Contact Dermatitis. 1994.31.249-255Printed in Denmark . Ali rights reserved

Copyright @ Munksgaard 1994

CONTACT DERMATITISISSN 0105-1873

Nickel release from nickel-plated metaIs andstainless steeIs

P. HAUDRECHY., J. FousSEREAu2, B. MANTOUT1 AND B.BAROUX.

IUgine Research Centre, 73400 Ugine, France2Serviced'Allergie, Clinique Dermatologique, Hôpital Civil, Strasbourg, France

Nickel release from nickel-plated metals often induces allergic contact dermatitis, but, for nickel-containing stainless steels, the effect is not well-known. ln this paper, AISI 304, 316L, 303 and430 type stainless steels, nickel and nickel-plated materials were investigated. 4 tests were per-formed: patch tests, leaching experiments, dimethylglyoxime (DMG) spot tests and eIectrochemicaltests. Patch tests showed that 96% of the patients were intolerant to Ni-plated samples, and 14%to a high-sulfur stainless steel (303), while nickel-containing stainless steels with a low sulfurcontent elicited no reactions. Leaching experiments confirmed the patch tests: in acidic artificialsweat, Ni-plated samples released about 100 jlg/cm2/week of nickel, while low-sulfur stainlesssteels released less than 0.03 jlg/cm2/week of nickel, and AISI 303 about 1.5 jlg/cm2/week.Attention is drawn to the irrelevance of the DMG spot test, which reveals Ni present in the metalbulk but not its dissolution rate. Electrochemical experiments showed that 304 and 316 gradesremain passive in the environments tested, while Ni-plated steels and AISI 303 can suffer significantcation dissolution. Thus, Ni-containing 304 and 316 steels should not induce contact dermatitis,while 303 should be avoided. A reliable nitric acid spot test is proposed to distinguish this gradefrom other stainless steels.

Key words: allergic contact dermatitis; electrochemical tests; leaching experiments; nitric acid spottest; potentiodynamic polarization; nickel; stainless steels; prevention. (Ç)Munksgaard, 1994.

Accepted for publication 16 December 1993

Nickel (Ni) contact dermatitis has been the subjectofmany medical publications (1-12). The responsi-bility of Ni in cases of contact between it and theskin is not doubted. However, the harmfulness ofskin contact with Ni-containing stainless steel isdenied by some authors (1) and, when implicatedby others, the stainless steel grade is not specified(2). This point is of particular interest, since thereexist many different grades of stainless steel, someof them containing Ni in significant amounts andothers not (13), the corrosion resistance of whichin aggressive environments may differ significantly.Moreover, when used under appropriate con-ditions, a stainless steel is known as "passive", asit is covered with a thin iron (Fe)-chromium (Cr)oxide film (thickness of the order of a fewnanomet-ers), which inhibits cation dissolution. Thus, thecations that can, at a very slow rate, be releasedtowards the environment are those that are presentin the passive film (Fe and Cr) and not those that,such as Ni, are contained in the steel bulk but notin the protective passive film. Under these con-

ditions, the safety of stainless steels in contact withthe skin, even those containing Ni, is insured solong as the passive film remains stable with respectto sweat. Considering the composition of humansweat (pH, chloride content, etc.), it is very likelythat the commoner stainless steels obey this re-quirement. This is probably untrue for Ni and Ni-plated alloys, the passive films of which are knownto be much less stable in chloride media.

It was intended in this study to shed light onthese points, by assessing the skin tolerance ofdifferent stainless steels in patients allergie to Ni,and evaluating the Ni and cation release of thesematerials. As a reference, we first investigated a Ni-free 17%Cr ferritic steel. Then, some Ni-containingaustenitic grades were considered, containing ornot containing molybdenum (Mo), since this ele-ment is known to improve corrosion resistance inchloride-containing media. ln contrast, sulfur isknown to decrease the corrosion resistance drasti-cally. Therefore, a high-sulfur free machining steelwas also characterized.

250 HAUDRECHY ET AL.

The following studies were carried out:(a) Clinical evaluation of skin tolerance of differ-

ent stainless steels, using patch testing with thesemetallic samples.

