1

Initiation and formation of electroless nickel-boron coatings on mild steel: effect of 1

substrate roughness. 2

3

V. Vitry1

, A.-F. Kanta2, F. Delaunois

1 4

1 Université de Mons, Service de Métallurgie, Place du Parc, 20, 7000 Mons, Belgique 5

2 Université de Mons, Service de Science des Matériaux, Place du Parc, 20, 7000 Mons, 6

Belgique 7

8

Corresponding author 9

Tel: +32 65 37 44 38 10

Fax: + 32 65 37 44 36 11

E-mail address: veronique.vitry@umons.ac.be 12

13

Abstract 14

The initial deposition and growth of electroless nickel-boron deposits on mild steel was studied: 15

the films were prepared in an electroless plating bath using sodium borohydride as reducing 16

agent. Samples were immersed in the plating solution for times from 5 s to 1 h and the 17

morphological evolution of the deposit was followed by scanning electron microscopy (SEM) 18

observation of the surface and prepared cross sections. Energy dispersive X-ray spectrometry 19

(EDX) and glow discharge optical electro spectroscopy (GDOES) analysis were used to obtain 20

information about the chemistry of the deposits and their results were correlated with the 21

morphology of the coating. The initiation mechanism of electroless deposition on mild steel was 22

identified. The effects of substrate roughness variation on the morphology and growth rate of the 23

coatings were investigated by reproducing the experiment on samples with various surface 24

preparation (grinding) states. 25

26

We observed that the increase of substrate roughness favours the deposit initiation: the density of 27

nickel nodules increases with increasing roughness of the substrate. Longer immersions in the 28

bath lead to homogenization and densification of the coating and the nodules are clearly 29

distinguishable. 30

31

2

Keywords: Film deposition, Metals, Surface morphology, Surface roughnesses, Electroless 1

nickel-boron. 2

3

1. Introduction 4

Electroless coating is a well-established surface engineering process that has developed by 5

Brenner and Riddel in 1946 [1]. It involves deposition of a metal-metalloid alloy coating on 6

various substrates (including dielectric materials) by electrochemical reactions in aqueous 7

solution. Electroless nickel deposits possess a number of interesting properties such as uniform 8

thickness, high hardness, good corrosion resistance, etc. [2-5]. 9

10

Electroless nickel deposits are usually classified according to the nature of the reducing agent. 11

Nickel-phosphorous deposits (based on reduction by the hypophosphite ion) are the most studied 12

and used but the properties of nickel-boron deposits are of very great interest for several 13

industrial applications like aeronautics, petrochemical industry, food industry, firearms, etc.: their 14

hardness is higher than nickel-phosphorous, and their electrical and tribological properties are 15

very promising [6-14]. 16

Most of the papers focused on electroless nickel and particularly on nickel-boron describe the 17

optimization of plating parameters, the coating properties or the effect of heat treatment. There 18

are very few papers dealing with deposit formation [7,8]. 19

20

Electroless plating occurs by the reduction of nickel ions at the surface of the active substrate 21

immersed into the plating solution and further growth of the coating is due to a catalytic action of 22

the deposit itself [15,16]. There are 3 known ways of inducing the initiation of electroless nickel 23

deposition. Either the substrate is spontaneously catalytic for the oxidation of the reducing agent, 24

or it is more easily oxidable than nickel, in which case a thin catalytic layer of nickel is 25

spontaneously deposited, or finally it can be activated by dipping in a solution of catalytically 26

active metal salts (like Pd) or by galvanic coupling [4]. Among the few studies dedicated to the 27

initiation mechanism of electroless deposition, are focused on systems in which catalysts, such as 28

