Passivation

From Brewing Forward
Stainless kettle

Iron and other metals are chemically reactive. Upon exposure to air, a metal surface will react with the oxygen, which results in the formation of a thin metal oxide film. This occurs completely spontaneously. The metal oxide film makes the surface more passive (resistant) to other chemicals in the environment. This is called "passivation" by a chemistry definition.[1][2][3][4]

Stainless steel is mostly iron, although it also contains a significant percentage of chromium. The chromium oxide in the passive surface layer adds a good deal more resistance to chemical reactions compared to other steel alloys, which means it is less susceptible to rusting. This is what makes the stainless steel "stainless".[2][4][5][6][7][8] However, most stainless steel used in the brewery is only about 18% chromium. As such, the relatively high percentage of iron on the surface still leaves it vulnerable to corrosion, and iron can negatively affect beverage flavor stability. To avoid these problems, the majority of surface iron can be removed with an acid bath. The resulting chromium-rich surface is superbly resistant to corrosion and has lower reactivity with the beverage. This iron removal process is called "passivation" by an industry definition.[3][5][9][1][6][7][10][8][11]

An undue amount of contention in the home brewing world has been caused by using these two definitions of passivation interchangeably. It's clear that they refer to different processes, which results in confusion when the word "passivation" is not well-defined in a particular document or discussion. It's understood that stainless steel naturally "passivates" in a chemistry sense by spontaneously forming an oxide layer. However, the definition of passivation used in this article is the process of using an acid treatment to selectively remove iron, which greatly improves the natural passive layer.

Passivation should also not be confused with pickling.

Purpose[edit]

Enriching surface chromium strengthens the natural metal oxide (passive) layer against chemical reaction, which ultimately acts in the ways:

  1. Increased flavor stability
  2. Prevention of metallic off-flavors
  3. Decreased possibility of corrosion
Rust on a new stainless kettle

New stainless steel equipment is often not passivated (to a degree that chemical passivation can accomplish), as evidenced by the frequent presence of machine oils and/or rust spots on brand new equipment.[12][13] Often stainless steel is pickled by the manufacturer, which does not provide the degree of surface chromium enrichment that passivation does.[14] Also, welding and physical abrasion during manufacturing or transportation removes the effect of any previous passivation or pickling in the affected areas, even if the steel had been passivated at some point earlier in the process.

Passivation treatment is not just beneficial for new equipment. The thin chromium rich layer can erode under normal usage conditions, necessitating that the equipment be passivated periodically.[15][4] Stainless steel is not immune to physical or chemical attack from conditions in the home brewing environment. Chloride or chlorine compounds in water sources, acid sanitizers, organic acids (e.g. in wine or sour beer), and carbon dioxide dissolved in the beer (or other fermentation) all chemically attack stainless steel over time.[13][7][16][17] An existing chromium-rich passive layer can also be damaged or removed by physical abrasion or scraping. In fact, stainless steel is relatively easy to scratch, which may be caused by use of abrasive cleaning pads, cleaning products, and contact with other metal tools. Many household cleaning products are abrasive, such as Comet, Ajax, Barkeeper’s Friend (BKF), and Soft Scrub. All of these products are capable of scratching stainless surfaces and should usually be avoided.[18] The passive layer can also be weakened by physical damage due to expansion and contraction caused by heating and cooling.[6]

Flavor stability[edit]

Metals ions can be released from metal surfaces in contact with the liquid, which may unacceptably change the sensory characteristics of the beverage, or pose a risk for consumers. In particular, iron, chromium, nickel, and manganese can be released from a stainless steel surface that hasn't been properly passivated.[19] Iron ions as well as other metals are known to have a negative influence on beer and wine flavor stability, primarily by increasing the rate of oxidation.[20][21] Specifically, iron catalyzes the formation of oxygen radicals (via Fenton and Haber-Weiss reactions), which then quickly react with components in the beverage to cause staleness (oxidation). Prevention of this staling mechanism is of particular importance to brewers looking to preserve fresh grain and hop flavors. See Iron for more information.

