ZINC PLATING 101

 

“Zinc Plating – The Corrosion-Prevention Workhorse”

Whether you are walking through a hardware store, looking under the hood of your car, or mowing the grass, you will find zinc plating at work protecting steel from corrosion in the products we use every day.  Zinc plating has found wide acceptance as a surface finish throughout all consumer, industrial, and commercial products.  Although it is very common in our daily lives, few of us have paused to contemplate this important engineering finish, much less understand how it works.

 

Zinc is a bluish-white metal, which, if mechanically polished, or electrodeposited with appropriate brighteners, somewhat resembles chromium in appearance.  However, the reflectivity of the polished surface is soon lost in most atmospheres.8   This quick tarnishing and corroding, is the property that makes zinc plating work so well in providing  sacrificial” protection for steel.  To learn more about the sacrificial nature of zinc plating, please read the section on “Galvanic Series of Metals in Seawater” elsewhere in this report.

 

The relatively low cost, protective nature, and attractive appearance of zinc plating make it a popular coating for nuts, bolts, washers, metal stampings, and automotive components, fabricated parts for industrial applications, and also serves as an effective undercoat for paints.8  

 

Electrolytic zinc coatings are used to protect and improve the appearance of ferrous metals, (i.e. iron & steel) as a corrosion barrier, and then as a sacrificial coating.  The application of chromate conversion coatings over zinc plating, and post-plate sealers, give additional protection against corrosion particularly under high humidity and moisture conditions.  For additional information on Chromate Coatings and Post-Plate Sealers, please read, “Heal Thyself! – How Chromates on Zinc Plating work”, elsewhere in this report.

 

In dry air, a protective layer of oxide soon forms on an untreated zinc surface, and subsequent attack is slow.  In moist air, zinc hydroxide forms first on the surface, and is then converted to zinc carbonate.  If the surface has not been chromated, the carbonate takes the form of a bulky, loose layer, often described as white rust, or wet storage stain.8  In confined spaces, zinc is attacked by organic acid vapors emitted by woods, plastics, and various insulating materials 8. 

 

Commercially, zinc is deposited in thick nesses ranging from 0.0001”- 0.0005”, depending upon the intended application and the corrosion protection required, the majority of which is 0.0001”-0.0003”, commonly known as “Commercial Zinc”.  Commercial Zinc has a high coefficient of friction, low strength, moderate abrasion resistance, poor impact resistance, brittle at room temperature, but malleable at 212-302°F. 

 

To relieve the potential for Hydrogen Embrittlement in hardened steels electro-plated with Zinc, a baking procedure after the plating is required to remove, or diffuse the hydrogen throughout the basis metal, reducing the risk of embrittlement.  For more information on Hydrogen Embrittlement, please read the section titled, “You Crack Me Up!  Hydrogen Embrittlement is No Laughing Matter”, located elsewhere in this report.

 


“Heal Thyself! – How Hexavalent Chromates on Zinc Plating work”

 

Hexavalent Chromate coatings exhibit a phenomenon called “self healing” – the ability to protect metal on areas where some of the coating has been removed as the result of a scratch, or an abrasion.  Chromate conversion coatings on zinc plated steel are often used as a final finish to retard the formation of white or gray products of corrosion of the zinc during environmental exposure, and also prevents surface discoloration from fingerprints and perspiration as the result of handling.  From this consideration alone, application of a chromate coating is advisable.

 

Figure 1

 
In general, chromates are applied by immersing the zinc-plated parts in a solution containing dichromate, or chromic acid and an activator (usually, nitrate, sulfate, chloride, formate, or fluoride).  An oxidation-reduction reaction occurs on the metal surface with the formation of substrate metal ions and trivalent chromium ions.  An accompanying increase in the pH of the solution immediately adjacent to the metal surface results in the precipitation of a gelatinous film, comprised largely of chromic hydroxide, and in which soluble chromates are incorporated.  This freshly formed coating, after rinsing and drying, is rather soft and vulnerable to damage, but soon hardens in less than 48 hours.  In addition, the chromate coating itself also contributes some protection by presenting a barrier between the metal and the environment.  The protective value of a chromate finish increases with increasing thickness.  The final appearance of the chromate film depends on the base metal smoothness and the quality of the plated deposit.  The duration of protection provided by zinc coatings is a function of: coating thickness, exposure conditions, post plating treatments, and chromate post sealers.

