Mechanism of Saccharin Transformation to Metal Sulfides And

Journal of The Electrochemical Society, 153 �8� C586-C593 �2006�C586

Mechanism of Saccharin Transformation to Metal Sulfidesand Effect of Inclusions on Corrosion Susceptibilityof Electroplated CoFe Magnetic FilmsIbro Tabakovic,*,z Steve Riemer,* Katmerka Tabakovic,* Ming Sun,* andMark Kief

Seagate Technology, Recording Heads Research and Development, Bloomington, Minnesota 55435, USA

The electroplated magnetic alloys �1.0T Ni80Fe20, 1.6T Ni45Fe55, 2.4T Co40Fe60�, obtained in the presence of saccharin, andsputtered magnetic alloys of the same composition showed dramatically different corrosion properties at pH 5.9. The highercorrosion susceptibility of electroplated magnetic alloys, known for many years, was generally attributed to sulfur inclusions intothe deposit. However, there was no direct evidence of the structure of sulfur-containing molecules included in deposit. We haveanalyzed electroplated, EP-CoFe, and sputtered, SP-CoFe, magnetic films using electrochemical, secondary ion mass spectros-copy, X-ray photoelectron spectroscopy �XPS�, and high-pressure liquid chromatography �HPLC� techniques. The analysis ofelectroplated CoFe films obtained in the presence of saccharin revealed saccharin, benzamide, o-toluenbenzamide �HPLC� andmetal sulfides �XPS� in EP-CoFe deposit. The proposed mechanism for saccharin transformation to metal sulfides involves foursteps: �i� a reductive cleavage of C-S bond in saccharin giving rise to benzamido sulfinate, �ii� a desulfurization step leading tobenzamide and sulfur dioxide, �iii� an electrochemical reduction of sulfur dioxide to hydrogen sulfide, and �iv� a reaction betweenH2S and M+2 �M = CO, Fe� to metal sulfides. The higher corrosion susceptibility of EP-CoFe magnetic alloys than SP-CoFemagnetic alloys is discussed in terms of the mechanism of sulfur-assisted corrosion.© 2006 The Electrochemical Society. �DOI: 10.1149/1.2207821� All rights reserved.

Manuscript submitted December 15, 2005; revised manuscript received April 10, 2006. Available electronically June 22, 2006.

0013-4651/2006/153�8�/C586/8/$20.00 © The Electrochemical Society

Saccharin is an additive that has been widely used for more thanthree decades in the industry for electrodeposition of magnetic al-loys such as 1.0T NiFe,1 1.6T NiFe,2 and 1.8T CoNiFe,3-5 used aswriter materials in the recording heads. It has been reported thatsaccharin reduces tensile stress,1,6 grain size,6 roughness,7 andcoercivity1-6 of magnetic materials. However, it is also known thatelectroplated magnetic films in the presence of saccharin showhigher corrosion susceptibility than magnetic films deposited with-out saccharin or sputtered by vacuum deposition.8-11 The higher cor-rosion susceptibility of electroplated magnetic alloys was generallyattributed to sulfur inclusions into the deposit. Notably, the sulfur inthe deposit was associated with saccharin present in the plating bathbut there was no direct evidence about the structure of the sulfur-containing molecules included in the deposit.

The CoFe magnetic alloys with 50-70% Fe have the highestmagnetic moment of 2.4 Tesla12 and can be obtained electrochemi-cally in the presence of saccharin as an organic additive. Such CoFealloy has also very high corrosion susceptibility and when used as awriter element in recording heads it can have detrimental effects onperformance. Attempts were made to electrodeposit 2.4T CoFe al-loys without additives13 or by replacing saccharin with another or-ganic additive.14-16 However, replacing saccharin is a difficult task.It is very often a trade-off between good corrosion resistance on oneside, and high stress, roughness, grain size, and coercivity on an-other side.

