Stainless steel or galvanised steel: which to specify, and why
Stainless steel and galvanised steel are not two grades of the same product. They are two different answers to rust, and they fail in different ways. Galvanised steel is carbon steel wearing a zinc coat that is slowly consumed on its behalf. Stainless steel is an alloy that carries its own protection through the full thickness and keeps it as long as the passive film can reform. Almost every practical difference below follows from that one distinction.
Two ways of stopping rust
The comparison is easier once the mechanisms are on the table, because they explain the behaviour rather than describing it.
| Aspect | Galvanised steel | Stainless steel |
|---|---|---|
| What protects it | A zinc layer on ordinary carbon steel | Chromium in the alloy, forming a passive oxide film |
| How deep the protection goes | The coating only, typically about 20 micrometres per side on coil and up to 85 on batch work | Through the full section |
| Does it get used up | Yes, the zinc is consumed at a rate set by the environment | No, the film reforms as long as oxygen reaches the surface |
| Life expectancy | A predictable clock, decades in most inland air | A condition rather than a clock: it lasts until chlorides break the film |
| Base metal | Structural grades such as S235JR and S355J2 | Alloys such as 1.4301 and 1.4401 |
This is why the two materials answer different questions. Galvanised steel gives a known service life in a known environment, and how much zinc is present decides how long that life runs, which is set out in the comparison of hot-dip and electro-galvanised coatings. Stainless steel gives a material that needs no such reckoning, provided the environment does not defeat its chemistry.
What each one does when it is damaged
Damage is where the difference stops being theoretical. Both materials get drilled, scratched and knocked on site, and they respond in opposite ways.
Zinc is sacrificial, so bare steel exposed at a scratch or a drilled hole is still protected by the zinc around it. That protection reaches roughly 2 mm across in ordinary urban air and up to about 5 mm in industrial or marine conditions, where the current between the metals is stronger. A hole through galvanised sheet is therefore not automatically a defect, while a wide unprotected area is.
Stainless steel has no such mechanism. A scratch does not need one, because the exposed metal is the same alloy and repassivates within minutes. What does cause trouble is contamination: grinding dust or tooling that has touched carbon steel leaves free iron embedded in the surface, and that iron rusts and stains the stainless around it. The full set of causes is in the guide to why stainless steel rusts.
The practical rule is short. Protect galvanised steel from losing zinc, and protect stainless steel from picking up iron.
Chlorides settle most arguments
Inland, both work. The choice there comes down to budget, appearance and hygiene, and either material will see out the structure. Near the coast, or within reach of de-icing salt, the comparison changes shape.
Zinc does not stop working in chlorides, but it is consumed faster, so a coastal specification is really a question of how much zinc is present and whether a duplex system of galvanising plus paint is worth the extra step. The consumption itself is described in the guide to galvanised steel corrosion protection, and the zinc classes available on coil products in the guide to DX51D and galvanised steel. Stainless behaves differently. Chlorides attack the passive film locally, and once a pit starts it does not heal, which is why grade selection near the sea is not a preference and why the molybdenum bearing grades exist. The distinction between the two common grades is set out in the guide to 304 and 316 stainless steel.
Both routes start from the same input: the measured corrosivity of the site, not an impression of it. How the categories are defined, and how to work out yours without test panels, is covered in the guide to corrosivity categories C1 to CX.
Strength: the base metal is not the same metal
This one is regularly assumed the wrong way round, because stainless sounds like the premium option and premium sounds like stronger.
Galvanised structural products are made from ordinary structural steel, which yields at 235 MPa in S235JR and at 355 MPa in S355J2. Austenitic stainless 1.4301 has a specified minimum 0.2 percent proof strength of 210 MPa, below both, although its tensile strength of 520 to 720 MPa is higher and it work hardens considerably as it is formed. For a beam, a purlin or a gate frame designed on yield, galvanised carbon steel carries more load for the money.
