INITIAL DEPOSITION OF ELECTROLESS NICKEL FROM BOROHYDRIDE-REDUCED BATH ON VARIOUS FERROUS ALLOYS.

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1 INITIAL DEPOSITION OF ELECTROLESS NICKEL FROM BOROHYDRIDE-REDUCED BATH ON VARIOUS FERROUS ALLOYS. Véronique VITRY, Adeline SENS, Abdoul-Fatah KANTA, Fabienne DELAUNOIS University of Mons (UMONS), Engineering Faculty, Metallurgy Department, 20 Place du Parc, 7000 Mons, Belgium, Abstract Electroless plating is based on the chemical reduction in aqueous solution of metallic ions, in the present case, nickel ions, that are reduced by sodium borohydride. The initial steps of the deposition and growth process were studied in depth during this work on substrates made of various ferrous alloys chosen for their chemistry: mild steel, cryogenic steel with 9% Nickel, austenitic and duplex stainless steel (with both nickel and chromium). The samples were prepared by grinding to 1200 mesh with silicon carbide abrasive paper and activated by acidic etching. They were then immersed in the plating solution for times varying from 5 s to 1 h. The morphological evolution of the deposit during the early stages of plating was observed by scanning electron microscopy (SEM) on the surface of the sample and on prepared cross sections. Growth rates were measured by weight gains as well as by SEM measurements. Energy dispersive X-ray spectrometry (EDX) and glow discharge optical electro spectroscopy (GDOES) analysis were used to obtain information about the chemistry of the deposits and their results were correlated with the morphology and growth rate of the coating. Initiation mechanisms were compared with the mechanism previously indentified on mild steel. Differences in initiation delay, growth rate and deposit morphology during growth were observed and related to the substrate chemistry. Keywords: Electroless nickel, coatings, Coating formation, SEM. 1. INTRODUCTION The electroless plating process, developed in 1946 [1], allows the synthesis of metal-metalloid alloy coatings by chemical reduction of nickel salts in aqueous solution. Contrary to electroplating, this process is suitable for plating dielectric materials because it does not use external current sources. The most popular alloys deposited by this method is nickel, which can be obtained in a variety of alloys including pure nickel [2, 3], nickel-phosphorous (Ni-P) [4] and nickel-boron (Ni-B) [5]. Electroless nickel coatings present several properties of great interest for industrial applications such as homogeneous thickness, high hardness and good corrosion and wear resistance [2, 3]. Ni-B alloys are less popular than Ni-P, but they have a higher hardness [5]. While there is an abundant literature on electroless nickel, most research is focused on properties and applications [5-7], plating optimization [8], or heat treatments [9-10]. There are only a few papers dealing with phenomena during the plating process [11-13], even if the comprehension of those could lead to better control of the plating bath. The process begins by the reduction of nickel ions on the immersed surface of an active substrate. Further reaction is enabled by the catalytic activity of the deposit itself [14]. Four ways are possible to induce initiation of electroless nickel deposition: (i) using a substrate with spontaneous catalytic activity towards the oxidation of the reducing agent, (ii) using a more oxidable metal than nickel as substrate, (iii) activating the

2 substrate by dipping in a solution of catalyst (such as Pd salts), (iv) activating the substrate by galvanic coupling [4]. Most literature on electroless plating initiation is dedicated to the use of catalyst [15]. As a result, initiation mechanisms are rarely known in other cases. In this work, the initial deposition of electroless nickel-boron (from a borohydride-reduced bath) was observed on various ferrous substrates chosen for their chemistry: mild steel, cryogenic steel with 9% Nickel, austenitic and duplex stainless steel (with both nickel and chromium). The onset time of the process, deposit morphology and growth rate were monitored during the first minutes of the plating process, allowing to get insight about the influence of the substrate chemistry on the plating initiation. 2. EXPERIMENTAL DETAILS 2.1. Sample preparation The initiation and beginning of the plating process were studied on 4 ferrous alloys whose chemistries are shown on table 1. The samples used for the study were small cut sheets with a size of 2*2 cm and a thickness of 1 mm. The substrates were prepared for plating using a classical procedure for ferrous alloys that has been described elsewhere [13]. After grinding to 1200 mesh with silicon carbide abrasive paper, the most important step of this procedure is activation etching in a solution of 30 vol. % HCl for 1 to 5 minutes. Table 1: chemistry of ferrous substrates Designation mild steel austenitic stainless steel duplex stainless steel cryogenic steel St 37 AISI (Ur 45N) ASTM A353 C 0.17 < 0.08 < 0.03 < 0.13 Cr Ni Mo Mn < 2 < 2 < 0.9 Si 0.3 < 1 < P 0.05 < < 0.03 < S 0.05 < 0.03 < 0.02 < Fe balance balance balance balance The electroless plating process took place with a borohydride-based bath developed by Delaunois et al. [6, 16]. This bath uses nickel chloride as a nickel ion source and lead tungstate as a stabilizing agent. It operates at 95 1 C and has a ph of 14. For ideal operation, the bath is continuously agitated and the bath load is fixed at 25 cm². l -1. All experiments were carried out in a thermostable, Teflon coated, cell with a volume of 8 l and a new bath was used for each experiment. The samples were immersed in the solution for times ranging from 5s to 60 minutes as follow: 5s, 15s, 30s, 60s, 90s, 4 min, 7 min, 10 min, 30 min and 60 min. After immersion, they were rinsed with deionised water and dried in hot air. A separate experiment was designed to ascertain the initiation mechanism on mild steel: a sample was immersed for 4 minutes in a bath without reducing agent Analysis of the samples The surface of samples was investigated by SEM using a Jeol JSM 5900LV scanning electron microscope. Thicker samples were also investigated on cross section after mirror polishing and nital etching.

