Cobalt electroless

The Fe, Co, and Ni deposits are extremely fine grained at high current density and pH. Electroless nickel, cobalt, and nickel—cobalt alloy plating from fluoroborate-containing baths yields a deposit of superior corrosion resistance, low stress, and excellent hardenabiUty (114). Lead is plated alone or ia combination with tin, iadium, and antimony (115). Sound iasulators are made as lead—plastic laminates by electrolyticaHy coating Pb from a fluoroborate bath to 0.5 mm on a copper-coated nylon or polypropylene film (116) (see Insulation, acoustic). Steel plates can be simultaneously electrocoated with lead and poly(tetrafluoroethylene) (117). Solder is plated ia solutioas containing Pb(Bp4)2 and Sn(Bp4)2 thus the lustrous solder-plated object is coated with a Pb—Sn alloy (118). 

Electroless nickel deposition may then be carried out directly onto steel, aluminium, nickel or cobalt surfaces. Surfaces of copper, brass, bronze, chromium or titanium are not catalytic for deposition of nickel-phosphorus and the reaction must be initiated by one of the following operations ... 

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

Mechanism 3 involves NiOH in at least three reactions, and Ni(OH)2 as the active Ni reactant in solution. Since increasing the concentration of the complex-ant(s) in solution will reduce the concentration of both unhydrolyzed and hydrolyzed metal ions, arguments of complexation cannot be readily employed to either support or discount this mechanism. However, it has been this author s experience in formulating electroless Co-P solutions with various complexants for Co2+ that improper complexation which results in even a faint precipitate of hydrolyzed cobalt ions yields an inactive electroless Co-P solution. Furthermore, anodic oxidation of hypo-phosphite at Ni anodes does not proceed at a significant rate under conditions where the surface is most probably covered with a passive film of nickel oxide [48], e.g. NiO.H20, which would be expected to oxidize the reducing agent via a cyclic redox mechanism. 

Despite H/D kinetic isotope studies, application of modern techniques such as atomic force microscopy (AFM), electrochemical mass spectrometry (EMS) [60], and electrochemical quartz microbalance (EQCM), the mechanism of electroless nickel and cobalt, whatever reducing agent is involved, continues to be the subject of much discussion and varying opinions. 

Figure 8.14. Rate of electroless cobalt deposition as a function of pH at 30 and 100 g/L citrate one-factor experiments. (From Ref. 65, with permission from the Electrochemical Society.)...

It is interesting to note that Brenner and Riddell (2-4) accidentally encountered electroless deposition of nickel and cobalt during electrodeposition of nickel-tungsten and cobalt-tungsten alloys (in the presence of sodium hypophosphite) on steel tubes in order to produce material with better hardness than that of steel. They found deposition efficiency higher than 100%, which was explained by an electroless deposition contribution to the electrodeposition process. 

Although the concept of phase is well defined thermodynamically, here phase refers to a mechanically separable homogeneous part of an otherwise heterogeneous system. The concept of phase change refers here to a change in the number present or in the nature of a phase or phases as a result of an imposed condition such as temperature or pressure. To clarify and illustrate the topic at hand, we use the specific cases of electrolessly deposited nickel and electrodeposited cobalt. 

TABLE 16.1. Influence of Solution Hypophosphite Concentration on Preferred Orientation and Magnetic Coercivity of Electroless Cobalt-Phosphorus Eilms... 

Electrolessly plated cobalt-phosphorus thin films are usually characterized by low- to medium-coercivity values (300 to 900 Oe). For some apphcations it would be desirable to have films with coercivities greater than 900 Oe. Table 16.1 shows that films with minimum preferred orientation exhibit maximum coercivity values. The ratio of the intensity of the (002) and (101) diffraction rings changes with hypophosphite concentration of the plating solution, with zero preferred orientation obtained at solution hypophosphite concentrations of 5 to 6 wt %. 

WTI, with Ensco Environmental Services, has recently won a Farr Grant to install two ARO systems in the Bay Area for long term testing and evaluation. One is to be installed at Hewlett Packard (on acid copper and nickel sulfamate) and the other, at Domain Technologies (on electroless nickel and electroless cobalt). The project should begin in late Summer, 1989. 

It is interesting to conclude this section with an example that, in a sense, brings the chapter full circle. The metallization of plastic materials used as metal substitutes is a process with actual and future commercial potential. Usually, plastics are plated after appropriate sensitization by an electroless process which involves reduction of metal ions (e.g. Ni2+, Cu2+) by chemical rather than electrical means.19 There seems no reason why the reducing agent should not be incorporated in the polymer and Murray and his collaborators101 have demonstrated that copper, silver, cobalt and nickel may each be electrodeposited on to films of [poly-Ru(bipy)2(4-vinylpyridine)2]2+ coated on to platinum electrodes. The metal reductions are mediated by the Ru1 and Ru° states of the polymer. 

Electroless plating — An autocatalytic process of metal deposition on a substrate by reduction of metal ions from solution without using an external source of electrons. It is promoted by specific reductants, namely formaldehyde, sodium hypophosphide, sodium boro-hydride, dialkylamine borane, and hydrazine. Electroless deposition has been used to produce different metal (e.g., nickel, cobalt, copper, gold, platinum, palladium, silver) and alloy coatings. It can be applied to any type of substrate including non-conductors. Some substrates are intrinsic catalytic for the electroless deposition other can be catalyzed usually by sensibilization followed by Pd nucleation also, in some non-catalytic metallic substrates the electroless process can be induced by an initial application of an appropriate potential pulse. In practical terms, the evaluation of the catalytic activity of a substrate for the electroless deposition of a given metal is... 

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