Tea staining vs pitting: how to tell which you have
Tea staining is cosmetic. It shows as a brown or tan film, often streaked in the direction water runs, on the surface of an otherwise sound sheet. It appears first on the faces rain never reaches, under sills, behind columns and on the underside of handrails, because nothing rinses the salt off them. The name comes from the colour, which looks like a tea ring on a table.
Pitting is structural. A pit is a small, deep hole where the passive film has broken down at one point and the metal beneath has dissolved. Pits often hide under a spot of rust product and look like a dark freckle until you clean the surface and find a hole. They start at crevices, under gaskets and washers, in an uncleaned weld root, in the scratch left by a grinding disc, and around any fragment of carbon steel pressed into the surface during fabrication. Once a pit has started it is self-sustaining, because the chemistry inside the pit becomes more acidic and more chloride-rich than the surface around it.
Clean the area with a mild non-chloride cleaner and look again under good light. Discolouration that lifts and leaves a bright surface is tea staining. A dark point that stays, or that you can feel with a fingernail, is a pit. Tea staining is a maintenance problem; pitting is a specification problem, and it tells you the grade, the finish or the fabrication route was wrong for the exposure.
Why chloride is the problem and why the coast delivers it
Stainless steel resists corrosion because chromium in the alloy forms a thin, invisible oxide layer on the surface. That passive film repairs itself as long as oxygen can reach it. Chloride ions attack the film locally, and where the film cannot re-form fast enough a pit begins. The mechanism is the same in every grade; what differs is how much chloride, at what temperature, the film can withstand. What stainless steel is and how the passive film works covers the underlying metallurgy.
The coast delivers chloride in the air. Sea spray breaks into an aerosol of salt droplets that the onshore wind carries inland. The droplets settle on every surface and dry into salt crystals. Salt is hygroscopic, so in the humidity Jeddah and Dammam see for most of the year it pulls moisture from the air and forms a concentrated brine on the metal, even with no rain. Rain would rinse it off; on the Saudi coast rain is rare, so the salt film stays and thickens.
Heat makes it worse. Metal in direct Saudi sun runs far hotter than the air around it, and chloride pitting becomes more likely as temperature rises. The daily cycle of a hot dry afternoon and a humid night repeatedly concentrates and re-wets the salt film. A handrail in Riyadh sees the same sun without the salt, which is why the same 304 lasts there and stains in Jeddah.
The places that generate their own chloride, swimming pool surrounds, cooling towers and any plant handling brine or seawater, behave like the coast wherever they stand. Desalination and brine service has its own grade logic, and it starts well above 304.
The three levers: grade, surface finish, washing
Grade is the lever most people reach for first. Molybdenum and nitrogen raise the alloy's resistance to chloride pitting, and the pitting resistance equivalent number, PREN = Cr + 3.3 Mo + 16 N in weight percent, ranks grades on exactly those elements. 304 has no molybdenum; 316L carries a minimum of 2 % and its PREN rises accordingly. Moving from 304 to 316L is the usual coastal upgrade, and 304 vs 316 stainless steel sets out the full comparison, but it is not the whole answer. 316L will still tea-stain on a sheltered coastal face if it is rough and never washed.
Surface finish is the lever that costs least and is most often ignored. A smooth surface holds fewer salt crystals, dries faster and gives a pit fewer places to start. A coarse brushed finish is a field of parallel grooves that trap chloride; a fine hairline or a polished mirror surface is far easier to keep clean. Finish designations such as 2B, No. 4 and BA are defined in ASTM A480, and how the finishes differ explains which is which. Where a brushed look is required on a coastal building, specify the finer hairline finish and lay the grain vertically so water and salt run off rather than sit in the grooves. A mirror finish is the smoothest mechanical option.
Fabrication belongs with finish. A weld leaves heat tint, an oxide that is thicker and less protective than the natural passive film, and the area under it pits first. Pickling removes the tint and the chromium-depleted layer beneath it; passivation to ASTM A967 then restores the passive film chemically. Grinding with a disc that was used on carbon steel, or handling stainless on a carbon steel bench, embeds iron particles that rust on the surface and open a path for chloride. Pickling, passivation and electropolishing after fabrication remove those weak points before the metal is exposed.
