Key takeaways
- Steel at a C5-M coastal site loses 80-200 micrometres in the first year, so a 20 micrometre scratch through to bare metal undercuts more steel than a 100 micrometre film protects.
- ISO 12944-2 puts a coastal installation in C5-M and a high-durability system needs a primer and topcoat, typically 200-240 micrometres total, not the 60-90 micrometre single-coat powder used inland.
- Chloride on the surface before coating is the most common cause of blistering; the Bresle test limit is 50 mg per square metre, and a month of coastal storage can exceed it.
- Powder pulls back at a sharp edge to roughly a third of the flat film, so a 2 mm edge radius does more for coastal life than another 20 micrometres of film.
- Ganesh runs seven-tank pretreatment, a 16 ft x 16 ft blast chamber and an 18 ft x 14 ft powder booth with DFT and cross-hatch records, and does not hot-dip galvanise in house.
A coastal installation is not an inland installation with more rain. Salt changes the chemistry of the failure. Chloride deposits on the surface, holds a film of moisture against the steel for longer each day, and drives corrosion under any coating defect at a rate that turns a scratch into a hole in a couple of seasons. The same powder, the same substrate and the same drawing can give ten years inland and three years on a site 400 m from the sea.
Coastal site corrosion is therefore a specification problem before it is a coating problem. The design has to name the corrosivity category, the pretreatment has to remove the chloride that arrived during storage, and the coating stack has to be thick enough at the edges, the fixings and the cut faces, which is where the film is thinnest and the salt sits longest.
This article sets out the category numbers, the failure order, the chloride and dew-point limits that decide whether the coating bonds, and the drawing fields that let a fabricator price the job honestly. The wider coating family is in the surface treatment hub, and the specific failure of outdoor powder is covered in why powder coating fails on outdoor enclosures.
What makes a coastal site different?
Salt, humidity and time. ISO 12944-2 classifies atmospheric corrosivity from C1, very low, to CX, extreme, and the marine band is C5-M. The classification is not a label for the view; it is a measured loss of steel. Standard specimens of low-carbon steel lose 80-200 micrometres in the first year at C5, against 25-50 micrometres at C3 and 50-80 at C4.
That number sets the stakes. A 100 micrometre powder film over steel that corrodes at 100 micrometres a year is not a ten-year system once the film is breached, because the exposed steel at the breach loses the same depth every year that the neighbouring film measures in total. The coating has to stay continuous, and the design has to stop the breach from starting at an edge or a fixing.
ISO 9223 adds a second measure that is more useful on site: chloride deposition, in milligrams per square metre per day. S0 is below 3, S1 runs 3-60, S2 runs 60-300 and S3 runs 300-1,500. A sheltered inland town sits at S0. A coastal industrial site commonly lands in S2, and a site with breaking surf or onshore wind can reach S3. The deposition class explains why two sites in the same city, 2 km apart, need different systems.
Where does coastal site corrosion start?
The failure order is consistent, and it is not the flat face.
Edges and cut faces. Powder and paint pull back from a sharp edge by surface tension. A 90-degree laser-cut edge typically carries 30-50% of the flat film, so a 120 micrometre specification can be 40 micrometres at the edge. Corrosion starts there, creeps under the film and lifts it. A 1-2 mm edge radius, called out on the drawing, is worth more than another 40 micrometres on the face.
Weld seams and spatter. A weld toe is a geometric discontinuity with a thin film over it, and spatter that is painted over leaves a shadow and a pinhole. Weld dressing on visible coastal parts is a coating decision as much as a cosmetic one.
Crevices. Two sheets bolted together with an unpainted gap hold water by capillary action. The gap stays wet long after the face has dried, and the corrosion product jacks the joint open. Sealant, a continuous weld, or a designed drain gap removes the crevice.
Fixings and dissimilar metals. A zinc-plated bolt against a powder-coated steel panel is a small galvanic cell once the film is damaged. Stainless fixings with isolating washers, or a coating system that includes the fastener, remove the couple. Coastal work should not use electroplated zinc hardware outdoors.
Trapped water in closed sections. A tube that is welded shut at both ends condenses water inside and rusts from the inside out. The first sign is a stain at a weld and a bubble in the film. A drain hole at the low point and a sealed top are drawing items.
