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PLATING/BAKING

PLATING METALS   Zinc   Cadmium   Aluminum

PLATING THICKNESS

LIFE EXPECTANCY

PLATING DISTRIBUTION

CHEMICAL CONVERSION COATINGS

OTHER PLATINGS AND COATINGS

 

Plating/Baking

Plating

Description

Electro-Zinc & Clear

Zinc is the most popular of all commercial platings because it is relatively economical and offers good corrosion resistance in environments not subject to excessive moisture.  Commercial zinc plating has a standard minimum thickness of 0.00015 inches.  However, Class 2A thread allowances in sizes No. 8 and smaller may not accommodate this thickness.  To avoid any reduction in the strength properties of these screws, a thinner coating may be acceptable.  A clear or blueish chromate finish is applied on top of the zinc to provide additional protection against white oxidation spots which can form due to moisture.  Electroplating is the most common way of applying zinc coating to fasteners.  It is recommended by certain industry experts that case-hardened parts which are electro-plated should be baked after plating to minimize the risk of hydrogen embrittlement (see below).

Electro-Zinc & Yellow

Commercial zinc-yellow plating has a standard minimum thickness of 0.00020 inches.  However, Class 2A thread allowances in sizes No. 8 and smaller may not accommodate this thickness.  To avoid any reduction in the strength properties of these screws, a thinner coating may by acceptable.  Yellow chromate offers a greater degree of protection from white corrosion than does clear chromate.  Electroplating is the most common way of applying zinc coatings to fasteners.

Electro-Zinc & Wax

A wax lubricant is added to the zinc coatings of certain fasteners to improve the ease of assembly.  This is the standard plating for thread rolling screws including the PlastiteR and TaptiteR II, as well as two-way reversible center-lock nuts.  Case-hardened parts are still recommended to be baked after plating (see below).

Mechanical Zinc & Clear

Mechanically applying zinc to fasteners reduces the risk of hydrogen embrittlement forming within the parts.  This minimizes the need for the precautionary practice of baking the parts soon after plating.  A clear or bluish chromate finish is applied on top of the zinc to provide additional protection against white oxidation spots which can form due to moisture.  It is common for lock washers made from spring steel to be plated this way to avoid brittleness after baking.

Mechanical Zinc & Yellow

This finish is identical to mechanical zinc but with a yellow chromate finish.  This is the standard plating for high-alloy split lock washers and for tooth lock washers used with zinc yellow machine screws.

Electro-Zinc & Clear for Sockets

Socket cap screws can receive a zinc plating of 0.0002 inches thickness.  A clear chromate finish is applied on top of the zinc to provide additional protection agains white corrosion.  The manufacturer must be told prior to the thread rolling process that the parts are to be plated.  The plated parts are then baked at 3750F for 24 hours within 1 hour of plating, then subjected to a 72-hour stress test.

Black Phosphate

This is the standard finish for most drywall screws, particle board screws and retaining rings.  It can have either a dull or bright appearance.  No additional oil treatment is added.

Black Phosphate & Oil

The most common standard coating of black phosphate and oil is 1100 mg per sq./ft. minimum.  The oil serves as a rust inhibitor and a lubricant.  Some fasteners with this plating are required to pass a salt-spray test, the duration and cost of which must be agreed upon between buyer and seller prior to the sale.  Floorboard screws, frame bolts, Grade-GT locknuts and spring nuts are usually supplied with a black phosphate and oil finish.

Nickel

Nickel has more of a silver color to it than zinc and has similar corrosion resistant characteristics.  It is the standard finish of cap nuts and countersunk finishing washers.

Cadmium & Wax

Cadmium plating results in a smoother surface and greater resistance to white oxidation spots than zinc plating.  However, cadmium is a much more toxic metal than zinc, which makes the plating process more difficult and costly.  The standard most commercial platers use when applying cadmium is a minimum thickness of .0002 inches.  A supplemental wax coating is often added as a lubricant when cadmium is used on prevailing torque lock nuts.

Hot-Dip Glavanized

Hot dip galvanizing is generally the most effective way to apply a sufficient thickness of zinc to threaded fasteners for the zinc to serve as a corrosion protectant in harsh environments.  During the galvanizing process, steel reacts with molten zinc, forming layers of zinc-iron alloy layers which are metallurgically bonded to the steel surface.  This hard barrier has a low corrosion rate and resists mechanical damage.   Bolts and nuts 3/8 inch diameter and smaller shall have a zinc coating with an average thickness of 0.0017 in. with no individual bolt having a coating of less than 0.0014 inc.  Bolts and nuts over 3/8 inches diameter, and all sizes if washers shall have a zinc coating with an average thickness of 0.0021 in. with no individual bolt having a coating of less than 0.0017 in.

