Figure 9: (COURTESY OF AUTHORS.)
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Figures 9a and 9b show the microstructure of the alloy N10276 that led to the deficiency in the electropolishing shown in
Figure 5. The microstructure consists of significant banding, thought to be a result of precipitation of topologically close-packed
intermetallic phases, mainly μ-phase. The μ-phase is a molybdenum-nickel rich phase containing chromium, tungsten, and iron.
The microstructure clearly shows that the segregated μ-phase microstructure intersects the surface of the plate. Due to the
difference in the composition between the μ-phase and the surrounding matrix region, a potential difference exists between
the two regions leading to preferential attack of the more active component, μ-phase. It is not surprising that the metal
surface did not exhibit brilliant luster and reflectivity. Electropolishing is essentially a mild corrosion process, and any
inhomegeneity in the microstructure of the alloy at the surface will result in preferential corrosion.
Figure 10: (COURTESY OF AUTHORS.)
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The photomicrographs in Figure 10 show the morphology of the intermetallic phases observed in the 6% moly (UNS N08367) plate
material. In the 6% moly superaustenitic alloys, the two principal elements that improve corrosion resistance, chromium, and
molybdenum, also participate in the formation of many of the damaging intermetallic phases that form in these alloys. Sigma
phase is the primary phase that affects the corrosion resistance and mechanical properties. Because high chromium and molybdenum
are an essential feature, minimizing the occurrence of sigma phase can be a significant factor in the successful production
and fabrication of the 6% moly superaustenitic alloys. All these grades were developed to be free of sigma phase in the solution-annealed
condition. Traces of sigma are not uncommon in solution-annealed austenitic grades because of segregation in the starting
cast slab or ingot. Homogenization heat treatments and electro-slag refining is important in minimizing the formation of sigma
phase.
Figure 11: (COURTESY OF AUTHORS.)
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Fortunately, both the nickel–chromium–molybdenum alloys and the 6% moly superaustenitic alloys can be produced with excellent
microstructure if the mills take the necessary steps in controlling chemistry and using the appropriate melt technology and
thermomechanical processing. Figure 11 shows the microstructure of an UNS N06022 plate that has an excellent clean microstructure.
This type of microstructure has electropolished well and will perform to the desired requirements for the biopharmaceutical
industry.
For the biopharmaceutical industry, it is prudent that the microstructure of the alloy be checked and samples of the alloys
electropolished before fabrication of equipment is contemplated. In addition, significant testing should be done on new alloys
that are introduced to the biopharmaceutical industry.
Hira Ahluwalia, PhD, is president of Material Selection Resources, Pennington, NJ 08534, hira@doctormetals.com
Brian J. Uhlenkamp* is vice-president of engineering and research and development at DCI Inc., 600 North 54th Ave., St. Cloud, Minnesota 56303,
tel. 320.252.8200, bjuhlenkamp@dciinc.com
* To whom all correspondence should be addressed.
Submitted: Jan. 20, 2008. Accepted Jan. 31, 2008.
References
1. Corrosion Costs and Preventive Strategies in the United States, Transportation Equity Act for the 21St Century (TEA-21), US Congress, 1998.
2. Code of Federal Regulations, Title 21, Food and Drugs.
3. American Society of Mechanical Engineers (ASME) Section VIII Div. 1, Boiler and Pressure Vessel Design Code (ASME, New York, 2007).
4. American Society of Mechanical Engineers (ASME) Section IX, Welding (ASME, New York, 2007).
5. American Society of Mechanical Engineers (ASME) Bioprocessing Equipment (ASME, New York, 2005).
6. H. Ahluwalia and B. Uhlenkamp, "The Influence of Microstructures in the Fabrication and Electropolish Finish of Corrosion
Resistant Equipment for the BioPharmaceutical Industry," presented at Corrosion 2007, National Association of Corrosion Engineers,
Nashville, TN, March 2007.
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