Everybody knows it, and to some extent interacted with it. Global economy would be very different if there was no corrosion – just imagine heavy industry or transportation in the world without rust. Our cars wouldn‘t have to be rust-protected, many daily use appliances would serve for years without breaking, many disasters wouldn‘t happen, including Erika oil spill by the shores of Brittany in 1999. Mill industry would be much less important and electrical and electronic equipment would contain much more copper.
According to NACE International, the global cost of corrosion is estimated to be 2.5 trillion USD annually (yes, that trillion with twelve zeros), which makes around 3.4% of the global GPD. Sure, the amounts are breathtaking, yet the laws of thermodynamics make this „illness of material“ unstoppable and all we can do is to only slow its progress and minimize its impact. The use of corrosion control practices can save 15 – 35 % of the cost of corrosion, which amounts to 375 – 875 billion USD globally each year. It is important to realize, that these astonishing, hair-raising sums do not include the cost of environmental impact – for instance, the above-mentioned tanker Erika disaster caused the release of tens of thousands tons of heavy fuel oil to the waters of the Atlantic Ocean, contaminating over 28 000 km squared and killing marine life. The costs are impossible to estimate!
So, what is this invisible monster, which is not scared of even the hardest materials? From the chemical point of view, it is a natural, electrochemical process between the surface and the environment, converting a refined metal into a more stable form of oxides, hydroxides or sulfides.
To take up the most effective fight with this ever-eating beast, one must understand it. That‘s why corrosion has been divided into the following categories and groups:
Atmospheric Corrosion is the most studied and described form of corrosion and has been the subject of many publications, started by E. Wilson who studied and described the observable changes on the electrical cables in London in the early days of the 20th century. It could be described as the attack of an alloy by the atmospheric environment. Degradation of materials is caused by air and the pollutants it contains as well as electrolyte in a form of rain, dew or, in general – humidity. As water is the universal solvent, the air containing chlorides and sulfur dioxide dissolves in it, forming said electrolyte. Chlorides usually come from sea water, but also from salt brine sprayed on the roads for deicing. Sulfur dioxide is the product of burning fossil fuels – coal and oil (once again, environmental pollution has made its mark on us).
Atmospheric corrosion is the most common cause of material degradation, which reflects on the cost of corrosion protection, which is about 50% of the total cost of all other corrosion measures. It shouldn‘t be surprising, as this is the most destructive form of corrosion, impacting from small elements up to large mega structures, such as skyscrapers and bridges.
Titanium undergoes passivation – it spontaneously creates a thin layer of dioxide on its surface, which provides an effective protection against corrosion.
Galvanic Corrosion occurs locally, in the contact zone of two metals and its intensity drops as the distance from the touch-point increases.
This type of corrosion is caused by the process occurring due to the differences in electrochemical potentials between dissimilar metals coupled in an electrolyte. The potential difference, or in other words – the voltage is the driving force, as current flows through the electrolyte to the more noble metal (cathode) and the less noble (anode) metal will corrode. The conductivity of electrolyte also plays an important role in this process.
Pitting Corrosion is another form of a localized corrosion, confined to a small point, over time growing into cavities and perforating the surface. It could be usually found on metals with thin passive layer, which, due to chemical or mechanical impacts, gets damaged. Pitting corrosion is a very dangerous form of corrosion. It can be easily omitted, as the pits are very small and in some cases may be discovered only through metallography. Even a small cavity with a small loss of metal can cause the collapse of an entire structure. The most known disaster caused by pitting corrosion was a series of explosions in 1992 in Guadalajara, second biggest city in Mexico.
Pitting corrosion of titanium occurs rarely at temperatures below 100°C. However, if emerged in concentrated Cl solutions, it may appear quite fast, even in the range of 100-200°C.
Crevice corrosion appears on a metal surface at the gap (crevice) joining two materials.
Intergranular Corrosion appears between the grains (or crystals) in an alloy, drastically decreasing materials durability and toughness. One way to avoid it is to add titanium or niobium as an alloying element.
Exfoliation Corrosion is a little sister of intergranular corrosion, appearing at grain boundaries, parallel to the surface, forming corrosion products which force plates of metal off the surface, making puff pastry-like appearance.
Stress Corrosion Cracking requires constant tensile stress imposed on the material and a corrodent. This is due to the fact that the surface of the metal exposed to a constant stress is not electrically uniform.
Filiform Corrosion appears under a thin coating creating random, thread-like filaments. It can be visually recognized.
Last, but not least, there is also mighty Corrosion of Conformity, but that’s a whole ‘nother story…
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