THE ABC’S OF METAL FINISHING

What is “Faraday’s Law”?

When discussing electroplating and understanding how the technology works there is often a reference to Faraday’s Law of Electrolysis.

“Faraday’s Law of Electrolysis” says that one Faraday of electricity (96,487 ampere-seconds) will electro-deposit one gram molecular weight of metal on the cathode. Therefore, knowing the surface area of the part, density of the plated metal, efficiency of the plating bath and desired average thickness of the plating, you can calculate the required ampere seconds to deposit this thickness very accurately.

Sheffield Platers utilizes two types of metal finishing, Electrolytic plating, which utilizes “Faradays Law” and Electroless plating. Below are two brief descriptions of Electrolytic and Electroless plating.

Faraday's Law

Electro Plating

In electroplating, an ionic metal is supplied with electrons to form a non-ionic coating on a substrate. A common system involves a chemical solution with the ionic form of the metal, an anode (positively charged) which may consist of the metal being plated (a soluble anode) or an insoluble anode (usually carbon, platinum, titanium, lead, or steel), and finally, a cathode (negatively charged) where electrons are supplied to produce a film of non-ionic metal.

Electroless Plating

Electroless plating, also known as chemical or auto-catalytic plating, is a non-galvanic plating method that involves several simultaneous reactions in an aqueous solution, which occur without the use of external electrical power. The reaction is accomplished when hydrogen is released by a reducing agent, normally sodium hypophosphite (Note: the hydrogen leaves as a hydride ion), and oxidized, thus producing a negative charge on the surface of the part. The most common electroless plating method is electroless nickel plating, although silver, gold and copper layers can also be applied in this manner, as in the technique of Angel gilding.

The chart below shows a comparison of Electrolytic and Electroless Nickel:

Electrolytic Nickel

Non-uniform thickness of deposit. Thicknesses dependent on current density at different locations on part surface. Fixturing may be required if differences in thickness cannot be tolerated. Thickness controlled by current density and time in tank.


Deposit is essentially pure nickel.

Deposit has a micro-hardness in the range of 250–450 VHN.

Deposit tends not to have lubricity and non-galling properties.

Thin deposits are porous.

Causes hydrogen absorption, a problem with high-strength steels.

Electroless Nickel

Uniform thickness of deposits even on parts with a complex shape as long as the part is properly prepared. No current density issues. Thickness usually controlled by time in the tank.


Deposit is a nickel/phosphorus alloy and does not have the same physical properties as electrolytic nickel.

Deposit has micro-hardness of 340–600 VHN. If deposit is heat treated, hardness can be increased to 1000–1150 VHN range.

Deposit has some inherent lubricity and non-galling properties.

Deposits are less porous and give better barrier corrosion protection to ferrous materials.

Causes reduced hydrogen absorption estimated to be less than 20% of that caused by electrolytic nickel.

An explanation of Electrolytic Plating:

Electroplating can be defined as the deposit of a very thin layer of metal “electrolytically” to a base metal to enhance or change its appearance. Electroplating is done in a liquid solution called an electrolyte, also known as a “plating bath”. The plating bath is a specially designed chemical bath that has the desired metal (i.e. silver, gold) dissolved as microscopic particles (positive charged ions) suspended in solution.

The plating bath solution serves as a conductive medium and utilizes a low DC voltage (direct current). The object that is to be plated is submerged into the plating bath and a low voltage DC current is applied to the bath. Generally located at the center of the plating bath, the object that is to be plated acts as a negatively charged cathode. The positively charged anodes that will complete the DC circuit are carefully positioned at the edges of the plating tank. A power source known as a rectifier is used to convert AC power to a carefully regulated low voltage DC current.

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The resulting circuit channels the electrons into a path from the rectifier to the cathode (object being plated), through the plating bath to the anode (positively charged) and back to the rectifier. Since electrical current flows from positive to negative, the positively charged ions at the anodes flow through the plating bath’s metal electrolyte toward the negatively charged cathode. This movement causes the metal ions in the bath to migrate toward extra electrons located at the cathode’s surface outer layer. By means of electrolysis, the metal ions are taken out of solution and are deposited as a thin layer onto the surface of the object.

This process is called electrodeposition. Theoretically, the thickness of the electroplated layer deposited on the object is determined by the time of plating, and the amount of available metal ions in the bath relative to current density. The longer the object remains in the DC activated plating bath, the thicker the electroplated layer will become. Typically, plated metal thicknesses can range from .10 microns for flash gold plating to 20 microns or more for heavy silver plated flatware.

The inherent shape and contour of the object can affect the thickness of the plated layer. Metal objects with sharp corners and edges will tend to have thicker plated deposits on the outside corners and thinner deposits in the recessed areas. This occurs because the DC current flows more densely around the outer edges of an object than the less accessible recessed areas. Items such as watches with sharp faceted corners are difficult to re-plate uniformly. This is due to the excessive buildup of plating deposits that occurs on the sharp corners and prominent surfaces of the watch case. Careful placement of the anodes and modification of the current density are necessary to offset this effect.

