Every time we spec a fastener there is one question underneath it, and it is not which one is stronger. It is how long the corrosion protection has to last, and whether that part is allowed to be the thing that fails.
Zinc plated and stainless are not two grades of the same idea. They are two completely different strategies, and the difference decides where each one belongs.
Zinc does not protect the way most people think
Zinc plated steel is ordinary mild steel with a thin layer of zinc on it. The zinc works two ways at once. It is a physical barrier, and it is also sacrificial: zinc is less noble than steel, so in the presence of moisture the zinc corrodes in preference to the steel underneath. That second part is the clever bit, because it keeps working across a scratch. Bare steel showing through a small nick is still protected by the zinc around it. The Galvanizers Association of Australia demonstrates exactly that with test panels where deliberately exposed circles of steel stay clean.
The catch is in the word sacrificial. The zinc is being spent. It is a fuel tank, not a shield, and when it runs out the steel underneath is bare and starts rusting like any other untreated steel.
Stainless has no coating to run out. It is an alloy carrying at least about 10.5 per cent chromium, and that chromium reacts with oxygen to form an extremely thin passive chromium oxide film across the whole surface. Scratch it and, given oxygen, it reforms. There is no thickness budget to spend, which is why a stainless part does not have a coating life the way a plated part does. What stainless has instead is a set of conditions where the passive film cannot reform, and that is a different failure mode entirely.
So how many years is a zinc coating actually worth
This is answerable with published numbers rather than vibes, and the answer is shorter than most people expect.
Start with how much zinc is there. ASTM B633, the Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel, sets out four standard thickness classes running from 5 to 25 microns, matched to service conditions from mild indoor use up to very severe. General purpose plated hardware sits toward the thin end of that.
Then work out how fast it goes. AS 4312 sorts Australian locations into atmospheric corrosivity categories, and on a temperate surf coast it is largely a question of how far you are from the water. The Galvanizers Association of Australia publishes the first year corrosion rates behind those categories, drawn from AS 4312 and AS/NZS 2312.2. Divide one by the other and you get this.
| Category | Roughly where | Zinc loss, first year | 8 microns lasts | 25 microns lasts |
|---|---|---|---|---|
| C2 | Dry inland, rural | 0.1 to 0.7 µm/y | 11 to 80 years | 36 to 250 years |
| C3 | 1 to 10 km from surf. Most coastal towns and suburbs | 0.7 to 2.1 µm/y | 4 to 11 years | 12 to 36 years |
| C4 | 500 m to 1 km from surf | 2.1 to 4.2 µm/y | 2 to 4 years | 6 to 12 years |
| C5 | 100 to 500 m from surf | 4.2 to 8.4 µm/y | 1 to 2 years | 3 to 6 years |
| CX | Ocean front, the first 50 m | 8.4 to 25 µm/y | under a year | 1 to 3 years |
Two honest caveats on that table. The first year rate is the standard basis for a durability estimate and the GAA describes it as a deliberately conservative one, because zinc builds a patina that slows the rate down after the first year. And these are rates for an exposed flat surface. A fastener sitting in a crevice that holds salty water and never dries will do worse than the band suggests, not better.
The arithmetic checks out against the GAA's own worked example: 85 microns of hot dip galvanising in a C4 environment gives approximately 20 years to first maintenance in the worst case and up to 40 in the best. Same sum, much bigger number, because hot dip puts on ten times the zinc.
Where zinc plating is still the right answer
None of the above makes zinc plating bad. It makes it specific.
- It is cheap. For a screw that lives in a dry cavity, paying for stainless is paying for nothing.
- It fits the thread. Hot dip galvanising adds tens to hundreds of microns and distributes it unevenly, heavier on edges and thinner in recesses, so it is not usable on small threaded fasteners. Electroplating is the only practical zinc process at M4 and M5.
- Indoors it lasts effectively forever. The mildest service condition in ASTM B633 is explicitly the indoor case, and the C1 and C2 numbers above run to decades.
304 or 316, and why we do not just use 316 everywhere
Both are austenitic stainless. The practical difference is molybdenum: 316 carries roughly 2 to 3 per cent of it and 304 carries none. Molybdenum is what buys resistance to chlorides, and chloride is the thing that breaks down the passive film and starts pitting.
The usual shorthand is the pitting resistance equivalent number, PREN, calculated as the chromium content plus 3.3 times the molybdenum plus 16 times the nitrogen. That puts 304 at around 19 and 316 at around 24. It is a comparison index rather than a lifespan, but the ranking it gives is the right one.
In practice the line falls about here. Grade 304 handles wet, it handles rain, and it is comfortable anywhere that gets washed with fresh water reasonably often. Grade 316 is what you want once salt is allowed to concentrate: spray that dries and leaves the salt behind, crevices that never rinse out, anything that spends its life over salt water. That is the position the Australian Stainless Steel Development Association takes on grade selection for marine and sea front work.
