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Electroplating

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Quick
Electroplating, also known as electrogalvanizing or electrochemical plating, is an electrolytic process in which metal ions in an electrolyte solution are deposited onto a cathode workpart. Electroplating is the process of putting a metallic coating on a metal or other conducting surface by using an electric current. It is used to improve the appearance of materials, for protection against corrosion, and to make plates for printing.


Equations

(Eq2)     V = ECIt	Volume of metal plated
(Eq3)     d = V A	Plating thickness

Nomenclature

A	surface area of plated part, mm2 (in2)
C	plating constant, which depends on electrochemical equivalent and density, mm3/amp-s (in3/amp-min)
d	plate thickness, mm(in)
E	cathode efficiency
I	current, amps
t	time during which current is applied, s (min)
V	volume of metal plated, mm3 (in3)

Details

Electroplating may also be known as electrogalvanized zinc, electrochemical plating, or zinc plated. Electroplating is a process where zinc is applied by using a current of electricity. It is a thinner coating than hot dip galvanizing making it unsuitable for outdoor applications. Its advantages are its brightness and uniform color making it more aesthetically appealing. This process involves placing steel in a solution of zinc sulfate and water. A flow of electricity causes the zinc in the solution to form a thin layer on the surface of the steel. Electroplating is used chiefly to galvanize a continuous piece of steel.

Electroplating takes place in a cold, electrolytic bath rather than a molten zinc bath for hot dip galvanizing. Traditionally the plating/coatings are thinner than hot dipped and not suitable for extended outdoor exposure. In this process a layer of pure zinc is applied. The thickness of plating ranges from a few microns on cheap hardware components to 15 microns or more on good-quality fasteners. Technical and cost issues prevent the economical plating of components with heavier coatings.


The Process

The electroplating process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal. Electroplating is primarily used for depositing a layer of material to bestow a desired property like corrosion protection. The process used in electroplating is called electrodepositing; the process is analogous to a galvanic cell acting in reverse.

The part to be plated is the cathode of the circuit. In one technique, the anode is made of the metal to be plated on the part. Both components are immersed in a solution called an electrolyte containing one or more dissolved metal salts as well as other ions that permit the flow of electricity. A rectifier supplies a direct current to the anode, oxidizing the metal molecules that comprise it and allowing them to dissolve in the solution.

At the cathode, the dissolved metal ions in the electrolyte solution are reduced at the interface between the solution and the cathode, such that they "plate out" onto the cathode. The rate at which the anode is dissolved is equal to the rate at which the cathode is plated, vis-a-vis the current flowing through the circuit. In this manner, the ions in the electrolyte bath are continuously replenished by the anode.

Electroplating is an electrolytic process in which metal ions in an electrolyte solution are deposited onto a cathode workpart. The anode is generally made of the metal being plated and thus serves as the source of the plate metal. Direct current from an external power supply is passed between the anode and the cathode. The electrolyte is an aqueous solution of acids, bases, or salts; it conducts electric current by the movement of plate metal ions in solution. For optimum results, parts must be chemically cleaned just prior to electroplating.



The part to be plated is thoroughly cleaned of grease and dirt by dipping it in acid and alkaline cleaning solutions. It is then put in a solution of the metal with which it is to be coated. The metal exists in the form of positive ions (atoms that have lost one or more electrons). The article is connected to the cathode (negative end of a source of electricity). The anode (positive electric terminal) is connected to another conductor which is also dipped into the solution. The electric current acts on the metallic ions in the solution. The ions are attracted to the cathode, and the coating is deposited on the article's metal surface. If the metal in the solution and the metal of the positive terminal are the same, the electricity may remove metal from the terminal to replace metal taken from the solution.

Zinc electroplating is a method of depositing zinc metal on the surface of another metal, such as steel, by immersing the fasteners in an appropriate plating bath and applying electrical current. Zinc travels through the electrolytic bath from the zinc anodes and attaches to the surface of the fastener. The thickness of zinc plating depends on the time spent in the plating bath, the amount of current, and the chemical composition of the bath.

