Whitaker Finishing has provided phosphate coatings to parts for the following industries:
- Aerospace
- Air compressor
- Armament
- Automotive
- Bearings
- Bushings
- Castings
- Chain links
- Construction heavy equipment
- Fasteners
- Firearms
- Forgings
- Gears
- High torque screws
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- Hydraulic pumps
- Motor parts
- Nuts
- Pins
- Retaining rings
- Shafts
- Slides
- Springs
- Springs
- Stamping
- Transportation
- Washers
- Other various fabricated, machined or stamped products
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Zinc Phosphate is the treatment of iron or steel by immersion in a solution of phosphoric acid and other compounds. During the process the surface of the metal is chemically converted to form a protective layer of zinc phosphate crystals. The physical characteristics of the base metal and the pretreatment methods used determine the size and color of the crystalline finish. The color will be grey to black for Zinc Phosphate. Phosphate coatings are used as a base for subsequent coatings such as paint, waxes, water soluble oils, dry film lubricants and high corrosion resistance Mil-3150 mineral base oils.
Zinc Phosphate Coating from Whitaker Finishing
Zinc phosphate coating gives base metal surfaces non-conductive, non-metallic properties. It also provides an excellent base for other materials such as paint, plastic or rubber. Phosphate provides corrosion resistance and is an excellent subsurface for drawing compounds, lubricants and sealants. Phosphate is generally specified by coating weight, rather than thickness, and provides an excellent surface for oil absorption. For added corrosion resistance or additional lubricity, post coatings such as seals, waxes or lacquer can also be applied to phosphate. Phosphate also reduces break-in wear on adjacent moving surfaces, improves torque-tension relationships and provides anti-galling and lubrication during cold forming processes such as deep-draw extrusions.
This process provides a micro-crystalline Zinc Phosphate coating on ferrous metals. Zinc tends to be relatively uniform, non-conductive and corrosion resistant. When combined with a black pre-dip prior to Zinc Phosphating, the finish will be matte black in appearance. Zinc is used in applications which require reduced light reflection, or for appearance characteristics. Zinc is used extensively on fasteners with excellent results. It can be used as a base coat for painting and is applied at 160ºF-200ºF. It is a porous surface that can absorb and maintain a post application of oil or wax.
Heavy zinc phosphates are used extensively to improve corrosion resistance. Some proprietary oil and wax formulations now on the market are capable of withstanding up to 240 hours neutral salt spray (ASTM B117). In addition to their corrosion resistant properties, phosphates also improve the overall appearance of finished goods by imparting a uniform, light to dark gray finish (the exact shade of gray is primarily dependent on the alloy and prior heat treatment). Uniform black coatings can also be obtained by adding a proprietary pre-dip step before the phosphate stage.
Cost is another reason to consider phosphates. Hidden metal parts, fasteners, hinges, pulleys, springs, slides, etc., can be zinc phosphated at a lower cost when compared to alternative finishes like plating or painting.
Characteristics of the Zinc Phosphate Coating
Phosphates are normally applied by immersion and often-utilized bulk processing techniques. Coating weights usually range between 1,500-4,000 mg/ft2 and depend on the surface condition of the steel, type of alloy used, and prior heat treatment. Varying the phosphate bath parameters can also have a dramatic effect.
Grain refiners can be incorporated into the phosphating cycle to assist in the formation of smoother, more uniform coatings. This ultimately results in an improved corrosion protector.
Finish Properties
Zinc Phosphate is used in applications that require reduced light reflection, appearance characteristics, as a base coat for solid film lubricants and paint. Coating weight thicknesses are in the range of 1500 – 2500 mg/sq.ft. (.0002” to .0006”). Zinc Phosphate meets Military Specification MIL-DTL-16232G Type Z, Classes 1, 2, 3 and 4, MIL-STD-171 and many more military, commercial, aerospace and automotive specifications.
What are Zinc Phosphates?
Zinc phosphate coatings are the treatment of iron or steel by immersion in a dilute solution of phosphoric acid and other additives. In the resulting chemical reactions, the surface of the metal is chemically converted to an integral protective layer of insoluble zinc and iron and iron phosphate crystals. Depending on the physical characteristics of the substrate and the pretreatment methods used, the translucent crystals appear black to light grey in color for zinc phosphate.
Why are Zinc Phosphate Used?
