*3.5. Galvanic Corrosion*

Table 11 presents the corrosion potentials measured in artificial sweat (EN 1811-2011+A1:2015) [50] for the precious alloys and austenitic steels. The measured potential values enable to establish a relative comparison of the alloys nobility in the considered medium and to construct a galvanic series. Higher potential values mean higher corrosion resistance. The alloys series with higher negative potentials (anodic) generally tend to undergo greater corrosion in the event of a galvanic coupling, while other metals (cathodic) will generally undergo a reduced attack. According to Mansfeld and Kenkel, the corrosion potential of each alloy is a criterion in the analysis of galvanic corrosion behavior, but it is still insufficient. The electrical potential values can only indicate a trend and state absolutely nothing about the rate of corrosion and the type of control of the galvanic cell (mixed, cathodic or anodic) [63,64].


**Table 11.** Galvanic series established in an EN1811-2011+A1:2015 artificial sweat type environment.

The most frequent cases we encountered are precious metal-austenitic steel assemblies in watch straps. Thus millions of gold-steel links are produced to assemble straps, this is the ideal case for the formation of a galvanic cell, a significant difference in electrical potential being involved. In the case of gold-steel, a difference in electrical potential of around 300 mV can be calculated.

According to Gilbert and Mali, while corrosion per se may not be of great concern, when combined with mechanical effects, restricted crevice-like geometries or any combination thereof, considerably amplified corrosion rates might arise [65].

One of the several available techniques to realize the gold-steel assembly is brazed gold caps. The brazed gold caps reveal a particularity due to brazing. The solder acts as the anode (a small area) and the steel and gold parts are the cathode (large area). Thus, the corrosion process results in the dissolution of the solder (Figures 22 and 23). In such a type of assembly it is particularly important to make the right choice of solder.

**Figure 22.** Galvanic corrosion in a gold-steel assembly.

**Figure 23.** Corrosion of the transverse surface, at the level of the gold-steel interface.

Two aspects of great importance have to be considered:


Table 12 presents a series of tests carried out on the same types of gold-stainless steel links. The 14441/316 LM steel originated from five different steelmakers from the EU, USA, Japan and China.


**Table 12.** Tests results for gold-stainless steel links.

In case of #6 (gold-steel 904L), the nickel release was greater than 2 <sup>μ</sup>g·cm−2·week−1, despite the absence of visible corrosion. This was due to a different behavior compared to a medical 316L steel. The comparison of the EDX profile between a gold-904L steel (Figure 24) and a gold-316LM steel (Figure 25) revealed a very different nickel profile with the disappearance of the nickel peak in the gold-904L steel solder (Figure 24). In the gold-904L system, the solder was in the anodic position (gold and 904L steel being cathodic), with a very unfavorable surface report. It revealed a selective corrosion morphology powered by a galvanic battery; this would explain the significant nickel release from the gold-904L steel coupling, despite the absence of visible corrosion.

**Figure 24.** Sample #6. (**a**) EDX profiles of the gold-904 L steel solder for iron, nickel, silver, gold, copper and zinc; (**b**) EDX Ni profile.

**Figure 25.** Sample #4. (**a**) EDX profiles of the gold-316 LM steel solder for iron, nickel, silver, gold, copper and zinc; (**b**) EDX Ni profile.

SEM examination of sample #4 (5N18) showed that the corrosion was localized and did not develop at the level of the steel, but of the brazing, causing its dissolution (Figure 26). This demonstrates that the solder represents the weak point of the gold-steel assembly.

The steels which are being used for watch straps are of grades 316 and 904L. The other steel grades, such as 304, 304L, 316LS, and 316Ti, are not usable; their rate of Ni release does not respect the limits imposed by the EU directives, or other countries legislations (USA, Japan, China, Korea, Canada). The difficulty consists in eliminating the corrosion process and achieving a rate of nickel release which respects the legislation: max 0.5 <sup>μ</sup>g·cm−2·week<sup>−</sup>1.

The use of a Ni-Cr-P solder involves the risk of increasing the nickel amounts by chemical dissolution or corrosion of the solder. In the case of a gold base solder (melting range 750–850 ◦C) the risk is to initiate corrosion in the steel; with a Ni-Cr-P based solder (melting range 800–950 ◦C) the risk is to start corrosion in the solder. To find the best compromise, testing the link assemblies is necessary. Because the quality of 316 LM steel is highly dependent on the supplier, most straps manufacturers use steels they have exclusivity for.

**Figure 26.** SEM examination of sample #4.
