Corrosion Resistance of Stainless Steels Intended to Come into Direct or Prolonged Contact with the Skin

The biocompatibility of materials in contact with a living tissue becomes a puzzle in the overall picture of assessing the toxic effects of chemicals that come into contact with us. Allergic reactions to substances are a significant and growing health problem affecting large parts of the population in Europe. Wristwatches are objects worn in prolonged contact with the skin, being subject to localized corrosion, especially pitting and crevice types, in sulfide-chloride medium, and high wear in the bracelets joints. Watches of medium quality are usually made of stainless steels. The X2 CrNiMo 17-12-2 316L grade as well as X1 CrNiMo 20-25-5 Cu 1 or 904L are commonly used, having good resistance to generalized corrosion. The passive layer is nevertheless insufficient to ensure complete immunity in all cases of localized corrosion encountered during wear. For this reason, a high-corrosion-resistant steel: X1 CrNiMo 18-15-4 N 0.15 or 317LMN, from three different suppliers was evaluated. Metallographic characterization was carried out. The corrosion behavior evaluation was performed for the generalized corrosion, pitting and crevice corrosion and galvanic corrosion. Galvanic couples steel 317LMN-gold 18K alloy 3N and gold 18K 5M were used. The results of the generalized and pitting corrosion test indicated three basic groups. All of the 317LMNs were similar. The 316L variants tested noticeably worse. The 904Ls were difficult to discern, but certainly easier than the 316Ls and, possibly, at least comparable to the 317LMNs.


Introduction
Since 2006, Europe has a new vision regarding chemicals toxicology to humans, because of the REACH regulation (registration, evaluation, authorization, and restriction of chemicals) [1,2].
Thus the biocompatibility of materials in contact with a living tissue becomes a puzzle in the overall picture evaluating the effects of chemicals toxicity that come into contact with us [3][4][5].
ECHA (European Chemical Agency) [6] has developed a plan for the implementation of the substances of very high concern (SVHC) [7], namely endocrine disruptors (ED), carcinogens, mutagens, toxics for reproduction (CMR), and sensitizers (skin sensitizers and respiratory sensitizers). Therefore, we have to reconsider our system for assessing the toxicology of substances, mixtures of substances and devices in our homes. Among the SVHC substances incriminated by ECHA, the most frequent  The manufacturers deliver the steels with nominal compositions. In accordance with the manufacturer, the compositions of the metals may vary as in shown in Table 1.
The chemical compositions were analyzed by XRF spectrometer, carbon analysis by IR carbon analysis, and sulfur and nitrogen by IR elemental analyzer.

Gold Base Alloys Used in Galvanic Coupling Measurements
For manufacturing jewelry, watches and other objects to wear, so called "gold jewelry" alloys are used. The amount of gold in the alloy is perfectly defined and the standard of proportions varies from one country to another [26,27]. In Europe, the title of 18K (750 Gold) is predominant in the jewelry market.

Selection of the 317LMN Stainless Steel
The austenitic stainless steels considered are characterized by very low carbon content and significant additions of nickel, chromium, and molybdenum. They have the property of developing a passive layer under anodic conditions which is formed mainly by chromium and molybdenum oxides. Molybdenum plays a decisive role in the stability of this layer. Improving corrosion resistance cannot be achieved by increasing the amount of these elements. In fact, the most highly alloyed stainless steels can develop intermetallic precipitation during heat treatments. These are at the origin of a loss of stability of the passive layer by depletion of the chromium and molybdenum elements.
Developing grades with improved corrosion resistance and metallurgic stability can be achieved by the combination of additions of nitrogen and manganese. It is thus possible to equal the super austenitic grade 904L with a much less alloyed and mechanically stronger grade, namely 317LMN.

Metallography
The samples were embedded in a self-curing methyl methacrylate resin, then polished with SiC paper and finally with diamond spray (6/3/1 microns). Electrolytic etching was carried out in a bath of 100 mL H 2 O, 10 mL HCl, and 5g Cr(VI)-oxide during 5 s under 0.4 V and 0.3 A. The alloys microstructures were observed using a metallographic microscope (Polyvar Met, Reichert-Jung, Vienna, Austria). The scanning electron microscopes (Sigma, Zeiss, Jena, Germany) with an Oxford X-MAX EDX Instrument for microanalysis were also used. The analyzed samples were covered with a gold flash.

Evaluation of the Corrosion Behavior
The corrosion behavior evaluation of the alloys was carried out based on techniques specific to the type of corrosion considered [28,29]: electrochemical evaluation of the generalized corrosion by the technique of the rotating electrode [30] and taking into account, for evaluation, the American Society for Testing and Materials ASTM G3-89 [31] and ASTM G59-97 [32] standards; -pitting and crevice corrosion according to the ASTM F746-04 [33] standard; -galvanic corrosion [34] taking into account the mixed potential theory [35].

