|STAINLESS STEEL (316)
|Mechanism of protection
|A protective oxide layer which self-repairs in the presence of oxygen, granting long-term corrosion resistance.
|A protective zinc coating is applied to the steel during manufacturing. When damaged, surrounding zinc cathodically protects the exposed steel.
|The stainless steel protective layer is more durable and is able to 'heal' itself. Stainless steel protection does not diminish with material loss or thickness reduction.
|Wide range of finishes available from very bright electropolished to abrasive linished. Appealing high quality look and feel.
|Spangles possible. Surface not bright and gradually changes to a dull grey with age.
|Aesthetic design choice.
|Very smooth and can be slippery.
|Coarser feel which becomes more apparent with age.
|Aesthetic design choice.
|May be re-used in new structures. Valuable as scrap after lifetime of structure - high recycling rate because of collection value.
|Carbon steel generally scrapped at end-of-life and is less valuable.
|Stainless steel is extensively recycled both within manufacturing and at end-of-life. All new stainless steel contains a substantial proportion of recycled steel.
|Heavy metal run off
|Significant zinc run off especially early in life.
|Some European highways have changed to stainless steel railings to avoid zinc contamination of environment.
|Indefinite, provided surface is maintained.
|Slow general corrosion until zinc dissolves, red rust will appear as zinc/iron layer corrodes, and finally the substrate steel. Repair required before ~2% of surface has red spots.
|Clear life-cycle cost benefit for stainless steel if extended life intended. The economic break-even point can be as short as six years, depending on the environment and other factors.
|Excellent for austenitic stainless steels with reasonable strength and deflection during fires.
|Zinc melts and runs which may cause failures of adjacent stainless steel in chemical plant. Carbon steel substrate loses strength and suffers deflection.
|Stainless steel offers better fire resistance and avoids risk of molten zinc if galvanised is used.
|Welding on site
|Routine for austenitic stainless steels, with care about thermal expansion. Welds can be blended into surrounding metal surface. Post-weld clean up and passivation essential.
|Carbon steel readily self-weldable but zinc must be removed because of fumes. If galvanised and stainless steel are welded together, any zinc residue will embrittle the stainless steel. Zinc-rich paint less durable than galvanising. In sever marine environments, a crusty rust can appear in three to five years and steel attack at four years/mm afterwards.
|Short-term durability similar but zinc-rich coating at joins required upkeep. In severe conditions, galvanised will get rough rust - even holes - and possible hand injury especially from unseen seaward side.
|Contact with damp porous material (e.g. wooden wedges) in a salty environment
|Likely to cause rust stains and crevice attack, but not structural failure.
|Similar to storage stain leading to rapid zinc loss and longer term due to perforation.
|Not desirable for either, but in long term can cause failure at base of galvanised poles.
|Can suffer tea staining and micro-pitting if not adequately maintained.
|Can suffer general zinc loss and subsequent corrosion of the steel substrate if not adequately maintained.
|Rain in open areas or washing in sheltered areas is required for both.