The Australian/New Zealand Standard AS/NZS 2312 provides guidelines for the selection and specification of coating systems for the corrosion protection of structural steelwork. It is arguably the most important Australian standard on corrosion protection, being the main guide for coating specifiers in this part of the world since its predecessor first appeared in 1967. (The new standard is described in the article Important Coating Standard Update for AS/NZS 2312.)
Issues Regarding Paint Coating Selection
With metal coatings now being covered by separate documents, factors influencing the selection of paint coatings were moved to Section 5 for the updated standard. There is additional information on color and factors to be taken into account when selecting colors for paint coatings. The clause on the influence of previous experience has been expanded. Paint coating systems can be divided into three categories:
- High-risk systems are those paint systems for which premature failures have arisen or those for which there is limited experience, such as systems applied with a standard of surface preparation less than Sa 2½.
- Medium-risk systems are those with good performance, but can be difficult to apply, have restricted application conditions or limited long-term experience. Examples are catalyzed acrylic systems, water-borne inorganic zinc and polysiloxane systems. (For further information about inorganic zinc, see Surface Preparation for Inorganic Zinc Silicate Coatings.)
- Low-risk systems are those widely specified and used, for which there many successful case studies and proportionally few failures have been noted. Examples are high-build epoxy systems, polyurethane top coat systems and single-coat solvent-borne inorganic zinc systems.
Comparison with ISO 12944
As with the previous Standard update, the committee looked at the ISO equivalent series of standards to see if it could be adopted in our part of the world. However, its omissions and differences are still significant and the issues discussed in a comparison published by the author (ref. 6) are still largely valid. These include:
- The absence of paint systems used here, such as:
- Catalyzed acrylic and polysiloxane
- Maintenance coating systems applied to power-tool-cleaned surfaces, such as those based on epoxy mastic
- Single-coat, water-borne inorganic zinc silicate coating systems
- The dry film thicknesses (DFT) specified are in multiples of 40 microns, not 25 microns as used here
- The method used to designate systems is complex, with five tables with different numbering for the same system
However, Table 6.3 in the new Standard now contains the ISO 12944-5 designation,3 where there is a similar paint system (with a total DFT within ±25 microns). But only about one quarter of the paint systems listed have ISO "equivalents." Also, new material in the application and inspection section on references areas, and information on issues regarding volatile organic compounds (VOCs), have been based on material from the ISO 12944 series of standards.
Paint Coating Systems
As with the previous version, Table 6.3 is still the key part of the Standard, listing the paint coating systems along with expected durability. Section 6 clearly explains that the durability figures will only be obtained under optimum conditions. This means planar surfaces where edges, ponding areas and other regions are likely to break down prematurely have additional treatment such as rounded edges and stripe coats.
The figures also apply to coatings applied to new steel, and reduced durability must be expected for maintenance work. The coatings must be applied in strict accordance with the manufacturer’s requirements for maximum durability.
The new version has also clarified the nominal dry film thickness figures given for the specified systems. In AS/NZS 2312, it is recommended that AS 3894 Part 3 be used for measurements. This requires the average of the readings to be greater than or equal to the nominal DFT (NDFT) with no individual reading <80% of the NDFT.
However, for the catalyzed acrylic and polyurethane color systems, the NDFT should be considered as an "absolute" minimum, to ensure good opacity and coverage of the thin top coat. If the measured DFT is greater than any maximum DFT specified, then "parties should find agreement." Because different coating types have difference tolerances to over-thickness, a blanket policy on maximum thicknesses would be difficult to justify. (Discover more in the article The Impact of Minimum & Maximum DFT Values on Coating Performance.)
Some coating systems have been removed from Table 6.3. These include some rarely specified alkyd systems and chlorinated rubber systems that are no longer used because of availability and concerns over VOC levels. Ultra-high-build epoxy and vinyl ester systems have been deleted as they are rarely used for atmospheric exposure and would not be economic for such exposures. They are listed in the non-atmospheric exposure coating system in Appendix C.
The Committee recognizes that only a small proportion of possible atmospheric coating systems can be included, but members wanted to include those most widely used, as well as examples of a range of generic types.
In the previous version of the Standard, a high-build epoxy was defined as an epoxy that could be applied at 125 to 500 microns per coat. This DFT range was viewed as rather large for a single generic type, and epoxy systems are now divided into two main groups:
- High-build epoxies (125 to 200 microns per coat)
- Very high-build epoxies (250 to 500 microns per coat)
While this delineation may not be widely used at present, it should assist specifiers in selecting epoxies from the wide range that's available on the market.
A new single coat solvent-borne inorganic zinc silicate applied to a NDFT of 125 microns has been added. This will provide a high durability system to complement the 125-micron water-borne system, but is easier to apply and can be used in humid environments. While this system is controversial and solvent-borne products are rarely specified to this thickness, there are now suppliers who will allow their products to be applied to this higher thickness. In fact, to meet the requirements of AS 3750.15 Type 4, solvent-borne products must be able to be applied to this thickness without defects.
The durability of both the water-borne and solvent-borne systems applied to 75 microns are now identical, as are the two 125 micron systems. There is now sufficient evidence that the performance of the two types is largely the same for the same thickness, although a note to the Table 6.3 does accept that the performance of some water-borne products may have better durability when applied to the same thickness.
The popular zinc-rich primer/epoxy mid-coat and polyurethane top coat systems dominate coating specifications around the world, and are probably the most important paint coating system where color is required.
