Corrosion and rust

Subject:

  • Corrosion and rust
  • Fighting Rust
  • Prevent rust
  • Pitting corrosion
  • Contact corrosion

Corrosion and rust:
The concepts of corrosion and rust are closely related. Rust is the result of corrosion. However, these two terms are often used interchangeably.

  • Corrosion is an umbrella term for the deterioration of materials through (chemical) reaction with their environment;
  • Rust occurs on the surface of iron or steel as a result of contact with water and oxygen. Rust refers to the oxidation of iron, which is also a form of corrosion.

As soon as metals are exposed to the normal atmosphere (the outside air), this metal will form a chemical bond with oxygen. In this case, the metal will rust. Rust can be recognized by the reddish-brown layer that appears on the metal and is formed when iron reacts with oxygen, in the presence of moisture from the outside air or water. It is a mixture of iron oxides and iron hydroxide. Rust continues to develop and can completely eat away metal. Over time, all the iron is converted to rust and thus loses its beneficial properties.

In case iron is exposed to oxygen and water, oxygen and iron will bond at the atomic level over time. This creates a new compound: iron oxide. Water speeds up the entire process because water acts as a catalyst. The small water molecules penetrate the metal and form even more acids, exposing the metal even more to the oxide formation. Rusting is even faster in seawater due to the higher concentration of sodium chloride ions.

Rust spots can form under the car paint. This is because the paint is also more or less porous and can therefore let moisture and oxygen through. To keep the paintwork sealed, it is good to wax the car regularly. Especially in winter, with a lot of humid conditions and brine, the latter is a good way to prevent rust as much as possible.
If there are deep scratches or stone chips, the protective lacquer layer will be gone in this place and it can also rust around it. It is therefore wise to touch up these spots as soon as possible with a touch-up pen, or to have them sprayed again.

In the picture on the left we see a VW Beetle with rusty body panels. This vehicle comes under the heading “Rat rod”. Vehicles that have been converted as Rad rot consist of intentionally worn and unpainted parts (in this case rust) and discarded parts.

Example of a so-called Rat rod

In very few cases one can appreciate rust. However, sooner or later every vehicle has to deal with it. In addition to the humid climate in the Netherlands, the brine on the road surface in winter has an increased risk of rust. In addition to the cosmetic parts, parts of the chassis or body can also be affected. In the longer term, this can lead to an unsafe vehicle and to rejection during the MOT.

Car manufacturers try to make the materials in and around the car as rust-resistant as possible. Here are some of the anti-rust treatments:

  • galvanizing the body;
  • apply wax treatments;
  • apply primer and base coats;
  • apply high-quality coatings.
Rust side shield
Rust sill
Rust undercarriage / wheel suspension

Fighting rust:
Rust occurs because iron has bonded with oxygen. It is incorrect to think that rust can be combated by preventing oxygen from reaching the iron. Steel is not a homogeneous substance, but a mix of materials that are brought together during recycling, for example old car wrecks. The steel also contains other metals. There is a small voltage difference between the crystals of two different materials, in which a conductive liquid can ensure contact between the crystals. The resulting electrical current ensures that the least precious metal goes into solution. For example, if a current flows between iron and copper, the iron will dissolve. The dissolved iron particles combine with oxygen. With the smallest amount of water in the body shell, even if it is just one water molecule, the process is maintained.

If you suffer from rust under the paint, it makes no sense to spray over rust. The rust continues to eat under the paint layer. Because the volume of rust is greater than the steel, the paint layer will eventually crack. This creates an opening where extra water can penetrate and the rusting process accelerates. To combat rust, steel is derusted by blasting the surface. A rust-resistant primer layer is then applied as soon as possible. You should not wait too long with this, because otherwise the moisture from the outside air will bind to the iron particles.

Vehicle halfway through the blasting process

Rust forms under the coating. Incipient rust is therefore not always easy to recognize. The four images below show a box girder of a VW Golf IV where the rusty metal has been ground away. Before it was started, there was only talk of bitumen peeling on the edge under the sill. The box girders showed no irregularities. The brown edges became visible when the bitumen was removed. The metal around it was already corroded to such an extent that it broke off easily. The photos show how rust can behave under bitumen.

Preventing rust:
In the first paragraph, the list of measures to prevent rusting referred to “galvanizing”. Manufacturers apply a thin layer of zinc during the production of the car. This offers the so-called cathodic protection. Because electrons are more likely to escape from zinc, the zinc dissolves earlier than the iron. Zinc bonds more easily with the oxygen in the air. The zinc is quite hard and forms a sealing layer. In case of deep scratches through the lacquer and zinc layer to the steel, the steel will hardly react. With enough zinc nearby, the electrical current will go in the right direction. For this reason, zinc offers long-term protection against corrosion and rust.

Galvanized steel does not only have advantages. Due to its increase, the amount of the pure steel decreases. Producers need zinc-free steel to make engine parts, among other things. Zinc has the property of forming bubbles in the steel, resulting in weak spots.

Pitting corrosion:
Pitting corrosion is a local attack on the material surface. Pit corrosion occurs with materials that protect themselves against corrosion by means of an oxide layer and mainly occurs where this oxide layer is damaged. Pit corrosion eats into the material quickly and deeply. This is the most dangerous, because the material apparently still looks good, and it is not possible to see the depth of the corrosion.

The image below shows the cross-section of a steel article with pitting corrosion. Only a small amount of rust can be seen on the top. The “dimple” in the rust betrays the opening that has arisen in the steel.

We can encounter pitting corrosion in brake lines, for example. The moment the opening becomes deeper, there is a chance that the brake fluid can leak out.

Pitting corrosion

Contact corrosion:
This occurs when two different metals come into close contact with each other. A voltage difference is generated between these two metals, causing a reaction. We see this when, for example, a stainless steel screw is screwed to a galvanized steel plate. The more precious metal encourages corrosion and therefore starts to rust. This is called contact corrosion.

The following figure shows two examples of a magnesium component, for example a gearbox, which is attached with a bolt made of an iron alloy. When the contact surface is exposed to water, an electric current is created between the two metals and this leads to contact corrosion. At the same time, the magnesium is broken down. Contact corrosion can be prevented by inhibiting the electric current between the two metals by coating the bolt with a non-conductive, ie insulating layer.

Iron bolt without and with coating in contact with magnesium component

The table below shows the different combinations of materials (for example, engine parts or the gearbox housing) and how the choice of bolt or screw materials influences the contact corrosion.

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