Does Nickel Plating Have to Be Removed Prior to Plating Again

Nickel Plating

Practise shows that nickel plating can reduce the friction coefficient of the slide plate and improve the clothing and rust resistance of the slide bedplate.

From: Design of High-Speed Railway Turnouts , 2015

Binders

Laurence W. McKeen , in Fluorinated Coatings and Finishes Handbook (Second Edition), 2016

four.one.9.7 Electroless Nickel Plating

Electroless nickel plating works without the external current source used by galvanic electroplating techniques. Electroless nickel plating is also known as chemical or autocatalytic nickel plating. The process uses chemical nickel plating baths. The nigh common electroless nickel is deposited by the catalytic reduction of nickel ions with sodium hypophosphite in acrid baths at pH iv.5–5.0  at a temperature of 85–95°C. The bathroom can comprise PTFE. The resulting plating contains typically 3–13% phosphorus by weight and mayhap xx–25% of PTFE by book. PTFE powder is usually used considering dispersions are normally unstable at 85°C. Figure 4.28 is a micrograph of an electroless nickel plating with PTFE.

Figure 4.28. Micrograph of an electroless nickel (designated NP3^®) PTFE coating used on a firearm.

Photo courtesy of The Robar Companies, Inc. https://robarguns.com/custom-firearm-finishes/np3/.

Coatings of this type have low friction, exceptional wear, and adept corrosion resistance that depends on the phosphorous content. This technique is commonly used to plate kitchen and bath fixtures and door knobs.

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Fundamentals of Electrodeposition

Thousand. Zangari , in Encyclopedia of Interfacial Chemistry, 2018

Iron Group Metals (Nickel, Cobalt, Iron)

Nickel plating is among the nearly common electrodeposition processes and is used mostly every bit a function of a multilayered organization designed to increase wear and corrosion resistance, and also able to part equally a diffusion barrier. Annotation that Ni is more noble than depression carbon steel (essentially fe) and cannot corrode, i.e., provide sacrificial protection to steel parts. This multilayer coating is made of Ni, coated by a sparse layer of Cr. Ni is also used as a structural fabric in electroforming, a near net shape electroplating process that uses a shaped mold (or mandrel) to grade the final office in a single piece. Electroplating is uniquely suitable to faithfully reproduce the shape of the mandrel due to the diffusional nature of the metal ion deposition process. The virtually popular electrolyte for Ni plating is the Watts bath, which contains Ni sulfate, chloride, and boric acrid, as a buffering agent. Decorative bright Ni is obtained past the addition of a diverseness of proprietary additives and brighteners, mostly circuitous carbon and/or sulfur compounds.

Cobalt has properties similar to nickel but information technology is much less used for the aforementioned purposes due to its much college toll. Co is instead used mostly in magnetic applications, since information technology exhibits a high saturation magnetization and a big magnetic anisotropy, which makes Co and its alloys suitable for manufacturing permanent magnets.

Electroplating of Fe is less important than Ni and Co, due to the limited corrosion resistance, even if its cost is lower. Yet, FeNi alloys tin be electroplated to grade very soft magnetic materials with extensive uses in magnetic and electronic applications at microscale, such equally transformers, inductors, and electromagnets. Amidst TM alloys, NiCo alloys are also of involvement for the potential increase in hardness and the potentially hard magnetic properties. The hardness and toughness of this material is utilized in the formulation of improved electroforming processes. NiPd in addition is used as a high-performance contact with high conductivity and hardness.

Fe, Co, and Ni are inert metals, with an intrinsic low degradation rate. All exhibit a standard redox potential more negative than that of hydrogen, resulting in electrodeposition of these metals occurring in parallel with hydrogen evolution. The accepted mechanism for electrodeposition of these metals in sulfate-based electrolytes involves 3 main steps:

(20a) ${\mathrm{Me}}^{2+}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Me}{\left(\mathrm{OH}\right)}^{+}+{\mathrm{H}}^{+}$

(20b) $\mathrm{Me}{\left(\mathrm{OH}\right)}^{+}+\mathrm{e}\to \mathrm{Me}{\left(\mathrm{OH}\right)}_{\mathrm{ads}}$

(20c) $\mathrm{Me}{\left(\mathrm{OH}\right)}_{\mathrm{ads}}+{\mathrm{H}}^{+}+\mathrm{e}\to \mathrm{Me}+{\mathrm{H}}_{\mathrm{ii}}\mathrm{O}$

where the metal ion interacts strongly with water, to class a metallic monohydroxide, which is eventually reduced and adsorbs at the electrode; finally, a further reduction mediated by the hydrogen ion leads to metal reduction.

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MOLD Blueprint

Roy J. Crawford , James L. Throne , in Rotational Molding Technology, 2002

5.1.3 Electroformed Nickel

The nickel plating process has been modified to produce molds for the blow molding, thermoforming, and rotational molding industries. ⁱ The procedure begins with the part pattern, as described above. The departing line is defined and half the pattern, along with additional blueprint construction of the parting line geometry, is advisedly isolated from the other one-half. This portion is so coated with an electrically conducting grease or polyurethane onto which a fine blanket of graphite has been air-diddled. This is then immersed in a common cold plating bath, where nickel is laid downward at the rate of 4 μm/h until a uniform layer of about one.5 mm or 0.060 inch thickness has been built onto the pattern surface. Hot plating techniques lay nickel at the rate of 10 to 20 μm/h, but produce a coarse-grained porous surface. Normally this surface is wearisome and cannot exist polished. The electroformed nickel mold produced by hot plating has most half the toughness of the cold plated electroformed nickel mold. Electroformed nickel molds are used where extreme item is required, as with plastisol PVC for doll parts. A typical case is shown in Figure 5.5.