Metallic flavor prevention[edit]

Sanke kegs

The use of citric acid for passivation of stainless steel was first discovered over thirty years ago (in the early 1980s) by the Coors Brewing company in Germany. As soon as Coors had begun using stainless steel kegs for their beer, they discovered that iron was leaching into the beer, causing a metallic taste. This was only occurring the first time a new keg was filled, but a lot of beer was being wasted as a result. Looking for a solution, Coors ran a study testing various chemicals for stainless steel passivation. Citric acid emerged as the clear winner by successfully eliminating the metallic taste from new kegs.[3][22][9]

Nowadays, anecdotal reports of metallic taste in home brewed beers are still somewhat common. Some of these are likely related to use of stainless steel home brewing equipment that hasn't been properly passivated.[23][24][25][26][27] However, it's impossible to say with certainty that the lack of passivation treatment to blame in all of these cases.

Corrosion resistance[edit]

Many people unfamiliar with metallurgy are surprised to hear that stainless steel can rust. Since the main constituent of stainless steel is iron, it is still vulnerable to corrosion. Chemical passivation treatments have repeatedly been demonstrated to be very effective in improving the corrosion resistance of stainless steel.[3][28][29] The passivation process also removes chips and "free iron" contaminations left behind on the surface from different fabrication operations. These contaminants are potential corrosion sites that can ultimately result in damage.[14]

Even microscopic corrosion can be detrimental. It has been shown that increasing surface roughness (e.g. by pitting corrosion) makes the surface more vulnerable to further corrosion (causing a vicious cycle).[28][30] A rough surface can also lead to a buildup of difficult-to-clean soils, ultimately leading to variety of problems.[13] One of those problems is that residue buildup can cause further corrosion (another pathway in the vicious cycle).[7][29] For all of these reasons it's better to employ good equipment maintenance techniques—including passivation—to prevent corrosion rather than react to it after it occurs.

Passivation Methods[edit]

Surface chromium enrichment is accomplished by exposing clean stainless steel to an acid solution. There are two acids used in the metal finishing industry for this purpose: citric acid and nitric acid.[1][17]

Citric acid[edit]

Citric acid is a powerful metal complexing agent (the chemistry term is chelation - key-LAY-shun). This causes it to dissolve iron from the stainless steel surface and also complex with iron in solution, without affecting the chromium or other metals.[19][16][31][6][3][32][22] Besides removing surface iron, citric acid passivation also works to remove sulfide inclusions, which are implicated in causing pitting corrosion.[33] Citric acid is not hazardous, it does not cause any corrosion or etching, and it is suitable to passivate all types of stainless steel.[1][34][5][31]

Where to purchase citric acid:

Instructions
The process we recommend is based on strong scientific evidence that high temperature, low concentration, and longer processing time provide a very high level of chromium enrichment and corrosion resistance.[5][9][3][35][36] Studies that tested lower temperature, concentration, and/or duration (than what we recommend) commonly found lower effectiveness.[37][14][33][28][38][31]

  1. Clean the surface - The material must be free of all surface contamination before it can be passivated. Presence of any grease (e.g. from manufacturing), soil, fingerprints, etc. on the surface will prevent iron removal and oxide layer formation.[4][39][40][41][2][42] See Cleaning for quality methods.
  2. Add water - Fill a brewing vessel with water.
    As a bonus you may also add any other stainless steel items that need to be passivated. Do not include items made from any metal other than stainless steel (such as carbon steel, aluminum, brass, or copper), because they will corrode.
  3. Apply heat - Heat the water to 140-180°F (60-82°C).
  4. Add acid - Stir in enough citric acid to make a solution of at least 4%. In other words, use anhydrous citric acid at a concentration of 40g/L, or 5.4 oz per US gallon.
  5. Allow to sit - Wait 2 hours or longer.
  6. Rinse - Drain the citric acid solution and rinse with clean water. Rinsing ensures the surface has full access to oxygen in the air, which creates the oxide layer.[43][1][4]
  7. Wait - The surface should be exposed to air for 8–24 hours before use. Passive film growth begins in seconds or minutes, however it takes several hours to stabilize.[44][2][42][45]

Ideally, water for the acid solution and rinsing should contain less than about 50 ppm (mg/L) of chlorides. Tap water is usually adequate, although TDS should be low if possible.[46][1]

More info
A citric acid solution containing iron after passivation will have a slight hue that varies depending on the pH of the solution.[47][48] This serves as a visual indicator that the passivation treatment is removing surface iron from the brewing equipment.

