Coastal and industrial sites do not fail the way inland sites fail. The structure is not immersed, it is not under constant load, and the coating does not wear out evenly. It fails at a cut edge, at a drilled hole, or under a fastener head — and it usually does so years before anyone expects it. That is why the useful question is not “which material is best” but “how do we specify the environment and verify what we were sold”.
This page is for EPCs, installers and asset owners specifying mounting structure for a site near salt water or heavy industry. It covers why coastal exposure is a different problem, what a salt spray test actually measures and what it does not, where coated steel really fails, how fasteners and dissimilar metals are chosen, and what belongs on the enquiry and on the delivery check.
Why coastal exposure is a different problem
A structure near the coast is not sitting in salt water. It is repeatedly wetted by airborne salt aerosol and then dried, and that cycle is what does the damage. Three consequences follow:
- Deposition falls off with distance, but not evenly. How far the site is from the water, which way the prevailing wind comes from, and whether anything shelters it all change the answer. “Near the coast” is not a specification.
- Time of wetness matters as much as salt load. A surface that dries quickly and is washed by rain behaves differently from a shaded underside that stays damp.
- The failure is local, not general. Coating thickness measurements taken mid-span will look fine while a cut edge three metres away is already rusting.
Start with the corrosion class, not the coating
Corrosivity of atmospheres is classified in bands — the common scheme runs from C1 (very low) through C5 (very high), with an additional extreme category for the most aggressive environments. The class is derived from the environment, not chosen from a catalogue, and it is the number that drives coating specification, fastener grade and inspection interval.
⚠️ What to do: state the corrosion class on the enquiry, and say how far the site is from salt water and which way the wind comes from. A supplier asked to quote “coastal” will quote a mid-grade solution and be technically compliant; a supplier told the class and the distance can specify against something.
What a salt spray test actually is
Salt spray testing is an accelerated laboratory corrosion test. A specimen is placed in a closed chamber and exposed to a continuous fog of salt solution at a controlled temperature; the result is normally reported as the number of hours until a defined amount of corrosion appears.
Two widely used references are ISO 9227 and ASTM B117. Both describe the apparatus and the operating conditions rather than a pass mark, and within them there are variants — a neutral salt fog, an acidified variant, and a copper-accelerated variant that is harsher and runs shorter. Which variant is relevant depends on the coating family and on what the buyer’s own specification calls for.
What a salt spray result does not tell you
This is the part that gets misused, and it is worth being blunt about it.
- It is a comparative test, not a life prediction. A result of “1,000 hours” does not mean 1,000 hours of service, or any multiple of it. Correlation between accelerated salt fog and real outdoor performance is weak, and it is weakest for the coated steel products the test is most often quoted on.
- It does not test the geometry that fails. Test coupons are flat and undamaged. Real structure is cut, drilled, bent and clamped, and those are the places corrosion starts.
- It does not cover the assembly. A coated rail bolted to a stainless fastener touching an aluminium clamp is a system, and the test does not evaluate the interface.
- Hours can be bought. A thicker or different coating raises the number without necessarily changing how the assembly behaves on a roof.
Used properly, the test is a quality-control check — evidence that a batch was produced the way previous batches were. Used as a service-life claim, it is misleading, and it is worth treating any offer framed that way with caution.
Coating, and what to actually measure
For steel structure the coating family and its thickness are the specification, and both should be stated in writing. Hot-dip galvanising to an applicable coating standard is the baseline for ground-mount and tracker work; zinc-aluminium-magnesium coatings give better cut-edge protection on thin sections, which is where it matters most.
On delivery, thickness is the thing that can be checked on site with a gauge, and it is worth doing on a sample rather than trusting the paperwork. What you are looking for is consistency across the batch and conformance to the figure that was specified, not a number that sounds large.
The cut edge and the drilled hole
Almost every early corrosion failure on a steel mounting structure starts at a place the coating was interrupted.
Reference table
Swipe horizontally to view all columns →
| Where it starts | Why | What helps |
|---|---|---|
| Cut ends | The coating does not cover the exposed steel at the cut | A coating family with cut-edge protection; cut before coating where the process allows |
| Drilled holes | Drilling on site removes coating and leaves a bare bore | Pre-drilled sections, or a specified touch-up procedure |
| Under fastener heads | Crevice, trapped moisture, and a different metal | Compatible fastener grade; isolation where metals differ |
| Scratches from handling | Damage that happens before the structure is even installed | Handling and storage that does not drag sections across each other |
Two of those four are site-handling issues rather than product issues, which is why the specification has to reach the people unloading the truck, not just the person placing the order.
Fasteners: the smallest item that fails first
Fasteners are the most common early failure on coastal projects, and they are the item most often left to whoever is assembling the structure.