Neither grade is maintenance free in a marine environment. Both will tea stain, which is a surface discolouration rather than structural rust, and both do better if they get rinsed.
What we actually put in the box
We do not pick one metal and apply it to the whole catalogue. Each part gets the grade its job needs.
| Part | Metal | Why that one |
|---|---|---|
| Anti Rattle Tow Hitch Shim Kit | 304 stainless | Shims live in the wettest, grittiest part of the tow bar and get abraded every time the tongue moves. A plated shim loses its coating to wear, not just to weather, and then blooms rust. |
| M8 roof rack channel nuts | 316 stainless | They sit in a channel that collects salt and grit and never dries properly, and nobody pulls a rack apart to inspect them. A seized channel nut wrecks the rail it is in. |
| Rod safety leash clip | 316 stainless | This one is not near salt water, it is in it. Textbook case for molybdenum. |
| Assembly screws across the range | 304 stainless, some with a black oxide finish | The finish is cosmetic. Our black BARRY30 screw is built from the same bare stainless screw with a blackening step added, so the corrosion resistance is coming from the steel and not from the colour. |
| Internal screws in dry assemblies | Zinc plated and black zinc passivate mild steel | Cheaper, and there is nothing there for the zinc to fight. |
The clearest example is a single twenty dollar kit. The 50A connector mounting kit ships with four screws and they are not the same screw. Two are 304 stainless self tappers, the ones that go into your canopy wall or battery tray and then face the weather for the life of the vehicle. The other two are zinc plated mild steel, because they live behind the bracket. Specifying stainless for all four would have added cost for no benefit on half of them.
The mistake that eats either one
Putting the two together in the wrong order will destroy a fastener faster than the weather ever would.
Stainless and zinc sit a long way apart on the galvanic series. Wet them, connect them, and you have built a battery: the zinc becomes the anode and corrodes, the stainless becomes the cathode and does not. How fast depends almost entirely on relative surface area. The American Galvanizers Association recommends an anode to cathode surface ratio of at least 10 to 1, and the illustration it uses is a zinc rivet in a stainless plate, which fails rapidly, against a stainless rivet in a zinc plate, where more area is affected but the depth of attack stays small.
Salt spray hours mean less than the marketing suggests
You will see hardware advertised with a salt spray figure, usually a few hundred hours under ASTM B117. It is worth knowing what that number is and what it is not.
ASTM B117 is a constant warm salt fog cabinet. It is a genuinely useful process control check: if a plating line drifts, the hours drop and you catch it. What it is not is a service life prediction, and the standard says so itself. Its significance and use section states that prediction of performance in natural environments has seldom been correlated with salt spray results when used as stand alone data, and that correlation should only be attempted where matching long term outdoor exposure work has been done alongside it.
The reason is that real corrosion is driven by wet and dry cycling, temperature swings, UV and varying chloride levels, and a cabinet that is permanently wet reproduces none of that. So 500 hours of salt spray does not translate to five years anywhere. Use the corrosivity category and the coating thickness instead. That is the calculation further up, and it is the one we run.
The rule we use
- Does it see weather? If yes, stainless. The coating maths does not work outdoors in this country.
- Does salt get to concentrate and dry on it, or does it live in salt water? 316.
- Wet but rinsed, and inland or a kilometre back from the surf? 304 is enough and it costs less.
- Dry, enclosed, never touched by weather? Zinc plated, and no guilt about it.
- Mixing metals? Put the stainless in the small part and the zinc in the big one, never the other way round.
If you are within a kilometre of the surf and you are buying hardware with no grade printed on it anywhere, assume it is thin zinc plate and assume you are replacing it inside two years. That is not pessimism. That is just the table.
Sources
- Galvanizers Association of Australia, Performance in Various Environments. Estimated first year corrosion rates of steel and zinc in Australia, developed from AS 4312 and AS/NZS 2312.2, plus the worked durability example for an 85 micron coating.
- Standards Australia, AS 4312:2019 Atmospheric corrosivity zones in Australia. Category definitions and the distance from coast mapping.
- ASTM International, ASTM B633 Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel. The four standard thickness classes and their service conditions.
- ASTM International, ASTM B117 Standard Practice for Operating Salt Spray (Fog) Apparatus. The correlation caveat quoted above is from its significance and use section.
- American Galvanizers Association, Dissimilar Metal Corrosion with Zinc. The 10 to 1 anode to cathode ratio and the rivet illustration.
- Australian Stainless Steel Development Association, 316: the first step up. Grade selection for coastal and marine exposure.
Everything in this article about our own products comes from our bills of material and supplier specifications, not from an estimate.