Steel · Fire, corrosion & serviceability
Corrosion allowance & coating life calculator
First-year corrosion rate by category, long-term thickness loss per side with the ISO 9224 power law, remaining section and indicative galvanising life.
Standard: AS 4312 / ISO 9223 categories · ISO 9224 (D = r·t^0.523)
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Corrosivity category | C3 medium | |
| Design life | 40 | yr |
| Initial element thickness | 8 | mm |
| Exposed sides | 2 | |
| Galvanised coating | 85 | µm |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| First-year steel rate (upper bound of band) | 50.00 | µm/yr |
| First-year zinc rate (upper bound) | 2.10 | µm/yr |
| Steel loss per side D = r t^0.523 | 0.344 | mm |
| Remaining thickness | 7.31 | mm |
| Section loss | 8.61 | % |
| Indicative galvanising life | 40.48 | yr |
- ✓ Section loss ≤ 10%
- ✓ Galvanising outlasts design life
Method, formulas and assumptions
- First-year steel rate (upper bound of band) = CHOOSE(3,1.3,25,50,80,200,700) = 50 µm/yr
- First-year zinc rate (upper bound) = CHOOSE(3,0.1,0.7,2.1,4.2,8.4,25) = 2.1 µm/yr
- Steel loss per side D = r t^0.523 = 50*40^0.523/1000 = 0.34423 mm
- Remaining thickness = MAX(0,8-2*0.34423) = 7.3115 mm
- Section loss = 100*(8-7.3115)/8 = 8.6057 %
- Indicative galvanising life = 85/2.1 = 40.476 yr
- Rates are the upper bounds of the ISO 9223 category bands; use site data where available.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.