Corrosion is defined as;
“Destroy or damage (metal, stone, or other materials) slowly by chemical action.” – Oxford Dictionary.
Put more simply, Corrosion is the process by which something deteriorates because of oxidation, a chemical action with the air, that creates oxides on the surface of the parent material. Although the word is usually associated with the rusting metal, the erosion of rock by wind and water is also a form of corrosion.
When Does Corrosion Happen?
The main form of corrosion that people talk about (rusting of metals) is a natural process that will occur when metals are exposed to moist oxygen rich environments. As such metals are often painted to prevent exposure of the parent metal to the air. However corrosion may occur even underneath paints or other coatings resulting in it being hidden from view.
A good example of this is Filiform corrosion. This is corrosion of a material when water gets under the surface coating, such as paint. It is very common to see on mine sites cracked and peeling paint. If you peel off one of these flakes you will likely find the material underneath has significant surface corrosion. The reason the paint cracks and peels from the surface is the corrosion process. The oxides (or rust) created in the corrosion process adds another layer of material slightly expanding the material.
Another type of corrosion commonly seen on mine sites is Galvanic corrosion. Galvanic corrosion occurs when 2 different materials are close to each other with an electrolyte (any fluid that allows the transfer of electrons) in between. One of the material’s electrons are attracted to the other material, slowly causing a material loss. Galvanic corrosion though can also be used as a protection method. By putting a ‘sacrificial’ block of material that is more susceptible to corrosion in the vicinity of the material you are trying to protect, the sacrificial material corrodes first.
Although these are two common forms of the corrosion they are not the only two, and corrosion should always be considered both in the design, and the maintenance and structures or plant made of metals.
Why Does Corrosion Matter?
“The global cost of corrosion is estimated to be US$2.5 trillion annually, which is equivalent to 3.4% of the global GDP (2013)” – NACE International
As you can see, corrosion can cost a lot of money. Unlike steel, iron oxide (rust) is not a good material for withstanding load, as such as steel is corroded away and converted to rust, a steel structure or item of plant will be able to support less load or pressure until it eventually fails, and as we all know failures can be extremely costly.
So How Can I Stop it?
Corrosion should be planned for in the design and maintenance of all steel structures. There are multiple things that can be done to prevent corrosion such as applying coatings, galvanic protection, changing the operating environment, cathodic protection and more. Rapallo’s team of engineers are experienced with the design and maintenance of plant and machinery operating in extremely corrosive environments and are prepared to assist if required.
But what if something is already corroded?
Corrosion doesn’t necessarily mean something is unfit for purpose. Surface corrosion often protects the steel underneath from further corrosion, additionally many steel structures or pressure vessels are designed with a corrosion allowance which allows them to be safe to operate and use even after a small amount of corrosion has occurred.
In events where there are high levels of risk however, its often worthwhile getting a integrity assessment of the structure or metal in question. Using a variety of methods, such as Ultrasonic thickness testing, visual inspection, engineering analysis, finite element analysis or otherwise the team at Rapallo is fully equipped to determine exactly what condition your asset is in. Call 6279 9000 to speak to one of our team members about the corrosion and maintenance of your assets.