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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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Managing time with a data-driven approach offers incredible value in process management and customer service on electroplating and anodizing applications. Analytical data, awareness of chemicals and deposit properties with a historical context add value to engineers and aspiring scientists in the metal finishing field.

 

Spot and quick tests offer incredible insights for the metal finishing processes. What is a spot test? Are these tests reliable?

 

High temperature oxidation resistance is a valuable elemental and deposit property. What does it mean? Which elements possess this property? We will review the value of this physical property.

 

Surface and metal finishing offer a variety of options such as plating, anodizing, coloring of metals and electropolishing. Plating on several substrates such as plastic, steel, stainless steel, Invar, Kovar, nickel, aluminum, and titanium alloys are common. Elements like gold, silver, nickel, chromium, zinc and cadmium are plated. We choose the deposit based on consideration for cost, corrosion resistance and tribology properties.

 

Other than plating, we can electropolish and anodize metals such as stainless alloys. Applicators anodize on aluminum, magnesium, titanium, niobium, tantalum, etc.

 

Anodising is an electrolytic process in which we make an Al alloy anodic with a metal cathode on an acidic electrolyte. There are three classes of coating. We anodize with and without sealing.

 

This short paper explains which element to choose as deposit or substrate. It lists spot and quick tests available in the market. Review brightener chemistry and history. Discuss inhibitors, chemicals used for coloring metals and chromium conversion coatings.

 

Spot Tests

Several wet and instrumental analyses methods are used to conduct qualitative and quantitative analysis of elements. The advantage of these methods are accuracy and data reliability. But these test methods are time consuming and some are expensive. When you need a quick feedback, consider conducting spot tests on deposits and effluents. The sensitivity of spot test reaction can be at ppm levels. Elements such as aluminum, stannous tin, gold, silver, iron, nickel, palladium, lead, zinc, cadmium, chromium and copper can be tested. Many of these tests can take less than 5 minutes.

We can detect heavy metals, hexavalent ions and cyanide content in the effluent. You can distinguish between cadmium or zinc plating deposits using spot tests.

 

Here is a list of some organic chemicals used for spot tests:


Diphenyl carbazone

Dimethyl glyoxime

Tri-ammonium aurine-tricarboxylate

Nitro-bruciquinone hydrate

p-Dimethylamino benzylidene rhodamine

diphenyl thiocarbozone

sym-Diphenyl-carbazide

(1-2-Hydroxy-5-sulpho-phenyl)-3 phenyl-5-(2-carboxy-phenyl)-formzan) sodium salt

1-(2-Pydridyl-azo)-2-naphthol


Refer to Chem Spider for further information on any chemicals.

 

For further information on spot tests, read the book - Analysis of Metal Finishing Effluents and Effluent Treatment Solutions. This is a book written by Duncan MacArthur, Fred Stevens, and G. W. Fischer.

 

Chemicals

Did you know tobacco and licorice were one of the earlier brighteners used? Several decades ago, or even a century back, the use of brighteners or additives were very limited. Even the awareness of organic brightener science did not exit. Like many inventions, use of organic chemicals as a brightener was an accident. Tobacco was one of earlier recorded chemical used as additive. More than a century ago a plating operator who had the habit of chewing tobacco drooled the juice onto a plating solution during electroplating. Later noticed that a plated lot had a brighter appearance. Further investigation revealed the brightness influence of tobacco on plating deposit.

People used licorice during earlier days. It had a presence in the industry for some time and even now to an extent. Licorice extract is a carbohydrate and its chemical name is glycyrrhizin. They used it as an additive.

 

The additive was prepared by weighing a known quantity of licorice root and steep in a boiling water until colour saturation occurred. For 100 L plating solution, 100 grams of root was steeped in 0.5 L of boiling water.

