Choosing stainless steel for a metering system means making three separate decisions. The first is the grade: chloride content, acidity, temperature and whether the assembly is welded decide between the 304 and 316 families and their low-carbon L versions. The second is fabrication quality: even the right grade underperforms if weld heat tint is left in place. The third, which must be kept apart, is hygienic design. Material decides whether a surface resists corrosion; it does not decide whether that surface can be cleaned, drained and shown to be clean.
Meter type and sanitary connections for food liquids are covered on the sanitary metering skids page, and meter technology for chemicals on the chemical metering skids page. This page deals with the material itself.
Where stainless protection comes from, and where it fails
Stainless steel resists corrosion because a thin, chromium-rich passive oxide film forms on its surface whenever enough oxygen is available. That film does not protect under every condition. In small oxygen-starved areas such as mechanical joints, tight corners and incomplete or poorly finished welds, the surface can turn active and crevice or pitting corrosion can start. In aqueous service this localized attack is most often linked to chloride ions, and acidity and higher temperatures make it more likely. On a metering skid, gasket seats, threaded instrument connections, thermowells and weld seams are exactly such places.
304 or 316, and what the L grades change
The key difference between the two families is molybdenum: 316 types contain roughly 2 to 3 percent, 304 types none. Molybdenum makes it harder for chloride ions to break down the passive film locally, and higher chromium, nickel and nitrogen help as well. Where the product, a brine or a chlorinated cleaner brings chlorides into contact with the steel, the 316 family is the first candidate.
The letter L means low carbon: a maximum of 0.030 percent for 304L and 316L against 0.08 percent for 304 and 316. Standard-carbon grades held for several minutes at roughly 450 to 850 °C, as in a weld heat-affected zone, become prone to intergranular corrosion (weld decay) in corrosive environments; below 0.030 percent carbon this does not occur. Hence low-carbon grades for welded skid piping and meter bodies.
Grades are often ranked by the pitting resistance equivalent number (PREN), commonly Cr + 3.3Mo + 16N. Typical ranges are about 17.5 to 20.8 for 304 type (EN 1.4301) and about 23.1 to 28.5 for 316 and 316L types (EN 1.4401 and 1.4404). PREN ranks; it cannot predict whether a grade will resist pitting in a given application. That decision rests on actual product composition and temperature.
How common stainless grades are chosen
Grade | Why it matters in design | Effect on selection |
304 | No molybdenum; carbon up to 0.08 % | Mild, chloride-free service; weld decay risk assessed for welded parts |
304L | Carbon up to 0.030 %; no weld-zone intergranular corrosion | Preferred over 304 when welded; no added chloride resistance |
316 | About 2–3 % molybdenum; higher PREN than 304 | Chloride-bearing or more aggressive media |
316L | Molybdenum plus low carbon | Common for welded piping and wetted parts; checked against product data |
Higher alloys | Duplex, nickel alloys, titanium, tantalum offered for meters | Where 316L is not enough, with the manufacturer's guide |
Meter wetted materials need their own check
For meters with vibrating flow tubes, the material decision is separate from the piping decision. A Coriolis manufacturer's corrosion guide notes that piping selection usually ignores cyclic loading, that general corrosion tests miss localized damage, and that localized corrosion can start fatigue cracks in a vibrating tube. One hygienic Coriolis meter for food service has wetted parts in grade 1.4435, a 316L type: a sound starting point, not proof of suitability for every product. The chemical metering skids page covers this in more depth.
Weld heat tint, pickling and passivation
Discoloration next to a weld, known as heat tint, is more than cosmetic. Beneath it lies a chromium-depleted layer that lowers the corrosion resistance of the surface. The process that removes it is pickling, which typically uses nitric and hydrofluoric acid mixtures to take off a thin layer of metal. Passivation removes no metal; it uses an oxidizing acid such as nitric or citric acid to promote the passive film. Nitric acid on its own does not work as a pickling agent.
The practical upshot is that passivation alone does not remove heat tint. The recommended order is to descale and pickle off the tinted layer, clean off oil and dirt, and then passivate; for pipework carrying corrosive liquids, circulation pickling reaches the inside surfaces. A food technology fact sheet also calls initial passivation critical. In service, iron contamination, high chlorides, unrinsed cleaning chemicals and sudden temperature changes can still undo this work, as discussed on the CIP / SIP compatible systems page. A skid specification should therefore state how post-weld treatment will be done and documented.