Zinc plating involves the electrolytic application of zinc by immersing clean steel parts in a zinc salt solution and applying an electric current. This process applies a layer of pure zinc that ranges from a few microns on cheap hardware components to 15 microns or more on good quality fasteners. Technical and cost issues prevent the economical plating of components with heavier coatings.

In-line galvanised coatings are applied during the manufacturing process of a hollow or open section, with a cleaned steel section exiting the mill and passing into the galvanizing bath. This process applies a coating of zinc to the surface that can be controlled in thickness. This coating is usually measured as coating mass in grams per square metre and ranges from a minimum of about 100 g/m2 upwards, with an average around 175 g/m2.

The thickness of the layer deposited on the article depends on the strength of the electric current, the concentration of metallic ions, and the length of time the article has been in the solution. The terms triple-plated and quadruple-plated indicate various thicknesses of plating, not separate layers deposited on the surface.

Ornamental and protective platings are very thin, usually from 1/1,000 to 2/1,000 of an inch (0.03 to 0.05 millimeter) thick. For plating gold, silver, copper, zinc, and cadmium, cyanide solutions of the same metals are often used. Copper and zinc may also be plated by acid-sulfate solutions. Chromium is plated with a chromic-acid solution and nickel is plated with nickel sulfate. Other metals plated for commercial use include platinum, lead, and tin. Alloys of two or more metals may be deposited by using a solution of salts of the metals that make up the alloy. Examples of alloys used for plating are brass, black nickel, lead-tin, and bronze.

Electroplating is also used to reproduce medals or other objects in a process called electroforming. This process was formerly known as galvanoplasty. One kind of electroforming, called electrotyping, is the reproduction of type forms and engravings for the printing industry.

Zinc metal has a number of characteristics that make it well-suited for use as a coating for protecting iron and steel products from corrosion. Its excellent corrosion resistance in most environments accounts for its successful use as a protective coating on a variety of products and in many exposure conditions. The excellent field performance of zinc coatings results from their ability to form dense, adherent corrosion product films and a rate of corrosion considerably below that of ferrous materials, some 10 to 100 times slower, depending upon the environment. While a fresh zinc surface is quite reactive when exposed to the atmosphere, a thin film of corrosion products develops rapidly, greatly reducing the rate of further corrosion.

High-strength metals, unless otherwise specified, including high-strength steels having a tensile strength greater than 247 ksi or hardness = RC 46 are not electroplated.
Stress relieving of all parts with ultimate tensile strength = 145,000 psi at minimum 375°F for three hours or more is recommended.

If these facts are considered and adhered to in specifying and ensuring that the vendor complies with the process and follows with the test results, the desired level of corrosion protection can be achieved.

More and more products are being introduced that are galvanised by high-speed, in line galvanizing technology. This allows a thin zinc coating to be applied to the steel at low cost. These thin zinc coatings are frequently coated with clear polymer topcoats to enhance their storage characteristics and in some cases, claims have been made that the addition of these polymer topcoats significantly improves the durability of the coating compared to a conventional galvanised coating. The addition of organic coatings to zinc plated parts is also a common technique that manufacturers claim improves the corrosion resistance of their products.


Formulae

Electrochemical plating is based on Faraday's two physical laws. The laws state:

1. The mass of a substance liberated in electrolysis is proportional to the quantity of electricity passed through the cell; and

2. The mass of the material liberated is proportional to its electrochemical equivalent (ratio of atomic weight to valence).

The effects can be summarized in the equation:

(Eq1)

V = CIt

The product It (current × time) is the electrical charge passed in the cell, and the value of C indicates the amount of plating material deposited onto the cathodic workpart per electrical charge.

For most plating metals, not all of the electrical energy in the process is used for deposition; some energy may be consumed in other reactions, such as the liberation of hydrogen at the cathode. This reduces the amount of metal plated. The actual amount of metal deposited on the cathode (workpart) divided by the theoretical amount given by Eq1 is called the cathode efficiency. Taking the cathode efficiency into account, a more accurate equation for determining the volume of metal plated is:

(Eq2)

V = ECIt

The average plating thickness can be determined from the following:

(Eq3)

d = V A

Accelerated Weathering Testing

Accelerated weathering testing of coatings has traditionally been done in salt spray cabinets. This testing technique has been largely discredited with respect to metallic coatings as it does not reflect the way metallic coatings weather in atmospheric exposure conditions where the development of stable oxide films gives these coatings there excellent anti-corrosion performance. The addition of polymer topcoats to metallic coatings will significantly improve their apparent performance in salt spray tests but field performance will not necessarily reflect this.