  • Green at pH > 2.0

    Green at pH > 2.0

  • Orange at pH < 2.0

    Orange at pH < 2.0

Home brewers that passivate very frequently have observed a light blue hue on their stainless gear. One hypothesis is that the color is due to a particularly thick oxide layer causing light refraction.[49] Another possibility is that highly effective chromium enrichment leads to especially high surface levels of chromium (III) hydroxide, which has a strong bluish-green color.

Algae or bacteria can grow in a citric acid solution, so we do not recommend storage of the passivation solution, especially since citric acid is relatively inexpensive.[32][50]

Citric acid is effective in restoring corrosion resistance along a weld,[5] however it is not strong enough to remove the "heat tint" discoloration.[3][42][11]

Disposal:
Citric acid is mild enough that it can go directly down the drain, along with plenty of tap water. If you have a septic system and are concerned with pH, just mix it with the alkaline cleaning solution that you used prior to passivation, and/or you can adjust pH upward with baking soda if needed. If you don't have a pH meter or strips, you can add baking soda until it stops fizzing.

Nitric acid[edit]

Nitric acid is commonly used to passivate stainless steel in various industries, mainly out of tradition because nitric acid passivation was discovered before citric acid passivation. However, nitric acid is hazardous, potentially corrosive to your equipment, and harmful to the environment.

We do not recommend the use of nitric acid for passivation by home brewers. Citric acid is cheaper, safer, and it works better.[3][51][9][5][31][1][6][17]

FYI: Older sources suggest that since nitric acid is an "oxidizing acid", it not only removes iron, but it also oxidizes the chromium at the same time.[6] However that notion is questionable because nitric acid can remove beneficial heavy metals (nickel, chromium, etc.) that give stainless steel its desirable properties,[51][9] as well as cause pitting and corrosion, which is mutually exclusive with the idea that it maintains the oxide layer. The oxide layer forms spontaneously upon contact with air regardless of which passivation treatment is used.[17]

Other acids[edit]

A common misconception among home brewers is that passivation can be done with any acid. Many brewers have an oxalic acid, phosphoric acid, and/or any abrasive based cleaner handy and think that it can be also used to passivate their stainless steel. These cleaners can help remove residue, remove rust, or bring the stainless steel surface down to bare metal, but they do not selectively remove iron. The stainless steel surface will form the oxide layer (as it normally does), but without a correct acid treatment the chromium only covers about 18% of the surface. In other words, there's no passivation benefit from these acids.

ASTM logo
ASTM logo

Formed in 1898, ASTM International is an international standards organization that develops and publishes technical standards for a range of materials and systems, including passivation.[52] ASTM's stainless steel passivation specification A967 is the industrial standard for how stainless steel is passivated. ASTM specifications are used across all industries from military, to pharmaceutical production, to food and beverage production—including commercial brewing.

The ASTM A967 specification does not recognize any other acids for the purpose of passivating stainless steel (as of 2017),[1][53][43] and neither does the British Stainless Steel Association.[54]

There are no studies supporting the use of phosphoric acid or oxalic acid as a good alternative to citric or nitric acids for passivation. Phosphoric acid provides inferior chromium enrichment compared to nitric and citric acid methods.[55][56][15] Oxalic acid is well known to corrode stainless steel, and does not enrich surface chromium.[34][57][58] The reactivity of chromium oxides to oxalic acid is remarkably high, causing it to strip a significant amount of the chromium and also leave behind ferrous oxalate embedded in the surface. Lastly, both phosphoric acid and calcium oxalate (the less corrosive conjugate base of oxalic acid) were tested by Coors in the original study that discovered the benefit of citric acid passivation, and unlike citric acid, neither of those provided satisfactory results.[3][22]

Sulfuric acid appears to be capable of passivating stainless steel (perhaps rather slowly), although it is not typically used for passivation so data regarding its usage is very limited.[59][14]

Repassivation and Testing[edit]

It is often thought that once a surface has been passivated it will remain in that state forever. Unfortunately, as described above, there are multiple mechanisms for the deterioration of the chromium-rich layer since it is very thin.