Figure 5.five. Electroplated nickel mold of mannequin head,

courtesy of Queen'south University, Belfast

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Crystallisation of nickel–phosphorus (Ni–P) deposits with high phosphorus content

W. Sha , ... K.Thou. Keong , in Electroless Copper and Nickel–Phosphorus Plating, 2011

x.ane Introduction

Electroless nickel plating is used to deposit nickel without the use of an electric electric current. Electroless nickel-phosphorus (Ni–P) coating consists of an blend of nickel and phosphorus. The autocatalytic degradation process occurs in an aqueous solution with chemical reactions caused past the catalytic reduction of the nickel ion with sodium hypophosphite in acid baths. The electroless nickel procedure was invented by Brenner and Riddell in the 1940s. Since the first discovery, electroless Ni–P deposits accept been widely used in various industries equally engineering protection coatings to protect the inner surface of pipes, valves and other parts, or as functional films.

The amount of phosphorus can be classified as loftier, medium and low. The microstructure of the electroless Ni–P deposits changes from having a mixture of baggy and nanocrystalline phases to a fully amorphous phase when the phosphorus content increases. Both high (10–16   wt%) and medium (five–8   wt%) phosphorus deposits have the same microstructure or it may consist of one or more compound phases. High phosphorus deposits take very skilful corrosion protection and abrasion resistance. Medium phosphorus deposits can be fully amorphous or consist of a mixture of amorphous and nanocrystalline nickel phase. Low phosphorus deposits consist of nanocrystalline nickel with a small amount of baggy phase or simply crystalline nickel phase. Microcrystalline was used in earlier publications, but we at present refer to the same structure as, rightly, nanocrystalline, afterwards the significance of nanostructure was found and nano has become trendy. At low phosphorus levels, the electroless nickel coatings have high wear resistance and excellent corrosion resistance.

As phosphorus levels increase, the hardness at as-deposited condition increases. Hardness can exist improved past estrus treatment that can crusade the formation or precipitation of nickel phosphide. In club to attain full hardness, the electroless nickel deposits are estrus-treated at 400 °C for one hour in an inert temper. High hardness and natural lubricity enable the electroless nickel coatings to take proficient wear resistance. The uniformity of the electroless Ni–P deposits makes them ideal wearable surfaces in many sliding-wearable applications. The presence of the amorphous phase increases the electrical resistivity of the deposits. Electroless nickel with loftier phosphorus deposits are not-magnetic equally-plated. Magnetism can be increased past rut treatment. Electroless Ni–P deposits are easily soldered with a highly agile acid flux. Those with low phosphorus content are more easily soldered after plating. The melting temperature decreases with an increase in phosphorus level. Depression phosphorus deposits melt at approximately 1300 °C whereas medium and high phosphorus deposits melt at approximately 890 °C.

Crystallisation and phase transformation behaviour of electroless-plated Ni–P deposits during thermal processing have been the subject area of diverse investigations (Martyak and Drake, 2000). These processes have vital roles in determining the cloth properties. Investigations on electroless Ni–P deposits have shown that dissimilar eolith compositions and rut treatment conditions could affect both the microstructural characteristics and the crystallisation behaviour of the eolith. Different results were reported regarding the microstructures in the as-deposited status and the stable phases after heat treatments (Martyak and Drake, 2000).

This chapter examines the effect of the continuous heating process and phosphorus content on the crystallisation kinetics and phase transformation behaviour of electroless Ni–P deposits with high-phosphorus content. For this purpose, deposits are heated in the differential scanning calorimeter under different heating rates, and the crystallisation processes are studied. For farther analysis of stage transformation, an 10-ray diffractometer and a scanning electron microscope are used to identify the phases formed in the deposits heated to different terminate temperatures under a constant heating rate.

Electroless–plated nickel picture has peachy potential to be used as a solder improvidence barrier in low cost flip-flake packaging. Since the bulwark event is due to the lack of grain boundaries in amorphous stage, i.east. the lack of diffusion path, the control of the formation of crystalline stage and grain size becomes crucial in such applications. The strain/stress level in the nickel film should as well be controlled considering of its detrimental event on the fatigue life of the plating. Based on Ten-ray diffraction (XRD) profiles, line-broadening technique can exist applied to judge the grain size and microstrains in the Ni–P deposits (Martyak and Drake, 2000). Investigation of the development of grain size and microstrain in Ni–P deposits isothermally annealed at temperatures between sixty °C and 600 °C shows that the lattice disorder in the crystalline stage increases with increasing phosphorus content. Grain size varies from 5 to 60   nm and increases sharply above 300 °C. A study was carried out by Martyak and Drake (2000) on a Ni–P film with medium phosphorus content (5–half dozen   wt%), isothermally annealed at temperatures between 100 °C and 600 °C. Pregnant grain size increase was observed at temperatures above 400 °C. Study of electrodeposited Ni–P deposits containing 1.2-15.4   wt% phosphorus using the line broadening technique shows that the grain size in the as-deposited status decreases with the increase of phosphorus content whereas microstrain increases.

In this chapter, XRD line broadening technique is used to study the evolution of grain size and microstrain in Ni–P platings after they are subjected to unlike heating processes. Continuous heating is the estrus treatment procedure. The influence of heating rate on grain size and microstrain is shown.

This chapter will too testify the microstructures and phase transformations caused by heating, past analysing scanning electron microscopy (SEM) images. This furthers research on the heat handling of Ni–P coatings and their characterisation using SEM, for examples by Rahimi et al. (2009) and Vojtech et al. (2009).

