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MANUFACTURING TECHNOLOGY


Exceptional position of ZnNi alloy plating for automotive engineering


By Rainer Venz, COVENTYA GmbH


Components, connecting parts and fasteners need to be protected against corrosion so that their function can be assured, uniform appearance can be ensured and corrosion initiated by connected parts avoided.


against atmospheric corrosion additional performances are also demanded such as temperature resistance, consistent coefficient of friction, electrical conductivity, and even more homogeneous appearance.


I


Quality of the layer The higher the requirements for ZnNi surfaces the more


complex process management and quality control of the layer are required. State of the art, automated analysing and dosing equipment is essential; as well as cooling, heating and filtration units. Laboratories must also be equipped with modern analytical capabilities, like atomic absorption spectroscopy (AAS), and have qualified specialists to operate them.


Corrosion protection ZnNi coatings can reliably reach the corrosion protection


targets defined by DIN EN ISO 9227 (neutral salt spray test), which meets major automotive OEM specifications. In fact substrate corrosion protection is much higher than the requirements in the standards. To simulate more complex corrosion stresses, however, alternative climate test methods are increasingly being utilised.


Coupled substrate


Stainless Z 10 Cr Ni 18-9 Brass Tin


Al 2017


Cast Iron Steel


Al 2011 Al 6060 Steel


Al 5754 7049


Magnesium


Cu Zn 39 Pb Al Cu 4 Mg XC 10


Al Cu 5 Pb Bi Al Si 10 Mg


XC 80 XC 90 with HT Al Mg 3


Al Zn 8 Mg Cu Zinc


1270 760 600 460 460 400 400 335 305 300 175 -660


Zinc Nickel 12-15%


970 460 300 160 150 100 100 35 5 0


-125 -860


Combining different materials in vehicle - especially light


metals to deliver weight reduction – can, depending on the material combination, cause galvanic corrosion. Avoiding galvanic corrosion or reducing the risk of it, is one advantage of ZnNi, particularly in combination with aluminium substrates.


n today’s automotive industry, warranty periods are now very long, especially for non-perforation corrosion. The requirements of corrosion protection layers have constantly increased to fulfil this demand. In addition to protection


Friction behaviour An addition sealant can be applied on top of passivated ZnNi


layers and integrated lubricants can be used to ensure uniform friction values. Depending on the type and quantity used of the various waxes, different target values can be achieved. A range of different FINIGARD topcoats is available to fulfil typical OEM friction requirements.


Appearance For design reasons, many fasteners or other functional parts need, in addition to achieving defined technical performance, to present a uniform silver or black appearance. In a few cases, in order to differentiate parts that may look very similar, it is necessary to introduce a third colour. In such cases yellow iridescent passivates may be used. All of these variations are based on trivalent chromium technologies that fulfil ELV requirements.


Substrates


The use of ZnNi can also successfully contribute to construction design objectives. The need to reduce weight results in increased use of high-strength steel, which permits components to be made smaller while maintaining comparable mechanical performance. High strength steel, however, is susceptible to delayed fractures due to hydrogen embrittlement. Here, electrolytic application of ZnNi coating can provide clear benefits. Development of the coating


0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6


0 0 10 20 30 Time (h) Scientific investigations have proven there is no risk of


delayed hydrogen-induced embrittlement fractures in materials of strength grade 10.9, as long as all the technical rules for the ZnNi plating process and preparation are respected. The industry is currently collecting practical experience in different field trials.


Conclusion ZnNi technology provides a combination of many positive


properties, which mean ZnNi coated parts and fasteners can be used in an extensive range of automotive applications.


www.coventya.com 130 Fastener + Fixing Magazine • Issue 79 January 2013 40 50 60 70 Loading steel XC68 Loading coating / Steel XC68 Zn / Steel


Zn-Ni / Steel Ni / Steel


JA (µA/cm²)


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