electrodeposition of sacrificial tin–manganese alloy coatings

Electrodeposition of sacrificial tin�/manganese alloy coatings

Jie Gong, Giovanni Zangari �Department of Metallurgical and Materials Engineering, Materials Science Program, The University of Alabama, A129 Bevill Building, PO Box 870202,

Tuscaloosa, AL 35487-0202, USA

Received 19 February 2002; received in revised form 4 June 2002

Abstract

Sn�/Mn coatings have been electrodeposited on steel substrates from simple ammonium sulfate baths, with or without the

addition of citrate, tartrate, EDTA or gluconate additives. The effect of current density and additives on the coating composition,

microstructure, crystallography and corrosion resistance of Sn�/Mn deposits has been investigated. It is found that ammonium

sulfate brings Sn2� and Mn2� discharging potentials closer, allowing codeposition of manganese with tin. Sn�/Mn coatings

obtained from simple ammonium sulfate baths at low current density contain a large amount of oxygen and are microstructurally

heterogeneous, while at high current density amorphous, bright and homogenous Sn�/Mn coatings can be obtained. The addition of

tartrate, EDTA or gluconate can improve coating quality at low current density and suppress Mn2� reduction, with a

corresponding decrease of Mn content in the alloy. Sn�/Mn coatings show an anodic potentiodynamic behavior intermediate

between that of pure manganese and pure tin, and their electrochemical characteristics can be adjusted by varying alloy composition

and structure. Coatings with a high percentage of the intermetallic Mn1.77Sn phase show good sacrificial protection for steel.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Electrodeposition; Characterization; Sn�/Mn coatings; Sacrificial protection

1. Introduction

Electrodeposited Cd films are extensively used as

sacrificial coatings for steel. However, Cd is an extre-

mely toxic metal, and Cd coatings are mainly obtained

by electrodeposition from cyanide-based electrolytes, a

hazardous process currently subject to increasing reg-

ulations in various countries. Until now, there is no

single coating that possesses all the attributes needed to

replace cadmium [1]. It is thus important to develop

novel alternative, environmentally friendly processes/

materials capable to substitute Cd as sacrificial coating

to steel.

Due to their low redox potential, adequate tribologi-

cal behavior, and suitable mechanical characteristics for

coating steel products, electrodeposits of Mn and Mn-

alloys have been studied as potential sacrificial coatings

for steel [2�/7]. However, pure Mn is highly chemically

reactive and a coating of this material may provide

sacrificial protection only for a limited time when

immersed in an electrolyte or exposed outdoors [2,6,7].

Tin and tin alloys on the other hand have also been

studied as protective coatings for steel [1,8,9], due to

their outstanding nontoxicity, corrosion resistance, low

friction coefficient and solderability. Electrodeposited

tin�/manganese coatings are thus of great interest, as

they potentially combine the barrier properties of tin

with the sacrificial protection afforded by manganese.

Despite this appeal, few investigations have been con-

cerned with a detailed study of the electrodeposition of

Sn�/Mn alloys and the structure and properties of its

deposits; the only report in the open literature is the

review by Brenner [2]. This is probably because,

manganese is the most electronegative metal

(E0(Mn

2�/Mn)�/�/1.421 VSCE) that can be electrodepos-

ited from aqueous solutions, and only at pH above 2.0

[6,7], while tin has a much higher standard redox

potential (E0(Sn

2�/Sn)�/�/0.377 VSCE) and tin coatings

of good quality are difficult to grow from nontoxic,

slightly acidic electrolytes without organic additives.

� Corresponding author. Tel.: �/1-205-348-7074; fax: �/1-205-348-

2164

E-mail address: [email protected] (G. Zangari).

Materials Science and Engineering A344 (2003) 268�/278

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In this paper, an electrochemical investigation of Sn�/

Mn ammonium sulfate electrolytes with and without

additives is conducted, and the results are used to

prepare coatings of Sn�/Mn under different depositionconditions. The morphology, crystal structure and

corrosion properties of the resulting coatings are thus

evaluated and the various electrolyte chemistries are

compared.