Evaluation of the General Corrosion
A potentiostat-galvanostat Eg&G PARSTAT 4000 with a cell of three electrodes (the reference electrode in saturated calomel (SCE) and the counter electrode in platinum), adapted for rotating electrode measurements was used. The potentiostat is equipped with a low current interface so extends the current measurement to 80 fA range(2.5 aA resolution).The tests were carried out at the ambient temperature in an artificial sweat electrolytic solution according to European Standard EN 1811-2011 [36], with the following composition: 1 ± 0.01 g·L −1 urea, 5 ± 0.05 g·L −1 NaCl, 1 ± 0.01 g·L −1 the corrosion potential (E corr ) and the corrosion current density (i corr ) determined from the Tafel plots in the domain ±150 mV SCE vs. E oc ; -the breakdown potential (E b ) from the potentiodynamic polarization curve between −1000 mV and +1200 mV/SCE; -coulometric analysis by zones, the first zone between E (I = 0) and +300 mV and the second zone between +300 mV and +600 mV.

Evaluation of the Localized Corrosion
According to ASTM F746-04 [33] standard, the tests were carried out in NaCl 9 g·L −1 at the room temperature. The samples used were cylinders (Ø = 5 mm and L = 20 mm), polished (paper P600) which are inserted on the conical rings in polytetrafluoroethylene (PTFE).
The test consisted first of all in an anodic excitement at +800 mV SCE for 10 s. Secondly, the potentiostat imposes a value of potential E oc . The potentiostatic curve is traced for 15 min. If the current registered remains in the cathodic domain, the potentiostat resumes the anodic excitement at +800 mV and will then impose a potential increase of +50 mV compared to the previous one. The cycles are repeated each time for a higher potential until the current measured remains in the anodic domain. Thus the crevice potential, E crev is the potential that corresponds to the penultimate measurement where the current is positive.

Evaluation of the Galvanic Corrosion
Steel-gold alloy assemblies are ideal for the formation of galvanic cells [11,28]. We are dealing with significant electrical potential differences (E couple ), which can be measured up to approximately 300 mV. A release of cations to the skin will take place. This is the typical case of nickel release, which is currently regulated by Annex XVII REACH [17]. There is also the aspect of the cathode-anode report. The surfaces of precious metals will be the cathode, and the other less noble parts will be the anode. Constructions with large cathodic surfaces and small anodic surfaces are very dangerous. The galvanic cell will discharge a strong anode current which will induce the rapid degradation of the anodic part by crevice or pitting corrosion mechanisms. This is probably the type of corrosion found most frequently in bracelets. Typical assemblies of the gold-steel links are shown in Figure 1. anodic part by crevice or pitting corrosion mechanisms. This is probably the type of corrosion found most frequently in bracelets. Typical assemblies of the gold-steel links are shown in Figure 1. There are two ways to investigate this type of galvanic corrosion: by direct measurements (Ecouple) or by prediction techniques. In the present study we used direct measurements and calculated the mixed potentials.
Direct Measurements [34]  There are two ways to investigate this type of galvanic corrosion: by direct measurements (E couple ) or by prediction techniques. In the present study we used direct measurements and calculated the mixed potentials.
Direct Measurements [34] The most direct procedure involves immersing the two different metals in an electrolyte and electrically connecting them, using a zero resistance ammeter to measure the current. This technique is very accurate for time-dependent polarization, but is expensive and time consuming. In general, the couplings are measured for three to four days to obtain reliable information. Individual samples are weighed before and after the test to determine the corrosion rate as a function of potential, and thus make the necessary corrections using Faraday's law. For this type of measurements we used the electronic assembly indicated by EG & G PAR equipped with a low current interface so extends the current measurement to 80 fA range (2.5 aA resolution). The measurements are carried out in two steps: in the first step, the open-circuit potentials of each coupling partner are recorded and in the second step the circuit is closed and the electric current discharged into the coupling is recorded. All samples were 11 mm diameter discs which are mounted in PTFE caps designed for this technique. They were obtained by spark machining (Electrical discharge machining (EDM)) technique transversely cutting from a 6 mm band. The samples are polished, washed with a mixture of acetone and ethanol, rinsed with deionized water 18 MΩ·cm and dried.
The measurements were made in the galvanic measurement cell, the electrodes being the samples used in the direct coupling measurements. According to EN 1811-2011 [36], the electrolytic solution used is artificial sweat, at room temperature. The volume used for the electrochemical test is 600 mL.

