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HYGIENIC DESIGN · MATERIALS · CORROSION
Key facts
- The three numbers chloride in mg per litre, surface temperature, pH range
- 304 envelope, commonly cited pH 6.5 to 8, chlorides to 50 mg per litre, up to 25 degrees C
- When 316 when chloride or temperature go beyond that envelope
- When duplex high chloride plus a need for strength or thinner section
- Surface finish Ra 0.8 micrometres is commonly specified on product contact stainless
- Ask at order mill certificate, surface finish figure with its method, passivation record
Choose the grade with the numbers, not the catalogue
Module 2 of Intermediate Hygienic Design covers grades, the four corrosion mechanisms, surface finish and topography, and elastomers and seals, with a practical application video that walks you through choosing a grade from your own water report. Nine hours, online.
Get the three numbers before you ask for a quote
Chloride first. It comes from two places: the municipal or borehole water you clean with, which is on your water report, and the product itself. Brines, cured products, sauces and anything with added salt carry their own chloride, and that figure is often far higher than the water.
Temperature next, and this is the one that gets stated wrong. What matters is the surface temperature, not the room. Clean in place at seventy five degrees, a steam jacket, a hot fill line: those set the duty. A circuit running at sixty two degrees against a seventy five degree set point is a different duty again, and usually one where somebody has been compensating with more chemical.
Then pH, from the product and from the cleaning chemistry both. Caustic and acid cycles move the surface between extremes every night.
Write the three at the top of the specification. Most specifications never do, which is why most of them get answered with whatever the fabricator had in the rack.
The commonly cited envelope, and how to quote it
| Grade | Commonly cited envelope | Where it fits |
|---|---|---|
| AISI 304 and 304L | pH 6.5 to 8, chlorides to 50 mg per litre, up to 25 degrees C (EHEDG Guideline 8, 2004) | Dry areas, low chloride wet areas, structures and frames |
| AISI 316 and 316L | Where chloride or temperature exceed the 304 envelope | Product contact in wet processing, brine, dairy, sauces, anything cleaned hot |
| Duplex | High chloride, and where strength or a thinner section matters | Tanks and vessels, high chloride process streams, structural applications |
Cite the document and the edition when you use these figures. Guidance differs between documents and between editions, and a supplier who checks and finds the citation wrong will discount the rest of your specification too.
The four mechanisms, and why three of them are really geometry
Pitting. Chloride breaks the passive layer at a single point. The pit then grows underneath its own corrosion product, so it is always deeper than it looks from the surface.
Crevice corrosion. Starts wherever two surfaces meet and oxygen cannot reach: under gaskets, under deposits, under lapped joints. This is a design fault wearing a corrosion label.
Stress corrosion cracking. Needs chloride, tensile stress and heat together. Hot chloride near a weld is the combination to look for.
Microbially influenced corrosion. Under a biofilm, in water that is standing still. Which means it starts wherever your drainability is poor.
Notice that crevice, stress and microbially influenced corrosion all begin with something the drawing decided: a lap joint, a weld, a low point that holds water. Upgrading the steel without fixing the geometry buys you a more expensive version of the same failure.
Surface finish, and why one number does not describe a surface
Ra 0.8 micrometres is commonly specified on product contact stainless, and it is a reasonable default. It is also not a full description. Roughness average tells you about height. It tells you almost nothing about shape, and shape decides whether a cell is sheltered or exposed. Four quite different surfaces can share one Ra figure.
So specify the method as well as the number: mechanically polished, electropolished, bead blasted, or as rolled. The method is what the fabricator actually controls.
The three documents to ask for, at order
Ask at the order, not at delivery. At delivery you have no leverage left.
- The mill certificate for the grade actually supplied, not the grade quoted.
- The surface finish figure and its method, per line, kept on file.
- The passivation record, after fabrication and again after any welding done on your site. Site welding undoes shop passivation, and that is where the first pits appear.
Those three cost the supplier almost nothing to provide at order and are close to impossible to obtain two years later when a surface starts blackening.
What to do on Monday
Pull your water report and write the chloride figure at the top of one page. List every surface on one line that runs above twenty five degrees. Mark anything in 304 that sits outside the envelope. Put those on the risk register with an interim control and a date. Then never approve another equipment quote that does not carry the three numbers.
Frequently asked questions
Is 316 always better than 304 for food equipment?
Not always, and not everywhere. 316 resists chloride better, which matters in wet processing, brine and hot cleaning. In dry areas, on frames and on structures, 304 is often the right choice and the money saved is better spent on geometry. Choose from chloride, temperature and pH rather than by default.
What chloride level can 304 stainless steel handle?
EHEDG Guideline 8 in the 2004 edition is commonly cited for chlorides up to 50 mg per litre, at pH 6.5 to 8 and up to 25 degrees C. Above that on chloride or temperature, move to 316. Quote the document and edition when you specify it.
What is the difference between 316 and 316L?
The L grades have lower carbon, which reduces carbide precipitation at the weld and the sensitisation that follows. For fabricated food equipment with a lot of welding, the L grade is the usual specification.
Do I need electropolishing?
It depends on the duty, not on fashion. Electropolishing improves cleanability and corrosion resistance and costs more. Specify the finish and the method against the process, and be able to say why. On many applications a controlled mechanical polish to Ra 0.8 micrometres is sufficient.
Our stainless is going black in patches. What is happening?
Blackening and rouging usually point to a passivation problem, often after site welding, sometimes with a chloride or temperature duty the grade was never chosen for. Establish the three numbers for that surface, then ask for the passivation record. Treat it as a material and geometry question together, because deposits and crevices often keep it going.
Choose the grade with the numbers, not the catalogue
Module 2 of Intermediate Hygienic Design covers grades, the four corrosion mechanisms, surface finish and topography, and elastomers and seals, with a practical application video that walks you through choosing a grade from your own water report. Nine hours, online.