Pd are used [17-22]. This means that if no catalyst is used, the actual mechanism that allows 29

initial deposition on a determined substrate is rarely known and is at best proposed by 30

assumptions. 31

3

1

The initiation and the growth of nickel-boron coatings have a non negligible influence on their 2

properties. In the case of experimental, unreplenished baths, the structure and the composition of 3

the coating are not homogeneous over the coating thickness but change during the deposition, as 4

was showed in the extreme case of a non agitated bath by Rao et al [23]. However, the 5

observation of the formation of nickel-boron deposits has not been studied extensively yet, 6

opposite to the growth of nickel-phosphorous coatings that has been studied on various substrates 7

with and without catalytic activation [17-22, 24-28]. 8

9

It is also well known by the electroless platers that the state of the substrate has a great influence 10

on the plating process, but also on the coating’s properties [29, 30]: surface roughness can for 11

example affect the coating’s appearance. Although this may appear to be the most trivial of the 12

problems caused by surface roughness, it is often important in terms of potential lost revenue 13

[31]. It is thus clear that controlling surface roughness is important in terms of quality and 14

functionality. 15

The substrate roughness is a parameter that is sometimes difficult to control in an industrial 16

process: Most parts are supplied by the client without particular surface preparation. They also 17

often have complicated geometries (because the plating of this kind of parts is a major application 18

of electroless plating). For this reason, mechanical preparation (such as grinding) may be difficult 19

to implement. As such, it is of great interest for the final application of the process to gain the 20

most extensive knowledge of the influence of the substrate surface condition on the initiation. 21

22

The aims of this work were to observe and describe the initiation and formation of electroless 23

nickel-boron deposits on mild steel, to relate the properties of the coating to this formation and to 24

evaluate the effects of substrate roughness on the formation of the deposit. To study the influence 25

of roughness on the initiation, samples were submitted to different mechanical preparation 26

processes, using varying grades of SiC grinding paper. 27

28

2. Experimental details 29

2.1. Sample preparation 30

4

The morphological and chemical evolution of the deposited material during the plating was 1

investigated on the same substrate (St 37 steel). Samples (steel sheets with a thickness of 1mm) 2

were prepared for deposition by mechanical grinding, acetone degreasing, acid etching 3

(activation) in 30% hydrochloric acid and deionised water rinse. They were then immersed in the 4

electroless nickel boron bath. The bath was based on nickel chloride and sodium borohydride, 5

with lead tungstate as a stabilising agent. More details on the bath composition have been given 6

by Delaunois et al. [32-33]. The same bath composition was used for all the process. 7

The samples were immersed in the plating solution from 5 s up to 60 min (Tab. 1). After 8

immersion in the electroless bath, the samples were rinsed with deionised water and then dried in 9

hot air. 10

A specific experiment, using a bath without reducing agent, was designed for the identification of 11

the initiation mechanism. 12

To study the influence of roughness on the initiation, mild steel (St 37) samples were submitted 13

to different mechanical preparation processes (with varying grades of SiC grinding paper) before 14

the chemical step of the surface preparation, Tab. 2. 15

16

2.2. Deposit analysis 17

To assess the nucleation sites and the initiation mechanism, the progressive spreading of the 18

deposit on the surface, and other parameters linked to the first stages of the deposition process, 19

the surface was investigated by several analytical techniques, such as SEM, EDX analysis and 20

roughness measurement. 21

The roughness of the samples was measured by the mechanical stylus method, using a Zeiss 22

Surfcom 1400D-3DF apparatus. 23

A Jeol JSM 5900 LV scanning electron microscope (SEM) apparatus was used to characterize the 24

structure and superficial morphology of the coatings. Cross sections were ground to mirror polish 25

using a diamond paste (1/4 micron) and then etched using 10 % nital before SEM observation. 26

During the SEM experiment, the average nickel and iron content of the surface was measured by 27

