American dairy processors adhere to the standards published by 3-A while the European manufacturers are operating under guidelines set up by the European Hygienic Engineering & Design Group (EHEDG). Currently there are efforts underway for both organizations to work together. 3-A’s adoption of the Third Party Review for approvals does bring it closer to the EHEDG procedures. However, 3-A operates under detailed standards that by definition rule out the use of most gear-driven tank cleaning devices. This means that dairy processers and some food plants in the United States are barred from using such devices. In Europe, EHEDG requires lab testing of the devices to analyze potential for contamination with the equipment itself.  In this evaluation they have found many gear-driven devices to be sanitary and compliant for use in dairy and other food and beverage applications.

There should be a better way to bridge the principles of one with the practices of the other, resulting in a safe, common operating ground for dairy and food manufacturers both in Europe and the United States. This will allow US dairy processors to take advantage of the economic benefits of modern cleaning devices, while also reaching their sustainability goals of reducing water usage at the plant level.

The real question is, “Are we trying to only passivate the surface or are we trying to produce a corrosion resistant cleanable surface?”  Chemical passivation will attain a chrome to iron ratio above 1.0 (Cr/Fe), without producing any measureable change in the finish characteristics of the surface. Electropolishing is designed to remove surface damage from mechanical polishing and produce a more cleanable, featureless, smooth surface finish.  The need to electropolish a surface is dependant upon the desireable surface finish requirements, while passivation is required by all austenitic stainless surfaces to improve their corrosion resistance, whether they are electropolished or not. Formation of a passive film on the surface does not require electropolishing.

Chemical passivation will improve the corrosion resistance of all austenitic stainless steel surfaces, no matter what the surface profile or roughness condition. It chemically removes the iron and iron oxide from the surface and leaves the chromium oxide at the surface to protect the alloy. The effectiveness of the passivation process can be quantified or measured in terms of the Cr/Fe (chrome to iron) ratio. The more chromium in the surface, the more corrosion resistance will be present. Chemical cleaning and passivation will improve the surface corrosion resistance and remove surface contamination, but will not remove surface area, roughness, or cold work damage from polishing.

Chemical passivation is required after electropolishing, since electropolishing passivates the surface only to a condition typically attained by phosphoric acid. Passivation with citric acid based chelant systems or nitric acid will effectively improve the Cr/Fe ratio and double the corrosion resistance of an electropolished surface alone. Comparison of passivation to electropolishing is difficult, since they effectively accomplish two different things. Passivation improves the chemistry of the surface (increases the Cr/Fe ratio) while electropolishing removes surface damage, improves the surface profile and cleanability, or smoothes the surface. Although electropolishing passivates the surface, it doesn’t meet the Cr/Fe ratio levels attained by comparative passivation processes.

In summary, if attainment of a set Cr/Fe ratio is the goal, passivation is the process to use. If a cleanable corrosion resistant surface is desired, then electropolishing followed by chemical passivation is the best choice. I hope this helps answer the question.

The short answer? If a 1.0 Cr/Fe ratio is all you are trying to achieve, no it is not. Now for the long answer. First, the 1.0 or greater Cr/Fe ratio indicated in the ASME BPE standard is a minimal requirement. The best passive and corrosion-resistant surfaces will have a Cr/Fe ratio in excess of 1.5/1, again achievable by passivation alone. The pharmaceutical industry, in most cases, requires a 15-25Ra value typically achieved through a mechanical polishing procedure. It is this procedure that, in my opinion, causes many of the problems experienced today with the formation of the “gray residue” and Class 1 rouge that has plagued end users for years.

Mechanical polishing, by definition, is a hand sanding process that uses various forms of abrasive media to remove scratches, gouges, and other damage from the surface of materials. The media is applied to the surface using hand-held power equipment, resulting in surfaces compliant with ASME-BPE surface finish standards. However, the truth of the matter is that mechanical polishing is actually damaging the surface of stainless steel, leaving behind scratches and contamination. This damaged surface is known as the “Bielby Layer” and is usually in the range of .0003” to .0005” in depth. (See Figure 1.1)

Figure 1.1

Figure 1.2

The damages in the upper most grains can be seen in the illustrations above. Both are a 180 grit mechanical polished surface reading a 20Ra using a profilometer. They provide a clear indication of the actual surface left behind. In a study done by J. Wulf, this damaged layer was identified to have up to seven distinct layers. The illustration below, Figure 2, depicts these results.

Figure 2

This study looked at three distinct surfaces: honed, ground (or mechanically polished), and electropolished. The study found the honed surface had up to three distinct layers and the mechanically polished surface had up to seven distinct layers, while the electropolished surface demonstrated only one layer of pure austenite.