Similar polysiloxane top coat systems are increasingly being specified. Two new polysiloxane three-coat systems have been added to the existing two-coat system:
- PSL2 with a zinc-rich primer
- PSL3 with a zinc-free primer
These are the equivalents of the popular PUR5 and PUR3 systems, with similar durability. The durability figures of some systems have also been revised. The polyurethane and polysiloxane systems in the new version are summarized in Table 2 below. In the table, for ease of comparison, this discussion considers the performance in only one environment: the C4 high corrosivity environment. The durability will be proportionally lower in more severe environments and greater in less severe environments.
Table 2: Comparison of polyurethane and polysiloxane top coat systems:
The main change to Section 7 on paint application is a new clause of qualifications for coating contractors. The Standard recognizes that surface preparation and coating application are skilled activities and qualified personnel should be used at all stages. It gives examples of acceptable qualifications. The clause also requires the contractor to have a quality plan in place and method statements for each stage of the work. Evidence for such a plan can be provided by a recognized QA system such as the PCCP. (Learn more about QA systems in the article What Does Quality Control Mean in the Corrosion and Coatings Industry?)
This section also provides further detail on stripe coating, painting around bolt holes and repairs to damaged coatings.
Maintenance of Protective Coatings
Section 10 in AS/NZS 2312 covers the maintenance of protective coatings. In an earlier paper (ref. 6), the amount of coating breakdown (0.2 to 2%) before maintenance given in the earlier standard was criticized as being unreasonably conservative, as was the 1% level given in ISO 12944-5. This figure has been slightly increased to 1–2% in the new version of AS/NZS 2312, which is still lower than a typical level of 5% or so used in industry. The Committee believe that surface preparation costs will be lower overall if coated with a lesser degree of breakdown, and owners should be encouraged to carry out maintenance before excessive breakdown has occurred.
The new version also provides a table from the lead removal standard AS 4361.1, showing additional criteria for the feasibility of repair7 with suggested levels of substrate corrosion, adhesion and dry film thickness of existing coatings, which will indicate whether repair of an existing system is likely or not.
The main additions to the inspection section of the Standard are additional comments on inspection requirements, who is to carry out such work, and two suggested levels of inspection. The contractor’s QA protocols should require inspection of the work at every stage.
In addition, the owner may require additional independent third-party inspection to provide an additional level of confidence in the overall quality of the work. This additional inspection could be as little as some auditing through to complete inspection of the entire works. The Standard points out that, regardless of the extent of inspection, all defects are unlikely to be identified and some minor maintenance must be expected.
Two levels of inspection are suggested:
- Level 1 inspection for C1 and less critical C2 environments, which would rely only on the contractor’s quality control, other than a final inspection by the principal.
- Level 2 inspection for all other work and work where abrasive blasting or water jetting are carried out. An inspector engaged directly by the principal should carry out all necessary tests and checks.
The Standard points out that some tests will depend on the specific job. For example, surface salts would only be measured for previously rusted steel or in a contaminated atmosphere. Adhesion is normally only carried out before maintenance painting, or in the event of a problem or dispute.
Curing may be required before over-coating inorganic zinc, regardless of environment, but may not be necessary for other coatings. The specifier needs to be aware of the tests and their relevance, and specify accordingly.
The Standard recommends that reference areas be used on a job to provide a means to determine responsibility for coating failure between coating specification, coating material and workmanship. Reference areas should be located in an environment that is typical for the structure concerned.
All surface preparation and paint application work on reference areas should be carried out in the presence of representatives of all parties concerned—such as the owner, paint manufacturer, sub-contractors and the main contractor—who should all provide their agreement in writing when the reference areas are in accordance with the specification. All reference areas should be accurately documented, and clearly and permanently marked on the structure itself.
Coatings for Non-Atmospheric Environments
Metallic coatings have been dropped from the informative Appendix C covering coatings for non-atmospheric environments. There are also some changes to the terminology and thickness of some coating systems, largely in line with changes in the main part of the Standard. Two new generic systems have been included for these environments:
- Elastomeric urethane systems for potable water, soil and sewage
- Epoxy novalac for acid and alkaline splash environments
These conclusive remarks can be made regarding the 2014 version of the Standard:
- AS/NZS 2312 is the main guide for selection and specification of protective coatings in Australia and New Zealand, and continues to provide much useful information for specifier and applicator.
- The main change to the Standard is that metal coatings (galvanizing and thermal spray) are now covered by separate parts to the Standard.
- All sections in the Standard now relate solely to paint coatings.
- The 2014 version has updated paint systems and more information on selection, and should make selection easier and result in better paint coating specifications.
AS/NZS 2312 has been developed by Australian Standards Committee MT14/2 (Corrosion Protection of Steelwork). The members of sub-committee MT14/2 have developed the Standard and their valuable work is acknowledged.
- P. Golding, This conference, paper 128.
- R. A. Francis, "Development and use of an Australian atmospheric corrosivity Standard," ACA Annual Conference, November 2007, Sydney, Paper 037. Also R A Francis, "AS 4312: An Australian Atmospheric Corrosivity Standard," Steel Construction, Vol 45(1), December 2011.
- ISO 12944 Paints and varnishes – Corrosion protection of steel structures by protective paint systems – Part 5: Protective paint systems, International Organization for Standardization, Switzerland, 2007.
- AS 4312 Atmospheric corrosivity zones in Australia, Standards Australia, 2008.
- ISO 9223 Corrosion of metals and alloys – Corrosivity of atmospheres – Classification, determination and estimation, International Organization for Standardization, Switzerland, 2012.
- R. A. Francis, "A critical comparison between AS/NZS 2312 and ISO 12944 standards for coating selection," Corrosion and Prevention 2008, Wellington, NZ, Paper 076.
- AS 4361.1 Guide to lead paint management, Part 1: Industrial applications, Standards Australia, 1995.