Combatting metal corrosion in Coastal Environments – an explanation for Building Owners and Surveyors.

What is a coastal environment?

The Metal Cladding and Roofing Manufacturers Association (MCRMA) define a “coastal environment” as the area extending to a minimum of 1km from the shore or edge of tidal water. This may extend significantly further (up to 5km) depending on the topography and prevailing wind.

What does that mean for my building?

A building in a coastal environment will be exposed to significantly more salt water (or vapour) than a building in a typical urban location. Salt water corrodes metal five times faster than fresh water and is an excellent catalyst for metal oxidation, or in other words – corrosion.

How does salt water corrode steel faster than fresh water?

We first need to understand the process of oxidation. Oxidation is a chemical interaction involving oxygen.

When oxidation occurs in some metallic elements, a thin film is formed, such as the green patina that copper acquires. The oxidised layer then halts further degradation.

The oxidation of iron – and metals related to iron, such as steel – is different to copper, zinc and aluminium. It is progressive and destructive. When a ferrous metal (containing iron, such as steel) is exposed to an environment that is both oxygen rich and lightly conductive, the oxidation process begins. Molecules of iron at the surface of the metal exchange atoms with the oxygen to form a new substance – iron oxide, which is the reddish-brown rust.

Oxidation is an electrochemical process requiring oxygen. Water assists this process by allowing electrons to move more easily between the two elements.

However, salt (Sodium Chloride – NaCl) is an electrolyte and when this is dissolved in water to form salt water it separates into sodium ions (Na+) and chloride ions (Cl-). These ions have an electrical charge, giving salt water electrolytic properties which accelerate oxidation reactions.

Therefore prime rust conditions occur when metals are regularly exposed to salt water and air.

To protect metal structures against rust, the oxidation process must be interrupted by application of a protective coating to stop water vapour coming in contact with the metal. Without oxygen and without water to help the elements exchange electrons the oxidation process cannot begin.

Other contributing factors to corrosion in coastal environments:

Anaerobic corrosion is caused by the metabolic activity of bacteria in seawater and occurs when metal is submerged in salt water for an extended period of time. One of the most destructive species is Sulphate Reducing Bacteria (SRB) which causes severe accelerated corrosion to ships and offshore steelwork.

Air borne sand particles are very abrasive, and you need to ensure that a protective coating with high abrasion resistance is used for your property.

A coating with a high chemical resistance should also be specified to help withstand the strong uric acid present in bird droppings. There is no substitute for regularly cleaning roof ridges to ensure that the droppings do not accumulate!

The solution

Don’t let corrosion take hold on your building. By treating corrosion early, your roof and walls will remain watertight and structurally sound for many years.

Rooflock has a range of BBA certified systems for metal roofing and cladding refurbishment. Contact the Rooflock Technical Team today who can assist you with technical advice and approved contractors to refurbish your building.