Stainless fasteners are graded by environment: 304 is generally acceptable inland, while 316 is the usual specification for coastal and industrial atmospheres because the addition of molybdenum improves resistance to pitting in chloride environments. Where the environment is aggressive or the consequence of failure is high, the grade should be named explicitly on the enquiry rather than left as “stainless”.
Matching the fastener to the structure matters as much as the grade. A stainless fastener into aluminium makes the aluminium the sacrificial side of the couple, and the corrosion appears around the fastener rather than on it.
Dissimilar metals in practice
When two different metals are in electrical contact in the presence of an electrolyte — which salt-laden moisture certainly is — the less noble one corrodes faster than it would alone. On a mounting structure this is usually aluminium against steel, or aluminium against stainless.
The mitigation is isolation: a non-conductive barrier between the two, a sleeve where a fastener passes through, and a design that does not leave water sitting at the joint. What does not work is relying on the coating alone, because the coating is thinnest exactly where parts are clamped together.
What to put on the enquiry, and what to check on delivery
Reference table
Swipe horizontally to view all columns →
| Stage | What to state or check |
|---|---|
| Enquiry | Corrosion class; distance from salt water and prevailing wind direction; coating family and thickness; fastener grade; isolation method |
| Submittal | Coating specification in writing; salt spray test report and which variant was run, for comparison between batches |
| Delivery | Coating thickness on a sample; no bare cut edges or un-drilled field damage; fastener grade marked or documented |
| Installation | Touch-up procedure for any cut or drilled section; isolation fitted where specified; nothing dragging across finished surfaces |
| Handover | Photographs of the as-built interface; the coating and fastener spec recorded for later maintenance |
The last row is the one that pays for itself. Coastal structure gets inspected, and without the original specification there is nothing to inspect it against.
How this fits the rest of the structure decision
Corrosion is one input to choosing a structure, alongside strength, load and roof type. The material and coating comparison is set out in the structure selection guide; what the coating has to survive structurally is covered in the load calculation guide, because uplift at the roof perimeter is usually the governing case on exposed coastal roofs.
The products these specifications attach to are the roof mounting system and ground-mount racking. For the roof context, see commercial rooftop solar; what is committed when equipment does not perform is set out under service quality and claims.
Coating and fastener specifications are issued per project against the class and the site conditions given at enquiry. If you are pricing a coastal or industrial site, send the location and the distance from the water, and the specification can be stated in writing before the order.
Salt spray is not the only aggressive environment we get asked about. Livestock and poultry buildings produce ammonia-driven corrosion, which attacks a different set of materials and fails in a different place. If the site is agricultural rather than coastal, see ammonia corrosion on solar equipment.
BUYER CHECKLIST
Buyer considerations for this scope
Is the corrosion class stated, or just the word coastal?
Coastal is not a specification. Ask for the class on the enquiry together with the distance from the water and the prevailing wind direction, and require the coating family, its thickness and the fastener grade in writing. Without those there is nothing to check on delivery and nothing to inspect against later.
Is the salt spray report being used as a comparison or as a promise?
Ask which variant of the test was run and what the number is meant to demonstrate. As a batch-to-batch quality check it is legitimate; as a service-life claim it is not. The two are easy to confuse in a quote that shows nothing but a number of hours.
What happens to a section that gets drilled on site?
This is the most common gap between specification and reality. Ask for a touch-up procedure to be stated before installation, and for pre-drilled sections where the design allows it. A correct coating on a section that is then drilled through is not a coating specification.
PRACTICAL ANSWERS
Questions buyers ask about this
What does a salt spray test result actually tell us?
It tells you that a batch was produced consistently with previous batches, and it gives a comparative number between coatings run under identical conditions. It does not predict service life, because correlation between accelerated salt fog and outdoor performance is weak, and it does not test cut edges, drilled holes or the interfaces between different metals, which is where real structures fail.
Which corrosion class should we specify?
The one derived from the site rather than one picked from a list. State the class on the enquiry together with the distance from salt water and the prevailing wind direction, because deposition falls off with distance and shelter changes it. A supplier told only that a site is coastal will quote a mid-grade solution and still be technically compliant.
Stainless 304 or 316 for a coastal site?
316 for coastal and industrial atmospheres, because the molybdenum addition improves resistance to pitting in chloride environments; 304 is generally acceptable inland. Name the grade explicitly on the enquiry rather than writing stainless, and match it to the structure, since a stainless fastener in aluminium makes the aluminium the sacrificial side of the couple.
Where does coated mounting structure fail first?
Where the coating was interrupted: cut ends, holes drilled on site, under fastener heads where moisture is trapped, and scratches from handling. Two of those are site-handling issues rather than product issues, so the specification has to reach the people unloading and installing, not just the person placing the order.
EXPLORE FURTHER