(b) Comparative determination of Ni releasefrom these stainless steels in artificial sweat andsodium chioride solutions of neutral or acidic pH(6.6 and 4.5), and evaluation of this release, a rateless than 0.5 ,ug/cm2/week being considered as safeby Menné et al. (1).

(c) Dimethylglyoxime (DMG) spot test. This testis commonly used to assess the "safety" of a ma-terial with regard to Ni allergy (2, 3, 5, 10-12), butits validity is very questionable and we will showthat it is not relevant in case of stainless steels.

(d) Potentiodynamic polarization experiments indeaerated chloride media, to compare qualitativelythe cation release of the materials studied, thewidth of the passivity region and the level of theanodic current in this passivity region both beingtaken into consideration.

Experiments were conducted at the Ugine Re-search Centre, except for the patch tests, whichwere performed by J. F. at the Strasbourg civilhospital.

Materialsand MethodsM ateria/s studied

6 materials were studied: pure nickel, Ni-platedsteel, and 4 stainless steel grades for which chemicalanalyses are given in Table 1. These grades will bereferred to using the AISI (American Iron AndSteel Institute) standard.

Stainless steel (SS) grades are basically iron-chromium alloys, this last element being added insufficient amount to form, in aqueous solution, aprotective Cr (or sometimes, Fe-Cr) oxide layer(the "passive layer"), the thickness of which is ofthe order of a few nanometers. As long as SS doesnot corrode (that is to say its passive layer isstable), the dissolution rate of the steel is negligible.Moreover, when considering the residual cationflux throughout the passive film, it is worth notingthat only the cations present in the film (mostly Crand possibly Fe) can dissolve, even if other alloyingelements are present in the steel though not in the

passive layer. ln contrast, when SS corrodes, suchas in very aggressive media, the corrosion rate de-pends mostly on the composition of the steel andnot just on that of the passive film. The questionof the safety of SS, regarding their cation releasein a given environment, is therefore directly relatedto their corrosion resistance and thus to their pass-ive film stability.

Other elements may also be added to stainlesssteels for various purposes, including Ni and Mo,which change both the metallurgical structure andthe mechanical properties of the steel and improveits corrosion resistance. A typical Ni-free SS is thegrade AISI 430, the crystalline structure of whichis bcc (body-centered cubic) (13), referred to as a"ferritic" structure. This steel does not contain Niin a significant amount, from the viewpoint underconsideration. Adding Ni up to typically 8% wL(grade AISI 304) makes the structure become fcc(face-centered cubic) (13), which is referred to asan "austenitic" structure. Adding Mo to such steel(typically up to 2%) does not change its crystalllinestructure but strongly improves its corrosion resist-ance. ln each case, the passive film remains a Fe-Croxide (or hydroxide). It must be noted that thepassive film formed on Ni (or Ni-plated alloys) isa Ni oxide, that is to say completely different frompassive films formed on SS.

Another metallurgical point must be considered:for various reasons, industrial steels contain manyminor elements or impurities. ln particular, even asmall amount of suifur combines with manganese(Mn) to form Mn sulfides, which, in very aggressivechloride-containing media, act as pitting corrosioninitiation sites. Furthermore, there exists a specialaustenitic grade(AISI 303) to which sulfur is addedat a very high level (typically 0.2% to 0.3%) forfree machinability purposes. It was necessary totest the behaviour of such an SS grade regardingNi release, and to propose a simple test for determi-ning its corrosion resistance under relevant con-ditions.

Table 1 shows the chemical composition of thestainless steels studied. Experiments were per-formed either on polished surfaces (SiC paper,grade 1200) or on surfaces as received (annealedand pickled sheets for AISI 304, 316L and 430, andmachined rods for AISI 303). Prior to experiments,

Table 1. Chemical composition (wg %) of the stainless steel grades and of the nickel used in our study

Grade Cr Ni Mo C S Mn Si Ti Nb P

AISI 303 17.3 8.459 0.26 0.064 0.275 1.79 0.54 0.002 0.012 0.033AISI 304 18.2 8.65 0.26 0.036 0.007 0.82 0.49 0.002 0.004 0.023AISI316L 17.9 11.3 2.15 0.021 0.002 1.67 0.61 0.004 <0.002 0.023AISI 430 16.6 0.11 0.11 0.037 0.001 0.43 0.33 0.007 <0.002 0.017Nickel 99.8 0.002 0.01

NICKEL RELEASE FROM STAINLESS STEELS 251

samples were cleaned ultrasonically in a solutionof acetone+ethanol (50/50 in volume ratio).

