The assumption that costs plants money
Walk into most specification meetings and the reflex is the same: stainless steel is “proper,” plastic is “cheap.” For a huge share of nozzle duties that ranking is correct — stainless shrugs off mechanical abuse, holds a tight orifice, and looks like it will outlive the plant. But in a specific, common family of duties the ranking inverts. Put 316L stainless into a chloride-bearing or strongly oxidising stream and it can fail faster than the polypropylene part it was meant to replace. A plastic nozzle is not the budget fallback in those duties; it is the engineering answer.
This guide is for the buyer or engineer who has to decide between metal and plastic and suspects the usual “stainless is better” rule does not apply to their medium. We will look at exactly why stainless fails in these services, which plastics take over, and a simple medium check that settles most arguments.
Why 316L stainless fails here
Stainless steel does not resist corrosion by being inert. It resists corrosion because its surface forms a thin, invisible passive oxide film that self-heals in air. As long as that film stays intact, 316L is superb. The problem starts when the environment strips the film faster than it can reform.
Chloride ions are the classic offender. They punch through the passive layer at discrete points and, once through, attack the bare metal underneath. The result is not smooth, even thinning you could measure with a caliper. It is pitting — deep, localised holes — and, once a pit concentrates stress, stress corrosion cracking (SCC), where the part splits along a crack rather than wearing through. Both are inspection nightmares: they start small, hide under deposits, and announce themselves when a nozzle snaps off or a header starts weeping.
That is the core point: in these duties stainless fails catastrophically and quietly, while a plastic nozzle simply keeps running. The “cheaper” material turns out to be the durable one.
The chloride family: seawater, brine, hypochlorite
The mediums that trigger this are more common than people expect. Seawater and brackish cooling water carry chlorides by definition. Brine and salt solutions do the same. Hypochlorite dosing — sodium hypochlorite used for disinfection and bleaching — is aggressively chloride-bearing and oxidising at once, which is about the worst combination for 316L.
Then there are the process streams: pickling lines in steel finishing, many electroplating and anodising baths, flue-gas scrubbers that catch chloride, and any duty recovering or handling chlorinated intermediates. Ferric chloride, used in etching and water treatment, is a well-known stainless killer. If your stream contains free chlorides and the stainless is also under tension or welded (welds are SCC-prone zones), assume the metal is at risk until proven otherwise.
Acids and oxidisers where plastic wins
It is not only chlorides. Hydrochloric acid is hostile to 316L across most concentrations and temperatures; a plastic nozzle in PP or PVDF is the standard choice there. Hot, concentrated caustic (sodium hydroxide) also attacks stainless through a different mechanism and is routinely handled in plastic or a nickel alloy instead. Strong oxidisers — such as hypochlorite, chlorine dioxide, peroxides and many bleaches — strip the passive film and outpace its repair.
The practical pattern: when the medium is acidic, chloride-rich, oxidising, or a combination of those, reach for plastic before you reach for 316L. The exceptions (hot concentrated oxidisers that even plastic struggles with, or duties needing high mechanical strength at temperature) are narrow and usually obvious once you list the medium’s properties.
What “plastic” actually means: PP, PVDF, PTFE
“Plastic nozzle” is not one material. Three thermoplastics cover almost all corrosive nozzle duties, and they trade temperature ceiling against cost:
- Polypropylene (PP) handles the widest range of everyday acids, alkalis and salts at the lowest cost. Its practical ceiling is roughly 60 to 80 degrees C; above that it softens and creeps.
- PVDF buys temperature — about 120 to 140 degrees C — plus better resistance to oxidisers and halogens, with noticeably more mechanical strength than PP. It is the usual step up when PP is chemically fine but thermally marginal.
- PTFE has the broadest chemical resistance and the widest temperature window of the three, but it is mechanically weak and prone to cold flow (slow deformation under load). Reserve it for duties where almost nothing else survives.
So the decision is rarely “plastic or metal.” It is “which plastic, and is the temperature inside its window.” For the overwhelming majority of acid and chloride duties the answer is PP or PVDF, not stainless.
The trade you accept with plastic
Plastic is not free of downsides. Compared with metal it has lower mechanical strength, a lower temperature ceiling, and — the point most people under-rate — worse dimensional stability. A metal nozzle machined to a given orifice keeps that orifice. A plastic nozzle creeps: thermal expansion, sustained line pressure and fluid absorption all nudge the orifice, which shows up as spray-angle and flow drift, especially across a multi-nozzle header.
You live with that trade by running well below the material’s temperature limit, not over-tightening threaded joints, and treating plastic nozzles as scheduled replacement items rather than fit-and-forget parts. None of that is hard; it just has to be designed in. For a closer look at the material options and their limits, see the BoreJet plastic nozzles page.
How to decide: a quick medium check
When a buyer asks “which nozzle — plastic or stainless?” the answer almost always comes from four questions about the medium:
- Does it contain free chlorides, or is it an oxidiser or strong acid? If yes, lean plastic.
- What is the continuous operating temperature? If it sits above 80 degrees C and you need chemical resistance, PP is out and you are choosing PVDF or PTFE.
- Is there mechanical load — high pressure, abrasion, impact — that demands metal strength? If yes, plastic needs careful grading or a metal insert.
- Is the duty abrasive? If so, the orifice material matters more than the body, and hardened or ceramic inserts enter the picture.
If the medium is neutral, clean water at moderate temperature and pressure, stainless is usually right and cheaper over the life. If the medium is corrosive, plastic is usually right and more reliable.
Where a plastic nozzle is the safer engineering answer
To be blunt: in hydrochloric acid, hypochlorite dosing, ferric chloride, seawater and hot caustic, specifying 316L is the riskier call, not the safe one. A plastic nozzle in the right polymer will outlast the stainless, cost less, and fail by slow wear you can schedule around rather than by sudden crack. The “proper metal” instinct is correct for most of the plant and wrong for exactly this corner of it.
Common duties where this decision is mis-made
A few services trip people up because they look benign. Cooling towers and once-through seawater cooling read as “water,” but the chloride load is what attacks 316L, not the temperature. Electrolysis and chlor-alkali sidestreams carry both chloride and oxidiser. Many clean-in-place return streams run warm caustic and nitric acid in sequence, each hostile to stainless in its own way. And plating rinse lines look like water until you remember the bath chemistry they carry over. In all of these, a plastic nozzle is the conservative choice, not the economy one.
Validating the choice before you commit a header
You do not have to guess. The cheap validation is a coupon test: hang a small sample of the candidate polymer in the actual stream for a week and measure its weight and dimension change. If it holds size and mass, the orifice will hold too. For temperature-marginal duties, run the coupon at the peak temperature plus a margin, because creep only shows up near the limit. This step turns “we think PP is fine” into “PP survived our medium,” and it costs far less than replacing a stainless header that cracked in service.
If your medium is corrosive and you are not certain which polymer survives it, send the fluid properties — concentration, temperature, any solids — to our application team. We will point at the polymer that runs, not the one that looks robust on paper.