At a recent “Food Plant of the Future” webinar co-sponsored by Hixson, one attendee wondered whether a glass bead surface finish might have a detrimental effect on stainless steel, adding that he “frequently sees such surfaces rust out.” Although stainless steel is considered to be a non-reactive and non-rusting surface, regardless of the surface finish specified, rust can form if this metal alloy is contaminated with free iron, is damaged by heat treatments (including welding), or is put into service without proper passivation.

To prevent rust (the oxidation of the iron component of the alloy), stainless steel must be passivated. Through this process, free iron or iron compounds are removed from the surface of stainless steel and the steel then spontaneously forms a protective layer (a chemically inactive surface) when exposed to a mild oxidant such as air. Last issued in 2006, the ASTM A380 Standard for passivation for the construction of new stainless steel parts, equipment and systems ensures a consistent product from multiple suppliers, allowing a standard basis for bidding. However, changes and variations in the methods used in stainless steel passivation have created confusion regarding what is, and what is not, compliant with this standard. To further complicate matters, the ASTM A380 Standard practice and the ASTM A967 Standard specification allow different methods for passivation as defined.

Unless otherwise indicated, the ASTM A380-06 standard defines passivation narrowly by specifying treatment with a Nitric Acid solution. Yet despite this rigid definition, nitric acid passivation is not the food industry standard. In fact, there are multiple options available for passivation or cleaning and treatment of stainless steel, including but not limited to:

• “Passivation by ASTM A380.” To attain this, the most rigorous standard, suppliers must follow all ASTM A380 guidelines and create passivation in the stainless steel with a Nitric Acid treatment. The advantages of nitric acid passivation includes the ability to remove welding and heat treated scale as well as metallic contamination. The disadvantages of nitric acid are many, including danger to personnel, food safety concerns, storage, use, and disposal of a hazardous chemical and higher cost. Under most circumstances, this standard does not provide additional value warranted by the costs and disadvantages it presents.

• “Cleaned and Acid Treated with Citric Acid or Nitric Acid as per ASTM A380” or “Passivation by ASTM A967.” These standards mandate a Nitric or Citric Acid treatment. Current food industry best practice would typically fall under these standards, and may include in-house testing.

• “Cleaned and Acid Treated as per ASTM A380 Table 2.1.” This standard mandates compliance with one of the ASTM A380 validated definitions of cleaning and acid treatment of stainless steel. This standard would be applicable to most industry best practices, with a range of chemical treatments being allowed.

• “Cleaned and treated so as to pass test 7.2.5.3 Copper Sulfate Test (for example) as detailed in ASTM A380-06.” This standard does not specify a cleaning or treatment method, but the final product must pass a test (or tests) as specified by the buyer. The test(s) need to be appropriate for the specific project.

Two white papers from Hixson on the topic of stainless steel passivation are available. Click here for more information or e-mail Hixson to learn more.

Common Stainless Steel Surface Finishes.

Surface Roughness Reference

Roughness Average (Ra) in

Micro-inches (μ-in)

Mill 125-250

2B Mill 4-16

#4 32-64

#4 Dairy 16-32

Electropolish 4-32

See the April 2006 From Experience for a discussion regarding Ra and how this

value is determined.

Passivation of Stainless Steel

For more from experience,visit Hixson on the web at:www.hixson-inc.com/FE

Direct Comments/Questions to:Warren Green, Manager

Process Engineeringwgreen@hixson-inc.com

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ARCHITECTURE ENGINEERING INTERIORSHIXSON

APRIL 2010

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