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Metal release and corrosion resistance of different stainless steel grades in simulated food contact
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
2016 (English)In: Corrosion, ISSN 0010-9312, E-ISSN 1938-159X, Vol. 72, no 6, 775-790 p.Article in journal (Refereed) Published
Abstract [en]

A new technical guideline has been implemented by the Council of Europe (CoE) to ensure the stability and safety of food contact articles of metals and alloys, using 5 g/L citric acid (pH 2.4) and artificial tap water DIN 10531 (pH 7.5) as food simulants. The objectives of this study were: (i) to quantify the extent of metal release from austenitic (grades AISI 201, 204, 304, and 316L), ferritic (grades AISI 430 and EN 1.4003), and lean duplex stainless steel (grade EN 1.4162) in citric acid (5 g/L, pH 2.4) and in artificial tap water (pH 7.5); (ii) to compare the release of metals to the surface oxide composition, the open circuit potential-time dependence, and the corrosion resistance; and (iii) to elucidate the combined effect of high chloride concentrations (0.5 M NaCl) and citric acid at pH 2.2 and 5.5 on the extent of metal release from AISI 304 with and without prior surface passivation by citric acid. Exposures of all stainless steel grades in citric acid and artificial tap water up to 10 d (at 70°C/40°C) resulted in lower metal release levels than the specific release limits stipulated within the CoE protocol. For all grades, metals were released at levels close to the detection limits when exposed to artificial tap water, and higher release levels were observed when exposed to citric acid. Increased surface passivation, which resulted in reduced metal release rates with time, took place in citric acid for all grades and test conditions (e.g., repeated exposure at 100°C). There was no active corrosion in citric acid at pH 2.4. Fe (in citric acid) and Mn (in all solutions, but mostly tap water) were preferentially released, as compared to their bulk alloy content, from all stainless steel grades. Ni was released to the lowest extent. 0.5 M NaCl induced a very low (close to detection limits) metal release from grade AISI 304 at pH 5.5. When combined with citric acid (5 g/L) and at lower pH (2.2), 0.5 M NaCl induced slightly higher metal release compared to citric acid (pH 2.4) alone for coupons that were not pre-passivated. Pre-passivation in 5 g/L citric acid (pH 2.4) at 70°C for 2 h largely reduced this solution dependence. Pre-passivation resulted in an up to 27-fold reduced extent of metal release in solutions containing citric acid and/or NaCl at pH 2.2 to 5.5, and resulted in improved reproducibility among replicate samples.

Place, publisher, year, edition, pages
NACE International, 2016. Vol. 72, no 6, 775-790 p.
Keyword [en]
Chloride, Citric acid, Corrosion resistance, Food, Food safety, Metal release, Stainless steel, Surface oxide
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-191469DOI: 10.5006/2057ScopusID: 2-s2.0-84973616656OAI: oai:DiVA.org:kth-191469DiVA: diva2:956616
Note

QC 20160901

Available from: 2016-08-30 Created: 2016-08-30 Last updated: 2016-09-01Bibliographically approved
In thesis
1. Metal Release and Corrosion of Stainless Steel in Simulated Food Contact
Open this publication in new window or tab >>Metal Release and Corrosion of Stainless Steel in Simulated Food Contact
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Knowledge on metal release behaviour of stainless steels used in food processing applications and cooking utensils is essential within the framework of human health risk assessments. Recently, a new European test guideline (the CoE protocol) has been implemented to ensure safety of metals and alloys in food contact, such as stainless steels. This guideline suggests 5 gL-1 citric acid (pH 2.4) as a food simulant for acidic foods of pH ≤ 4.5. So far, limited assessments exist that investigate the correlation between the bioaccessibility, material characteristics, corrosion behaviour and surface chemistry of stainless steel for food application tests using citric acid. Therefore, this doctoral thesis comprises an in–depth interdisciplinary and multi–analytical research effort to fill this knowledge gap.

This work includes thorough investigations of a range of stainless steel grades in simulated food contact as a function of different important parameters such as grades, surface finish, temperature, pH, solution composition, metal complexation and buffering capacity, concentration of the complex forming agents, loading, and repeated usage. This is accomplished by kinetic studies of metal release, electrochemical, and surface analytical investigations. Another focus of this thesis is to assess the dominating metal release process in citric acid or chloride containing solutions of varying pH.

This study suggests protonation (at acidic pH) and surface complexation (at weakly acidic and neutral pH) as the predominant metal release mechanisms for stainless steel in citric acid solutions. Solution complexation may also play a role by hindering metal precipitation at weakly acidic and neutral pH, and metal release from surface defects / inclusions may initially be important for non-passivated surfaces.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. 63 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2016:32
National Category
Materials Chemistry Metallurgy and Metallic Materials
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-191474 (URN)978-91-7729-067-4 (ISBN)
External cooperation:
Public defence
2016-09-22, F3, Lindstedtsvägen 26, KTH Campus, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20160831

Available from: 2016-08-31 Created: 2016-08-30 Last updated: 2016-08-31Bibliographically approved

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