Indirect Measurements
The method is based on the mixed potential theory. Galvanic corrosion can be described in terms of mixed potential theory [35], as schematically illustrated in Figure 2. However, in the case of a bimetallic or multi-metal galvanic coupling in which two or more metals are electrically in contact, according to the theory there will be at least two cathodic and two anodic reactions. One of each of these reactions occurs on each metal. In this case, the most noble of the two metals will be cathodically polarized in the electronegative sense, and its anodic rate of reaction will be suppressed. Reciprocally, the less noble, or more sacrificial, more anodic material will be anodically polarized, and so the anodic rate of reaction will be accelerated. The resulting galvanic current can be determined solely from the sums of all anodic and cathodic currents for each material at each potential when the following condition is met: where i a ·A a is the sum of anodic currents (current density multiplied by area), and i c ·A c is the sum of the cathodic currents.
In the first step, the potentiodynamic polarization curves over a range of ±150 mV vs. SCE were drawn in the Tafel domain. In the second step, from the drawn polarization curves, a mathematical model based on the Stern-Geary equations and PAR Calc where β a and β c are the Tafel slopes calculated with PAR Calc . According to the mixed potential theory, we sum the two anodic slopes (example 3N + #8) and then the two cathodic slopes (example 3N + #8). The intersection of the anodic slope sum with the cathodic slope sum is theoretically i coupling and E coupling. According to the Evans diagrams, the galvanic cells can work under a) mixed, b) cathodic, and c) anodic control (Figure 2b). E oc and E coupling are the open circuit and coupling potentials. Under these conditions, it can be seen that the E AB coupling potential can approach the open-circuit potential of the E A or E B metal or remain somewhere between E A and E B .
Materials 2018, 11, x FOR PEER REVIEW 6 of 21 anodic material will be anodically polarized, and so the anodic rate of reaction will be accelerated. The resulting galvanic current can be determined solely from the sums of all anodic and cathodic currents for each material at each potential when the following condition is met: Where ia·Aa is the sum of anodic currents (current density multiplied by area), and ic·Ac is the sum of the cathodic currents. In the first step, the potentiodynamic polarization curves over a range of ±150 mV vs. SCE were drawn in the Tafel domain. In the second step, from the drawn polarization curves, a mathematical model based on the Stern-Geary equations and PARCalc the anodic and cathodic Tafel slopes are calculated. We retrace the rights I(E) = Icorr 10 (E − E corr )/βa (3) and I(E) = Icorr 10 (E − E corr )/βc (4) where βa and βc are the Tafel slopes calculated with PARCalc. According to the mixed potential theory, we sum the two anodic slopes (example 3N + #8) and then the two cathodic slopes (example 3N + #8). The intersection of the anodic slope sum with the cathodic slope sum is theoretically icoupling and Ecoupling. According to the Evans diagrams, the galvanic cells can work under a) mixed, b) cathodic, and c) anodic control (Figure 2b). Eoc and Ecoupling are the open circuit and coupling potentials. Under these conditions, it can be seen that the EAB coupling potential can approach the open-circuit potential of the EA or EB metal or remain somewhere between EA and EB.
In a) we have a situation where the oxidation of the anode or the depassivation of the cathode is not really important compared to the coupling potential, thus the rates of corrosion are relatively weak. The galvanic cell is in the desirable situation of a mixed control. In b), there is a significant depassivation of the coupling partner which is in the cathode position. In c), there is a strong oxidation due to the potential difference EB−AB.
To apply the mixed potential theory, the polarization curve (±150 mV) must be plotted for the calculation of the Tafel slopes, the corrosion potential (Ecorr) and the corrosion current (Icorr), with a scanning speed of 0.1 mV/s. The measurements were carried out by the microelectrode technique on bracelet links ( Figure 3). In a) we have a situation where the oxidation of the anode or the depassivation of the cathode is not really important compared to the coupling potential, thus the rates of corrosion are relatively weak. The galvanic cell is in the desirable situation of a mixed control. In b), there is a significant depassivation of the coupling partner which is in the cathode position. In c), there is a strong oxidation due to the potential difference E B − AB .
To apply the mixed potential theory, the polarization curve (±150 mV) must be plotted for the calculation of the Tafel slopes, the corrosion potential (E corr ) and the corrosion current (I corr ), with a scanning speed of 0.1 mV/s. The measurements were carried out by the microelectrode technique on bracelet links ( Figure 3).

Nickel Release-EN 1811
It is well known that Ni contact can result in an allergic response. In Europe the prevalence is 10-15% for female adults and 1-3% for male adults who are allergic to Ni. 40-70% of Ni contactdermatitis develops acute or chronic hand eczema, estimated at 42 million people. Consequently, for

Nickel Release-EN 1811
It is well known that Ni contact can result in an allergic response. In Europe the prevalence is 10-15% for female adults and 1-3% for male adults who are allergic to Ni. 40-70% of Ni contactdermatitis develops acute or chronic hand eczema, estimated at 42 million people. Consequently, for the protection of the general population, in contact dermatitis, the European Union (EU) imposed the following restrictions of use: -Ni release from parts in direct and prolonged contact with the skin must be lower than 0.5 µg/cm 2 /week [17] -the metallic parts that are inserted into pierced ears and other parts of the human body must not have a Ni release rate greater than 0.2 µg/cm 2 /week [17].
The release of nickel cations was measured in artificial sweat according with EN 1811 (Section 2.4.1). The solutions were filtered before use over a sterilized Falcon ® 0.22 microns cellulose acetate membrane; the release flasks used were of Falcon ® sterile type made of polypropylene. The samples were in the rectangular shape of 14 mm × 35 mm and were firstly cleaned in ethanol p.a. under ultrasound. The ratio of release solution volume/total sample surface was equal to 1. The extraction was carried out at 37 • C shielded from light for 168 h. Three samples of 317LMN** 18-14-4-MnN, 317LMN* 18-15-4N (standard), 316L*** 17-13-3, 904L***20-25-5 Cu steel alloys were used. The solutions were analyzed by ICP-MS Perkin Elmer. Two blank samples were measured as a reference.

