A replacement of boric acid brings clear advantages for the surface coating
Lead, cadmium, PFOS, Cr (VI). We have long since come to terms with arsenic and hydrazine. The list could be continued as long as you like. But boric acid? It is hard to believe that boric acid is one of the substances that could be banned because of its hazard potential. Well, lead has been known to be toxic since ancient times, and we don’t really want fluorinated or poly-halogenated hydrocarbons as part of our biosphere. Boric acid, on the other hand, is ubiquitous as an additive in glasses, pesticides and paints, and even the application submitted by Germany and Slovenia to declare boric acid as a SVHC (substance of very high concern) toxic to reproduction describes that epidemiological studies in humans have been lacking or insufficient to rule out adverse effects on fertility. Basically it means that to date there is no study proving that boric acid is toxic for human reproduction.
Nevertheless, the recent past has shown that if the legislator threatens a ban, sooner or later it will come into force. In addition, there are also examples of boric acid leading to growth and fertility disorders in aquatic organisms in particular, if exposure is high enough. It is therefore justified to evaluate alternative systems.
Since boric acid is commonly referred to as a buffer substance in nickel baths, and these nickel baths are generally operated in a pH range of 4 to 4.5, it was obvious to replace boric acid with carboxylic acids. Their pKa values (a measure for the strength of the acid) are usually between 4 and 5 and since the buffering capacity of an acid-base pair is best in the range of its pKa value, carboxylic acids were the seemingly logical choice to keep the pH stable. In fact, such baths are characterized by an extremely advantageous pH stability, which is much more pronounced than when using boric acid. In addition, nickel layers from these baths are largely non-porous, which is an indication that no or practically no solid nickel hydroxide is formed during deposition. This means that one of the main requirements placed on a boric acid replacement is fulfilled. Unfortunately, however, these electrolytes have a very serious disadvantage: The brigthness and especially the leveling known from boric acid baths has largely disappeared, so that these baths are unfortunately unsuitable for decorative applications. Attempts to counteract this deficit with higher brightener concentrations were initially successful, but sooner or later failed in direct comparison with boric acid electrolytes.
In search of alternatives to boric acid
Classic nickel baths consist of a mixture of nickel sulfate and nickel chloride, boric acid, wetting agents, so-called basic brighteners, such as sodium saccharine, as well as leveling substances, which generally have an alkyne group as functionality. Now it is a simple rule of thumb for the coordination chemist that not only buffer capacities in the range of the pKa value are highest, but also complex stability between complexing agents and metal ions. And here is a decisive difference between nickel and boric acid on the one hand and nickel and carboxylic acids on the other. In the working range of a nickel bath, a pronounced complex formation between nickel and carboxylic acids takes place, whereas with boric acid this only occurs at higher pH values due to the higher pKa value. Although it is well known that, depending on the medium, reactant, concentration and temperature, boric acid can assume pKa values comparable to those of carboxylic acids, the operating parameters of nickel baths tend to favor higher pKa values, which make the strong complex formation at pH 4 to 4.5 between nickel and boric acid improbable.
It seems therefore not so important what boric acid does, but much more what it does not do, namely to saturate Ni2+ coordinately and thus make it inaccessible for leveling substances.
In summary, a boric acid substitute must meet two conditions: on the one hand, the lowest possible tendency to bind nickel in the range of the working pH, which in turn allows a direct bond between leveler and nickel, but on the other hand – at and slightly below the pH at which nickel hydrolysis begins – to bind nickel very strongly to prevent the formation of nickel hydroxide.
The riag Oberflächentechnik AG has identified an entire class of compounds that combines the above-mentioned conditions almost perfectly. After a successfully completed test phase, the products derived from this class are now in the process of gradually replacing processes containing boric acid. The following features are particularly noteworthy:
Extension of the working window: The new compound class allows coating at higher pH values. This allows coating between pH 3.8 and, in extreme cases, pH 5.5 without observing separations in the high current density range.
Improved ductility: The efficient suppression of nickel hydroxide formation largely prevents embrittlement of the nickel layer.
Reduced additive consumption: Since the degradation of additives is dependent on the pH value, among other things, it is open to the coating process to adjust the coating parameters in such a way that a significantly reduced consumption of additives results while retaining the same decorative properties.
Improved coating thickness distribution: Probably the most interesting property for the surface coater is the more uniform coating thickness distribution, which allows significantly shorter coating times.
Full compatibility of the new compounds with boric acid: This ensures a seamless transition from boric acid to the new processes without having to fear production interruptions.
In conclusion, it can be said that what was initially perceived as state-imposed coercion is now proving to be a stroke of luck for technology. While the ban on lead and chromium(VI) is accompanied by considerable quality losses in the end product, the opposite seems to be the case with boric acid substitution.