Pickling and acid activation are a very common process at a steel mill, metal foundry, and a metal finishing processing plant. Conditional on activation and deposition bonding requirements, a substrate such as copper alloys or steel alloys would require activation under strongly acidic conditions. Use of strong acids such as hydrochloric acid, sulfuric acid and hydrofluoric acid can etch the substrates.

 

We commonly use sodium fluoride as an inhibitor. On picking applications, industry used antimony trioxide as an inhibitor.

 

Copper alloys such as brass tarnish in the presence of oxygen from atmosphere. Brass plated deposit do the same. An organic coating can prevent tarnish. Benzotriazole coating forms a thin layer in an immersion process, and the layer protects copper alloys from tarnish.

 

Brass plating is an alloy deposition process. The electromotive force potential of copper and zinc makes cyanide brass plating one of the most complex electrolytic processes. Attainment of a consistent and durable colour is tricky. Proper use of current density with an excellent choice of rectifier (IGBT or SCR) with an organic coating ensures great cosmetic appeal with durability.

We can also colour brass. One such colour is blue. An immersion process at high temperature in the presence of sodium sulphite and lead acetate colours brass substrates. Other than brass, we can colour stainless steel alloys using dichromate salts.

 

Yellow chromate on zinc deposit is one of the popular choices. We recognize the yellow chromate for brilliance of colour and corrosion protection. There is a subtle colour difference different between hexavalent and trivalent chromates. Chromates sometimes leaves iridescent finish. A protective coating similar to benzotriazole layer reduces iridescent streaks.  One can get a reddish tone on the yellow chromate formulation. Use of a sulfate ions and nitric acid offers a reddish yellow chromate finish.

 

Occasionally people refer to chromate or chromium plating as chroming. Chroming is a colloquial speech term and we prefer you avoid using informal terms to avoid confusion and for use of clear language communication. Hexavalent Cr and (cyanide) cadmium is listed by Registration Evaluation Authorization and Restriction of Chemicals (REACH). We now replace cadmium plating with zinc / nickel alloy. Sacrificial protection of cadmium under saline conditions are inimitable.

 

Deposit Properties

Observing electromotive force series of elements and their potentials (negative or positive / active or noble) suggests evidences on fascinating deposit, material or electrolyte properties. We are referring to properties such as high temperature oxidation resistance and conductivity.


Pilling and Bedworth conducted a seminal work on high temperature oxidation.


Chromium, tantalum, zirconium and gold possess exceptional high temperature oxidation resistance properties. What is high temperature oxidation resistance?


The volume of oxide is greater or lesser than the parent metal, it produces or cannot produce an effective protective property.


What does a reference to protective property mean? It refers to the formation of an oxide layer, such as tantalum oxide, chromium oxide and zirconium oxide on the metal or the deposit. The refractory metals offer oxidation resistance up to ~ 600ºF. Oxidation and healing property are the principal reason hexavalent hard chromium plated components had gained wide popularity. Many applications of aerospace and automotive industries require components to posses tribological properties at a higher temperature. Wear, lubrication, and friction are such examples. The oxides can re-form and withstand high temperatures during these mechanical transformations. Tantalum, zirconium, niobium, and chromium metals are a few among the best to possess such a property. Other than oxidation properties, these elements are susceptible to corrosion resistance from acids, alkalis, organic media and other reagents. We measure high temperature oxidation in ratio and Pilling - Bedworth (PB) ratio of corrosion resistant metals range between 0 and 4. Higher the number better the corrosion resistance. PB ratio is the ratio of the metal oxide volume divided by the metal volume. Chromium PB ratio is 2. All chromium electroplating deposits do not have the same corrosion resistance properties. Corrosion resistance of decorative Cr plating is because of nickel undercoat. Most hard chromium deposit do not have any corrosion resistance. However, a few formulations containing fluoride ions in the electrolyte possess high corrosion resistance. Some hard-hexavalent Cr deposits pass 500 hours of neutral salt spray test (NSST). We mainly attribute the variations to formulation, processing, and process control.