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Electroplating baths and anodes used for industrial nickel degradation

J.One thousand. Dennis , T.Due east. Such , in Nickel and Chromium Plating (Third Edition), 1993

Watts nickel bathroom

Well-nigh commercial nickel plating solutions are based on the 1 named afterwards Watts who first introduced a bath having the conception:

Nickel sulphate	NiSO4·7H2O	240 g/l
Nickel chloride	NiCltwo·6H2O	20 g/l
Boric acrid	H3BOthree	20 chiliad/l

The term Watts Bath is now used to cover a range of solutions whose compositions vary within the range shown in Table three.1, the chloride ion sometimes beingness introduced in the class of sodium chloride. Sodium chloride is cheaper than nickel chloride and is satisfactory for most purposes, although information technology has been reported that sodium ions are detrimental in the presence of some organic add-on agents; this cannot exist then in the bulk of cases since many organic compounds are added in the form of their sodium salts.

Table 3.one. Concentration ranges of ingredients of Watts bath

Chemical	Concentration range, one thousand/l
Nickel sulphate, NiSOfour·6H2O*	150 to 400
Nickel chloride, NiCl2·6H2O †	xx to eighty
or
Sodium chloride, NaCl	10 to 40
Boric acrid, HthreeBO3	15 to l

*

Commercially available nickel sulphate has a composition between NiSO₄·6H_twoO and NiSO₄·7H₂O but BS564 ^one   states that the textile shall not contain less than 20.9% of nickel + cobalt, the cobalt being not more than than 0.5% of the cloth.

†

BS564 ⁱ   states that nickel chloride to exist used for electroplating shall contain not less than 24.5% of nickel + cobalt, with the aforementioned proviso regarding the cobalt.

Nickel sulphate ^one   is the principal ingredient; it is used as the main source of nickel ions because it is readily soluble (570 g/l at l°C), relatively cheap, commercially bachelor and is a source of uncomplexed nickel ions. Yet, it is known that a certain amount of ion association occurs in concentrated solutions due to ions of opposite accuse existence held together by coulombic forces. This reduces the constructive concentration of costless ions and the action coefficient is a measure of the extent to which association takes place. In nickel plating solutions the action of nickel ions is governed by the concentration of nickel salts in solution, their degree of dissociation and the nature and concentration of other components of the solution. If the concentration of Ni^{2   +}  available for deposition is low, burnt deposits volition exist produced at a relatively low current density, and in add-on the limiting current density will be depression. For these reasons the concentration of nickel sulphate must be high.

The presence of chloride has two main effects: it assists anode corrosion and increases the improvidence coefficient of nickel ions thus permitting a higher limiting current density. Saubestre ²   quotes values for the improvidence coefficients of nickel ions in sulphate and chloride solutions at specified conditions and shows that the limiting current density at a cathode in a chloride bathroom is approximately twice that in a sulphate bath, other factors beingness equal. Earlier, Wesley et al ³   had calculated the limiting current densities in i M NiSO₄  solution and i M NiCl_two  solution and obtained similar results.

Boric acid is used every bit a buffering agent in Watts nickel solution to maintain the pH of the cathode at a predetermined value. Boric acid solutions of the strength used in Watts nickel solutions accept a pH of most 4.0 due to the nickel ions ⁴ . From this, it would appear that boric acid should be most suitable equally a buffer at virtually pH 4, which is rather convenient, since nigh nickel solutions are operated most this value. Even so, information technology is satisfactory over the range of pH 3 to 5, probably due to the formation of complexes of boric acid and nickel. The buffer action of boric acid is peculiarly important in solutions of low pH (high activity of hydrogen ions), since hydrogen discharge occurs and consequently the pH increases in the cathode film with the possibility of co-degradation of nickel hydroxide. Hoare ⁵   has suggested that boric acid can act as a catalyst during loftier speed deposition. Other buffers such as acetate ^{iv , half dozen}   and formate can exist used successfully, particularly at the lower pH values.

Cathode efficiency of nickel deposition

The standard electrode potentials ${E^0}_{{\mathrm{Ni}}^2+/\mathrm{Ni}}=-0.25\mathrm{V}\mathrm{and}{E^0}_{{\mathrm{H}+/\mathrm{i}/2\mathrm{H}}_{\mathrm{two}}}=0.00\mathrm{V}$ betoken that thermodynamically hydrogen discharge should take place in preference to nickel ion belch when the ions are present at unit activity. However, the cathode efficiency for nickel deposition from a Watts bath is ≈   95%, and this is due to the much higher activity of the nickel ion (≈   1 grand ion/l) compared with that of the hydrogen ion (10^{−   iii}  – x^{−   6}  g ion/l corresponding to pH iii–six), which affects the reversible potentials and the rate of diffusion of the two species into the cathode layer. In addition, account must be taken of the respective overpotentials.

Saubestre estimated values for deposition potentials that are in good agreement with actual values past taking into business relationship activities of the discharging species and their overpotentials. On the basis of these factors it tin can be shown that cathode efficiency increases with increase in activeness of nickel ions, pH, temperature and current density. Saubestre has also shown that a mixed sulphate/chloride solution gives a similar cathode efficiency to a sulphate solution. The deviation between the 95% cathode efficiency for nickel degradation and the anode dissolution efficiency will result in a steady increase in nickel ion concentration in those baths from which 'drag-out' and other liquid losses are depression.

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Bright nickel electroplating

J.K. Dennis , T.Due east. Such , in Nickel and Chromium Plating (Third Edition), 1993

Porosity

In present solar day nickel plating practice, porosity should non occur, except in coatings of less than approximately 20 μm. Diverse tests take been developed which are claimed to illustrate porosity in deposits, but in many instances these are aggressive and lead to the formation of pores at the more than agile regions in the blanket; discretion is therefore needed when interpreting such results. Two types of porosity occur, i.e. intrinsic and that due to bad housekeeping, the latter beingness a microscopic form of the pitting defects caused by the aforementioned effects described for 'Roughness and pitting' earlier. Porosity in some thin coatings is inevitable; chromium deposits are an example of this. Intrinsic pores can exist troublesome in thin nickel deposits used for sure purposes such as an undercoat for gold.