Patch tests

These tests were performed in 50 patients alreadyknown to be sensitive to nickel (nickel sulfate 5%pet.), with circular samples of the 4 SS grades andof the Ni-plated steel (1.5 cm in diameter) in the as-received condition. Prior to experiments, sampleswere washed with distilled water and cleaned firstwith ethanol and then with ether (10). Tolerancewas tested with a Roc Neodermotest applied onthe back and read after 2 and 3 days, according toInternational Contact Dermatitis Research Grouprecommendations (3). Sensitivity to Cr and cobalt(Co) for these patients was also known (Cr aspotassium dichromate 0.5% pet. and Co as cobaltchloride 1% pet.).

Leaching experiments

These were mainly in artificial sweat solutions andin 0.05 M sodium chloride (NaCI) solutions at pH4.5 and at ambient temperature. pH 4.5 was chosenfrom a paper by Hemingway & Molokhia (4) asthe lowest pH value found quoted in the literaturefor the biological sweat. Composition of the arti-ficial sweat solution was as follows (4): NaCI 0.3%,Na2S04 0.1%, urea 0.2% and lactic acid 0.2%.Samples of the same grade, in the as-received con-dition, were immersed for 1 week in 100 ml of thenaturally aerated solution.

Tests were also conducted in artificial sweat solu-tions, at pH 6.6 for AISI 303, 304 and the Ni-platedsteel, and at pH 3.0 with NaCI concentrations of0.05 M and 0.1 M for AISI 304 and the Ni-platedsteel, to study the influence of pH and chloride(Cl-) concentration on the leaching of Ni. Area ofspecimens exposed to the above solutions were thefollowing. AISI 303: 7.07 cm2 (pH =4.5) or 10.3cm2 (pH=6.6 and 3.0); AISI 304, 316L and 430:72 cm2; Ni-plated steel: 14.14 cm2 or 28.3 cm2;nickel: 14.14 cm2.

After immersion, samples were rinsed with dis-tilled water. The test solution was added with therinsing solution, then analysed using atomic ab-sorption spectroscopy (detection limit= 10 ,ug/l) todetermine the Ni concentration, from which theleaching level (in ,ug/cm2/week) was deduced.

Dimethylglyoxime (DMG) spot tests

This test is based on the complexation of Ni ionswith DMG to give a chromophoric compound.ln ammoniacal solution, Ni is complexed in theoxidation state + 2, which develops a pink color-

ation. This coloration is characteristic of a positivereaction and thus supposedly reveals the presenceof Ni. Thus, it is commonly used by determatolog-ists to reveal the presence of Ni in alloys and soto determine if they can elicit Ni allergy in Ni-sensitive patients (2, 3, 5, 10-12).However, this testseems to be neither very sensitive nor relevant. Forexample, "false" -negative reactions, due to insuf-ficient oxidation of Ni, can appear. It is then re-commended that the alloys should be etched witha strong acid such as hydrochloric acid (HCl). Thisprocedure leads to metal dissolution in the activestate and therefore reveals the presence or not ofNi in the alloy composition. However, if the steelis used in the passive state (no corrosion), whichis the normal situation if the steel is sufficientlyresistant in the environ ment considered, its passivefilm forms a protection against such dissolutionand then the test is probably not relevant. More-over, "false" -positive reactions can occur becauseDMG is not a specific reagent for Ni. It also reactswith Fe. Thus, for alloys containing Fe, it is necess-ary to apply hydrogen peroxide, which decolorsthe complexes of Fe, but not those of Ni, withDMG.

