a b s t r a c tTo 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°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.
Differences in surface oxide characteristics and extent of nickel release have been investigated in two thoroughly characterized micron-sized (mainly <4 μm) nickel metal powders and a nickel oxide bulk powder when immersed in two different synthetic fluids, artificial sweat (ASW-pH 6.5) and artificial lysosomal fluid (ALF-pH 4.5) for time periods up to 24h. The investigation shows significantly more nickel released from the nickel metal powders (<88%) compared to the NiO powder (<0.1%), attributed to differences in surface properties. Significantly more nickel was released from the nickel metal powder with a thin surface oxide predominantly composed of non-stoichiometric nickel oxide (probably Ni(2)O(3)), compared to the release from the nickel metal powder with a thicker surface oxide predominantly composed of NiO and to a lesser extent Ni(2)O(3) (88% and 25% release after 24 h in ALF, respectively). Significantly lower amounts of nickel were released from the nickel metal powders in ASW (2.2% and <1%, respectively). The importance of particle and surface characteristics for any reliable risk assessment is discussed, and generated data compared with literature findings on bioaccessibility (released fraction) of nickel from powders of nickel metal and nickel oxide, and massive forms of nickel metal and nickel-containing alloys.
Industries that place metal and alloy products on the market are required to demonstrate that they are 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 sheets 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 of 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.
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 KNO 3 ). 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 KNO 3 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 KNO 3 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 Citric acid-stainless steel interactions are of high relevance in food contact, 1-5 for surface cleaning, 6,7 and surface passivation. 8 Metal release (release of metal species) from stainless steel surfaces into citric acid containing solutions can generally be governed by chemical or electrochemical oxide dissolution (protonation, ligand/complexationinduced dissolution, and reductive or oxidative dissolution), corrosion (metal oxidation), and physical processes (e.g. due to friction).9 Metal release investigations are not only important for the area of risk and safety assessments, but can also be a sensitive measure of dissolution and corrosion processes for passive metals and alloys, where electrochemical activity is limited. 9 We have earlier investigated metal release from stainless steel (different grades) into citric acid containing solutions from a food safety perspective, 10,11 as citric acid is one of the food simulants suggested in a relatively new European guideline 1 for testing metals and alloys used in food contact. 10 It became clear that citric acid caused an initial metal release that results in surface passivation (hindering further release), and that the mechanism does not involve any active corrosion (metal oxidation) in solutions containing citric acid and no chlorides.10 Combined release and wettability studies on stainless steel (grade AISI 304) suggested an adsorption-controlled complexation-induced metal release mechanism.12 This study was however conducted at pH 2.4, where metal-citrate complexation exists, but not to a large extent (except for chromium, Cr).9 It has been debated whether complexation-induced metal release plays a major role for metal release from stainless steel in weakly or non-acidic biological environments and whether it can contribute to other mechanisms, such as corrosion or other chemical dissolution processes.9...
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