With the recent interest in superaustenitic stainless steels and nickel-base alloys for materials of construction in pharmaceutical applications, specifications have been created to assure the best possible finish after electropolishing. These specifications require extensive microstructural examinations for the presence of inclusions and secondary phases. By excluding these phases, electropolishing defects will be minimized. It has been shown that banding of secondary phase will cause significant indications during electropolishing, but there has been minimal evidence to determine the effect of other inclusions. This paper will discuss some of the microstructural defects and their correlation to surface defects. Furthermore, some of the causes for these microstructural defects will be discussed.
The pharmaceutical industry and especially the biopharmaceutical industry rely heavily on metal vessels for reaction, storage, and conveyance of material. These vessels are fabricated from a variety of materials, but in all cases, the contact surfaces must meet stringent industry standards. Of course, material must be selected that has essentially zero corrosion rate in the application to minimize any toxic or unwanted effects from metal contamination, but also, the vessels must be able to be cleaned and reused with other solutions. Although off the shelf material can be found with corrosion resistance required in many of the applications, the surface of this material from the mill contains many small crevices, bumps, and texture. This surface can entrap very small quantities of either a previous media or a cleaning agent that could wreak havoc on the next process. In the vast majority of industrial applications, the mill surface is not an issue, but when in the pharmaceutical arena, this surface is as important as fabricability, corrosion resistance, and strength. Unfortunately, no mill can provide an adequate surface for this application, but through electropolishing, the mill finish can be replaced with a mirror-like finish that facilitates cleaning. The electropolished surface removed the features from the metal and eliminates peaks and valleys that trap both media and the material used to clean the vessels. This featureless surface is ideal for the application since contamination from one batch to the next could easily ruin millions of dollars of product. During the early development of these vessels, there were no guidelines for the material requirements and the end users (and fabricators) might end up with material that certainly met the standard ASTM properties, but would display unacceptable electropolished results. Material from simple austenitic stainless steels (UNS S31603) to complex Ni-Cr-Mo alloys exhibited defects after electropolishing that led to rejections. These defects manifested themselves as hazy or streaky marks on an otherwise reflective finish. Under slight optical magnification, these anomalies appear as pits in the surface. Pitting defeats the purpose of the electropolish as these pits serve as areas that can either escape cleaning while harboring contamination or trap cleansers that could effect future processes. Furthermore, these pits were associated with microstructural defects in Ni-Cr-Mo alloys, namely the Mo/Cr rich mu phase. Microstructurally, defects known to cause electropolish indications appeared as banding (Figure 1-2).
Brossia, C.S. (CC Technologies ) | Scott, C.S. (CC Technologies ) | Beavers, J.A. (CC Technologies ) | Edgemon, G.L. (ARES Corporation ) | Berman, H. (Perot Systems Corporation ) | Frankel, G.S. (Ohio State University ) | Boomer, Kayle (CH2M Hill - Hanford Group )
Wiersma, B.J. (Savannah River National Laboratory ) | Mickalonis, J.I. (Savannah River National Laboratory ) | Beam, D.C. (Savannah River National Laboratory ) | Poirier, M.R. (Savannah River National Laboratory ) | Herman, D.T. (Savannah River National Laboratory ) | Fink, S.D. (Savannah River National Laboratory ) | Pareizs, J.M. (Savannah River National Laboratory ) | Fondeur, F.F. (Savannah River National Laboratory )