To verify the sensitivity and validity of this test,we applied it to the Ni-plated steel and to the fourSS grades, according to the method described invarious papers (3, 10). Tests were conducted on as-received samples in different ways, 3 times for eachtype of steel: (i) a few drops of NH3 and of theDMG solution were placed successively or rubbedon the samples, then drops of H202 were depositedto detect "false" -positive reactions; (ii) to detect"false" -negative reactions, samples were alsoetched with 3N HCl before applying NH3 andDMG.

Electrochemical investigations

Corrosion and passivity of metals and alloys arebasically electrochemical processes (13). Thus, elec-trochemical tests enable comparison of the stabilityof materials, in terms of dissolution. ln fact, for agiven metal-solution potential difference, one ob-tains a current density (i) that is the sum of 2currents: (i) the cathodic one (iK), which is con-nected to the electroreduction of some ions of thesolutions (for example H+ or OH-); (ii) the anodicone (iA),which reflects the oxidation of the metal,that is to say its dissolution in deaerated media(for example Fe-+Fe2++2e).

Under open-circuit conditions, the total currentdensity equals zero since liKI= liAI,whichthen rep-resents the so-called corrosion current density. Therest potential or corrosion potential (Erest), is thenthe corresponding electrode potential. To study the

252 HAUDRECHY ET AL.

electrochemical behaviour of materials, it is con-venient to apply the electrode potential (E) atvalues different from Erest and to record the totalcurrent density (i = iA+ id. Thus, one obtains thei= fiE) curve. Therefore, comparing the i= fiE)curves measured for several materials, enablescomparison of the importance of the reaction ofdissolution of the metal, and so the cation releaserate. For example, for metais like Ni or Ni-platedsteel, the surfaces of which are composed only ofNi, this current density reflects directly the dissol-ution of Ni.

Tests were conducted in 0.05 M NaCI solutionsat pH 4.5 and 6.6, and in artificial sweat solutionsat pH 4.5. They were performed on circularsamples (diam. = 30 mm except for AISI 303:diam = 15 mm). The holder was designed so as toavoid any risk of creviee corrosion, which wasverified throughout the testing procedure. Testsolutions were maintained at 23°C and deaeratedwith N2+ 3% H2flux for 2 h before and throughoutthe experiment. After 15 min at the rest potential,polarization curves were obtained from the restpotential to the pitting potential, using a 10 mV/min scanning rate. SS were tested in the as-receivedcondition and polished (followed by 24 h ageingin air), except in NaCI at pH 4.5 in which theywere tested only in the as-received condition. Ni-plated steel was tested as received and Ni afterpolishing.

ResultsPatch tests

Results of the patch tests are presented in Table 2.They dearly reveal that the low sulfur SS studiedhere, induding austenitic SS, elicited no allergiereactions in Ni-sensitive patients. ln contrast, 96%of the patients reacted to Ni-plated metal and 14%to the resulfurized austenitie stainless steel (AISI303), whieh has the lowest corrosion resistanceamong SS studied here, as will be seen below.

~eaching experiments

Results obtained for the tests run in artificial sweatsolutions are shown in Fig. 1. They are averagedover several tests, the number of which is given inparentheses. (Ni)Reis the Ni quantity leached inthe test solution by the metallic samples during the1-week experiment. For each type of solution, ablank was also analysed by atomic absorptionspectroscopy and, when traces were found, valuesobtained for the test solutions were corrected.

It appears from these results that leaching levelsof Ni from Ni or Ni-plated steel are very high, ofthe order of 100,ug/cm2/week, while they are much

Table 2. Results of the patch tests performed in 50 patientsalready sensitive to nickel

Patch test results (reading at D2)

AISI AISI AISI AISI Ni-platedSex Ni Cr Co 430 304 316L 303 Steel

F* +M + - +F* +F + - +F* +F* +F* + - +F +F +F* +F ++M ++F* ++F* ++F* ++F* ++ - ++F* ++F ++F* ++F* ++F* ++F* ++F ++ - +F* ++F ++F ++F* ++F* ++F ++F* ++ - +++F* ++F ++F* ++M ++F ++ +F* ++F ++F ++F* ++F ++F* + + + + +F* + + + - +F* + + +F* + + + - +F* + + +F +++F* +++ - ++F +++F* +++ - ++M +++* Previous contact dermatitis from costume jewellery.

