Latest News on Nickel

STEEL AND COMPLIANCE WITH THE NICKEL REGULATIONS.....continued page 2

There is no suggestion that this steel (316L) would never ever release nickel due to its composition. The factors listed above, particularly surface finish and metallurgical aspects could have an adverse effect on nickel release values. Research has shown that the surface finish has a significant influence on the release of nickel ions, irrespective of composition. Where the metal surface is cold rolled, bright, and reflective, the nickel release decreases.

Although 316L alloys contain much more nickel than those specified above, the presence of high Chromium and Molybdenum and low carbon causes nickel to remain within the alloy and usually very little is actually released. Product components identified as ‘Non-compliant’ or ‘Re-submit to EN1811’ were not tested for the presence of sulphur, but there is a strong possibility that high sulphur may be present in these articles. Sulphur combines with manganese, initiating pitting corrosion sites in the presence of artificial sweat solution. Pitting corrosion could account for the elevated levels of nickel release resulting from the roughened surface creating ‘pits’.

Technical explanation: it is well known that the first stage in skin sensitisation by materials is a corrosion process and the formation of a soluble metal ion. The formation of soluble metal ions in artificial sweat solution depends upon the ability of the solution to corrode the metal. Very little or indeed no nickel release will be observed if the metal has high corrosion resistance properties.

CHROMIUM

316L Stainless Steel is known to be highly corrosion resistant. The term ‘Stainless Steel’ relates to iron-based alloys of high corrosion resistance.  To be called ‘Stainless’, the steel must contain more than 10.5 wt% chromium. The corrosion resistance of steel at about 10.5%Cr is weak and affords only mild atmospheric protection but its corrosion resistance increases significantly with increasing chromium content. However, for sufficient corrosion protection, a minimum of 17-18 wt% chromium is desirable. At this level, the chromium in the alloy reacts with oxygen dissolved in the artificial/natural sweat (Refer: EN1811 reference Standard for detailed procedure) and forms a very thin chromium rich oxide layer, known as a ‘passive film’ on the surface of the steel. This passive film is continuous, non-porous, and insoluble under normal conditions and effectively separates the alloy material (i.e. component being tested) from the sweat which ultimately results in very little or indeed no nickel release.

Chromium is the essential element for the formation and stabilization of this passive film. Other alloying elements may influence the effectiveness of chromium in forming or maintaining this film. For example, nickel which is enriched in a thin alloy layer below the passive film promotes re-passivation, especially in reducing environments; and molybdenum stabilizes the passive film in the presence of chlorides rich environment.

Increasing the chromium content, from the minimum of 10.5 wt% to 17-18wt% for stainless steel, greatly increases the stability of the passive film.

As nickel has to pass through the passive film, it acts as a strong barrier to nickel release. The little, or no nickel release observed from 316 Stainless Steel widely used in post assemblies, can be attributed to the high stability of the passive film formed as a result of the high chromium content.

In order to minimize the possibility of this happening in the future; that is to reduce the incidence of failure in products incorporating steel components, Customers may wish to consider replacing their existing grade of steel (possibly containing low chromium, high manganese and sulphur) with 316L Stainless Steel.

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