 

Other than refractory metals, precious metals such as gold plating deposit possess high temperature oxidation resistance. Industry uses gold plated components on space applications because of high temperature oxidation resistance, conductivity, high surface stability, high resistance to tarnish, and chemical corrosion. Both trivalent and monovalent salts are used to deposit gold from electrolytes. Precious metals like gold, silver and palladium can be plated on several substrates such as stainless steel, Kovar, Inconel, magnesium, aluminum and titanium alloys efficaciously.

 

Besides lightweight, the stubborn oxide layer makes aluminum and titanium alloys indispensable in our daily lives.

 

Previous paragraph mentioned about the conductivity of deposit. What about electrolyte conductivity? Change of ion activities with concentration affects electrolyte conductivity. Understanding interionic attraction theory of electrolytes are essential to improve conductivity.

 

Value

Whether you are an engineer or a research scientist, understanding element and deposit properties, electrolyte capabilities and limitations, and vitality of unique chemicals are important. A process design engineer with a good understanding on these characteristics can design products with superior corrosion and tribological properties. The matters covered in this paper such as high temperature oxidation resistance can help a designer identify suitable metal as substrate and a deposit.

 

We identify some chemicals listed in this paper as hazardous or carcinogen per Registration Evaluation Authorization and Restriction of Chemicals (REACH). REACH is a European Union regulation. You can also find additional information on carcinogens by visiting the website of National Institute of Environmental Health Sciences (NIEHS) under the National Toxicology Program (NTP). Observing regulatory compliance and identifying risk mitigation plan will drive an organization’s governance.

 

REACH, quality demand, and customer requirement will call for a transformed focus on a few facets such as colouring of metals, plating solution additives, chromium conversion coating and anodizing. An ardent electroplating specialist must consider all the services, test methods and fundamental concepts.  

A leader must prepare electroplating companies to manage complex processes. An agile metal finishing organization aiming to survive even under adverse conditions shall be data driven, ensure speed of business is appreciable, work faster, and inculcate easy-to-use test methods. We know many that plating companies who are not data driven do not grow or adapt to developing changes.

 

Spot testing of effluent, electrolyte or deposit is an under used method, and will help gather data with speed and ease. Other quick tests like pH measurement, specific gravity, litmus, refractometer, and profilometer are all easy and inexpensive. This do not mean volumetric and instrumental analysis offer less value. Both methods are vital for many electroplating operations. The choice depends on their technical ability, product testing requirements and financial capability. The examples of these instruments are atomic absorption spectroscopy, scanning electron microscopy, induced couple plasma (ICP), x-ray diffraction, x-ray fluorescence and electron microprobe analysis. We use these units for elemental analysis at lower concentration with a top-level accuracy. XRD and EPMA can detect light elements such as lithium, oxygen and carbon with an outstanding repeatability and reproducibility.  

We deliberated chemicals, methods, properties, and data driven approach. Cognizance of these matters without a long-term analysis and reaction plan is not noteworthy!  Consider use of run chart, control chart and Process Development and Control (PDC) tools with a visual dashboard.

Readers can find a value on other short papers written on this page previously. Please read articles on electrode potential, IGBT and SCR power supplies, current distribution, throwing power, periodic table, Time Change Management (TCM), Process Development & Control (PDC) tools, and communication. A complex electrolysis process requires a multidimensional approach on disciplines such as science, mathematics, technology and management. There are many vital aspects involved in this field such as automation, process control, business development methodologies, and so on. Fundamentals, laws, equations, and concepts govern electrolysis. Though not needed on a day-to-day basis, these are important to be aware and apply. Discipline, observation, data, patterns, and behaviors are critical for one to succeed at a higher level. Though there will be a scientific explanation for all outputs, one needs to treat the work as art! This is imperative because of our limitations – time and knowledge. Hence, at Advint we offer on guidance on subjects related to laboratory practices, equipment engineering, automation, productivity, lean, statistical process control (SPC) tools, and management. Advint’s virtual Electroplating Training explains all these subjects comprehensively. 

