Thermal Diffusion Coatings Protect Automotive Lock Parts

By Dan Riter | October 1, 2009

The new Greenkote GmbH coatings plant in Germany delivers improved corrosion protection for automotive parts.

The automotive industry faces some of the strictest regulations when it comes to meeting environmental and safety standards for materials and parts for manufactured vehicles. Those restrictions became even more challenging for auto makers when the European Commission adopted the ELV (End of Live Vehicles) Directive in 1997. Put in place as a means to make vehicle dismantling and recycling more environmentally friendly, the ELV and other regulatory EU directives force auto makers to search for ways to maintain the highest level of quality for metal finished parts while meeting the growing demands for environmentally safe materials.

Greenkote GmbH's new coatings facility in Germany has over 2,000 square meters of operations and office space.
As a provider of thermal diffusion coatings services for the automotive and fastener industries, Greenkote GmbH serves European auto manufacturers and their Tier 1 suppliers in meeting the demand for improved environmental and technological advantages. In September 2008, the company opened a new coatings plant near Munich, located in Sauerlach, Germany.

Greenkote is a thermal diffusion coatings process that modifies and improves the basic surface characteristics of metals, and is thus applicable to metal finished parts in many industries including automotive. Because thermal diffusion coatings do not contain any hazardous heavy metals, this "green" technology can help meet the demanding specifications of the automotive industry in the EU market.

Greenkote developed PM-10 and PM-21 coatings for the automotive industry mainly as a base for corrosion protection and low friction topcoats. PM-10 is zinc-aluminum and PM-21 is a zinc-aluminum-magnesium chemical composition. Duplex coating PM-10 with e-coating is approved by various auto manufacturers including Volkswagen, Audi and GM.

In January 2009, Greenkote started coatings production for Audi's Tier 1 suppliers. The Greenkote PM-10 plus e-coat topcoat was approved by Audi and meets VW specification TL-196, delivering improved protection for Audi's automotive lock parts. More on the specifics of those test results are detailed below.

Duplex coating and green technology

Greenkote PM zinc TD (thermal diffusion) coatings meet the ELV Directive requirements because there are no hazardous materials used in the coating. Also, the thermal diffusion process used in Greenkote technology produces no HAPS (hazardous air pollutants) and no industrial wastewater discharge.

The proprietary Greenkote coatings are purchased and processed as a dry powder, so there are no solvents or water to be removed during the coating process. The little water used for post rinsing after coating can be recycled.

In addition to its green aspects, Greenkote zinc TD coatings offer a high level of corrosion protection due to their good adhesion proprieties for paint, e-coat and powder coatings topcoats. The sacrificial zinc layer in the thermal diffusion basecoat is another reason for good performance against corrosion.

Figure 1
In most automotive applications, there is a need for a top coat over the Greenkote process. Currently the best topcoat combination is a black electro-coat topcoat, commonly called "e-coat," or "KTL" in Europe. The Greenkote PM-10 coating paired with e-coat offers several synergies and is a solid replacement for zinc nickel plating with e-coat. Greenkote duplex coatings protect all sorts of lock parts including base plates, strikers and latches (see Fig. 1).

The thermal diffusion process and how it works

Greenkote TD patented PM coatings work by thermo-chemical surface modification. Carbon steel, cast iron and sintered metal parts can be coated with zinc TD coatings. The coating temperatures range between 360�C and 410�C (680�F and 770�F).

The coating layer forms in steps:

Figure 2
1. Zinc in the coating diffuses into aluminum in the coating and into the iron in the substrate (see Fig. 2).
2. Iron in the substrate forms intermetallics of Zn-Fe-Al (see Fig. 3).
3. Aluminum inclusions are formed on the coating surface (see Fig. 4).

The thermo-chemical process forms a protective layer that partially diffuses into the substrate and cannot be separated by physical or environmental testing. The aluminum inclusions on the surface are a key in healing any micro cracks or porosity in the coating while the zinc provides a sacrificial layer.