With rare exception, electroplating processes will not conceal preexisting surface blemishes such as scratches, dents, or pit. In fact, the plating process has a tendency to make most surface imperfections even more noticeable. It is therefore necessary to remove any undesirable surface marks prior to the plating process.

Since most typical plating thicknesses are so thin, surface treatments such as chemical etching, glass bead blasting and other applied textures are unaffected in surface character or dimensional depth by the electroplating process.

For decorative finishes, the metals typically used by the jewelry and decorative metals industry for electroplating are gold, silver, nickel, palladium, platinum, ruthenium and rhodium.

For engineering finishes, the most commonly plated metals are brass, cadmium, stainless steel, copper, gold, nickel, silver, tin, and zinc. These types of finishes are primarily concerned with functionality of the plated finish rather than surface beauty.

The generic sequence of operations required prior to the electroplating process would include the thorough cleaning of the object, stripping of old plating and surface contaminants, refurbishing and final polishing. The surface of the object is then cleaned and activated by a mild acid rinse prior to final plating. Depending on the metal object, sometimes a thin strike plate (i.e. nickel over stainless steel) is required as a pre-plate to allow the plating process to occur.

An explanation of Electroless Nickel Plating:

Electroless nickel (EN) plating is a chemical reduction process which depends upon the catalytic reduction process of nickel ions in an aqueous solution (containing a chemical reducing agent) and the subsequent deposition of nickel metal without the use of electrical energy. Due to its exceptional corrosion resistance and high hardness, the process finds wide application on items such as valves, pump parts etc., to enhance the life of components exposed to severe conditions of service ,particularly in the oil field and marine sector. With correct pretreatment sequence and accurate process control , good adhesion and excellent service performance can be obtained from EN deposited on a multitude of metallic and non-metallic substrates.

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In the EN plating process, the driving force for the reduction of nickel metal ions and their deposition is supplied by a chemical reducing agent in solution. This driving potential is essentially constant at all points of the surface of the component, provided the agitation is sufficient to ensure a uniform concentration of metal ions and reducing agents. Electroless deposits are therefore very uniform in thickness all over the part’s shape and size. This process offers distinct advantages when plating irregularly shaped objects, holes, recesses, internal surfaces, valves or threaded parts. Distinct advantages of EN plating are:

  • Uniformity of the deposits, even on complex shapes
  • Deposits are often less porous and thus provide better barrier corrosion protection to steel substrates, much superior to that of electroplated nickel and hard chrome
  • The deposits cause about 1/5th as much hydrogen absorption as electrolytic nickel and about 1/10th as much hard chrome
  • Deposits can be plated with zero or compressive stress
  • Deposits have inherent lubricity and non-galling characteristics, unlike electrolytic nickel
  • Deposits have good wettability for oils
  • In general low phosphorus and especially electroless nickel boron are considered solderable. Mid and high phosphorus EN’s are far worse for solderability
  • Deposits are much harder with as-plated micro-hardness of 450 – 600 VPN which can be increased to 1000-1100 VHN by a suitable heat-treatment
  • The versatility of electroless nickel plating is demonstrated by the wide range of coatings possible. The following are important types of coatings industrially available

Low Phosphorous (Hard):
A unique bath providing an as-plated deposit hardness of up to 60 Rockwell This bath provides a deposit nearly as hard as Hard Chrome, with the advantage of a uniform thickness inside complex configurations, as well as outside. The deposit is so uniform that grinding after plating is eliminated. Low Phosphorous Electroless Nickel offers excellent resistance to alkaline corrosive environments.

Medium Phosphorous (Bright High Speed):
This is a workhorse electroless nickel. It has proven itself over the years. Steel parts plated with Medium Phosphorous electroless nickel will in many cases perform like stainless steel. Electroless nickel will not build up on edges or ends, and it plates inside and out giving uniform total coverage. With heat treatment, medium phosphorous electroless nickel can be hardened from 45 Rockwell C to as high as 68 Rockwell C.

High Phosphorous:
This finish provides maximum corrosion resistance. High Phosphorous electroless nickel is standard in industries that require resistance to strongly acidic corrosive environments like oil drilling and coal mining. High Phosphorous electroless nickel has a low degree of solderability. It will remain solderable for only a brief period of time after plating. This makes it a desirable finish for electronics parts such as connector housings and semiconductor packaging.

Electroless Nickel/Teflon Composite:
Teflon adds to the already slick surface of the electroless nickel, yielding a very low friction surface. This product is a relatively new one. It consists of microscopic beads of Teflon co-deposited up to 20% with the electroless nickel. This finish can be the solution to sticking, galling or drag problems with moving parts, or heated seal surfaces. In some cases, liquid lubricants can be eliminated with the use of Nickel/Teflon plating.