3 Spreading of the coating on the substrate was followed by EDX during SEM experiments. This technique was used on a surface of 1 mm² at a magnification of 100 times, during 60s. The results obtained this way represent the composition of the top 1 µm of the sample and give information about the progressive accumulation of nickel on the surface. In the case of nickel containing substrate, a correction factor was used to discount contributions from the substrate. Used this way, EDX is not quantitative. Profile composition of the coating was obtained by glow discharge optical electron spectroscopy (GD-OES) with the help of a Horiba-Jobin-Yvon GD-profiler 2 apparatus. 3. RESULTS AND DISCUSSION 3.1. Initiation mechanism on mild steel No catalytic activation of the substrate was used in this study. As such, only two mechanisms are possible for the initiation of the electroless deposition process: oxidation of sodium borohydride by catalytic activity of the substrate or formation of a thin layer of catalytic nickel by displacement with iron. The second mechanism is not dependent on the presence of sodium borohydride in the bath. The first, however, is. Observation of samples placed in a bath without reducing agent (sodium borohydride), all other parameters kept identical, allows thus to identify the acting mechanism on a particular substrate. Figure 1 present a SEM micrograph of the surface of a sample immersed for 4 minutes in a bath without reducing agent. On this sample, no trace of nickel could be detected but small cubic lead crystals were formed on surface defects. This shows that nickel deposition by displacement with iron is not possible in plating conditions. The initiation mechanism is thus, by elimination, catalytic oxidation of sodium borohydride on the substrate. The presence of lead on the samples can however be attributed to a displacement reaction, lead being more noble than nickel and iron (-0.47 V for Fe 2+ /Fe; V for Ni 2+ /Ni; V for Pb 2+ /Pb). Fig 1: SEM micrograph of a sample immersed for 4 minutes in a bath without reducing agent. Fig 3: Surface morphology of coatings after 4 minutes of immersion on (a) mild steel; (b) austenitic stainless steel; (c) duplex stainless steel; (d) cryogenic steel 3.2. Initiation of electroless nickel-boron plating of ferrous substrates SEM micrographs taken after 15, 30 and 90 seconds of immersion are presented on figure 2. On mild steel, the surface stays unmodified up to at least 15 s. However, an important amount of nodules is observed after 30s and the surface is fully covered after only 90 s. Images taken later in the process indicates that the coating appears fully densified after 4 minutes (see figure 3a).

4 On austenitic AISI 304 stainless steel, initiation is already detectable after 15s but the coverage after 30 and 90 s is not complete (figure 2). However, after 4 minutes, the substrate is fully covered and the coating appears dense (figure 3b). The propagation of the coating is thus less quick than on mild steel. The situation on Duplex stainless steel is similar to austenitic steel but with an even quicker initiation (first evidence is observed after 5 seconds), as can be seen on figures 2 and 3c. On cryogenic steel, the coverage is always higher than on any other steel. However, roughness of the sample may be a factor in this case as initial samples were very rough and polishing could not suppress all surface defects. Quicker initiation has been reported on rougher samples [13]. The quicker initiation observed on stainless (austenitic and duplex) steels compared to mild steel is attributed to the following factors: (i) freshly activated (non-passivated) stainless steels are very easily oxidizable. A displacement reaction could thus take place very quickly between chromium and nickel, which would induce a quicker initiation of the deposition process. (ii) nickel, which is present in non-negligible amounts in all samples except mild steel, is a known catalyst for the oxidization of sodium borohydride. This would also help explaining (together with the higher roughness) the very quick initiation observed on cryogenic steel, which contains nickel in comparable amount to the tested stainless steels but does not contain chromium. In the case of duplex steel, it is possible that galvanic coupling between the austenitic and ferritic phases also plays a role in the quicker initiation. It has not yet been possible to determine the relative importance of the different factors. Fig 2: surface morphology at the beginning of the deposition process on various substrates