2. Experimental

2.1. Setup

The electrodeposition experiments were carried out in

a three electrode, two-compartment cell. The counter

electrode was a platinum foil placed in the same

compartment as the working electrode, and covered by

a diaphragm to avoid contamination of the solution

from MnO2 formation at the anode. The Saturated

Calomel Reference Electrode (SCE) was in a differentcompartment, separated from the working electrode by

a Luggin capillary mounted on a syringe barrel. The

same cell was used for potentiodynamic experiments and

galvanostatic electrodeposition. The experiments were

performed, and potential/current curves as well as

galvanostatic transients were recorded, using an

EG&G PAR A273 potentiostat/galvanostat.

2.2. Substrate preparation

The tin�/manganese coatings were plated on stainless

steel #304 plates with an active surface area of 3 cm2

(1.5�/2.0 cm). The substrates were mechanically po-

lished with various grades of alumina powder papers

and degreased first with an alkaline solution (NaOH, 23

g l�1; Na2CO3, 22 g l�1; Na2SiO3 (anhydrous), 10 gl�1; Na3PO4, 10 g l�1; Sodium lauryl sulfate, 1 g l�1;

90�/95 8C, 3�/5 min) and successively with acetone.

Subsequently, they were electropolished in concentrated

phosphoric acid (85%) and pickled in mixed nitric (5%)

and hydrochloric acid (25%) just before use.

2.3. Electrodeposition

Ammonium sulfate based electrolytes were used in the

electrochemical experiments and for galvanostatic elec-

trodeposition. The base solution contained SnSO4 (0.01

M), MnSO4 (0.59 M) and (NH4)2SO4 (1 M). Citrate,

tartrate, EDTA and gluconate additives were added

separately in the form of sodium compounds and with

variable concentration. pH was adjusted between 2.5

and 3.0 by adding concentrated ammonium hydroxideor sulfuric acid. No attempt was made to control the pH

during deposition, but pH was measured and, if

necessary, adjusted, after each deposition experiment.

The solutions were prepared with analytical grade

reagents and triply distilled water. All experiments

were carried out at the temperature of 25 8C.

2.4. Characterization

Surface morphology of the coatings was examined by

Scanning Electron Microscopy (SEM) using a PHILIPS

XL30 instrument. Chemical analysis of the deposits was

performed by an attached Energy Dispersive X-ray

Spectrometer (EDX) with a CDUTM LEAPTM detector,

which can semi-quantitatively determine the relativeamounts of elements heavier than boron. Incorporation

of oxygen and the chemical states of Mn and Sn were

further studied by X-ray Photoelectron Spectroscopy

(XPS), using a Kratos Axis 165 system. Crystal structure

was determined by X-ray diffraction (XRD), using a

PHILIPS APD 3520 diffractometer with Cu Ka radia-

tion.

2.5. Electrochemical measurements

Cathodic potentiodynamic behavior of the various

electrolytes was determined using a scan rate of 5 mV

s�1 on stainless steel substrates. To evaluate the

corrosion resistance and possible passivation behavior

of the coatings, anodic potentiodynamic curves were

measured in solutions mimicking seawater. All the

measurements were performed at a temperature of25 8C. No attempt was made to purge the solution

with an inert gas and the solutions were not stirred.

During the anodic potentiodynamic sweeping experi-

ments, the samples were first immersed into a 3% NaCl

solution at pH 3.0 (adjusted by hydrochloric acid) for

about 10 min to stabilize the open circuit potential Ecorr.

Subsequently, potentiodynamic curves were recorded by

sweeping the electrode potential from �/250 (withrespect to Ecorr) to �/1200 mV at a rate of 2 mV s�1.

The experimentally determined potentiodynamic curves

were fitted using the Stern�/Geary equation [10] to give

the values of the corrosion potential Ecorr and the

corrosion current Icorr. The measurements were per-

formed with an EG&G PARC 273 Potentiostat/Galva-

nostat, and the data were manipulated using proprietary

software.

