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Metal Release Mechanisms for Passive Stainless Steel in Citric Acid at Weakly Acidic pH
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. Karolinska Institutet, Sweden.
2016 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 163, no 10, C686-C693 p.Article in journal (Refereed) Published
Abstract [en]

Metal release investigations from stainless steel into citric acid (CA) solutions at near-neutral pH are relevant for food applications, cleaning, and passivation. This study investigated metal release from abraded stainless steel grade AISI 304 into 5 g/L CA at pH 3.1, 4.8, and 6.4 at 40°C, as compared to a control solution (10 mM KNO3). Polyacrylic acid (PAA) was used as a model solution with and without separation from the stainless steel surface by a membrane. No significant difference was found for the released amounts of Fe and Mn between CA, PAA, and KNO3 solutions at pH 3.1, suggesting other mechanisms than complexation. At pH 4.8 and 6.4, a significantly higher release was found for CA and PAA solutions compared with KNO3 solution, but not for PAA solution when PAA molecules could not reach the stainless steel surface due to membrane separation, implying a dominant complexation-induced metal release mechanism that requires adsorption and/or close vicinity of the complexing agent to the surface. Cr was enriched in the surface oxide (surface passivation) in complexing solutions and the release of Cr was most dependent on complexation by CA at pH 4.8 and 6.4.

Place, publisher, year, edition, pages
Electrochemical Society, 2016. Vol. 163, no 10, C686-C693 p.
National Category
Materials Chemistry
URN: urn:nbn:se:kth:diva-191472DOI: 10.1149/2.1041610jesOAI: diva2:956622

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.
TRITA-CHE-Report, ISSN 1654-1081 ; 2016:32
National Category
Materials Chemistry Metallurgy and Metallic Materials
Research subject
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)

QC 20160831

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

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