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The metal finishing industry plates hard and decorative hexavalent chromium processes from 1920s. It is an acknowledged industry standard and preferred choice because of its exceptional physical characteristics. US Environmental Protection Agency (EPA) and Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH) had listed chromium trioxide as a hazardous chemical because of its carcinogenic property.

 

Applications

Many aerospace components use hard chromium, the automotive sector primarily uses decorative and other sectors use both decorative and hard chromium plating deposits. A deposit thicker than 1.2 micron are functional (hard) and any lesser are decorative. The industry favors chromium because of its oxidation resistance properties. Among groups 4, 5, and 6 elements of periodic table we plate only chromium using an aqueous solution. Ionic liquids can deposit most of the transition elements. Aqueous hexavalent Cr deposit has exceptional tribological and corrosion resistance (not all formulations) properties. We plate the deposit on high strength steels and nickel alloys (with Wood’s nickel strike). This deposit exists in alpha phase, is crystalline and forms limited compounds or components with occlusion of hydrogen and carbon developing internal deposit stress (refer ASM Handbook for more information). The electrolytes’ low cathode current efficiency allows greater tribological properties. This is because the deposit has hydride and carbide compounds. These hydrides and carbides develop deposit intrinsic stress and effect deformation property of the deposit (Hooke’s law describes elastic properties of materials or deposit).

 

Extensive applications and good properties make environmental directions a challenge to meet. Before we get deeper, let us get a historical perspective.

 

History

Around 1910, a researcher accidentally developed the original hexavalent chromium plating formula comprising chromium trioxide and sulfuric acid. He assumed chromium trioxide was a trivalent salt until another scientist corrected the misinterpretation within two years.  

 

Research

From that time substitute methods such as trivalent chromium plating, cobalt alloy deposits, electroless Ni deposits with P or B alloys were developed and are a focus of continuing research. High temperature and room temperature ionic liquids for deposits such as trivalent chromium, niobium, aluminum, molybdenum is in study.

 

Alternate Choices

Chromium Plating

Trivalent Chromium Plating

Decorative trivalent and hexavalent deposits have similar properties because of the thickness limit and electrolysis mechanisms of the respective electrolytes. Electrolysis mechanisms change as electrolysis progresses and the deposit characteristics vary with thickness. There are scientific papers on this phenomenon. On hard Cr applications major variation is on macro-cracks, which develops after baking. When analyzing macro-cracks, a seldom adhered to practice is to compare the microstructure on transverse sections. Refer to ASTM E3 – Microstructure and Properties for more information. A few applications use nickel undercoat to negate the effect of macro-cracks.

 

Other Electrolytic Methods

A few specialists recommend electroless Ni-P, electroless Co – P and electroless Co as substitutes to hexavalent hard chromium plating deposit. But the author of this paper doesn’t consider these as dependable alternatives. Only electroless Ni-B (mid boron) deposit possesses tribological properties, but it doesn’t offer comparable wear and corrosion resistance properties of hard hexavalent Cr plating deposit.

 

Ionic Liquid Methods

Room temperature ionic liquid electrolysis is an effective alternate. Aluminum deposit offers many unique advantages. However, it is still an emerging technology.

 

Vapour Deposition Methods

There are two types of vapour deposition methods – physical vapour deposition (PVD) and chemical vapour deposition (CVD). We can apply CVD on several transition metals. Of particular interest to this topic are CVD deposits of Ta and Nb.

 

Thermal Spray Coating

On economy, versatility and diversity of options, the thermal spray coating processes is the best alternate to hexavalent Cr plating method. There are five different methods available in the market – oxyfuel wire (OFW) spray, electric arc wire (EAW) spray, oxyfuel powder (OFP) spray, plasma arc (PA) powder spray, and high velocity oxyfuel (HVOF) powder spray. Refer to ASM Handbook Volume 18 for more information on this subject. However, on many applications the line-of-sight characteristic will limit the thermal spray method. There is continuous research in this field, and recently a few companies have taken the processes to a new level.

