In this article, I will introduce what 304 and 316 stainless steel are, how the molybdenum in 316 changes chloride service, when each grade is specified, and what a buyer should send before a mill quote.
What are 304 and 316?
304 and 316 are both austenitic stainless steels. They stay non-magnetic in the annealed condition in ordinary shop practice, they form well, and they are the two grades most general fabrication starts from. The chromium in each grade lets a thin oxide film reform on a clean surface and slow rust. The nickel keeps that structure austenitic. Neither name is a coating. The corrosion resistance is in the alloy.
Under ASTM A240, type 304 is UNS S30400, with chromium about 18 to 20 percent and nickel about 8 to 11 percent, and carbon 0.08 percent maximum. Type 316 is UNS S31600, with chromium about 16 to 18 percent, nickel about 10 to 14 percent, and molybdenum 2 to 3 percent. The low-carbon L grades, 304L (S30403) and 316L (S31603), keep carbon at 0.030 percent maximum. EN 10088 names often set beside these are 1.4301 and 1.4307 for the 304 pair, and 1.4401 and 1.4404 for the 316 pair. Those are trade equivalents, not an automatic substitution. The standard printed on the drawing is the one to quote.
How the molybdenum difference works
Chlorides, from salt, coastal air or a chloride wash, attack the passive film at weak points and start pits. Molybdenum makes that film harder to break in chloride water. A pitting-resistance number built from chromium, molybdenum and nitrogen is only an index. It is not a guarantee that a tank will last a stated number of years. It does explain why a drawing moves from 304 to 316 when the service is chlorides, and why it does not move for a dry indoor part.
The move is a corrosion choice, not a strength choice. In the annealed flat-product condition the two grades are ordered for similar minimum strength. Read the minimums on the named standard rather than treating 316 as the stronger plate. 316 also does not cover every acid. Hot sulfuric acid, hydrochloric acid and some reducing media need a different alloy, such as a duplex grade or a nickel alloy, and the drawing should already name that grade.
What the grade choice protects
The grade is chosen for the media and for the weld, not for the product photo.
The surface in chloride water
304 is the usual grade for indoor equipment, dry structure and many food-plant parts when the drawing says so. 316 is the usual step up for marine fittings, coastal plant and lines that are cleaned with chloride solutions. Stagnant seawater and tight crevices can still attack 316. A severe marine duty is a reason to read the specification again, not a reason to assume 316 is immune.
The weld on an L grade
If austenitic stainless is held in the range where chromium carbides can form, often cited as about 425 to 815 °C, chromium can leave the grain boundary and the weld area can corrode. That is sensitization. The L grades limit carbon so fewer carbides form during a normal weld. If the drawing says 304L or 316L, the enquiry should keep the L. A higher-carbon 304 or 316 is a different order.
The standard that belongs to the form
Sheet and plate are often ordered to ASTM A240. Austenitic pipe is often ordered to ASTM A312. Mechanical or sanitary tube may be ASTM A269. Bar may be ASTM A276. The finish, such as 2B, BA or No. 4, is a separate line. A 316 sheet in 2B and a 316 sheet in No. 4 are not interchangeable on a visible part.
When 304 is the grade to order
Order 304 or 304L when the drawing names it and the part is not a chloride service. Indoor equipment, dry structural parts, kitchen equipment in fresh water and many general fabrications are specified this way. There is no benefit in writing 316 on the enquiry to sound safer if the end user designed the part in 304.
Order 316 or 316L when the drawing names it, or when the agreed service is salt, coastal air or a chloride cleaner and the end user has accepted that grade. Do not downgrade a 316 drawing to 304 to save cost unless the end user agrees in writing. Do not upgrade a 304 drawing to 316 on the same assumption.
304 versus 316 on the same drawing
The two grades can share a thickness, a finish and a form and still be different orders, because the chemistry is different. A mill test certificate for 304 will not satisfy a 316 line, and the reverse is also true. Compare offers on the UNS or EN number, the product standard, the size and the finish, not on the word stainless.
Other grades sit beside this pair for duties it does not cover. 321 adds titanium for a higher temperature. Duplex 2205 is a different structure, used where strength and chloride resistance are both on the drawing. 904L is a higher-alloy austenitic grade for media that 316 is not selected for. If the drawing names one of those, the enquiry should not be folded back into 304 or 316.
FAQ
These are the questions that usually decide whether the first offer matches the drawing.
Is 316 always the better grade?
No. It is the usual step up for chlorides. On a dry indoor part it is a different, usually more expensive, alloy, not a better 304. The drawing picks the grade.
What does the L mean?
L is the low-carbon version, 0.030 percent carbon maximum under ASTM A240, used when the part will be welded. 304 and 304L are not the same order. 316 and 316L are not the same order.
Can 304 replace 316 to lower the price?
Only if the end user agrees. The molybdenum is there for the chloride service. Removing it changes the corrosion behavior the drawing selected.
Does 316 handle seawater?
316 is widely used for marine fittings and coastal plant. It is not immune to crevice corrosion in stagnant seawater. A seawater duty should be read from the specification, not assumed from the grade name.
Does the grade decide sheet, coil or pipe?
No. The same grade is rolled as sheet, wound as coil or made as welded or seamless pipe. The enquiry has to name the form and the standard as well as 304 or 316.
What to send with the enquiry
Copy the grade exactly, including the L if it is printed, and the standard, such as ASTM A240 or ASTM A312, or the EN number the drawing uses. Name the form: sheet, plate, coil, strip, welded pipe or seamless pipe. Add thickness or wall, width or outside diameter, length, finish, quantity and whether a mill test certificate is required. Those points are confirmed on the enquiry before the material is rolled.