Not all 316 stainless steel is created equally. In this blog post we compare the corrosion resistance of 2 stainless steel manufacturers. And we find out how to level the playing field.
Comparing the Corrosion Resistance of 316 Stainless Steel
SilcoTek® has previously shown that not all stainless steel is created equally. A study involving bleach corrosion, found that two different heats of stainless steel had dramatically different corrosion rates. The two lots can be leveled onto the same “playing field” by coating them with Dursan®. In this study we found the corrosion rates, despite manufacturer, became similar after coating, whereas one manufacturer's bare disc was clearly superior to another manufacturer.
In this blog post you will learn:
Stainless steel is known to be corrosion resistant, but its performance can vary depending on the type and duration of chemical exposure. What you may not know is that 316 stainless steel corrosion rates can vary depending on manufacturer.
The composition of 316 stainless steel
Go to any materials handbook and you'll find the composition of 316 and 316L stainless steel. The graph below from an AK Steel product data sheet is an example of a typical summary of the composition of 316 stainless steel. Notice that stainless steel (and other alloys) are specified in allowable ranges. This is a reflection of manufacturing capability, measurement practicality and cost efficiency which all roll up to a specification that when the material composition falls within the specified range will be called 316 stainless steel, or 316L stainless steel.*
|Element||Type 316 (%)||Type 316L (%)|
|Carbon||0.08 max.||0.03 max.|
|Manganese||2.00 max.||2.00 max.|
|Phosphorus||0.045 max.||0.045 max.|
|Sulfur||0.03 max.||0.03 max.|
|Silicon||0.75 max.||0.75 max.|
|Nitrogen||0.10 max.||0.10 max.|
Unfortunately that practical error results in product and heat lot variation that can significantly impact corrosion resistance. That's why not all stainless steel is created equally. The graph below shows both the high degree of variation in corrosion resistance between manufacturers of 316 stainless steel and the corrosion benefit of the Dursan coating. The left side of the graph compares 2 different manufacturers of 316 stainless steel test coupons, sample AQ and AR (blue and red respectively). The AQ sample manufacturer had significantly better corrosion resistance when immersed in a 15% bleach solution over a 72 hour period compared to the AR sample.
Leveling the Playing Field: Comparing and Testing a Corrosion Resistant Coating
Now let's take a look at the right side of the graph. We coated both AQ and AR lots with our corrosion resistant Dursan coating. Notice that the Dursan coated 316 stainless steel samples performed much better in our 72 hour bleach immersion test ASTM G31. The AQ sample performed about 10x better while the AR sample improved by about 35x. The graph shows that Dursan essentially leveled the playing field by minimizing corrosion rate variation.
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Here's another example of how Dursan reduces variation. In this test we compared the coating performance of a common coating and Dursan. Several samples were immersed in 6M hydrochloric acid (HCl) for 24 hours per ASTM G31 standards. The common coated samples showed significant variation in corrosion performance as shown by the error bars in the graph. The Dursan coated samples showed minimal variation.
Why is this important?
Is variation important? Well it's not if you like surprises. If you're like me though you'll feel a lot better using a product that has predictable corrosion performance. Nobody likes to install a probe or sample device in their process and have it fail unpredictably. It's always better to understand the corrosion performance of a product in order to plan for and budget replacement and maintenance. Ultimately this will avoid unexpected process or product failures. That's where a level playing field counts most.
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*Source: AK Steel Product Data Sheet. 316/316L Stainless Steel