by Oleg GAVRILIN, Cand. Sc. (Chem.), Russian Chemical-Technological University named after D. Mendeleev
Parts of machinery and mechanisms, other items made of steel and copper alloys, have as a rule to be protected from corrosion, electromagnetic and other damaging impacts. With this aim in mind, and also in order to increase wear resistance of rubbing surfaces and decorating elements, they have to be coated or plated with a fine layer of nickel.
The history of this technology covers quite a number of decades. Back in the first half of the last century-a period of rapid accumulation of chemical knowledge - there already existed techniques of galvanic plating, or coating, of metals. Specialists were also able to produce mirrors made of gold, silver and copper. This was done by placing into water solutions of metal salts glass articles upon which films of these metals precipitated. But using this technique for platings with nickel or cobalt failed to achieve the expected results.
It was only in the middle of the 19th century that specialists succeeded in obtaining nickel metal powder from its salt solutions in the presence of a reducing agent-hypophosphite of sodium. For chemical experts this was a sensation, but the effect was confirmed by numerous experiments. As for the practical applications of the discovery, they were not yet "on the agenda" and the find was shelved for the next half a century. But research in this general field went on, and in 1904 American chemical expert Prof. Roux was able to observe nickel coating forming on the surface of a steel plate placed into the aforesaid solution. But even then the novelty failed to attract due attention of industrial experts.
And it was only 40 years later that the effect was "rediscovered" and that was quite by chance. Later on experts developed appropriate technologies which were immediately claimed by industrial users. As for researchers, they focused on the structure of the new coating, and its performance in different conditions. And they established that the coating consisted not of pure nickel, as was thought before, but of its "melt" with phosphorus (at a level of some 10 percent, which is quite a lot). But the atomic mass of the latter component was twice lower than of the first - the ratio was five-six atoms of metal to one phosphorus atom. This being so, the material in question is so much different from pure nickel by its physico-mechanical and chemical properties. For example, its structure is no longer crystalline, but amorphous and friction factor is twice lower than that of nickel. The "melt", or alloy, is chemically inert which accounts for its high resistance to corrosion. And the most interesting fact is that it possesses increased hardness, which reaches some 1,000 kg/mm2 when heated to 400°C.
This can only be "matched" by tempered steel or galvanic chromium plating. In the past that belonged to the one and only method of increasing the wear resistance of rubbing parts. But it has an appreciable drawback which, incidentally is shared by all technologies using direct current: the metal film is distributed unevenly on surfaces with complicated "relief.
As for nickel alloy produced by chemical methods (without using electric current), its thickness is the same all across the treated area. What is more, it screens HF electromagnetic emissions, has low transitional electric resistance and is easily soldered. Platings of this kind find a range of applications in the automobile, aerospace, petrochemical, electronic, food and textile industries and is quite indispensable in mining, railway transport, naval transport, medicine, pharmaceutics, etc.
But even that technology was not flawless. If the temperature of the process happened to be just a few degrees higher than the prescribed, the solution of salts (incidentally, very costly) disintegrated and had to be dumped out and the waste water had to be "cleaned". Another negative factor was the inadequate knowledge of the mechanisms of the reactions involved which necessitated frequent analyses and took more time and more money.
Bearing all this in mind, Russian specialists decided to do away with the aforesaid flaws by means of preserving the stability of the electrolyte solution in water even in overheating (up to boiling temperature), and increase the rate of precipitation to 20 mcm/h and more. And it has to be constant in order to control the plating thickness without using measuring instruments. And it was also necessary to conduct precision measurements of the changing composition of the solution during the time of its use. This would make it possible to "correct" the quantity of components of chemical transformations without additional analyses.
And in pursuing these objectives we have run into many problems. We established that the aforesaid decomposition of the liquid phase was caused by the constant presence therein of solid inert particles which could not be "filtered out" because of their small size (tens and hundreds of micrometer). This being so, it was necessary to select a catalytic substance which would "poison" such impurities. And in order to protect the surface of treated parts it was necessary to ensure a rather low concentrations of the "poison" (in case of chemical nickel plating-of the stabilizer). Its other function was to ensure good morphology of the coatings which had to be smooth and shiny. And putting such "additives" into the electrolyte one should guard against deteriorating mechanical and chemical properties of the platings.
Another important question is how the rate of the process under discussion depends on the temperature, acidity, nickel salt concentrations and other components of the solution. In practice one has to be dealing with solutions of different compositions. As physical chemists put it - it was necessary to determine the effective energy of activation and reactions succession. With this abundance of factors, which influence the kinetics of the process and the properties of the platings, the number of the required experiments would exceed all reasonable limits and an analysis of the vast volume of related data would be really hard to fulfil. This being so, it has been necessary to turn to the methods of what we call mathematical planning of experiments, dispersion analysis and partial derivatives equations. Still and all, it took us several years to collect the necessary data and perform its mathematical processing. At long last all our problems were solved, though originally only on paper. This was followed by "production testing". The result was an improved technology of chemical nickel plating.
And we wanted even more and decided to broaden the spectrum of properties of platings by alloying the nickel-phosphorus matrix with other metals with different concentrations of the new components. And we were able to cope also with this problem so that our industrial users now have at their disposal several methods of chemical applications of alloys of nickel-phosphorus, nickel-cobalt-phosphorus, nickel-tungsten-phosphorus and nickel-cobalt-tungsten-phosphorus. All of these have proper mathematical descriptions. That means that putting into your computer data on the desirable composition of plating and the rate of precipitation, one can obtain all parameters of the process: components of the solution and their concentrations, the regime of reaction and the range of its rates.
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