Looking back at the mechanical polishing illustration, figure 1.2, notice the layers of the material folded over on the surface. Studies have shown that underneath the “folds” there are trapped particles of abrasives, oxides, polishing compounds, dyes, greases, and other contaminants all embedded in the distorted crystal structure. Studies further show that no amount of cleaning or passivation can remove these contaminants. Only when the system is placed into service, with normal operating conditions of heating and cooling cycles, does the material expand allowing these contaminants to release onto the surface and into the product.

With stainless steel being an alloy that contains approximately 64% iron, it is only logical to conclude that the grinding dust released during this process will contain iron particles that are distributed and then deposited downstream on piping and equipment walls, contributing to Class 1 rouge.

Electropolishing offers the ultimate product contact surface by providing an optimum micro-surface finish, a reduction in total surface area, and providing pure alloy without contamination or damage at the material’s product contact interface surface. Electropolished surfaces offer optimum cleanability, sterility, corrosion resistance, and a reduction to rouge formation.

During the electropolish process, approximately .0005” of material is actually removed from the surface of the steel.  This ultimately removes all of the damaged layer and subsequent contaminants trapped under the smeared material on mechanically polished surfaces. (See figures 3.1 and 3.2)

Figure 3.1

Figure 3.2

In addition to offering an “UltraClean” surface, electropolishing also offers a reduction in total surface area as shown in figures 4 and 5. In these examples, samples were provided for a white light interferometric (WLI) surface analysis to look microscopically at the surface profile.

Figure 4 shows the cross section and surface profile on mechanical polish 11.5 Ra stainless steel sheet metal. The red line in the box shows the actual surface profile, highlighting the peaks and valleys of the surface.

Figure 5 shows the cross section and surface profile on mechanical polish 11.5 Ra stainless steel sheet metal followed by electropolishing to a final Ra of 2.3. The red line in the box shows the actual surface profile, displaying the lack of peaks and valleys of the surface and indicating a microscopic, featureless surface and reduction in the total surface area.

When comparing figure 4 with figure 5, the improvement to the surface is undeniable. The surface area is reduced and the damaged layer from the mechanical polishing process is eliminated, along with the sub-surface contaminants.

In addition to the obvious benefits to the surface via the electropolish process, ASTM B-912-02 specification recognizes electropolishing and electrochemical cleaning as an acceptable form of passivation.  In order to meet ASTM-B-912-02, a nitric or citric acid and water passivation solution is applied at ambient temperature to a surface and is a very fast and effective alternative to conventional passivation procedures.  Following passivation, a final rinse using deionized (DI) water at ambient temperature is performed. The duration of the rinsing process will be determined by testing the water to ensure that the effluent conductivity is within 1μS of the influent.

In conclusion, processors must be more concerned with product contact surfaces well beyond the Cr/Fe ratio. By proper material selection and surface conditions, the actual need for repetitive passivation treatments to correct iron contamination and cleaning inefficiencies could be reduced.

There are many alloys available to address the corrosion issues; but what makes super-austenitic stainless steel alloy AL-6XN^® and nickel alloy C-22 the favorite materials for highly corrosive environments? Are the properties more attractive, or is the availability of the material in the required forms more desirable? Is one better than the other?

Both AL-6XN and C-22 have a face-centered cubic lattice structure and contain chromium, molybdenum, and nickel as the major ingredients for providing corrosion resistance. However, C-22 has more of each of these elements to give better resistance to chlorides, mineral acids, and high temperature oxidation. More chromium gives more resistance to pitting, stress corrosion cracking, and crevice corrosion. More molybdenum provides more resistance against reducing media such as hydrochloric, formic, and phosphorous acid mediums. More nickel resists more attacks from chlorides and other halides. Both AL-6XN and C-22 are materials that can withstand very corrosive environments, but they each have material design limitations and application restrictions.

Though AL-6XN was originally developed for sea-water applications, it is also an excellent material for food and beverage products such as sports drinks, ketchup, soy, barbeque, and salsa where the corrosive product eats away the material. It is also found to resist corrosive attacks from various personal care products such as deodorant and hair care products. AL-6XN is probably the best choice not only for corrosion resistance bust also for applications that require excellent formability, strength, and weldability that is not achievable by any conventional stainless steel alloy.

C-22 is designed to tolerate extremely corrosive environments. The advantage of this material is that it can withstand both oxidizing and reducing acids at high temperatures exhibiting high resistance to localized corrosion. C-22 has been used in chemical, food, beverage, and pharmaceutical industries where high temperatures and high concentrations are utilized. Some of the processes use extremely high concentrations of chlorides at high temperatures. C-22 works effectively in these aggressive conditions.

Remember, every material has a threshold limit, and AL-6XN will not hold well after reaching its threshold. So, it is beneficial to opt for C-22 beyond this limit.

