Read our complete guide ‘The Importance of the Correct Specification of Metallic Coatings for Steel’
It is important to understand the difference between different zinc coatings – because they have different levels of corrosion performance and different suitability in their applications.
Only batch hot dip galvanizing, to EN ISO 1461, provides complete coverage of the steel component with a thick, strongly bonded layer of zinc by immersion of a fabricated steel article into molten zinc.
The process is also known as ‘general galvanizing’, ‘post galvanizing or ‘galvanizing after fabrication’ – but the most certain way to get the correct specification is to refer to EN ISO 1461.
The life expectancy of a zinc or zinc alloy coating is largely determined by its thickness. Thicker coatings give longer life. Batch galvanizing to EN ISO 1461 gives fabricated steel products maximum protection through complete coverage with a thick, tough and metallurgically bonded coating.
Steel sheet or strip can be coated with zinc (or zinc alloys) before fabrication. Steel sheet or strip is passed through a bath of molten zinc in a continuous process. When the steel is withdrawn from the molten zinc, the coating is mechanically wiped to produce a thin layer of zinc or zinc alloy. Sheet steels that are coated in this way are usually ordered according to EN 10346. Coatings with zinc are given the designation ‘Z’ ; zinc-aluminium alloys ‘ZA’ and zinc-aluminium-magnesium alloys are designated with ‘ZM’. Steel products that are manufactured from these ‘pre-coated’ steels will have thinner coatings and will have exposed steel at the cut edges and holes that are created during manufacturing – giving points of weakness in the future. It is important to recognise during specification that these types of continuously coated steels have lower durability and are less robust than a steel product that is galvanized after manufacture (to EN ISO 1461).
Other types of zinc coating for steel include thermal sprayed zinc; electroplated zinc and zinc-rich paints. Each of these have their own specific applications and exhibit inferior characteristics to batch galvanizing in terms of coating thickness, adhesion and efficiency of application. It is important to recognise that these differences when specifying for corrosion protection. These other coating types are not equivalent to batch hot dip galvanizing to EN ISO 1461.