Do You Know What’s in Your 316 Stainless Steel?Do You Know What’s in Your 316 Stainless Steel?

Stainless steel is the alloy of choice for a wide variety of both gas and liquid chromatography systems, making it very important for users of stainless steel to understand their material choice as well as some of the variability that comes with using it.

Stainless steel has a mixture of different elements that make up its composition. Notable elements include mostly Iron as well as between 10.00 – 13.00 (percent composition) Nickel and between 16.50 – 18.50 (percent composition) Chromium. However, 316 stainless steels can also contain trace amounts of elements like Carbon, Silicon, and Phosphorus. The allowability and range of different elements can create very different metals based on elemental percent compositions.

Figure 1 below shows the allowable percent composition of elements in 316 stainless steels:

FIGURE 1: Allowable percent composition of elements for a metal to be classified as 316 stainless steel.

Depending on the quantities of trace elements, the composition of 316 stainless steel can look as different as the 2 metal make-ups shown in Figure 2 below:

FIGURE 2: Elemental combinations can be very different in makeup and still be within allowable percent compositions to be called 316 stainless steels.

Trace elements have the following effects on stainless steel:

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Stainless steel is typically created by melting all the previously mentioned elements together to form its composition and then decarburizing that mixture to burn off more carbon. From there, companies have many opportunities to increase the variability of their metal including turning the melt into bulk metal, choosing to roll or extrude that bulk metal into sheets or tubing, or turning the melt into powder and choosing to sinter the material into filters or 3D print stainless steel through additive manufacturing. After that, manufacturers can choose to do annealing, machine their parts, or incorporate surface treatments like electropolishing or passivation. Finally, end users will receive the final product — and will pay more for that entire process to be as consistently high quality as possible.

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Achieving a drastic reduction in variability and interaction.

When quality and consistency are not prioritized, there can be a large variability in the composition of parts that are manufactured which leads to different reactions and resistances to corrosive environments and samples.

Dr. Jesse Bischof, a senior scientist at SilcoTek Corporation, found a significant difference in corrosion resistance between two 316 stainless steel samples when using ASTM G31 standard testing guidelines. After immersion in 15% bleach for 72 hours at room temperature, Sample 1 measured a corrosion rate of only 3.09mpy while Sample 2 measured a significant 51.29mpy. Considering both metals were purchased as 316 stainless steel, Dr. Bischof began to consider the impact of trace elements in chromatography and analytical science.

A well-passivated 316 stainless steel chromatography part should have a layer of chromium oxide that allows sample analytes to easily move through the system without interaction. These parts, like tubes, valves, and columns, all work together to create an inert pathway to ensure the analytes will not interact with the problematic trace elements that lie below the chromium oxide layer. However, inclusions are a common issue that passivation cannot always solve. The sample analytes will move through the parts and get stuck on the trace elements exposed on the part surface. The analyte sticking leads to slower response time and inaccurate analysis.

Dr. Bischof has spent years researching surface preparation for accurate chromatography and through this research has found that the allowed variability in 316 stainless steel creates very unreliable and inconsistent chromatography parts like columns, tubing, frits, and valves. He emphasizes the importance of finding reliable manufacturers and machine shops that prioritize consistency and quality in their parts and post-production processes like passivation and electropolishing.

In Dr. Bischof’s most recent research, which he presented at the Pittcon Conference in 2024, he notes that surface treatments like SilcoTek’s CVD coating process are a reliable way to create a barrier between the sample analytes and trace elements in the metal part surface. SilcoTek’s CVD coating technology is a uniform, silica-like coating that creates an inert surface resistant to corrosion, leaching, and analyte sticking. For more information on Dr. Bischof’s presentation and SilcoTek coatings, please visit www.SilcoTek.com.