Cut Edge Corrosion
Causes of Cut Edge Corrosion
The long term performance of factory applied coatings on profiled metal roofing and cladding is dependent on a number of key factors, particularly sunlight and UV, with moisture, location, pitch and the quality of the coating itself all playing a part. Of all these, the dominant factor is UV radiation.
Cut Edge Corrosion originates at the interface of organic coating, zinc galvanising and the steel substrate. Water is almost invariably trapped at the edges, largely through capillary action and the overlapping sheets, making it difficult for rainwater to evaporate. Eventually the now ‘trapped rainwater’ begins to dissolve the protective zinc layer and allows corrosion to creep and lift the organic coating (PVC/PVF2) from the steel substrate.
When to take action to stop Cut Edge Corrosion
The quality of coatings on metal profiled sheets has improved significantly in recent years. Nevertheless, the cut edge and overlap joint remain especially vulnerable due to the effects of capillary action drawing the contaminated rainwater into the joint causing the double hazard of ‘topside’ and ‘reverse side’ corrosion.
Edge corrosion is a progressive problem and, if neglected, can develop away from the cut edge, destroying the weather sealed lap joint and gutter overhang. Resulting in more extensive and expensive sheet repairs as the integrity of the roof has now been compromised.
There will be clear evidence in many areas of the roof including the lap joint, gutter overhang and flashings where the sheet has succumbed to UV or surface attacks, commonly recognised as mechanical damage. This will result in delamination, corrosion and coating failure where the steel substrate has been left exposed to the environment.
Providing the edge corrosion and peel back has not yet spread to the first fixing point of the roof sheet and is not affecting the basic integrity of the roof, all areas are suitable for treatment without the need for disturbing the roof sheets.
Cut Edge Corrosion and its treatment
SEAMSIL® works by creating a tenacious bond with the surface, leaving a permanent residual ‘wetted’ layer across the area of application. This chemical bond limits the oxidisation by allowing water vapour to vent through its breathable membrane. Preventing standing vapour on the surface of application negates the whole process and cycle of corrosion.
The Silecium/oxygen bond within Seamsil forms a totally stable chemical barrier that cannot be attacked by UV, this helps retain its long-term physical properties. SEAMSIL® was developed by Delvemade over 20 years ago, it is the UK’s Number 1 treatment for Cut Edge Corrosion. Designed to provide a fully flexible external seal. All areas are suitable for treatment without the need for disturbing the roof sheets. Edge corrosion repairs commonly include Flashings and Rooflight tops.
Zinc reacts with atmospheric oxygen forming a protective layer. However, zinc salts are slightly soluble in water and as further rainwater exposes more zinc, more zinc oxide (salts) form to maintain the protective barrier. As more and more zinc is called upon to replenish the depleted barrier, stocks are called from further up the sheet, resulting in thinner and thinner protection until eventually the steel substrate lies exposed and the unsupported coating having lost its initial adhesion, begins to ‘peel back’.