MANUFACTURING TECHNOLOGY
High efficiency plating for the highest corrosion protection:
Acidic zinc-nickel for fastener applications Dr Björn Dingwerth, global product line manager, corrosion resistant coatings, Enthone GmbH
Over recent years new car warranties have evolved to include well beyond five years anti-corrosion guarantees. Such extended warranty times call for improved corrosion protection on numerous automotive components. Within those components that contribute to the structural stability and functionality of a car, fasteners play a dominant role.
S
ufficient fastener quality throughout the service life is a crucial prerequisite to be met in order to achieve the required high overall stringent OEM quality standards of today’s automotive production.
This need for higher quality throughout the in-service life finds
itself expressed in an ongoing change in the surface finishing industry, which is moving more and more away from pure zinc layers towards zinc-alloy based sacrificial coatings. The zinc-alloy that generates the highest protection under corrosive exposure is zinc-nickel – particularly the so called zinc-nickel gamma-phase alloy, which can be achieved with a 12% – 16% (weight) nickel alloy content from qualified plating solutions. Zinc-nickel alloys are widely plated from alkaline electrolytes
onto fasteners and many other automotive components. Alkaline ZnNi plating solutions offer an excellent thickness distribution even on complex formed parts and are the best process, for example, when rack plating stamped mild steel parts. Alkaline plating solutions are also widely employed for barrel
application of bulk parts. Here, the advantage of an excellent thickness distribution over the part is less relevant than in rack plating operations. Due to the nature of the barrel application process the different thickness distribution of the electrolytic process has only a minor effect, particularly on fasteners. Technically, a good thickness distribution translates into a
lower cathodic current efficiency at higher current densities. Therefore, to enjoy a good thickness distribution one trades off some plating efficiency on certain areas of a work piece for a more even plated layer thickness. An outstanding feature of the latest generation of acidic
zinc-nickel electrolytes such as the newly developed ZINCROLYTE®
KCL-Ni IV is the very high efficiency (>90%),
which is consistently maintained over long-time production. While for pure
zinc coatings, acidic electrolytes are generally used in barrel application and alkaline electrolytes predominantly are run in rack application, due to the aforementioned reasons, alkaline zinc-nickel remained nearly exclusively the
Figure 1 – plating rates obtained from an acidic zinc-nickel electrolyte (red) and from an alkaline one (blue) plotted versus current density
recommendation for both rack and barrel application. One main reason for this was the better control of nickel
incorporation, which in the past was only achieved from alkaline electrolytes. The nickel incorporation is of paramount importance to achieve the desired high corrosion protection. Strict control of 12% – 16% of Ni in the alloy is crucial to obtain the optimum corrosion protection. With the latest generation of acidic zinc-nickel electrolytes
a narrow ranged nickel incorporation rate is achieved in rack and barrel application on the same level as obtained from application of an alkaline electrolyte. The structure of the electroplated layer from either electrolyte
can best be assessed by X-ray diffractometry. The diagrams obtained for layers from acidic ZnNi ZINCROLYTE® and from alkaline zinc-nickel ZINCROLYTE®
KCL-Ni NCZ 315 Plus
do not show significant differences (Figure 2), thus the resulting zinc-nickel layer is the same zinc-nickel material showing the same crystallographic structure and texture, independent from choosing acidic ZINCROLYTE® ZINCROLYTE®
KCL-Ni or alkaline NCZ 315 Plus.
on mild steel from acidic ZINCROLYTE® KCL-Ni NCZ 315 Plus
Figure 2 – XRD diagrams (Cu Kα electrolyte (top) and ZINCROLYTE®
(bottom) plated at 15% Ni. Both layers bear the same texture, preferred (330), (600) orientation
106 Fastener + Fixing Magazine • Issue 89 September 2014
-radiation) of ZnNi
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