Seawater and chlorine: where corrosion starts

The first corrosion path at a desalination plant is salt spray and seawater splash. Chloride penetrates the resin surface and spreads along the fibre-resin interface, and once water has opened that interface the glass fibre loses its protection, with strength and chemical resistance falling together.

The second source is the chlorine the process itself uses. Pretreatment often doses sodium hypochlorite to control biological fouling, so walkways, trenches and grating near the dosing point meet chlorine-bearing droplets and mist for years. Ordinary isophthalic polyester gradually loses surface resin in that environment and shows pale fibre.

Ultraviolet light is the third factor. A coastal plant has little shade, and the grating takes salt spray, UV and wet-dry cycling at the same time, so the resin chalks far faster than it would inland. The three effects together are the real duty, and judging by salt spray alone understates the requirement.

Offshore platforms and marine projects meet the same combined attack, and their practice is set out in FRP grating for marine environments

The intake: wet, slippery and exposed to waves

The platform around the intake is permanently wet, and the splash zone can be hit directly by the sea. The surface has to drain freely, with bearing bars running down the slope so water leaves instead of sitting in the hollows and between the cross bars.

Marine growth is specific to the intake. Shellfish and algae attach to the supports and to the underside of the grating, adding weight, blocking drainage and making inspection difficult. Bar spacing and support detail have to allow washing and scraping, with no inaccessible pockets left.

Wave and swell impact also count at the intake. Fixings cannot be chosen from panel weight alone; they must be checked against the storm condition and the maintenance load, and edge panels, corners and hatch covers are the first places to lift.

Pretreatment: chemicals and washdown

Floors in coagulation, sedimentation and filtration are washed often, and the grating catches polyaluminium chloride, sodium hypochlorite, sodium bisulphite and scale inhibitor. Each chemical affects the resin differently, so the selection list should carry the highest concentration and the highest temperature rather than the word corrosive.

Grating near dosing rooms and tank bunds must also survive short immersion during a spill. A bund can hold water, and the underside and cut edges of the panel will meet the chemical, so edge sealing and support protection have to be complete.

Walkways in pretreatment carry sludge and algae and are wet underfoot. A concave surface copes with clean wash water, but sludge fills the hollows, so these areas usually need grit and a fixed cleaning interval; the protection detail is covered under chemical plant grating protection

High-pressure pump and RO platforms

Platforms around high-pressure pumps and energy recovery devices see continuous vibration. FRP grating is light and does not amplify high-frequency vibration, but fixings loosen and hold-down plates grind dust from the bearing bars, so resilient washers and periodic re-torquing are needed.

The pump area also carries oil and the risk of high-pressure leakage. When a pump seal fails, water mist reaches the platform and the underside of the grating, while oil makes the surface more slippery. The panel must drain quickly and the surface should be grit for the oil-water mix.

Maintenance platforms above and beside RO membrane housings are often narrow. Cutting and openings should be finished at the factory and site cutting kept to a minimum; where a site opening is unavoidable, its edge must be sealed and the hole kept away from the middle of a bearing bar.

Slip resistance and drainage in wet areas

Most of a desalination plant is wet, so slip resistance cannot be chosen from dry-floor experience. Grit is more reliable with salt water and oil, concave suits frequently washed and lightly contaminated routes, and stairs and ramps should follow one standard based on the worst case.

Drainage matters more than the surface. There must be enough clearance and fall below the grating, with bearing bars running with the flow, so the panel does not become a tray. Standing water accelerates support corrosion and leaves salt crystals after a shutdown.

Junctions with handrails and kick plates trap water. Leave a drainage gap at installation instead of pressing the panel onto a solid surface, and use a transition support at corners so no step or dirt pocket forms.

Fasteners and supports

How the grating is fixed decides how long it lasts. Stainless clips can pit in chloride, carbon steel hold-down plates rust and stain the panel, so fasteners must be chosen with the grating and the support, with an insulating barrier at the contact.

Support spacing, bearing width and fixing positions come from the load table. A fixing too close to the panel edge concentrates stress on a few bearing bars, and edge distance and opening limits must follow the manufacturer's data, as set out in FRP grating fasteners and installation

Concrete and steel supports need different details. Chemical anchors suit concrete, clips or bolts suit steel beams, and both interfaces must allow small thermal movement instead of locking all four edges.

UV and the coastal atmosphere

UV is strong on a coastal site, and the resin surface chalks and fades. Good products include UV absorbers and a surface veil, but colour change still occurs over long exposure; it is an appearance issue, not structural failure.

The chalked layer hides cracks and exposed fibre, so clean before inspection. If surface resin is being lost continuously and fibre is visible, ageing has reached the structural layer and replacement should be assessed rather than painted over.

Salt spray also crystallises on the underside of the grating and on top of the supports. The crystals hold moisture and keep the area wet, which speeds fastener corrosion, so the washdown plan must cover the underside and the supports, not just the walking surface.

Maintenance and inspection

Inspection intervals should be graded by area: quarterly at the intake and dosing areas, twice a year at the pumps and membranes, annually on dry routes. The check covers cracks, whitening, exposed fibre, lost grit, loose fasteners and support corrosion.

Wash with low-pressure fresh water and a neutral detergent. A high-pressure jet held close strips grit and surface resin, chlorine residue keeps attacking the resin, and rinsing with clean water is needed afterwards. General wet-area practice is described under wastewater plant grating experience

Replacement should be planned in batches. Panels from one batch age together, and scattered replacement mixes old and new, creates joint steps and mismatched fixings; a shutdown replacement spreads the scaffold and downtime cost.

Selection and inquiry points

The inquiry should state the area, medium, concentration, temperature, washdown method, slip requirement and support conditions. Conditions inside a desalination plant vary widely, and the intake, dosing area and pump area cannot share one specification.

Resin, surface veil, edge sealing and fasteners should be specified together. Saying seawater resistant alone leaves the supplier unable to choose between isophthalic, vinyl ester and bisphenol A resin, or to set the veil grade.

If drawings are not fixed, give the span, support spacing, loads and chemical list first and ask the supplier to work back to panel depth and process. Choosing a panel before the supports usually means rework on deflection and fixings.

• List the medium and temperature separately for each area.
• State the washdown frequency and the detergent type.
• Use one slip class for wet areas and pump areas.
• List fasteners and support isolation as separate items.