++ ++

++++++++++

++++

++ +++++

+ ++++++++++++

+++++++++++++

+ +++++++++

++++

+ +++++++++

+ ++++

+ ++++

++++

+++++

+++++

+++++

lower for the low-sulfur-containing SS studied here:around 0.1 ,ug/cm2/week in the most corrosivesolutions and often beyond the detection limit of1.4 10-2 ,ug/cm2/week (for AISI 304, 316L and430). Besides, the AISI 303 grade exhibits a higher(Ni)Revalue than the other SS, though weIl below

NICKEL RELEASE FROM STAINLESS STEELS 253

1000

100.Ni -p1ated(2) xnidœJC1

10

8303(3)

0.1

0.01

0,001

pH-3 ,8-4,5 p9-6,6

1 . 303 . 304 -.-3UiL 430 -x-nickel 8 Ni-platedl

Fig. 1. Amounts of Ni released, (Ni)Re(llg/cm2/week), fromAISI 304, 316L, 430, 303, pure Ni and Ni-plated steel, after al-week immersion in artificial sweat solutions of variable pH,at 23°C.

those observed on pure nickel and Ni-plated steel.Nickel release is close to the value of 0.5 J.1.g/cm2/week, which is currently considered as being a sen-sitization limit, below which only weak reactivityin Ni-sensitive patients is expected (1).

ln 0.05 M NaCI solution of pH 4.5, (Ni)Revaluesfor SS are similar to those obtained in artificialsweat solutions of pH 4.5, while the Ni-plated steelshows a lower (Ni)Revalue in NaCI (20 J.1.g/cm2/week), though still higher than the limit mentionedabove.

ln artificial sweat of pH 3.0 with a 0.1 M NaCIconcentration, results are similar for the Ni-platedsteel, while (Ni)Reis slightly higher for the AISI304 grade (0.1 J.1.g/cm2/week).

Thus, we observe an increase in Ni release fromSS when the corrosivity of the leaching solutionincreases (lower pH and/ or higher Cl- concen-tration). ln contrast, pH and Cl- concentrationshave no noticeable influence on Ni release fromNi-plated steel, because of the low corrosion resist-ance of that material in each of the leaching solu-tion used.

Dimethylglyoxime (DMG) spot tests

The DMG test applied to Ni-plated metal samples,without acid etching, is always positive. A pinkcoloration appears and remains when we applyH2û2. Applied to SS, the test is negative: withoutacid etching, no coloration appears; with acid etch-ing with HCl, a pink coloration occurs on eachsample, but disappears with H2û2. This shows,then, that the coloration is due to complexation ofFe with DMG, and not of Ni with DMG.

Results obtained show that this test does not

differentiate between the SS studied here, whilethey do not release Ni in the same amount andwhile their harmfulness regarding Ni allergy is verydifferent: for example, according to this test, theAI-SI 303 grade should be harmless, which does notagree with what was observed. Moreover, to detectfalse-negative reactions, it is recommended that thesamples be etched with a strong acid. For an SS,such an attack results in destruction of the passivefilm and in the loss of its corrosion resistance.Thus, the test is then applied on the bulk of thealloy, but no longer on the surface which is actuallyin contact with the skin.

The irrelevance of this test led us to proposeanother test that would differentiate between SS,that is to say SS susceptible to release, in a corros-ive environment, low or high quantities of metallicelements, Ni among them. Since, among austeniticSS, suifur content is the main parameter that exertsan influence on corrosion resistance, we applied avery simple test with nitric acid, which reveals thepresence of suifur. We used a solution of 3N HNû}and applied a drop of it on each SS grade for 5min. The AISI 304, 316L and 430 grades showedno reactions, while, on the AISI 303 samples, ablack coloration appeared that indicated the pres-ence of a high quantity of sulfur.