EDX analysis. This allowed us to obtain an approximation of the nickel coverage ratio of the 28

surface and subsurface (EDX analysis is not limited to the surface of the sample but gives an 29

average composition for the upper part of the sample). 30

5

EDX analysis was used to determine a ’nickel coverage ratio’ of the coating. To obtain consistent 1

results, a surface of 1 mm2 was analyzed at a magnification of 100 times, during 60 s. The results 2

of this analysis represent the composition measured on the surface of the sample. However, this 3

does not represent the uppermost surface because the penetration depth of the EDX analysis is of 4

the order of 1 m. Moreover, as boron is not detected by this technique, the nickel/iron ratio 5

cannot be used as a quantitative tool. Nevertheless, this analysis allows detecting the complete 6

absence of nickel on the surface and gives qualitative indications about the formation of a 7

continuous coating. 8

Profile composition of the coating was determined by GDOES using a Horiba-Jobin-Yvon GD-9

Profiler 2 apparatus. 10

11

3. Results and discussion 12

13

3.1. Initiation and formation of the deposit on polished mild steel substrates 14

The first step of the study was the observation of the formation and morphological evolution of 15

the nickel-boron deposit on substrates submitted to the standard surface preparation method (that 16

was used for most of our studies: mechanical polishing of all surfaces up to the grade 4000 SiC 17

paper [34]). All the samples studied in section 3.1 and 3.2 were synthesized in a complete 18

deposition bath containing the reducing agent, as opposed to the bath used in section 3.3. 19

20

3.1.1. Surface observation 21

Immersion in the bath up to 15 s brought no evidence of nickel deposition detectable by SEM 22

observation. This period can be considered, as far as the microscopic observation is concerned, as 23

an induction period. The first nodules of electroless deposit are observed after an immersion of 24

30 s and are preferentially concentrated on the scratches and defects of the surface. As can be 25

seen on Fig. 1a, they do not form a continuous layer yet and their size is in the range of 0.1-0.2 26

µm. After 60 s, the islands have colonized the whole sample surface but they do not form a 27

continuous layer yet (Fig. 1b). At very high magnification, it is possible to see that, on some 28

places of the samples, several layers of nodules are present (Fig. 1c). The nodule size is close to 29

0.5 m. After an immersion of 90 s (Fig.2a), a thickening of the nodules is observed and the 30

surface seems to be levelled. This phenomenon is still observed after 4 min of deposition (Fig. 31

6

2b). On the sample that was plated for 7 min (Fig. 2c), the beginning of a refining of the surface 1

texture is observed. This refining is achieved after 10 min of plating (Fig. 2d). After a deposition 2

time of 30 min, a slight thickening of the columns and levelling of the coating is observed and the 3

intercolumnar interstices are very pronounced. After 1 h, the interstices seem to be nearly filled 4

out and the apparent size of the cells is very small. 5

6

3.1.2. Cross section observation and thickness 7

For technical reasons, there are no observable cross sections for samples immersed in the bath for 8

less than 90 s. After an immersion of 90 s, a very thin and continuous layer of nickel is observed 9

near the interface. Over this layer, nodules of various sizes are growing (Fig. 3a). This suggests 10

that some nickel is deposited before the formation of nodules and columns during the so-called 11

induction period. After 4 min, the nodules have colonised the whole surface and begun to 12

coalesce. The coating is now about 1 m thick and the porosities are beginning to close (Fig. 3b). 13

After an immersion of 7 min, the nodules are turning into columns and a secondary germination 14

induces the refining of the columns size (Fig. 3c). The structure of the columns has a nearly 15

fractal quality: the initial column divides into several smaller columns that can be further divided 16

during the next stages of deposition but becomes less marked after a long deposition time because 17

of the progressive densification of the coating (Fig. 3d and Fig. 3e). 18

After an immersion of 1 h, several layers of columns, generated by the alternating germination 19

and growth phases, are observed (Fig. 3f). 20

Thickness of the deposits was measured, when possible, on SEM micrographs, as shown on Fig. 21

4. At the beginning of the process (up to 4 min), the thickness increases very quickly (with a 22

growth rate exceeding 40 µm.min-1

). However, SEM observation shows that the deposit is far 23

from being dense. Between 4 and 7 min of plating, the thickness does not increase because the 24

coating is progressively densifying. After this, a quasi linear thickness increase is observed up 1 h 25

of immersion. The mean growth rate is 18 mm.h-1

. 26

27

3.1.3. EDX analysis 28

EDX analysis carried out on a 1 mm² area of the coating was used to follow the beginning of the 29

deposition process. EDX results are not representative of either the upmost surface of the coating 30

or the average composition because EDX has an interaction depth of the order of 1 µm in the 31