Interest in intrinsic porosity has lessened since it has been institute that the corrosion of nickel plated metals occurs primarily at points where external corrosive attack has penetrated the blanket rather than at previously existing pores. Even so, in the past, much effort was devoted to attempts to plant reasons for this intrinsic porosity in nickel electroplate, Watts nickel baths usually being employed for ease of reproducibility. The American Electroplaters' Society was especially active in this respect and Ogburn and Benderly produced a comprehensive report ^thirteen . An before A.E.S. Research Written report by Thon and his coworkers ¹⁴   included results on the varying intrinsic porosity of nickel on copper pretreated in different ways, the porosity being measured by the gas permeability method devised by that team. Brook ¹⁵   later on carried out similar work using an autoradiographic technique to reveal pores. Information technology is possible that their results were influenced past the then-called zoning effect found at different microscopic distances from the surfaces of metals, depending on their type and previous metallurgical history, as kickoff reported for steels by Clarke and Britton ¹⁶   and subsequently for copper and brass past Clarke and Leeds ¹⁷ . More recently, Fan et al ¹⁸   accept investigated the human relationship betwixt plating overpotential and porosity of thin nickel plate.

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Control and purification of nickel electroplating solutions

J.K. Dennis , T.East. Such , in Nickel and Chromium Plating (3rd Edition), 1993

Analysis for inorganic impurities

The estimation of inorganic compounds in nickel plating baths often presents difficulties ⁹   and ofttimes their presence is diagnosed by the symptoms they cause rather than by straight identification. Fortunately, the coating methods favoured for removal of metal impurities are sufficiently embracing to remove almost offending metals, whatever they may be. Notwithstanding, recognition of the specific deleterious chemical element or elements might enable more positive steps to exist taken to prevent a recurrence of the contamination. While the use of a Hull prison cell will, with feel, enable the presence of some metals to be positively identified ¹⁷ , this is not e'er and so, and the presence of more than one metal impurity complicates their detection. Nonetheless more than difficulties are experienced in relating the advent of panels plated in the standard Hull cell to those that are obtained from nickel plating barrels. For this reason and because of the inherent limitations of the standard Hull prison cell, other types of plating test jail cell have been devised ^xviii   in attempts to simulate more closely the mass-transfer conditions occurring most to the cathode(s) in plating tanks or barrels. These cells oft incorporate rotating cathodes and work on these was about recently described by Lu ¹⁹ .

In the case of brilliant nickel solutions which are devised so as to mitigate the effect of unwanted metals on their appearance, there may be a deterioration in some other properties before the presence of the impurity produces a visible effect, fifty-fifty at low current densities. For case, the Rousselots ²⁰   accept demonstrated that the presence of contaminating metals may affect the chromability of the nickel plate. Too, the codeposition of certain metals, in particular copper, may adversely affect the corrosion resistance of nickel coatings. Thus, it would exist an advantage to be able to determine rapidly the concentration of metals in a nickel plating solution, to be able to assess the benefit of any purification treatment performed.

Many investigations have been conducted with this aim in heed. Xxx years ago, most of them were based on colorimetric techniques. With these methods some prior moisture separation is usually required and considering this tin can be tedious and crave the use of very pure reagents, analysis for metallic impurities has neither been a rapid nor rewarding practise. Serfass and his colleagues ²¹   published the nearly comprehensive text on this topic. While the development of continuous liquid/liquid extraction apparatus showed promise for automating the colorimetric method, some disadvantages of this method withal remained and hindered its widespread awarding. Nevertheless, the colorimetric method is sometimes useful for metals whose salts absorb lite of a very different wavelength to nickel salts, e.chiliad. cobalt ²² .

An improvement to the classical dc polarographic technique is differential pulsed polarography ²³ , which is both more selective and sensitive. This method appears suitable for determining the concentrations of many metals present in nickel plating solutions, whether in major or trace amounts, and likewise many of the organic compounds used as brighteners and levellers. Ion chromatography is some other useful belittling technique ²⁴   that detects, identifies and quantifies many mutual cations and anions nowadays in nickel baths, although it appears to take been most used in analyzing chromium baths ²⁵ . Ion-selective electrodes ^{26 , 27}   can exist used to determine certain elements such equally chloride.

The invention of the Diminutive Absorption Spectrometer (AAS) radically changed the analytical position. This instrument is then simple to use and so specific that nigh any metallic element can exist adamant in nickel plating solutions, providing suitable lamps are available. Manifestly, only lamps for the commonly encountered elements are essential for routine control analysis.

When using the get-go generation of AAS instruments, the sample of nickel plating solution was diluted only v or ten times with distilled h2o and then that impurities could be detected but for the nowadays generation, which has much greater sensitivity, twenty or 100 times dilution is more usual. Precise limits of detection for metals vary according to the particular instrument used, the more sophisticated and costly models (such equally those with double-beams and furnaces instead of burners) beingness the most sensitive and authentic.