In the present work we have thoroughly analyzed the composi-tion and corrosion properties of electroplated CoFe magnetic filmsobtained with and without saccharin using electrochemical, second-ary ion mass spectroscopy �SIMS�, high-pressure liquid chromatog-raphy �HPLC�, and X-ray photoelectron spectroscopy �XPS� tech-niques. Based on the analysis of the results we have proposed amechanism for saccharin transformation to metal sulfides. We havealso found that a higher amount of metal sulfides co-deposited inCoFe films results in corrosion susceptibility.

Experimental

The CoFe films were electroplated on 6 in. round alumina coatedAlTiC wafers using 2000 Å copper seed layers. The thickness ofCoFe films used for further measurements was in the range 0.5-

* Electrochemical Society Active Member.z E-mail: [email protected]

Downloaded 05 Sep 2009 to 130.127.56.12. Redistribution subject to E

1.0 �m and they were deposited using the standard paddleconfiguration17 with a 1000 Oe external magnetic field at constantcurrent density. The voltammetric and corrosion measurements havebeen described previously.5,7 The plating solution, bath A �sulfate/chloride�, contained 0.3 M NH4Cl, 0.4 M H3BO3, and metal ionsobtained by dissolution of CoSO4·7H2O and FeSO4·7H2O. Bath B�all chloride� was a modified bath A in which CoCl2·6H2O andFeCl2·4H2O are used as a source of metal ions. Bath C �all sulfate�was a modified bath A in which 0.3 M NH4Cl was replaced with0.3 M �NH4�2SO4. The additives used in electrodeposition of CoFefilms were sodium lauryl sulfate �NaLS� and saccharin �Sacc�.

The analysis of foreign elements in CoFe films was determinedusing dynamic SIMS depth profile analysis. SIMS analysis was ob-tained from Charles Evans & Associates on their in-house system.18

SIMS results were calibrated using a sample where major elementswere determined by X-ray fluorescence �XRF� and hydrogen contentwas determined using hydrogen forward scattering �HFS�. Thechemical state of elements was determined using XPS. Films weresputtered with 500 eV argon ions during XPS analysis to expose thebulk of the films. HPLC analysis was carried out on a Waters 2695UV–photodiode array �PDA� detector and 15 cm Symmetry C18from Waters. A gradient phase of acetonitrile and trifluoracetic acidwas used. All solutions were prepared from analytical grade chemi-

Figure 1. Polarization curves of EP-Co40Fe60 and SP-Co40Fe60 films in0.1 M NaCl solution at pH 3.0 and 5.9, 1 mV/s.

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C587Journal of The Electrochemical Society, 153 �8� C586-C593 �2006� C587

cals and ultrapure water. Agilent liquid chromatography/mass spec-trometry �LC/MS� system with Micromass triple quad was used forthe LC/MS analysis.

Results and Discussion

Electrochemical analysis.— Figure 1 shows the polarizationcurves of electroplated �EP� and sputtered �SP� Co40Fe60 films mea-sured in quiescent 0.1 M NaCl solutions at two different pH 3.0 and5.9, respectively. Electroplated and sputtered Co40Fe60 films of thesame composition show dramatically different corrosion propertiesat pH 5.9. The corrosion potential of the SP alloy is −0.1 V vs SCEcompared to −0.6 V vs SCE for the EP alloy. Also, the corrosioncurrent density of the SP alloy is over two orders of magnitudelower than that of the EP alloy. At pH 3.0 the corrosion potentials ofthe EP and SP alloys are near −0.6 V vs SCE. The cathodic pro-cesses are principally mass transport dominated proton reduction atpH 3, mass transport dominated oxygen reduction at pH 5.9 for theEP alloy, and is kinetically controlled for the SP alloy at pH 5.9.Because for the EP alloy the cathodic processes are dominated bymass transport �at both pH 3.0 and 5.9� the anodic processes, whichare very similar at pH 3.0 and 5.9, determine the corrosion poten-tials. The corrosion current density of the EP alloy at pH 5.9 is twoto three times smaller than that of the SP and EP alloys at pH 3.0.

Figure 2 shows anodic polarization curves for EP-Co40Fe60 andSP-Co40Fe60 alloys. The pitting potential of EP-Co40Fe60 alloy oc-

Figure 2. Anodic polarization curves of EP-Co40Fe60 and SP-Co40Fe60 filmsin 0.1 M NaCl solution at pH 5.9, 10 mV/s.