3. Results and discussion

3.1. Potentiodynamic behavior

As pointed out in the literature [7,11], the addition of

ammonium sulfate to electrolytes for manganese elec-trodeposition is essential to grow manganese coatings

with good coverage. Ammonium sulfate prevents the

precipitation of manganese hydroxide, improves the

J. Gong, G. Zangari / Materials Science and Engineering A344 (2003) 268�/278 269

conductivity of the electrolyte, increases current effi-

ciency and in general widens the operation window of

manganese electrodeposition. In order to electrodeposit

Sn�/Mn coatings, a small amount of stannous sulfate

(SnSO4) was added to a solution containing MnSO4

(0.59 M) and (NH4)2SO4 (1 M) [7]. Although manganese

coatings of good quality can be electroplated from the

above solution at pH 2.0�/7.0, our preliminary experi-

ments showed that Sn2�, even with concentration as

low as 0.01 M, was not stable and would hydrolyze and

precipitate in the above solution at pH over 3.0 when no

other complexing agents were added. Therefore, pH of

all the solutions studied here was pre-adjusted to 2.5�/

3.0.

The cathodic potentiodynamic behavior of pure Sn,

pure Mn, and Sn�/Mn alloy grown from simple ammo-

nium sulfate solutions without other additives and

stirring is shown in Fig. 1 and compared with that of

Sn/Mn solutions without ammonium sulfate. The dis-

charging potential of Sn2� in pure SnSO4 solution

(about �/0.47 VSCE) is close to the calculated Sn2�

reduction equilibrium potential (�/0.436 VSCE, using the

Nernst equation). The addition of ammonium sulfate

polarizes the Sn2� discharging potential to a more

negative value (about �/0.6 VSCE) and shifts H� and

Mn2� reduction reactions to more positive potentials,

thus bringing the reduction potential of Sn and Mn

closer. This makes codeposition of Sn and Mn possible,

even if Sn still deposits preferentially and Mn codeposi-

tion is always accompanied by hydrogen evolution.

Mn2� reduction seems to be enhanced by codeposition

of tin, which means that manganese can be deposited at

a lower current density in alloy deposition than in pure

manganese deposition (this was confirmed also by

galvanostatic electrodeposition results). It is also shown

that the reduction reaction of tin is already under

diffusion control before manganese is electrodeposited.

Codeposition of Sn and Mn, and the synthesis of Sn�/

Mn alloys of different composition by varying the

current density appear thus possible.

Figs. 2 and 3 show the potentiodynamic behavior of

Sn, Mn and Sn�/Mn electrolytes with the addition of one

of various complexing agents without stirring. Citrate,

tartrate, EDTA and gluconate sodium compounds were,

respectively, added at the concentration of 0.01 M, the

same as [Sn2�]. The various complexants shift the

potentiodynamic curves for Sn deposition in the nega-

tive direction to a relevant extent (Fig. 2a), with EDTA

and citrate complexes inducing the strongest complexa-

tion. On the contrary, the polarizing effect on Mn

deposition is almost non-existent at low over potentials

(Fig. 2b), probably due to the high ratio [Mn2�]/

[complexing agent]. The effect of the complexants is

similar for alloy deposition (Fig. 3), with the curves

being shifted in both directions with the addition of

different additives (Fig. 3b). In this case, the citrate

Fig. 1. Potentiodynamic behavior of Sn, Mn and Sn�/Mn solutions

with and without (NH4)2SO4.

Fig. 2. Potentiodynamic behavior of ammonium based SnSO4 and

MnSO4 solutions with various additives.

J. Gong, G. Zangari / Materials Science and Engineering A344 (2003) 268�/278270

addition is the only one shifting the potentiodynamic

curve in the cathodic direction.

3.2. Galvanostatic electrodeposition and characterization

of the coatings

Sn�/Mn deposits were grown galvanostatically from

ammonium sulfate solution with or without additives at

a temperature of 25 8C, with current density varying inthe range 10�/600 mA cm�2 at pH 2.5�/3.0. All

electrodeposition processes are accompanied by hydro-

gen evolution, which is consistent with the potentiody-

namic results and implies current efficiencies below

100%. Deposition was carried out in a quiescent

solution, electrolyte stirring being provided by hydrogen

evolution at the cathode. All coatings had a nominal

thickness (calculated assuming 100% efficiency) of 10mm. Potential transients were recorded during each

measurement. After deposition, each sample was im-

mediately removed from the bath, washed by distilled

water and air-dried, then characterized by SEM, EDX,

XPS and XRD.

SEM micrographs of Sn�/Mn coatings electroplated

from simple ammonium sulfate electrolytes at differentcurrent densities are shown in Fig. 4. The atomic

composition of the coatings obtained at different current

densities, as determined by EDX, is shown in Fig. 5. In

addition to Sn and Mn, a varying amount of oxygen

(25�/65 at.%) is incorporated into these coatings at

different current densities above 20 mA cm�2 (Fig.