AL-6XN is a cost-effective alternative to 316L for higher corrosion resistance and strength. It is considerably less expensive when compared to C-22, and yet it provides a comparable level of corrosion resistance.

Not only are the properties attractive, but the material is available in many product forms. AL-6XN and C-22 are readily available in forms such as tube, pipe, sheet, plate, bar, billet, and forgings from which components such as fittings, valves, fasteners, and castings can be obtained. Because many sanitary piping components made from AL-6XN alloy that are readily available you should always check for commercial availability of the form of product (tube and fittings) you may need before planning a project.

————
“AL-6XN” is a registered trademark of ATI Properties, Inc., licensed to Allegheny Ludlum Corp.
“Hastelloy” and “C-22” are registered trademarks of Haynes International.

Devices like the Rotary Jet Head or Rotating Spray Head for tank cleaning provide the ability to reduce your cleaning time as well as your energy, water, and chemical consumption. On top of that, being able to find such a device that is 3A and EHEDG compliant sounds too good to be true.

The following article, written by Bo Boye Busk Jensen, helps explain why this is not only feasible but is being done successfully all over the world. As a member of EHEDG I thank them for working with us to get this information to you.  If you find value in this article feel free to repost.

Tank cleaning technology: Innovative application to improve clean-in-place (CIP)

What has your experience been with welding this alloy? We’d love to hear from you!

What is purging?

“Purging” is the process of removing air that contains oxygen. Welding is a high-temperature process, so it is very important to keep oxygen and any other gases away from hot metal. We do this to avoid any kind of reaction between the metal and the atmospheric gases.

Oxygen has a tendency to react with metals, forming metal oxides that can lead to discoloration or leave pores on the welds. Hydrogen has a tendency to cause embrittlement (loss of ductility) of the metal, especially in the presence of titanium and some steels. Therefore, it is very important to purge before your welds are tacked.

Why purge when welding AL6XN?

• AL-6XN is selected for its high corrosion resistance. This corrosion resistance is obtained by the formation of a very thin, stable, oxide film that is rich in chromium. This film forms at normal temperature and pressure. To maintain the corrosion resistance, it is very important to protect the chromium oxide film from contamination during the fabrication process. This is why we must purge the backside of the weld in AL-6XN material.
• Purging prevents the formation of oxides and, by doing so, protects the material from losing its corrosion resistance.

How to purge?

• Apply an Argon shielding gas to the ID of the assembly. To do this, both ends of the tube should be plugged. The gas purging time will vary depending on the volume of atmosphere that needs to be evacuated. We typically allow enough gas-flow time to exchange 4 times the original held volume. For high purity applications, there are other methods that can be used to ensure that the ID of the assembly has been properly purged. In some cases you may want to monitor the escaping gas with an oxygen meter.
• Purging time should also continue until the weld seam cools down to avoid any other oxidation.
• Skin tacks should be very light to avoid penetrating the wall of the tubing. Full penetration tacks requires a high purity of purging gas and should be avoided.

What has your experience been with purging? Do you recommend it, or do you think it’s a waste of time? Join the discussion!

The most commonly used unit for surface finish is Ra (Arithmetic Average Roughness).  A device called a “profilometer” is used to measure Ra.  Measurements are taken by drawing a stylus along the surface to be measured.  The motion of the stylus is perpendicular to the surface and registers the mean of the peaks and valleys along said surface.  This reading is typically expressed in microinches or micrometers.

The following table gives examples of commonly specified finishes in both microinches and micrometers.

Unit Conversions:
1μm = 39.37μin
1μin = 0.0254μm

Another surface designator used in the past, but typically not used today is “Grit”.  Grit refers to the grit size of the finishing materials.  There is no mathematical conversion between Grit and Ra, but the chart below is commonly agreed upon by most in the industry.

Surface finish measurement procedures, general terminology, definitions of most parameters and filtering information can be found in American Standard ASME B46.1 – 2002, Surface Texture, ASME BPE Part SF, and in International Standards, ISO 4287 and ISO 4288.

In recent years, disposables have also found their way into processes for pilot and contract manufacturing plants.  Today they are available for practically every aspect of biopharmaceutical manufacturing including fermentation, buffer, media, filtration, purification, and separation applications. As volumes reach over 1,000L, the trend today is a marriage of single-use components and reusable stainless steel.

Single-use components and systems make sense for clinical development and contract manufactures to employ, because the production runs are small and varied.  However, at this time large-scale commercial production still uses predominately reusable stainless steel, due to the fact that the facility has been validated, and it does not make commercial sense to change the process.

When we look 5-7 years into the future, how will a large-scale bioprocessing plant handle processes upstream and downstream?  Will they continue to utilize reusable stainless steel, will they employ all single-use processes, or will we see a combination of disposable and stainless steel?

Please weigh in with your thoughts on how production will be handled in state-of-the-art biotech facilities in 2020.