Electroplated Tin-Nickel Coatings as a Replacement for Nickel to Eliminate Nickel Dermatitis
By P. Møller, Technical University of Denmark; J.B. Rasmussen and S. Köhler, Elplatek A/S; and L.P. Nielsen, Danish Technological Institute
Editor’s Note: This paper is a peer-reviewed and edited version of a paper delivered at NASF SUR/FIN 2013 in Rosemont, Ill., on June 12, 2013. A printable PDF version is available by clicking HERE.
Nickel dermatitis (skin allergy) is a growing problem in numerous countries. The alarming frequency of sensitization to nickel, especially in the US, has caused nickel to be selected as the “Allergen of the Year” in 2008 by the American Contact Dermatitis Society. Nickel as a coating in contact with skin has already been regulated by the nickel EU directive [94/27/EC] since 1994. In this paper, tin/nickel alloy coatings (66.9 wt% Sn) electrodeposited from a chloride/fluoride-containing alloy electrolyte, are presented as an alternative for both nickel and bright chromium coatings. The main focus will be on the corrosion properties where the following corrosion investigations will be covered: corrosion potential measurements for the different coatings, estimation of corrosion rates for materials in galvanic coupling with tin/nickel coatings, salt spray test, medical tests and immersion test of tin/nickel coatings in artificial sweat.
Keywords: nickel dermatitis, tin-nickel alloy electrodeposits, nickel substitute
Introduction to Nickel dermatitis
Nickel allergy or nickel dermatitis is one of the most common types of contact allergic dermatitis. In many countries, electroplated nickel coatings have been banned in applications involving direct and prolonged skin contact. This is also the case when a nickel substrate or a nickel plated surface is coated with a protective layer of another metal such as chromium, gold, silver or a thin organic coating/paint. The reason is that even small pinholes, scratches or other defects in the protective layer may cause the release of nickel ions by corrosion. This is especially the case when the corrosion is accelerated by a galvanic coupling with other metals and in the presence of a “sweat induced electrolyte,” containing among others chloride, lactate or ammonia ions. Nickel ions form a lactate complex as well as ammonium complexes, which significantly increases the corrosion speed of nickel by reducing the thermodynamic stability of nickel.
The affected skin areas become intensely itchy and may become red and blistered (acute dermatitis) or dry, thickened and pigmented (chronic dermatitis) as a consequence of the allergic reaction.
The European Nickel Directive regulates the use of nickel products that come into direct and prolonged contact with the skin. These limits, known as migration limits, are:
0.2 μg/cm2/week for post-assemblies, which are inserted into pierced ears and other pierced parts of the human body,
0.5 μg/cm2/week for other products intended to come into direct and prolonged contact with the skin.
Medical treatments can reduce the symptoms of nickel allergy. However, once a person has developed a nickel allergy, that person will always have an increased sensitivity towards nickel and the need to avoid future contact with the metal.
Figure 1 shows nickel dermatitis (skin allergy) caused by a bright chromium plated ABS polymer. The thickness of the chromium layer is normally below 1.0 µm and many small defects in the chromium layer make it possible to establish a galvanic coupling with the bright nickel layer below. Sweat which contains ammonia, chloride and organic acids such as lactate forms a chemical complex with nickel and accelerates the anodic dissolution process. The cathode reaction takes place on the chromium layer and can be described as a reduction involving oxygen. The red coloring of the cotton swab indicates that nickel is corroding and is caused by the indicator dimethylglyoxime [Fig. 2 and Ref. (8)].