Electrochemical investigations

Curves obtained in deaerated artificial sweat solu-tions of pH 4.5 are shown in Fig. 2: SS and Ni-plated steel are in the as-received condition, Ni ispolished. A large domain of passivity is shown forthe 2 austenitic grades (AISI 304 and 316L), witha low passive current density (less than 1 J.1.A/cm2)in each case, while AISI 430 presents only a very

a.socl

0.300

-O.tOO

AISI 303

.0.300

_1.soo -8.500 -7.100 .'.500

101(1) - 1 ln R.-c"

Fig. 2. Polarization curves of AISI 303, 304, 316L and 430, andof Ni-plated steel and pure nickel, measured in a deaeratedsynthetic sweat solution of pH 4.5 (T=23°C). Samples are inthe as-received condition except pure Ni which is polished withSiC paper (grade 1200).

254 HAUDRECHY ET AL.

smaIl passivity domain (from 227 mVI SCE until247 mVI SCE), not visible in Fig. 2 because of thescale used for the y axis. FinaIly, for AISI 303 nopassivity domain is seen.

The Ni-based materials present a peak of activityat about - 225 mVI SCE (SCE: saturated calomelelectrode) followed by a short passive domaincharacterised by a "high" current density (l0 pAlcm2 for Ni, ~200 j.1A/cm2 for Ni-plated steel).From these results, it follows that AISI 304 and316L have a much higher passivation ability thanthe other materials in that solution.

ln NaCl solution at pH 4.5, results are basicallysimilar. For AISI 304, 316L and 430, we observea higher rest potential (AErest~ 80 mV for AISI304, AErest~20 mV for AISI 316L and around 10mV for AISI 430) and a lower pitting potential(AEp~ 160 mV for AISI 304 and AEp~90 mV forAISI 316L). Thus, the width of the passivity poten-tial range is smaller than in artificial sweat, but thepassive current density, which varies as the cationre1ease rate throughout the passive film, remainsof the same order of magnitude. For the AISI 303grade and Ni-plated steel, there is no noticeabledevelopment of passivity. For pure Ni, the maxi-mum anodic current density is significantly largerthan in artificial sweat (850 j.1AIcm2 instead of 50j.1AIcm2). Thus, the chemical components addedto the NaCI solution to form the artificial sweatseem to improve the corrosion resistance of someof the materials, which can be explained by aninhibitory effect.

pH effect: acidification of the NaCI solutionfrom pH 6.6 to 4.5 has the following consequences:(i) an increase in the rest potentials for aIl themetals, due to an increase of the redox potentialof the solution; thus, for AISI 304, 316L and 430grades, the passive domain is reduced and it disap-pears for AISI 303; (ii) an increase in the passiveand in the active current density for the AISI 303grade and the Ni-based materials.

Mechanical polishing decreases the rest potentialwith respect to the as-received condition, in neutralNaCI solution as weIl as in acid artificial sweatsolution (in NaCl, AErest= -130 mV for AISI 304,- 290 mV for AISI 316L, - 280 mV for AISI 430and -100 mV for AISI 303. ln artificial sweat,AErest=-630 mV for AISI 304, ...:.570mV for AISI316L, -790 mV for AISI 430 and -70 mV forAISI 303), while pitting potentials seem fairly un-changed. It results in a larger passive domain, butwith a higher passive current density.

Thus, it appears from these results that AISI 304and 316L have the highest corrosion resistance,AISI 430 shows an intermediate behaviour, andAISI 303 and the Ni-based mate rials have the low-est corrosion resistance. Qua1itatively, this is weIl

correlated with the respective level of Ni releasemeasured previously. However, it is difficult toquantitatively relate Ni release and anodic currentmeasured in electrochemical test, since the Ni re-lease rate probably decreases with time during theleaching experiment.

DiscussionThe maximum amount of nickel that can be re-leased in a corrosive environment is proportion alto the anodic current density, which correspondsto the cationic current flowing from the metal tothe corrosive solution.This current density, i, de-pends on the rest potential Erest, which is itselfdetermined by the cathodic reaction that is effectiveat the metal-solution interface. ln aerated neutralmedia, the cathodic reaction is that of oxygen re-duction, which roughly determines the rest poten-tial. This remains true at lower pH (such as pH4.5), when the rest potential is not controlled bythe hydrogen evolution reaction. Thus, studyingthe current density i(E) versus electrode potentialE, in deaerated media, allows estimation of thecurrent density in aerated media as being roughlyi(Erest).FinaIly, the polarization curves in deareatedmedia allow a qualitative comparison of the cationrelease tendency of the various materials, takinginto consideration both the width of the passivityregion and the level of the anodic current in thispassivity region.