5a). The relative atomic ratios of Sn and Mn are

extracted and shown in Fig. 5b. Typical XRD patternsare shown in Fig. 6. At very low current density (10 mA

cm�2), the coatings contain 97.1 at.% tin, 2.9 at.%

oxygen and no manganese. The appearance is semi-

bright, compact and uniform. XRD shows a nearly pure

b-Sn (body-centered tetragonal, BCT) structure. The

absence of Mn in the deposits obtained at low current

density is due to the fact that the discharge potential of

Mn2� (�/1.487 VSCE according to the Nernst Equation)has not been attained at 10 mA cm�2 (Fig. 1).

As current density increases to 20 mA cm�2, the

content of manganese and oxygen increases abruptly to

30.59 and 46.34 at.%, respectively. Tin content decreases

correspondingly. Further increase of current density (up

to 100 mA cm�2) leads to a small increase of Mn and O

content and a corresponding decrease of Sn content in

the coatings (Fig. 5). Generally, Sn�/Mn depositsobtained in this current density range (20�/100 mA

cm�2) exhibit a similar morphology: spongy, porous

and dark in appearance, with either a dendritic or a

cauliflower-like microstructure. EDX spot analysis

shows that the spongy, bright and dendritic structure

is tin-rich, while the underlying dark region is Mn-rich

(Fig. 4, 40 mA cm�2). XRD (Fig. 6, 40 mA cm�2)

shows that, in addition to b-Sn and MnSn2, some formsof Sn hydroxide and/or oxide (SnO�/SnO2) exist in these

coatings. The incorporation of over 50 at.% oxygen

confirms the presence of Sn and Mn oxides. This

behavior is quite different from pure Mn electrodeposi-

tion, where the incorporation of oxygen into bulk

coatings due to local pH variation occurs only above

100 mA cm�2 [6,7]. In binary alloy systems, this

characteristic was also observed in Co�/Mn alloyelectrodeposition [12], where it was attributed to the

influence of H3BO3. However, in Sn�/Mn electrodeposi-

tion, neither a high current density is applied nor

chemicals similar to H3BO3 are present. It was also

proposed that the incorporation of oxygen may be

related to the galvanic corrosion/oxidation of manga-

nese during Mn-alloy electrodeposition [2]. This me-

chanism alone, however, cannot explain the presence oftin oxides. Therefore, the incorporation of oxygen into

Sn�/Mn coatings at low current density may imply

another mechanism connected with the presence of

Sn2�, in addition to the galvanic corrosion/oxidation

Fig. 3. (a) Potentiodynamic behavior of Sn�/Mn ammonium solutions

with various additives; (b) The area circled in (a) is enlarged.


of Mn. From the potentiodynamic curves (Fig. 3b), the

reduction of Sn2� starts at �/0.6 to �/0.8 VSCE. Due to

the low concentration of Sn2�, the limiting current

density for Sn2� discharge is about 1 mA cm�2,

corresponding to the first plateau of the respective

cathodic potentiodynamic curve. As the current density

increases above the diffusion limiting current for Sn2�,

hydrogen evolution starts according to the following

reaction:

2H��2e 0 H2 (1)

The limiting current density of reaction Eq. (1) is

about 3 mA cm�2 (the second plateau of the cathodic

potentiodynamic curve Fig. 3b). This reaction tends to

increase the local pH near the substrate surface with

respect to the bulk solution. However, because of the

buffering effect of ammonium sulfate, the actual pH

excursion at this current density range is very small. Byfurther increasing the current density, hydrogen evolu-

tion by direct reduction of water may occur, providing

for the increment of current density observed at the

Fig. 4. SEM micrographs of Sn�/Mn electrodeposited from base solution at different current densities and pH 2.5�/3.0.

Fig. 5. Atomic composition of Sn�/Mn coatings electrodeposited from

the base solution at different current densities; (a) atomic percentage of

Sn, Mn and O; (b) relative atomic fraction (%) counting only Sn and

Mn.

