Measuring Thick Nickel on Aluminum or Steel

Q: I often need to plate thick electrolytic and electroless nickel on steel and aluminum. We want to use our XRF for these measurements. However, on aluminum we struggle to get accurate and consistent readings above 1–1.2 mil (1,000–1,200 µ”, 25–30 µm). On steel we start seeing difficulties above 1.4 mil (1,400 µ”, 35 µm). Are there any other test methods for measuring thick nickel plating?

A: Over the past decades, X-ray fluorescence (XRF) has become the go-to instrument for measuring electrolytic and electroless nickel coating thicknesses. Measurement with XRF is fast, nondestructive, accurate and repeatable. However, as you have seen, XRF does have limitations when it comes to measuring thick coatings. There is a maximum thickness of material XRF can “see through” and get reliable readings. If too thick, the X-rays cannot penetrate deeply enough to give accurate or repeatable results. The good news there are other measurement techniques available able to measure thick nickel deposits. 

Electrolytic and low phosphorus electroless nickel on aluminum and copper

Elemental nickel is paramagnetic. This means nickel is weakly attracted to magnetic fields; nickel will not ‘stick’ to steel, but a magnet will stick to nickel.  Magnetic methods can measure nickel on non-magnetic bases, such as aluminum and copper. As the nickel thickness increases, so does the magnetic attraction. The increasing magnetic attraction is then correlated to a coating thickness. Magnetic methods may also apply to electroless nickel having a phosphorus content below ~7-8 wt%.

The paramagnetic properties of nickel, and low phosphorus electroless nickel, can change over time. To ensure accuracy it is best practice to create your own gage calibration standards from materials you have plated. This will ensure the paramagnetic properties of the calibration materials match those of what is being plated. Measurements of electrolytic and low phosphorus nickel coatings, up to ~ 6 mil (6,000 µ”, 150 µm), may be possible using magnetic methods.

Electrolytic nickel on magnetic steels

As mentioned above, electrolytic nickel is paramagnetic. Therefore, magnetic methods cannot measure nickel on magnetic steel (both the coating and the base are magnetic). Phase-sensitive eddy current is appropriate for these applications. Phase-sensitive eddy current (PSEC) measures eddy current phase angle shifts, created by changing nickel thicknesses, to correlate to a coating thickness. As with the magnetic method, the user must make their own calibration standards to ensure the eddy current properties of the calibration standard match those of the actual parts. Measurement of nickel coatings, up to ~ 4 mil (4,000 µ”, 100 µm), are possible using the Phase Sensitive Eddy Current test method. Unfortunately, PSEC is not appropriate for low phosphorus electroless nickel. 

Other methods

Beta backscatter: The beta backscatter can be an effective, nondestructive test method to measure a thickness of nickel coatings, both electrolytic and electroless. Beta backscatter is limited to applications having at least five atomic numbers between the base material and the coating. Measurement of nickel thicknesses on aluminum is possible with beta backscatter. Beta backscatter is not appropriate for nickel on steel or aluminum because they are too close together on the periodic table. Measurement of nickel coatings up to 4 mil (4,000 µ”, 100 µm), may be possible using beta backscatter.

Coulometric: This technique uses an electrolyte solution and an electrical current to remove the nickel coating, a reverse of the plating process. The electric current applied is directly proportional to the mass removed, and the resulting deplating time is correlated to a coating thickness. Therefore, the coulometric method is considered destructive, leaving a small ~ 1-3 mm Ø (~ 0.04 “ – 0.12” Ø) circle where the plated layer has been removed. 

Advantages of the coulometric method is that is can be used to measure multiple nickel layers, using what is know as the STEP test, and measure multiple, different layers, such as chrome-nickel-copper. 

Measurement of nickel thickness up to ~ 2 mil (2,000 µ”, 50 µm) are possible using the coulometric method.

Summary: XRF is often the go-to for measuring nickel and electroless nickel coating. However, XRF does have upper thickness limitations, with nickel coatings becoming challenging to measure starting around 1.2 mil (1,200 µ”, 30 µm). Other methods do exist to help platers and coaters measure thick nickel coatings. I recommend speaking with your instrument vendor’s applications team to review your measurement tasks.