 

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Surface profile of the substrate, specifically surface activity and its receptivity to accept electroplating deposit, spontaneously determines the morphology.


A deposit with good morphology will possess an adherent deposit. There is a myth and inadequate science work related to surface profile and adhesion. A few sectors do well with improving surface activity, and a sector is vulnerable. This short paper intends to shine light, dissipate a myth, and suggest improvements with supporting information.

 

Cleaning & Activation

It is a normal practice to blast a metal surface to improve cleanliness. This approach predates the 60s, when cleaner science had made minimal progress. Over the years proprietary plating supply houses like Atotech, MacDermid Enthone and Coventya had conducted extensive research and offer an array of cleaning products. We released a short paper explaining the importance of cleaning on April 1, 2019.

 

In the absence of an effective cleaning process, blasting the surface is pertinent. It is an important treatment method on applications where we require higher Ra values or unique aesthetic appeal. Rather, if one blasts a surface merely to improve adhesion, then it is time to debunk the myth! Now, I’ll back up the purpose with facts and information.

 

To get an adherent deposit the metal surface must be clean, active and receive nucleation (form a unimolecular layer) within the first 10 or 20 seconds regardless of the current distribution pattern. A spontaneous and uniform deposit formation is important. The critical nucleation time varies between substrates and the deposit element’s electrode potential.


A truly active surface allows effective nucleation.


A poor nucleation layer will disrupt crystal growth and cause re-nucleation of the crystals. This disruption leads to deposit non-coherence and inconsistency in physical characteristics, resulting in premature product failure. Impregnated blasted media is very difficult to obliterate the surface and in most cases leaves a residue, hinders nucleation or continuity and uniformity of the deposit. An electropolished surface address this concern and enables epitaxial or pseudomorphic growth when and where applicable. This surface possesses very low Ra values, are active and free from foreign materials. If it drives you to get the most adherent deposit, electropolished surface is one of the best means to achieve this endeavor.

 

electropolishing electroplating surface profile

 

Surface Profile & Adhesion


A truly active metal surface can form a thick intermetallic layer and develop a single domain deposit morphology (columnar structure) through the process. A single domain columnar structure deposit will possess a distinct grain boundary.


 

This mechanism is not independent of process control. The author of this paper had conducted extensive research over a decade on this matter, and so did a few other scientists from our society during the 80s and till now. A deposit with an intermetallic layer forms the most adherent deposit. In order for a non-electropolished surface to form an intermetallic layer, it must possess an active surface. We know that an electropolished surface possesses lower Ra value, but we require more controlled studies to validate the relationship between the surface profile and adhesion. On this subject, good surface profile implies a clean, smooth and active surface.

 

Morphology & Physical Characteristics

Most adherent deposits possess good morphology. A deposit with an intermetallic layer and undisrupted columnar structure will have greater than 20% improvement in physical characteristics such as hardness, tribology, and corrosion resistance properties. 

 

Summary

Blasting is a valuable pretreatment method, but an ambitious applicant must recognize that residue left on the surface affects the deposit characteristics, and it is not a certain choice in pursuit of an adherent deposit. Electropolishing is not practical on many applications. Exceptional cleaning and activation are viable and important.

 


Accomplishing an intermetallic layer on all applications is an unreasonable expectation, but a clairvoyance can set that as an aim. This aim is akin to lean’s one-piece flow and six-sigma.


In short, be mindful of the relationship between activation, morphology and physical characteristics.

 

Notes:

Material scientists and electrochemists now and then use different terminologies when referring to the same concept. Some terms used in this paper are no exception.


I used material scientist as a substitute for metallurgist. The global technical community replaced the term metallurgy with physical characteristics in 2010.

 

You are welcome to post a comment or email with questions to adviser@theadvint.com if a concept is abstruse.

