High-performance corrosion-resistant materials
Stainless steel 316L has traditionally been the workhorse of the biopharmaceutical industry. Because of more demanding applications
and the unforeseen decrease in the quality of 316L stainless steel due to chemistry changes and heat-treatment practices,
end users have been selecting higher performance corrosion-resistant alloys for many new applications.
Figure 3: (COURTESY OF AUTHORS.)
Microstructure quality is a major issue with 316L stainless steel. Figure 3 shows the typical microstructure of the 316L plate
that is currently on the market. This microstructure consists of bands of delta ferrite stringers. Figure 4 shows the microstructure
of a 316L bar product that contains large stringers of manganese sulfide inclusions. The metallurgical quality of alloys is
an important issue since it has a direct impact on the corrosion resistance and therefore on product contamination. The presence
of discontinuities on surfaces resulting from removal of inclusions that intersect the surface can release contaminants that
in turn affect product quality and yields. Since most equipment destined for biopharmaceutical applications needs to be of
good quality, it is essential to have a pit-(mechanical-) free surface.
Figure 4: (COURTESY OF AUTHORS.)
Over the past 10 years, increased use of high-performance corrosion-resistant alloys has involved superaustenitic stainless
steels, commonly referred to as 6% moly alloys and nickel base alloys in the family of nickel–chromium–molybdenum alloys.
The most common alloys in these groups are AL6XN (UNS N08367) and Alloy 22 (UNS N06022). Table I shows the typical composition
of some high performance alloys used in the industry.
Table I: The chemical composition ranges of some of the alloys used in the biopharmaceutical industry.
The melting and processing method for each alloy can vary, which can ultimately affect the metallurgical and corrosion performance
of the alloy. The alloys listed in Table I are generally produced in electric arc or induction furnaces. From the electric-arc
furnace, the molten and precarburized heat is transferred in the liquid state either to an argon oxygen decarburized (AOD)
converter, vacuum induction melter (VIM) or to a vacuum oxygen decarburized (VOD) unit. In the AOD and VOD, the alloying elements
additions are adjusted, and the carbon content is reduced to less than 0.03%. After decarburization and deoxidation, extensive
desulfurization is also done in the AOD and VOD processes. To achieve low-segregation characteristics of the ingot, subsequent
electro-slag remelting or remelting in a vacuum-arc furnace is necessary.
Generally, AL-6XN (UNS N08367) and 254SMO (UNS S31254) alloys are produced using a continuous cast method where the slabs
are bottom-poured continuously from the AOD furnace. This technique can result in significant segregation of intermetallic
phases in some of the slabs.
The alloying elements in the superaustenitic stainless steels must be in solid solution to maintain optimum corrosion resistance
and fabricability of these alloys. Precipitation of intermetallic compounds, particularly sigma, but also chi and Laves phases,
in the superaustenitic alloys can result in depletion of chromium and molybdenum in adjacent areas. These areas can serve
as sites for pitting and, in some cases, intergranular corrosion. For high-purity applications, these alloys must undergo
electro-slag remelting to avoid the segregation effects.
Almost all nickel–chromium–molybdenum alloys are poured from the AOD furnace into ingots and subsequently electro-slag remelted.
These alloys are expected to have good microstructure. Because of thermomechanical-processing problems, however, these materials
have shown tendencies to form intermetallic phases that are deleterious to corrosion resistance and electropolish quality.