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Contempo Developments in Loftier-Speed Surface Modification in Metallic Finishing Manufacture—Electrodepositing Nanocrystalline Nickel Direct on Aluminum Without Whatsoever Pre-treatment - A Review

Mohammad Due south. Hussain , in Nanomaterials in Chromatography, 2018

six Detailed description of the invention: depositing nanocrystalline nickel on aluminum alloy surfaces without any pretreatment

The present invention discloses a method of depositing nickel onto an object having an external aluminum oxide layer comprising the step of performing electroplating through an electrolyte solution continuously flowing at a charge per unit of 1.5–3.0 ms⁻ⁱ, preferably two.7 ms⁻ⁱ in between surfaces of an anode and a cathode, wherein the cathode is the object. The cathode is preferably aluminum. The anode, on the other hand, can as well be nickel or an inert metal. The rate at which the electrolyte solution passes over the surface of the cathode has an outcome on the maximum current density at which satisfactory electrodeposits are obtained. Furthermore, the charge per unit of electrodeposition that is governed by Faraday's police depends on the current density. The current density applied in this electroplating process is from 0.1 to 1.0 Acm⁻², preferably between 0.1 and 0.6 Acm⁻². The rate of degradation can be increased past increasing the cathode electric current density as this would increase the transfer rate of nickel atoms during the plating process. Hence, more nickel atoms are deposited onto the aluminum surface, which is the aluminum oxide layer [2–4,7,8].

The electrolyte solution plays the role of an aqueous electrolyte bath in the electroplating process. This solution can exist any nickel solution, for case, Watts-blazon nickel-plating solution, nickel sulfamate, nickel fluoborate, all sulfate solution, or all chloride solution. The temperature of the nickel electrolyte correlates with the rate of plating every bit the rate of plating increases with the solution temperature until it reaches a certain temperature. The preferred temperature is in the range of 50–65°C, most preferably at 55°C. The equipment of the preferred apotheosis of this electroplating procedure is illustrated in Fig. 16.1. The electrolyte solution from the bath is continued to a pump, filter, flow meter, and an enclosed container that contains the anode and cathode. The electrolyte is pumped in this hydraulic circuit in the direction equally shown in Fig. 16.1 until the desired thickness of the nickel blanket has been attained. The electrolyte solution is in a closed loop circuit and hence it flows continuously. The purpose of the filter is to keep the electrolyte clean as it flows into the container. Information technology removes whatsoever suspended articles from being codeposited with nickel deposits. Nonetheless, as would be done with any plating solution, the chemistry of the nickel electrolyte needs to exist monitored regularly and maintained at a desired level at all times and then that the solution can be reused.

Figure 16.ane. Schematic of the new high-speed procedure for directly plating of nickel on aluminum [iv].

The flow meter is used to measure the menses rate. The pump controls the flow charge per unit past pumping the electrolyte solution at a desired flow rate. Besides the pump, other devices can also be applied to control the menses rate. The solution flows into the container in which the anode and cathode are housed and then flows back into the bath. When electric current is applied to the solution that flows at a rate of i.5–3.0 ms⁻¹, preferably two.7 ms^−ane, nickel-plates onto the surface of the aluminum oxide layer directly. A power generator (DC power supply—rectifier) connects to the container to provide electric current for the electroplating process to occur. A normal DC ability supply is used to provide electric current for the electrolysis to take place. Once electrical current is applied, the current passes through the anode and cathode. A disquisitional aspect of this procedure is the proximity of the conforming electrodes. The preferred two mm spacing provides a high potential gradient. Since the rectifier provides the current required for the electroplating, at about x V, the potential gradient is typically 5 kV k⁻¹. A preliminary report of the process variables of direct plating of nickel on aluminum has established that the electrodeposition is increased by current density (up to 0.6 Acm⁻²), electrolyte temperature (upwards to 65°C), and electrolyte catamenia rate (upwardly to five ms⁻¹).

Here, the process of directly plating of nickel on aluminum, using high-speed (turbulent) electrolyte flow between conforming electrodes in proximity is discussed. Nosotros find that the diffusion-limited two-nm thickness of aluminum oxide at ambient conditions has been increased to virtually 45 nm (Fig. 16.2). High-resolution transmission electron microscopy (Hour–TEM) images clearly show that the nickel is deposited, with good adhesion, on this thickened aluminum oxide layer. We propose that it is in the start-upwards phase, in the few minutes prior to plating, that the conditions are created for effective nickel-plating. The elevated temperature increases thermal diffusion of Al³⁺ and O²⁻ ions. Turbulent period places fresh electrolyte at the surface, increasing the chemical potential that becomes a further driver for the ionic diffusion. The period of the acidic electrolyte also interacts with the thickening layer to form an irregular surface morphology. We suggest that this irregular and active oxide surface is conducive to nickel nucleation, since we observe that the nickel, as deposited, is nanocrystalline in structure. Turbulent catamenia is an important aspect of this new industrial procedure for plating nickel on aluminum. We believe that the turbulent menstruation of the acidic electrolyte contributes to the oxide thickening and to the training of the amphoteric oxide layer for the nickel electrodeposition.

Figure 16.2. Hr-TEM image showing the existence of an oxide layer (40–47 nm thick) between the aluminum substrate and the electrodeposited Ni.

An aluminum surface absorbs oxygen within milliseconds of exposure to air. It and then forms an Al₂O₃ oxide layer that continues to abound because in that location is a potential between the metal substrate and the captivated oxygen that enables aluminum ions to motility through the film. Information technology is known experimentally that humidity is one parameter that can profoundly influence the oxidation rate [30]. It is therefore expected that the oxide growth rates will generally be increased past crystalline imperfections. This provides a surface on which nickel is readily nucleated under the applied electric field. Further nucleation and growth is the machinery by which an adherent nickel layer is formed on this aluminum oxide. The limiting thickness of this stable film at ambient conditions is typically ii nm for aluminum. This limit is reached when aluminum ions can no longer cantankerous the movie past diffusion, as Mott says, without the aid of an electrical field [31]. If the environment of the surface is changed, this motion-picture show can grow further. Factors that can cause this film to thicken include increasing the temperature, applying an electric potential, the presence of water and oxygen, and modifying the pH (acidic or bones, since aluminum oxide is amphoteric). Aluminum is the but metallic that has been commercially prepared for reception of adherent electrodeposits by anodizing [32]. Although anodizing has been a pretreatment for plating on aluminum for more 70 years, the process is not equally common every bit the zincate and stannate processes [19,33]. Anodizing is a process used to thicken the oxide layer on aluminum, used industrially since 1923. In anodizing, there is an electrical field, acidity, oxygen, and h2o. With these variables, anodized coatings of aluminum oxide, ranging from a few microns to more than than 100 μm, tin can be created. The porosity of these anodized coatings is due to the equilibrium between the acid dissolution of the oxide and the electrolytic growth of the oxide layer.

