The Process of Structural Steel Corrosion
Structural steel corrosion is an electrochemical process that needs the concurrent presence of oxygen and moisture. Basically, what produces rust is the iron present in steel being oxidised as it occupies six times the volume of the material in approximation. And the rate the corrosion process advances depend on determined factors including the micro-climate encompassing the structure.
Steel corrosion process works in stages. Initially, the process occurs on the surface of anodic areas where ferrous ions mix into the solution. Electrons are freed up from the anode moving through the metallic structure to the surface adjacent to cathodic sites, where they combine with water and oxygen to produce hydroxyl ions. These ions will then react with the ferrous ions from the anode to form ferrous hydroxide. When further oxidised in air, what’s produced is hydrated ferric oxide.
After a period, the growth of corrosion products on the surface, which is a polarisation effect, causes the process to be restrained. With this, reactive and new anodic sites may be formed, promoting corrosion. And over long periods, “general corrosion”, which refers to the metal loss that’s fairly uniform over the surface, will occur.
When two different metals are combined and then exposed with an electrolyte, an electric current will pass between them, making corrosion happen on the anodic metal. There are metals, such as stainless steel, which cause low alloy structural steel to corrode more favourably, whereas other metals, including zinc, corrode themselves protecting it. Keep in mind that the tendency of differing metals to bimetallic corrosion depends partly upon their positions in the galvanic series. The further apart the metals are, the greater the tendency of such corrosion to occur.
One more aspect that encourages bimetallic corrosion is the electrolyte’s nature. It is in buried or immersed structures that bimetallic corrosion is most severe, while the effect is minimal when in less aggressive environments such as in stainless steel brick support angles that are attached to mild steel structural sections.
To guide you with the galvanic series, see the list below.
Anodic end (where corrosion is more prominent)
- Carbon and Low Alloy Steels
- Cast Iron
- Copper, Brass, Bronze
- Stainless Steel
In some conditions, the attack on the anodic area continues deep into the metal as it is not stifled. This forms what they call as corrosion pit. More often, this occurs with low alloy structural steels that are either buried in soil or continually wet conditions compared to those that are exposed in air. Hence, pitting corrosion is rarely encountered on modern steel bridges or buildings.
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