How to prevent metal corrosion
What causes corrosion?
Corrosion is the process where metal degrades as it comes into physical contact with a gas or liquid. This contact causes an electrochemical reaction which results in oxidisation on the metal’s surface and is often visible to the naked eye. Most metals are susceptible to corrosion.
Metals are used extensively in harsh environments such as automotive, marine, construction, mining, nuclear, and oil and gas industries, as well as in almost every aspect of daily life. Corrosion can have a catastrophic effect if the materials in use degrade and become unfit for purpose which means there are important safety, legal, and financial implications of metal corrosion and degradation of materials.
Types of corrosion.
Corrosion Engineering, authored in 1967 by Mars Fontana and Norbert Greene, identified eight forms of corrosion; these included general attack corrosion, localised corrosion, and environmental cracking. The most frequently encountered form of corrosion in metals is general attack corrosion where an electrochemical reaction affects the surface of the metal causing it to break down. Once the surface is broken down, the metal weakens which brings about its ultimate failure.
Preventing corrosion.
Choosing the correct metal
One of the easiest ways to prevent corrosion is to select the most appropriate metal for the job. Aluminium and stainless steel are both corrosion resistant. Aluminium does not corrode because it has protection from the oxide layer which occurs naturally, and stainless steel is resistant to corrosion because of the addition of chromium to the alloy. Chromium protects the material by creating a film which prevents gas or liquid from coming in to contact with the surface. Other alloys, such as aluminium alloy, can be protected by anodising. This is where a thick aluminium oxide layer is created by a process of controlled oxidisation. You can read more about anodising in our article available here.
Cathodic or anodic protection
Protection from corrosion through cathodic or anodic methods are collectively known as ‘sacrificial coatings.’ This is where the base metal is coated with another metal that is more – or less – susceptible to corrosion.
To protect metals cathodically, a base metal such as steel is galvanised with another metal like zinc. Zinc is a metal that corrodes faster than steel so when oxidisation of the zinc takes place, oxidisation of the steel is inhibited; because the zinc oxidises first, the zinc is sacrificed and the steel benefits from its protection. This process of protection is useful for metals used in pipelines and in marine and oil industries.
With anodic protection, a less active metal is used to coat the important base metal. Tin is a less active metal than steel and oxidises at a slower rate than steel. The steel will be protected from oxidisation as long as the tin coating remains in place, as the tin is the first line of defence.
Paint or powder coatings
Paint or powder coatings are the most cost-effective way of preventing corrosion of metals. The paint or powder creates a barrier between the metal and the corrosive gas or liquid. Powder coatings can be applied to aluminium, steel, bronze, copper, brass, or titanium and are available in a range of presentations including acrylic, epoxy, and polyester. The powder coating is applied by spraying it on to clean metal then heating the metal until the powder fuses with it, the resulting finish is a clean, smooth surface that gives protection over a lengthy period.
Corrosion inhibitors
Corrosion inhibitors chemically suppress corrosion and are applied either as a protective surface coating or as a solution that creates a chemical reaction to form a film on the surface of the metal to prevent oxidisation. Protective coatings include metal oxides which are applied by a process called passivation. Corrosion inhibitors are most commonly used commercially in the manufacture of vehicle chassis and should be applied to both inner and outer faces of the metal to ensure maximum protection.
Maintenance and monitoring
Monitoring the condition of metals, especially those being used in harsh environments, can help to prevent corrosion. Regular inspection and monitoring of the surface condition, looking for cracks and pitting, alongside a proactive maintenance programme can extend the lifespan of metals in use. Treatments can be applied or components replaced before long term damage to structures or component parts takes place.
Environmental corrosion protection
Corrosion can be prevented or reduced by calculated environmental measures. When measures are taken to control aggressive environments, the chemical reactions that cause corrosion can be dramatically reduced. Measures to reduce the threat of corrosion can include controlling other external chemicals and reducing the exposure to saline solutions. By ensuring the correct metal is selected for a particular job, or that the grade chosen has the chemical properties suited to use in a particular environment, damage to structures can be minimised or avoided. Taking the time to research the correct grade can save valuable metal resources, time, and money.