Sun-facing horizontal surfaces. UV does not cause rust directly, but it chalks and erodes the topcoat, and a horizontal ledge collects salt that is not washed off by rain. The top of a canopy or a louver hood fails before the vertical panel beside it.
Coastal exposure against the coating system
The table links the exposure class to the steel loss, the deposition class and the film build, so the specification can be argued from numbers rather than from a colour card.
| Exposure | First-year steel loss | Chloride class (ISO 9223) | Typical system | Ganesh limit or note |
|---|---|---|---|---|
| C3, inland urban | 25-50 µm | S0-S1, under 60 mg/m²/day | Single-coat powder, 60-90 µm | Seven-tank pretreatment; DFT and cross-hatch per batch |
| C4, industrial | 50-80 µm | S1, 3-60 mg/m²/day | Powder or wet, 120-160 µm | 16 ft x 16 ft blast chamber; profile 40-70 µm |
| C5-M, coastal | 80-200 µm | S2-S3, 60-1,500 mg/m²/day | Primer plus topcoat, 200-240 µm | Wet booth and curing oven; DFT recorded at 5 points per part |
| CX, surf zone | 200-700 µm | S3 and above | Duplex galvanised plus coating | Galvanising not in house; we coat a supplied substrate |
| Cut edge | Film 30-50% of flat | Local cell | 1-2 mm radius, edge stripe | Laser-cut edge quality controlled; deburr before coating |
| Fixings | Galvanic couple | Local | Stainless 304 or 316 with isolation | Masking drawing controls the contact faces |
| Closed section | Internal condensation | Trapped | Drain hole plus sealed top | Drain position must be on the drawing, not left to the shop |
Why does chloride control decide the bond?
A conversion coating bonds to clean metal, not to a salt film. The seven-tank process exists to deliver that clean surface in a sequence: alkaline degrease, rinse, derust or acid clean, rinse, conversion coating, rinse, and a final rinse on demineralised water before drying. Each rinse is a stage where contamination is diluted and carried away, and each stage has a temperature, a concentration and a dwell time that the line controls.
Chloride is the contaminant that survives a casual wash. It sits in pits and under loose scale, and it is invisible once the part is dry. The Bresle method extracts it with a patch and measures conductivity, and the working limit before coating is 50 mg per square metre. A part that has been stored outside near the coast for a month can exceed that limit without showing rust, because the humidity has not yet been high enough to start corrosion.
Blast cleaning removes scale and chloride together, which is why a C5-M part is usually blasted even when the drawing does not ask for a profile. The 16 ft x 16 ft chamber takes a full enclosure or canopy frame, and the profile it leaves, typically 40-70 micrometres for a powder system, gives the film a mechanical key as well as a chemically clean surface.
Dew point is the other pretreatment limit. Blasting and coating should not proceed when the steel surface is less than 3 °C above the dew point, or when relative humidity is above 85%. Below that margin, moisture condenses on the part between stages, and the conversion coating forms unevenly or not at all. On a coastal site the dew point can sit within a degree or two of the steel temperature for hours after sunrise, which is why blasting is scheduled for the afternoon rather than the early shift.
What changes in the coating stack?
The first change is film build, and it is not a matter of adding powder to the same single coat. A thick powder coat applied in one pass can trap outgassing volatiles and produce pinholes, and it does not fix the edge coverage problem. The C5-M answer is a system: a blast profile, a conversion coating, a primer that wets the profile, and a topcoat that carries the UV and the colour. The total is usually 200-240 micrometres for high durability, defined by ISO 12944-2 as 15-25 years to first major maintenance.
The second change is edge treatment. Specify a radius, then specify that the edge is stripe-coated or that the powder is applied with an electrostatic wrap that reaches the edge. A 2 mm radius on a 2 mm sheet sounds trivial; it triples the edge film compared with a sheared square edge, because the coating sees a curved surface rather than a knife edge.
The third change is the back face. A panel coated on one side only is a one-sided barrier, and on a coastal site the uncoated face corrodes and pushes the coating off from behind. If the drawing says "coated one side", the part needs a reason, such as a concealed face inside a sealed box, and that reason should be written down.
The fourth change is the hardware. Stainless fixings, neutral-cure silicone at joints and isolating washers are small cost items that decide whether the first rust spot appears at a bolt head in year two. The coating system should name them, because the painter cannot.
What Ganesh runs for coated coastal parts?
