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An Introduction to the Stainless Steel Finishes of Mixers Used in Sanitary Processes

By David Wenzel

In order to meet the high standards of purity and cleanliness demanded by the pharmaceutical and biopharmaceutical industries, it is necessary to use 316 stainless steel as the material of construction for mixing and blending equipment, not only for the wet end portion of the mixer, but also for portions of the drive end, which may include the motor, bearing frame or gear box.

It is also important that the surfaces of the mixers and blenders are smooth and clean, to minimize microbial contamination, reduce erosion corrosion and prevent the adherence of particulates to the surfaces of the equipment. The required surface characteristics may be obtained by electropolishing or by using a mechanical or chemical treatment.

A comprehensive inventory for sanitary standards and accepted manufacturing practices for the dairy, beverage and food processing industries has been established by 3-A Sanitary Standards Incorporated (3-A SSI) [1]. The 3-A symbol displayed on process equipment shows compliance with regard to the materials of construction, design and fabrication. New P-3A standards are being developed specifically for the pharmaceutical and biotechnology industries and expected to be issued within the next year [2].

In 2003, 3-A introduced the Third Party Verification (TPV) program to verify that process equipment meets established sanitary standards. The updated 3-A TPV number indicates that compliance has been independently verified by a certified conformance evaluator, resulting from a detailed inspection of all components on each machine at the manufacturer’s facilities [3].

Within the 3-A manuals it is stated that the surfaces of process equipment - particularly those surfaces that contact the materials being processed - should be smooth, corrosion resistant, impervious, free of cracks and crevices, non-absorbent, non-toxic and cleanable. To meet these guidelines, stainless steel alloys have become the preferred materials of construction, being readily available and easily fabricated into complex structures. Furthermore the alloys provide superior corrosion resistance compared to mild steel or carbon steels, and the required surface properties can be obtained by a combination of mechanical or chemical polishing, electropolishing and passivation.

Many equipment manufacturers favor 316 stainless steel alloy for the construction of blenders and mixers, particularly for use in the pharmaceutical and biopharmaceutical industries. The 316 alloy contains molybdenum (2-3%), added to provide resistance to general corrosion that is superior to that of the 304 and 304L alloys more commonly used in the processing industries. The high purity requirements of the pharmaceutical and biopharmaceutical industries justify this position. The new standards that are being developed [2] will be directed to the equipment used in the production of Active Pharmaceutical Ingredients (APIs) and related materials and will overlap with ASME-BPE standards (directed to bioprocess equipment) and 3-A sanitary standards (directed to the food, beverage and dairy industries).

Why are smooth surfaces important?

Not only visually attractive, smooth surfaces can minimize the accumulation of products at the metal surfaces during mixing and other processing steps. Furthermore, the surfaces can be cleaned easily and effectively, since there are fewer sites available for the growth of microbial contaminants. It is also easier to visually confirm the cleanliness of a very smooth surface due to its optical properties. Perhaps more significantly, the rate of general corrosion of the metal depends upon the real surface area that is exposed to the corrosive environment. A smooth surface has a significantly lower, real area and therefore a lower rate of corrosion. In addition, the smooth surface reduces erosion corrosion, ie. the loss of metal from the surface due to the flow of the process fluids.

How are smooth surfaces obtained?

The surfaces of the stainless steel parts may be polished to a smooth finish by mechanical or chemical treatments or by electropolishing.

Mechanical Surface Treatments
Mechanical surface treatments of the stainless steel alloys include the use of abrasive compounds, grinding and buffing processes, sandblasting with silica, as well as shot, grit and wire blasting using stainless steel media. These methods are carried out carefully to avoid contamination of the polished surface with iron-containing materials. Furthermore, the parts that are polished mechanically are not as resistant to corrosion as those treated chemically or electropolished, due probably to the loss of chromium from the surface layer.

Several of the basic stainless steel mill plate finishes may be obtained by mechanical polishing. For example, the #4 polished finish (required to meet the basic 3-A standards for the processing industries) is obtained using a 120-150 mesh abrasive and is a general purpose bright finish. The #7 and #8 finishes are highly reflective finishes that are obtained by polishing and buffing the surface with a series of fine abrasives.

Chemical Surface Treatment
Chemical surface treatment, also referred to as chemical polishing and steel pickling [4], is required to remove the scale formed at the surface of the steel during the hot rolling and annealing processes. Pickling is carried out by either the immersion of the part in a pickling bath or by coating the surface with a pickling paste. Presently, a solution of hydrochloric acid is the preferred pickling bath, providing faster reactions, greater flexibility, better quality control and lower costs. The pickling bath is typically heated to a temperature between 35 and 45¢ªC and the length of time of the treatment is dependent upon the thickness and composition of the scale.

Chemical treatment is also used to remove the debris (scale) from the surface of the steel, debris from grinding operations, abrasives and buffing compounds that can promote pitting and crevice corrosion. The presence of particles of iron on the alloy surface results in the formation of “rouge,” a red-brown film of iron oxide. The presence of surface “rouge” is a concern when high purity is required and the part is usually chemically cleaned. The presence of impurities in or on the surface can also interfere with passivation of the steel substrate.