 

Summary

Bottom line, trivalent chromium plating, vapour deposition, and thermal spray methodologies are operative substitutes to hexavalent hard chromium plating process. Application demand, cost and the required physical characteristics determine the value of a specific method.

 

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Acknowledgement of the influential in electroplating develops our learning of the fundamentals, and it improves our research.


There are thousands of contributors who served the electroplating industry and academia for over a century.

Who tops your list?

What did they do?

Why is it important?

What did we learn?


This short paper lists 8 innovators, two focused on electrochemistry and the rest on industrial plating (electrolytic, autocatalytic, ionic & aqueous). Their contributions enabled our understanding, improved our applications, and helped us to advance the technology.

 

Let us see who they are, when and what did they offer to the field and its benefits.

 

Michael Faraday

During 1832 Faraday published a paper relating the quantity of electricity with the amount of metal liberated at the electrodes resulting in two laws. When we think of electroplating, Faraday’s laws are probably the first to come to our mind. Mastering a process or shining at customer satisfaction does not end without due consideration of an electrolyte’s current efficiency and the feasibility of a deposit.

 

Walther Nernst

Nernst equation is the fundamental equation of electrochemistry and for the electrode processes. An ardent electroplating researcher shall begin and ensure a thorough understanding of Nernst equation and its concept. We can plate several transition metals, post transition metals, and metalloids. Some using aqueous electrolytes and room temperature ionic liquids, and most using high temperature ionic liquids. A modest approach to choose the electrolyte and the element for the deposition process is to comprehend Nernst equation and its concept. An interstitial or interatomic alloy electrodeposit choice is no exception to this subject.

 

Abner Brenner

Brenner conducted many studies on electroplating deposits. We conspicuously recognize for his preliminary contributions on electroless nickel plating invention around 1946. Though he got black non-adherent dendrites in the initial testing, his research allowed significant growth in electroless Ni-P and Ni-B deposition processes, and plating on plastics.

 

Richard Hull

Hull cell device is common in most electroplating laboratories. Most platers gain Hull cell testing skills or aspire to become good at it! Hull around 1930s invented a testing cell and derived a formula to calculate the effective current density of an electrolyte. Hull cell unit and its formula is quite easy to use! Is the design of the unit and its formula an ordinary achievement?

 

Oliver Watts

Professor Watts reported on 1931 his work on nickel plating bath comprising nickel sulfate, nickel chloride and boric acid at a higher temperature. Many used nickel-plating electrolyte using the same inorganic constituents at room temperature or at 120ºF. Watts was the first to report the benefits of thick and uniform deposit at 145ºF and 160ºF. Since then we began calling this electrolyte a Watts nickel plating solution.

 

Donald Wood

Wood was an expert in cyanide silver plating, and that is what he did for most of his career. We know him for his invention of chloride-based nickel strike bath during early 1940s. The Wood’s nickel strike formulation enabled the global industry to plate on all ferrous, nickel, titanium and aluminum alloys.

 

Donald Cook

Even if you are well read in the industry, you might not have heard about Dr. Donald Cook! Cook coined the term ‘metaliding’. Metal finishing industry did not see the effects of his research. But if I didn’t mention his name with others, it would be amiss as he is alike Nernst and Brenner. No one would have worked on more transition elements and diffused into others than him! His knowledge in electrochemistry and chemistry of halides was impeccable, and he distinguished the ins and outs of transition metals of group 3 to 11 of the periodic table. But his only focus was in high temperature ionic liquids.

 

Seymour Senderoff

We know Dr. Senderoff for his invention of spiral contractometer used in hard hexavalent chromium and nickel sulfamate plating applications to detect internal stress in the deposit. He worked for Dr. Brenner for several years and later focused on high temperature ionic liquid tantalum plating. It was my source of pride to continue on his research and improvise his formulation.

 

electroplating leaders

 

So, what is the learning?

Recognizing these pioneers and their work must transcend awareness and dwell on a deeper understanding of their explanations and research outputs.