 

Chromated Zinc:

·        Bright Zinc:  Single-dip bright 8-24 hours to white corrosion product   

·        Yellow (Iridescent) Zinc: A typical iridescent chromate coating prevents the appearance of white salts from corrosion of the underlying metal for more than 96 hours of salt-spray exposure.

·        Black (Bronze) Zinc: 96 hours to white corrosion product

·        Olive Drab Zinc: 120-172 hours to white corrosion product 

 

Post-Treatment Sealers: After the zinc plating, and chromate has been applied, a post-plate “sealer” can be applied that will significantly enhance the corrosion protection.  The Sealer chemically bonds with the chromate film to seal and harden chromate films as well as increase their adhesion to zinc surfaces.  It will also reduce chromate leaching and fingerprints while dramatically improving corrosion resistance.  Sealers may be applied over bright (clear), yellow, olive drab, or black chromate conversion coatings.  Salt Spray results have shown a 50%-100% increase in corrosion protection after the addition of a post-plate sealer, and red rust protection up to 300-500 hours.  Additionally, the cost for the sealer can be very economical, especially when considering the importance of the enhanced corrosion protection provided.

 

“Can’t Stand the Heat?”

Heating of chromated zinc adversely affects corrosion resistance, as the heat causes a decrease of the available (leachable) inhibitive hexavalent chromium by an irreversible dehydration phenomenon and cracks appear in the surface film.  The adverse effects are worsened as the temperature increases, and after heating to above 212°F, the protective nature of the chromate film may be nullified.  Since zinc plated high-strength steel (above Rockwell C-40) requires heating to relieve hydrogen embrittlement, the chromating operation is deferred until after baking.


FAQ’s About Zinc Plating

 

1.        What Is Commercial Zinc?  Commercial Zinc is the name or label given to a zinc finish specification that is commonly used in finishing metal parts. When specifying “Commercial Zinc”, you get a basic range of zinc finish protection. The normal composition has a thickness of .0002” of electroplated zinc.  Additionally, some commercial zinc formulations add a chromate top covering to protect the zinc finish.

 

2.      Why Use A Chromate On Zinc? Post-Plate Chromate treatments are used primarily to improve corrosion resistance, improve paint or adhesive bonding properties, and provide a decorative or colored finish.

 

3.      What can Post-Treatment Sealers do for zinc plating? After the zinc plating, and chromate has been applied, a post-plate “sealer” can be applied that will significantly enhance the corrosion protection.  The Sealer chemically bonds with the chromate film to seal and harden chromate films as well as increase their adhesion to zinc surfaces.  It will also reduce chromate leaching and fingerprints while dramatically improving corrosion resistance.  Sealers may be applied over bright (clear), yellow, olive drab, or black chromate conversion coatings.  Salt Spray results have shown a 50%-100% increase in corrosion protection after the addition of a post-plate sealer, and red rust protection up to 300-500 hours.  Additionally, the cost for the sealer can be very economical, especially when considering the importance of the enhanced corrosion protection provided.

 

4.      What about Zinc Alloy Plating? There are several alloys of zinc that are used throughout the industry.  The more common types include Zinc-Cobalt, but others are Tin-Zinc, Zinc-Nickel, and Zinc-Iron, all of which provide better corrosion protection than zinc alone.

 

5.      Will E-coat and Paint Adhere To A Zinc Finish?  In short, Yes!  E-coat and Paint will adhere to a zinc finish, and provide superior corrosion protection, as the protective value of the combined finishes provides excellent protection for the base metal.  We recommend Zinc with a Yellow chromate to provide the best adhesion.

