Peer-Reviewed Topical Review: The Importance of Quality in Corrosion-Resistant Alloys in Biopharmaceutical Manufacturing - Pharmaceutical Technology

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Peer-Reviewed Topical Review: The Importance of Quality in Corrosion-Resistant Alloys in Biopharmaceutical Manufacturing
In this topical review, the authors discuss the rationale behind microstructural requirements for biopharmaceutical equipment and problems that may be encountered during the fabrication of high-performance corrosion-resistant equipment.


Pharmaceutical Technology
Volume 3, Issue 32

High-performance corrosion-resistant materials


Figure 3: (COURTESY OF AUTHORS.)
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 4: (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.


Table I: The chemical composition ranges of some of the alloys used in the biopharmaceutical industry.
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.

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.


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