The Difference Between 304 and 316 Stainless Steel

Like with any standard steel, stainless can and will eventually rust in the right environment, and a marine environment (especially saltwater) is one of those that is quite harsh on stainless alloys and really puts them to the test. While a galvanized steel, or a plain steel fastener might seem to work at first, a stainless steel alloy will stave off rust and corrosion much longer and is the right choice for marine uses as well as many other uses where corrosion resistance is important.

What is stainless steel?

Steels are all primarily composed of iron and carbon and can be traced back to the 13th century when blacksmiths discovered that iron became harder and stronger after being left longer in their coal furnaces. It took until 1913 when Harry Brearley discovered stainless steels. Brearly added chromium to gun barrels to make them last longer. That addition of chromium is what makes stainless steel stain less and last much longer.

Now there are numerous grades of stainless steels, each possessing a slightly different alloy composition, and therefore slightly different physical and corrosion resistance qualities.

To be a true stainless steel the alloy must contain at least 10.5 percent chromium. Depending on the grade, it may contain much higher chromium levels, and additional alloying ingredients like molybdenum, nickel, titanium, aluminum, copper, nitrogen, phosphorous or selenium.

Common stainless steels

The two most common stainless steel grades are 304 and 316. The important difference is the addition of molybdenum and more nickel, both alloys which drastically enhance corrosion resistance, especially for more saline or chloride-exposed environments such as in saltwater or even in an environment that is close to salt air.

Another important difference between the two most common grades of stainless is the cost. 304 stainless steel is far more common and cheaper to manufacture. The fact that it's produced on a larger scale combined with it's lack of expensive molybdenum and less nickel makes it a much less expensive grade of stainless. 

Stainless steel is an ideal corrosion-resistant material, but it will only withstand long-term exposure if the grade is appropriate for the environment that it will be used in. While 304 is an economical and practical choice for most environments, it doesn’t have the chloride resistance of 316 and will therefore corrode much faster. The higher price point of 316 is well worth it in areas with high chloride exposure, especially near the ocean or by heavily salted roads. 

304 Stainless Steel

304 stainless steel is the most common form of stainless steel used around the world, due to it's general corrosion resistance and value for money. It contains between 18 percent chromium and 8 percent nickel (where it's other name, 18-8 comes from), as well as small amounts of carbon and manganese.

304 (or 18-8) stainless steel can withstand corrosion from most oxidizing acids such as vinegar or other acids used in cleaning chemicals. It's resistance to cleaning chemicals makes 304 easy to sanitize, and therefore ideal for kitchen and food applications. You'll often find 304 stainless steel used in cutlery, on appliances, home furnishings, or indoor architectural elements.

304 stainless steel does have that one weakness we've already mentioned: it is susceptible to corrosion from chloride solutions, or from saline environments like the coast - better known as salt water! Chloride ions can create localized areas of corrosion, called "pitting," which can spread beneath protective chromium barriers to compromise internal structures. For example, the head of a stainless steel fastener might only have a small pit in it, but the interior part of the fastener that cannot be seen might be completely compromised.

When purchasing fasteners in particular, it's important to know the grade of the stainless alloy that they are comprised of. Fasteners, as their name of course implies, fasten together important and structural components. Using 304 stainless steel fasteners in a marine environment will likely leave your project with a compromised structure over a short amount of time. 304 stainless steel fasteners, being cheaper and far more easily available, can be found in local hardware stores and big box retailers, while proper marine grade 316 alloy stainless steel fasteners can be much harder to source. Pay attention to how they are labelled! If the alloy is not named on the packaging they are likely the cheaper 304 stainless steel.

316 Stainless Steel

316 stainless steel is the second most common form of stainless steel behind 304. The same physical and mechanical properties of 316 stainless are similar to that of304 stainless steel, and contains a somewhat similar material make-up, but small differences make it far more corrosion resistant. The key difference to 316 stainless steel is that it incorporates about 2 to 3 percent molybdenum as well as slightly more nickel. The addition increases corrosion resistance, particularly against chlorides in a marine environment and other industrial solvents.

316 stainless steel is commonly used in many industrial applications involving processing chemicals, as well as high-saline environments such as coastal regions and outdoor areas where salts are common such as near salted roads. Due to its non-reactive qualities, 316 stainless steel is also used heavily in the manufacture of medical surgical instruments.

Common uses for 316 stainless steel:

• Marine grade fasteners
• Industrial equipment that is used in:
  • Pharmaceutical manufacturing
  • Chemical manufacturing
• Pressure vessels
• Cisterns and pipes for chemical applications
• Medical equipment
• Marine equipment, parts, fittings, and fixtures
• Outdoor furnishings
• Commercial kitchens
• Food production and processing
• Commercial appliances

Stainless Steel provides natural corrosion resistance

Corrosion is a natural phenomenon. Pure elements always react with the surrounding environment, which is why so few elements are naturally found in their pure form. Iron is no exception.

In wet or humid conditions iron reacts with the oxygen contained in water to form rust (iron oxide). The red flaky oxide deteriorates easily and then exposes more of the material to oxidation. Iron and standard carbon steels are highly susceptible to rusting.

Stainless steel has the innate ability to form a passive layer that prevents corrosion. The secret? Chromium.

The chromium found in all stainless steels reacts quickly with oxygen environments, much the same as iron. The difference, however, is that only a very fine layer of chromium will oxidize (often only a few molecules in thickness). Unlike flaky and unstable iron oxide, chromium oxide is highly durable and non-reactive. It adheres to stainless steel surfaces and won't transfer or react further with other materials. It is also self-renewing, if a bit of the oxidized layer of chromium is scratched or damages, more chromium within the stainless steel will react with oxygen to replenish the protective layer. The higher the chromium content, the faster that protective layer repairs itself.

Keep in mind though, we just learned oxygen is needed to form the protective layer that protects stainless steel from corrosion. In an environment lacking oxygen or where oxygen cannot freely circulate, stainless steel is highly susceptible to crevice corrosion, and more can be read about that effect here. For example, if a fastener is going to be used on a vessel's hull underneath the waterline, then stainless steel should not be used and an alternative alloy for your fasteners such as silicon bronze should be considered.