1. For Lubrication During Cold Forming
A phosphate-based lubricant soap system is superior to all other lubricants for severe extrusions because of the ability of the coating to flow with the metal as it is deformed, reducing metal to metal contact. This characteristic is the result of strong metal-to-phosphate bonds, which are formed during coating and the excellent extreme-pressure properties of the soap used as a supplementary treatment. Phosphate-soap coatings extend tool life, permit faster drawing speeds, allow for maintenance of closer dimensional tolerances, and produce a smooth, glossy surface on the finished parts. Low iron, light zinc phosphate coatings are typically used in this application.
2. For Corrosion Resistance
Alone or in combination with rust-inhibiting oils, waxed, or other organic coatings, zinc phosphates are a low-cost alternative to electroplating. Depending on the supplemental coating, it can withstand from 48-480 hours of neutral salt spray exposure per ASTM method B-117.
3. To Reduce Break-In Wear On Adjacent Moving Surfaces
Nonmetallic, oil absorptive zinc phosphate coatings on bearing surfaces permit rapid break-in of moving parts without scuffing or welding by preventing metal to metal contact, as the friable coating is easily crushed when a load is applied. Additionally, the oil held by the crystals improves the lubrication of the treated surface and imparts corrosion resistance. After break-in, and even and fine distribution of oil "reservoirs" remains, assuring continued lubrication.
4. For Torque-Tension Requirements
Zinc phosphated fasteners sealed with a lubricating oil guarantee repeatable torque-tension relationships in applications where automated assembly equipment cannot be conintually adjusted to compensate for varying surface conditions. The excellent uniform lubrication provided by our special "TORQ-TECH", zinc phosphate coatings eliminate stick-slip conditions and galling during assembly. Additionally, the parts are protected from oxidation during storage.
5. For Bonding Organic Coatings to Metals
Reducing the electrochemical activity of the ferrous surface and forming strong physical and chemical bonds with organic coatings, zinc phosphates are widely used as a bare under adhesives (in rubber to metal bonding), paints, and other organic coatings. The highly absorbent phosphate crystals allow paint to penetrate into tiny fissures and also increase the total surface area available for bonding. Acting as a dielectric film that insulates the active anode and cathode centers which exist over the surface of the base metal, phosphate coatings retard corrosion "creep", extending the life of the finished product. Light zinc phosphate coatings with small, uniform crystal sizes produce the best bond.
How are Zinc Phosphates Applied?
Material to be coated is cleaned by immersion in a hot alkaline solution that removes most oils and loose soil. If surface oxides are present, the parts are then stripped in an acid-cleaning step that undercuts the rust or scale, exposing the bare metal beneath. The work is then rinsed thoroughly and coated in a chemically balanced hot phosphoric acid solution via an autocatalytic reaction. The temperature, time and chemical composition of this bath must be carefully controlled to produce consistent results. After coating is completed, excess acid is neutralized and a supplementary treatment is applied if required. Material to be zinc phosphated may either be racked or bulk-processed in barrels, depending on the specifications of the end user. Normally threaded parts, soft alloys, or parts which weigh in excess of 6 ounces are racked to avoid nicks, distortion, and coating damage which result from bulk handling practices.
Are all Zinc Phosphates the Same?
Depending on the intended purpose of the finish, the crystal size, shape, coating weight, and color can be modified to meet many industrial military specifications. Because the hardness, composition, and surface condition of the raw material greatly influence the results of a given processing cycle, it is often necessary to use different cleaning cycles, refiners, and phosphate bath formulations to effect identical results on dissimilar substrates.
How are Zinc Phosphates Tested for Quality?
The primary indicators of zinc phosphate quality are coating weight measurement and neutral salt spray resistance. Crystal size and form are important for some applications, but due to the expense of the electron microscopy necessary to make an accurate determination, this characteristic is not commonly checked on a production basis and is normally limited to first article (layout) inspection. Some specifications also include thickness requirements, but much controversy exists regarding the accuracy and methods of testing because the phosphate crystals are extremely friable and crush under even the slightest pressure. Diagnostic tests for coating integrity, torque-tension relationships and for the presence of zinc are also available. All tests performed on zinc phosphated material are destructive.
Whitaker Finishing can provide zinc phosphate coatings to the following specifications: Mil.Std. 171, Mil-DTL-16232G, TT-C-490 and most corporate specifications.
Please contact Whitaker Finishing at (419) 666-7746 or fill out our on-line form for a quote for your zinc phosphate Coating requirements.
