NCC 2022 - Corrosion protection for structural steel

The NCC 2022 Volume 2 provides a transition period enabling users to continue to use the corrosion protection requirements in NCC Volume Two 2019 Amendment 1 until adoption on 1 May 2024.

How the changes are found in the new look NCC

NCC 2022 Volume Two Part H1D6 provides several deemed-to-satisfy (DTS) options for steel structures, which satisfy performance requirement H1P1 and cover corrosion protection: 

• ABCB Housing Provisions Standard Clause 6.3.9, or 
• AS 4100 for steel structures, or 
• AS/NZS 4600 for cold-formed steel structures, or 
• The NASH Standards for residential and low-rise steel framing 

As the NCC is a performance-based code, it is also valid to develop a Performance Solution supported by appropriate evidence of suitability. 

The ABCB Housing Provisions Standard

Clause 6.3.9 of the ABCB Housing Provisions Standard (HPS) replaces the old Clause 3.4.4.4 and contains extensive changes to the corrosion protection tables to include greater detail and additional options.  

The environmental corrosion classifications have been updated to align with corrosion categories used in other DTS provisions and AS 4312. Four environmental classifications are defined depending on proximity to the coast – Low, Medium, High and Very High. Users should refer to the new NCC table to ensure the appropriate corrosion protection options are selected for their location. 

The provisions now require structural steel not built into a masonry wall to be protected against corrosion using one of the options provided in Table 6.3.9a. Options for galvanising and duplex coatings per AS 2312.2, and paint coatings per AS 2312.1 are provided for the four different environment classifications. 

New Tables 6.3.9b and 6.3.9c provide more detail about the paint and duplex coating systems nominated in Table 6.3.9a. 

The HPS clause also provides information on surface preparation and repair to on-site damage including being free of rust prior to painting, preparations for the surface where zinc is applied, and recoating with the required coatings when cut or welded on-site.  

What do the new galvanising options mean? 

Table 6.3.9a recommends four different minimum hot-dip galvanising classifications. The coating mass in g/m2  for each environment ranges from Low (HDG75) to Very High (HDG900). 

However, these classifications do not necessarily reflect actual products available on the market. The Australian Galvanisers Association has advised that when approaching a galvaniser the following specifications are applicable (based on AS/NZS 2312.2 and AS/NZS 4680): 

• Standard hot-dip galvanising achieves the following minimum average coating mass for the following steel thickness ranges: 
• Steel ≤1.5mm – HDG320 
• Steel >1.5 to ≤3mm – HDG390 
• Steel >3 to ≤6mm – HDG500 
• Steel >6mm – HDG600 
• Therefore, for Low and Medium corrosion environments, structural steel of any thickness that is hot-dip galvanised to AS/NZS 4680 will achieve the requirements. 
• For High corrosion environments, standard hot-dip galvanising will meet the requirements for steel more than 3mm thick. Thinner steels will require 2D duplex coatings over the galvanising. 
• Very High corrosion environments will need 5D duplex coating after hot dip galvanising for steels up to 6mm thick. Steels more than 6mm thick will require 4D duplex coating after hot-dip galvanising. 
• Standard hot-dip galvanising cannot achieve the HDG900 rating should be discussed with your galvaniser or supplier.  

AS 4100 Structural Steel

AS 4100 has undergone revision and the 2020 version of the Standard has now been adopted.  Corrosion protection requirements are covered within Clause 3.8.6 and guidance in Appendix C, refers to the following secondary reference standards: 

• For classification of environmental corrosion category - AS 4312 
• For coatings system selection, based on corrosion environment and desired durability (years to first maintenance): 
• Paint systems - AS 2312.1 
• Galvanising and duplex coatings – AS/NZS 2312.2. 
• For surface preparation and application of corrosion protection - AS/NZS 5131 : 2016 (see Section 9 of that standard). 

Using AS 4100 will generally align with the HPS provisions but provides a broader range of options for coating systems using secondary reference standards. 

What about structural steel located internally to the building?

HIA notes that HPS Clause 6.3.9 no longer references internal environments, which has prompted several questions to HIA from members. AS 4100 provides options for structural steel elements located in dry internal environments: 

• AS 4100 Appendix C refers to AS 4312 for classification of environmental corrosion category. 
• AS 4312 includes Corrosion Category C1 “dry indoor” and for C1 environments coating systems include: 
• Paint coatings – AS 2312.1 includes a range of options depending on the desired durability (years to first maintenance), including simple single-pack acrylic latex systems. 
• Galvanising – according to AS/NZS 2312.2 any zinc coating system with a thickness more than 36 micron will achieve a 50-year durability to first maintenance; or hot-dip galvanising to AS/NZS 4680 for general structural steel products also achieves this. 
• Duplex coatings – per above are unnecessary for C1 environments. 

To find out more, contact HIA's Building Services team.

Email us