The three variables: chloride, temperature and crevices
Stainless steel resists chloride water by keeping a thin chromium oxide film intact. Chloride ions attack that film at its weakest points, and once a pit starts the chemistry inside it turns acidic and feeds itself. Three things decide whether the film holds, and a grade has to be chosen against all three at once.
Chloride concentration is the first. Seawater is already a chloride solution; the brine that leaves a reverse osmosis membrane carries the feed's salt in a smaller volume, so its chloride runs well above the seawater it came from, and thermal plants concentrate the same way in their brine loops. A grade that is comfortable at the intake can be marginal in the concentrate line.
Temperature is the second. For every grade there is a temperature, in a given chloride solution, above which pitting starts, and a lower one above which crevice corrosion starts. A plant on the Red Sea or the Gulf sees warm feed for much of the year, and external metalwork in a Saudi summer reaches surface temperatures around 45 °C, which is what an uninsulated brine line in the sun runs at.
Crevices are the third, and the one specifications forget. Under a gasket, behind a flange face, in the root of a partial-penetration weld or beneath scale, the oxygen that repairs the passive film cannot reach the metal while the chloride concentrates. Crevice corrosion starts at lower temperatures than open-surface pitting on the same grade, so the joint, not the pipe, sets the grade. Stagnant water belongs here too: a train shut for maintenance leaves brine in dead legs with no flow.
PREN as the sorting tool, and what it leaves out
The pitting resistance equivalent number ranks grades by the three elements that resist chloride attack: PREN = Cr + 3.3 Mo + 16 N, all in weight percent. The PREN calculator runs that expression in the same engine the material pages use, and every figure in the table below came from it.
The figures are computed from the specification minimums in ASTM A240, the flat-product standard each grade is supplied against, not from a typical analysis. That is the safe basis for selection: the lowest PREN a heat may have and still be certified to the grade. The calculator's own worked example, a duplex composition of 22 Cr, 3.2 Mo and 0.18 N, returns 35.44 against the 34.14 the 2205 minimums give. Where the standard sets no nitrogen minimum, as for 316L and 904L, nitrogen enters as zero.
| Grade | EN number | Spec minimum Cr / Mo / N, wt.% | PREN at spec minimum | Corrosion class | Typical service position (industry practice, not a standard) |
|---|---|---|---|---|---|
| 304 | 1.4301 | 17.5 / none / none | 17.5 | Good | Not for wetted seawater or brine; dry-side equipment and buildings |
| 316L | 1.4404 | 16 / 2 / none | 22.6 | Very good | Intake screens, low-pressure pretreatment, product water |
| 904L | 1.4539 | 19 / 4 / none | 32.2 | Very high | Brine and concentrate lines where an austenitic is preferred |
| Duplex 2205 | 1.4462 | 22 / 3 / 0.14 | 34.14 | Very high | High-pressure RO trains, seawater piping at ambient |
| Super duplex 2507 | 1.4410 | 24 / 3 / 0.24 | 37.74 | Excellent | Brine reject, concentrate, warm seawater, high-pressure trains on warm feed |
PREN says nothing about crevice geometry, temperature or flow, so the same number can pass in a welded straight run and fail behind a gasket. It is an index of composition, not of the metal as delivered: a duplex weld with the wrong ferrite balance, or a 316L surface left with heat tint, corrodes below the number it was sold on. PREN sorts the candidates; the three variables and the fabrication route choose between them.
The calculator page gives common reference points of around 24 for 316, 35 for 2205 and above 40 for super duplex. Those are typical-composition figures and sit above the minimums in the table because a real heat carries more than the standard demands. When a specification asks for a PREN of 40 or more, the order has to say so and the mill certificate has to show it, because the 2507 minimum does not reach it.
Where 316L still works in a desalination plant
316L earns its place where chloride is at seawater level or below, temperature is at ambient and the geometry is open. In industry practice that is the intake side, meaning screens, strainers, low-pressure pretreatment piping and cartridge filter housings, and the product-water side, where the chloride is a fraction of the feed. It is the grade of the pretreatment building.
