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STEEL CORROSION: BASIC FACTS YOU NEED TO KNOW

Part 1: The Different Types of Corrosion

 

Rust. Whether you’re a builder or a building owner, a steel user or a steel supplier, this can be a major problem. Rust not only degrades the appearance of steel, it can also wear down the quality and consequently, durability of structural steel supplies and steel structures.

rust

What most don’t realise is that rust is simply the by-product of corrosion, which is the real cause of the metal’s degradation

 

What is Corrosion?

Corrosion is an electrochemical process wherein the iron in steel is oxidised and produces rust. In order for corrosion to occur, both oxygen and water must be present. The chemical reaction can be summarised as follows:

Fe + 3O2 + 2H2O = 2Fe2O3H2O
(Steel) + (Oxygen) + (Water) = Hydrated ferric oxide (Rust)

There are some cases however, where even without the presence of water, corrosion can occur. This is called dry corrosion and happens when the metal oxidizes only with the atmosphere, and intensifies in higher temperatures.

Wet corrosion is the most common form of corrosion, taking place in environments with a relative humidity of more than 60%. It is most accelerated with water containing salts, since these have high conductivity and chlorides can also increase corrosion rates.

 

Different Types of Corrosion

Contrary to what most people know, there are several types of corrosion which differ according to form of occurrence and combined causes:

  • General Corrosion – Also known as uniform corrosion, this is when the metal corrodes in a uniform manner throughout its surface.
  • Local Corrosion – This is when a part of a steel structure corrodes at a much faster rate than usual. Local corrosion can be further classified as:
    • Pitting corrosion – Corroded areas are concentrated and localised, which leads to pitting. This often occurs with steel that are continually exposed to wet conditions or are buried in soil
    • Crevice corrosion – Corrosion that often occurs in crevices, gaps, recesses and such spaces due to the lack of oxygen and the presence of corrosive substances or other deposits, such as dirt
    • Galvanic corrosion – This type occurs when two metals of different electrode potentials come in electrical contact, causing one metal to corrode in preference to the other
    • Intergranular corrosion – Corrosion that occurs along the boundaries of metal crystallites or grains
    • Selective corrosion – Also known as selective leaching, dealloying and demetalification, this occurs when, in certain conditions, a component of the alloy is leached from it.

     

When combined with mechanical factors, corrosion can also be classified as:

  • Stress corrosion – This occurs when a metal is exposed to a corrosive environment as well as static stress, resulting in a fracture
  • Corrosion fatigue – This results from corrosion and various states of stress
  • Erosion corrosion – This is corrosion accelerated by the presence of high velocity liquid, and/or solids carried in the liquid
  • Cavitation corrosion – This is caused by the combination of corrosion and the pressure from the breaking of gas bubbles formed in liquid
  • Fretting corrosion – This happens when two metals rub against each other in corrosive conditions

 

It’s important to know these different types of corrosion in order to prevent or minimise their occurrence. This is significant not just for steel fabricators, but builders and even owners of steel structures as well.

In our next post, we’ll discuss the different factors that affect the rate of corrosion and what can be done to slow it down, so stay tuned.

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