THE LASTING DECISION
Stainless Steel vs Galvanised — Understanding the true cost of corrosion resistance and design life.
Whole of Life Cost Comparison
Two papers provide guidance about the durability and life expectancy of stainless steel. The first study provided Gold Coast City Council with information on materials degradation and the justification to consider stainless steel from a perspective of life cycle costing.
Research Study — Griffith University & Gold Coast City Council
Whole of Life Cost Comparison and Cost Benefit Analysis for Steel Structures Constructed in the Foreshore Zone. Final Report. Jordan Cocks, 2009.
A study undertaken at Kure Beach, North Carolina has shown the strength of stainless steel’s corrosion resistance. Specimens of grade 316 and 304 stainless steel were exposed for 15 years, situated 250m from the mean high tide mark.
During this time the average corrosion in mm/year was less than 0.000025 for stainless steel (Housaka, 2001). This makes stainless steel almost 700 times more resistant to corrosion than HDG steel (Cocks, 2009).
Life Cycle Cost (LCC) Findings
The LCC of stainless steel structures are comparable or less than mild steel structures:
- A HDG coating will cease to exist after 14 years
- Paint systems do not provide protection if the coating is damaged
- The only maintenance required for stainless steel is washing
- Stainless steel structures have an extremely long design life
Based on the cost/benefit analysis, stainless steel is a viable option for structures with a design life over:
- 16 years for HDG steel
- 11 years for painted steel
- 23 years for duplex coated steel
Durability & Life Expectancy
British Stainless Steel Association
The most important corrosion consideration with stainless steel is pitting. Micropitting is manifested as surface rust staining and is generally considered unacceptable for a material basically selected for its aesthetic appearance and corrosion resistance.
Factors Affecting Durability
- Stainless steel grade
- Environment
- Surface finish
Chloride ions are the most aggressive environmental hazard when assessing the pitting corrosion risk. Marine and coastal sites tend to be the most aggressive and so give the lowest projected durability. Acid conditions, sometimes found in industrialised atmospheres, are also aggressive. It is also important to consider the effects of local ‘micro climates’ that may influence how aggressive an environment is.
Estimated Pit Penetration Times
The pitting life predictions shown in the table are based on a linear pit growth rate model.
| Location | 304 Grade Years to Penetrate 1mm |
316 Grade Years to Penetrate 1mm |
|---|---|---|
| Marine | 145 | 260 |
| Semi-industrial | 135 | 525 |
| Rural | 770 | 1,200 |
The marine and rural site samples had ‘mill’ finishes and the semi-industrial site results had dull polished finishes. The 1mm-perforation times for the 304 type are consistent with the projected design lives of most buildings.
Further Information
For more details, please refer to the British Stainless Steel Association (BSSA) — Durability and life expectancy for stainless steel in external environments.