Preventing corrosion is something that engineers around the world spend many hours dedicated to. According to NACE International today, every year, about US$2.5 trillion is lost to corrosion globally – that’s more than 3 percent of the global GDP.
While it does obviously have significant financial implications, concerns over metal corrosion also pose safety and health concerns as well. Which brings us back to why it is so important to try and prevent corrosion as much as possible.
Metal corrosion is a natural phenomenon that requires three conditions: moisture, a metallic surface, and an oxidizing agent called an electron acceptor. Corrosion converts the reactive metal surface into a different form which is either oxide, hydroxide, or sulphide. A commonly known form of corrosion is rust.
Corroded metal not only affects the metallic structure, but it can also affect the people using the item or things that are within the vicinity of the metal as well. In worst case scenarios, corroded metal can result in buildings and bridges collapsing, pipes leaking, and medical implants poisoning people’s blood.
While all metals are susceptible to corrosion, some metals, like pure iron, corrode much more quickly than others. However, iron can be combined with other alloys to make stainless steel which has the ability withstand corrosion much more.
It is thought that an approximate 25-30% of corrosion could be prevented using suitable protection methods.
In general, you can prevent corrosion by selecting the right Metal Type, Protective Coatings, Environmental Measures, Sacrificial Coatings, Corrosion Inhibitors, Metal Plating and Design Modification for your project.
Choose the right Metal Type
One of the simplest ways to prevent corrosion is to use a corrosion resistant metal such as stainless steel, duplex, super duplex, nickel alloy or 6% Moly.
These metals are so well made that they have a higher ability to resist corrosion and using them reduced the need to implement addition corrosion protection measures.
At Special Piping Materials, we supply products made from some of the most high-performing materials available – stainless steel, duplex, super duplex, 6% Moly and Nickel Alloy. Different materials are selected by our clients for different environments, with one of the main factors being considered is the probability of corrosion.
Another way to prevent corrosion is to apply a coating of special protective paint. Paint coatings can act as a barrier that works by preventing the electrochemical charge transferring to the corrosive solution and the metal underneath.
Another way of doing this is to apply a powder coating to the clean metal surface. The metal is heated to fuse the powder into a smooth unbroken film that acts as a corrosion resistant barrier. Many different powder compositions can be used, such as acrylic, polyester, epoxy, nylon, and urethane.
Corrosion is certainly something that is caused by the environment that the metal is in as the chemical reaction that takes place is due to the metal reacting to the liquids and gases in the surrounding area.
Controlling the environment therefore can help to minimise these reactions. This can be as simple as reducing the exposure to rain or seawater or could be steps taken to reduce the amounts of sulphur, chlorine, or oxygen in the area. As an example, treating water in water boilers to adjust hardness, alkalinity, or oxygen content, before exposing the metal to that water would go a long way in helping to prevent corrosion.
Sacrificial coating to prevent corrosion means coating the metal with an additional metal type that is likely to oxidise – you sacrifice this top layer in order to protect the metal underneath.
There are two main techniques for achieving sacrificial coating:
- Cathodic Protection: Cathodic protection works by making the steel the cathode of an electrochemical cell. The most common example of cathodic protection is the coating of iron alloy steel with zinc – this process is known as galvanizing. Zinc is more of an active metal and therefore when it corrodes it inhibits the corrosion of the steel. Cathodic protection is regularly used for steel pipelines carrying water or fuel, water heater tanks, ship hulls, and offshore oil platforms.
- Anodic Protection: Anodic protection is the opposite of cathodic protection and works by making the steel the anode of an electrochemical cell. A common way to do this is to coat the iron alloy steel with a less active metal, such as tin. Tin will not corrode, so the steel will be protected as long as the tin coating is in place. Anodic protection is often used on carbon steel storage tanks that are used to store sulfuric acid and 50% caustic soda.
Corrosion inhibitors are chemicals that are chosen to react with the surface of the metal or the surrounding gases and therefore suppress the electrochemical reactions that can lead to corrosion. When they are applied to the surface of a metal, they form a protective film. Inhibitors can be applied as a solution or as a protective coating using dispersion techniques.
Corrosion inhibitors are commonly applied via a process known as passivation. An example of passivation is the Statue of Liberty where the signature blue-green hue of the metal actually is there to protect the copper underneath.
Plating is very similar to coating as a thin layer of metal is applied to the metal you actually want to protect. As well as preventing corrosion, the metal layer provides good aesthetic finishes.
There are four types of metallic plating:
- Electroplating: Where a thin layer of metal such as chromium or nickel is put on the substrate metal via an electrolyte bath.
- Mechanical Plating: this involves cold welding metal powder to the substrate metal.
- Electroless: A coating metal like nickel or cobalt is placed on a substrate metal using a non-electric chemical reaction.
- Hot Dipping: The simplest coating technique which involves immersing the substrate in a molten bath of the protective metal.
Modifying the design of a project can have a significant impact on preventing corrosion as it works by removing the possible causes of corrosion. Not only can it inhibit corrosion, but it can also significantly improve the durability of any protective anti-corrosive coatings that have been applied to the products being used.
Generally, designs using metals that can be corroded should be optimised to ensure that dust and water can’t be trapped, the movement of air is encouraged, and open crevices are avoided.
Designing a structure to ensure that the metal is easily accessible for regular maintenance is also good practice and will help to prevent corrosion and increase the life of the metals being used.
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