Fig. 6. XRD patterns of Sn�/Mn coatings electrodeposited from base

solution at different current densities. ‘S’ denotes ‘substrate’.


potential of about �/1.2 VSCE (where Mn2� discharge

does not yet take place):

2H2O�2e 0 H2�2OH� (2)

It is believed that local pH could still be buffered by

ammonium sulfate at the beginning of reaction Eq. (2)

occurrence, because, the coatings obtained at 10 mA

cm�2 have negligible oxygen content as determined by

EDX. In the base solution studied here, it was found

that Sn2� stability is highly dependent on pH, and Sn2�

is quickly hydrolyzed to tin hydroxide precipitates even

at a pH slightly higher than 3.0:

Sn2��2H2O 0 Sn(OH)2�2H� (3)

or with O2 present,

2Sn2��O2�6H2O 0 2Sn(OH)4�4H� (4)

while, Mn2� hydrolysis occurs at pH�/7.5 in 0.59 M

ammonium sulfate solution [7]:

Mn2��H2O 0 Mn(OH)2�H� (5)

Therefore, whether codeposition of tin hydroxide/

oxide and/or manganese hydroxide/oxide in parallel

with Sn�/Mn occurs depends on the extent of the local

pH excursion. The increase of oxygen content in the

coating at current densities over 10 mA cm�2 indicates

that local pH excursions due to reaction Eq. (2) are out

of the buffering range of ammonium sulfate. As the

current density increases to about 60 mA cm�2,although the total tin content (Sn�/SnOx) lowers a

little, the amount of oxygen increases due to the

increment of the fraction of tin existing in the form of

SnOx . Afterward, the variation of the amount of oxygen

observed in the coatings roughly corresponds to that of

Sn. At high current density, more MnOx or Mn(OH)x

replaces SnOx in the coatings, with the result that

oxygen content does not vary much even if the amountof Sn decreases.

Upon further increase of the current density, the rate

of Sn�/Mn electrodeposition increases and becomes

predominant over the galvanic corrosion of manganese.

In such conditions, even the g-Mn phase can be found

inside the coatings (Fig. 6, 100 mA cm�2). Correspond-

ingly, the oxygen content decreases. The microstructure

is similar in shape, with cauliflower-like grain sizedecreasing as current density increases. When current

density increases above 150 mA cm�2, the peak

centered at 2u about 42�/42.58 becomes increasingly

broadened and enlarged (Fig. 6, 330 and 600 mA cm�2),

which is a characteristic of the amorphous or nanocrys-

talline structure also observed in pure Mn electrodeposi-

tion [6,7]. At current densities above 400 mA cm�2, Sn�/

Mn coatings become bright, glossy and compact,composed of fibrous tiny crystallites. The appearance

and microstructure are close to that of pure Mn deposits

obtained at high current density [6,7]. Tin content

becomes lower than 2 at.% and no b-Sn can be clearly

discerned by XRD (Fig. 6). Oxygen content also

decreases.

XPS depth profiling was used to characterize coatings

obtained at 600 mA cm�2 in detail. Fig. 7 shows high

Fig. 7. High resolution XPS spectra at various depth of Sn�/Mn

coatings obtained at 600 mA cm�2; (a) Mn2p binding energy window;

(b) Sn3d binding energy window; (c) O1s binding energy window.


resolution XPS scans of the Mn2p, Sn3d and O1s

regions at various depths in the coatings. The Mn:Sn:O

atomic ratio changes from 55.0:3.5:41.5 at the surface to

93.0:0.7:6.3 after 80 etching cycles (about 160 nm). Bydeconvoluting the Mn2p peak using three components

(corresponding to Mn(0), Mn(II), and Mn(III/IV)), [13],

the existing forms of manganese could be quantified.

