The Z number shows the total amount of zinc on both faces of the sheet in grams per square metre: Z100 = 100 g/m², Z275 = 275 g/m². As the number rises the coating gets thicker and protection against corrosion lasts longer.
The choice follows how aggressive the environment will be; "the more the better" is wrong — "appropriate to the environment" is right.
Classes and typical use
| Z100 | Indoors, sheltered environment, light sections |
|---|---|
| Z140 | Indoors plus partial outdoor exposure; general purpose |
| Z200 | Outdoors, roof and façade applications |
| Z275 | Aggressive environments: coastal, humid industry, agricultural buildings |
How the coating protects
Zinc protects in two ways: as a physical barrier it cuts the steel off from the air, and through cathodic protection it sacrifices itself at exposed points such as scratches.
Thanks to that second property small scratches do not rust straight away; the zinc spreads from around the point and protects the exposed spot.
Estimating life
As coating thickness rises, life lengthens in proportion; but the environment is the decisive factor. Salt-laden damp at the coast, sulphurous air in an industrial zone and ammonia in agricultural buildings consume zinc quickly.
So the same Z class gives very different lives in two different regions. Make the choice knowing the environment.
A common mistake
Choosing a low coating class in a coastal region to reduce cost. The difference gained in the short term is repaid many times over in the cost of early replacement.
The second mistake is confusing coating with sheet thickness: thickness determines strength, the coating determines corrosion life.