Dacrotized

Dacrotizing is a pollution-free ceramic coating for fasteners used with treated lumber.  The coating offers corrosion protection comparable to hot-dip galvanizing without discoloring the wood.  Screws with a proper dacrotized coating can typically withstand a 500-hour salt-spray test.  Dacrotizing minimizes greatly the risk of hydrogen embrittlement so baking the parts is not required after the finish is applied.

 

Baking of Case Hardened Parts

Electroplated screws which are case hardened should be baked for a minimum of 4 hours within the temperature range of 375-4500F no later than 4 hours after the plating operation.  However, this process does not guarantee that hydrogen embrittlement will not still be present after baking or that it will not occur at a later date while in service.  Specialized testing or a substitute part may be required, depending on the application.  This heat treatment practice is recommended for tapping screws, drywall screws, SEMS screws, clinch nuts and clinch studs.

 

 

PLATINGS AND COATINGS

 Approximately 90 percent of all carbon steel fasteners are plated, coated, or furnished with some other type of supplementary finish.  Although the principal reason is to protect against corrosion, such treatments also enhance appearance, control installation torqu-tension relationships, minimize thread seizing, and assist product identification.

 PLATING

Platings are the deposition of an adherent metal onto the surface of a base metal.  For commercial fasteners (non-aerospace), practically all deposition is accomplished by electroplating, hot-dipping or mechanically.  Other processes – such as, spraying molten metal, vacuum metalizing, chemical vapor deposition, ion plating and enameling, - are special-purpose and economically impractical for commercial fasteners. 

Electroplating is carried out in a water-based solution containing a chemical compound of the metal to be deposited.  The parts to be plated are immersed in this bath and an electrical current passed through which causes the plating metal to precipitate out and be deposited onto the parts. 

Hot-dip galvanizing is accomplished by submerging carbon steel parts for a few minutes in a bath of molten zinc at about 9600F (5100C).  The result is an iron-zinc alloy at the steel surface, gradually changing to pure zinc at the part’s exterior.  Hot-dip aluminizing is a similar process with aluminum substituted for zinc. 

In the mechanical deposition process, a metal coating is applied by impacting particles of the plating metal against the parts and cold welding a coating to their surface. 

PLATING METALS

Zinc is by far the most widely used plating metal followed in popularity by cadmium and by aluminum, which has modest use.  Copper, tin, nickel, chromium, lead and silver are used to a lesser degree – all for special reasons. 

Zinc, cadmium and aluminum are favored as plating metals because in the Galvanic Series (Table 1, page B-xx) they are less noble than carbon steel, stainless steel, and most other nonferrous metals used in fastener applications.  In an electrochemical reaction, the plating metal corrodes, and through its sacrifice, the base metal remains protected.  Only after the plating metal has been significantly lost to corrosion does corrosion of the base metal begin.  Other plating metals are more noble than carbon steel.  When the coating is breached, the base metal comes under immediate attack. 

Zinc

Zinc is popular as a fastener coating because it is the least expensive, can be applied in a broad range of thicknesses, has good-to-excellent corrosion resistance, and is relatively non-toxic. 

Zinc plated fasteners require more tightening torque to develop equivalent preloads in threaded fasteners.  Also, zinc coatings without some supplementary protection develop a dull white corrosion product on their surface which is nicknamed “white rust”.  Because of its unsightly appearance, most zinc plated fasteners are given a chromate treatment, which is a chemical conversion process to cover the zinc surface with a hard non-porous film.  This added coating effectively seals the surface, protects it against early tarnishing and reinforces the fastener’s resistance to corrosion attack.  Chromate coatings are available clear, iridescent, or in a variety of colors. 

Cadmium

Cadmium is extremely toxic and while investigations have established that cadmium plated fasteners are not a high risk hazard, except when in contact with food or beverages, use of cadmium as a plating metal is phasing out.

Cadmium’s environmental protection threat related primarily to the plating process and the subsequent handling of plating effluents.  The strictness of government regulations that define effluent disposal controls has discouraged many job platers from continuing their cadmium plating services. 

The high cost of engineering systems to remove cadmium from effluents, coupled with the declining availability of subcontracting plating sources, has added greatly to cadmium-plated fastener costs.  Consequently, extensive research effort has been under way during the past few years to discover alternative plating and coatings with equivalent performance properties at a reasonable costs.  Until these efforts are successful, the need for cadmium as a fastener plating metal will continue. 