Metallographic Characterization
The metallographic structures, analyzed with optical microscopy in transverse section, are presented in Figure 4. Table 2 shows grain size and hardness values for reference steels.
The metallographic structural characteristics of the steels studied were mandated to Laboratoire Dubois, La Chaux-de-Fonds, Switzerland.

Nickel Release-EN 1811
It is well known that Ni contact can result in an allergic response. In Europe the prevalence is 10-15% for female adults and 1-3% for male adults who are allergic to Ni. 40-70% of Ni contactdermatitis develops acute or chronic hand eczema, estimated at 42 million people. Consequently, for the protection of the general population, in contact dermatitis, the European Union (EU) imposed the following restrictions of use: -Ni release from parts in direct and prolonged contact with the skin must be lower than 0.5 µg/cm 2 /week [17] -the metallic parts that are inserted into pierced ears and other parts of the human body must not have a Ni release rate greater than 0.2 µg/cm 2 /week [17].
The release of nickel cations was measured in artificial sweat according with EN 1811 (Section 2.4.1). The solutions were filtered before use over a sterilized Falcon ® 0.22 microns cellulose acetate membrane; the release flasks used were of Falcon ® sterile type made of polypropylene. The samples were in the rectangular shape of 14 mm × 35 mm and were firstly cleaned in ethanol p.a. under ultrasound. The ratio of release solution volume/total sample surface was equal to 1. The extraction was carried out at 37 °C shielded from light for 168 h. Three samples of 317LMN** 18-14-4-MnN, 317LMN* 18-15-4N (standard), 316L*** 17-13-3, 904L***20-25-5 Cu steel alloys were used. The solutions were analyzed by ICP-MS Perkin Elmer. Two blank samples were measured as a reference.

Metallographic Characterization
The metallographic structures, analyzed with optical microscopy in transverse section, are presented in Figure 4. Table 2 shows grain size and hardness values for reference steels.  The metallographic structural characteristics of the steels studied were mandated to Laboratoire Dubois, La Chaux-de-Fonds, Switzerland.
All microstructures are of recrystallized, equiaxed type. The grain size was evaluated according  All microstructures are of recrystallized, equiaxed type. The grain size was evaluated according to ASTM E112. All steels have an average grain size of index 5. The inclusion cleanliness was evaluated according to DIN 50602 method K and the composition of the inclusions was analyzed by microprobe EDX. None of the steels has inclusions within the standard range. Inclusions of globular oxides are nevertheless present with an index less than K0 = 0. The Vickers hardness was measured by micro durometer according to ISO 6507. The measured samples show a homogeneous hardness. The average hardness ranges from 140 to 180 Vickers under load of 1 kg. This extent is to be compared to the different rates of nitrogen and molybdenum of the different grades.

Evaluation of the General Corrosion
Electrochemical parameters measured and calculated for the different grades of steel are given in Table 3. The breakdown potential, E b is the potential for which the anodic current increases strongly. The open-circuit potential, E oc characterizes the electrochemical state of the interface after 15 h of immersion in the absence of any polarization. The values of E oc are given in Table 3.
The highest open-circuit potentials are observed for #7 and #11 steels. Resistance to polarization, R p , characterizes the stability of the surface in the vicinity of the potential in open circuit (E oc ). The resistance to polarization is calculated from the polarization curves recorded in the vicinity of the potential E oc . The values of R p are given in Table 3. The highest polarization resistance is observed for grade #6 (2162.5 kΩ/cm 2 ). We note the following classification: high R p values (>600 kΩ/cm 2 ) for grades #6, #2, #1, #5, and #10; intermediate R p values (250 to 350 kΩ/cm 2 ) for grades #12 and #7; low R p values (<150 kΩ/cm 2 ) for grades #4, #3, #11, #9, and #8.
Particularities: The R p of the grade 316L, the #8 is the lowest. The R p of the grade 316L (#8 and #9) are similar. The R p of the grade 317L (#1-#7) are dissimilar. The R p of the grade 904L (#10-#12) are dissimilar.

Tafel Plots
The calculated values of the corrosion potential, E corr and the corrosion current density, i corr , are given in Table 3. The corrosion potential E corr characterizes the zero current electrochemical state of the Tafel scanning interface. The corrosion current density, i corr , characterizes the corrosion intensity at Tafel's scanning corrosion potential (i corr = i an − i cat ).
Corrosion potential (E corr ).The highest corrosion potential is observed for grade #7.We note the following classification: a high positive E corr value (

Potentiodynamic Curves
Potentiodynamic curves characterize the general electrochemical behavior of the scan interface from −1000mV to +1200mV/SCE. For the interpretation of the results two representations are used: a semi-logarithmic coordinate representation and a linear coordinate representation. Figure 5 represents the semi-logarithmic global polarization curves plotted for the samples tested in generalized corrosion. Passivation levels are noted for all samples tested for generalized corrosion. The best behavior is shown by grade #2. Intermediate behavior is revealed by grades #1, #6, and #7. Their anodic curves move to the right with currents five times higher than for alloy #2. The worst behavior is shown by grade #8. Its anode curve moves strongly to the right at currents of the order of tenth microamperes.