Figure 3. SIMS depth profiles of EP-Co Fe film deposited in bath A in t
40 60

curs basically around its corrosion potential �−0.6 V� while the pit-ting potential of SP-Co40Fe60 alloy occurs around +0.3 V vs SCEwhich is well above its corrosion potential. �−0.1 V�

The corrosion behavior of two other electroplated alloys obtainedin the presence of saccharin, i.e., 1.0T Ni80Fe20 and 1.6T Ni45Fe55,is similar to the described behavior of electroplated 2.4T Co40Fe60alloys, i.e., EP NiFe alloys are more susceptible to corrosion at pH5.9 than SP NiFe alloys. Generally, the corrosion potentials of EPand SP alloys with the same elemental composition of major ele-ments measured at pH 3.0 are practically the same �see Table I�.However, the corrosion potentials of EP alloys measured at pH 5.9are more negative than the corrosion potentials of SP alloys. Thereasons for inferior corrosion behavior of EP alloys compared to SPalloys is discussed later.

SIMS analysis.— Analysis of light elements in the bulk of EP-CoFe and SP-CoFe films, i.e., S, O, H, Cl, C, N, and B was carriedout by means of SIMS. A typical SIMS depth profile of EP-Co40Fe60 film is shown in Fig. 3. The amount of light elements inCoFe films is dependent on the plating bath conditions including theconcentration of anions, presence of organic additives, and pH �seeTable II�. To gain insight into the origin of light elements found inCoFe films, obtained by electrodeposition, without the presence ofadditives in plating Bath A at pH 2.0 �Table II�, we have calculatedthe distribution of various species in solution. The equilibrium for-mation of various species was examined as a function of pH in therange from 2.0 to 6.0. It is well established that the surface pHincreases at the cathode during metal deposition.19 The bulk concen-trations were calculated using equilibrium constants, material con-centration equations, and electroneutrality conditions.20 Math-ematica 4.0 was used to solve for the concentrations. Thedistribution of major Fe+2 and Co+2 electroactive species vs pH is

Table I. Corrosion properties of electroplated (EP) and sputtered(SP) magnetic alloys.

EP �mV vs SCE� SP �mV vs SCE�

Alloy pH 3.0 pH 5.9 pH 3.0 pH 5.9

2.4T Co40Fe60 −600 −580 −600 −1001.6T Ni45Fe55 −400 −260 −440 −101.0T Ni80Fe20 −360 −240 −390 −30

sence of Sacc and NaLS.
he pre

C588 Journal of The Electrochemical Society, 153 �8� C586-C593 �2006�C588

shown in Fig. 4. These calculations show that by far the highestconcentrations are uncomplexed ions �Fe+2, Co+2� and chloridecomplexes �FeCl+, CoCl+�. In addition we found that ammonia andborate complexes becomes significant for Co+2 as well as FeOH+

complexes at pH values approaching 6.We assume that the origin of O, H, B, and Cl light elements in

CoFe deposit can be explained through reductions of metal com-plexes �Eq. 1 and 2�

ML+ + e → MLads or MLs �1�

MLads + e → Ms + L− �2�

where M = Co or Fe and L− = OH−,Cl−, or H2BO3−.

The one electron reduction of complex ML+ �Eq. 1� can lead toan adsorbed complex which can be further reduced to metal, Ms,�Eq. 2�, or entrapped in the deposit as a one-electron product, MLS.It seems appropriate to view the reduction of M+2 and ML+ speciesas separate reactions occurring in parallel with each other. A similarmechanism for inclusion of oxygen and other elements in CoNiFeand NiFe films has been discussed.20-22 The total amount of lightelements is 10-15 times higher in CoFe films compared to CoNiFeor NiFe films.20 This observation coincides with 5-10 higher Fe+2

ion concentration in a CoFe plating bath compared to those in NiFeor CoNiFe plating bath. It seems that electrochemical reduction ofFeL+ complexes �Eq. 1� and/or chemical reduction of SO2 by Fe+2