Cabrera and Mott [34] and Fehlner and Mott [35] have described a loftier field mechanism for oxide film formation and growth. This theory starts with adsorption of oxygen on bare titanium surfaces to create a monolayer of oxide. Subsequent electron tunneling from the titanium, through the oxide, to adsorbed oxygen creates oxygen ions, which human action as electron traps at the surface. As these traps accumulate, the potential drop across the flick is increased. This, in plough, establishes an electric field across the flick, which acts to lower the activation energy for ion transport through the film.

For further oxide to be formed, the titanium and the oxygen need to be brought into contact. This implies that either titanium ion or oxygen ion (or both) must be mobile inside the film. The oxide moving-picture show formed on titanium is classified every bit an N-type semiconductor, which means that anion mobility is the dominant machinery of ion transport [35]. Therefore, the oxygen ions movement through the oxide film to the titanium surface to course new oxide. This machinery of formation is referred to as growth at constant field. Since the mobile ions and the field-creating ions are the same, the charge per unit of anion transport must exist balanced by the charge per unit at which new field-creating anions are formed at the surface. In this model, it is further causeless that as the oxide motion-picture show thickens, the activation energy for ion transport increases and eventually limits further oxide formation. If the potential drop beyond the moving picture is increased, the electric field is increased and this provides a ways to continue oxide growth. This word described the atomistic mechanism-giving rise to oxide motion-picture show formation.

In this new process of direct nickel-plating of aluminum, even before we apply the electric field, there is a alter in the chemic environment of the aluminum—in that location is an increased temperature (preheated—hot electrolyte is circulated), acidity, oxygen, and water. The electrolyte is circulated until information technology, and the apparatus, attain the preset temperature, for instance, 65°C. During this initial, offset-up phase, having a duration of just a few minutes, we accept two dimensions of oxide development (Fig. 16.3):

Figure sixteen.3. Elemental map of morphology showing the clear aluminum, oxide, and nickel layers.

1.

There is an environment conducive to the thickening of the oxide layer ("vertical" growth), from the diffusion of aluminum ions through the oxide motion-picture show, and oxygen availability from the surface.

ii.

The high speed of the acidic electrolyte, flowing turbulently over the top surface of the oxide, creates an irregular or porous morphology, similar to that observed in anodizing ("horizontal" morphology evolution).

The change in the surface layer thickness and morphology is illustrated schematically in Fig. 16.4. We believe that the existence and importance of this start-up (preelectric field) phase, in the procedure of the plating of aluminum, has not been previously identified and reported. When the electrical field is practical for nickel-plating, the surface of this thicker oxide layer is already in a very active state considering of the ionic move.

Effigy sixteen.4. A schematic diagram showing the thickening of the existing oxide layer between the electrodeposited Ni and the aluminum substrate.

Thompson and Wood [36] have shown that the surface morphology develops during this conventional anodizing process. Every bit shown in Fig. 16.5, they have schematically shown that the moving picture surface morphology develops during this conventional anodizing process. We accept observed a morphology in our process that seems to correspond with the growth in the film thickness and the evolution of the surface morphology that they observed in the initial and midstages of anodizing, schematically shown in Fig. 16.6.

Effigy 16.5. Schematic sections of anodic films showing the evolution of the flick surface morphology during the anodizing process.

Information taken from After G.East. Thompson, G.C. Wood. Corrosion: aqueous processes and passive films, in: Treatise on Materials Science and Technology. Academic Press, London, 1983.

Figure sixteen.six. Schematic sections showing the proposed evolution of the film surface morphology during the early stages of electrolyte period process without any current flow.

Imaging (Fig. 16.7) of the surface of the nickel that was deposited past high-speed plating, using a loftier-resolution scanning electron microscope, shows the particulate morphology, ranging from just-nucleated particles (<50 nm) through to fully grown grains that are micron-sized. The nickel forms a continuous dense structure without whatsoever surface cracks in spite of the use of a high electric current density during electrodeposition. Figure 16.8 shows cross-sections of nickel-plated on aluminum at different temperatures and current densities. The thickness of the blanket increases by increasing the current density.

Figure xvi.7. Loftier resolution-SEM image showing equally-deposited nanocrystalline nickel morphology on aluminum.

Figure 16.8. Illustrative cantankerous-sections showing nickel-plated on aluminum surfaces at different temperatures and current densities.

(A) Plated at 55°C and 0.iii Acm⁻²; mag × 2000; coating thickness, 12 μm. (B) Plated at 60°C and 0.3 Acm⁻² electric current density; mag × 2000; coating thickness, 15 μm. (C) Plated at 65°C and 0.three Acm⁻² electric current density; mag × 2000; coating thickness, 22 μm. (D) Plated at 60°C and 0.6 Acm⁻ⁱⁱ current density; mag × 1500; coating thickness, 36 μm.