Faraday and Nernst focussed on the fundamental of electrochemistry. Cook and Senderoff concentrated mostly on high temperature ionic liquid electrolysis (plating and diffusion). Brenner, Hull, Watts and Wood dedicated their research on commercial electrolysis. Nevertheless, all played a revolutionary role in electroplating applications.

 

Here are the examples of teachings from a few of these pioneers’ work:

Nernst’s invention allowed us to relate electrode potential, valency of the metal ion and current. Hull’s work brought to light Tafel’s, Butler’s and Volmer’s work. Wood’s invention signified electromotive force (emf) series and distinguished strike, flash plating, and plating. Cook’s and Senderoff’s developments emphasized the importance of eutectic temperature, phase diagram, liquidus temperature of salts, fluxing effect and ionic conductivity of electrolytes.

 

Hope you find this paper valuable and you dig deeper on these matters!

Post your comments and write about who tops your list.

 

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Cyanide based electrolytes of silver and copper deposits possess excellent throwing power property.


None of the nickel or chromium electroplating electrolytes contain cyanide ions.


The paper discusses the polarization, coordination chemistry, and thermodynamic reasons for these phenomena.


Did you ever think about why nickel does not possess a simple cyanide-based plating electrolyte? Why can’t we electrolyze a nickel cyanide salt [Ni (CN2)] to get a deposit? Won’t a cyanide nickel electrolyte with a good active polarization and throwing power property be of use?

 

We will deliberate the evidences of asking these questions.

 

Cyanide based electrolytes such as silver and copper deposits possess excellent throwing power. Copper possesses good leveling characteristics, while nickel without organic molecules has negative throwing power. Bright nickel with class 1 and 2 brighteners can get a uniform deposit on the cathode, but it is not comparable with copper.

 

Coordination chemistry plays an important role in electrolysis. It is important to distinguish thermodynamic parameters such as stable and unstable, and kinetic parameters such as inert and labile. These terms refer to stability. Nickel and chromium cyanide complexes like [Ni (CN)4]2- and [Cr (CN)6]3- are (extremely) thermodynamically stable. Unlike kinetically inert compounds, thermodynamically stable coordination compounds become very difficult to break a bond or ligand during electrolysis.

 

cyanide copper plating

 

Silver and copper cyanide baths possess good throwing power because of the presence of simple cyanides like sodium or potassium cyanide. Cyanide ions during electrolysis effects total polarization. The concentration polarization increases at the cathode interface because electrolysis liberates cyanide ions. The concentration polarization and cathode current efficiency work in tandem and distribute electrodeposition based on primary and secondary current distributions factors. This behavior is a part of tertiary current distribution phenomenon.

 

Ni and Cr are not incomparable with their limitations. Platinum and gold have unique advantages and restrictions. Like Cu and Ag, brass plating containing cyanide ions produce beautiful deposit colour. In aqueous electrolysis Cu, Zn, Ag & Cd deposits differ from Ti, Zr, V & Nb. These points emphasize distinct properties and limitations of elements, chemicals and media (aqueous or ionic).

 

Purpose

The short paper does not offer a solution or recommend an alternate method. It explains the fundamental benefits of cyanide ions and distinguishes unique elements of the periodic table like Ni and generates an awareness on their chemical properties.

 

Understanding the characteristics of an element, position in the periodic table, and their chemical properties are important to research scientists and advanced engineers.

 

If you are a scientist formulating a bath recipe or a forward-thinking engineer choosing a process to meet deposit characteristics’ fundamental concepts, advantages and limitations are important to understand. This learning will take full advantage of the prospect or prevent an issue in the long-term.

 

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A profound analysis of periodic table will help ardent electroplating professionals understand the effectiveness of the deposition of elements from the outlook of electrode potential, current efficiency, and the deposit characteristics. Use of the table provides added knowledge and supports to decipher the precise information swiftly.


