Life to first maintenance for hot dip galvanised steel to AS/NZS 4680:2025, and for duplex (HDG + paint) systems per AS/NZS 2312.2:2014 using the Altex / Carboline Generic Duplex Specifications 2026 (Job 48247). Rev 3.1 · 01 Sep 2026 · Estimates only — not a guarantee.
1 · Steel and galvanised coating
Steel section thickness
Designations per AS/NZS 2312.2:2014 Table 6.2; minimums per AS/NZS 4680:2025 Table 3.
Steel reactivity
Mean coating thickness to assess (µm)
85 µm
Defaults to the Table 3/4 mean minimum. Slide to model a measured or thicker coating (Table 6.2 Note 2).
2 · Corrosivity
Site guide (sets the category)
Descriptions from AS/NZS 2312.2:2014 Table 6.1 and AS/NZS 2312.3:2025 Appendix A. NZ site-specific classification is in SNZ TS 3404 (not in the knowledge base) — confirm for tenders.
Corrosivity category (ISO 9223)Micro-environment
Table 6.2 excludes micro-environments (Cl. 6.3.1). The one-step upgrade is a conservative judgement, not a figure from the Standard.
Results — HDG only
Designation
—
mean minimum
Life to first maintenance
—
years (min – max)
Durability class
—
average of min & max
Zinc corrosion rate
—
µm / year (Table 6.1)
Std mean minimum
—
µm
Parameter
Value
Source
Life to first maintenance = coating thickness ÷ first-year zinc corrosion rate, evaluated at both ends of the category's rate band (AS/NZS 2312.2:2014 Cl. 6.3.1 worked example). Repair or re-galvanise when about 2 % of the surface in any area shows rust (Cl. 6.3.2). Standard designations use the published Table 6.2 figures; other thicknesses are calculated and rounded.
1 · Galvanised base coat
Steel section thickness
Steel reactivity
Mean HDG thickness (µm)
85 µm
As galvanised — measured before any sweep blasting.
Surface preparation approach — GHB recommended order
Zinc removed by sweep blasting (µm)
10 µm
Deducted from the galvanised thickness before the HDG life is calculated. AS/NZS 2312.2:2014 Cl. 7.5.3.2: a correctly executed sweep blast removes no more than 10 µm (GAA AN 01.3: "around 10 µm"). Set 0 if the thickness above was measured after blasting; 20–40 models over-blasting.
Blast parameters assumed (Cl. 7.5.3.2)
Fine non-metallic abrasive 150–180 µm (ilmenite or garnet) · venturi nozzle 10–13 mm · pressure ≤ 275 kPa · nozzle 350–400 mm from the work at ≤ 45° · no dwell · measure coating thickness before and after (GAA AN 01.3). Altex specs state none of these — write them into the GHB issue.
Chemical adhesion route (GHB preferred). Zero zinc loss — the HDG life below is calculated on the full galvanised thickness. Steps: degrease SSPC-SP 1 / AS 1627.1 · remove all traces of dichromate/quench passivation (Altra~Etch PDS: thorough scouring with a synthetic pad or mechanical sander where present) · apply the adhesion primer within its windows (Altra~Etch: 15–20 µm DFT, never over 30 µm; topcoat with other finishes from 4 h @ 24 °C; maximum 90 days unweathered) · then the specified build coats. Caveat, shown for honesty: AS/NZS 2312.2:2014 Table 7.1 writes systems 2D–5I against sweep-blast preparation, and the six Altex 2026 specs do the same — so have Altex nominate the chemical-prep variant on the project specification, as they did on TG-2025-189791 (Glen Innes bridge: Altra~Etch 20 µm under Carboguard 636 XT). Paint-component durability shown from Table 7.1 is then indicative pending that confirmation.
2 · Paint system over the galvanising — Altex / Carboline 2026
Six specifications issued 29 July 2026 (TG‑2026‑194639 to 194644). Five map directly to AS/NZS 2312.2:2014 Table 7.1 systems; the sixth is a single-coat manufacturer option outside Table 7.1. Every system starts with Carbozinc 15‑625 spot repair (75 µm) on damaged galvanising only.