Lani [37] and Masrur [38], working with the author and using exactly the aforementioned equipment adult by the author, concluded that it was possible to plate nickel directly on aluminum without any pretreatment. They used Watts-type, all sulfate, and nickel sulfamate solutions, and varied solution speed, current densities, and plating solution temperature. Increasing current density and temperature of the solution both finer increased the weight and thickness of the electrodeposited nickel coatings for both nickel solutions. Table 16.2 shows for all three types of nickel solutions used that the nickel sulfamate solution gave the highest rate of plating when the solution was rotated at a speed of 500 rpm, at a temperature of 75°C, and electric current density of 0.1 Acm⁻². Experimental results indicated that a good level of adhesion has been obtained between the coating and the substrate material. Electron microscopic characterization (Figs. 16.7 and 16.eight) of the electrodeposits shows no gap, porosity, or whatever other sign of failures at the interface. The charge per unit of electrodeposition increased as the cathode electric current density, temperature, and the electrolyte movements were increased and the relationship between those parameters are almost linear in the experimental range that was used in this piece of work. As deposited, nickel has been found to exist nanocrystalline.

Table sixteen.2. Thickness of Nickel Coating Showing the Consequence of Different Types of Solutions Used [37]

Charge per unit of solution movement (rpm)		500
Plating time (min)		3
Current density (Acm−ii)		0.1
Temperature (°C)		75
Types of solution	Watts	All sulfate	Nickel sulfamate
Boilerplate coating thickness (μm)	7.iii	nineteen.six	22.9

Size of component plated was 1 cm in diameter.

All the same, the sulfate-based nickel solution gave a college rate of deposition compared to that of the Watts solution as the plating temperature and electric current densities were increased. Mansoor and Lani had carried out adhesion tests using 2 types of tests and had ended that the level of adhesion between the substrate and the plated nickel was very good.

Imaging (Fig. 16.7) of the surface of the nickel that was deposited by high-speed plating, using a loftier-resolution scanning electron microscope, shows the particulate morphology, ranging from just-nucleated particles (<l nm) through to fully grown grains that are micron-sized. The nickel forms a continuous dense structure, without any surface cracks, in spite of the use of a high electric current density during electrodeposition. A cross-section of a nickel-plated aluminum sample shows, for all current densities used, that there is no debonding or cracking between the substrate and the plated nickel (Fig. 16.8). These micrographs support the adhesion tests by pocketknife (ASTM D6677-07) and tape (D3359-09) on these samples, which confirmed a expert level of adhesion of the nickel to the aluminum substrate. This good adhesion has been farther substantiated.

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Electroless Plating Fe-Based Alloys

Bangwei Zhang , in Amorphous and Nano Alloys Electroless Depositions, 2016

v.7 Summary

After describing, analyzing, and discussing the EP Iron–B-based and Fe–P-based deposits more in particular, the following remarks can be made.

Well-nigh twoscore years later the discovery of chemic nickel plating past Brenner and Riddel, the EP Atomic number 26–B-based alloy deposits appeared. The reasons and the process for studying the EP Atomic number 26–B-based alloys have been described in this chapter.

Collectively, many papers on EP Fe–B-based alloys have been published, including several ternary and quaternary Atomic number 26–B-based alloys. Not just the manufacturing technique (bathroom composition and operating conditions), simply also the fabricating principle, structure, composition, formation mechanism, and of import properties of the Fe–B-based alloy films have all been studied systematically. However, papers investigating EP Fe–P deposits are rather fewer to date, for instance, only Fe–Westward–P in the ternary but no quaternary Fe–P-based alloys accept been reported. However, information technology can be said that currently we already have a considerable understanding of EP Fe-based alloy films.

Knowledge of the nature and characteristics of both EP Atomic number 26–B and Fe–P-based alloys are needed to carry out in-depth and all-encompassing research. This is peculiarly then for enquiry on EP nano Atomic number 26-based alloy films, which we accept not nonetheless begun to study. Such a field will undoubtedly yield relevant ideas and volition be well worth studying.

Unfortunately, there are few applications of EP Fe-based blend films, particularly in industry. Of class, looking for such applications is non only necessary but also very meaningful. Studies in such field are definitely welcomed.

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Films and Coatings: Engineering science and Recent Development

A. Yli-Pentti , in Comprehensive Materials Processing, 2014

4.eleven.half-dozen.1.ane Bath Types

Nickel metal works as a catalyst for electroless nickel plating. If noncatalytic surfaces are to exist plated, they must beginning be treated with a goad. Polymers ordinarily also require etching in an acidic solution earlier catalyzing for providing anchor spots for goad metal and wetting ability for aqueous catalyst solutions, encounter Figures 14 and 15.

Effigy 14. A microscopic picture of an etched ABS plastic surface.

Reprinted from McCaskie, J. East. Plating on Plastics. Met. Finish. May 2006, 31–39, with permission from Elsevier.

Figure 15. A schematic figure of an etched ABS plastic surface.

Reprinted from McCaskie, J. E. Plating on Plastics. Met. Cease. May 2006, 31–39, with permission from Elsevier.

After etching, the surface is sensitized, which ways absorbing ions that can be oxidized past the actual catalyst ions. For case, acidic Sn(2) or Sn(iv) solutions or more than seldom silverish nitrate or gold(three) chloride is used to sensitize the surface that is absorbing the metal ions. Thereafter a catalyst treatment with, for example, palladium chloride solution is required. Palladium would settle as a metallic class with sensitized surface by an exchange reaction with the ion used in sensitizing. If the metal that is to be plated likewise reacts by an exchange reaction (i.e., copper), no split catalyst treatment is required. Another popular way is to use the colloidal technique to catalyze and activate surfaces (Figure 16). Typical polymers that are metal plated with an electroless method are ABS, ABS/PC, Nylon, and POM (59–61).

