Not long after its initial development in the first decades of the 20th century, stainless steel was soon incorporated into the food making process. Its resistance to corrosion, the ease with which it can be cleaned and sterilized with steam, and its lack of a need of a protective surface coating have all led to its adoption throughout the industry. While most within the industry are familiar with how wide-spread the use of stainless steel is within the daily operations, a lesser number of people know that there are 150 varieties of stainless steel, let alone the differences between two of the most commonly-used food-grade versions.
Broadly speaking, the two versions of food grade stainless steel; Grade 316 and Grade 430, share the corrosion resistance that is common to all stainless steel, both being made of an alloy consisting of iron, nickel and chrome. Beyond that, they both share these other qualities which are important to food processors:
- Impermeable surface which prevents contamination
- Smooth surface allows for easy cleaning
- Highly resistant to nicking, denting or breakage
- Available in a number of finishes appropriate to specific use or product
- Resists acid erosion
- Bacterially neutral
- Stain and transfer resistant
- Fire resistant
- Chemically neutral to prevent absorption or transfer of flavors or odors
- Capable of withstanding frequent cleaning with strong detergents
What Determines “Food-Grade”?
For a stainless steel to be Food Contact Substances (FCS) approved, the Food and Drug Administration (FDA), National Science Foundation (NSF) and American National Standards Institute (ANSI) require that it must have minimum chromium content of 16%. In general, three series of stainless steel meet these requirements; SAE 200 (chromium-nickel-manganese alloys), SAE 300 (chromium-nickel alloys) and SAE 400 (chromium alloys).
Substitutes manganese for most of the chromium content as a cost saving measure at the expense of corrosion resistance so it is not used long-term.
Has a higher nickel content of the three series, giving it the highest corrosion resistance and a correspondingly higher cost. Grades 304 and 316 are both part of this series and the most commonly used varieties of stainless steel. Additionally, both are austenitic or non-magnetic. Of the two, 304 is the most commonly used in general and has a composition of 18% chromium and 8% nickel, with the remainder of the alloy consisting of steel. However, it is not as resistant to pitting by salt as 316 which has a composition of 16% and 10% of chromium and nickel, respectively, along with the added 2% molybdenum, which brings more chromium to the surface. Grade 304 can be commonly used when processing dairy and beer as well as being utilized in sanitation applications. Because of its higher nickel content, 316 is more frequently used in commercial food production.
Unlike SAE 200 and 300, 400 Series stainless steel can be magnetic. It is important to note that not all of the 400 Series stainless steels meet the FCS certification as they have less than the 16% minimum content of chromium. Some also have little or no nickel content. And, some of the alloys can contain lead. Of the 400 series, 430 Grade meets the FCS standards with a chromium content of 16-18% and a nickel content of no more than 0.5%. The advantage that 430 stainless steel offers is that it can be worked into much more complex shapes than 316. And, because the chromium and nickel content is lower by comparison, items made of 430 are less expensive than those made of 316. However, 430 is more commonly used in paneling, kitchen utensils, table tops, back splashes and equipment housings among other applications in the food preparation facilities themselves.
Comparing 316 and 430 Grade Stainless Steel
The question of which food grade stainless steel to use is answered by its intended application. A manufacturer specializing in foods with a high-acidity content, such as tomato-based products will have different needs than one whose products are baked goods.
Heat: Both Grade 316 and 430 resist oxidation at high temperatures. Of the two, 316 performs better with a continuous use temperature of 1472°F (800°C) while 430 is good for intermittent temperatures of 1598°F (870°C) and continuous temperatures of 1499°F (815°C). But, 430 can become brittle after long use at 752-1112°F (400-600°C), requiring annealing to reverse the effects. In applications where the stainless steel part is regularly stressed, 430 is far more resistant to stress corrosion cracking which is when surface sees crack formations which can cause sudden failure in a corrosive environment.
Corrosion: Because they have the same chromium content, 316 and 430 stainless steels have strong resistance to organic and nitric acids. However, 316 has additional resistance to a broader range of acids, alkalis and pitting corrosion caused by salts – a frequent ingredient in many foods. Where 430 has increased resistance to sulfur and oxidation, 316 is more resistant to diluting acids. Overall, 430 is acceptable where it is only exposed to acidic substances for a brief period of time, while 316 is able to withstand prolong repeated exposure. Additionally, 430 must be wiped down soon after exposure to moisture to avoid rusting and it must be kept well-polished to maintain its functionality.
Cleaning: Stainless steel’s ability to be quickly and efficiently cleaned makes it an essential material in any food processing application, especially in this era of tightened oversight regarding food safety standards. A cleaning regimen is an essential part of the Hazard Analysis and Risk-Based Preventative Controls (HARPC) mandated by the Food Safety Modernization Act (FSMA) of 2011. In order for food grade stainless steel to maintain its qualities, its surface must not provide a location for food to stick or for bacteria to gain a foothold. For this reason, abrasive pads or cleansers should never be used on a surface that comes in contact with food as they will damage the surface and perhaps embed metal fragments that will promote corrosion. Similarly, chemical sanitizers that contain chlorine bleach or chlorides should not be utilized in combination with heat as the causticity is enhanced and will lead to corrosion. However, such damage can often be reversed through timely electropolishing or chemical passivation. In fact, performing such processes as part of a regular maintenance procedure can enhance the performance of the parts as the process smooths out the microscopic surface irregularities while enhancing stainless steel’s natural protective layer.