Metal release from stainless steel powders and massive sheets – comparison and implication for risk assessment of alloys

Abstract

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.

Influence of Citric Acid on the Metal Release of Stainless Steels

“…This is in agreement with previous investigations of metal release from these grades in citric acid and other aqueous acidic or neutral solutions [11][12][13][14][15][16][17][18][19] . The release pattern is related to the passive mixed surface oxide of Cr 2 O 3 and Fe 2 O 3 with metallic nickel present in the alloy surface layer beneath the surface oxide 11,20) . Part of this nickel is only released initially and is related to the initial presence of surface defects (as in this study) 21) or released in any case of active corrosion (not observed in this study).…”

“…Generally, the amounts of all released metals were relatively low. This indicates no active or metastable corrosion, which is expected from previous studies at similar conditions 11) (Mazinanian, Odnevall Wallinder, Hedberg, unpublished results).…”

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

“…Higher release for abraded/aged surfaces compared with as-received surfaces, and similar trends, are in agreement with previous studies on metal release from austenitic stainless steel grade 304 exposed into ALF at 37°C (Herting et al, 2006) and from ferritic stainless steel grade 430 immersed into 3 vol% acetic acid (pH 2.4) for 10 days at 40°C (Herting et al, 2008a). A preferential release of Fe and Mn, compared with Cr and Ni was observed for all exposures, independent of sample preparation, in agreement with literature findings on stainless steels (Galván et al, 2012;Hedberg et al, 2013;Herting et al, 2008b;Okazaki and Gotoh, 2005;Virtanen et al, 2008). Comparable released amounts of Cr from abraded compared with as-received surfaces, Fig.…”

“…These findings are in agreement with previous chromium speciation measurements for released chromium from chromium-containing alloys in synthetic biological fluids (Flint et al, 2007;Hedberg et al, 2013;Hedberg and Odnevall Wallinder, 2014).…”

“…absence of peaks assigned to Fe, Cr, Mn, and Ni in their metallic form after 2 and 26 h (data not shown). Reduced metal release rates with time are expected for stainless steel due to the passive surface oxide properties (Hedberg et al, 2013;Herting et al, 2008a;Jensen et al, 2003;Karimi et al, 2012). However, this effect may be less significant at elevated temperatures due to altered oxide properties (Neville and Hodgkiess, 1996) and increased tendency toward pitting (Sedriks, 1996), increased ligand-induced metal release (Essington, 2004), and/or changed chemical equilibrium.…”