Ganesh coats sheet metal assemblies at the Mumbai (Vasai) and Nagpur units, with seven-tank pretreatment ahead of an 18 ft x 14 ft electrostatic powder booth and a wet paint booth, both feeding a curing oven. A 16 ft x 16 ft shot blast chamber takes the large frames, canopies and enclosures that will not fit a standard cabinet, and DFT is checked with a gauge and adhesion with a cross-hatch test, recorded against the batch.
The five units hold ISO 9001:2015 and ISO 14001:2015, and the effluent treatment plant handles the pretreatment waste stream. Customers include Godrej & Boyce, TKE, Eros Elevators, Minda, Gunnebo and Indian Railways, and coated enclosures and frames for those programmes carry the same film-build records as the painted parts on a genset canopy.
The paint shop capability sets out the line sequence, and the shot blasting capability explains the profile and the chamber limits. What the plant does not do is hot-dip galvanise, run a salt spray cabinet for a customer's qualification, or warrant a service life; those belong to the galvaniser, the test house and the product owner respectively.
What belongs on a coastal coating drawing?
- Corrosivity category to ISO 12944-2, with the distance from the shoreline and the chloride class where known.
- Durability band and the years to first major maintenance.
- Substrate and surface preparation: blast cleanliness and profile range.
- Pretreatment sequence and the chloride limit before coating.
- Coating system by layer, with the dry film thickness for each layer and the total.
- Edge radius and the edge-coverage requirement.
- Faces to be coated and faces to be masked, including gasket lands and earth points.
- Fixing material and isolation method.
- Drain hole positions for every closed section.
- Inspection: DFT points per part, adhesion test frequency and the record that ships.
When is powder the wrong answer?
Immersion duty, splash zones and buried parts are not powder jobs. A part that sits in salt water or in the splash zone needs a barrier designed for continuous immersion, usually a heavy epoxy or a duplex galvanised system, and often a different substrate. Ganesh does not hot-dip galvanise, so a duplex specification means the substrate arrives galvanised and we coat over it after the appropriate pretreatment.
Stainless steel is the other route. A 316L part with a pickled and passivated finish removes the coating maintenance question entirely for many coastal fittings, at a material and machining cost that is usually justified on small brackets rather than on large panels. The trade-off is covered in 304 stainless corrosion and finish compatibility, which explains why 304 is not a coastal substitute for 316L where chloride is present.
For a painted steel enclosure, the economics usually favour the coating system, provided the drawing states the category, the edges are radiused and the chloride is controlled before the first tank. Send the drawing with the site distance and the expected life through the quotation route; a coastal job quoted without those three facts is a guess with a film thickness attached.
Frequently asked questions
What corrosion category is a coastal site?
Most coastal installations fall in C5-M under ISO 12944-2, the marine category, where steel loses 80-200 micrometres in the first year. Sites within a few hundred metres of breaking surf, or with salt deposition above 300 mg per square metre per day, sit at the top of that band or in CX.
Is C5-M powder coating enough on its own?
A single powder coat at 60-90 micrometres is a C3 system. C5-M at high durability usually needs a blast profile, a conversion coating, a primer and a topcoat totalling 200-240 micrometres, with edge and fixing protection treated separately.
How important is washing the part before coating?
It is the difference between a coating that blisters in two years and one that lasts ten. Chloride left on the surface draws water through the film by osmosis; the Bresle test limit before coating is 50 mg per square metre, and parts stored near the coast can exceed it in weeks.
Can I specify salt spray hours instead of a coating system?
Salt spray to ISO 9227 is a screening test, not a service-life prediction. A 480-hour pass does not mean five years at a coastal site, because UV, chloride cycling and mechanical damage are absent from the cabinet. Specify the system, the DFT and the pretreatment, then use salt spray as a check.
Does Ganesh hot-dip galvanise?
No. Ganesh pretreats, blasts and coats in house, and can work to a drawing that specifies a galvanised substrate supplied by others. Duplex systems and immersion-zone work belong with a galvaniser, not a powder line.
Sources and further reading
- ISO 12944-2:2017 - Corrosivity categories and durability ranges for protective paint systems · reviewed 3 Nov 2026
- ISO 9223:2012 - Corrosion of metals, classification of corrosivity of atmospheres by deposition rate · reviewed 3 Nov 2026
- BIS - IS 513 cold reduced steel sheet and IS 4759 zinc coatings on steel · reviewed 3 Nov 2026
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