Rust forms around scratches

There is no simple rule that says when a piece of equipment must be passivated, especially since usage conditions can vary greatly.[6] It is generally recommended that stainless steel surfaces be passivated initially and then at a defined frequency thereafter, as well as after any surface repair, abrasive cleaning, polishing, or other modification.[8][12] Some commercial breweries passivate every 3 months, while other breweries never passivate.[60][17] Our arbitrary suggestion for home brewers is to passivate your stainless gear once a year, and after any modifications or abrasive cleaning.

The ASTM A967 describes several methods for testing whether a part is passivated. However, these methods only check for the presence of "free iron" or other contaminants blocking the formation of the oxide layer. They don't quantitatively measure surface iron or chromium percentages. Therefore when a surface "passes" the test, it doesn't mean that it's passivated to our desired level (with a high surface chromium percentage).[1] As a result, we don't recommend testing. It's easier just to passivate on a regular basis.

Myth Busting[edit]

There is plenty of misinformation being spread on the Internet that stems from simple misunderstanding. We have addressed the common passivation myths in this article by providing accurate information supported by many scientific and professional references. However, one myth remains the most troubling since it leads home brewers to believe that they are passivating their equipment when in fact they are not:

Myth: Star San (phosphoric acid) and/or Barkeeper's Friend (oxalic acid) can passivate stainless steel.
Certain prominent hardware vendors as well as popular blogs have been spreading this misinformation. We challenge those vendors to provide any scientific information to support the position that these acids enrich surface chromium to a similar extent as a citric acid treatment.

Spike Brewing cites a study (Wegrelius and Sjödén 2004) to suggest that passivation isn't needed, and we feel this deserves a more detailed analysis:

  • Introduction: The authors significantly overstate the hazards of citric acid. It is well known to be environmentally friendly and it doesn't create heavy metal waste.
  • Methods: The tested steel was grade 316L, which is not what comprises most home brewing gear. The molybdenum in a 316 alloy provides additional corrosion resistance compared to 304. The "L" signifies low carbon, which even further improved its natural corrosion resistance compared to 304.
  • Methods: The citric acid methods used was immersion at room temperature for 30 minutes. This is definitely not the most effective temperature or duration for passivation with citric acid, according to multiple other studies cited above.
  • Methods: Sulfuric acid dissolves iron, which explains the increased surface chromium content after 3 days in the sulfuric acid bath for all methods (fig. 7). This method is obviously inappropriate for testing the experimental variable (and explains why it's not used to test corrosion resistance in any other passivation studies, nor is it a testing method described by the ASTM A967).

In summary, these multiple design flaws make the authors' conclusions completely invalid and inapplicable for our purposes. If anything, when you compare the surface chromium percentage right after the various treatments (fig. 7), it pretty clearly demonstrates that a correct passivation method does in fact drastically increase surface chromium content.

References[edit]

