The type of stainless steel you choose can make a long-lasting system become a money pit in a matter of years. Choosing the wrong grade means that the materials can not withstand the corrosive media, elevated temperatures, and cyclic pressure, and that mistake will not be cheap to fix.
Most facilities start with austenitic stainless steel, grade 316 or 316L. In comparison to the other common choices, these grades have molybdenum, which gives them better resistance to chlorides and other mildly aggressive process chemicals. However, 316 grade stainless steel has its limitations. If you know that pitting or crevice corrosion will happen, then your hoses will fail with 316 stainless steel. When plants are operational, they can be very difficult to inspect, and, therefore, flanged joints and welds are often the first place to look.
Duplex and super duplex stainless steels play a prominent role in procurement strategy because of a combined austenitic-ferritic microstructure. They provide significantly greater strength than standard austenitic grades, which also facilitates the specification of thinner wall sections throughout for the same pressure rating, as well as significantly improved resistance to stress corrosion cracking. For companies dealing with chemicals in processing vessels, agitators, or pipeline limbs, specifying duplex grades at the design phase becomes more cost-effective over the life of the asset than continual repair or replacement of 316 components.
When evaluating vendors, procurement teams should also consider surface finish and traceability. Equipment for chemical processing often requires the vendor to provide a complete material certs to meet the applicable pressure equipment and process safety regulations, so a vendor who provides mill certs with the correct grade is streamlining the process significantly during the commissioning and inspection of the equipment. For welded assemblies, low carbon versions, such as 316L or 317L, are preferred because they minimize the risk of carbide precipitation in the heat-affected zone and maintain corrosion resistance around welded joints, which typically is where failures occur first in service.
Stock planning is another area where time and money can be saved for facility managers and buyers. Normally chemical processing projects run to tight commissioning schedules, and to source a rare grade or size on short notice can hold up an entire installation. In plants that regularly have scheduled maintenance or an expansion program, maintaining an agreed forward stock plan with a metals supplier for certain grades and forms such as plates, tubes, or bars, helps to remove a common bottleneck and often results in improved pricing compared to multiple individual orders.
Each time there is a change in process, it is worth looking at the choice of material again. A shift to a different feedstock, an operating temperature change, or the use of a new cleaning-in-place chemical can move a system outside the corrosion resistance envelope of the grade originally specified. It is good practice to periodically check the corrosion resistance of the existing metallurgy of the system rather than just assuming the original specification is correct. This is premature diligence that often is not done until there is some kind of failure.
When it comes to choosing stainless steels for certain chemical processing applications, it is an oversight to base the decision purely on cost per kilogram. Factor in the specific chemical environment, the method of fabrication, the level of certification required, and the long-term stock availability, and you can optimize your system and reduce unplanned downtime, which is far more costly than the savings gained from under-specifying materials.
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