Metal in its pure form does not contain organic subjects and do not
give the lichen any nutrition. Affection is therefore not possible.
Damp and dirt gives on the other hand the metal organic qualities –and contains nutrition for the lichen spores.
For long time it has been an extensive problems to get metal roofs or thin plate and walls free from lichen, which demands large amount of maintenance at a high cost.
As long as the metal is kept free from damp, dirt, spores and old affections, metal roofs and walls can last for decades. Thereby saving both time and money. Simple.