The results of accelerated corrosion tests indicate that the expected life of the continuously galvanised and lacquer coated samples will not be essentially different from the commercially continuously galvanised sheet material. Test results demonstrate that the expected life exhibited by the standard hot-dip galvanised panels (zinc coating thickness approx. 100 microns) can be considered to be significantly superior to the continuous galvanised/lacquer samples. The lacquer coating appears not to be fully effective in inhibiting the onset of corrosion under damp conditions due to porosity.

It is well known that the zinc/iron alloy layers of standard hot-dip galvanised coatings are hard in nature (in excess of 200HV - often harder than the base steel itself). Conventional hot-dip galvanised coatings, consisting of alloy layers with a soft zinc outer layer, therefore provide in essence a buffer stop coating which withstands knocks and abrasion. The soft nature of continuous galvanised lacquer coating (75 HV) coupled with the low coating thickness indicates that these coatings will not have the same ability to withstand rough handling compared to conventional hot-dip galvanised items."


Poor Performance from Plated Coatings

Zinc plated coatings are not suitable for exterior exposure applications. Zinc plated bolts and hardware fittings such as gate hinges will not provide adequate protection from corrosion, and will rarely last more than 12 months in exterior exposures in most urban coastal environments.

Zinc plating has been used in industrial coating applications from time to time, with very poor results. Industrial Galvanizers joint venture galvanizing operations in Bakasi, Indonesia, PT Bukit Terang Paksi Galvanizing (BTG), was commissioned in March 1998 to reprocess a large tonnage (approx. 400 tonnes) of cable trays that had been electroplated. The zinc electroplated coating had failed prior to delivery to the project resulting in the rejection of the entire consignment.

The client requested an extra-heavy hot-dip galvanised coating to replace the zinc plating, and BTG was able to apply a 100 micron coating to the 3 mm thick cable tray sections - this is around 50% above the required minimum standard for hot dip galvanised coatings applied to steel of this thickness of and over 10 times the thickness of the zinc plating.

Zinc plated products have an attractive appearance when new as the zinc coating is bright and smooth, where a hot dip galvanised coating has a duller and less smooth surface. There is typically about 10 times as much zinc applied to small parts in the hot dip galvanizing process as with zinc plating. A bright, shiny smooth zinc finish on builders hardware (bolts, nuts, hinges, gate latches, post shoes) indicates a plated coating that will not provide adequate corrosion resistance and will rarely provide more than 12 months protection in most of the coastal population centres.


ASTM B633 - 07 Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel.
This specification establishes the requirements for electro-deposited zinc coatings applied to iron or steel articles for corrosion-protection purposes. The coating is provided in four standard thickness classes in the as-plated condition or with one of three types of supplementary finishes. The surface of the article is cleaned as pre-plating basis metal, pre- and post-coating treatment is given to reduce the risk of hydrogen embrittlement. Coating is sampled, prepared, tested and conform accordingly to this specification the general criteria is the appearance (luster and workmanship), thickness, adhesion, corrosion resistance, and hydrogen embrittlement.

Electroplating is a process by which a coating of zinc is deposited on the steel by electrolysis from a bath of zinc salts. A rating of SC3 provides a minimum zinc coating thickness of .5 mils (excluding hardware, which is SC1 = .2 mils). When exposed to air and moisture, zinc forms a tough, adherent, protective film consisting of a mixture of zinc oxides, hydroxides, and carbonates. This film is in itself a barrier coating which slows subsequent corrosive attack on the zinc. This coating is usually recommended for indoor use in relatively dry areas, as it provides ninety-six hours protection in salt spray testing per ASTM B117.


Benefits of Zinc Electroplating: Zinc Electroplating provides corrosion resistance to the steel fastener by acting as a barrier and sacrificial coating. Zinc is more electrochemically reactive than steel, so when exposed to a corrosive environment, the zinc plating corrodes sacrificially, delaying rust formation on the fastener even after portions of bare steel are exposed.

