion.
Other metals can also be passivated and thus protected against corrosion, but not all of them also form an oxide layer. Well-known examples are nickel, titanium and silicon. In the case of silicon in particular, no modern technology would be possible without sophisticated passivation processes, as semiconductors in cell phones and computers would corrode quickly.
Image source: siam4510 / stock.adobe.com
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