Electropolishing
Electropolishing [5] uses an electric current to uniformly dissolve the surface layers of the metal and has been termed “electroplating in reverse.” Large and irregularly shaped parts are rapidly and efficiently polished electrolytically, though not necessarily at a lower cost than mechanical polishing.

In electropolishing, the substrate acts as the anode in the cell and is immersed in an electrically conductive solution, which is usually an aqueous solution containing mixtures of inorganic acids and organic additives. A counter electrode (the cathode in the cell) is also immersed in the electrolyte to complete the circuit and allow current to flow. The rate of dissolution of the metal is proportional to the applied current and the amount of metal electrochemically dissolved depends upon the composition of the electrolyte, time, temperature and the current density (the applied current divided by the surface area of the part).

Electropolishing is particularly suitable for components that have complex geometries, eg. the impellers in mixers, or components that are easily damaged. The configuration of the counter electrode can be critical, particularly when the part is an irregular shape, since it is necessary to provide the required current density across the whole surface being polished. Experience also teaches that the effects of gas evolution from the cathode can adversely affect the surface finish and it is necessary to use anions or solvent molecules that are difficult to discharge or to provide controlled agitation.

What is passivation?

Passivation refers to the formation of a film of chromium oxide, by the reaction of chromium atoms in the surface of the steel with atmospheric oxygen [6]. This film acts as a protective layer on the steel substrate and inhibits general corrosion reactions, enhancing the long-term performance of the equipment. The stainless steel alloys are often passivated to ensure that the process equipment meets the required standards for purity. The presence of impurities in or on the surface of the alloy can interfere with the formation of the passive, chromium oxide layer and therefore passivation is usually preceded by a chemical treatment or electropolishing.

How is surface smoothness measured?

Surface smoothness or, conversely, surface roughness has been described in various ways, eg. grit numbers, USA finish numbers, RMS, Ra (microinches) or Ra (microns). However, to allow specification of surface finishes for stainless steel equipment, a method has been adopted that conforms to the ANSI/ASME standard, B46.1. The surface roughness is measured with a profilometer, an instrument that uses a sensitive, diamond-tipped probe to trace the peaks and valleys as it moves across the surface. The roughness is then expressed as the arithmetic mean of the departure of the peak heights and valley depths from a centerline and the profile is recorded over several sampling lengths to obtain an average value. This average is termed the Ra value and is expressed in microinches [7]. The table below compares grit numbers, USA finish numbers and common names of surface finishes with the Ra number obtained using the profilometer.

Table 1. Terminology for Surface Finishes.
Polish
Grit
Ra (Microinch)
Ra (Micron)
#3
#80
50
--
#4
#150
30-35
0.76-0.89
#4
#180
20-25
0.50-0.65
#6
#240
15-20
0.375-0.50
#7
#320
8-12
0.20-0.30
#8
#400
4-8*
0.10-0.20
* Ra (Microinch) values of 4-8 are typically obtained by electropolishing.

Which surface treatment is preferred by the process industries?

The high purity of pharmaceutical and biopharmaceutical processes and their sensitivity to traces of metal impurities requires the surfaces of the mixing and blending equipment to be very smooth (“mirror finish”). As stated earlier, electropolishing can be used effectively with large, irregularly shaped parts and the resulting surfaces are bright, very smooth, stress relieved and free of impurities. However, economics may dictate the use of abrasives and buffing compounds; if so, it is then essential to clean the surfaces of iron oxide and debris using a chemical treatment. Passivation may be selected as a final treatment to improve the long-term performance of the equipment.

References

  1. Website, 3-A Sanitary Standards Incorporated, McLean, Virginia Retrieved 4/25/08 from www.3-a.org

  2. Website, 3-A Sanitary Standards Incorporated, McLean, Virginia Retrieved 4/25/08 from www.3-a.org/pharma/index.htm

  3. Website, 3-A Sanitary Standards Incorporated, McLean, Virginia Retrieved 4/25/08 from www.3-a.org/symbol/tpvmanual.pdf

  4. “Steel Pickling – A Profile” Report by J.Robson, December 1993 EPA Contract Number 68-D1-0143 Retrieved 4/25/08 from http://www.epa.gov/ttn/ecas/regdata/IPs/Steel%20IP.pdf

  5. “Electrochemistry, Principles and Applications” by E.C.Potter, Chapter XV, Pages 352-353, Published by Cleaver-Hume Press, London (1956).

  6. “Why is Stainless Steel Stainless?” A paper published online. Retrieved 4/25/08 from http://chemistry.about.com/cs/metalsandalloys/a/aa071201a_2.htm

  7. In “Stainless Steel Tubing for the BioTechnology Industry” By Michelle Gonzalez. April 2001 Retrieved 4/25/08 from www.asepco.com/docs/PDF_files/Stainless_tubing.pdf

David Wenzel has been with Admix Inc. for over eight years. Presently he is a Technical Sales Manager focused on being a process partner for food, pharmaceutical and sanitary customers. He holds a BSME degree with an emphasis on fluids from Western New England College. He has served in applications and system engineering functions for pump and mixer companies for over 16 years. David can be reached at (800) 466 2369 Ext 209 or email dwenzel@admix.com

Page last updated: 5 March 2009