7

conditions we used. This technique gives only information about the presence of nickel on the 1

surface (i.e. the presence of boron was not detected). 2

However, as it is difficult to obtain thickness measurements for electroless coatings in the early 3

stages of deposition, the evolution of the amount of nickel detected by EDX can be used as a 4

qualitative indicator to detect the initiation and follow deposition rate. The EDX results are 5

summarized in Tab. 3. 6

7

After an immersion of 15 s, a non negligible quantity of nickel is already measured on the 8

surface. The induction period is thus shorter when measured in term of nickel content on the 9

surface than when measured by microscopic observation. It can be due to the deposition of very 10

small nuclei on the substrate before the formation of the first nickel-boron nodules. This 11

hypothesis is reinforced by the presence of a very thin continuous layer of nickel under the 12

nodules (Fig. 3a). 13

After an immersion of 30 s, the amount of nickel detected on the surface and subsurface is close 14

to 20%, indicating the presence of a non negligible amount of nickel outside of the nodules 15

detected by the SEM. This is attributed to a selective growth phenomenon. This phenomenon 16

may reflect heterogeneities in nucleation and growth rate but it is possible that it may be caused 17

by growth inhibition on some of the nuclei that were deposited during the very first moments of 18

the process, by lead adsorption, causing a slower growth rate and a growth that is characterized 19

by an unmodified surface topology. On other places, the growth is not inhibited and the formation 20

of nodules can be observed during SEM analysis. 21

After 1 min, the amount of Ni detected on the subsurface is close to 60% meaning that a nearly 22

continuous layer of nickel is formed. After 4 min, a continuous layer is formed, as indicated by 23

the SEM observations. 24

25

3.1.4. Chemical heterogeneities 26

The profile composition of the coating was measured by Glow-discharge Optical Emission 27

Spectroscopy (GD-OES). The nickel and boron contents of the coating are relatively stable across 28

the whole deposit. However, the evolution of the lead content is really different: it is higher near 29

the substrate/coating interface and close to the sample surface than in the bulk of the coating. 30

8

Relation between the local lead content of the coating and the morphology were established by 1

superimposing the profile chemical analysis of a coating with its SEM cross section image, as 2

shown on Fig. 5. 3

The lead content of the coating is close to 0.45 wt.% at the substrate/coating interface then 4

decreases quickly down to 0.3% in the first 2-3 µm of the coating. The content stays then low 5

(0.25-0.3 wt.%) for the next 10 µm of the deposit before going up in near the surface (at the end 6

of the deposition process). The lowest lead concentration of the coating can be matched with the 7

stage of quick growth observed after 7 min of deposition, while the high concentration detected 8

in the coating that is first deposited can be linked with the plateau in growth due to densification 9

of the coating. This suggests an explanation for the plateau phenomenon: during the first 90 to 10

250 s of deposition, the deposition rate is high but the material that is deposited is not very dense 11

and possesses thus a very high specific surface. At this moment, an important amount of lead is 12

adsorbed on this surface and slows significantly the deposition process until the majority of the 13

cavities formed by the initial deposit are filled. The growth continues then at a more controlled 14

rate because the process enters a steady state. At the end of the experiment, a lowering of the 15

deposition rate is observed, that can likewise be linked to the higher lead content observed near 16

the surface of the coating. 17

Comparison of the lead content and morphology shows that the lower lead content observed in 18

the center of the coating coincides with the presence of wider columns, while the columns appear 19

to be smaller at the very beginning and the very end of the process, when the lead content is 20

higher, which is in good agreement with the SEM observation of the surface of the samples. 21