Potentiodynamic Curves
Potentiodynamic curves characterize the general electrochemical behavior of the scan interface from −1000mV to +1200mV/SCE. For the interpretation of the results two representations are used: a semi-logarithmic coordinate representation and a linear coordinate representation. Figure 5 represents the semi-logarithmic global polarization curves plotted for the samples tested in generalized corrosion. Passivation levels are noted for all samples tested for generalized corrosion. The best behavior is shown by grade #2. Intermediate behavior is revealed by grades #1, #6, and #7. Their anodic curves move to the right with currents five times higher than for alloy #2. The worst behavior is shown by grade #8. Its anode curve moves strongly to the right at currents of the order of tenth microamperes. The anodic currents of 316L are very high (Figure 6), which corresponds to a very fast corrosion compared to the other grades considered. The anodic currents of 316L are very high (Figure 6), which corresponds to a very fast corrosion compared to the other grades considered. The anodic currents of 316L are very high (Figure 6), which corresponds to a very fast corrosion compared to the other grades considered.  The breakdown potential E b is the potential for which the anodic current increases strongly. The range of potential situated between the E corr or E (i = 0) and E b represents the immunity zone in which the corrosion is weak, indeed insignificant. The more this zone is extended, the less the alloy is likely to be found in a situation that may lead to severe corrosion, by polarization or by galvanic effect, as a result of another alloy's presence. Figures 7 and 8 represent in linear coordinates the part of the polarization curve between −500 mV and +1000 mV in the cathodic-anodic current scale of between −10 µA/cm 2 and 10 µA/cm 2 to observe the breakdown potential (E b ).
The E b of the grade #8 is around 450 mV. The immunity domain is restricted to median potentials. Corrosion will start from 450 mV without following passivation. The E b of the other grades are around 800 mV. The immunity domain reaches high potentials with low anodic currents of the order of the micro-amperes.
E b values for all studied alloys are presented in Table 3.

Materials 2018, 11, x FOR PEER REVIEW 11 of 21
The breakdown potential Eb is the potential for which the anodic current increases strongly. The range of potential situated between the Ecorr or E(i = 0) and Eb represents the immunity zone in which the corrosion is weak, indeed insignificant. The more this zone is extended, the less the alloy is likely to be found in a situation that may lead to severe corrosion, by polarization or by galvanic effect, as a result of another alloy's presence. Figures 7 and 8 represent in linear coordinates the part of the polarization curve between −500 mV and +1000 mV in the cathodic-anodic current scale of between −10 µA/cm 2 and 10 µA/cm 2 to observe the breakdown potential (Eb).
The Eb of the grade #8 is around 450 mV. The immunity domain is restricted to median potentials. Corrosion will start from 450 mV without following passivation. The Eb of the other grades are around 800 mV. The immunity domain reaches high potentials with low anodic currents of the order of the micro-amperes.    Eb values for all studied alloys are presented in Table 3. The results of the generalized corrosion test point out three basic groups. All of the 317LMNs were similar. The tested 316L steels were noticeably worse. The 904Ls were difficult to discern, but The results of the generalized corrosion test point out three basic groups. All of the 317LMNs were similar. The tested 316L steels were noticeably worse. The 904Ls were difficult to discern, but certainly better than the 316Ls and likely at least comparable to the 317LMNs. These conclusions are supported by data shown in Table 3.

Coulometric Analysis
Coulometric analysis characterizes the amount of corrosion developed in the given anodic scan range. The area of E (i = 0) at +300 mV corresponds to the usual corrosion range. The area from +300 mV to +600 mV corresponds to a severe corrosion area. The results are presented in Table 3. The results are coherent between the two ranges considered. They confirm the observations and remarks made on the electrochemical magnitudes presented previously. In particular (for the second zone): the lowest electrical consumptions (<1.00 mC/cm 2 ) are observed on grades #2, #6, #7, and #1; the intermediate electrical consumptions (1 to 250 mC/cm 2 ) are observed on grades #5, #10, #4, #3, #12, and #11; the highest electrical consumption (>250 mC/cm 2 ) is observed on grade 316L (#8 and #9).

Evaluation of the Localized Corrosion
The crevice potential, E crev is the potential that corresponds to the penultimate measurement where the current is positive (potentiostatic static curve in red color). For a better understanding of the evaluation procedure of E crev , the potentiostatic scan curves for #2 and #11 steels are shown in Figure 9. The values for E crev are summarized in Table 4. certainly better than the 316Ls and likely at least comparable to the 317LMNs. These conclusions are supported by data shown in Table 3.

Coulometric Analysis
Coulometric analysis characterizes the amount of corrosion developed in the given anodic scan range. The area of E(i = 0) at +300 mV corresponds to the usual corrosion range. The area from +300 mV to +600 mV corresponds to a severe corrosion area. The results are presented in Table 3. The results are coherent between the two ranges considered. They confirm the observations and remarks made on the electrochemical magnitudes presented previously. In particular (for the second zone): the lowest electrical consumptions (<1.00 mC/cm 2 ) are observed on grades #2, #6, #7, and #1; the intermediate electrical consumptions (1 to 250 mC/cm 2 ) are observed on grades #5, #10, #4, #3, #12, and #11; the highest electrical consumption (>250 mC/cm 2 ) is observed on grade 316L (#8 and #9).

