Corrosion in Metals

When choosing a material for a metal structure in a harsh environment, there are many factors to consider. Some of these include the Durability of the material, the materials specific characteristics, and ultimately, the price. When discussing the durability of types of metal, the overarching question is around corrosion. So, what is corrosion?...


When choosing a material for a metal structure in a harsh environment, there are many factors to consider. Some of these include the Durability of the material, the materials specific characteristics, and ultimately, the price.

When discussing the durability of types of metal, the overarching question is around corrosion. So, what is corrosion? How are different metals affected by this? and what other major factors are there to consider when choosing a material?

Corrosion is a process by which a metal is broken down by exposure to certain elements.

Corrosion rates vary from place to place and during different times at the same location. These are influenced by a large array of factors, including –

  • The reactivity of the metal
  • Temperature
  • Exposure of the metals to air containing gases or harsh chemicals
  • Exposure of metals to moisture, especially salt water (which increases the rate of corrosion)

How are different metals affected by corrosion?

Mild Steel.

Many of us are familiar with the term ‘rust’ which is a form of corrosion specific to iron (mild steel) and is how the metal breaks down as oxygen attacks the surface. Oxygen creates iron oxide, which flakes away from the body of the metal, exposing fresh metal to oxygen, and thus continuing to corrode the metal. Unprotected steel is very prone to rapid and continuous corrosion in almost any environment and would not be a good option for a structure subjected to a harsh environment.

Galvanised Steel.

Galvanised steel has a zinc coating for enhanced corrosion protection when compared with mild steel. Zinc can form protective layers to completely cover the surface, resulting in corrosion that proceeds at a greatly reduced rate. This is generally quite successful, however there are several scenarios in which galvanised steel will continue to corrode. These include –

  • High humidity
  • Salt in water or air. (Marine or coastal environments.)
  • Situations where the coating is frequently wet.
  • Sulphur dioxide pollution in urban atmospheres.
  • Exposure to hydrogen sulphide from volcanoes, hot springs, natural gas, and sewer gas.
  • Outside a pH of 5-12
  • Moss and lichen, which will hold moisture and create a high humidity micro-climate.

Other drawbacks of galvanising include the reliance on the thin zinc layer, the danger with the process and environmental impact associated with it.

Relying on the zinc coating as protection means that if the coating is damaged then it leaves the underlying metal vulnerable to corrosion that will then spread. A deep scratch on a galvanised structure could lead to rapid localised corrosion.

Danger with galvanization also lies in the process itself. Hot-dip galvanising involves molten solutions, which pose risks to worker safety. Exposure to harmful fumes from the process can also be a risk.

Finally, the environmental effects of galvanization can’t be dismissed. Areas near industrial plants can see higher levels of zinc in their water supplies if wastewater isn’t treated properly. This could lead to a rise in acid levels which would affect nearby wildlife.

Therefore, galvanised steel remains an option when it comes to structures in harsh environments, but it does remain vulnerable in various ways.

Stainless Steel.

Stainless steels contain chromium and are much more resistant to corrosion than mild steel or galvanised steel. This significantly increased corrosion resistance is due to the passive film of chromium oxide spontaneously formed on the stainless steel surface. When exposed to harsh environments such as damaging chemicals, saline, grease, moisture, or heat for prolonged periods of time, this protective film could be damaged, leading to localised corrosion.

Stainless steel corrodes in two main forms. Pitting appears in the form of dark brown pits in the steel’s surface. You might also see a form of corrosion that creates a large crevice in the steel’s surface. Neither of these forms of corrosion affects stainless steel’s mechanical properties, but they are unattractive.

Stainless steel’s protection against corrosion is largely dependent on the amount of chromium present. If there is not enough chromium content near the surface of the stainless steel, a new chromium oxide layer cannot be formed when the top layer is scratched off. This also could leave the material vulnerable to corrosion.

Although stainless steel is much more resistant to corrosion than mild or galvanised steel, it does come with its drawbacks, most significantly price. Stainless steel can cost up to five times more than its galvanised counterpart, which is a stumbling block for many. This leads us onto a final material choice, and one that is becoming increasingly popular for many reasons – Aluminium.


Like stainless steel, aluminium when in the presence of oxygen rapidly forms an adherent, continuous and corrosion-resistant oxide layer onto its surface. This results in very good corrosion resistance, generally requiring no additional protection in normal atmospheres, such as –

Atmospheres (rural, industrial, marine) Fresh Waters (natural, treated, reclaimed) Sea Waters (quay side, open water)

Aluminium also offers good corrosion resistance to: Most soils, Most foods, and Many Chemicals.

A study in South Africa on the surface corrosion of aluminium in coastal areas found that whilst it corroded very slightly more than stainless steel, it was five times slower than that of galvanised steel, and one hundred times less than uncoated steel.

In addition to the corrosion resistance of aluminium, it comes with other advantages. To name a few of these, aluminium is –

Lightweight – is only 30% the weight of Steel.

Easy to work with – cutting/drilling can be easily done onsite without the risk of damaging protective layers.

Sustainable – Very recyclable - it is estimated 75% of all aluminium ever produced is still in circulation.

Cost effective – Aluminium is very cost effective when building structures, a smart design in aluminium can often be much cheaper than a stainless or even galvanised structure.

Given the natural corrosion resistance of aluminium and the many and varied advantages that come with its use, it is becoming increasingly popular in many different industries including construction, putting forward a challenge to established norms in the industry.

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