As we celebrate the 150th anniversary of periodic table, we will dedicate this short paper on the properties of elements and their position in the periodic table. Regarding electroplating of elements and alloys there are a few significant relationships with the groups and periods of the table. Though these are important for all plating specialists, it particularly interests those involved in research and formulation of an electrolyte and the alloy deposit.

 

Here are some properties:

 

Water:

The presence or absence of water affects the plateability and the current efficiency of the deposit. Chromium, the group 6 element of periodic table is a good example. Ability to plate Cr using the aqueous (presence of water) chromic acid-based electrolyte is an exception. Low current efficiency property of the electrolyte shows the reason for the struggle. On other hand, one can conveniently plate neighboring elements like Ti, Zr, Hf, V, Nb, Ta, Mo and W using ionic liquid (absence of water) electrolyte, with other challenges.

 

Position of elements:

The position of elements in the table and their properties are insightful. The position and proximity with each other can also help determine suitable alloying elements. On another point, let us look at indium, tungsten and boron. The relevant properties of this example are hardness and melting point. The position and the properties of these elements seen with their respective neighboring elements will show a trend.

 

Oxide (passive) layer:

The group of elements in the table say a lot about the oxide layer of the metal or the deposit. Examples are Ti, Ta, W and Cr. The formation of an oxide layer protects the metal from corrosion and make it tough to form an adherent deposit using the conventional electrolytes. Electrode and decomposition potentials of these elements also contribute significantly towards effective plating or the challenges thereof.

 

periodic table nickel electroplating

         

There are nearly 35 elements suitable for plating and their effective deposits offer a unique and superior properties. But aqueous electrolytes can conveniently plate only ≤ 20 elements (read metals) and alloys. Certain ionic liquid formulations can electroplate all or most of the 35 elements. We will discuss the reasons and mechanism of ionic plating in another short paper. Looking at the elements in groups and periods of the periodic table will offer unique understanding on plateability and desired functional properties such as corrosion resistance, tribology, and electrical conductivity.

 

Refractory elements, precious metals, sacrificial protection metals, and conductive ions (cations) are in proximity among their groups on the periodic table. They are in decreasing or increasing order within the groups or periods.

 

Summary:

Periodic review of the periodic table will enable electroplating professionals strengthen their understanding on the feasibility and properties of the deposit. Greater effort to interpret different properties are definitely worth it!


 

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Donald Wood contributed significantly to the surface finishing industry by inventing (Wood’s) nickel strike formulation around the 1940s. The invention of this formulation and subsequent improvements from the 1960s enabled plating elements of different electromotive force (emf) potential on stainless steel (SS420) and nickel alloys (Inconel) effectively.


Mr. Wood was an expert in cyanide-based silver-plating process and had used silver strike formulation with low free cyanide content.


Mr. Wood was an expert in cyanide-based silver-plating process and had used silver strike formulation with low free cyanide content. In a verbal communication on this subject he had mentioned, “…. a strike solution is generally designed to operate at low cathode efficiency so that a liberal evolution of hydrogen will perform it surface scouring function before metal is deposited”.


Both nickel (cation) and cyanide (anion) ions posses a unique value in plating. We use many elements to develop a strike layer on a substrate, and deposit other elements where electrode potential is different. Gold, copper, iron, and cobalt are a few examples. But most professionals consider the nickel strike deposit the best, and it is most commonly used to form an adherent strike layer. We consider nickel as an element which possesses many refractory properties, though it isn’t a refractory element!

Other than cyanide ions, sulfates, chlorides, and fluoride ions possess good transportation or conducting properties. Plating baths which use simple cyanides (base) for many reasons provide superior physical characteristics than other types of anions.

 

electroplating chemicals electroless nickel


The point driven above is not about the use of nickel or cyanide. It is about choosing suitable cation and anion(s) in a process at the design phase to achieve sustainable quality.


What is unique about nickel? What is the effect of cyanide in the electrolysis, it can produce exceptions results? What other variables influence quality at the design phase? This blog post introduced a few fundamental electroplating terms. What do they mean? I will answer these questions. Stay tuned for the next post.

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Venkat Raja
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May 1, 2021
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