5 Fig 4: substrate coverage in the early stages of plating Fig 5: coating thickness The quicker initiation but slower growth observed by MEB is confirmed by EDX analysis, as shown on figure 4. Nickel is detected much quicker (event after correction) on duplex and austenitic stainless steel than on mild steel. However, continuity of the coating (materialized by maximal nickel content) is attained after only 4 minutes on mild steel and after at least 10 minutes on the other substrates. It appears that growth is slightly slower on duplex stainless steel than on austenitic stainless steel. During later stages of the process, thickness measurements carried out by SEM can be used to confirm those results. Results are presented on figure 5 for mild steel and austenitic stainless steel. The coating deposited on mild steel is always thicker than the one on austenitic stainless steel. However, both coatings present a phase of very low growth in the first 10 minutes of plating that is associated with coating densification Relation between coating chemistry and morphology Average chemistry of the nickel-boron deposit is 6 wt.% B, close to 1 wt. % Pb, balance Ni [17]. To compare coating chemistry and morphology, the profile composition, obtained by GD-OES, was superposed to a SEM cross section image of the coating (figure 6). While Ni and B contents are stable across the coating, the Pb content has a more noticeable evolution. There is nearly 0.45 wt.% Pb at the coating/substrate interface but this values decreases quickly in the first 2-3 µm of the coating, stays low for the next 10µm and increases once again near the surface. Morphology of the coating appears influenced by the Pb content: lower Pb content (at the center) coincides with wider columns and smaller columns are observed at the beginning and the end of the process, when lead content is higher. Fig 6: GDOES profile analysis of a Ni-B sample superimposed with a cross section image of the sample.

6 The presence of finer columns (and thus of higher Pb content) at both extremities of the coating is associated with lower net growth rate. This is coherent with the evolution of chemistry as lead is used to slow deposition. 4. CONCLUSIONS The initiation and growth of electroless nickel-boron deposits on various ferrous alloys was studied in this work. The following results were obtained: The initiation mechanism on mild steel is a catalytic effect and not nickel deposition by displacement. Initiation occurs quicker on cryogenic steel, austenitic and duplex stainless steel than on mild steel. Several effects are proposed to explain this but their importance is not yet known. Growth of the coating is quicker on mild steel than on other substrates. The columns constituting the coating are smaller where the lead content of the coating is higher (near the interfaces). LITERATURE [1] BRENNER A., RIDDEL G., Nickel plating on steel by chemical reduction, J. Res. Nat. Bur. Stds. 37 (1946) 31. [2] HARI KRISHNAN K., JOHN S., SRINIVASAN K.N, PRAVEEN J., GANESAN M., KAVIMANI P.M., An overall aspect of electroless Ni-P depositions - a review article, Met. and mat. Trans., 37A (2006) [3] RIEDEL A., Electroless nickel plating. Finishing Publication LTD., London, [4] GUTZEIT G., KRIEG A. Process of chemical nickel plating and bath therefor. US Patent , [5] RIDDLE Y.W., BAILER T.O., Friction and wear reduction via an Ni-B electroless bath coating for metal alloys JOM (April 2005) [6] DELAUNOIS F., LIENARD P., Heat treatments for electroless nickel-boron plating on aluminium alloys Surf. Coat. Technol.160 (2002), [7] SANKARA NARAYANAN T.S.N., SESHADRI S.K., Formation and characterization of borohydride reduced electroless nickel deposits J. All. Comp., 365 (2004) [8] ORAON B., MAJUMDAR G., GHOSH B. Parametric optimization and prediction of electroless Ni - B deposition. Materials and Design, 28: , [9] ORAON B., MAJUMDAR G., GHOSH B.. Improving hardness of electroless Ni B coatings using optimized deposition conditions and annealing. Materials and Design, 29: , , 35, 45, 179 [10] ZIYUAN S., DEQING W., ZHIMIN D., Surface strengthening of pure copper by Ni-B coating, Appl. Surf. Science, 221 (2004) [11] CLERC M.-A., PhD Thesis, Besançon, [12] VITRY V., KANTA A.-F., DELAUNOIS F., Influence of local composition on morphology of electroless nickelboron deposits on mild steel. 'Praktische metallographie' 42, septembre 2010, [13] VITRY V., KANTA A.-F., DELAUNOIS F., Initiation and formation of electroless nickel-boron coatings on mild steel: effect of substrate roughness. Materials Science and Engineering B 175 (2010) [14] MALLORY G.O., HAJDU J.B., Electroless Plating: Fundamentals and Applications, American Electroplaters and Surface Finishers Society, Orlando, FL (1990) pp [15] LI L., MAOZHONG, J. Alloy. Compd. 461 (2008), [16] DELAUNOIS F., PETITJEAN J.P., LIENARD P., JACOB-DULIERE M., Autocatalytic electroless nickel-boron plating on light alloys. Surf. Coat. Technol. 124 (2000), [17] VITRY V., KANTA A.-F., DELAUNOIS F., Mechanical and wear characterization of electroless nickel-boron coatings. Surface & Coatings Technology 206 (2011) (special issue: proceedings ICMCTF 2011)