Key takeaways
- 316L gives a PREN of about 25 against 18.6 for 304L, and that gap buys chloride pitting resistance only between roughly 20 and 60 °C.
- Powder bonds to a blasted and converted surface, not to the passive film, so blast to Ra 2.5–4 µm and skip passivation when the part will be coated.
- Hot-dip galvanising does not wet a chromium oxide surface; a drawing that says galvanised is a CRCA part, not a stainless part.
- A 70 µm powder film on each face closes a joint by 0.14 mm, so mask threads, earth studs and sliding faces before the part reaches the booth.
- Keep the film at 60–80 µm: at 120 µm the thermal strain mismatch with 316L is enough to craze the coating at the edges.
316L stainless corrosion and finish compatibility is a surface decision before it is an alloy decision. The same sheet leaves a plant with a passive film a few nanometres thick, a powder film sixty micrometres thick, or a primer and topcoat system over a conversion layer, and each route changes which number decides the part.
The number is the pitting resistance equivalent, PREN, calculated as chromium plus 3.3 times molybdenum plus 16 times nitrogen. For 316L at 16.5% Cr, 2.5% Mo and 0.04% N that gives 16.5 + 8.25 + 0.64 = 25.4. A 304L sheet at 18% Cr with no molybdenum lands near 18.6. The 6.8-point gap is what the nickel and molybdenum in the alloy buy, and it is the gap any coating has to be worth.
Most of the confusion in a finish specification comes from treating 316L as a painted material like CRCA. It is not. The passive film that makes the alloy corrosion resistant is the same film that makes powder struggle to bond, and the coating routes that work on mild steel do not all transfer.
What does 316L stainless corrosion resistance actually cover?
It covers chloride pitting, crevice attack and general rusting in wet service up to about 60 °C, and it covers nothing else. It is not a wear surface, it is not stronger than mild steel, and it does not resist hot concentrated acids without a higher alloy.
The mechanism is a chromium oxide film roughly 2–3 nm thick that reforms in air within minutes of being scratched. Molybdenum is what makes that film stable in chloride: it slows the breakdown of the passive layer at a pit site and helps it repassivate. Nickel stabilises the austenitic structure, which is why 316L is tough and weldable, and also why it work-hardens so quickly.
Three limits decide real parts:
- Pitting starts where chloride concentrates and the film breaks down. In a 6% ferric chloride test to ASTM G48 the critical pitting temperature is typically 20–25 °C for 316L, 10–15 °C for 304 and below 0 °C for a ferritic grade such as 430.
- Crevice corrosion starts at a gap under about 0.5 mm, where oxygen cannot reach and the chloride concentrates. A gasket line, a lap joint, a threaded insert pressed into a panel and a powder-coated lap are all crevices.
- Chloride stress corrosion cracking is a different failure and starts around 60 °C in austenitic grades. It is why an insulated hot pipe in a coastal plant fails at the insulation gap while the same grade survives cold seawater.
The ISO 9223 corrosivity categories are the right way to scope the environment rather than guessing from a postcode. C1 is heated indoor, C2 is unheated dry indoor, C3 is urban or industrial with moderate sulphur, C4 is coastal or heavily industrial, C5 is splash and salt fog, and CX is tropical marine or offshore.
Which finish route fits 316L, and what each one needs
The table is the working set. Anything in the last column that reads "not in-house" or "not possible" is a route change, not a process tweak.
| Finish route | What it adds | Preparation it needs | Ganesh limit or note |
|---|---|---|---|
| Bare, passivated | 2–3 nm passive film, no colour control | Degrease, pickle or citric passivate per ASTM A967 | Laser cut to 20 mm; passivation stated as a drawing callout |
| Blast and powder coat | 60–80 µm polyester, colour and abrasion resistance | Alumina blast to Ra 2.5–4 µm, acid etch, conversion coat | 16 ft × 16 ft blast chamber, 18 ft × 14 ft booth |
| Wet paint over primer | 25–40 µm epoxy primer plus topcoat, best edge coverage | Blast plus conversion coat, primer within 8 h | Wet booth in-house, DFT checked per coat |
| Hot-dip galvanised | A zinc-iron alloy layer | None that works on stainless | Not possible: zinc does not wet chromium oxide |
| Electropolished | Bright, low-Ra surface for cleanability | Dedicated polishing line | Not in-house |
| Anodised | A decorative oxide on aluminium only | Anodising line | Not applicable to steel |
Two rows in that table account for most of the enquiries we reject. Galvanising a stainless bracket is a specification error, because the zinc bath at 450 °C has nothing to react with. Electropolishing a pharmaceutical panel is a valid requirement that we do not run, and we say so before quotation rather than after.
Bare 316L: passivation is a process step, not a default
A laser-cut and welded 316L part is not automatically passive. Cutting and welding leave three things on the surface that must come off: free iron from tooling and fixtures, heat tint oxide from the weld, and a chromium-depleted layer under that tint.