Compared to zinc, cadmium provides superior corrosion protection in marine and other aggressive corrosion atmospheres.  It can be soldered, it doesn’t produce “white rust”, and it has a smoother appearance with greater luster.  Most importantly, it has lubricity.  Lubricity lowers frictional coefficients and narrows the scatter range of torque-tension relationships.  These properties are particularly important in applications using prevailing-torque nuts. 

Aluminum

Aluminum as an alternative to zinc in the hot-dip deposition process has two distinct advantages.  It provides superior corrosion resistance in severe industrial and marine exposures, and it withstands service temperatures to 9000F (4820C) without discoloration or scaling.  Zinc, at temperatures above 5000F (2600C), tends to alloy or peel.  The main disadvantages of aluminum are higher cost and limited sources of plating facilities with equipment and capability.   



PLATING THICKNESS

As a general rule, fastener service life, in a corrosive atmosphere, is proportional to the thickness of its plating.  The thicker the plating the longer it will survive. 

Electroplated fasteners have plating thicknesses ranging from a “flash” coating of insignificant thickness, to a “commercial” thickness of 0.00015 in., through to 0.0005 in.  Thicker electroplatings are possible but, from an economics viewpoint, quite impractical. 

Hot-dip galvanizing produces much thicker coatings, which in engineering standards are expressed in terms of ounces of plating metal deposited per square foot of plated surface.  Standard hot-dip galvanized fasteners have an average plating thickness of 1.25 oz/sq ft (0.0021 in. in thickness).  Heavier coatings to 2.00 oz/sq ft (0.0034 in.) are feasible, but such coatings may necessitate adjustments in mating thread fits to a degree that the fastener’s strength properties may be adversely affected. 

Mechanically plated coating thicknesses are available through the full range offered by either electroplating or hot-dip galvanizing.   



LIFE EXPECTANCY

For several years, the relative corrosion combating performance of zinc electroplated and hot-dip galvanized fasteners compared with mechanically plated fasteners has been under investigation.  A range of exposure environments indicated equivalent performance for fasteners having the same plating thickness. 

Useful service life expectancies of zinc plated fasteners in various environments are: 

-                     zinc plated with chromate treatment, 0.00015 in. plating thickness: up to 20 year indoors, about 4 years in a rural atmosphere, 2 years in coastal locations, and less than 1 year in heavily polluted industrial atmosphere.

-                     hot-dip galvanized with an average thickness of 1.25 oz/sq ft: over 40 years in a rural atmosphere, 25 to 30 years in coastal locations, and 5 years or longer in heavily polluted industrial atmospheres. 

Survivability is almost a direct function of plating thickness.  However, plating is expensive.  Costs – and attendant problems increase with increasing plating thickness.  Consequently, the prudent engineer is advised to specify only that thickness of plating required to satisfy the application.   



PLATING DISTRIBUTION

The build up of plating on fastener surfaces occurs differently with each of the principal deposition methods. 

Electroplating deposits the plating metal unevenly with exterior edges and corners receiving thicker coatings.  In the fastener’s threaded section, the thickest plating is located at the thread crests and becomes progressively thinner on the thread flanks, with the thinnest deposits in the thread roots.  With hot-dip galvanizing, it is just the opposite, with thicker coatings being deposited at interior corners and in the thread roots.  Because of clogging of thread roots is difficult to control, it is usually impractical to hot-dip galvanize fasteners of nominal sizes smaller than 3/8 in.  Mechanical plating tends to deposit the plating metal similarly to hot-dip galvanizing but more smoothly and considerably more uniform in thickness over the entire surface.   



PLATING PROBLEMS

Two serious problems are directly attributable to plating – thread assembly and hydrogen embrittlement. 

1)Thread Assemblability 

The addition of a plating to its surface increases the size of the fastener.  When the plating thickness exceeds certain limits – generally one-fourth of the specified allowance for the class of thread fit – there is a distinct possibility the internally and externally threaded parts will not assemble.  When interference between mating threads is likely, some accommodation must be made prior to plating.  Recommended practices for adjusting thread fits of plated fasteners are discussed in the earlier discussion of screw threads. 

2)Hydrogen Embrittlement 

High strength, high hardness carbon steel fasteners have susceptibility to embrittlement, which evidences itself in various mechanisms.  Plated and coated fasteners, especially those that are electroplated, are vulnerable to the one known as hydrogen embrittlement. 