Preparation before the zinc TD coatings process varies depending upon the condition of the part itself. While some parts may be ready for coating as received, other parts such as those with heavy oils or lubricants on the
Figure 3
surfaces need to be cleaned in a parts washer. There is no need to rinse after alkaline cleaning-simply dry the parts. If the parts arrive with significant rust or scale, usually from high temperature heat treating, the parts must be mechanically blast cleaned.

The heart of the zinc TD coating process is the heating time in a steel retort. These closed steel cylinders come in varying sizes depending on the size and volume of the parts. One end opens for easy loading and unloading. The parts, ceramic media and zinc TD coating powder are then added based upon weight from a predetermined loading menu for each part.

The heating cycle consists of the time it takes to ramp up, the time it takes to hold the temperature for one hour, and then cool-down time for unloading. The cycle time is typically three hours with convection heat, but
Figure 4
can be can be less than two hours with IR (infrared) heat.

Finally, the cooled parts are rinsed with ambient city water and then passivated with a zinc phosphating rinse before drying and packing. The rinse step removes a light amount of coating dust left from the retort operation. Both the rinse water and phosphating solution are reused in a closed loop system, adding to the eco-friendly process.

Corrosion test results

With the environmental-friendly thermal diffusion process that supports the ELV Directives and the excellent performance provided by the duplex coating, the Greenkote process is gaining recognition and acceptance through a series of successful tests run by some of the major automotive manufactures and Tier 1 suppliers.

In June 2003, VW qualified Greenkote PM coating with e-coat topcoat for its TL-196 specification "Duplex Coating for Small Parts" for corrosion protection. In July 2007, Audi approved Greenkote PM-10 plus e-coat topcoat for its lock parts after passing the TL-196 specification of 60 cycles of corrosion testing.

Figure 5
An independent lab tested an automotive clamp to the ASTM B117 neutral salt spray test. The clamp had Greenkote PM-10 zinc TD coating at 15 microns film thickness and black e-coat at 18 microns film thickness, for a combined total thickness of 33 microns (1.3 mils). The final test report gave a ten rating for the scribe with 0.0 mm creepage and a rating of 10 for no surface defects (see Fig. 5).

Additional testing shows that Greenkote performs well on fasteners as well as stamped parts. In 2006, PPG tested M6 and M8 fasteners using Greenkote PM Coating with Powercron XP electrocoat and reported no red rust after 960 hours of salt spray testing.

The added value of TD coatings

Greenkote PM coatings provide high performance corrosion protection at a competitive cost when compared to other zinc coatings such as zinc plating, hot dip galvanizing and zinc metal flake coatings. With zinc TD coatings there isn't any hydrogen embrittlement, which can be a concern with electroplating.

Additionally, Greenkote PM coatings provide excellent rubber to metal bonding surfaces, eliminate porosity in sinter metals and produce a thin, uniform thickness on threads, blind holes and even tube interior walls. They also provide a hard, wear resistant surface. Wear testing on striker wire loops showed normal wear and gouging after 5000 cycles with another duplex coating, while the Greenkote PM and e-coat barely showed any signs of wear after 10,000 cycles.

As the automotive industry continues to seek out new products and technology to stay abreast of the ever-tightening restrictions and regulations placed upon it, Greenkote will continue to respond to stringent environmental standards by providing an eco-friendly alternative to traditional coatings.

With the opening of the Sauerlach facility in Germany, the reach of the Greenkote thermal diffusion coatings process is now more widespread than ever. As Audi and its Tier 1 suppliers can attest, Greenkote PM zinc coatings can provide the corrosion protection for automotive lock parts as a cost-effective solution for meeting environmental-based directives.
About the author: Dan Riter is director of engineering at Greenkote Plc located in Cleveland, OH. For more information on thermal diffusion coatings, contact Dan Riter at 440-243-2938 or dan.riter@greenkote.com.

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