Demir grubu alaşımların anormal kaplama davranışına genel bir bakış

Özet

Demir grubu metal (Fe, Ni ve Co) alaşımlarının sahip oldukları manyetik ve termofiziksel özelliklerden dolayı elektrokaplanmış demir grubu metal alaşımlarının endüstride önemli kullanım alanları mevcuttur. Söz konusu alaşımların endüstriyel öneme sahip olmaları yanısıra anormal kaplama olarak tanımlanan olayın, demir grubu alaşımların elektrokaplamasında görülmesi de bu sistem üzerinde yoğunlaşan çalışmaların bir başka nedenidir. Demir grubu elementleri olan Ni, Fe ve Co arasında termodinamik olarak en soy olanı Ni, en az soy olanı ise Fe elementidir. Bu üç metal arasında ikili ya da üçlü olarak gerçekleşecek alaşım kaplama reaksiyonları kinetik olarak karşılaştırıldığında Ni’in hem Co hem de Fe’e göre öncelikli olarak kaplanması beklenebilir. Ancak anormal kaplama davranışı gereği Ni kaplama, Co ve/veya Fe kaplama tarafından yasaklanmaktadır ki benzer durum Co ile Fe arasında da mevcuttur. Normal alaşım kaplamaya göre ilginç olan bu kaplama davranışı sonucunda endüstriyel anlamda istenilen manyetik özelliklere sahip ikili ve üçlü demir grubu alaşımlar üretilebilmektedir. Anormal kaplama olarak ifade edilen bu sıradışı kaplama olayının mekanizmasına açıklık getirebilmek adına günümüze kadar süregelen çalışmalar yapılmıştır. Yapılan çalışmalarda araştırmacıların hemfikir olduğu nokta termodinamik olarak soy olan metalin kaplanmasının daha az soy olan metal tarafından engellendiğidir. Bu çalışmada alaşım kaplama yöntemlerinden biri olan elektrokaplama yöntemi kısaca tanıtılmıştır. Bunu takiben literatürde yeralan ikili ve üçlü demir grubu alaşımların elektrokaplamasına özgü anormal kaplama davranışını açıklamaya yönelik çalışmalar özetlenmiştir.

Abstract

Electrodeposition, also known as electroplating, refers to the process where a thin film of metal is deposited onto a conductive substrate under an applied current. The electrodeposition of alloys on a substrate requires the simultaneous deposition of two or more metals. Electrodeposition is both a costeffective and simple method to produce thin film materials (such as NiFe, CoNi, CoFe and NiCoFe). Nickel, cobalt and iron, ( iron-group metals), and their alloys are important engineering materials in many applications because of their unique magnetic, corrosion and wear-resistance, thermophysical and electrocatalytic properties. For example, NiFe and NiCoFe alloys are strong candidates for application in high density magnetic recording heads due to their high saturation flux density and low coercivity. These functional materials are usually prepared by electroplating because this technique is often much cheaper and simpler than other methods. In addition, advantages of electrodeposition include low deposition temperature and the simplicity of the required apparatus. The magnetic properties of these materials are seriously affected by their composition and structure thus, reliable control of these properties for the iron-group alloys is important for their wide applications. The standard equilibrium potentials of Ni, Co, and Fe are –0.25, –0.27, and –0.44 V vs. the normal hydrogen electrode (NHE), respectively. Therefore from a thermodynamic consideration Ni is the most noble of the three metals. On the basis of these values, in normal codeposition, nickel is expected to electroplate preferentially to cobalt and cobalt preferentially to iron. When these metals are plated individually, the kinetics of the electrodeposition of individual metals follows the trend expected from the standard equilibrium potential. However, when Fe is present in the same solution with either Ni, Co or both ions, there is often potential range where the rate of Fe deposition exceeds the rate of Ni and Co deposition occurs. The called anomalous codeposition of iron group metals is characterized by the less noble metal deposition preferentially to the nobler one. In other words, the reduction of nickel is inhibited while the deposition of iron and/or cobalt are enhanced when compared with their individual deposition rates. The study on the anomalous deposition of the irongroup alloys can be traced back to 1927. The mechanism of anomalous codeposition of iron group metals has been studied by many researchers and several hypotheses have been presented in the literature. Although many models have been proposed in the literature, the exact mechanism of the anomalous codeposition process is still not well understood. Anomalous codeposition is a term first introduced by Brenner for binary alloy system, such as NiCo, NiFe and ZnNi. Among these, the NiFe system is most studied. One of the earlier explanations of anomalous codeposition in NiFe binary system was put forth by Dahms and Croll. Their model based upon the inhibition of nickel reduction due to the formation of the hydroxide of the less noble metal, Fe(OH)2. The hydroxide is considered stable because of the high pH calculated at the electrode surface. Andricacos and coworkers were the first to systematically examine the role of hydrodynamics in the deposition process of NiFe binary alloy. In their study, iron came under mass transport limitations at high enough currents. The nickel deposition inhibition was seen to increase with increasing rotation rate. Recent studies of NiFe have utilized this theory; both Hesssami and Tobias and Grande and Talbot have focused on the discharging of the monohydroxyl ion on the electrode surface. The most widely accepted explanation for the anomalous codepostion is that precipitation of ferrous hydroxides on the cathode, caused by an increase in the pH due to hydrogen evolution, inhibits Ni and/or Co reductions. The anomalous codeposition of iron group metal is still not completely understood. Therefore, it would be of academic interest to conduct research of the anomalous codeposition of the iron group metal in order to obtain understanding of the factors that control the anomaly of Fe and Co deposition in the codeposition process. This could allow researcher to control the rates of deposition of the individual metal component in the NiCoFe alloys. As a result, it would lead to better control of the physical, mechanical and magnetic properties of electrodeposited iron group alloys.