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Conversion coatings are used in manufacturing for a few main reasons: to protect parts from corrosion, to provide additional lubrication, to enhance appearance, and to allow additional coatings to adhere better to the part surface.

Chromate plating, zinc plating, and black oxide coatings are common conversion coatings in automotive applications, among other industrial uses. These processes are typically used as a final step in producing a finished part to enhance its final appearance and provide varying degrees of corrosion resistance. 

Zinc phosphating, however, is ideal as an intermediate step in the metal finishing process that improves the adherence of other coatings, oils, and lubricants. Heavy phosphate coatings are also used to provide corrosion protection for fasteners and other small items. In this article, we’ll take a deeper dive into the zinc phosphating process and discuss its advantages for automotive applications.

Advantages of Zinc Phosphating

Zinc phosphating transforms the metal surface of a component into a polycrystalline coating that allows additional coatings to adhere well to the part’s surface. Zinc phosphating provides high alkaline resistance and is preferred as a base for paint because of its superior adhesion and durability. 

It is also widely used for dry-to-touch oil and wax oil lubricants for automotive parts, because the zinc phosphate layer allows the oil to better adhere to the part surface, protecting it from corrosion while it sits on a shelf waiting to be installed into a larger assembly. Zinc phosphating is also a common metal finishing process for parts such as automotive seating components that require plastic insert molding or overmolding, because just like paint and lubricants, the phosphate coating helps the plastic form a stronger bond to the metal insert.

In addition to promoting the adhesion of coatings and paints, zinc phosphating also reduces conductivity for metal components that could otherwise misdirect electrical current away from where it is supposed to go within an assembly. 

Finally, zinc phosphating provides an improved, more consistent appearance for metal parts. It can even yield the “space gray” appearance that is highly desirable in today’s markets (in addition to black). 

The Zinc Phosphating Process

The first step in nearly any metal finishing process is to thoroughly clean and rinse the surface of the parts. This sets up the parts to have an even, consistent coating applied to the surface without any contaminants getting in the way of the coating’s adherence.

The phosphating part of the process starts with preparing a solution of zinc phosphate salts in phosphoric acid. The coating can be applied either by spraying onto metal components or by immersion in a barrel. Immersion is ideal for small components such as screws, bolts, and fasteners, while spray application is better for larger parts such as automotive structural components. 

During the conversion coating process, the crystals that bond to the steel surface are a solid solution series between hydrated Zn3 (PO4)2 (also known as hopeite), and hydrated Zn2Fe (PO4)2 (also known as phosphophyllite). The thickness of this conversion coating builds steadily until the substrate is completely covered by the phosphate crystals until there is no iron left to react with the chemicals in the phosphating solution, at which point the reaction stops.

After the coating has been applied to the parts, they are rinsed and dried before the next operation is performed.

Heat Treating and Zinc Phosphating for Automotive Applications

At Paulo, our phosphating lines are specialized for an immersion process that is ideal for large volumes of small parts because of our in-house continuous austempering and through-hardening capabilities. Our continuous belt austempering furnaces and continuous belt atmosphere furnaces can process large quantities of parts at once, providing faster cycle times and cost efficiency for our high volume customers.

Having both high-volume heat treating and zinc phosphating capabilities under one roof helps simplify your supply chain, consolidate your thermal processing data into a single place, and make it easy to maintain accountability for quality results.

Common automotive parts we process using a combination of heat treating and zinc phosphating include:

  • Nuts
  • Bolts
  • Threaded and non-threaded fasteners
  • Weld studs
  • Seating components such as springs and latches
  • Clips
  • Strikers

Many of the parts we phosphate are used in internal assemblies on cars and trucks that aren’t seen from the outside. These parts are often stored by our customers until they are ready to complete the assembly, which means that they must be protected from corrosion in the atmosphere while they are awaiting installation. Our teams can apply dry-to-touch oil and wax oil coatings right here at Paulo for these parts to ensure they maintain their integrity during the waiting period and beyond.

The phosphating lines at Paulo utilize drums measuring three feet high and two feet in diameter. We can accommodate parts as long as 12 inches in length weighing up to one pound per piece. Parts that weigh more than one pound run the risk of damaging each other during the tumbling that occurs during processing. 

Zinc Phosphating Specifications

Depending on the application, prints will specify different coating weights for zinc phosphating. Light phosphates, classified as featuring 400–800 milligrams of coating per square foot, take less paint to cover and may be specified for applications in less demanding environments.

For more corrosive environments where greater protection is required, a heavy phosphate coating can be specified. Heavier coatings require more materials (be it paint or oil) to cover the part because the larger crystals on the part surface absorb more liquid. This extra absorption lends the finished part greater corrosion resistance and durability, making the heavy phosphate coating better suited for more demanding applications. A heavy phosphate coating is considered to be 1,100 milligrams per square foot or greater, with the maximum achievable coating thickness topping out at 1,800 milligrams per square foot.

At Paulo, we process to several leading automotive OEM specifications for zinc phosphating, including the following: 

  • General Motors GMW3179
  • General Motors GM4435M
  • Daimler Chrysler PS-80
  • Ford ESS-M3P20-A

Quality Testing & Automation 

Automation and computerized controls offer zinc phosphating lines greater precision and more efficient cycle times. At Paulo, our hopper system automatically moves barrels from tank to tank according to the part recipe. Our team members monitor the processing on an HMI (human-machine interface), ensuring proper voltage and checking the pH levels and chemical composition of the phosphating solution at regular intervals.

Salt Spray Testing

During process development and while obtaining PPAP certification, our team will perform salt spray testing according to your specifications or according to ASTM B633 standards. Typically, for parts that are zinc phosphated and subsequently oiled, specifications dictate that parts withstand 24–48 hours or 48–72 hours of salt spray before forming red rust. Phosphated parts that are coated with a non-chromic seal to prepare for painting usually call for 8–12 hours of continuous salt spray before red corrosion should form.

Coating Weight Measurement

The frequency of coating weight measurement that we perform depends on the specifications of our customers, so our team performs these quality control checks at a variety of different intervals depending on the requirements of each job. To determine the thickness of the coating, raw parts are weighed prior to phosphating and then again after coating. Using the total part surface area from the part print, we can then calculate the thickness of the zinc phosphate coating in milligrams per square inch of surface area.

Quality Control Best Practices

In addition to testing parts according to our customer’s specifications during the development and production phases of each job, our team also performs independent testing of our systems on q-panels at frequent, regular intervals. We perform both coating weight measurements and salt spray testing to ensure that our phosphating lines are producing precise, repeatable results.

Choosing a Zinc Phosphating Partner

Vertical integration is an advantageous characteristic to look for when selecting a zinc phosphating partner. When your supplier has the ability to perform multiple operations under one roof, it makes your job easier and helps you deliver parts to your customers faster. Especially during times of supply chain disruption, any efficiencies that can be gained in shipping and processing times help insulate your business from risk and fortify your ability to meet your commitments to customers.

Paulo can provide continuous austempering, continuous neutral hardening, zinc phosphating, and oil coatings all under one roof in our Murfreesboro division, making us an ideal partner to help you streamline your automotive part supply chain. Our PICS system makes your entire quoting, ordering, and tracking process easy, so you’ll never have to wonder where your parts are in the production process.

Advance Your Zinc Phosphating Results

If you’re looking to simplify and elevate your outsourced heat treating and metal finishing operations, then Paulo can help. Start your quote online today to get connected with a Paulo expert who can help you achieve precise, repeatable results that meet your specifications, time after time.

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