Electrodeposition of chromium and its alloys from trivalent salt solution

In this thesis work, trivalent chromium electrodeposition has been studied as an alternative to hazardous hexavalent chromium electroplating. Aqueous solutions of Cr (III) are significantly less dangerous, in terms of human health and environmental impact, as compared to the traditional Cr (VI) baths employed for this purpose. Hexavalent chromium is highly toxic and a known carcinogen, especially when carried through the air as a vapor. Transport of chromium particles occurs during plating because a number of by-products are generated, such as hydrogen and oxygen gases, water vapor and chromic acid mist. Hydrogen and oxygen gases are generated due to the inefficiency of electrolytic reaction during chromium plating. Chromic acid mist is generated by the bursting action of the hydrogen and oxygen gases escaping at the surface of the solution and air interface. Nowadays, there are several new investigations for improvement of trivalent chromium, although it is limited by inability to deposit thick coatings with acceptable quality. In order to overcome problems regarding the reduction of Cr (III) in aqueous solution; choice of a specific ligand for Cr3+ ion, which can generate easily reducible complexes at the metal-solution interphase, was followed as an approach. The sufficient complexing agents for trivalent chromium are generally based on organic compounds which are able to capture chromium ion weaker than hydrated molecules. These organic compounds, which have the ability to react with chromium metal ions, promote the electro-reduction of Cr3+ and accelerate the deposition rate. There are several types of complexing agents such as formic acid, urea, malonic acid, glycine, citric acid, glycolic acid, acetic acid and oxalic acid which can be used as ligand in trivalent chromium electrodeposition bath. In this research; effect of bath composition and electrolysis conditions on the current efficiency, electroplating rate and surface morphology of Cr(III) coatings was studied by using a chloride trivalent chromium bath. As it has been already studied in literature; even if formic acid and urea are the most favorable complexing agents in trivalent chromium electrodeposition, especially because of its toxic effect it is avoided to use formic acid in the process. Therefore, glycine is decided as a ligand during the experiments. The bath is operated using titanium oxide anodes and copper cathodes which trivalent chromium was deposited on. The effects of plating parameters such as current density, bath pH and plating time on structure and morphology of deposited coatings were investigated. The optimum conditions for good quality chromium metal deposition, which has a higher hardness value, good wear and corrosion resistance while having a smooth and bright aspect, are determined. Moreover, the possibility of co-depositing chromium with phosphorous and palladium to improve mechanical and superficial appearance properties were investigated as another approach. The approach of co-deposition of chromium was born as the target to produce an alternative layer which can be used in jewelry industry. As the main target of this research; trivalent chromium deposition was studied based on same bath ingredients with different amount of buffer agents to observe the effect of buffer agents on deposition. Hence; effects of buffer agents are observed on mechanical, chemical properties of coating and its appearance. Furthermore; basic trivalent chromium deposition was compared with results of deposited Cr(VI), Cr(III)-P and Cr(III)-Pd. The effects of bath parameters, bath contents and different co-deposition applications were investigated using hardness measurements, wear and coefficient of friction tests, optical microscopy, scanning electron microscopy (SEM), X-ray diffraction analysis (XRD) and corrosion behavior of coating is analyzed by potentiodynamic analysis.

In questa tesi si è studiata la possibilità di elettrodeporre da bagni di cromo trivalente come alternativa ai depositi ottenuti dai più pericolosi bagni di cromo esavalente Le soluzioni a base di Cr(III), per impatto sull’ambiente e sull’uomo, sono meno pericolose se paragonate a quelle contenenti Cr(VI). Il Cr(VI) è carcinogenico e tossico sia se ingerito, ma soprattutto se inalato: durante l’elettrodeposizione i numerosi co-prodotti gassosi, come l’idrogeno e l’ossigeno, e l’evaporazione medesima dei bagni trasportano nell’ambiente microgocce che lo contengono. Oggigiorno numerosi sono gli studi che coinvolgono il cromo trivalente, ma il problema maggiore rimane la difficoltà di ottenere un deposito sufficientemente spesso e con buone qualità a corrosione e meccaniche. Per ovviare alla difficoltà di riduzione del Cr(III) nelle soluzioni acquose, sempre più specifici agenti complessanti sono stati testati: molti sono di origine organica. Questi complessi legano più debolmente il cromo rispetto ad altre molecole inorganiche, promuovendone al medesimo tempo le riduzione ed accelerando la velocità di deposizione. Acido formico, urea, acido malonico, glicina, acido citrico, acido glicolico, acido acetico, ed ossalico sono i più comunemente utilizzati. In questa ricerca sono stati evidenziati gli effetti legati alla composizione del bagno ed alle condizioni di deposizione rispetto: ad efficienza e velocità di deposizione, alla morfologia del deposito. Il bagno sviluppato è a base di cromo cloruro esente acido formico ( quest’ultimo è fortemente tossico per la salute ), ma con glicina come agente complessante. Si è poi scelto di operare con un anodo insolubile ed inerte di titanio e di depositare su rame. Modificando densità di corrente , pH , tempo e temperature di deposizione si sono quindi determinate le migliori condizioni di deposizione rispetto a: morfologia di superficie ( lucida e compatta ), microdurezzza, usura e resistenza a corrosione. Inoltre è stata vagliata la possibilità di co-deporre elementi come fosforo e palladio per migliorare le caratteristiche meccaniche e di corrosione o per ottenere strati barriere per il settore della gioielleria. Determinata una composizione di base del bagno di lavoro è stato modificato il contenuto di agente tampone e di un secondo componente per studiare come si modificano le proprietà meccaniche, chimiche e l’aspetto superficiale. E’ stato inoltre condotto uno studio comparativo tra i depositi di Cr(III), Cr(VI), Cr(III)-P and Cr(III)-Pd attraverso morfologia e composizione di superficie ( SEM-EDS, XRD ), microdurezzza, usura e resistenza a corrosione.