Hydrogen embrittlement causes fastener failures, the actual fracture of the fastener into two separate pieces.  The failure occurs in service (i.e., after the fastener has been installed and tightened in its application), it usually happens within hours, it’s sudden, there’s no advance warning or visible indication of imminence. 

During fastener manufacturing and processing, particularly during acid pickling and alkaline cleaning before plating and then electroplating, atomic hydrogen is absorbed into the fastener’s surface.  The deposited metallic coating then entraps the hydrogen.  When the fastener is tightened, the hydrogen migrates towards points of highest stress concentration.  Pressure builds until the strength of the base metal is exceeded and minute surface ruptures occur.  Hydrogen is exceptionally mobile and quickly penetrates into the newly formed cracks.

This pressure-rupture-penetration cycle continues until the fastener fractures, usually within hours of first stress application.

To neutralize the threat of hydrogen embrittlement, fasteners are thermally baked early as possible after plating.  Time delays seriously jeopardize the effectiveness and benefits of the baking.  The purpose of the baking – generally at 3750F to 4000F for 3 to 24 hours dependent on plating type and thickness – is to drive out the hydrogen by bleeding it through the plating.  Baking is always done prior to chromating or application of any other supplementary coating. 

In broad terms, fasteners with hardnesses less than Rockwell C32 have a low risk of embrittlement.  Those with higher harnesses should always be suspect. 

Because mechanical plating is nonelectrolytic, the hydrogen embrittlement threat is virtually eliminated.  In fact, parts with hardnesses up to Rockwell C55, mechanically plated without post baking, have performed satisfactorily without evidence of embrittlement. 

Hot-dip galvanized fasteners are rarely subject to hydrogen embrittlement.  The primary reason is that engineering standards strongly discourage the hot-dip galvanizing of fasteners with hardnesses higher that Rockwell C35 – i.e., fasteners stronger than SAE Grade 5, ASTM A449, and ASTM A325.  The reason is that galvanized fasteners of higher strengths have a susceptibility to another embrittlement mechanism known as stress corrosion or stress corrosion cracking.  



CHEMICAL CONVERSION COATINGS

Chemical conversion coatings are adherent films chemically formed on a metal’s surface when immersed in a bath of appropriate solution.  Chemical conversion coatings popularly specified for fasteners are chromate treatments on electroplated parts (mentioned earlier) and zinc and manganese phosphate coatings. 

Zinc phosphate coatings, or manganese phosphate often used as permitted alternative, are extensively specified for fasteners, particularly those intended for use in automotive applications.  The phosphate base provides an excellent substrate for painting and for retention of oils, waxes or other organic lubricating materials.  Most zinc phosphated fasteners are additional oiled to enhance corrosion resistance and to help control torque-tension relationships.  Dry zinc phosphate is often used as a base for non-metallic locking elements on externally threaded fasteners. 

The corrosion resistance of zinc phosphated and oiled fasteners is reasonably good in non-aggressive atmospheres.  Significant improvements are possible through secondary treatments, such as painting. 

Although phosphate-coated high strength fasteners are not immune from hydrogen embrittlement, susceptibility and frequency of occurrence are less than similar fasteners which have been electroplated.  Unlike deposited plating, phosphate coatings do not significantly increase fastener size.  Class 2A/2B screw thread fits are usually adequate to permit assembly.  Rarely is it necessary to make adjustments in thread size limits prior to coating. 

One of the more important considerations when evaluating the possible us of a phosphate coated fastener is cost.  Phosphate and oiled coatings are less expensive than zinc electroplating with chromate treatment.  However, the packaging and handling of phosphate and oiled coated fasteners has a degree of sensitivity because the oil may be removed by absorption into the packing materials. 



OTHER PLATINGS AND COATINGS

This discussion has concentrated on the basic platings and coatings applicable to carbon steel fasteners. 

There are a great many other plating and coatings available – mostly special-purpose and each with some unique performance enhancing feature.  There are those which greatly lengthen service life in corrosive atmospheres; those which provide protection at elevated temperatures; others which drastically lower frictional coefficients and reduce torque-tension relationship scatter; some give wear resistance; others help overcome galling and seizing between mating threads.  Some are multi-purpose – for example, the family of fluorocarbons, which enhance corrosion protection while simultaneously reducing and more closely controlling frictional variations.  Most of the special platings and coatings are proprietary, using patented processes and trademarked materials.  All are more expensive than standard electroplating and phosphate coating. 

Plating and coating technology is dynamic with new developments and improvements being introduced with remarkable frequency.