From this viewpoint, the polarization curves ob-tained in various chloride solutions (artificial sweatof pH=4.5 and NaCI of pH=4.5 and 6.6), showa much better behaviour of the SS 304, 316L and430 grades compared to pure nickel or Ni-platedmetal. This can be attributed to the stability ofthe passivation of SS in those solutions, while Nipassivity is clearly destroyed in such conditions.Thus, using SS in conditions where the passivefilm remains stable should not lead to any risk ofcontact dermatitis.

The case of the high-sulfur grade AISI 303 mustbe considered in a different way. It is well-knownthat manganese sulphide inclusions, while highlyfavourable to free machinability, are very detri-mental to corrosion resistance. Our studies showthat the corrosion resistance of AISI 303 is notsufficient in the considered solutions to avoid anyrisk of contact dermatitis, even if the risk is muchlower than for Ni-plated steels. Thus, the use ofNi-containing SS in prolonged contact with theskin should be restricted to low-sulfur-containingSS (typically less than 0.03%, while AISI 303 con-tains of the order of 0.3%). The nitric acid spottest proposed in this paper allows distinction be-tween the 2 types of steel.

NICKEL RELEASE FROM STAINLESS STEELS 255

Conversely, the DMG spot test appears irrel-evant in determining which alloys can elicit nickelintolerance, because it includes acid etching. lnfact, in the case of SS, this attack destroys theirpassive film and so greatly reduces their corrosionresistance, a fundamental property that enablesthem to be safe with regard to nickel allergy. Tobe valid, the test must be applied to the surfacethat really cornes into contact with the skin andnot to the bulk of the alloy. Thus, instead of thistest, it seems more appropriate to apply a test thatallows differentiation between SS with regard totheir corrosion resistance, such as the nitric acidspot test.

Patch tests and leaching experiments confirmthese conclusions. Results of the patch tests showedthat no allergic reactions occurred with low-sulfurSS, while some of the patients (14%) reacted to theAISI 303 grade and most of them were intolerantto Ni-plated metal (96%). It is also interesting tocompare the Ni release value in artificial sweatsolution to the sensitization limit of 0.5 f.1g/cm2/week. According to that limit, the low sulfur SSshould elicit no reactions in Ni-sensitive patients,while the AISI 303 grade should provoke reactionsonly in patients with an acid sweat, since in theacid leaching solution (pH=4.5) we found a (Ni)Revalue of 1.4 f.1g/cm2/week,but at pH=6.6, it wasbelow 0.5 f.1g/cm2/week (0.3 f.1g/cm2/week). FinallyNi-plated steel, as weIl as pure nickel, should elicitstrong reactions in the majority of patients. Patchtest results confirm these assumptions and thusvalidate the value of 0.5 f.1g/cm2/week as a sensitiz-ation limit.

Thus, a good correlation appears between cor-rosion resistance in neutral or slightly acid NaClsolutions, Ni release levels in the same solutions,and the probability that the material will elicitallergic reactions in Ni-sensitive patients. Our mainconclusion is that the AISI 304, 316 and 430 gradescan be used without any problem' in prolongedcontact with skin, while Ni-plated metais and, afortiori, pure nickel, should be prohibited. Finally,the use of an appropriate nitric acid spot test allowsselection of an appropriate low-sulfur-containingSS, while the dimethylglyoxime test is irrelevantfor this purpose.

The results presented in this paper can be gener-alized to various uses of Ni-containing SS in bio-logical applications or in the food industry. (i) Solong as the steel remains passive in the actual en-

vironment, a Ni-containing stainless steel does notrelease Ni, since it is protected by a Ni-free passivefilm. (ii) Tests to be used for deciding whether asteel is safe or not should be carefully evaluated.It is meaningless to use a test that gives informationon the alloying elements that are contained in thesteel: chemical analysis does the same, and moreaccurately.The relevant question to know theanswer to is whether these elements dissolve or donot dissolve in the environment under consider-ation.

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