Free iron is the one that surprises people. A carbon steel brush, a shared guillotine or a fixture that has held CRCA will embed iron particles in a stainless surface, and those particles rust in days while the surrounding alloy stays clean. ASTM A967 covers the chemical treatments that remove free iron and restore the film; citric acid is adequate for light contamination and nitric acid is used where the surface is heavily worked.
Laser cutting leaves a different condition depending on the assist gas. Cut with nitrogen, the edge is bright and essentially oxide-free, and a light passivation is enough. Cut with oxygen, the edge carries a dark oxide and a chromium-depleted zone a few micrometres deep, and that edge will stain and pit before the faces do. If the edge is visible, specify nitrogen cutting and pickle the edge before passivation.
The check is simple and does not need a laboratory. A water-break test shows a continuous film on a clean surface; a copper sulphate test shows free iron as a copper deposit within a minute. Both belong on the first article, not on every part.
Powder coating 316L: adhesion is the whole problem
A polyester powder film at 70 µm is about a hundred and forty times thicker than the 0.5 µm conversion layer beneath it, and it holds on because of that layer, not because of the alloy. On stainless the conversion layer is harder to form, because the surface has no free iron for a conventional zinc phosphate bath to react with.
There are two workable routes. The first is mechanical: blast with angular alumina at 100–120 grit to a profile of Ra 2.5–4 µm, which gives the film a key and exposes fresh metal. Glass bead is the wrong choice, because it peens the surface and closes the profile it was meant to create. The second is chemical: an acid etch that removes the passive film and leaves iron available, followed by a zirconium or silane conversion coating rather than a thick phosphate.
We run the first route on the seven-tank line, with the acid stage doing the etch, and prove it with cross-hatch adhesion to ASTM D3359 Method B on the first article. A part that passes the tape pull at 4B or better, with no coating lifted from the lattice, is ready for production.
Film thickness is where coated stainless goes wrong at the other end. Polyester expands about 40–60 × 10⁻⁶ per kelvin against 16 × 10⁻⁶ for 316L. Across a cure and cool cycle of about 175 K the mismatch is roughly 0.6% strain, which a 70 µm film absorbs without complaint and a 150 µm film does not: thick films craze at corners, lose impact resistance and chip when the panel is handled. Keep the specification at 60–80 µm and put the extra protection into the pretreatment, not the film.
Edge coverage deserves its own line on the drawing. A laser-cut edge is square and sharp, and powder pulls away from a sharp corner during cure. A 40 µm minimum on cut edges, checked on the first article by microscopy or a destructive section, is a realistic callout.
Wet paint, galvanising and the routes that do not apply
Wet paint is the better route when the part is large, when the edges are exposed, or when the substrate cannot take a 200 °C cure. A two-pack epoxy primer at 25–40 µm gives a barrier and a chemical bond that tolerates more surface variation than powder, and an air-drying primer can be used on thin panels that would distort in an oven. The trade is solvent handling, longer throughput and a finish that is softer than polyester.
Galvanising is simply not available. Zinc at 450 °C does not wet a chromium oxide surface, forms no zinc-iron intermetallic layer, and runs off the part. The temperature is also high enough to distort a thin panel that was flat when it went in. A drawing that specifies "316L, hot-dip galvanised" has two contradictory requirements, and the cheaper resolution is almost always to change the material to CRCA and keep the galvanising.
Where stainless meets a galvanised part, the galvanic couple matters more than most drawings admit. Stainless is cathodic to zinc, so the zinc becomes the anode and corrodes at a rate set by the cathode area. A large stainless panel bolted to a small galvanised bracket is the bad case: the bracket loses metal quickly in wet service. Keep the stainless fastener small relative to the galvanised part, and isolate the joint with a nylon washer where water will sit.
Masking and fit: the 70 µm that changes the assembly
Coating thickness is a dimension. At 70 µm per face, a bolted lap gains 0.14 mm of compressible film, a 6.5 mm clearance hole for an M6 fastener closes to about 6.36 mm, and a tab that was drawn with 0.2 mm of clearance will not enter its slot.
Mask these zones on every coated stainless part:
- Threads and threaded inserts, because cured powder in a thread is a tap and a rework ticket.
- Earth studs and a 10 mm ring around them, because the bond has to land on bare metal.
- Sliding and locating faces where the drawing calls for metal-to-metal contact.
- Weld zones on parts that are coated before a later weld, which is common on frames that are coated in sections.
Masking plugs and tape rated to 200 °C survive the cure without leaving adhesive. Anything rated lower leaves a residue that shows as a dull patch and fails the adhesion test at exactly the place the customer inspects.