�Scheme 1� play an important role in the precipitation of light ele-ments in the CoFe deposit. Note that the atomic ratio of O/H inCoFe deposit obtained without organic additives �Table II, baths Aand B, pH 2.0� is close to one which indicates that FeOH compoundprecipitated in deposit. The reduction of protons increases the pH atthe electrode surface, which brings about the increase of the concen-

Table II. SIMS analysis of light elements in atom % of EP-CoFe an

EP-CoFe fi

Elements/Ecorr

Bath A/pH 2.0No additive

Bath A/pH 2.0NaLS + Sacc

Bath B/pH 2.0No additive

S Traces 0.34 TracesO 1.1 0.18 0.804H 1 0.419 0.783CI 0.67 0.03 1.96C Trace 0.011 TracesN 0.12 0.09 0.062B 0.001 Traces Traces

CoFe �XRF wt %� Co32Fe68 Co40Fe60 Co33Fe67

Ecorr at pH 5.9�mV vs SCE�

−200 −600 −210

Figure 4. Distribution of major Fe+2 and Co+2 species in the plating Bath A


tration of FeOH+ complex in solution �Fig. 4� which is reduced atthe electrode to FeOHs. The amount of chlorine in CoFe deposit,presumably included as a MClS �M = Co, Fe� compound, increasesas the chloride concentration in bath increases �Table II, sulfate/chloride bath A vs all chloride bath B�. The incorporation of nitro-gen in the CoFe films obtained without the presence of organicadditives is associated with NH4Cl present in the plating bath. Itseems that the addition of NaLS into the plating bath does not havea large effect on the amount of O, H, Cl, and N elements in CoFefilms. However, the addition of saccharin into the plating bath de-creases the amount of oxygen and chlorine but increases sulfur andcarbon in deposit. It is possible that the polarizable saccharin mol-ecule is preferentially adsorbed on the deposit surface which inhibitsentrapment of MCl and FeOH species in deposit according to theEq. 1 and 2.

The amount of sulfur and carbon in EP-CoFe films is associatedwith the presence of saccharin in the plating bath. The concentrationof sulfur in the deposit, produced in bath A, decreases from 0.34atom % at pH 2.0 to 0.20 atom % at pH 2.8. The concentration ofsulfur increases with increase of chloride concentration in the plat-ing bath in order: 0.269 �all sulfate bath C�, 0.34 �chloride/sulfatebath A�, and 1.04 �all chloride bath B� atom %, respectively. Theconcentration of carbon in the deposit shows the dependence on pHand chloride ion concentration in the plating bath similar to theinclusion of sulfur. However, the S/C atomic ratio found in theEP-CoFe deposits is 20-350 times higher than the S/C atomic ratioin saccharin molecule, depending on the solution conditions. Thisindicates that the main source of sulfur in the deposit is the cathodictransformation of saccharin to metal sulfides co-deposited in EP-CoFe films.

The sputtered CoFe film contains much less impurities than elec-

CoFe films.

SP-CoFe filmB/pH

NaLSBath B/ pH 2.0NaLS + Sacc

Bath C/pH 2.0NaLS + Sacc

Bath A/pH 2.8NaLS + Sacc

.06 1.04 0.269 0.206 Traces

.98 0.39 0.096 0.54 0.023

.25 2.1 0.375 1.75 0.061

.02 0.47 Traces 0.124 Traces014 0.079 0.005 0.007 NA094 0.32 Traces 0.44 0.0026012 0.0015 0.028 0.004 NA

4Fe66 Co38Fe62 Co39Fe61 Co40Fe60 Co40Fe60

00 −620 −540 −400 −100

d SP-

lm

Bath2.0

0012

0.0.

0.0Co3

−2

.



troplated CoFe films and its corrosion potential �−100 mV vs SCE�indicates a higher corrosion resistance compared to EP-CoFe films.The results in Table II show that corrosion potentials for EP-CoFeand SP-CoFe films measured at pH 5.9 coincide with the amount ofsulfur present in CoFe deposit.

HPLC analysis.— Because SIMS analysis does not give infor-mation about the chemical nature of the present impurities in EP-CoFe films, we have used the HPLC technique to analyze the or-ganic compounds included in CoFe films.