The results show that Mn(0) content is as high as 60

at.% in the bulk, a result quite different from pure Mn

electrodeposition where Mn(0) was in the range 6�/8

at.% [7]. This justifies the low oxygen content in Sn�/Mn

coatings obtained at high current densities. By decon-voluting the Sn3d peak using three components (corre-

sponding to Sn(0), ‘quasimetallic’, and Sn(II�/IV))

[14,15], the existing forms of tin could also be quanti-

fied. Over 70 at.% tin exists under the ‘quasimetallic’

oxide form, 20 at.% in the Sn(II�/IV) forms and only

about 8 at.% in Sn(0) form. Due to the low content of

tin (average 0.7 at.%), the effect of oxygen in combina-

tion with tin on the O1s peak can be neglected. There-fore, by deconvoluting the O1s peak using three

components (corresponding to H�/O�/H, Mn�/O�/H,

and Mn�/O�/Mn) [13], the existing forms of oxygen

could be quantified. Most of the oxygen exists in the

form of hydroxide (66 at.%), while the Mn�/O�/Mn form

is very rare (1.5 at.%). This shows that at high current

density, the reaction Eq. (5) of Mn hydrolysis occurs

near the cathode in addition to the reduction of Mn2�.Correspondingly, the hydrolysis product Mn(OH)2 is

incorporated into the alloy coatings.

In order to obtain compact and uniform Sn�/Mn

coatings with lower hydroxide/oxide content at low

current density (B/100 mA cm�2), the hydrolysis of

Sn2� must be retarded or prevented. Various ligand

additives were used to stabilize Sn2� [8,9,17]. In this

study, small concentrations (0.01 M) of citrate, tartrate,EDTA and gluconate sodium were separately added to

the base electrolyte. All these additives can form more

stable ligands with Sn2� than with Mn2� as shown by

the stability constants of the respective complexes

reported in [16]. Consequently, addition of the additives

polarizes the reduction of Sn2� to more negative

potentials, while it has only a small influence on the

discharge of Mn2� (this is probably also due to a large[Mn2�]/[ligand] ratio) (Fig. 2). The logarithms of

cumulative stability constants of these complexing

agents with Sn2� are 15.34 [17], 9.91 [16], 18.3 [16]

and 5.29 [8], respectively. These values are qualitatively

consistent with the observed cathodic shift in the

polarization curves reported in Fig. 2a.

SEM micrographs of typical coatings grown from

electrolytes containing various additives are shown inFig. 8. Addition of sodium citrate to the base electrolyte

in the concentration of 0.01 or 0.05 M yielded coatings

with a high oxygen content of 50�/60 at.%. XRD of

these films shows a relevant percentage of SnO2 and

SnO (not shown). Coatings grown at 20�/330 mA cm�2

appear dark, spongy and loosely attached to substrate.

Therefore, it is concluded that citrate ligands do not

improve the quality of Sn�/Mn coatings, while tartrate,EDTA and gluconate are effective to improve the

quality of the Sn�/Mn coatings, which appear more

compact and uniform. The bath with gluconate in

particular gives a silvery matte coating comprised of

small crystallites (Fig. 8, gluconate). EDX shows that by

the addition of complexing agents, oxygen content in the

Sn�/Mn coatings can be reduced to 20�/30 at.% for

current densities below 50 mA cm�2. The crystalstructure of the coatings is, however, rather compli-

cated, with various phases�/b�/Sn, MnSn2, Mn or Sn

hydroxides/oxides and sometimes Mn1.77Sn*/that can

be present under different deposition conditions and for

different compositions (Fig. 9).

Fig. 10 shows the variation of the relative manganese

content to tin in the Sn�/Mn alloy coatings with current

density for the electrolytes with the various ligands/complexing agents. Manganese content in general in-

creases with current density and the addition of any of

the ligands reduces manganese content in the Sn�/Mn

coatings with respect to the simple electrolyte, to a

varying extent. The first finding is reasonable because,

in the current density range studied here, Sn deposition

rate is controlled by Sn2� diffusion, while Mn deposi-

tion rate is not. Increasing current density increases bothMn deposition and H2 evolution rates. The decrease in

Mn content upon the addition of complexing agents is

possibly due to the fact that additives may be absorbed

on the cathode surface and could consequently suppress

Mn2� discharging. While additives also form ligands

with Mn2�, their stability is lower and their concentra-

tion is smaller, so that their influence on the potentio-

dynamic behavior of the electrolytes is very limited. Thishypothesis is substantiated by the following two experi-

ments: (a) Selected additives can completely inhibit the

electrodeposition of pure manganese at low current

density (30 mA cm�2), a condition where Mn can be

deposited in absence of additives [6,7]; (b) Further

increase in ligands concentration results in a further

decrease of Mn content.

