01Three mechanisms, three different rust patterns
The pattern tells you which one you have, which is why it is worth knowing before calling the supplier.
| Mechanism | What it looks like | Where it appears |
|---|---|---|
| Chloride pitting | Small dark pits, often with a rust bloom around them | Near sinks, welds, scratches, anywhere water stands |
| Free iron contamination | Patchy surface rust, brown and shallow | Anywhere a carbon steel tool or swarf touched the surface |
| Crevice corrosion | Rust in a line or a patch under a joint | Under silicone beads, clips, gaskets, stacked plates |
All three are about oxygen. The passive layer re-forms in air within hours; where there is no air — under water, under a bead, under a stacked plate — it cannot, and the layer breaks down.
02Chloride pitting
Chlorides attack the passive layer locally rather than uniformly. Salt, bleach, hydrochloric descaler and even strong dishwasher detergent all supply them. Once a pit starts it is self- sustaining: the pit interior becomes acidic, which accelerates the attack, which deepens the pit. A pit is often under 0.5 mm across when it first becomes visible, and its depth matters far more than its width — a shallow bloom the size of a coin is cosmetic; a pinprick through 5 mm of plate is a leak.
304 tolerates roughly 200 ppm chloride in continuous service at ambient temperature; a bleach solution or a strong sanitiser is well past that at the surface. 316L tolerates more because of the molybdenum, which is exactly what the PREN comparison is measuring.
Rinse and dry. Most pitting we are shown started with a sanitiser or a brine spill left wet over a weekend — and drying the surface costs nothing while changing the alloy costs real money.
03Free iron contamination
The most common cause of rust on brand-new stainless, and the most often misdiagnosed. A wire brush that has been used on mild steel, a grinding disc carrying carbon steel dust, a bench where swarf from another job was left, or a steel trowel — any of them can deposit free iron on a stainless surface. The iron rusts, the stainless around it is fine, and the customer concludes the material is wrong.
This is why fabrication shops keep stainless tooling separate from carbon steel tooling, and why a weld must be pickled and passivated afterwards: welding burns chromium out of the heat-affected zone, and grinding with a contaminated disc deposits iron back onto it.
| Source | How | Prevention |
|---|---|---|
| Wire brush / wheel | Used on carbon steel first | Separate stainless tools |
| Grinding disc | Iron particles embedded in the abrasive | Dedicated discs, marked |
| Work bench | Swarf from another job | Clean, covered surface |
| Handling | Gloves or trolleys with steel dust | Clean gloves, dedicated racks |
04Crevice corrosion
Two surfaces close together, water in the gap, no oxygen renewal: the classic. On a countertop it appears under a silicone bead that has partly failed, between a sink rim and the plate, between stacked parts in storage, or under a clip.
The answer is design rather than maintenance: welded joints instead of gasketed ones, a folded upstand instead of a bead at the wall, drainage holes at the low point, and letting parts breathe in storage rather than stacking them wet.
05Getting rid of it
For surface contamination, a mild acid gel or a proprietary stainless cleaner followed by a water rinse and a dry is usually enough, because the underlying material is intact. For pitting, cleaning the pit and re-passivating will stop the progress but will not fill the pit; if the pit is through, the part is scrap.
Order of operations when a complaint arrives: identify the pattern (pits, patches, or lines), find the chloride or the contamination source, then fix the cause. Replacing the top without finding the cause produces the same top in six months.
We do three things on our own parts that make this easier to answer. We pickle and passivate every weld and re-grain the machined edge after dressing, so the heat-affected zone is not the first place to fail. We keep stainless tooling separate from carbon steel tooling, so a grinding disc cannot deposit free iron. And we record the grade and the plate thickness against the part, so when someone sends us a photograph of a pit we can say whether the surface was 304 or 316L without guessing. On a 304 top the practical chloride limit is around 200 ppm in continuous service at ambient temperature; above that, the tolerance question stops being about the finish and becomes about the grade.
Delivery does not change any of this. Our lead time is 15 working days from a confirmed drawing, parts are inspected and packed on-edge with edges protected, and they ship with the inspection sheet — but no packing method prevents corrosion, and the warranty does not pretend otherwise. What the warranty covers is a defect we made, not a chloride somebody left on the surface.
304 vs 316L · Finishes · What the warranty does and does not cover