Evaluation of the Localized Corrosion
The crevice potential, Ecrev is the potential that corresponds to the penultimate measurement where the current is positive (potentiostatic static curve in red color). For a better understanding of the evaluation procedure of Ecrev, the potentiostatic scan curves for #2 and #11 steels are shown in Figure 9. The values for Ecrev are summarized in Table 4.     intermediate electrical consumptions (1 to 250 mC/cm ) are observed on grades #5, #10, #4, #3, #12, and #11; the highest electrical consumption (>250 mC/cm 2 ) is observed on grade 316L (#8 and #9).

Evaluation of the Localized Corrosion
The crevice potential, Ecrev is the potential that corresponds to the penultimate measurement where the current is positive (potentiostatic static curve in red color). For a better understanding of the evaluation procedure of Ecrev, the potentiostatic scan curves for #2 and #11 steels are shown in Figure 9. The values for Ecrev are summarized in Table 4.    These results reveal a good crevice corrosion behavior for the 317LMN alloy grades. If we classify the 317L steel grades according to their E crev , the best behavior is that of alloy #2, 317LM* 20-19-4. It is followed by grades #1, 317LMN** 18- Steels #8, 316L* (standard) and #9, 316L*** show the poorest behavior. In general, the 316L steels are sensitive to localized corrosion by pitting and crevices. Thus the quantities of nickel released are higher than the 0.5 micrograms/cm 2 imposed by the standards of protection concerning nickel allergies.
Alloy #10, 904L* (control) has a crevice potential of 350 mV, higher than the crevice potential of sample #11, 904L*** 20-25-5 Cu. Alloy #12, 904L*** 20-25-5 Cu exhibits a similar behavior (E crev = 350 mV) to #11. This steel grade begins to replace the type 316L grades in the high watchmaking range. They have good corrosion resistance and the quantities of nickel released are in compliance with the current legislation in the EU, USA, Australia, Canada, Asia, etc.

Non-Parametric Kendal Test
We measured and evaluated six electrochemical parameters: E oc , R p , E corr , i corr , and coulometric analyses for both zones. Since the experimental conditions are strictly the same for all the alloys tested, a global view of the corrosion behavior can be given by a non-parametric classification of the Kendall rank correlation type [37][38][39]. Table 5 is created in the following way: each quantity is I* Coulometric analysis E (i = 0) 300 mV; II** Coulometric analysis 300-600 mV.
The classification of grades after the generalized corrosion results allows the following observations: the best behaviors are shown by the 317LMN grades; the best 317LMN grades, #6, #2, and #7 are close to the 'standard' 18% chromium composition; grades 317LM and 317LMN'alloyed' with 20% chromium, #1, #4, #3, and #5 are in the intermediate position, together with the grade 904L #10; the grades 904L #11, #12 and the grades 316L #8 and #9 belong to the last classes. In the same way, we can classify the steels according to the results of crevice corrosion tests. The classification parameter is the crevice corrosion potential (E crev ). In Table 6, all the values of an equal series carry the same rank as the smallest index and the higher index ranks of this series are deleted. In addition, crevice corrosion is considered to be twice as serious as generalized corrosion on the products under consideration. The ranks of E crev are thus affected by a factor of 2. The ordering of the grades after crevice corrosion results leads to the following observations: the best behaviors are shown by the 317LMN grades; the best 317LMN grades, #2, #4, #5, #3, are the 317LM and 317LMN grades, 'alloyed' with 20% chromium; the grades 317LMN #6, #1, #7, of 'standard' composition with 18% chromium, are in intermediate position with the grades 904L #10 and #11; grades 316L #8 and #9 are in last position. Note the order inversion of the 317LMN 'standard' and 317LMN 'alloyed' grade groups between generalized corrosion and crevice corrosion.
To obtain an overall classification of generalized corrosion and crevice corrosion, the rank values obtained in crevice corrosion after multiplication can be added to the rank sum of Table 7.  Table 7 shows rank changes related to the influence of crevice corrosion: the best behavior is shown by 317LM #2 followed by 317LMN #6 and #1 (class I); steels 317LMN #4, #5, # 7, #3, and 904L #10 (control) are found in Class II; 904L steel and 316L steel keep their position in Class III; lower than the grades 316L Class IV.
The comparison of the 317LMN #1 and #7 grades, however similar in composition, shows a difference, more particularly in crevice corrosion. This result can be attributed to the presence of secondary phase inclusions (sigma) in grade #7 and to the ultrapure nature of the remelted grade #1. The role of manganese in grade #1 is uncertain. The difference between grades 317LM #2 and #3 shows the adverse effect of copper in crevice corrosion.
Two hypotheses make it possible to explain the fact that the grades 904L remain lower than the grades 317LMN: on one hand the proportion of the passivating elements in the 317LMN grades can be more favorable to the formation of a stable oxide layer than in the grade 904L (kinetic effect) and on the other hand, the 904L grade may have a lack of passivation in a diluted low-oxidizing salt electrolyte, such as that used for the test. These grades remain partially active at 'low potential'.
The difference between the grades 904L #10, #11, and #12 may be relative to a different elaboration method. A higher impurity level in grades #11 and #12 is suspected. In particular, shade #11 is developed to facilitate shavings removal.
Grade 316L #8 and #9 do not resist to crevice corrosion, as empirical observations on the products show. The proportion of passivating elements in these grades is too low to allow repassivation in crevice corrosion. These grades remain active in this corrosion configuration and have a higher probability of galvanic corrosion and nickel release.

