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

The need for electropolishing

The 300-series type stainless steels are often electropolished and passivated primarily to improve their corrosion resistance and cleanability. It can be argued that the high performance corrosion-resistant alloys do not need to be electropolished and passivated because the higher alloying content provides adequate corrosion resistance. Passivation procedures that are used to form a uniform passive film to enhance corrosion resistance are not necessary with the higher alloys. Electropolishing, however, is often specified for the high-performance alloys. Mechanically finished surfaces produce a cold worked surface and damaged layer that includes scratches, lapping, and gouges with possible embedded abrasive compounds. Electropolishing is used to remove this layer and impart a surface that is microscopically featureless and clean. It substantially reduces product contamination and adhesion as electropolishing reduces the effective active surface area on the metal surface and removes material imperfections such as inclusions. Overall, electropolishing allows better cleaning, sanitization, and sterilization, thereby, lowering cleaning costs, allowing easier validation that minimizes the potential for product contamination, along with lowering future maintenance costs.

Since electropolishing is a form of corrosion, any microstructural defects are preferentially corroded leading to an undesirable surface finish.

Microstructure quality deficiency

Over the past few years, several cases have been reported where vessels fabricated using the high-performance alloys showed a visual "hazing," frosting, or whitening of the surface (6). Figures 5 and 6 show some of the extreme cases of inadequate surface characteristics obtained after electropolishing the nickel–chromium–molybdenum alloy UNS N10276 plate. Figures 7 and 8 show the surface of an UNS N08367 plate after electropolishing. It is clear that the surface appearance in all these cases is due to the preferential corrosion of secondary phases in the microstructure. The defects have been observed on various product forms, including sheet, plate, tube, pipe, bar, and forgings. The defects are not limited to alloy N10276, but have been observed in UNS N06022, UNS N06200, UNS N06686 (Alloy 686), and UNS N06059 (Alloy 59) and also the superaustenitic stainless steels UNS N08367, UNS S31254, and UNS N08926 (Alloy 25-6Mo).

To determine the root cause of the deficiency observed during electropolishing, samples from the various alloy plates were cross-sectioned, and the microstructure evaluated. The montage of photomicrographs shown in Figure 9 show the microstructure of the various nickel–chromium–molybdenum alloys that cause undesirable electropolish quality and make these microstructures unacceptable for fabrication of biopharmaceutical equipment.


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