What Ganesh runs for 316L parts
Stainless work runs on the 12 kW fibre laser with the 40 ft × 16 ft bed, which handles 316L up to 20 mm and cuts a 3,000 mm panel with its full hole pattern in one pass. Forming goes to the 200 T, 5-axis brake with the 4,450 mm bed and Delem control, which holds 316L to 10 mm and stores a programme per part number so a repeat release bends to the same angle. Welded assemblies run on the three CLOOS robotic MIG cells or at the 30-plus manual stations, with fixtures designed and built in-house.
Finishing is on the same site: the 16 ft × 16 ft blast chamber, the seven-tank pretreatment line, the 18 ft × 14 ft electrostatic powder booth, a wet paint booth and a curing oven, with DFT gauge and cross-hatch adhesion testing in-house. The plant holds ISO 9001:2015 and ISO 14001:2015, and the custom parts range covers brackets, panels, enclosures and welded assemblies in 316L, with PPAP and first article inspection available where the quality plan requires them.
The coating capability page sets out the booth sizes and the pretreatment sequence. If the part is a bent stainless panel rather than a coated one, the stainless bending guide covers the tooling and film-protection side, and degreasing before powder coating covers the stage that decides whether any of the coating work survives.
What to put on the 316L drawing
- Grade and standard: 316L to ASTM A240 or IS 6911, with the mill certificate required on the first lot.
- Finish route: bare passivated, blasted and powder coated, or primed and painted. Name one.
- Surface condition: 2B, BA or No. 4, and the Ra if the part is visible or cleanable.
- Film thickness with a minimum edge value, and the cure schedule by metal temperature rather than oven setting.
- Adhesion and corrosion test method, with the acceptance level: cross-hatch to ASTM D3359, pitting test to ASTM G48 where chloride service is real.
- Masking schedule, listing every thread, stud, sliding face and weld zone.
- Colour standard under a named illuminant, because RAL 7035 under sodium light and under daylight are two different approvals.
Post the drawing with the service environment written on it: chloride level, maximum temperature, whether the part is washed down, and the expected life. Those four lines decide whether 316L is doing the work or whether the coating is being asked to do a job it cannot do.
When 316L with a coating is the wrong answer
It is the wrong answer in a splash zone, in a chlorinated wash-down, above 60 °C with chlorides present, and anywhere the coating will be damaged in service. In those duties the alloy must carry the corrosion resistance on its own, which means either a bare passivated finish or a higher grade.
It is also the wrong answer when the environment is dry and indoor. A C1 or C2 interior does not need molybdenum, and the comparison of what 316L costs against the alternatives sets out where 304, 430 or coated CRCA is the better buy. The materials and grades hub collects the rest of the grade-selection guides.
Send the drawing with the finish callout and the environment to request a quotation; the finish route is decided before the price, not after it.
Frequently asked questions
Does powder coating 316L stainless improve its corrosion resistance?
Only where the coating stays intact. At a scribe or a damaged edge there is no sacrificial metal under the film, so attack becomes a crevice that grows sideways under the coating. In chloride service the alloy has to carry the corrosion duty, and the coating is a colour and abrasion layer.
Can 316L stainless be hot-dip galvanised?
No. Zinc at 450 °C does not wet a chromium oxide surface and forms no zinc-iron alloy layer, so nothing adheres. If the specification calls for galvanising, the part is CRCA and the drawing should say so.
What surface preparation does powder need on stainless?
An abrasive blast to Ra 2.5–4 µm with angular alumina, an acid etch to expose fresh metal, and a conversion coating. A standard zinc phosphate bath formulated for mild steel needs the etch stage adjusted, and the first article is proved by cross-hatch adhesion to ASTM D3359.
Should a stainless part be passivated before powder coating?
No. Passivation rebuilds the passive film that the blast is trying to remove, and adhesion drops. Passivate parts that stay bare, and coat parts that are blasted.
How thick should the powder film be on a 316L enclosure?
60–80 µm, with at least 40 µm on a laser-cut edge. Above about 120 µm the difference in thermal expansion between polyester and 316L puts the film in enough tension to craze at corners during cure and cooling.
Does salt spray testing prove anything on 316L?
Very little. Neutral salt spray to ASTM B117 produces no red rust on a stainless surface, so a 500-hour pass says nothing about pitting. Ask for a ferric chloride test to ASTM G48 or a service-specific immersion test.
Sources and further reading
- ASTM A240 — Chromium and chromium-nickel stainless steel plate, sheet and strip for pressure vessels and general applications · reviewed 3 Nov 2026
- ASTM A967 — Chemical passivation treatments for stainless steel parts · reviewed 3 Nov 2026
- ISO 9223 — Corrosion of metals and alloys: corrosivity of atmospheres, classification, determination and estimation · reviewed 3 Nov 2026
Have a 316L part that needs a finish decision?
Send the drawing with the service environment, the finish callout and the quantity. We will tell you whether the alloy or the coating is doing the work, and quote the route that matches.
Send a drawing