Galvanic Coupling Measurements
After 12 h of immersion, the potentials of each coupling partner were collected. No wonder the open circuit potential (E oc ) values are specific for 18K gold alloys and steels. After gold-steel partner couplings, the galvanic current values were recorded for 25 h (Figure 11). For this relatively short period we can notice a trend, but to have more information it would be necessary to extend the measurements to 3-5 days. In this method, the galvanic current can be directly determined as a function of time. A reference electrode can be used in the usual manner to determine the galvanic couple potential. It is obvious that the galvanic currents are in the nano-amperes scale, the galvanic currents in the #10/Au couplings are larger than in the #1/Au couplings; #10/Au galvanic currents have a tendency to rise with time; the galvanic currents #1/Au, on the contrary, tend to decrease with time; the galvanic currents generated in the Au 5N coupling are greater than in the Au 3N coupling. This is an aspect that should be the subject of further study; to establish the influence of the elements in the composition of Au 18K and the kinetics of cations release with respect to the chemical composition.
The galvanic currents recorded can be positive or negative. This is a question of positioning the electrodes in the electronic measurement setup. It is very important to keep the same position in the assembly, the 18K gold alloy will be the cathode and the steel will be the anode. In the coupling, according to the electrochemical laws, the anode corrodes the weakest. Suppose that in this case the generated coupling current has a positive sign. If a negative current is recorded then the anode will be the 18K alloy and the cathode will be the steel. In other words the steel, passivated, becomes a cathode and corrodes Au 18K.
The galvanic currents of the #8/Au couplings are presented in Figure 12. Their order of magnitude is micro-ampers; so we built a graph in the micro-ampers scale. In the construction of an Au-steel watch bracelet, the use of a 316L type steel is not the best solution. The coupling currents generated are of the order of micro-ampers; therefore, the risk of triggering corrosion in the bracelet powered by a galvanic battery is very high. The quantities of nickel released in contact with the skin will also have to be taken into consideration [17]. It is obvious that the galvanic currents are in the nano-amperes scale, the galvanic currents in the #10/Au couplings are larger than in the #1/Au couplings; #10/Au galvanic currents have a tendency to rise with time; the galvanic currents #1/Au, on the contrary, tend to decrease with time; the galvanic currents generated in the Au 5N coupling are greater than in the Au 3N coupling. This is an aspect that should be the subject of further study; to establish the influence of the elements in the composition of Au 18K and the kinetics of cations release with respect to the chemical composition.
The galvanic currents recorded can be positive or negative. This is a question of positioning the electrodes in the electronic measurement setup. It is very important to keep the same position in the assembly, the 18K gold alloy will be the cathode and the steel will be the anode. In the coupling, according to the electrochemical laws, the anode corrodes the weakest. Suppose that in this case the generated coupling current has a positive sign. If a negative current is recorded then the anode will be the 18K alloy and the cathode will be the steel. In other words the steel, passivated, becomes a cathode and corrodes Au 18K.
The galvanic currents of the #8/Au couplings are presented in Figure 12. Their order of magnitude is micro-ampers; so we built a graph in the micro-ampers scale. In the construction of an Au-steel watch bracelet, the use of a 316L type steel is not the best solution. The coupling currents generated are of the order of micro-ampers; therefore, the risk of triggering corrosion in the bracelet powered by a galvanic battery is very high. The quantities of nickel released in contact with the skin will also have to be taken into consideration [17]. It is obvious that the galvanic currents are in the nano-amperes scale, the galvanic currents in the #10/Au couplings are larger than in the #1/Au couplings; #10/Au galvanic currents have a tendency to rise with time; the galvanic currents #1/Au, on the contrary, tend to decrease with time; the galvanic currents generated in the Au 5N coupling are greater than in the Au 3N coupling. This is an aspect that should be the subject of further study; to establish the influence of the elements in the composition of Au 18K and the kinetics of cations release with respect to the chemical composition.
The galvanic currents recorded can be positive or negative. This is a question of positioning the electrodes in the electronic measurement setup. It is very important to keep the same position in the assembly, the 18K gold alloy will be the cathode and the steel will be the anode. In the coupling, according to the electrochemical laws, the anode corrodes the weakest. Suppose that in this case the generated coupling current has a positive sign. If a negative current is recorded then the anode will be the 18K alloy and the cathode will be the steel. In other words the steel, passivated, becomes a cathode and corrodes Au 18K.
The galvanic currents of the #8/Au couplings are presented in Figure 12. Their order of magnitude is micro-ampers; so we built a graph in the micro-ampers scale. In the construction of an Au-steel watch bracelet, the use of a 316L type steel is not the best solution. The coupling currents generated are of the order of micro-ampers; therefore, the risk of triggering corrosion in the bracelet powered by a galvanic battery is very high. The quantities of nickel released in contact with the skin will also have to be taken into consideration [17].  Table 8. 20-25-5 CuN (control) and #11: 904L*** 20-25-5 Cu. The curves of Tafel and icorr were calculated with PARCalc routine EG&G PARC Application Model SOFT Corr TMII. The Scan rate was 0.1 mV/s. The calculated values of Ecorr, icorr in the Tafel domain are presented in Table 8. The values in Table 8 were used in the determination of Ecorr coupling and icorr coupling according to the theory of mixed potential. Figure 13 shows two coupling diagrams for steels 316L, 317LMN 3N and 5N.  Table 9 the coupling values I (Icoupling) and E (Ecoupling) calculated for each coupling are presented. It should be noted that the 316L/3N, 5N couplings confirm the current quantities measured directly and, on the other hand, the coupling values in 316L/Au 5N are larger than in 316L/Au 3N. The values in Table 9 are used to draw Evans diagrams. Thus we will be able to examine the control under which the galvanic battery functions (Figure 14).  In Table 9 the coupling values I (I coupling ) and E (E coupling ) calculated for each coupling are presented. It should be noted that the 316L/3N, 5N couplings confirm the current quantities measured directly and, on the other hand, the coupling values in 316L/Au 5N are larger than in 316L/Au 3N. The values in Table 9 are used to draw Evans diagrams. Thus we will be able to examine the control under which the galvanic battery functions (Figure 14).  Examination of the Evans diagrams of the couplings studied does not reveal any anodic or cathodic batteries. We can say that they are under mixed control. On the other hand, Ecoupling-Eoc differences are quite important. Taking into account that the measured coupling current values are of the order of 10 −6 A, they are low. The exception is the case # 8/Au for which the currents are of the order 10 −4 -10 −5 A.

