Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment. Marine structures are highly susceptible to corrosion.
The World Corrosion Organization estimates the global cost of corrosion to be about US$ 2.2 trillion annually. The world marine coatings market was forecasted to be US$11.88 billion by 2020. It would also fascinate you that as much as 25% of the problems caused by corrosion can be eliminated by applying simple, well-understood prevention techniques.
We all know from our everyday experiences that Steel –> Rust this is called corrosion. But let’s get a bit nerdy and see the chemical reaction of corrosion.
1)Fe → n electrons + Fe(n+)
The iron atom has lost some electrons and become a positively charged ion.
2) O2 + 2H2O + 4e- → 4OH(-)
Water reacts with Oxygen and free electrons to give negatively charged OH ions.
3) 2Fe + O2 + 2H2O → 2Fe(OH)2
Iron + Water with oxygen → Iron Hydroxide
4) 4Fe(OH)2 + O2 → 2H2O + 2Fe2O3.H2O
Iron hydroxide + oxygen → water + Hydrated iron oxide (brown rust)
This series of steps tells us a lot about the corrosion process.
(1) Ions are involved and need a medium to move in (usually water)
(2) Oxygen is involved and needs to be supplied
(3) The metal has to be willing to give up electrons to start the process
(4) A new material is formed and this may react again or could be protective of the original metal.
(5) A series of simple steps are involved and a driving force is needed to achieve them.
Now that we know how corrosion occurs let us look at some of the common types of corrosion you may encounter.
Types of Corrosion
Some of the most common types of corrosion are described below.
1. General Attack Corrosion:
This very common form, corrosion attacks the entire surface of a metal structure. It is caused by chemical or electrochemical reactions. While general attack corrosion can cause a metal to fail, it is also a known and predictable issue. As a result, it is possible to plan for and manage general attack corrosion.
2. Localized Corrosion:
Localized corrosion attacks only portions of a metal structure. There are three types of localized corrosion:
- Pitting — the creation of small holes in the surface of a metal.
- Crevice corrosion — corrosion that occurs in stagnant locations such as those found under gaskets.
- Filiform corrosion: corrosion that occurs when water gets under a coating such as paint weakness.
3. Galvanic Corrosion: Can occur when two different metals are located together in an electrolytic liquid such as salt water. In essence, one metal’s molecules are drawn toward the other metal, leading to corrosion in only one of the two metals.
4. Environmental Cracking:
When environmental conditions are stressful enough, some metal can begin to crack, fatigue, or become brittle and weakened.
In the coming articles, we will look into various methods used across the industry to prevent corrosion.