A salt plant corrodes everything it touches
Salt is produced by solar evaporation, by rock mining or by vacuum evaporation of brine. In each case the refining stage washes, dries, grades and packs the product — and in each case the building is exposed to salt in every form it can take.
Three forms, three mechanisms
Brine is the liquid form: it wets surfaces, it creeps by capillary action into joints and overlaps, and it leaves a salt deposit as it dries.
Salt dust is the airborne form: it settles everywhere, including on the top of purlins and in the roof profile, and it absorbs moisture from the air at quite low humidity.
Washdown water is the third: it dissolves the deposits and carries them to the floor and to the column bases, where the concentration is highest.
A building in a salt plant is therefore attacked from above by dust and from below by brine, and the two meet on the frame.
Standard coating selection does not survive here
Because salt is a food product, the instinct is to specify a food-grade coating. That is the wrong instinct: food-grade refers to contact with product, not to resistance to salt.
What is needed is a system selected for chloride exposure, applied to a surface preparation that is actually achieved on site, and detailed so that water cannot sit in a joint. The three failures in a salt plant are inadequate surface preparation, a coating chosen by label, and a detail that traps brine.
The roof is the most exposed surface
Salt dust on a steel roof is a permanent deposit. It is hygroscopic, so it stays damp longer than the surrounding air, and it is renewed every shift.
That makes the roof the first component to fail and the hardest to inspect. A shallow profile that sheds and a detail that lets rain wash the roof are worth more than a thicker coating.
Stainless is not automatically the answer
Chlorides attack common stainless grades, particularly in crevices where oxygen cannot reach the surface. Specifying stainless for a salt plant without specifying the grade and the detailing is how a plant ends up with a very expensive frame that fails at the fixings.
The practical answer is usually a coated carbon steel structure with the coating system matched to the exposure, and non-metallic fixings and separators where dissimilar materials meet.
Washdown and drainage carry the salt away or concentrate it
A floor with falls and gullies takes the brine away from the steel. A floor without them leaves it in place to concentrate as it dries.
That makes the floor the most important single element in the building's durability, and it is the element most often designed last.
What I want before drawings
- Whether the plant handles brine, dry salt or both.
- The washdown regime and the volume of water used.
- The exposure classification intended for the structure, and who has defined it.
- The floor falls, gullies and the route the brine takes out of the building.
- Whether any part of the structure is stainless, and if so which grade and why.
- The wind speed for the site in km/h from the applicable local code, not a round number.
Salt plants are the clearest case in food processing where the building's life is decided by the floor and the coating rather than by the frame. It is worth spending the design time there.
Written by Johnny Joe, who advises owners and project managers in Africa and South America on steel buildings. If this decision is live on a project, send the layout and the constraint that worries you and I will tell you what it changes.
Related: another salt that attacks steel · the hardest envelope in food processing · corrosion class as a selection input · a wet chemical building