A Co40Fe60 film was electrodeposited on an AlTiC wafer withRu-seed layer in bath A at pH 2.0 in the presence of Sacc + NaLSorganic additives. The wafer was transferred to a petri dish and filledwith 0.3 M HCl solution. After 1 h Co40Fe60 film was dissolved andthe Ru seed remains undissolved. The acidic solution was analyzedby HPLC. Figure 5 shows the chromatograms of the aged platingbath A solution �Fig. 5a�, for the purpose of comparison, and acidicsolution after dissolution of Co40Fe60 film �Fig. 5b�.

Figure 5. Typical HPLC chromatogram obtained from �a� aged bath solutionand from �b� acidic solution after dissolution of CoFe film in 0.3 M HC1.

Scheme 1. See text.


The chromatogram of the acidic solution after dissolution of theCo40Fe60 film �Fig. 5b� resembles the chromatogram of the solutionof aged plating bath A �Fig. 5a� in the range of retention times from6.5 to 11 min. Three compounds present in Co40Fe60 deposit involv-ing saccharin—1 �RT�6.92 min�, benzamide, 1R �RT�7.26 min�,and o-toluensulfonamide, 2R �RT�8.6 min�—were detected inacidic plating solution by comparing their retention times with purecompounds �1, 1R, and 2R� and also by spiking of their peaks. Inaddition, for each peak the UV-vis spectrum was recorded and com-pared with the spectrum of the authentic sample �Table III�. Thesame compounds detected in CoFe deposit were detected in theCoFe plating solution, which also contains benzylamine, 3R �Fig.4a�. Most of the saccharin by-products show absorption in the vis-ible spectrum indicating the formation of metal complexes with or-ganic ligands �Table III�.

To gain additional insight about the possible metal complexeswith organic ligands we have performed LC/MS analysis with elec-trospray ionization to analyze products under the peak with retentiontime 4.5 min �Fig. 4a�. Table IV lists the most likely matches for themasses that are present in these spectra. From the obtained LC/MSresults it seems that under the peak with RT = 4.5 min at least threecompounds were detected which include iron and cobalt complexesformed with organic ligands generated through reduction of saccha-rin present in the plating bath.

The by-products of saccharin, benzamide ando-toluensulfonamide, have been previously identifiedspectroscopically23 and by HPLC24 in Ni plating Watt’s bath solu-tion. In addition, 2,3-dihydro-benzothiazole-l,4-dioxide and benzylalcohol were identified in the chloroform extract of the platingsolution.25 Radiotracer techniques have demonstrated that 35S is in-corporated into Ni films electroplated from saccharin-containingsolution.26-28 Because in all experiments only the presence of S-35isotope was detected, no conclusion could be made concerning theform in which the additive or sulfur-containing by-products wereincorporated.

XPS analysis.— The low resolution XPS spectra of EP-Co40Fe60 and SP-Co40Fe60 films are shown in Fig. 6. The sampleswere sputtered for 12 min using 1 kV Ar+ beam and for 5 min using500 eV beam. The sputtering removed about 300 Å of materialwhich is much higher than the thickness of natural oxides �15-20 Å� on the CoFe surface. The samples were tilted and analysiswas performed at take-off angle of 75° to analyze material damagedby ion beam. The standard binding energies of different chemical

Table III. Assignation of peaks in HPLC chromatogram.

Retention time�min� UV-VIS �max �nm� Note

3.19 200.7, 253, 500-600 Sacc. by-product/metalcomplex

4.32 200, 220, 500-800 Sacc. by-product/metalcomplex

5.04 200, 500-700 Sacc. by-product/metalcomplex

5.87 266 Benzylamine �3 A�6.15 206, 337, 500-700 Sacc. by-product/metal

complex6.19 206, 337 500-700 Sacc. by-product/metal

complex6.43 210,228,296,500-800 Sacc. by-product/metal

complex6.75 219,301,500-800 Sacc. by-product/metal

complex6.92 204, 260 Saccharin7.26 200, 225, 270 Benzamide �1R�8.6 200,216,270 o-Toluensulfonamide �2R�8.73 215,256 Sacc. by-product/metal

complex



states of each element were analyzed according to literaturedata.29,30 The intensity of Fe 2p and Co 2p peaks in low resolutionspectra are the same in EP-Co40Fe60 and SP-Co40Fe60 which is ex-pected for the films of the same composition and can be regarded asa linear combination of the spectra of Co and Fe.

