Corrosion zones

This Issue This is a part of the NZS 3604:2011 and more feature

By - , Build 124

In the 2011 version of NZS 3604, some changes have been made to the corrosion map, mainly with the atmospheric corrosivity classification system.

The severity of atmospheric corrosion is now categorised into five groups based on the corrosion rates of mild steel given in AS/NZS 2312:2002 Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings and ISO 9223:1992 Corrosion of metals and alloys – Corrosivity of atmospheres – Classification.

Exposure zones aligned

These standards define the zones as category A (very low), category B (low), category C (medium), category D (high) and category E (very high). Category A is not applicable in New Zealand.

To align the standards, the exposure zones in NZS 3604 have been redefined:

  • Sea spray zone is now zone D.
  • Zone 1 is now zone C.
  • Zones 2 and 3 are now zone B.

Now the classification of New Zealand atmospheric environments is aligned with related standards, including AS/NZS 2312 and AS/NZS 2728, except they are now referred to as zones instead of categories.

Sea spray zone now zone D, or E in some places

The sea spray zone, including all offshore islands and the area within 500 m of the coastline, is defined as zone D in NZS 3604:2011.

However, in areas very close to beachfronts subjected to rough seas and surf beaches, the atmospheric corrosivity can be very severe due to the heavy deposition of airborne sea salt particles. These areas are classified as category E in AS/NZS 2312 and ISO 9223 but are not shown in the map in NZS 3604:2011 because the standard requires identical corrosion protection for metallic components within both zones D and E.

Geothermal zone out, geothermal hot spots reclassified

The geothermal zone (zone 4) used in NZS 3604:1999 is not in NZS 3604:2011. Most geothermal areas now fall into zone B of NZS 3604:2011. This does not imply that the selection of metallic components for buildings within these areas should always follow the recommendations for a mildly corrosive atmosphere.

It has been shown experimentally that mild steel and galvanised steel have much higher corrosion rates in areas adjacent to geothermal hot spots than in other inland regions. Geothermal hot spots are defined as being within 50 m of a bore, mud pool, steam vent or other sources. While they are mainly found in Rotorua-Taupo, they may occur elsewhere.

However, since the atmospheric corrosivity is highly variable within these areas, the geothermal zone has been removed and is to be considered as a micro-climate and subject to specific engineering design (SED).

Micro-climate considered

NZS 3604:2011 recognises that micro-climate, as well as macro-climate, affects the corrosion performance of a metal and the durability of a structure. Components corrode at quite different rates when installed at different locations on the same building because design features can significantly influence the local climate surrounding a component. For example, a sheltered surface may accumulate more airborne salt particles due to the lack of rain washing, making a mildly corrosive environment more aggressive toward metals. Pollutants, such as oxides of nitrogen and sulphur released by industry or agriculture, can also accelerate the deterioration of metals.

When assessing the aggressivity of a given environment, if micro-climatic factors are found to be significant and outweigh macro-climatic parameters, NZS 3604:2011 recommends a more severe corrosion category is used for materials selection (see clause 4.2.4).

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