 

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This is the second year of short papers release. Last year, Advint released papers on cyanide ions, properties and behaviours of anions, trends within the groups and periods of the periodic table, electroless Ni and current distribution (primary and secondary), quality and management. A year ago, I wrote about electrode potential and electromotive force (emf). As an example, nickel and gold possess distinct electrode potentials, and so are their deposition mechanisms.

 

Reel-to-Reel Plating

Electronic industry regularly uses gold, tin, palladium, palladium/nickel and copper on a continuous reel-to-reel plating application. Similar to gold, tin and indium possess comparable properties and advantages. Tin, silver, copper, indium, and gold all possess good conductive property. Indium, a precious metal, we primarily use as an alloying material in plating applications, though they use it as a deposit by itself (without alloying element).

 

Palladium and nickel are neighbors in the periodic table. We consider palladium to be a precious metal and it also possess properties comparable with nickel. It is broadly used in electronics and jewelry industries. On many applications, palladium or palladium/nickel acts as a good undercoat (an intermediate layer). Palladium/nickel deposit is recognized for its cosmetics, and wear and corrosion resistance properties. The ratio of palladium and nickel in the deposit varies between 80 and 20 to 95 and 5, respectively. Palladium also gained prominence because of nickel release and nickel allergic properties. Upsurge in dominance of palladium is apparent when we look at the price of the metal in the market (today’s value - ~ USD 2213 per troy ounce).

 

hard chrome plating

 

Plating on Plastics (PoP)

The demand for nickel on other hand is rapidly growing in the automotive industry, particularly in plating on plastics applications. The wide acceptability of nickel is because of its refractory properties, though it does not belong to elements of the refractory group (periodic table). Nickel release and its allergic properties are an issue in certain demographics and on applications such as eyeglass frame, earring, necklace, ring, bracelet.

 

On PoP applications, plastic substrate preparation using a colloidal catalyst is one of the most important steps. On acrylonitrile butadiene styrene (ABS) plastics, it follows preparation of the substrate with electrolytic copper, nickel and chromium deposits. Among Ni use, electroless Ni is gaining wider acceptance in recent decades. The industry also now replaces hexavalent Cr with trivalent chromium.

 

Electrode potentials are distinct between hexavalent and trivalent Cr plating applications. The trend is the same on other metals such as Ni, Au, Sn, In, Cu and Pd. The properties are distinct not only because of electrode potentials but also because of transportation of ions and ionic mobility. Similar to chromium, ionic mobility of electrolytes of electro-polishing and anodizing are also less. This is one reason these require a very high DC voltage from the rectifier during processing. Note: A paper on rectifier is coming soon.

 

Summary

Different metals deposition mechanisms vary because of their electrode potentials and other properties like ionic mobility and concentration of metal ions.


Among many elements of reel-to-reel plating and PoP applications reviewed in this paper, Pd is used on both applications, and has gained significant recognition in the recent decades.


The recognition goes beyond plating applications.

 

 

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Electroless Ni-P and Ni-B process management had improved significantly within the last two decades. In this paper I’ll write about the science of autocatalytic plating as practiced now and the criterions for advancement at a higher level.


The purpose is to offer reader’s deeper information to achieve consistent deposit physical characteristics and the functional properties of electroless nickel plating.


We will go over the functions of a reducing agent, stabilizer (or catalytic inhibitor), and complexing agent related to thermodynamic property of an electrolyte and mixed potential theory.

 

electroless nickel

 

Temperature (energy), electrolyte flow (current density), solution volume, and the concentrations balance determine the effectiveness of the deposition mechanism and consistency of the deposit in the long term. Gibbs free energy and its relationship with volume, temperature, and energy influence the required concentrations and ratios of a reducing agent, stabilizer, and the complexing agent. An understanding of Gibbs ∆G value with the stability, instability and absolute stability of the electrolyte is imperative. Per mixed potential theory, the deposition reaction is a combination of cathodic and anodic partial reactions taking place at separate electrodes. Redox potentials of sodium hypophosphite, dimethyl amine borane (DMAB), and sodium borohydride play a significant role in a reduction step and the concentrations of stabilizers and complexing agents.

