AISI 304 or 316 stainless steel

The most commonly used stainless steel grades in conventional sheet metal processing and construction are AISI 304(L) or 316(L). When should 304 grade be used, and when 316? The fact that 316 stainless steel offers higher corrosion resistance than 304 grade is common knowledge, but the price also plays a role. And yet, I often clash over misunderstandings with people who don’t deal with stainless steel on a daily basis.

This is understandable as, even for specialists, the choice is not always clear, because a qualitative advantage should be weighed against a more expensive price. It’s as if you’re wondering which car is better now: a Renault Clio or a Mercedes in the premium price range; of course, the latter is better, but it comes with a much higher price tag...

When making this choice, we prefer to give you advice personally, once we have been informed of your particular circumstances. Besides AISI 304 and 316, there are other stainless steel grades or other metals such as galvanized steel or aluminium; neither 304 nor 316 may therefore be the ideal material.

However, there are a few rules of thumb you can apply:

In a kitchen environment and indoors AISI 304.

Outdoors AISI 316 stainless steel, although in specific cases, the customer may also select the 304 grade, e.g. if placed somewhere on a roof and out of sight and if the customer is not bothered by the fact that over time some brown discoloration or “rust” will become noticeable. Maritime applications require special attention, as sea air is aggressive, and 316 grade is necessary in any case. Formerly, ‘experts’ would more or less apply the rule: within a 100 km radius from the coast or in polluted environment, 316 grade; but where in Belgium is there no more pollution nowadays?

In the food industry, AISI 304 grade is largely used and actually sufficient. Switching to 316 grade is necessary only in aggressive environments (e.g. contact with brine).

In swimming pools: chlorine is a great enemy of stainless steel, which means that a pool environment requires at least 316 grade. And preferably electropolishing, especially for a (public) indoor pool; for a private outdoor pool, where the use of chlorine is sometimes very limited, sometimes untreated 316 is an option.

Sulphur is also aggressive; corrosion tables should be consulted.

AISI 304 is suitable for ammonia and most organic acids such as fruit acids, but in this case, too, it is advisable to consult the corrosion tables.

In the pharmaceutical industry: 304 grade is often selected for parts which are not in direct contact with the product, while 316 grade is used for parts in contact with the product.

In aggressive environments: the best is to seek advice, but probably 316 grade or higher quality.

Evidently, 316 grade is of higher quality than 304 grade stainless steel, but even with 316 grade there is no 100% guarantee that corrosion won’t occur. Only the probability is smaller. Conversely, if you make the wrong choice (304 instead of 316), you shouldn’t fear the worst if corrosion occurs: you will always be able to find products on the market that can offer additional protection...

In order to test this in a simple way, we check the material for the presence of molybdenum using a molybdenum tester. While AISI 304 doesn’t contain any molybdenum, AISI 316 does. It is possible to implement Positive Material Identification (PMI) testing using an X-ray fluorescence (XRF) spectrometer to determine the exact chemical composition of the alloy.

N.B.: For practical reasons, AISI 304 and 316 may also be combined, because some standard products such as wheels are not available in 316 grade. These are of course relatively easily replaceable and never touch the product, which means that the cart is often made from AISI 316 and the wheels always from AISI 304. The latter can be easily replaced when worn out.

On this page you will find a summary of the types of stainless steel that are regularly used by Alinco.