Washing is the lever the building owner controls. Rain is the natural wash, and its absence is the reason coastal stainless in Saudi Arabia stains faster than the same grade on a European seafront. Rinsing external stainless with fresh water on a schedule, more often on the faces rain would never reach, removes the salt before it concentrates. A mild detergent and a soft cloth are enough.
What to specify for Jeddah and Dammam
The table ranks the grades a coastal specification usually considers. PREN is computed by the same calculation engine that runs the PREN calculator, using the specification minimum chromium, molybdenum and nitrogen for each grade as listed in ASTM A240, with no minimum taken as zero. These are spec-minimum values, the lowest PREN the grade is allowed to have, so a real heat on a mill certificate reads higher. The corrosion class is the qualitative rating from Hidayath's material master.
| Grade | EN designation | Cr min % | Mo min % | N min % | PREN (spec min) | Corrosion class |
|---|---|---|---|---|---|---|
| 430 | 1.4016 | 16 | 0 | 0 | 16 | Moderate |
| 304 | 1.4301 | 17.5 | 0 | 0 | 17.5 | Good |
| 316L | 1.4404 | 16 | 2 | 0 | 22.6 | Very good |
| 2205 duplex | 1.4462 | 22 | 3 | 0.14 | 34.14 | Very high |
| 2507 super duplex | 1.4410 | 24 | 3 | 0.24 | 37.74 | Excellent |
The EN designations in the second column are the equivalents listed in EN 10088-1, which carries the composition ranges for the European grade names. The ASTM and EN limits for a given grade are close but not identical, so the equivalence is a substitution with a warning attached, never an identity, and the mill certificate governs.
Read the table as a ladder. 304 at 17.5 sits close to 430, a ferritic grade rated only moderate, a reminder that 304 is a general-purpose grade, not a marine one. 316L at 22.6 is the sensible floor for exterior work in Jeddah and Dammam: handrails, cladding, signage and door furniture within sight of the water. 2205 duplex at 34.14 is for the harder cases: splash zones, jetties and marinas, components that see wetting by seawater rather than salt-laden air, and structural members where its higher strength lets the section come down. 2507 super duplex at 37.74 belongs to immersed and process service, and the marine and offshore page covers where that line falls.
One grade is missing from the table for an honest reason. 444, a molybdenum-bearing ferritic that the material master rates very good in chloride environments, has no composition row in the fact base yet, so its PREN is not printed here rather than taken from memory.
Grade alone is not a specification. For a coastal exterior the line on the drawing should read as four items together: 316L to ASTM A240 or 1.4404 to EN 10088-1; the finish, with a hairline or polished surface preferred over a coarse brush; pickling and passivation after welding; and a fresh-water washing schedule handed to the facilities team. Leave any one out and the other three carry the cost. Where the stainless meets aluminium or galvanised steel in the same façade, galvanic corrosion between the two metals adds a separation detail to the same drawing.
What 304 is still right for
Most of the stainless steel in Saudi Arabia is 304, and most of it is performing well. Indoors, in air-conditioned space, chloride is negligible and 304 is the right grade for lift cabins, kitchens, shopfitting, balustrades and wall cladding in every city including the coastal ones. Its formability, weldability and price are the reasons it is the default, and none of those change on the coast.
Outdoors inland, in Riyadh and the central region, 304 in a clean finish, washed occasionally, gives a long service life. Dust holds dew against the surface, so the same cleaning logic applies in a gentler form.
On the coast, 304 is still acceptable where it is out of direct spray, rain-washed or regularly cleaned, and where a little tea staining can be tolerated or polished out at intervals. A boldly exposed exterior face that gets washed by the occasional rain often looks better than a sheltered soffit a few metres away. What 304 should not be asked to do on the coast is sit in a crevice, under a rough finish, with a heat-tinted weld, unwashed. That combination produces the pits that give stainless steel a bad name.
Where Hidayath fits
Hidayath Metal Industries lists 304, 316L, 444 and the duplex grades in its material database and processes them at its branches in Jeddah, Riyadh and Dammam, three cities, ten locations and three factories in Saudi Arabia, a group that began in 1976. The finishing services that decide how a coastal installation ages are in-house: mechanical polishing to hairline and mirror, pickling, passivation and electropolishing after fabrication. The PREN calculator carries its result into a pre-filled enquiry.