  1. ↑ a b c d e f g h i ASTM A967 / A967M-17. Standard specification for chemical passivation treatments for stainless steel parts. ASTM International; 2017.
  2. ↑ a b c d Crookes R. Pickling and Passivating Stainless Steel. 2nd ed. Euro Inox; 2007.
  3. ↑ a b c d e f g h i Kremer R. Developments in citric acid passivation of stainless steel. Conference Proceedings of the National Association for Surface Finishing. 2007:68–75.
  4. ↑ a b c d e Cluett JD. Cleanability of certain stainless steel surface finished in the brewing process. Dissertation. University of Johannesburg; 2001. Accessed March 2022.
  5. ↑ a b c d e f Yasensky D, Reali J, Larson C, Carl C. Citric acid passivation of stainless steel. Slides presented at: 2011 Aircraft Airworthiness and Sustainment Conference; April 2011; San Diego, CA.
  6. ↑ a b c d e f g Passivation of stainless steel. JohnsonDiversey Tech Tips. 2007;15.
  7. ↑ a b c d Palmer JJ. Preventing corrosion in the brewery. MoreBeer website. 2015. Accessed March 2022.
  8. ↑ a b c Schmidt RH, Erickson DJ, Sims S, Wolff P. Characteristics of food contact surface materials: Stainless steel. Food Prot Trends. 2012;32(10):574–584.
  9. ↑ a b c d e Lewis, PL, et al. Alternative to Nitric Acid for Passivation of Stainless Steel Alloys. NASA, 2013.
  10. ↑ Process specification for passivation and pickling of metallic materials. National Aeronautics and Space Administration; 2021.
  11. ↑ a b Bornmyr A, Toesch J, Winkler F. Manual for surface treatment of stainless steels. 3rd ed. Voestalpine Böhler Welding; 2019.
  12. ↑ a b Take care of your equipment and investment-clean and passivate. Five Star Chemicals & Supply, LLC website. January 2020. Accessed January 2021.
  13. ↑ a b c Lawrence G. What is tank passivation? Country Malt Group website. 2018. Accessed March 2022.
  14. ↑ a b c d Wegrelius L, Sjödén B. Passivation treatment of stainless steel. Acom. 2004;4.
  15. ↑ a b Allard D. Passivation – natural protection for stainless steel. Sani Marc Group website. 2015. Accessed March 2022.
  16. ↑ a b Kilkeary, JJ, and Sowell, T. New Developments In Passivation Technology. Astro Pak Corporation, Presented at Interphex Pharmaceutical Conference, 1999.
  17. ↑ a b c d e Master Brewers Podcast Episode 126: Passivation of Stainless Steel (with John Palmer and Ashton Lewis). Master Brewers Association of the Americas (MBAA), 2019.
  18. ↑ Green, D. Stainless steel care: Tips from the pros. Brew Your Own website. November 2018. Accessed March 2022.
  19. ↑ a b Mazinanian N, Wallinder IO, Hedberg YS. Influence of citric acid on the metal release of stainless steels. Corros Sci Technol. 2015;14(4):166–171.
  20. ↑ Zufall C, Tyrell Th. The influence of heavy metal ions on beer flavour stability. J Inst Brew. 2008;114(2):134–142.
  21. ↑ Narziss L. Technological factors of flavour stability. J Inst Brew. 1986;92:346–353.
  22. ↑ a b c Olsson, H., et al. Stainless steel flavour contribution to beer. MBAA TQ vol. 20, no. 3, 1983, pp. 102–105.
  23. ↑ Citric acid and stainless steel. Cleaning and passivating a fermenter. Reddit website. 2019. Accessed March 2022.
  24. ↑ Metallic odor/aftertaste. HomebrewTalk website. 2020. Accessed March 2022.
  25. ↑ Iron/metallic flavor?! HomebrewTalk website. 2020. Accessed March 2022.
  26. ↑ Metallic taste from keg. HomebrewTalk website. 2012. Accessed March 2022.
  27. ↑ Metallic and bitter taste - SS Brew Bucket, Star San and oxydation. HomebrewTalk website. 2015. Accessed March 2022.
  28. ↑ a b c Yun J, Hwang H, Toor IU, Shon M. Study on the effect of surface finishing methods on pitting corrosion behavior of 304 stainless steel alloy. Corros Sci Technol. 2009;8(6):209–217.
  29. ↑ a b Roll DL. Passivation and the passive layer. Astro Pak Corporation. 2014.
