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Metal Release and Corrosion of Stainless Steel in Simulated Food Contact
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
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: urn:nbn:se:kth:diva-191474ISBN: 978-91-7729-067-4OAI: oai:DiVA.org:kth-191474DiVA: diva2:956623
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
List of papers
1. Metal release from stainless steel powders and massive sheets - comparison and implication for risk assessment of alloys
Open this publication in new window or tab >>Metal release from stainless steel powders and massive sheets - comparison and implication for risk assessment of alloys
2013 (English)In: Environmental Sciences: Processes and Impacts, ISSN 2050-7887, Vol. 15, no 2, 381-392 p.Article in journal (Refereed) Published
Abstract [en]

Industries that place metal and alloy products on the market are required to demonstrate them being safe for all intended uses, and that any risks to humans, animals or the environment are adequately controlled. This requires reliable and robust in-vitro test procedures. The aim of this study is to compare the release of alloy constituents from stainless steel powders of different grades (focus on AISI 316L) and production routes into synthetic body fluids with the release of the same metals from massive sheet in relation to material and surface characteristics. The comparison is justified by the fact that the difference between massive surfaces and powders from a metal release/dissolution and surface perspective is not clearly elucidated within current legislations. Powders and abraded and aged (24 h) massive sheets were exposed to synthetic solutions of relevance for biological settings and human exposure routes, for periods up to one week. Concentrations of released iron, chromium, nickel, and manganese in solution were measured, and the effect of solution pH, acidity, complexation capacity, and proteins elucidated in relation to surface oxide composition and its properties. Implications for risk assessments based on in-vitro metal release data from alloys are elucidated.

Keyword
Patch Test Reactivity, Particles In-Vitro, Ion Release, Nickel Release, Pure Metals, Orthodontic Appliances, Ferrochromium Alloy, Contact-Dermatitis, Oxygen Reduction, Aisi 316l
National Category
Environmental Sciences
Identifiers
urn:nbn:se:kth:diva-105520 (URN)10.1039/C2EM30818E (DOI)000315397700008 ()2-s2.0-84874439615 (ScopusID)
Note

QC 20130213

Available from: 2012-11-22 Created: 2012-11-22 Last updated: 2016-08-30Bibliographically approved
2. Comparison of the influence of citric acid and acetic acid as simulant for acidic food on the release of alloy constituents from stainless steel AISI 201
Open this publication in new window or tab >>Comparison of the influence of citric acid and acetic acid as simulant for acidic food on the release of alloy constituents from stainless steel AISI 201
2015 (English)In: Journal of Food Engineering, ISSN 0260-8774, Vol. 145, 51-63 p.Article in journal (Refereed) Published
Abstract [en]

To ensure the safety of metals and alloys intended for food contact, a new European test protocol (CoE protocol) using citric acid as a food simulant was published in 2013. This study investigated the influence of citric acid and exposure conditions on the metal release from an austenitic manganese stainless steel (AISI 201). Exposures in 5 g/L citric acid resulted in significantly lower metal releases compared with specific release limits set by the CoE protocol. 5 g/L (0.3 vol%) citric acid was more aggressive than 3 vol% acetic acid (Italian protocol) due to higher metal complexation. Studies on abraded surfaces revealed that most metals were released during the first 0.5 h of exposure due to surface passivation. Surface abrasion, increased temperature (40-100 degrees C), increased surface area to solution volume ratio (0.25-2 cm(2)/mL) and increased citric acid concentration (0-21 g/L) all resulted in increased released metal quantities.

Keyword
Austenitic stainless steel, Food contact, CoE protocol, Test guideline, Complexation, Metal release, Surface oxide
National Category
Food Engineering
Identifiers
urn:nbn:se:kth:diva-156095 (URN)10.1016/j.jfoodeng.2014.08.006 (DOI)000343379500007 ()2-s2.0-84907192007 (ScopusID)
Available from: 2015-01-09 Created: 2014-11-21 Last updated: 2016-08-30Bibliographically approved
3. Metal release and corrosion resistance of different stainless steel grades in simulated food contact
Open this publication in new window or tab >>Metal release and corrosion resistance of different stainless steel grades in simulated food contact
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
Keyword
Chloride, Citric acid, Corrosion resistance, Food, Food safety, Metal release, Stainless steel, Surface oxide
National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-191469 (URN)10.5006/2057 (DOI)2-s2.0-84973616656 (ScopusID)
External cooperation:
Note

QC 20160901

Available from: 2016-08-30 Created: 2016-08-30 Last updated: 2016-09-01Bibliographically approved
4. Influence of Citric Acid on the Metal Release of Stainless Steels
Open this publication in new window or tab >>Influence of Citric Acid on the Metal Release of Stainless Steels
2015 (English)In: Corrosion Science and Technology, ISSN 1598-6462, Vol. 14, no 4, 166-171 p.Article in journal (Refereed) Published
Abstract [en]

Knowledge of how metal releases from the stainless steels used in food processing applications and cooking utensils is essential within the framework of human health risk assessment. A new European standard test protocol for testing metal release in food contact materials made from metals and alloys has recently been published by the Council of Europe. The major difference from earlier test protocols is the use of citric acid as the worst-case food simulant. The objectives of this study were to assess the effect of citric acid at acidic, neutral, and alkaline solution pH on the extent of metal release for stainless steel grades AISI 304 and 316, commonly used as food contact materials. Both grades released lower amounts of metals than the specific release limits when they were tested according to test guidelines. The released amounts of metals were assessed by means of graphite furnace atomic absorption spectroscopy, and changes in the outermost surface composition were determined using X-ray photoelectron spectroscopy. The results demonstrate that both the pH and the complexation capacity of the solutions affected the extent of metal release from stainless steel and are discussed from a mechanistic perspective. The outermost surface oxide was significantly enriched in chromium upon exposure to citric acid, indicating rapid passivation by the acid. This study elucidates the effect of several possible mechanisms, including complex ion- and ligand-induced metal release, that govern the process of metal release from stainless steel under passive conditions in solutions that contain citric acid.

Place, publisher, year, edition, pages
The Corrosion Science Society of Korea, 2015
Keyword
metal release, stainless steel, citric acid, food, surface oxide
National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-191471 (URN)10.14773/cst.2015.14.4.166 (DOI)
External cooperation:
Note

QC 20160901

Available from: 2016-08-30 Created: 2016-08-30 Last updated: 2016-09-01Bibliographically approved
5. Metal Release Mechanisms for Passive Stainless Steel in Citric Acid at Weakly Acidic pH
Open this publication in new window or tab >>Metal Release Mechanisms for Passive Stainless Steel in Citric Acid at Weakly Acidic pH
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
National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-191472 (URN)10.1149/2.1041610jes (DOI)
External cooperation:
Note

QC 20160901

Available from: 2016-08-30 Created: 2016-08-30 Last updated: 2016-09-01Bibliographically approved

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