4. Corrosion resistance properties

A typical anodic potentiodynamic behavior of a

Sn47�/Mn53 coating (from EDTA-containing solution,

50 mA cm�2) in 3% NaCl solution (pH 3.0) is compared

with that of electrodeposited pure manganese and tin in

Fig. 11. For pure manganese, the corrosion potential

(Ecorr) is about �/1.35 VSCE, the corrosion current (Icorr)is 120 mA cm�2 for crystalline and 37 mA cm�2 for

amorphous coating. A maximum current region (10�/

100 mA cm�2) extends from �/1.2 to �/0.1 VSCE for


both manganese coatings. Above �/0.1 VSCE, a decrease

in current density of about one order of magnitude is

observed, probably due to formation of adherent

corrosion products. However, this is not a true passive

region since the anodic currents are still very high, about

10 mA cm�2, and it can not be compared with the

active�/passive transition observed in other metals [18].

The high anodic current density in this large potential

region indicates that the dissolution rate of pure

manganese is fast in most environments and the use of

pure manganese as sacrificial protective coating may not

be economical. According to the Pourbaix diagram [19],

tin has a large tendency to passivate and can be readily

passivated with or without an applied potential [20].

Experimentally, it can be found in Fig. 11 that tin has an

Ecorr about �/0.505 VSCE, a Icorr about 213.6 mA cm�2,

Fig. 8. SEM micrographs of Sn�/Mn coatings electrodeposited from the base solution containing different additives at the concentration 0.01 M.

Fig. 9. XRD patterns of electrodeposited Sn�/Mn coatings obtained at

different current density from the solutions containing (NH4)2SO4 (1

M), MnSO4 (0.59 M) and (1) [Tartrate]�/[Sn2�]�/0.01 M, 10 mA

cm�2; (2) [Tartrate]�/[Sn2�]�/0.01 M, 30 mA cm�2; (3) [Tartrate]�/

[Sn2�]�/0.01 M, 50 mA cm�2; (4) [Tartrate]�/[Sn2�]�/0.05 M, 50

mA cm�2.

Fig. 10. Dependence of relative Mn content (to Sn) on current density

for Sn�/Mn coatings obtained from the base solution with various

additives.

Fig. 11. Typical anodic potentiodynamic behavior of Sn�/Mn, pure

Mn and pure Sn coatings in 3% NaCl solution (pH�/3.0).


and a passive region extending from �/0.25 to 0.25 VSCE

with a passive current (Ipass) of about 100 mA cm�2. The

electrodeposited Sn�/Mn coatings generally have elec-

trochemical properties intermediate between those of

pure tin and pure manganese. In some cases (Fig. 11),

Ipass is even lower than for pure tin. Ecorr of Sn�/Mn

coatings is between �/1.35 and �/0.505 VSCE, depending

on manganese content, crystal structure and microstruc-

ture. Icorr of most Sn�/Mn coatings, however, varies to a

large extent depending on the composition and micro-

structure. All Sn�/Mn coatings, even with Sn content as

low as 0.66 at.% (600 mA cm�2), show passive behavior,

and their range of passivity and passive current density

are also dependent on tin content and microstructure.

The anodic potentiodynamic behavior of Sn�/Mn

coatings obtained from simple ammonium sulfate solu-

tions at different current densities (corresponding to

different composition, crystal structure and microstruc-

ture) is shown in Fig. 12. The extracted Ecorr, Icorr are

shown in Fig. 13 as a function of Mn at.%, deposition

current density and crystal structure. When the current

density is lower than 100 mA cm�2 (Type I coatings in

the following), the relative Mn at.% in the coatings is

lower than 86% (Fig. 5b). Ecorr of Type I coatings is

from �/0.9 to �/0.6 VSCE and reflects the mixed

corrosion potential of mainly MnSn2, Sn and their

oxides. Icorr (around 10�6 A cm�2) and Ipass (B/10�4

A cm�2) are quite low, probably due to the large

amount of conductive oxides formed during film

growth. As current density increases over 100 mA

cm�2 (Type II), Mn at.% also increases and g-Mn (or

Mn(0)) is present in the Sn�/Mn coatings. The presence

of g-Mn (or Mn(0)) (E0(Mn

2�/Mn)�/�/1.421 VSCE)

brings Ecorr down to a value lower than �/1.2 VSCE

and correspondingly Icorr increases about two orders of

magnitude (above 10�4 A cm�2). Ecorr of Type II

coatings reflects the mixed corrosion potential of g-Mn,

MnSn2, Sn and their oxides. In Fig. 12, the active

corrosion region of the Type II anodic potentiodynamic

curves is quite similar to that of pure manganese.