Figure 7 shows a high-resolution photoelectron spectra for EP-CoFe films which are essentially the same as SP-CoFe films �notshown here�. The Fe 2p peaks located at 707 and 720 eV correspondto Fe 2p3/2 and Fe 2p1/2 peaks in metallic state and Co LMM Augerpeak at 713 eV, respectively. The Co 2p peaks located at 778 and793 eV correspond to the Co 2p3/2 and Co2p1/2 in metallic state andFe LMM Auger peak at 784 eV, respectively. The O 1s spectrumshows peaks typical of metal hydroxide �MOH� at 532 eV and metaloxide �MxOy� at 530 eV, respectively.

The C 1s spectrum in Fig. 8 was plotted using normalized inten-sity as a plotting convenience to provide a full scale peak for ease ofviewing. The C 1s peak can be seen to be real when compared to thebaseline noise visible in the plotted spectrum. Multiple carbon spe-cies are represented in the C 1s peak because the observed fullwidth at half-maximum �fwhm� is �4.0 eV while single speciesC 1s peaks obtained under similar conditions have an approximatefwhm of 1.5 eV. Because the total carbon amount in the CoFe de-posit is only 0.011 atom % �Table III, bath A at pH 2.0� the C 1speak is weak and not well resolved. The component peaks cannot beassigned to specific binding energies, but the suggested organiccompounds �1, 1R, and 2R� have C 1s binding energies that wouldbe encompassed by the observed C 1s peak. The reported values forFeS �161.6 ± 0.1 eV�31 and for CoS �162.6 ± 0.2 eV�32 are inagreement with binding energies obtained. The increase of the in-tensity of the MS peak at 162.5 eV for EP-CoFe film obtained at pH2.0 indicates the higher concentration of metal sulfides than in theEP-CoFe alloy obtained at pH 2.8. This also coincides with their

Table IV. Mass matches.

Detected �m/z� Possible molecule

285.9 PhCONHSO3Fe-H+

272.1 Co�PhCH2NH2�2-2H+

287.9 Co�PhCH2NH2 + PhCONH2�

Figure 6. Low-resolution XPS spectra of �a� EP-Co Fe film, and �b� SP-
40 60 40

corrosion potentials, i.e., −600 mV vs SCE for EP-CoFe film ob-tained at pH 2.0 and −400 mV vs SCE for EP-CoFe film obtained atpH 2.8.

Mechanism of saccharin transformation to metal sulfides.— Wehave detected four compounds included in EP-CoFe films, i.e., sac-charin, benzamide, toluensulphonamide, and metal sulfides. The keyresults, i.e., detection of benzamide �HPLC� and metal sulfides�XPS�, allow us to propose the mechanism of saccharin transforma-tion to metal sulfides which is shown in Scheme 1. We propose thattransformation of saccharin to metal sulfides occurs in four mainsteps. The first step involves reductive cleavage of the C-S bond insaccharin molecule, 1, giving rise to benzamido sulfinate, 1a. Thereductive cleavage of C-S bond in the sulfonamide moiety of thesaccharin molecule involves two electrons and two protons. It isknown that sulfonamides X-C6H4SO2NH2 �where X is an electron-withdrawing group� are cleaved at the C-S bond.33 The carbonylmoiety in saccharin molecule acts as an withdrawing group andfacilitates the reductive cleavage of C-S bond. The second step isdesulfurization of benzamidosulfinate, 1a, which is presumably anacid catalyzed reaction. The protonation of the amide moiety in, 1a,occurs on the oxygen atom leading to the protonated intermediate,1b, which is fragmented to sulfur dioxide and to benzamide, 1R.The third step is reduction of sulfur dioxide to hydrogen sulfide,which is reduced at the electrode through six electrons and six pro-tons. This reduction can be carried out also in solution by Fe+2 ionsin the presence of chloride ions.34 We believe that the conversion:SO2 → H2S is the slowest step in the overall transformation of sac-charin to metal sulfides. This conversion includes numerous chemi-cal and electrochemical steps and could give a different intermedi-ates like H2SO2, H2S2O4, H4SO2, H2SO, etc. The fourth step is fastreaction between H2S and M+2 leading to metal sulfides, whichprecipitate in CoFe deposit presumably at the grain boundaries.