Where stainless earns its place structurally is duplex, which combines high strength with chloride resistance, and that is a different conversation from the everyday choice between a galvanised section and an austenitic one. Weight is not a differentiator here: both materials sit near 7.9 kg per cubic decimetre, so a stainless section is not the lighter option.
Heat: zinc has a ceiling
Temperature is the limit people discover late, usually after specifying galvanised steel near something hot.
For continuous exposure, 200 degrees Celsius is the accepted maximum for a hot-dip galvanised coating. Between 200 and 250 degrees the zinc iron alloy layers keep protecting the steel, but the outer layer of free zinc can separate from them, and above 250 degrees that separation accelerates. None of this touches the steel underneath, yet a coating that peels has stopped being a coating.
Austenitic stainless steel works far beyond that range, which is why flues, ovens, exhausts and hot process equipment are stainless rather than galvanised. If the part will be warm to the touch in service, this single line often settles the specification on its own.
What happens in the workshop
Fabrication costs differ as much as material costs, and in opposite directions.
Welding galvanised steel destroys the coating along the weld and releases zinc fume, so the joint has to be ventilated during the work and the coating restored afterwards with a zinc rich product. The method is covered in the guide to welding galvanised steel. The advantage is that repair is genuinely simple and can be done on site with a brush.
Stainless steel welds cleanly but demands discipline: dedicated tools and brushes that have never touched carbon steel, and cleaning of the weld and heat tint afterwards so the passive film reforms across it. Skipping that step produces a joint that rusts while the rest of the structure does not, as described in the guide to welding stainless steel. Repair after the fact is mechanical work followed by passivation, not a coat of paint.
When the two meet in one structure
Mixed structures are normal rather than exceptional: galvanised frames with stainless fixings, stainless balustrades on galvanised posts, solar structures that combine both with aluminium.
Zinc is anodic to stainless steel, so in a wet joint the zinc corrodes to protect the stainless. Whether that matters depends almost entirely on the ratio of the two areas. A few small stainless fasteners in a large galvanised member spread that loss over a wide surface and are normally accepted practice. Reverse the proportions, with galvanised fasteners holding stainless sheet, and the whole galvanic load lands on the small item that also happens to be carrying the structure.
The rule that follows is easy to apply: make the smaller, more critical part the more noble one. In permanently wet or coastal joints, add isolation with a washer or sleeve regardless of the ratio, and choose the fastener grade for the environment rather than for the frame. Fastener grades are explained in the guide to stainless steel bar.
Cost, and what the money buys
Stainless steel costs more than galvanised steel because the protection is alloyed through the whole section rather than applied to its surface. Chromium and nickel are priced as alloying elements on world markets, which is why stainless prices move with nickel while carbon steel moves with iron ore, scrap and energy. Those mechanics are set out in the guide to what drives the price of steel and aluminium.
Over a service life the comparison narrows, but not always in the direction people expect. Galvanised steel in a C3 environment can run for decades before first maintenance, and when it does need attention the repair is cheap. Stainless steel in the same place needs nothing at all, yet if the grade is wrong for the chlorides present it can stain within a season, and that is not a maintenance issue but a specification error.
Choosing between them by application
| Application | Usual choice | Reason |
|---|---|---|
| Structural frames, carports, storage structures | Galvanised | Yield strength per euro, long life in C2 and C3 |
| Fencing, gates, agricultural work | Galvanised | Weather and handling, repairable coating |
| Roof and purlin systems | Galvanised coil products | Coil economics with a defined zinc class |
| Balustrades, architectural and visible metalwork | Stainless | Appearance that does not depend on a coating |
| Food, dairy, laundry and wash-down areas | Stainless | Cleanable, no coating to breach |
| Coastal and de-icing salt exposure | Stainless in a molybdenum bearing grade, or heavier zinc plus paint | Chlorides drive both the grade and the coating decision |
| Hot equipment, flues, exhausts | Stainless | Zinc has a continuous service ceiling near 200 degrees |
| Fasteners into galvanised structures | Stainless | Favourable area ratio, and the fixing outlives the frame |
Both materials from ULAMEX
ULAMEX stocks galvanised steel and stainless steel side by side in Zawada, Poland, and supplies both across continental Europe, which means an enquiry can be answered with the material that suits the job rather than the one that happens to be on the shelf. Sheet, tube, bar, profiles and fence systems are held in both material groups.