Why a 316L skid is not automatically hygienic
Picture a skid where pipe, meter body and valves are all 316L, but the connections are standard flanges, the thermowell is threaded and the sample point sits at the end of a long branch. The material is right, yet the gasket gap, the threads and the stagnant branch are pockets that cleaning solution cannot reach. The skid resists corrosion but does not meet hygienic design criteria, which form a separate set of requirements:
· Cleanability: cleaning solution reaches every product-contact surface with enough flow, with no dead legs or crevices.
· Drainability: pipework and equipment empty completely by gravity.
· Surface finish: roughness of the flow path; one meter manufacturer, for example, offers a 0.8 µm Ra hygienic flow path option.
· Sanitary connections and seals: joint types that leave no gaps and seal materials that withstand the cleaning cycle.
· Food contact rules: materials, including seals and plastic parts, must not transfer harmful constituents to food and need compliance documentation.
Certification ties weld quality to hygienic design too. EHEDG states that fully welded components that fully meet hygienic design criteria, such as pipelines or a thermowell, can be granted a certificate without testing; the criterion is the weld and geometry, not stainless material as such. In the European Union, Regulation (EC) No 1935/2004 sets the framework for food contact materials, a further compliance layer separate from steel grade.
Stainless selection outside food service
In chemical, biofuel or specialty fluid service, product compatibility drives stainless selection rather than hygiene. A research report on biomass pyrolysis oils, for instance, notes that carbon steel cannot be used in pumps and lines for these acidic products, that 304L and 316L have been found suitable, and that gaskets and instruments must still be checked separately. Here concentration, temperature and impurities decide the grade, and where needed the move to a higher alloy.
Where TLY Enerji contributes
Depending on project scope, TLY Enerji provides engineering support in selecting measurement technology for the process conditions and can take part in equipment, valve and pump supply, system integration, installation, testing and commissioning. Checking wetted materials against product data and defining the material and surface treatment documents to request from suppliers can be part of that work. No food-industry experience or skid fabrication is claimed here; the supply boundary is agreed for each project.
Data for a material review
· Product composition, including chloride content, pH and impurities where known
· Operating and cleaning temperatures, and whether sudden temperature changes occur
· Cleaning and sanitizing chemicals and their concentrations
· Whether the application needs hygienic design or only corrosion resistance
· Whether a welded or flanged construction is expected
· Documents required: material certificates, post-weld surface treatment records, food contact declarations
Related pages
· Sanitary metering skids: Meter type, sanitary connections and what device certificates cover.
· Chemical metering skids: Meter technology and wetted material choice for chemicals.
· CIP / SIP compatible systems: How cleaning and sterilization cycles affect equipment.
· Hygienic transfer systems: Dead legs, slope and drainage design.
· Food-grade and sanitary liquid transfer facilities: Overall sector requirements.
Common questions on stainless steel selection
When should 316L be chosen over 304?
Two factors decide it. Chlorides in the product, cleaning chemicals or environment favor the roughly 2 to 3 percent molybdenum of the 316 family. A welded assembly favors a low-carbon L grade, which removes the risk of intergranular corrosion in the weld zone. Where both apply, 316L is a sensible starting point, still to be checked against actual product composition and temperature.
Does passivation remove weld heat tint?
No. Passivation removes no metal and only promotes the passive film. Heat tint and the chromium-depleted layer beneath it are removed by pickling, which takes off a thin layer of metal. Pickling and cleaning come first, passivation last, and a skid specification should state the method and how it will be documented.
Is a meter with a 316L body good enough for food service?
The material may suit many food liquids, but material alone does not make a meter hygienic. Connection type, mounting orientation, flow path finish and seal material do. One manufacturer states that its hygienic design approvals apply only with specific sanitary connections and housing option, so the same meter with flanges can fall outside that scope.
Is a grade with a higher PREN always safe?
No. PREN ranks grades by pitting resistance from their chromium, molybdenum and nitrogen content, but it cannot predict performance in a given service. Temperature, chloride concentration, pH and crevice geometry all change the outcome, so PREN is a screening tool and the final choice rests on product data and manufacturer guidance.