22

3.2. Influence of the substrate roughness 23

The influence of roughness on the initiation was investigated by following three same coating 24

properties than on polished steel: the morphology, the coverage (as measured by EDX analysis), 25

and the roughness. These were carried out on substrates submitted to different mechanical 26

preparation processes, as described in Tab. 1. 27

Roughness measurement carried out just after the surface preparation (Tab. 4) show that grinding 28

with grade 220 SiC abrasive paper (P1) induces a slight decrease of Ra with a non negligible 29

increase of Rp. Here follow the results of ordering the samples from the rougher to the smoother 30

according to Ra: NP, P1, P3, P2; and according to Rp: P1, NP, P3, P2. 31

9

The P3 sample, which has been ground only with 4000 grade paper, is rougher than the sample 1

ground with grade 1200 paper (P2) because the finer grained paper, when used without pre-2

grinding, provides a less effective surface preparation. The P1 sample has a higher Rp roughness 3

than the NP sample but this difference is not statistically significant because the standard 4

deviation for both values (close to 0.5) is higher than the difference between them. 5

The parameters chosen to represent the roughness in this study are Ra (arithmetic average of the 6

height of every point of the surface) and Rp (maximum peak height). Ra was chosen because it is 7

the most in use roughness parameter. However, as the electroless process takes place in solution, 8

we feel that the peaks will have a more important contribution to the deposit initiation and growth 9

than the valleys, thus we chose to use Rp conjointly with Ra. 10

11

After a plating time of 5 s, the increase of substrate roughness favors the deposit initiation, as 12

shown on Fig. 6: the density of nickel nodules increases with increasing roughness of the 13

substrate. Moreover, the size of the nodules present on the surface appears smaller when the 14

roughness is higher, with a nodule size of the order of 50 nm on the unpolished surface. This 15

shows that the nucleation of the nickel nodules on the substrate is easier when the substrate is 16

rougher. The higher density of nodules on the rougher substrate is in accordance with the findings 17

of Liu et al. [30] but the formation of bigger nodules on the rougher substrate was not observed in 18

this case. It is not surprising as the Ni-P does not form nodules on the Mg alloy substrate but 19

small cubic crystals, which denotes a different initiation mechanism. Moreover, low 20

magnification micrographs on the samples give the impression of bigger deposits on the rougher 21

substrate because aggregates are already formed. 22

After an immersion of 15 s in the plating bath, the density of nickel nodules is still decreasing 23

with decreasing roughness (Fig. 6) but the size of the nodules appears homogeneous on all the 24

samples and is close to 50 nm. After 30 s in the plating bath, the surface of the samples is nearly 25

completely colonized by the nickel deposit (Fig. 6). The size of the nodules is still similar for all 26

the samples. 27

After 60 s, all the samples are completely colonized (Fig. 7). However, the deposit appears more 28

homogeneous on the sample that submitted to the P1 surface treatment, which has the higher Rp. 29

Longer immersions in the plating bath lead to homogenization and densification of the coating, as 30

shown on Fig. 6. The size of the nodules is still in the same range as in the previous cases but 31

10

they tend to form aggregates, which are clearly distinguishable on low magnification images 1

while the nodules are only observables at high magnification. 2

3

To assess the deposition rate on substrates with varying roughness, EDX analysis was used 4

(qualitatively) in the first moments and thickness measurements were carried out by SEM on 5

thicker coatings. 6

The results of the EDX analysis are presented in Fig. 8. After 5 s, a similar amount of nickel is 7

detected on the samples with NP, P1 and P2 preparation while the nickel is slightly lower on the 8

P3 sample. This is in agreement with the SEM observation that roughness favors deposit 9

initiation. After 15 s, the P3 sample has reached the same level as the NP, P1 and P2 samples. 10

During the next time of immersion the P1 sample has a higher deposition rate (its nickel content is 11

already close to 90% after 90 s) than all the other samples. The NP sample on the other hand has 12

a faster initiation but grows slower and reaches similar value to the P2 sample after 90 s. 13