Our results indicate that the transformation of saccharin to metalsulfides is pH-dependent. A higher amount of metal sulfides in EP-CoFe was obtained when electrodeposition was performed in thesolution at pH 2.0 than at pH 2.8 �see Table II and Fig. 8b�. Becausethe mass transport of protons is diffusion-controlled, the supply ofprotons to the electrode surface is dependent on proton concentra-tion in solution and diffusion layer thickness. To verify the role ofprotons in transformation of saccharin to H2S we have deposited

e film.
Co F 60


CoFe films onto Au pad electrode �RDE, A = 1 cm2� in bath A atpH 2.8 and constant current density �5 mA/cm2� using differentrotation rates of RDE. The transport of all electroactive species insolution, which are competing for the free electrode surface, is en-hanced at the higher rotation rate of the electrode. The content ofiron in CoFe films was practically independent of rotation rate,while plating rate decreased by about five times as the rotation rateof RDE increased �Fig. 9�. The decrease of plating rate with anincrease of rotation rate is a result of decrease of diffusion layerthickness and enhancement of proton mass transport to the elec-trode, which lowers the current efficiency. The higher supply ofprotons realized at the higher rotation rate showed remarkable ef-fects on the corrosion properties of CoFe films �Fig. 10�. The in-crease of the mass transport of saccharin to the electrode wouldinhibit proton reduction35 and therefore result in an increase of theplating rate. The opposite experimental observation, i.e., decrease ofthe plating rate �Fig. 9�, can be explained through higher diffusivityof proton by an order of magnitude compared to additivemolecule.36

The CoFe films produced at higher rotation rate of Au-pad RDEare more susceptible to corrosion than films obtained at lower rota-tion rate. Although the electrodeposition of CoFe films was per-formed from bath A at pH 2.8 the value of corrosion potential ofCoFe alloy obtained at the rotation rate of 1000 rpm �−580 mV� isclose to the corrosion potential for the CoFe alloy obtained in thepaddle cell at pH 2.0 �Table II�. It seems that the higher corrosionsusceptibility is linked to the larger amount of metal sulfides co-deposited in CoFe films. This is confirmed by the increase of metalsulfide peak at 162.5 eV in XPS spectra of CoFe film obtained athigh rotation rate of 1000 rpm �Fig. 11b�. On the other side themetal sulfide peak of CoFe film obtained at low rotation rate of85 rpm is very small and almost buried in the noise of the XPSspectra �Fig. 11a�.


The results indicate that the conversion of SO2 → H2S is pH-dependent and is probably the slowest step in transformation ofsaccharin to metal sulfides. The experiments with RDE also confirmthat the larger amount of co-deposited metal sulfides in CoFe filmscoincides with their higher corrosion susceptibility. Therefore, theCoFe alloys electrodeposited in the presence of saccharin at higherpH and under hydrodynamic conditions that create a thicker diffu-sion layer will result in superior corrosion properties.

Sulfur-induced corrosion.— Sulfide inclusions, particularly thosecontaining traces of MnS added for better machinability, have beenknown for many years to be detrimental to the corrosion resistanceof steel alloys.37 The sulfide inclusions have a similar effect oncorrosion resistance of Ni38 and NiFe alloys.39 According to themechanism of sulfur-induced corrosion of Ni and NiFe alloys, pro-posed by Marcus,40-42 the sulfide inclusions are likely to initiate alocalized corrosion on heterogeneous surface defects �step edges,dislocations, grain boundaries, etc.� due to �i� a selective chemicalor electrochemical dissolution of sulfide inclusions, �ii� a sulfur en-richment at the localized surface �pit walls� during the pit growth,�iii� a catalytic effect of adsorbed sulfur on metal dissolution, and�iv� a blocking effect on passivation when the surface is coveredwith a monolayer of adsorbed sulfur. Therefore, a small amount ofincorporated sulfide can promote the dissolution of macroscopicamounts of materials.