 

6.      Why Is Salt Spray Testing Used?  Salt spray testing is a means to measure the relative protective value of a particular finish. The key word is relative. By rigidly controlling the exposure environment, a value can be derived to measure when corrosion starts. The American Society for Testing Methods (ASTM) salt spray specification B 117-90 is a detailed testing method for controlling the amount of salt spray solution, at what temperature, in what direction and much more. The results, normally in hours of exposure, allows for comparison of different finish formulations.  Corrosion resistance of a finish can be denoted in terms of the number of hours exposed to Salt Spray (Fog) Testing per ASTM B 117-90 Test Method. The results indicate the number of hours before white corrosion (the first stage of reaction) begins. Figure 1 reflects the differences in corrosion resistance abilities of some of the finishes offered.

 

7.      What Can I Do To Get More Protection For My Components?  First, you must define your application and environment requirements. What life do you expect from the finish and how should it look after 1, 3, or 5 years?  For instance, the finish protection required for outdoor road use is much more severe than for indoor office use.  Often times, combining finishes can result in extended protection, such as zinc plating with a chromate coating, and zinc under E-coat, spray paint, or powder coat.

 

8.      What Are The Different Classes Or Types Of Zinc Finishes?  The American Society for Testing Methods (ASTM) specification B-633 has four classifications for electroplated zinc finishes. They are based on coating thickness and type of application /environment that will be seen. Service Condition 1 is mild indoor applications and they move up to Service Condition #4 which is VERY SEVERE or exposure to harsh outdoor high abrasion applications. The basic idea is that protection increases as the finish thickness increases.

 

ASTM Type                                       Description                            

Type I ................................................ Zinc, as plated.........................

Type II................................................ Zinc, w/colored chromate coating          .

Type III............................................... Zinc, w/colorless chromate coating       

Type IV.............................................. Zinc, w/phosphate conversion coatings 

 

ASTM Service Condition                  Thickness

SC4.................................................... 0.001” min.

SC3.................................................... 0.0004” min.

SC2.................................................... 0.0003” min.

SC1.................................................... 0.0001” min

 

 

9.      “What is the difference between Sacrificial Protection & Barrier Protection?”  Sacrificial coatings are those deposits that give themselves up to the corrosive media, protecting the base metal.  Increasing the thickness on sacrificial coatings extends the life of the protection.  Barrier protective coatings (e.g. nickel, e-coat, powder coat, paint, chrome) are deposits that reduce or eliminate moisture, oxygen, and atmospheric gases from contacting the base metal. However, unlike sacrificial protection, any void or break in a barrier coating can lead to an immediate base metal attack. Therefore, pits or porosity in the base material can be highly detrimental to a barrier protector.  When a continuous zinc coating is present on steel, for instance, the zinc simply corrodes at its characteristic rate, which, incidentally, is considerably lower than that of steel, despite its greater activity in the Galvanic Series.  When the plating is discontinuous from pores or defects in the coating, the exposed steel areas are protected because the exposed steel becomes the cathodic member of the (steel/zinc) couple where oxygen discharge occurs and alkalinity is formed.  The zinc is, of course, anodic and corrodes at a faster rate than normal, particularly in the vicinity of the exposed steel.  Any tendency for the steel to corrode is counteracted by the flow of electrons from the corroding zinc to the steel surface (cathodic protection).

 

10.  “What are the Limitations of Zinc Plating?”  Zinc should not be used on critical steel parts that will reach temperatures of 500˚F, or higher, as zinc may diffuse into grain boundaries to embrittle the steel.   Zinc coatings can produce bulky corrosion products during exposure to marine or tropical environments and should not be used where the products may cause binding and prevent functioning of equipment that has moving parts in contact.  Rapid corrosion of zinc can occur in confined atmospheres where repeated condensation of moisture is likely and where certain organic vapors containing halogen can accumulate.