Electroless Nickel on Zinc Die Cast:
Electroless nickel can be applied directly to zinc die cast without a copper layer. This has many applications where corrosion resistance and resistance to chipping and flaking is necessary. The selection of a specific grade of EN-plating is done in accordance with the nature of application, where a high hardness and low coefficient of friction is desired, low phosphorous EN is preferred (1-3%P). For general applications where a bright finish is required and the operating conditions are not very corrosive, medium phosphorous (6-8% P) EN is used e.g.. Computer printer rollers, machine components, plastic molding dies etc. When the conditions of use for an EN plated components are severely corrosive, a high phosphorous EN (12-13% P) is usually selected. The high phosphorous EN is amorphous in nature and is compressively stressed unlike the low and medium phosphorous EN which are Crystalline and tensile stressed. Proper process sequence and maintaining the correct operating parameters helps ensure a virtually non porous deposit of high phosphorous ENP which finds wide application in areas such as valve components, aerospace industry, oil & gas and chemical industries etc.

Physical Properties

  • Surface Hardness: As plated 48-50 RC. After Heat Treatment (400°C, 1 hr) 62-63 RC
  • Melting Point :- 890°C
  • Density :- 7.85-7.95 gm/cm qb.
  • Coefficient of friction :- EN Vs STEEL 0.3
  • Coefficient of Thermal Expansion :- 0.13 microns /°c

Specifications and Testing
ASTM specifications are generally followed in evaluating EN plated components some of the relevant tests are as follows:

  • Hardness: The Hardness may be determined on a Micro-hardness Tester using a 100 gm load as per ASST. B-578
  • Thickness: The microscopic examination of the cross section of an article is tested in accordance with ASTM B-478. The EN plating thickness will vary from 5-125 microns depending upon the service conditions.
  • Corrosion Resistance: A 5% neutral salt spray test is carried out as per ASTM B-117 to determine the corrosion resistance of plated items. This is most important test in evaluating EN plated samples. The Corrosive conditions to which EN Plated components are exposed, can be classified as mild, moderate and severe. The bath used for EN Plating, varies accordingly to obtain alloy compositions varying from low to high phosphorous content. Generally a high corrosion resistance requires a high phosphorous content (10-12% ) and the relevant test to evaluate the performance of plated samples is the 5% Neutral salt spray test in accordance with ASTM B-117. With a proper operating procedure, high phosphorous deposits will show no rust spots, even after 1000 hrs of salt spray exposure for a plating thickness of 40-50 microns.
  • Adhesion: Several tests such as Bend test, Quench test, Ring shear test etc. Are carried out to determine the adhesion of EN-plating to the base metal in accordance with ASTM-B-571.

Areas of Application
Due to its unique properties of excellent corrosion resistance, combined with a high wear resistance and uniformity of coating, EN finds extensive applications in a number of fields. Some of the prominent areas of application are:

  • Oil & Gas: Valve components, such as Balls, Gates, Plugs etc. And other components such as pumps, pipe fittings, packers, barrels etc.
  • Chemical Processing: Heat Exchangers, Filter Units, pump housing and impellers, mixing blades etc.
  • Plastics: Molds and dies for injecting and low and blow molding of plastics components, extruders, machine parts rollers etc.
  • Textile: Printing cylinders, machine parts, spinneret’s, threaded guides etc.
  • Automotive: Shock Absorbers, heat sinks, gears, cylinders, brake pistons etc.
  • Aviation & Aerospace: Satellite and rocket components, rams pistons, valve components etc.
  • Food & pharmaceutical: Capsule machinery dies, chocolates molds, food processing machinery components etc.

Solderability of E/N coatings
Most suppliers of E/N now recommend using low phos. for the best solderability, and longest shelf life. Standards ISO 4527, DIN 50966 and ONORM C 2550 (Austrian) reference this important property.

A paper submitted at the Electroless Nickel conference of 1989 held in Cincinnati Ohio, Titled “Solderability Parameters of Electroless Nickel Bearing Electronic Finishes” By Louis Kosarek of STB Systems, Inc. report that “An electroless nickel deposit which contains a concentration of phosphorus ranging from 0.1% to 3.0% is readily solderable on an “As-plated Basis” per Mil-Std 883c method 2003. The frequency of solderability tests which fail per Mil-Std 883c will increase as the phosphorus content of electroless nickel alloy increases from 3.0 to 7.0% phosphorus. A solderability test conducted per Mil-Std 883c method 2003 incorporating an as-plated surface finish containing phosphorus in excess of 7%, the components will consistently fail. The mode of failure is non-wetting of the surface.”

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