Why the electroplated alloys are more susceptible to corrosionthan sputtered alloys �Table I� can be answered in terms of themechanism of sulfur-induced corrosion. In acidic media at pH 3.0the pasivating oxide layer is dissolved in both EP- and SP-CoFefilms and corrosion occurs at the CoFe surface. Because the compo-sition of both alloys is the same �Co40Fe60� the corrosion potential isaround −600 mV vs SCE for both alloys. At pH 5.9 the corrosionpotential of SP-CoFe alloys free from metal sulfides shifts to

Figure 7. Fe 2p, Co 2p, and O 1s XPSspectra from �a� EP-CoFe film obtainedfrom bath A.

Figure 8. C 1s XPS spectra from EP-CoFe film obtained from bath A at pH 2.0and S 2P spectra form EP-CoFe film ob-tained from bath A at pH 2.0 and pH 2.8.



−100 mV vs SCE while for EP-CoFe alloys it remains at −600 mVvs SCE �Table I�. The chemical dissolution of sulfide inclusions atthe grain boundaries of EP-CoFe alloy leads to the sulfur enrich-ment at the pit surface. The presence of sulfur apparently catalyzesanodic dissolution and blocks the passivation. It is likely that thesulfur enrichment at the pit surface causes an increase in the corro-sion current density for metal dissolution, which in turn will shift thecurrent-potential curve for EP-CoFe films to more negative poten-tials �see Fig. 1�.

Conclusions

Electroplated magnetic alloys, obtained in the presence of sac-charin, and sputtered magnetic alloys �1.0T Ni80Fe20, 1.6TNi45Fe55, 2.4T Co40Fe60� of the same composition show dramati-cally different corrosion properties at pH 5.9.

Figure 9. Effect of rotation rate of Au pad RDE on Fe-content and platingrate.

Figure 10. Effect of rotation rate of Au pad RDE on corrosion potential andcorrosion to current of CoFe films tested in 0.1 M NaCl solution at pH 5.9.


The amount of light elements �S, O, H, Cl, C, N, B� in the bulkof EP-CoFe alloys is dependent on the plating bath conditions in-cluding the concentration of anions, presence of saccharin, and so-lution pH.

The origin of O, H, B, and Cl light elements in CoFe deposit isexplained through one electron reduction of ML+ complex present inthe plating solution.

The addition of saccharin into the plating bath decreases theamount of oxygen and chlorine but increases sulfur and carbon inCoFe deposit.

The analysis of electrodeposited CoFe films revealed the pres-ence of saccharin, benzamide, o-toluenesulphonamide �HPLC�, andmetal sulfide �XPS� in deposit.

The proposed mechanism for saccharin transformation to metalsulfides involves four steps: �i� a reductive cleavage of C–S bond insaccharin giving rise to benzamido sulfinate �ii� a desulfurizationstep leading to benzamide and sulfur dioxide, �iii� an electrochemi-cal reduction of sulfur dioxide to hydrogen sulfide, and �iv� a reac-tion between H2S and M+2 �M = Co, Fe� to metal sulfides.

The reductive conversion: SO2 → H2S is dependent on protonconcentration in solution and diffusion layer thickness. The higherproton concentration and thinner diffusion layer thickness �more in-tense agitation� bring about the larger amount of metal sulfides inCoFe deposit which in turn leads to their inferior corrosionproperties.

The higher corrosion susceptibility of EP-CoFe magnetic alloysthan SP-CoFe magnetic alloys was discussed in terms of the existingmechanism of sulfur-assisted corrosion.

Seagate Technology assisted in meeting the publication costs of thisarticle.

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Mechanism of Saccharin Transformation to Metal Sulfides And

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