Effigy xvi. Plating of a plumbing fixture function; treatments cumulating from left to right are nontreated plastic, etched and activated, electroless nickel   +   electrolytic copper, bright nickel, and brilliant chromium electroplating.

There are a number of formulations for the electroless nickel bathroom. Nickel metal content is achieved past adding nickel sulfate NiSO_four·6H₂O or chloride NiCl_ii·6H_twoO into the bath, which is like for almost all formulations. The amount of the salts typically varies between 20 and 45   g   l⁻ⁱ, and a typical nickel metal content in a bathroom is 6   g   l⁻¹. Nickel metal is complexed with a combination of lactate, citrate, glycolic, or malic acids. The amount of complexing agents may be 30–50   thousand   fifty^−ane (62).

There are several possible reducing agents. The most common one is sodium hypophosphite NaH₂PO_three·H₂O, only also sodium borohydride NaBH₄ or dimethylamine borane (CH_iii)₂NHBH_iii could exist used. The reducing agent affects the properties of the coating, since part of information technology volition deposit into the coating.

The summary reaction from hypophosphite bath can exist written every bit

$\mathrm{two}{\text{H}}_{\mathrm{ii}}{{\text{PO}}_2}^{-}+2{\text{H}}_2\text{O}+{\text{Ni}}^{\mathrm{ii}+}\to \text{Ni}+{\text{H}}_2+4{\text{H}}^{+}+2{{\text{HPO}}_{\mathrm{iii}}}^{2-}$

Hydrogen evolution is occurring as a side reaction. From hypophosphite baths, phosphorus is codepositing along with nickel. In heat treatment it will form the NiP compound. The amount of phosphorus depends on the bath formulation and pH. Ordinarily, the bath types are classified equally high-phosphorus (10–14% P), eye-phosphorus (4–9% P), and depression-phosphorus (below three% P). The amount of hypophosphite may vary widely, 10–fifty   g   l⁻¹, depending on formulation (63). The pH has an consequence on phosphorous content, acidic, that is, low-pH, baths yield high-phosphorous coatings, and at the alkaline side the phosphorous content is low, even beneath i%. The acidic baths are more common, and they are typically operated at a pH range of iv–6, while alkaline baths are operated at nine–11. The temperature is at the range of lx–xc   °C. Acidic baths are commonly operated at slightly higher temperatures of 85–95   °C than alkali metal baths.

Sodium borohydride and dimethylamine borane baths yield boron into the coating. Usually, there are two types of baths, which produce 0.2–2   westward-% or three–viii   w-% boron into the coating. For borohydride baths, the required pH is 12–14 to avert nickel boride precipitation (63). Information technology limits the use of the bathroom to the materials withstanding loftier alkalinity. Thallium is used as a stabilizer in some formulations, simply information technology may also codeposit in substantial amounts, which is a drawback from an environmental standpoint. Bath temperature is 90–95   °C. The coating contains about 3–8% boron. The blanket is harder than nickel–phosphorous coatings, just corrosion resistance is not as skillful. Information technology does not, withal, deteriorate afterward heat handling. The dimethylamine or diethylamine borane reducing agents can be done in both acid and alkaline metal solutions with a varying pH in the range five–11. Boron content varies between 0.1 and 5   due west-%, and the electric conductivity of the blanket is much better than that of nickel–phosphorous coatings. When hydrazine is used, the coating contains Ni–Due north deposits. The amount of nitrogen, however, is some tenth of a percent. The commercial use of hydrazine baths is negligible, and there are potential work safety problems with hydrazine since it is carcinogenic.

Metal circuitous builders or chelating agents are used to go along the nickel metal in solution and to help maintain the stable operation atmospheric condition. They control the precipitation speed and assistance to continue pH in the specified range. The mutual complexing agents used in electroless nickel baths are citrates, acetates, ammonia, and pyrophosphate.

Stabilizers are used to forestall the bath from 'growing wild' that is, from experiencing a sudden depletion of the bath when the metal grows on fine particles inside the solution. The stabilizers may exist grouping IV compounds like sulfur and selenium or unsaturated organic acids like maleic acrid. Stabilizers may also exist heavy metal cations similar Sn²⁺. Previously, lead and cadmium were too used, merely their amount has been lowered from some hundreds of ppm to below 1–iii   ppm to make the baths compatible with the RoHS directive. Cadmium has also been used as a brightener. The fourth grouping of stabilizers consists of oxygen-containing compounds similar MoO_four ⁱⁱ⁻ which passivate the surface of metal particles in baths.

The pH has a smashing event on the deposition charge per unit and the phosphorous content of the blanket; therefore it is commonly kept equally stable as possible. The reaction scheme of the deposition is complex equally discussed above, merely there will form 4   mol of hydrogen ions for 1   mol of deposited nickel, so buffering agents are essential to go on the deposition rate and coating composition inside specification. The nearly common compounds used are acetic, propionic, and succinic acids. Boric acid and amines are formed in reactions of borane baths, and they piece of work as buffering salts too, which leads to a long operating life of that bath type.

The deposition rate depends on several factors: bath temperature, complexing agents and stabilizers, pH, the concentration of the reducing agent, and nickel concentration. A usual production charge per unit is 15–20   μm   h⁻¹. The more than the bath is used, the more reaction products volition accumulate into the solution, which will slow down the growth charge per unit. The amount of reaction products is virtually v   g sodium orthophosphate per ane   g of coating. Bathroom life is usually 6 to 8 metal turnover (MTO). Later on that, too the slower deposition charge per unit, tensile stresses will grade into the coating, which will cause deterioration of the corrosion-resistant properties.

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