High performing coatings for fasteners 22 January 2013

Here NOF METAL COATINGS GROUP looks at the development and benefits of anti-corrosion coatings when used in the automotive industry.

Introduction
In the 1970s zinc-flake anti-corrosion technology was introduced with the first applications on fasteners being carried out by American and European automotive manufacturers. Originally coatings were passivated throughout by chromium oxides.

Thanks to its anti-corrosion performance with thin coatings and giving no hydrogen embrittlement, this technology grew very fast. In the early 2000s, the European directive regarding recycling of End of Life Vehicles led to zinc-flake technology being produced without hexavalent chromium.

NOF METAL COATINGS GROUP used this opportunity to press ahead with its strategy of developing a fully water-based range. Today, automotive manufacturers have three overriding criteria for anti-corrosion coatings on fasteners, namely anti-corrosion performance, tightening control and environmental concern.

Anti-corrosion protection at low thickness
Anti-corrosion performance associated with low thicknesses has always been a determining choice for the automotive industry. Its requirements regarding anti-corrosion are met with coating weights ranging from 24g/m2 to 36g/m2. This low thickness enables most threaded fasteners to be coated easily. Automotive manufacturers have very specific test methods, particularly cyclic corrosion tests (ACT, SAE/USCAR) and salt spray tests (IS0 9227/ASTM B117).

A GEOMET® 321 without topcoat and with an average thickness ranging from 6μm to 7μm is able to reach 720 hours without red rust. A GEOMET® 321 without topcoat, this time with an average thickness ranging from 8μm to 10μm, is able to reach 1,000 hours without red rust. Results can vary according to substrate, geometry of parts and type of application process.

Protection principles
The anti-corrosion products developed by NOF METAL COATINGS GROUP are coatings that’s technology is based on a water-borne chemical composition. They are made of zinc and aluminium flakes in an inorganic matrix. This flake structure, where zinc and aluminium layers overlap, enables the protection of the substrate in three ways:

Barrier protection – Overlapping zinc and aluminium flakes provide an excellent barrier between the steel substrate and the corrosive atmosphere.

Sacrificial protection – The general principle of sacrificial protection is based on the fact that for metals in contact, one metal will corrode preferentially. Zinc contained in coatings oxidises before steel, which is therefore protected and does not corrode. The second mechanism is self-repair. If the coating is damaged, a small area of steel can be exposed. In this case, the products resulting from zinc corrosion will fill in damage and thus increase life span of parts before appearance of red rust.

Passivation – Metal oxides throughout the coating slow down the corrosion reaction of zinc and aluminium to provide a greater corrosion protection than with pure zinc.


Other properties of zinc-flake technology
Application processes based on zinc-flake technology do not incur any risk of hydrogen embrittlement once parts are coated, which is a key factor for security parts, for instance wheel bolts and chassis screws. Another key point is that there is no need for any external passivation process, as zinc-flake technology is an integral passivation.

The silver/grey appearance is the most used colour in the automotive industry. The black appearance is also very widespread, particularly for fasteners like door hinges or visible parts like bumper screws.

Tightening control
The coefficient of friction of threaded fasteners is a core parameter for tightening control. Automotive manufacturers have specific requirements regarding coefficient of friction. In order to get the required values, NOF METAL COATINGS GROUP develops and markets several water-based topcoats to apply onto the GEOMET® range, thus enabling compliance to the required tightening performance:

PLUS® XL: 0.06 – 0.09
PLUS® L: 0.08 – 0.14
PLUS® VL: 0.09 – 0.14
PLUS® ML: 0.10 – 0.16
PLUS® M: 0.12 – 0.18

Coefficient of frictions are validated on a reference screw with hexagonal head, M10 x 55 according to ISO 16047.
NOF METAL COATINGS GROUP also produces GEOMET® 500 with integrated lubricant. GEOMET® 500 allows values between 0.12 and 0.18 according to ISO 16047 to be obtained.

Today, some automotive metallic parts are made of light alloys like aluminium. The PLUS® topcoats allow under certain conditions, improvements performance regarding friction for specific difficult tightening cases, particularly for multitightening in aluminium and onto catophoretic paint. They are also effective in cases of loosening at high temperatures, while avoiding stick-slip problems when tightening.