From those results, it seems that the P1 surface treatment produces the best results, as far as 14

deposit initiation is concerned,. This is not surprising because the sample submitted to the P1 15

treatment has the higher Rp roughness. This sample has thus higher peaks that are preferred 16

nucleation sites. The NP sample shows a good comportment at the very first stages but the 17

deposit grows slower on this sample, probably because its higher roughness is due to valleys (the 18

Rv roughness, that measures the depth of valleys, is 1.98±0.4 for the NP sample and 1.75±0.6 for 19

the P1 sample) in which the deposition is slowed by the diffusion of reactive needed to attain 20

them. 21

22

We can conclude from those observations that the presence of peaks influences favorably the 23

initiation process (the peaks are the preferred nucleation sites) but that the presence of grooves or 24

valleys in the substrate decreases the growth rate because diffusion of the reactive inside the 25

valleys is difficult. 26

27

Thickness measurements at later stages of the deposition process (Fig. 9) showed that all samples 28

had similar growth rates. It is however interesting to note that the end thickness decreases with 29

the initial roughness of the substrate. 30

31

11

1

3.3. Initiation mechanism 2

Based on the theoretical knowledge of the electroless process [4], one can deduct that the 3

beginning of the process (the deposition of the very first atoms of nickel on the surface) on an un-4

catalyzed steel substrate can only be induced by 2 phenomena: (i) the formation of an ultra thin 5

layer of nickel by displacement (redox reaction between nickel ions and the substrate, 6

accompanied by dissolution of the substrate material, without intervention of the reducing agent), 7

(ii) oxidation of the sodium borohydride due to catalytic activity of the substrate (steel) and 8

subsequent reduction of nickel salts that form a very thin nickel layer. 9

Since the resulting nickel layer should be similar in both cases, simple SEM observation and 10

EDX analysis of the coatings cannot permit the identification of this mechanism during 11

deposition. Thus, experiments dedicated to the identification of the actual initiation mechanism 12

taking place in our bath for mild steel (St37) substrates were developed for this study. During 13

these experiments, mild steel (St37) substrates were immersed in a bath exempt of the reducing 14

agent, every other parameter (composition, temperature, pH and agitation) kept constant. The 15

samples were immersed in this bath for varying durations (Tab. 1) and were then observed by 16

SEM. 17

18

No nickel was detected on any of the samples immersed in the electroless bath without reducing 19

agent, even after an immersion of 4 min (Fig. 10). However, lead had been detected on the 20

surface by EDX and forms small cubic crystals that are concentrated on the surface defects i.e. 21

polishing scratches, etc. (Fig. 10). This allows excluding the displacement reaction as initiation 22

mechanism for the electroless deposition. The formation of lead crystals could be however 23

attributed to a displacement reaction between iron and lead, which is not really surprising because 24

the redox potential of lead is higher than nickel and iron (-0.47 V for Fe2+/Fe; -0.27 V for 25

Ni2+/Ni; -0.13 V for Pb2+/Pb). 26

27

28

Conclusion 29

. 30

12

The present work brings a lot of answers about the formation of electroless nickel-boron coatings 1

which is a really complex process. However the present knowledge of the process does not 2

permit yet to describe and to understand all the phenomena that happen during this formation. 3

This work will thus be completed by further investigations including structural characterization 4

of the growing coating, surface analysis by XPS of the deposits at different stages of their growth 5

and TEM observation. A study of the influence of substrate nature on the nucleation and growth 6

