We are often asked, why do metal roofs corrode?
The short and simple answer is, they are made of iron based metal and frequently exposed to rain and moisture, hence they suffer from rust corrosion!
Rust is an electrochemical process which occurs on metal surfaces that are exposed to the electrolytes. When rusting occurs the oxides expand and occupy more space than the original material. The forces in this process are enormously damaging to the structure under attack.
The Manufacturing Process
To better understand everything lets start with how metal roofs are actually made and the benefits of having a corrugated metal roof. Sheets of flat steel are fed from a roll onto a metal forming machine. The sheets are flattened, then formed and vary in size depending on the roof sheet specification. They are then cut to set lengths by guillotine and treated at each end. The cut ends or cut edges are a weak point that will require maintenance throughout the lifespan of the roof.
Watch the video below to see how steel roof sheets are made.
What causes rust and how does it form on metal?
Three elements need to be present in order for iron oxide, or rust to form. These elements are iron, water and oxygen. You won’t be surprised to know that the main cause for rusting is water! Oxygen in water and moisture creates iron oxide, commonly known as rust, on any iron mass such as steel.
Although steel structures appear solid water molecules will penetrate microscopic pits and cracks in exposed metal. The oxygen which is present in air simply dissolves in water which forms a layer of rust. The chemical composition of rust is Fe2O3.nH2O.
So basically rust corrosion is an attack that destroys the metal due to the way it reacts to the environment around it. Rust occurs more quickly with acid rain and salty water by the seaside. Given enough time the entire structure will fall apart and disintegrate. This is a worry and indeed a major cause for concern.
Are there different types of corrosion?
Yes, corrosion in iron and its alloys, commonly referred to as rust, is just one form of metal corrosion. All metals have a tendency to oxidize and dissolve, some more than others. Reinforcing bars in concrete pillars located underwater and exposed to chloride suffer from green rust. The most common colours are red, orange and black and only iron or alloys containing iron can rust in this way.
Other types of corrosion such as caustic agent corrosion occur when impure gases and liquids break down metallic materials. Galvanic Corrosion, while not as common General Attack Corrosion, is nonetheless a serious problem when it happens. It occurs when two different metals sit side by side and one metal is a cathode, the other an anode. The anode metal breaks down more quickly than the cathode metal.
Flow Accelerated Corrosion is one of the most common in roof tops due to their exposure to wind and rain. The protective layer is simply blown away or dissolved which exposes the metal beneath to corrosion. Stress Corrosion Cracking is another problem suffered by roof structures. High winds and activities inside a building such as dance halls and gymnasiums all contribute to extra tensile stress on a roof. Repetitive movement and vibration cause microscopic cracking and accelerate attack from water particles.
What can we do to stop rust?
Metal and steel should never be exposed to the harsh weather we experience in the UK and other parts of the world. As explained above, when water makes contact with metal the rusting process begins. If we cover the metal with a protective coating, we create a barrier between the metal and water in the atmosphere thereby preventing oxidation in iron.
Here at D&D Coatings we use a series of silicone based products to block attack from water and other substances that are harmful to steel roof sheets. This provides a strong flexible seal which encapsulates the area at risk. The products we use have a longer lifespan compared with the products used by many other industrial painting contractors.
To find out more about our services and how we can help to repair and protect your metal roof, visit our roof coating services page or call 01606 554040 for a friendly chat with our expert staff.