Corrosion resistance. While different factors play into grade selection for stainless steel, many would argue that this is the one that factors most heavily into the ultimate decision.
The emphasis on corrosion resistance is important due in large part to the varying types of environments in which the end products are used. Let’s start with a basic rule of thumb, which says the higher the chromium levels contained within the stainless steel, the more corrosion resistant the product. All stainless steels are iron-based alloys containing at least 10.5% chromium. The rest of the makeup is defined by various alloying elements, which control the microstructure of the alloy.
Today there are many different grades of stainless steel—which are broken out into five families:
1. Austenitic
2. Ferritic
3. Martensitic
4. Duplex
5. Precipitation Hardening
Each family has its own set of benefits in comparison to the others:
But when it comes to corrosion resistance, which stainless steel family ranks highest? Let's compare three of the five families:
3. Martensitic
In general, martensitic stainless steels are considered ‘moderate’ when it comes to corrosion resistance. Like low alloy or carbon steels, martensitic stainless steels are similar in structure to ferritic, but can be hardened or strengthened by heat treatment—which can also make them more brittle. This is due to the addition of carbon. The main alloying element of this family is chromium.
The oil industry is a heavy user of martensitic stainless steel, as is the medical field—primarily for surgical equipment.
Among the most widely used grades of martensitic is 410. This heat-treatable product is ideally suited for applications used in air, fresh water or with limited amount of chemicals and acids; environments where corrosive elements aren’t severe.
For applications where higher corrosion resistance is required, 416 is ideal. This grade is corrosion resistant to natural food acids, basic salts, water and most atmospheric conditions.
2. Ferritic
Consider the corrosion resistance of this family of stainless steels to be ‘moderate-to-good’ with those levels increasing with chromium content. Ferritic stainless steel contains iron-chromium alloys with body-centered cubic crystal structures—they are plain chromium stainless steels with varying chromium content between 12 and 18%. But that means they are a bit more difficult to produce than standard austenitic stainless steel.
Overall, the weldability and impact toughness of ferritic stainless steels are not as good as those for austenitic. The moderate-to-good corrosion resistance level of this family increases with chromium content. This makes it an ideal fit for such applications as appliances and cookware, among others.
One of the more widely used ferritic stainless steels is 430. Used in both industrial and consumer products, it offers a combination of good corrosion and heat resistance with good mechanical properties, along with oxidation resistance to 15008 F.
For applications where higher corrosion resistance is required, 416 is ideal. This grade is corrosion resistant to natural food acids, basic salts, water and most atmospheric conditions.
1. Austenitic
Taking home the title of most resistant to corrosion is austenitic. This is related to the fact it has the highest chromium levels amongst the families. The corrosion performance of this metal can even be adjusted to suit different environments through the adjustment of alloying elements—for example, varying the carbon or molybdenum levels.
In fact, you can say that austenitic is the most popular family of all stainless steels, as about 50% of the stainless steel used today comes from this family—AISI 304 type, to be exact, which contains 18% chromium and 8% nickel.
Look at your average food processing or dairy equipment and you will see an austenitic stainless steel-based product. Even such high-profile structures as the Chrysler Building in New York is constructed using this family of stainless—grade 302 (304with high carbon content) to be exact.
The popularity of this material is due to a variety of factors, including:
• Excellent weldability
• Good formability
• Good market availability—and available in a wide range of dimensions, product forms and surface finishes
One of the most widely used of all stainless steel grades, 304, is austenitic. This heat-resistant grade offers good corrosion resistance to many chemical corrodents, as well as industrial atmospheres. And with good formability, 304 can be readily welded by all common methods. An extra low carbon variation, called 304L, avoids harmful carbide precipitation due to welding. This variation offers the same corrosion resistance as 304, but with slightly lower mechanical properties.
When it comes to the question of being most resistant to corrosion, the winner is dictated by the added level of chromium content beyond the minimum 10.5%. Ultimately, there is a grade of stainless steel for every need—it just comes down to which member of the family has the right combination of properties (see chart above) for your end application.
