Corrosion – a short introduction

From nature’s perspective, almost everything will eventually break down. It is often said that everything will disappear completely after about 10,000 years. This is not entirely correct. In sedimentary rocks that have hardened, researchers have found bone remains that are more than a million years old.

For tens of thousands of years, humans have tried to preserve paintings, tools, weapons and other objects. In most cases this has only been partly successful.

We know, for example, that lanolin from sheep was used by Vikings to preserve weapons and sails. Lanolin is also mentioned by the historian Plutarch, who visited Egypt around the year 50 AD. He explained that the substance was used by Egyptian women as a skin treatment. Today lanolin is also used with good effect as an anti-corrosion treatment for vehicles.

It is commonly said that steel rusts when it comes into contact with salt. This is only partly true. Road authorities typically spread between 90,000 and 120,000 tonnes of salt each year, yet cars still continue to rust.

The main problem is not necessarily the salt itself, but the chlorine contained in salt – sodium chloride, the same substance we use on food. If this element is removed, the corrosion problem is greatly reduced.

Chlorine has some remarkable properties. Many years ago, motor-oil manufacturers used chlorine compounds in their oils. Tests showed that chlorine could improve the lubricating properties of the oil to nearly ideal levels. The sliding properties were excellent and the oil spread efficiently through hot and rotating engine components.

However, this practice ended when a number of engines began to rust while in operation. Today no oil manufacturers use chlorine in engine oils.

The same issue applies to seawater, which contains large amounts of sodium chloride. In earlier times one of the worst things that could be done was to use waste oil as rust protection on steel parts of vehicles.

Many car manufacturers also use steel of somewhat lower quality because it is cheaper to purchase. Such materials can be more vulnerable to corrosion.

Even when a car is treated with anti-corrosion products, rust can still appear. One reason is a phenomenon known as stress corrosion.

This type of corrosion often appears as small spots in the paint, for example on the bonnet or other exposed surfaces. Even if the affected area is sanded down and repainted, the rust may return after a relatively short time.

In simple terms, corrosion is a major loss of metal ions that occurs through chemical and electrochemical reactions. Iron will naturally attempt to return to the ore from which it originally came.

Not many decades ago corrosion was still a relatively poorly understood phenomenon. I personally had responsibility for parts of the corrosion protection work on the first H3 offshore drilling platform. In that project both construction and surface treatment were designed significantly stronger than the original calculations suggested.

In structures that experience repeated stress cycles, such as aircraft and helicopters, it took many years before engineers understood the cause of several unexplained accidents. The reason was dislocations inside the material.

Dislocations are microscopic disturbances in the steel structure that can bring the internal stresses close to the breaking point. In the 1950s five British passenger aircraft crashed within a short period before this mechanism was properly understood.

When steel used in structural applications becomes so hard that it moves into the plastic region, fracture can occur very quickly. Today this knowledge is widely used in the automotive industry, where deformation zones are designed to absorb energy during collisions.

Another historical example is the Titanic. One of the contributing reasons the ship sank was that the steel plates contained too much sulfur, which makes steel brittle.

Forms of corrosion

Corrosion can appear in several different ways.

Uniform corrosion occurs evenly across an entire surface.

Pitting corrosion appears as small localised holes or points in the material.

Stress corrosion occurs when internal stresses already exist in the material. These stresses can extend deep into a structure, and if ammonia is present it may accelerate the process.

Much of this type of corrosion can be reduced if steel structures are tempered to around 300°C.

The best preventive measures

The most important step is to use products that are proven to slow down corrosion. Today there are many effective solutions available.

If such materials are able to penetrate cavities and internal channels in a vehicle’s chassis and bodywork, it is also important that they prevent water from entering and keep oxygen away from the steel.

Steel normally rusts when both water and oxygen are present. However, some corrosion types – such as stress corrosion and pitting corrosion – cannot always be prevented by surface treatment alone.

In those cases the quality of the steel used in manufacturing becomes crucial.

On this particular issue, corrosion protection alone cannot always solve the problem.

It is nevertheless worth noting that offshore oil companies have approved the use of lanolin as an effective anti-corrosion treatment.