Zinc coating: what Z275 means
What the designation says
Section titled “What the designation says”In the designation of continuously coated steel coil the letter Z stands for the zinc coating, and the number gives the coating mass in grams per square metre, counting both faces together.
So Z275 means a thicker protective layer than Z140 or Z100.
Other coating types exist — zinc-aluminium-magnesium alloy coatings, marked with a different letter combination. For a given coating mass these can give better corrosion resistance.
How much coating is needed
Section titled “How much coating is needed”There is no general answer: the installation environment decides.
In a dry, closed structure
Section titled “In a dry, closed structure”Inside the wall of a heated building, protected by a vapour control layer, the structure is effectively in a dry environment. Here the standard coating already gives a long service life.
Where moisture is present
Section titled “Where moisture is present”Near the foundation, in unheated spaces, behind a ventilated facade, around bathrooms or in industrial air, the load is higher, so a thicker coating is justified.
On directly exposed external members
Section titled “On directly exposed external members”There galvanised thin sheet on its own is generally not enough: additional protection or a different material choice is needed.
What the coating does not solve
Section titled “What the coating does not solve”The cut edge
Section titled “The cut edge”During production the ends of the section and the edges of cut-outs are exposed. Zinc does give cathodic protection in the immediate vicinity, but on thicker material and under heavy exposure this is limited.
Poor detailing
Section titled “Poor detailing”Where water stands, where vapour condenses, where there is no ventilation — the coating only delays the problem. The first tool of corrosion protection is good detailing. Corrosion and service life.
Material pairing
Section titled “Material pairing”In contact with another metal, galvanic corrosion can start. Material pairing.
What the design has to state
Section titled “What the design has to state”The design must give the type and amount of coating, not just the steel grade. This can be checked against the declaration of performance, and it has to match the installation environment.
The point
Section titled “The point”The coating is not an “extra” but designed protection: service life depends on it. The amount needed is set by the environment, not by habit.