However, a broad passive region ranging from �/1 to

0.2 VSCE with Ipass from 10�5 to 2�/10�4 mA cm�2

appears due to the presence of tin. As the amount of tin

increases, the passive region becomes broader and

deeper.

For Sn�/Mn coatings obtained from bath with ad-

ditives, although the improvement in coating quality is

relevant, most of the alloys exhibit an anodic potentio-

dynamic behavior falling into Type I due to the

inhibiting effect of additives on Mn2� discharge, even

at current densities as high as 330 mA cm�2. The

resulting deficiency of Mn-rich intermetallics or g-Mn

(Mn(0)) in Sn�/Mn coatings prevents Ecorr from shifting

to a more negative value (Fig. 14), though Icorr can be

Fig. 12. Anodic potentiodynamic behavior of Sn�/Mn coatings

obtained from simple ammonium sulfate electrolytes in 3% NaCl

solution (pH�/3.0).

Fig. 13. The relationship between relative Mn content and Ecorr, Icorr

of the coatings electrodeposited from simple sulfate bath at different

current densities.

Fig. 14. The relationship between Mn content and Ecorr, Icorr of the

coatings electrodeposited from various additive baths at different

current densities.


low. For the purpose of sacrificial protection of steels,

coatings with sufficiently negative Ecorr and low Icorr are

preferred, because they can be anodic with respect to

steel while exhibiting low dissolution rate. From thispoint of view, both Type I (Ecorr too high) and Type II

(Icorr too high) coatings are not ideal for the sacrificial

protection of steels. It was found, however, that a few

samples, for example those obtained at 50 mA cm�2

from tartrate additive bath show a behavior intermedi-

ate between these two types, and thus give the best

results up to now (Fig. 15). These coatings have Ecorr

about �/1.054 VSCE, Icorr about 2.6 mA cm�2 and a widepassive region ranging from �/0.5 to 0.5 VSCE, while the

passive current is about 4�/10�4 A cm�2. These

coatings also have a Mn:Sn atomic ratio about 60:40,

and are mainly comprised of a Mn1.77Sn phase, with a

little fraction of MnSn2 and b-Sn. A high volume

fraction of Mn1.77Sn is probably the main reason for

these good electrochemical properties. Therefore, ma-

nipulating electroplating parameters to achieve a highpercentage of Mn1.77Sn seems desirable for the synthesis

of Sn�/Mn sacrificial coatings.

5. Conclusions

Sn�/Mn coatings were electrodeposited galvanostati-

cally on steel substrates from simple ammonium sulfate

baths, with or without the addition of one amongcitrate, tartrate, EDTA or gluconate additives. The

effect of current density and additives on the coating

appearance, composition, microstructure, crystallogra-

phy and corrosion resistance of Sn�/Mn deposits was

investigated. Ammonium sulfate could bring Sn2� and

Mn2� discharging potentials closer and manganese

could thus be codeposited with tin. Sn�/Mn coatings

obtained from simple ammonium sulfate baths at lowcurrent density contain a large amount of tin and

oxygen and exhibit a heterogeneous microstructure.

However, at high current density, sound and homo-

geneous Sn�/Mn coatings can be synthesized which

contain metallic Mn and a small amount of tin and

oxygen. The addition of tartrate, EDTA and gluconate

can improve the coatings appearance and microstruc-

ture at low current density by making them morecompact and homogeneous. The additives can also

suppress Mn2� reduction. Sn�/Mn coatings show an

anodic potentiodynamic behavior intermediate between

that of pure manganese and pure tin, and their electro-

chemical characteristics can be adjusted by varying alloy

composition and structure. Coatings with a high per-

centage of the intermetallic Mn1.77Sn phase have good

sacrificial protection for steel.

Acknowledgements

This work was supported by DOD-SERDP under

contract DACA72-99-P-0201. The use of MRSEC

shared facilities was supported by NSF-DMR 9809423.

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electrodeposition of sacrificial tin–manganese alloy coatings

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