 


Electrodeposited Zinc Technical Data

 

Characteristics8

 

Corrosion Protection8:

 

Basic Data for Zinc8:

Understanding the Galvanic Series of Metals in Seawater

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



“You Crack Me Up!  Hydrogen Embrittlement is No Laughing Matter”

 

What is HE?  Have you ever paused to consider when riding in a car, or walking across a bridge, whether the plated parts were professionally handled to remove Hydrogen Embrittlement (HE)?  Not paying attention to HE can cause failure of the plated components, resulting in very serious consequences, or personal injury.  When certain steels exposed to sources of hydrogen fracture at stress levels well below their theoretical strength, HE may be the cause.  Steels with hardness above Rockwell C40 are the most susceptible to HE, including heat-treated steels used to manufacture many bolts, screws, nuts, springs, lock-washers, and other fasteners.  Do you trust that your “plater” handles HE properly?

 

How Does HE Occur?  The zinc electroplating process utilizes electrical energy through the electrical reduction of aqueous solutions of zinc salts.  Because the part is negatively charged to attract the positively charged zinc ions, it also attracts positively charged atomic hydrogen ions.  Unable to eradicate hydrogen from our plating processes, we can take precautions to manage the negative effects of this hydrogen on the parts that are plated.4

 

Atomic hydrogen moves throughout the metal, following cracks and impurity lines until it suddenly comes to an open area, or void, in the crystalline structure encountering zero pressure, and begins to bounce around.  Along comes a second hydrogen atom, and soon the two collide, readily forming hydrogen gas (H2).  As the volume of H2 builds, the pressure increases because the larger H2 molecule does not readily move out of the base material, and is “entrapped”.  This process continues as more H2 molecules are trapped, and the resulting pressure increase causes the stress that we are concerned about.  Brittle Fracture occurs when the stress exceeds the yield point of the base material.  In practice, problems with HE are rare when dealing with low-strength steels, but there are a number of problems with high-strength steels. 

 

How to Solve the HE Problem? To relieve the potential for HE, a baking procedure after the plating either removes the hydrogen, or diffuses it throughout the basis metal, both reducing the risk of embrittlement.  Mechanically plated, hardened steel parts, when processed according to standard procedures, should be held 24 hours before use.  If particularly aggressive cleaning procedures are required to remove excessive amounts of heat-treat scale prior to mechanical plating, the waiting period should be extended to 48 hours.4 

 

Professionally Managed: Customers can’t easily check the residual hydrogen plated steel parts, so they must rely on their “plater” for assurance that hydrogen management practices have been instituted, including4:

·        Customer: Providing proper notations on part drawings requiring HE bake-out.

·        Customer: Identifying on purchase order documentation the need for HE bake-out.

·        Customer: Using a plater that follows a detailed cleaning and plating procedure.

·        Plater: Maintaining and calibrating oven equipment and controls.

·        Plater: Keeping quality records that are available for review.

 

In conclusion, where critical parts are involved and where high-strength steel is the substrate, the safest approach to eliminating HE is with proper baking by a professionally managed plating company, commonly known as “HE relief bake out”, providing you with peace of mind.5


Sources:

  1. “Selection & Applications of Inorganic Finishes: Metal Deposits”, by Fred Perlstein, updated by Dr. James H. Lindsay, Plating & Surface Finishing, February 2003.
  2. “Selection & Applications of Inorganic Finishes: Chromate Coatings”, by Fred Perlstein, updated by Dr. James H. Lindsay, Plating & Surface Finishing, December 2002.
  3. “Hydrogen Embrittlement in Coating Technology – Measurement & Testing”, by Dr. W. Paatsch and V.D. Hodoroaba, Plating & Surface Finishing, October 2002
  4. “Averting a Hydrogen Embrittlement Crisis”, by Megan Pellenz & Milt Stevenson, Jr., Plating & Surface Finishing, March 2002
  5. “Fundamentals of Hydrogen Embrittlement”, by N.V. Mandich and G.A. Krulik, Metal Finishing, March 2003
  6. “Chromate Post Treatments”, by Jack Horner, Metal Finishing, February 1990
  7. “Zinc Plating”, by Steve Schneider, Plating & Surface Finishing
  8. “Coating & Surface Treatment Systems for Metals – A Comprehensive Guide to Selection, by Joseph Edwards, Finishing Publications, LTD.