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.