Steel and compliance with the nickel regulations
The use of nickel is regulated in products intended to come into direct and prolonged contact with the skin as it may cause an allergic reaction. The Laboratory at the Birmingham Assay Office is a leading Nickel testing specialist in the UK.  Our experts regularly work with the Nickel Working Group, Nickel Development Institute, British Medical Association and the Nickel Producers Environmental Research Association to carry out ongoing research and evaluation of nickel testing and regulation. The Laboratory is also frequently called upon to offer technical advice and investigate possible reasons for a particular product being non-compliant.

Whenever any product component is marked as ‘Non-compliant’ or ‘Re-submit to EN1811’ after testing by the reference standard EN 1811 or by the New ‘Quick Nickel Release Test’, the Laboratory at the Birmingham Assay Office investigate  possible reasons for the failure/nickel release as a matter of routine. These investigations are undertaken purely out of academic interest and such information is passed on to Customers only when it is absolutely compelling and in the public interest.

Examination of testing data carried out recently clearly indicates that some steel product components; specifically the watch back’s and earring post of earring post assemblies, which were identified as being ‘Non-compliant’ as they were releasing nickel at a level greater than the threshold value specified in the EN 1811 reference standard exhibited the following approximate compositions:

7.0-8.0% Manganese, 14.5-15.5%Chromium, 4.0-4.2%Nickel, 1.5-2.0%Copper, Plus others (includes carbon, phosphorus, silicon, sulphur, nitrogen and iron)

8.5-10% Manganese, 15.0-16.0%Chromium, 0.8-1.2%Nickel, 1.5-2.0%Copper

Plus others (includes carbon, phosphorus, silicon, sulphur, nitrogen and iron)

0.5-1.0% Manganese, 11.5-14.0%Chromium, 3.5-5.5%Nickel, 0.50-1.0% Molybdenum Plus others (includes carbon, phosphorus, silicon, sulphur, and iron)

1.0% Manganese, 11.5-13.5%Chromium, 1.2-2.5%Nickel Plus others (includes carbon, phosphorus, silicon, sulphur, and iron)

There is evidence available in the literature that an alloy of the above composition(s) exhibits variable nickel release results, the rate of nickel release some time could be significantly lower or higher than the threshold value specified in the EN 1811 reference standard. The extent of this variability is largely dependent upon sulphur content, surface finish, nickel content, corrosivity and metallurgical aspects. Alloys of such composition usually fail to comply with the requirements of the Nickel Regulations stipulated for piercing post assemblies (i.e. less than 0.2microgram per square centimeter per week). An alloy of the above composition in majority of cases could therefore cause a health hazard to those sensitive to nickel to the highest degree. For this reason, manufacturers of nickel containing jewellery often prefer to use 316L steel (Approximate composition of 316L Steel: Fe, <0.03% C, 16-18.5% Cr, 10-14% Ni, 2-3% Mo, <2% Mn, <1% Si, <0.045% P, <0.03% S) in piercing post assemblies and other similar products, which in the majority of cases is known to comply with the requirements of the Nickel Regulations, provided they are used correctly, and the surface of the product is not contaminated by nickel during the fabrication process; for example by drawing a wire through a stainless steel die, pressing, plating with a nickel containing solution etc.

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.