304 vs 316 stainless: how to choose for plant work

What is the actual difference between 304 and 316?

Both 304 and 316 are austenitic stainless steels built on chromium and nickel, and 316 adds molybdenum on top. Everything else that gets said about the two grades (corrosion behaviour, price, where each belongs) follows from that one change in the recipe.

Stainless steel does not resist corrosion because it is somehow inert. It resists corrosion because chromium reacts with oxygen to form a thin, self-repairing oxide film on the surface. Damage that film and it reforms, provided there is oxygen available. Chlorides are a problem precisely because they attack that film locally, in small spots, faster than it can heal. That is what pitting corrosion is. Molybdenum makes the film considerably more resistant to that local attack, and that is the whole reason to pay for 316.

When is 304 the right choice?

304 is the sensible default for stainless work that is not exposed to chlorides. It is easier to source, costs less, and in the absence of chloride exposure will give the same service life as 316 in most plant applications.

Typical work where 304 is normally specified: structural stainless, walkways and platforms in clean environments, dry ducting and chutes, potable-water and general process pipework, equipment guards and frames, and hopper and tank work handling dry or non-aggressive product. Choosing 316 for these out of caution is not a technical error, but it is usually money spent on a property the job will never call on.

When do you need 316?

Specify 316 where chlorides are present, where the environment is wet and warm, or where a failure would be expensive to reach. Temperature and concentration both matter. The same water that is harmless cold can pit the same steel when it is hot.

The Australian Stainless Steel Development Association makes the practical point that the chloride concentration is what governs: 316 performs well in ordinary tap water but will corrode rapidly in seawater, where chloride levels are far higher. The grade is not a guarantee, it is a range.

Situations that normally push a specification to 316:

• Coastal or marine air: airborne salt reaches plant a long way inland.
• Chlorinated washdown or CIP chemicals: common through food and beverage plant.
• Brines and salted product: anywhere salt is part of the process.
• Hot, wet service: heat sharply worsens what chlorides do.
• Buried, lagged or otherwise unreachable pipework: the cost of getting to a failure dwarfs the cost of the material.

Are 304 and 316 food grade?

Both are standard grades for food and beverage plant. The Australian Stainless Steel Development Association’s code of practice for stainless process plant in the food and beverage industries says plant is generally built from 304/304L or 316/316L. It treats 304 as the grade for lighter duties, suitable for constant contact with chlorides up to 200 ppm, and 316 for heavier chloride duties, up to 1,000 ppm, both at ambient temperature and neutral pH. Acidic product or higher temperatures bring those limits down.

The same code leaves the choice of grade to the purchaser, and asks for certificates showing that every material used as a product contact surface complies with the applicable specification. So the grade is only part of the answer: ask for the material certificates too.

What do the “L” grades mean, and do they matter?

The L in 304L and 316L means low carbon, and it matters most when the steel is welded. For fabricated work, which is nearly all plant work, the L grade is frequently the one you want.

The reason is what happens in the heat of welding. Carbon in the steel can combine with chromium and come out of solution at the grain boundaries near the weld. The chromium tied up that way is no longer available to maintain the protective film, leaving a band next to the weld that corrodes more readily than the parent metal. Lowering the carbon content reduces that effect. If a line is being welded and then put into corrosive service, this is not a detail worth economising on.

What happens to stainless when you weld it?

Welding temporarily removes the corrosion resistance you paid for, and the fabricator's job is to put it back. This is the part of stainless work that separates a good installation from one that starts weeping rust in a year, and it is almost invisible on a finished job unless you know what to look for.

Three things matter:

• Shielding the root. On pipework, the inside of the joint has to be purged with inert gas while it is welded. Without it the root oxidises into a rough, granular surface (known as "sugaring"), which corrodes and, in hygienic service, harbours product.
• Controlling heat input. Too much heat widens the affected zone and increases distortion.
• Cleaning up afterwards. The coloured oxide film left beside a weld, called heat tint, sits above chromium-depleted metal. On anything going into corrosive or hygienic service it should be removed, by pickling and passivation or by mechanical means, rather than polished over.

A stainless weld that has been left with heavy heat tint on it has not been finished, no matter how neat the bead looks.

How should you decide?

Work from the service conditions, not from habit or from what is in the yard. Four questions settle most jobs without needing a metallurgist.

1. Are chlorides present in the product, the water, the cleaning regime or the air?
2. How hot and how wet does it get, and for how long at a time?
3. Is this being welded? If so, consider the L grade.
4. What would it cost to get at this if it failed, and would the line have to stop?

Where the answers are borderline, the extra cost of 316 is usually small next to the cost of reaching a failed component inside a running plant. Where there is plainly no chloride exposure, 304 is the correct specification, not a compromise, and paying for 316 buys nothing.

Related reading

Grade selection is one half of specifying stainless work; who welds it is the other. See what a coded welder is, and what the certificate covers. If the installation falls inside a planned outage, see how to prepare for a plant shutdown. Shorter answers are on the frequently asked questions page.