Welded load-bearing structures fabricated by ULAMEX carry CE marking to EN 1090-1 EXC2, with welding under an ISO 3834-2 quality system. A 3.1 inspection certificate under EN 10204 is available on request. The company has traded in steel and aluminium since 1988, that is 38 years.
There is no minimum order on stocked SKUs, and delivery within continental Europe usually takes 2 to 7 working days. Describe the application and the environment through our contact page, or see what the supply desk covers on the page of capabilities, and we will come back with a non-binding quotation.
Frequently asked questions
Will galvanised steel rust?
Eventually, yes, because the zinc is consumed rather than permanent. The coating corrodes slowly in place of the steel, and rust only appears once the zinc in that area is gone, which takes decades in ordinary inland air and much less in coastal or heavily industrial atmospheres. Small scratches and drilled edges are not a problem, since zinc protects exposed steel across roughly 2 mm in urban conditions and up to about 5 mm in industrial and marine ones.
Is stainless steel completely rust proof?
No, and the name promises more than the metal delivers. Stainless steel resists rust because chromium forms a passive film that reforms whenever it is damaged, but the film can be broken locally by chlorides, starved of oxygen in a tight crevice, or contaminated with free iron from carbon steel tooling. Where any of those happen, stainless can stain or pit while behaving perfectly on the surface next to it.
What is better for outdoor use, galvanised or stainless steel?
Inland, galvanised steel usually wins on value: it lasts for decades in C2 and C3 environments, and it carries more load per euro because the base metal is structural steel. Near the coast, in de-icing salt or in wash-down areas, stainless in a molybdenum bearing grade is the safer specification. The deciding input is the corrosivity of the site rather than a general preference for one material.
What two metals should not be used together?
Any pair far apart in the galvanic series will react in a wet joint, with the less noble metal corroding first, and the area ratio decides how serious it is. Zinc is anodic to stainless steel, so galvanised fasteners in stainless sheet are a poor combination, because a small item carries the whole galvanic load. The reverse, stainless fasteners in a galvanised frame, spreads the same load over a large surface and is normal practice. In permanently wet or coastal joints, isolate the metals with a washer or sleeve.
Can stainless fasteners be used in galvanised steel?
Yes, and it is the usual arrangement. The stainless fastener is small compared with the galvanised member, so the zinc loss around it is spread across a wide area and is slow enough to ignore in most environments. Choose the fastener grade for the environment rather than for the frame, and add an isolating washer where the joint stays wet or sits in coastal air.
Is stainless steel stronger than galvanised steel?
Not in the way the price suggests. Austenitic 1.4301 has a specified minimum proof strength of 210 MPa, which is below S235JR at 235 MPa and well below S355J2 at 355 MPa, the structural grades that galvanised sections are made from. Stainless has a higher tensile strength of 520 to 720 MPa and work hardens as it is formed, and duplex grades are genuinely high strength, but for a member designed on yield the galvanised carbon steel section is the stronger choice for the money.
Can galvanised steel be used at high temperature?
Only up to a point. The accepted maximum for continuous service is around 200 degrees Celsius. Between 200 and 250 degrees the zinc iron alloy layers continue to protect the steel while the outer layer of free zinc may separate, and above 250 degrees that separation accelerates. Flues, exhausts, ovens and hot process equipment are specified in stainless steel for this reason.
Which is cheaper over the life of a structure?
Galvanised steel is cheaper to buy and usually cheaper to own in inland environments, because the coating lasts for decades and can be repaired on site with a zinc rich product. Stainless costs more because chromium and nickel are alloyed through the whole section, and it pays that back where maintenance access is difficult, where appearance matters or where chlorides would consume zinc quickly. A wrong stainless grade removes the advantage entirely, since staining in the first season is a specification error rather than a maintenance item.