Nickel-based coatings are one of the most important industrial coatings due to their durability, corrosion resistance and aesthetics. Traditionally, electrolytic plating is the most common process used to deposit a nickel coating and it is still widely used. However, the use of an electroless nickel plating process has been increasingly used due to the improved quality and more environmentally friendly process.
Electroless nickel plating (ENP) is an auto-catalytic chemical process used to deposit a layer of nickel-phosphorus (Ni-P) alloy coating on a substrate. The nickel-phosphorus coating thickness varies typically from 1 to 40 μm depending on the application. Electroless nickel coatings can have varying phosphorus content which impacts specific metallurgical properties. The phosphorus range typically varies from 2% up to 15% level. The benefit of low phosphorous nickel coating is very uniform thickness inside and outside of complex parts; whereas, a higher phosphorous level offers corrosion resistance and ductility for demanding industrial use cases.
One of the key benefits of the nickel-phosphorus coating is that it provides a very consistent thickness that isn't dependent on part geometry. Electroless nickel plating can reach all hidden surfaces to offer complete coating coverage even on the most intricate parts. Electroless nickel plating can be applied to both conductive and non-conductive materials, meaning it can be used also for a wide range of non-metallic substrates such as plastic and textiles. Electroless nickel coating is used in many industries such as automotive (e.g. bearings and gear assemblies), electronics (e.g. printed circuit boards and connectors).
When tight quality control of metal coatings is required, X-ray fluorescence (XRF) analysis is the best overall solution. The modern micro-XRF instrument, such as the Bruker M1 MISTRAL can provide simultaneous coating thickness and coating composition measurement.
The M1 MISTRAL is equipped with a high-performance silicon drift detector (SDD), W or Rh microfocus X-ray tube and user-selectable collimator which allows spot sizes down to 100 µm. Together with the video microscope and motorizes and programmable XYZ stage, the user can ensure that measurement takes place at exactly the desired spot.
The nickel-phosphorus coating composition significantly influences the mechanical, corrosion resistance and magnetic properties which makes it critical to not only measure coating thickness but also phosphorus content. The energy of the characteristic Phosphorus K-line radiation is only 2 keV and information depth just under 1 μm which means that excellent instrument hardware and calibration are required for precise phosphorus analysis. For Ni-P application, the M1 MISTRAL with Rhodium target X-ray tube is an excellent choice as it provides superior light element excitation.
With M1 MISTRAL, it is possible to monitor phosphorus content with excellent accuracy and repeatability even when a small measurement spot (collimation) is required. This enables very tight quality control of the electroless nickel-phosphorus coating properties even for very small parts and measurement areas.