  30. ↑ Mathiesen T, Frantsen JE. Corrosion aspects for stainless steel surfaces in the brewery, dairy and pharmaceutical sectors. Paper presented at: NACE International Corrosion Conference Series; March 2009; Atlanta, GA.
  31. ↑ a b c d O'Laoire, C., et al. Analysis of the Acid Passivation of Stainless Steel. Analytical Letters, vol. 39, no. 11, 2006, pp. 2255–2271.
  32. ↑ a b Smith JR. Stainless steel passivation white paper. Allegheny Surface Technology website. Accessed 2020.
  33. ↑ a b Bragaglia M, Cherubini V, Cacciotti I, et al. Citric acid aerospace stainless steel passivation: A green approach. CEAS Aerospace Europe Conference. 2015.
  34. ↑ a b Speranzini, RA, et al. Corrosiveness of Decontamination Solutions to Sensitized AISI 304 Stainless Steel. Corrosion, vol. 43, no. 10, 1987, pp. 632–641.
  35. ↑ NASA and ESA Collaboration on Alternative to Nitric Acid Passivation. NASA and ESA, 2016.
  36. ↑ Gaydos, SP. Passivation of Aerospace Stainless Steel Parts with Citric Acid Solutions. Plating and Surface Finishing, vol. 90, no. 3, 2003, pp. 20–25.
  37. ↑ Zheng, ZB, and Zheng, YG. Effects of surface treatments on the corrosion and erosion-corrosion of 304 stainless steel in 3.5% NaCl solution. Corrosion Science, vol. 112, 2016, pp. 657–668.
  38. ↑ Karlsson, J. Pitting corrosion on stainless steel with or without passivation. Lunds University, Centre for Analysis and Synthesis, 2017.
  39. ↑ CitriSurf Citric Acid Passivation. Stellar Solutions, 2016.
  40. ↑ Passivating with Citric Acid. Logic Inc. (ecologiccleansers.com), Accessed June 2020.
  41. ↑ Troske, W. Nitric vs Citric Passivation Methods. Advanced Plating Technologies, 2019.
  42. ↑ a b c Maller, RR. Passivation of stainless steel. Trends in Food Science & Technology, vol. 9, no. 1, 2009, pp. 28–32.
  43. ↑ a b Aerospace Material Specification 2700 (AMS2700) Passivation of Corrosion Resistant Steels. SAE Aerospace, 2011 (not the most recent revision).
  44. ↑ Olsson, COA, and Landolt, D. Passive films on stainless steels—chemistry, structure and growth. Electrochimica Acta, vol. 48, 2003, pp. 1093–1104.
  45. ↑ What Is Passivation? Koslow Scientific Company, retrieved online June 2020.
  46. ↑ Debold, TA, and Martin, JW. How To Passivate Stainless Steel Parts. Modern Machine Shop, 2003.
  47. ↑ Additional passivation needed? Low Oxygen Brewing forum, 2018.
  48. ↑ Iron(III) citrate. Wikipedia, accessed June 2020.
  49. ↑ Boil kettle turned blue. Low Oxygen Brewing forum, 2020.
  50. ↑ Modern Brewhouse passivation. The Modern Brewhouse website. 2020–2021. Accessed January 2021.
  51. ↑ a b Passivating Stainless Steel with Citric Acid. Finishing.com, 2007.
  52. ↑ ASTM International. Wikipedia, The Free Encyclopedia, accessed June 2020.
  53. ↑ Passivation of Stainless Steel Brewery, Winery, and Distillery Tanks. Stellar Solutions, Inc., 2016.
  54. ↑ Selection of stainless steels for handling phosphoric acid (H3PO4). British Stainless Steel Association, accessed June 2020.
  55. ↑ Ekstrand, BJ. Comparison of Passivation Modalities. Astro Pak Corporation, 2015.
  56. ↑ H2SO4 as passivation solution for Stainless Steel. Finishing.com, 2003.
  57. ↑ Borghi, EB., et al. Cleaning of stainless steel surfaces and oxide dissolution by malonic and oxalic acids. Journal of Nuclear Materials, vol. 229, no. 2, 1996, pp. 115–123.
  58. ↑ Stopping rust/ passivating stainless steel. Brew School website. 2020. Accessed November 22 2020.
  59. ↑ Yue, Y., et al. Passivity of stainless steel in sulphuric acid under chemical oxidation. Corrosion Engineering, Science and Technology, vol. 53, no. 3, 2018, pp. 173–182.
  60. ↑ Cleaning. Optimism Brewing Company, accessed June 2020.
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