Influence of Citric Acid on the Metal Release of Stainless Steels

Abstract

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.

Surface characterisation of fine inert gas and water atomised stainless steel 316L powders: formation of thermodynamically unstable surface oxide phases

“…In contrast to the inert gas atomised powder with only trivalent chromium in the surface oxide, chromium seems to be primarily present in its hexavalent state within the surface oxide of the water atomised powder. Previous findings by some of the authors have shown that the chromium release occurs at very low levels from the WA<45 μm powder after exposure in aqueous solutions and in amounts of the same range as for the GA powders,46 which is in agreement that the oxidised chromium of the WA<45 μm powder is not present as chromate (Fig. 4), which is relatively high soluble.…”

Inhalation toxicity of 316L stainless steel powder in relation to bioaccessibility

“…The most important difference from a toxicological perspective is that the gas-atomized SS particles have a thin (a few nanometre) 32 surface oxide composed only of trivalent Cr and Fe, 10 whereas welding fume particles reveal surface oxides that are at least >10-fold thicker 33 compared with the particles investigated here and enriched in, for instance, chlorine, fluorine and potassium. 33 It has, in addition, been shown in numerous publications that welding fume particles are composed of a large amount of soluble chromate 34 –36 or even only chromate (no trivalent Cr), 36 which is dissolved as chromates into solution 33 at concentrations about 30,000-fold higher 37 in comparable solutions 38 as in this study for inert gas-atomized particles. Their release is however strongly depending on welding conditions.…”

Electrochemical Study of Three Stainless Steel Alloys and Titanium Metal in Cola Soft Drinks

“…For instance, the high corrosion resistance of stainless steel and titanium alloys used in dental devices is based on the formation of a passive oxide film at the metal surface when it is in contact with oxygen from the air, or is immersed in a water-containing media. This passive oxide films may experience dissolution in a strong acid media such as chloride and fluoride-containing products [7][8][9][10]. These are usually commercial products such as soft-drinks, toothpastes and mouthwashes.…”