COP:trash; corrosion

18.9 Corrosion 


18.9.1 Types Of Corrosion 

There are many types of corrosion that affect metal claddings but they can be divided into two main types:

  • atmospheric for a metal in isolation
  • electrochemical or galvanic, where the metal reacts with another metal or material

The various manifestations of corrosion within these two types are categorised in a different way, cannot be clearly defined and almost always overlap . Frequently the different types of corrosion typical of different metals and alloys do not develop separately, but are interdependent .

The nature of the interaction between various metals and electrolytes is influenced by the many permutations of the environment, the degree of pollution and the pH of the electrolyte.

It is the responsibility of both the designer and the Roofing Contractor to ensure that they do not cause corrosion by incorrect use of materials.

The major factors that affect metal corrosion are:
  • t he metal ,
  • t he atmosphere ,
  • t he rain , and
  • p ollutants

18.9.1.1 Balance 

Sacrificial protection and passivity can be mutually exclusive and passivity can be so well developed that sacrificial activity is suppressed. An AZ coating does not provide the same degree of sacrificial protection as a Z, ZA or ZM coating, which does not have the non-loss passivity of aluminium.

The differences between primarily zinc and primarily aluminium coatings lies in the balance between passivity and sacrificial protection, and for roofing and cladding products in most conditions, an AZ coating provides satisfactory sacrificial protection of cut edges although not as well as a zinc-based coating. The introduction of magnesium into coatings containing aluminium is intended to enhance the cut-edge corrosion resistance of the coatings to provide better all-round performance.

18.9.1.1.1 Cut Edges 

18.9.2 Durability 

As the requirements of the NZBC are performance based, it is necessary to make a subjective assessment specificallyconcerning durability of the building elements covered by this Code of Practice. While it could be assumed that roofor wall cladding can be easily accessed and therefore easily replaced, the same cannot be assumed for any flashingswhich may be embedded in plaster or could not be removed without the removal of monolithic claddings. Someflashings are half hidden and as this portion would not be subject to inspection or maintenance its failure could leadto structural degradation and therefore would not comply with the NZBC. Because replacement could be classifiedas a major reconstruction, the flashing material durability requirement is 50 years. This requirement also applies tounseen flashings and secret gutters.

The NZBC does not take into account the cost or aesthetics of replacement, so while replacement period would complywith Code, it may be not considered acceptable by the client or owner. Any pre-painted cladding will change colourover time and the replacement would be visible, and the cost of replacing one long length sheet from a distantsupplier would be disproportionate to its value.

All metal roof and wall cladding and accessories should be designed and installed to comply with the durability requirementsof the NZBC, but the economic and aesthetic consequences of replacement should also be considered.