‘Green’ coatings
The GEOMET® technology has been developed to comply with the 2000/53/CE directive regarding recycling of End of Life Vehicles. This directive bans the use of four heavy metals (lead, mercury, cadmium and hexavalent chromium) in vehicle components and materials. NOF METAL COATINGS GROUP has opted for an entirely chromium free anti-corrosion solution (neither trivalent chromium, nor hexavalent chromium).

Today, some other environmental issues have to be taken into account, for instance the reduction of carbon gas emissions.NOF METAL COATINGS GROUP’s strategy has always been based on developing waterborne technologies, unlike the other zinc-flake solutions that are solvent based.

Another key point is that NOF METAL COATINGS GROUP’s chemical compounds have a very low Volatile Organic Compounds (VOC) content. Practically odourless, they allow line operators to work in a healthier environment.

The low thickness compared to other processes allows the use of less material (zinc particles, aluminium particles), thus preserving resources. Global availability and production as close as possible to final customers allows the reduction of CO2 emissions during transport.

NOF METAL COATINGS GROUP’s products anticipate the substitution of substances subjected to regulations like ELV and VOC directives, as well as REACH regulation. NOF METAL COATINGS EUROPE has been found to be compliant to REACH by the German TÜV Rheinland Group organisation by having acquired an in-house know-how.

The technologies provided by NOF METAL COATINGS GROUP have received two awards by well-known organisations from the chemical sector. The Chemical Industries Union awarded the Pierre Potier prize in 2009 and the European Chemical Industry Council, the Responsible Care® Europe prize in 2010. These two major awards in the chemistry sector underline NOF METAL COATINGS GROUP’s contribution in providing industries with environmentally friendly technology.

The different application processes
NOF METAL COATINGS GROUP focuses its research efforts on two closely interlinked targets - chemistry of products and application processes. A well performing chemical and a specifically adapted application process are essential to achieve performing anti-corrosion coatings with thin layers. Thanks to this close combination efficient anti-corrosion solutions can be provided that take into account both size and more or less complex geometry of fasteners.

The most used application processes are:

Dip-spin in bulk application - This process is used worldwide for the protection of small metallic parts as screws, bolts, springs, etc. Parts placed into a basket are dipped into a GEOMET® bath then centrifuged to remove excess product. The wet film is then dried and cured.

Drum rotating spray application - Metallic parts are placed into a horizontal drum. Upon drum rotation, several spray guns mounted within the drum spray the chemical. This process is used very often for small screws with socket heads to prevent product filling the recesses.

Dip-spin application on racks - Metallic parts are attached to racks so that they can be centrifuged to remove excess product. This process is well adapted to large size parts such as wind turbine bolts or fuel filling tubes. 

Spray application - Metallic parts are attached to racks or frames in order to be able to spray the chemical. The product chemical properties provide a very good distribution of layers. This process is very adapted to the coating of heavy parts.


A Worldwide Training Centre
NOF METAL COATINGS EUROPE opened in 2012 a Worldwide Training Centre with experienced people and cutting-edge technical equipment.

It allows participants to specialise in zinc-flake anti-corrosion technology by encompassing many themes including corrosion mechanisms, zinc-flake protection, quality control or tribology. 

Customers can take part in tailor-made training modules with theoretical and practical parts. They are enabled to get an overview of the different knowledge linked to chemicals, application processes and coatings performance.

Conclusion
For over 30 years, water based zinc-flake anti-corrosion technology has evolved and developed further in order to meet the needs of the global automotive industry. It meets specifications of most automotive manufacturers. Choosing this technology is driven by its anti-corrosion performance with thin layers, its ease of use and application, and its global availability, thanks to a network of more than 500 application lines. The automotive industry takes increasingly into account environmental concern in their developments and automotive components as evidenced by the development of hybrid and electric vehicles. Water based zinc-flake technology provides very low VOC values, a product entirely without chromium and allows lower energy costs compared with other anti-corrosion technologies.

www.nofmetalcoatings.com

Claire Aldridge Deputy Editor t: +44 (0) 1727 743 889

Having joined the magazine in 2012, Claire developed her knowledge of the industry through the numerous company visits, exhibitions and conferences she attended both in the UK and abroad.

Claire prides herself on keeping readers well informed and up to date with the latest industry news.