Specifications and Performance: ASTM F1941, the “Standard Specification for Electrodeposited Coatings on Threaded Fasteners (Unified Inch Screw Threads (UN/UNR))”; and ASTM F1941M, “Standard Specification for Electrodeposited Coatings on Threaded Fasteners [Metric]” are the preferred standards to use when specifying zinc electroplating on fasteners. ASTM F1941 and F1941M also cover Cadmium, Zinc-Iron, Zinc-Nickel and Zinc-Cobalt electrodeposited coatings not covered in B633. The standards include information on zinc thickness, chromate coatings, salt spray performance and other quality requirements.

Corrosion Protection: When zinc corrodes, it develops a white powdery product on the surface, analogous to and just prior to the appearance of red rust on steel. To delay the appearance of this “white rust” on zinc plating, a chromate surface treatment is applied. This chromate film seals the surface to increase the corrosion protection of the zinc. The chromate coating can provide a range of appearances, as detailed in the tables above, with generally increasing corrosion protection from clear to black. The corrosion protection of zinc/chromate plating can be measured by salt spray hours. Salt Spray corrosion testing (per ASTM B117) is done in a controlled test chamber at 95°F with continuous exposure to an atomized fog of salt solution. Salt spray test results provide a method of comparison testing of coatings in a controlled environment, but may not directly predict corrosion protection in real world conditions. Consult the standards for specific minimum salt spray testing requirements for the finish thickness and supplementary chromate conversion coating specified or contact Nucor Technical Support Personnel for more information.


Hydrogen Embrittlement
During the acid cleaning and in the electroplating process, atomic hydrogen can be absorbed through the surface of the fastener. The electroplated coating traps the hydrogen inside the fasteners unless they are baked soon after plating to drive the hydrogen out. If the hydrogen remains in the steel, it can migrate to areas of high stress and cause small microcracks, which rapidly enlarge under load causing a brittle failure. Both standards recommend hydrogen embrittlement relief baking for fasteners above HRC 40 maximum hardness levels. We know high strength, high hardness parts are more susceptible to hydrogen embrittlement, therefore, Nucor Fastener bakes all Grade 8 cap screws and metric property class 10.9 electroplated fasteners to reduce the chance of hydrogen embrittlement from occurring, before reaching hardness levels requiring baking by the standards.


Electroplating for Fasteners
Some companies offer a 0.0002” minimum thickness of zinc (Fe/Zn 5) on electroplated fasteners. While a heavier zinc plating thickness can provide improved corrosion protection, potential interference with thread fit-ups can result from the build-up of plating on the threads. The geometry of a 60° thread results in a change to the pitch diameter equal to four to six times the plating thickness due to irregularities in plating uniformity. Fine pitch and smaller diameter fasteners tend to be less forgiving than coarse pitch and larger diameter products, so please consult Nucor Technical Support Personnel before specifying plating thicknesses greater than .0003”, to avoid needing special thread tolerances during manufacturing to allow for the increased thickness of the plating.


Commercial Chromate Conversion Coatings: Clear, blue-bright, designation A or B trivalent chromates are normally used for Grade 5 or metric property class 8.8 fasteners. Commercial Grade 8 and metric property class 10.9 fasteners are usually supplied with yellow chromate. Our standard yellow chromate is hexavalent free and designated CT. Other colored chromate conversion coatings can be special ordered to suit your application requirements.
The following tables denote thickness and chromate coatings for ASTM F1941 and F1941M (refer to actual ASTM document to confirm):

Coating Designation	Thickness μm (minimum)	Thickness Inch (minimum)
Fe/Zn 12	12	0.0005
Fe/Zn 8	8	0.0003
Fe/Zn 5	5	0.0002
Fe/Zn 3	3	0.0001

Designation	Type	Typical Appearance
A	Clear	Transparent colorless with slight iridescence
B	Blue-Bright	Transparent with a bluish tinge and slight iridescence
C	Yellow	Yellow iridescent
D	Opaque	Olive green, shading to brown or bronze
E	Black	Black with slight iridescence
F	Organic	Any type of the above plus organic topcoat