(carried out on various stainless steel grades) is also underway. 7

8

The following results were obtained concerning the initial deposition of Ni-B films, which were 9

electrolessly deposited from sodium borohydride baths. 10

The initiation mechanism on mild steel for those baths is a catalytic effect and not a 11

displacement reaction between nickel and iron. 12

The different phases of the deposition process are, as follow: 13

- A very short induction period during which some nickel is deposited on the coating 14

but not in the form of nodules. 15

- The formation of a very thin continuous layer. 16

- The formation of nodules. 17

- The densification of the coating. This phase begins after 4 min in the bath, when the 18

nodule layer is continuous. During this phase, the thickness of the coating does not 19

increase but the amount of deposited metal does. 20

- Several nucleation/growth phases, each one beginning by the refinement of the 21

columnar structure. The first re-nucleation happens between 4 and 7 min of 22

immersion. 23

The columns that form the coating are smaller where the lead content of the coating is 24

higher (near the interfaces). 25

Initiation occurs quicker on rougher substrates but the consecutive growth of the deposit is 26

slower. 27

28

29

Acknowledgments 30

13

One of the authors (V. Vitry) wishes to thank the FRIA (Fonds pour la formation à la recherche 1

dans l’industrie et l’agriculture) for funding. 2

The authors wish to thank Mr J. Dutrieux from INISMA for its help with the SEM work and the 3

University of Udine for the GDOES analysis. 4

5

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[32] F. Delaunois, P. Lienard, Surf. Coat. Technol.160 (2002), 139-148. 18

[33] F. Delaunois, J.P. Petitjean, P. Lienard, M. Jacob-Duliere Surf. Coat. Technol. 124 (2000), 19

201-209. 20

[34] A.-F. Kanta, V. Vitry, F. Delaunois, J. Alloy. Compd. 486 (2009) L21–L23 21

22

15

List of Tables 1

2

Table 1: Deposition time for the observation of the initiation of electroless deposits. 3

Sample 1 2 3 4 5 6 7 8 9 10

Deposition

time

5 s 15 s 30 s 60 s 90 s 4 min

(240s)

7 min

(420s)

10 min

(600s)

30 min

(1800s)

60 min

(3600s)

4

5

6

Table 2: Mechanical preparation processes for substrate roughness influence on deposit initiation. 7

polishing process description

NP unpolished substrate (as received)

P1 polished with grade 220 SiC paper

P2 polished with grade 1200 SiC paper

P3 polished with grade 4000 SiC paper

8

9

10

Table 3: Results of EDX analysis on ‘polished’ mil steel. 11

time (s) 5 15 30 60 90 240

Fe (at.%) 100 94.3 81 42 26 7

Ni (at.%) 0 5.7 19 58 74 93

12

13

14

15

Table 4: Substrate roughness after mechanical preparation processes. 16

samples Ra Rp

NP 0.674±0.111 1.896±0.502

P1 0.559±0.144 2.122±0.467

16

P2 0.294±0.084 1.103±0.515

P3 0.476±0.120 1.759±0.596

1

2

List of Figures 3

4

5

Figure 1: Surface of polished mild steel (St 37) samples after an immersion of 30s (a) and 60s (b 6

and c) 7

8

Figure 2 : Surface of polished mild steel (St 37) samples after an immersion of 90s (a), 4 min 9

(240s) (b), 7 min (c), 10 min (d), 30 min (e) and 1 h (f) in the nickel-boron deposition bath. 10

17

1

Figure 3: Cross section observation of polished mild steel (St 37) samples after an immersion of 2

90 s (a), 4 min (240s) (b), 7 min (c), 10 min (d), 30 min (e) and 1 h (f) in the nickel-boron 3

deposition bath. 4

5

Figure 4: Evolution of deposit thickness with immersion time (on polished St 37 steel). 6

18

1

Figure 5: Comparison of the composition and morphology across the deposit. 2

19

1

Figure 6: SEM observation of the surface of samples with varying roughness immersed in the 2

plating bath for 5 s, 15 s and 30 s. 3

20

1

Figure 7: SEM observation of the surface of samples with varying roughness immersed in the 2

plating bath for 560 s, 90 s and 4 min. 3

4

21

Figure 8: EDX analysis carried out on samples with varying substrate roughness, during the first 1

stages of the plating process. 2

3

Figure 9: thickness measurements on samples with varying substrate roughness, during the later 4

stages of the plating process. 5

22

1

Figure 10: Surface observation after 4 min of a sample immersed in a bath without reducing 2

agent. 3

4

5

6