 

To confirm sustained deposit characteristics, electrolysis mechanism and control or management of by products release are important. Geometrical properties such as

  • tank and pump design,
  • electrolyte volume, flow rate and pattern,
  • process control of temperature, pH, metal ion concentration, reducing agent, stabilizer, and complexing agent

will ensure stability.

 

When Gibbs free energy and partial electrode reactions (anodic and cathodic) are within the optimum range plating rate, deposit composition (Ni and P or B weight percentages), and formation of uniform eutectic compounds of Ni-P or Ni-B will be effective and consistent.  Corrosion and tribology properties depend on the consistency of the deposit composition and phase formation. Out of range ∆G value can lead to unstable or absolute stability of the electrolyte. This will lead to either electrolyte decomposition or very low plating rate. The concentrations of a reducing agent, stabilizer, and the complexing agent determine the stability of the electrolyte. Plating rate depends on electrolyte temperature and pH.

 

Summary:

 

Other than simple control and changes done to temperature and chemical constituents, due diligence on the following variables considering the aforementioned concepts will advance the attained deposit characteristics:

  1. Optimize tank design, flow pattern and temperature stratification
  2. Aim for consistent deposit composition
  3. Observe deposit phase formation and aim for consistency if it is economically viable.

One can improve the bath life and the effectiveness of any deposit by giving more attention to the fundamental concepts and conscientiousness on the input variables. 


Though the examples cited in this paper are for electroless Ni, the recommendations apply to all electroless processes including Cu, Ag, Pd, Au, etc.

 

Readers - You are welcome to post questions, comments or thoughts. 

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Formulating cleaners such as alkaline soak cleaner and electro cleaner are imperative, though not as significant and lucrative as formulating plating baths containing primary and secondary brightener systems.


Unlike quite a few decades ago, current cleaner formulations contain methodically investigated inorganic and organic ingredients which are effective and comply with the environmental regulations. 


Different sectors and industries use solvent, alkaline soak and electro cleaners uniquely. Does the industrial practice catch up with scientific advancement? Good formulations are available! Did the market seize the opportunity? 

This blog highlights the value of a good cleaner, identifies a simple step in choosing a cleaner, and list a few potential ingredients and their properties. 

A carefully chosen cleaner can remove organic residues from the substrate thoroughly. Decrease the cleaning time and (or) improve cleanliness. Cut cost by increasing the cleaner life and minimize environmental impact.

 

Go over a material safety data sheet (MSDS) to select a cleaner, however, a proprietary chemical supplier could throw a curve ball. Section 2 of an MSDS must contain hazardous ingredient listed with Chemical Abstracts Service (CAS) registry number and the concentration. The reader can find specific CAS related information on this link. 

 

formulating electroplating cleaners

 

Before considering the inorganic and organic ingredients, process owner must know their processes and supply chain well. Knowing and considering the substrate(s) and their corrosion properties in most cases are explicit. Sometimes information is hidden and implicit on aspects such as oils, lubricants, and kinds of paraffin used. Application time, heat treatment (if applicable) and their intricate factors on some instances might not be clear. Source and purity of water, properties such as temporary and permanent hardness are vital for the makeup of cleaner solution and rinsing.

Alkalinity, buffer, water softening, chelation, and surface tension are a few important solution properties to observe. The process owner should assess the presence and concentrations of sodium hydroxide, carbonate, phosphate, silicate, amines, and surfactants and relate to the application (s). A surfactant can be anionic, non-ionic and (or) amphoteric. It changes the surface tension and wettability of the solution.

 

Briefly put, irrespective of reasons and justifications, focussing on cleaners and improving the cleaning performance is worth the effort and not a hard nut to crack using the listed suggestions. 

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Venkat Raja
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May 1, 2021
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