Ni Release
The results for the extraction tests according to European Legislation EN 1811 are presented in Table 10.  As a result, the grades of steels studied are suitable for manufacturing items which are in prolonged contact with the skin, acceptance limit 0.5 µg/cm 2 /week. On the contrary, they are prohibited to be used to manufacture metallic parts that are inserted into pierced ears and other parts of the human body, acceptance limit 0.2 µg/cm 2 /week.

Conclusions
The corrosion behavior of all the 317LMNs was similar. All 317LMNs have proven better in general corrosion as well as in crevice and pitting localized corrosion. The 316L variants, least alloyed, behaved noticeably worse and occupy the last classes. The 904Ls were difficult to discern, but were certainly better than the 316Ls and at least comparable to the 317LMNs. 317LMN(standard) grades with 18% chromium, #6, #7, #1 are better in generalized corrosion while 317LMN grades 'alloyed' with 20% chromium, #2, #4, #3, and #5 proved better in crevice corrosion.
The comparison of the #1 and #7 317LMN grades, however similar in composition, shows a difference, more particularly in crevice corrosion.
The 317LMN grades are better compared to the 904L grades under the corrosion conditions applied. It is reasonable to assume a negligible occurrence of Au-steel galvanic corrosion and crevice Examination of the Evans diagrams of the couplings studied does not reveal any anodic or cathodic batteries. We can say that they are under mixed control. On the other hand, E coupling -E oc differences are quite important. Taking into account that the measured coupling current values are of the order of 10 −6 A, they are low. The exception is the case # 8/Au for which the currents are of the order 10 −4 -10 −5 A.

Ni Release
The results for the extraction tests according to European Legislation EN 1811 are presented in Table 10. Table 10. Results of ICP analysis of the artificial sweat. Average of three measurements, (µg/cm 2 /week). As a result, the grades of steels studied are suitable for manufacturing items which are in prolonged contact with the skin, acceptance limit 0.5 µg/cm 2 /week. On the contrary, they are prohibited to be used to manufacture metallic parts that are inserted into pierced ears and other parts of the human body, acceptance limit 0.2 µg/cm 2 /week.

Conclusions
The corrosion behavior of all the 317LMNs was similar. All 317LMNs have proven better in general corrosion as well as in crevice and pitting localized corrosion. The 316L variants, least alloyed, behaved noticeably worse and occupy the last classes. The 904Ls were difficult to discern, but were certainly better than the 316Ls and at least comparable to the 317LMNs. 317LMN(standard) grades with 18% chromium, #6, #7, #1 are better in generalized corrosion while 317LMN grades 'alloyed' with 20% chromium, #2, #4, #3, and #5 proved better in crevice corrosion.
The comparison of the #1 and #7 317LMN grades, however similar in composition, shows a difference, more particularly in crevice corrosion.
The 317LMN grades are better compared to the 904L grades under the corrosion conditions applied. It is reasonable to assume a negligible occurrence of Au-steel galvanic corrosion and crevice corrosion. In the case of 317LMN steels, the galvanic currents have a tendency to decrease with time, on the other hand, for 904L steel the tendency is to increase with time. There is a difference in steel/Au 3N and steel/Au 5N couplings, coupling currents with 5N are larger than those with 3N.
At present, metal watchmaking items in contact with the skin are manufactured using 316L and 904L steels. Unfortunately, when dealing with gold-steel bimetallic constructions, it is very difficult to respect the quantities of nickel released, according to international regulations. This is the reason why this research was oriented towards the development of new grades of steel that meet the new current legislation, for consumer protection.