Its limits are the ones a coastal engineer already knows from why 304 stains and pits near the Saudi coast: 316L is 304 with molybdenum, which moves the PREN from 17.5 to 22.6 and buys a real margin in open seawater at ambient, but not one that survives warm brine under a gasket. 304 versus 316 stainless steel sets out that gap; the gap between 316L and the duplex grades is a different order of change.
Three habits keep 316L where it belongs. Drain its dead legs and low points at shutdown so it never sits in stagnant brine. Specify full-penetration welds and no bolted joints in any wetted concentrate line, so there are no crevices to fail in. Pickle and passivate after welding, so the heat-tinted zone beside each weld has its film restored; a 316L weld left with its tint is the first place a plant pits.
The L matters. Carbon at 0.03 percent maximum in ASTM A240 keeps chromium carbides from forming at grain boundaries during welding. Straight 316 offers no chloride benefit; at specification minimum both compute to the same 22.6.
Duplex 2205 or super duplex 2507: the step and what it buys
Duplex 2205 is where most seawater and high-pressure RO piping specifications land in industry practice, and the reason is in the table: 22 percent chromium minimum, 3 percent molybdenum and a nitrogen minimum of 0.14 lift the PREN to 34.14, half again the 316L figure. The mixed ferrite-austenite structure also gives duplex a yield strength well above the austenitics, which designers often take as a wall-thickness saving; that decision sits with the pressure code. Duplex pipe is supplied against ASTM A790, austenitic pipe against ASTM A312, and sheet and plate for both families against ASTM A240 or EN 10088-2. In the material master the austenitic grades sit at 8000 kg/m³ and the duplex grades at 7800 kg/m³.
The step to super duplex 2507 is a step in every element: 24 percent chromium, 3 percent molybdenum and 0.24 percent nitrogen, for a PREN of 37.74 at the minimums and above 40 on a typical heat. That is the grade for concentrate and brine-reject lines, for warm seawater, and for high-pressure trains on warm feed where 2205 would be working at its crevice limit. Its corrosion class in the material master is the top band, shared with super duplex S32760, a copper-and-tungsten-bearing variant used in the same positions.
The price of the step is not only the material. Both duplex grades want tighter control in the shop: heat input within a window, interpass temperature watched, and a filler chosen to keep the weld metal's austenite share up. Welded by a procedure qualified for the grade, they behave as the table says; welded like carbon steel, they do not. The lifecycle argument in carbon steel versus stainless steel for structural work in the Gulf applies with more force in a plant that cannot afford to open a brine line for repair: the grade is bought once and the shutdown is paid for every time.
Welding, the heat-affected zone and the surface after fabrication
A stainless steel corrodes at its weakest point, and after fabrication that point is almost always the weld or the surface beside it. The heat-affected zone of an austenitic can lose chromium to carbides if the carbon is too high, which is what the L grades prevent. The heat-affected zone of a duplex can go too far towards ferrite if it cools too fast, or form intermetallic phases if it stays hot too long, and either drops its pitting resistance below the parent metal. On every grade the weld leaves a heat-tinted oxide that is chromium-poor beneath, plus any iron picked up from carbon-steel tooling.
The remedies are procedural. Weld procedures qualified for the grade and the joint. Full penetration on anything wetted by brine, because a root gap is a crevice. Backing gas on the root of a pipe weld so the inside surface, the one the brine sees, is not oxidised. Then pickling and passivation once fabrication is complete, so heat tint and embedded iron are removed and the passive film reforms. A welding and fabrication shop that works stainless daily treats this as routine; the specification makes it a contract.
A carbon-steel support strap on a stainless pipe forms a galvanic couple in seawater; galvanic corrosion between aluminium and stainless steel covers the mechanism, and the answer here is to keep wetted materials to one family and isolate the rest.
Where Hidayath fits
Hidayath Metal Industries operates from three cities, ten locations and three factories in Saudi Arabia, with branches in Jeddah, Riyadh and Dammam; the group began in 1976. The material pages for 316L, 2205, 2507 and 904L carry the chemistry and the standards each grade is supplied against. The Industrial Engineering division fabricates pressure piping, pressure vessels and skid packages in these grades; the water and desalination page is the entry point, and the quality page sets out how the mill test report travels with a delivery when the specification calls for it.