Corrosion of Galvanised Equipment

What is Galvanising?

Hot dip galvanising is a surface treatment used to protect mild steel from corrosion. The process involves dipping chemically clean steel into a bath of molten zinc which causes a metallurgically bonded coating of zinc to form on the surface of the steel.

Zinc, like stainless steel or aluminium does not rust like mild steel. An oxide layer on its surface is created on exposure to air, which resists further corrosion. Zinc is used in galvanising as it bonds well to mild steel.

How does Galvanising prevent corrosion?

Galvanising prevents oxygen and water from reaching the steel beneath using this zinc oxide layer on its surface. When there’s moisture present this turns into zinc carbonate. This gives the metal a dull grey patina that’s insoluble in water and stops any further chemical changes. In certain circumstances, the galvanising will offer cathodic protection, where the zinc corrodes preferentially to the steel.

So, does Galvanising last forever?

All IAE Hot Dip Galvanised products are galvanised to the highest standards, conforming to BS EN ISO 1461 however, depending upon the environment and location the equipment is within, there is always a possibility of galvanised steel corroding in the medium to long-term, particularly in aggressive environments.

What degrades the Zinc coating?

Typical environments or chemicals that accelerate corrosion of galvanised steel are:

  • Acids or ammonia’s found in animal feeds (e.g. silage) and excretion
  • Strong alkalis such as cement, (especially Portland cement,) as these contain chlorides and sulphates
  • Sodium Chloride (salt) in water or air (marine or coastal environments)
  • Situations where coatings are wet or soaked

Present within some of the above are corrosive gasses which catalyse the corrosion of the zinc coating through the formation of carbonic and sulphuric acid respectively. These acids modify the zinc coating causing its properties to change and therefore reduce the protection against corrosion. As a result of this, ferric and ferrous oxides occur in the form of rust. Marine and salty environments lower the lifetime of galvanized steel because the high electrical conductivity of sea or air increases the rate of corrosion, primarily through converting the solid zinc to soluble zinc chloride which is then washed away.

Why is my water trough rusting, but not my steel building?

The thickness of the galvanised coating is determined by the thickness of the material being galvanised. The two variables have a linear relationship and therefore as the thickness of the steel increases the thickness of the galvanised coating also increases. The thicker the layer of galvanise, the longer the lifespan of the product. Typically, a Rolled Steel Joist (RSJ) used in a steel building is significantly thicker in-depth than the steel used to generate our product range. Therefore it will be longer before the RSJ begins to rust compared with a water trough, for example.

What can be done to increase a galvanised item’s lifespan?

Applying a secondary layer of protection to a galvanised item that is particularly susceptible to corrosion is the primary method of increasing corrosion resistance. This is known as a “duplex system”. This method is used on all IAE Industrial fencing systems where parts are galvanised first then Polyester Powder Coated.

Agricultural applications where aggressive compounds are present can also benefit from a secondary layer of protection. Coating the lower section of posts with bituminous paint prior to installing into concrete is a recognised method of increasing a posts lifespan, protecting it from acids and ammonias present in animal excretion. Coating feed and water troughs with food safe paint or resin is also common practice. Where additional coatings cannot be practically applied, ensuring products are washed down frequently where exposed to aggressive environments, and keeping products dry where possible will both increase longevity.

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