Why doesn't  a galvanised sheet  rust at the edges?

In most normal circumstances in the UK it doesn't, but in Corrosive Industrial Environments and Coastal Regions the cut edge needs extra protection. Page down for details.

 The answer taken from a  British Steel ( Now Corus) information leaflet is:-

The corrosion rate of Zinc in most UK atmospheric environments is known to be only 1/10th to 1/30th the rate of mild steel. Furthermore if the surface is painted. then the protective effect of the duplex coating is synergistic (i.e. whereas, in a given environment the zinc coating might last six years and the paint coating on mild steel around 2 years then the life of the painted zinc would be not eight but more like 11 to 13 years)

Galvanic protection across small bare areas (cut edges) also occurs. This happens when two dissimilar metals are in electrical contact in the presence of a conducting medium (moisture). Galvanic corrosion of one takes place whilst the other is protected.

The most reactive of the two metals will become the anode in a natural electrical cell and if the anode does not form an insoluble high resistance oxide film (and zinc does not) it will oxidise and produce electrical current to protect the cathode from corrosion. Zinc protects steel by galvanic action  because it is anodic to steel at normal ambient temperatures.

EXTRA CUT EDGE PROTECTION.

with

Latex Edge Protection Lacquer.

After 28 months at the Swedish Corrosion Institute's outdoor research laboratories, the untreated metal (left above) was severely corroded while the metal (above right) protected by Latex Edge was un-corroded

LATEX EDGE PROTECTION LACQUER

A sheared edge is a potential weak link in cladding material. Exposure of the sheared edge to the atmosphere can result in damaging  corrosion, irrespective of the substrate used.

Thus is particularly so in aggressive environments, such as severe marine conditions, industrial corrosive environments, low pitched roofs and on drip edges.

RECOMMENDED

To combat the likelihood of corrosion and its effect on the cut edges of the cladding, Becker Industrial coatings has developed its Cut Edge Protection Lacquer System. The Lacquer has been tested by Corus and is recommended by them for use on any type of cladding as well as testing in the harsh conditions prevailing at the Swedish Corrosion Institutes outdoor testing centre at Bohus Malmoen on the West coast.

UNIVERSAL SYSTEM

The Edge Protection Lacquer is a universal edge protection system, suitable for use on the cut edges of pre-painted material, coated with PVF2, Plastisol, SMP, ARS or Polyester coatings.

The system is a water white, clear lacquer, with a water borne acrylic emulsion base. The lacquer air dries to touch in 2 - 4 hours.

APPLICATION

A one-coat system. Edge protection lacquer is designed for easy on site application. Brush or roller may be applied straight onto a clean dry surface.

There is no need to carry extensive stocks to colour match: the lacquer is water white transparent.

The above Lacquer is stocked by SLE Cladding Ltd.

SAFETY

It is a non-flammable product. However the usual health and safety precautions (e.g. us only in well ventilated space) should be observed. After skin contact the skin should be rinsed. After eye contact, the eye should be rinsed and medical advice sought.

GENERAL CHARACTERISTICS.

The Latex Edge Protection Lacquer is based on an air drying water borne acrylic emulsion (latex)

PHYSICAL CHARACTERISTICS.

Viscosity. 3,500 - 5,000 cp Brookfield RV4 20 rpm

Density  1.05

Resin Type  Acrylic emulsion

Clarity   Water White Transparent

Solids Content.  By weight 49 % v+/- By volume 45% +/- 2%               

Application. Brush or Roller in one coat on to surface free from grease, dirt and other extraneous matter.

Curing Air Drying.

Touch Dry. 2 - 4 hours

PRECAUTIONS

Protect from Frost. Do not store below 5 deg. C

Flash Point.  None.

Water based Product. do not apply below 5 deg. C

AVAILABILITY. Available in 1 Litre Cans.

COVERAGE RATE

200 Linear Metres per litre at 1 cm wide.