Coating stack (as specified)
Specification data
3 · Corrosivity and duplex settings
Site guide (sets the category)Corrosivity category (ISO 9223)
AS/NZS 2312.2:2014 Table 7.2 Note 1 states 1.5 for C5. The GANZ introduction to the Standard quotes 1.5–2.3 depending on environment; GAA GEN 14/1 quotes 1.5–2.7. Only 1.5 is in the Standard itself — leave at 1.5 for client documents.
Results — duplex system
HDG after preparation
—
µm remaining
HDG alone (post-prep)
—
years to first maintenance
Paint component
—
years to first paint maintenance (Table 7.1)
Duplex — corrosion
—
years to first maintenance
Durability class
—
duplex, corrosion basis
Gain vs HDG alone
—
years, conservative end
Parameter
Value
Source
Altex / Carboline products — data sheets and SDS summaries
Compiled from the Altex specification bundle (Job 48247): product data sheets (Q625, Q504, 379, Q1026, Q0870) and Chemwatch Safety Data Sheets issued under the NZ Health and Safety at Work (Hazardous Substances) Regulations 2017. These are summaries — the full SDS for every part in use must be current and on site before work starts. Source: Altex Coatings Ltd, Tauranga · altexcoatings.com · Altex 0800 258 390 · NZ Poisons 0800 764 766 · Chemwatch emergency 0800 700 112 (24/7).
Carboline maintenance guidelines (all systems)
From the Altex "Maintenance Guidelines for Carboline Coating Systems": wash all painted surfaces yearly with a proprietary cleaner and freshwater rinse, and areas sheltered from rain-washing quarterly with freshwater; treat moss, mould and lichen with diluted moss & mould killer and rinse; no abrasive rubbing; keep detergent run-off out of stormwater. Inspection schedule: year 1 full inspection and repair of any breakdown to the original specification; year 4 repeat; year 6/7 condition survey and repair; year 9/10 full survey by a qualified coatings inspector to set the maintenance plan beyond the initial system life.
Applicator QA record — set-up
Choose the specified system and fill in the job details; the record below rebuilds automatically. Cells with a dotted look are editable on screen (click and type), or print blank and complete by hand. The layout follows the Carboline Applicator Inspection Record (12 September 2018) from the Altex bundle, with GHB's galvanising handover and surface-preparation requirements added.
Paint system
Project
Client
Contractor / applicator
Item no. / description
Inspector
Select a paint system above (or on the Duplex tab) to generate the QA record.
Inputs
Mean HDG thickness as galvanised (µm)
85 µm
Paint system (optional)
Zinc removed by sweep blast (duplex only)
10 µm
Synergy factor
1.5 ×
All corrosivity categories
HDG: AS/NZS 2312.2:2014 Table 6.1 rates and Table 6.2 published lives for HDG390/500/600/900; other thicknesses calculated. Paint: Table 7.1. Duplex: factor × (HDG + paint), Table 7.2 Note 1; the Standard caps duplex estimates at ">25 years" because longer estimates increase the risk of failure. C1 uses the C2 paint column (Table 7.1 has no C1 column). CX has no Table 7.1 data. Duplex columns deduct the sweep-blast allowance from the galvanised thickness before calculating HDG life; Table 7.2 does not, so set 0 µm to reproduce it.
Figures verified against the uploaded documents
Each item below was checked against the documents in the GHB Duplex Coating project. ✓ = confirmed, ↻ = corrected in this revision, ⚠ = discrepancy to raise with the document owner.
Every Altex specification states "Durability of HDG600 in a C2 low / C3 medium environment is 40–60 years as per AS/NZS 2312.2:2014". Table 6.2 of that Standard gives HDG600 as >100 years in C2 and 40 to >100 years in C3; only the C4 figure (20–40 years) matches. This calculator uses the Standard's figures. Ask Altex to correct the notes before the specifications go to clients.
Not in the knowledge base
SNZ TS 3404 (NZ site corrosivity maps) · AS/NZS 2312.1 (paint systems, referenced by Cl. 7.5.1) · AS 4506 (powder coating over HDG) · "Duplex Coating Failures Associated With Galvanized Structures — 2026 Update" (Karakasch) is referenced in the project instructions but not uploaded as a file. Powder coating and generic non-Altex systems have been removed from this calculator because no uploaded document gives their durability.
GHB Galvanising is a member of Steel Construction New Zealand. Industry data in this calculator draws on Galvanizing Association of New Zealand (GANZ) and Galvanizers Association of Australia (GAA) publications — see the Sources & checks tab.
