Simultaneous Thickness and Electrochemical Potential Determination of Individual Layers in Multilayer Nickel Deposits
The 1981 AES Gold Medal Award was given to Ed Harbulak for the Best Paper appearing in Plating and Surface Finishing in 1980. His work involved the development of the well-known STEP test, a method for evaluating the performance of decorative Cu-Ni-Cr, measuring the electrochemical potential of nickel multilayers with a modified version of the venerable Kocour thickness tester.
Edward P. Harbulak
Editor’s Note: Originally published as E.P. Harbulak, Plating and Surface Finishing, 67 (2), 49-54 (1980), and was awarded the 1981 AES Gold Medal for Best Paper published in Plating and Surface Finishing in 1980. A printable PDF version is available by clicking HERE.
The corrosion protection provided by multilayer nickel deposits is related to the electrochemical potential difference between the individual nickel layers. A simple procedure for simultaneously determining the thickness, relative electrochemical potential and polarity of the individual layers of a multilayer nickel deposit is described. A commercially available thickness tester was modified to display results on a chart recorder. Representative results obtained for duplex and triple-layer nickel deposits are presented along with data on defective deposits.
Multilayer electrodeposits consisting of sulfur-free semibright nickel, sulfur-containing bright nickel and microdiscontinuous chromium constitute the preferred plating system for severe service applications such as exterior automotive hardware.1 The efficacy of duplex nickel coatings is a function of the electrochemical potential difference between the sulfur-free semibright nickel and the more active sulfur-containing bright nickel.2-4 A sufficient potential difference between the two nickel layers causes the bright layer to corrode preferentially and sacrificially with respect to the semibright nickel. This in turn retards the rate of penetration through the total nickel deposit and gives better (i.e., longer) corrosion protection to the basis metal than a single-layer nickel deposit of equal thickness. A 30.5 μm (1.2 mil) deposit of duplex nickel is superior to 51 μm (2.0 mil) of either semibright or bright nickel present as a single-layer deposit, each with 0.25 μm (0.010 mil) of chromium.5 The inclusion of a thin, very active nickel layer between the semibright and bright nickel layers can provide an additional increase in corrosion protection.6
In order to obtain the best possible corrosion protection from a multilayer nickel microdiscontinuous chromium deposit, certain conditions must be met. An adequate minimum thickness for each of the individual nickel layers as well as the proper thickness ratio of semibright to bright nickel are the most obvious requirements for satisfactory results. Of equal or even more importance, is a sufficient electrochemical potential difference between the various nickel layers together with the proper activity or polarity of the nickel layers. These last two items constitute the basis for the duplex effect. Finally, if a microdiscontinuous chromium deposit is used, it is essential that an adequate crack or pore count be achieved across the entire useful current density range of the plated part.
A variety of satisfactory procedures has been developed for measuring total deposit thickness. However, thickness measurements of the individual layers in a multilayer nickel deposit normally can only be made by sectioning, mounting, metallurgically polishing and etching a sample of deposit. The individual layers are then measured microscopically.7 The Dubpernell Test,8 or a recent modification,9 makes it easy to determine the crack or pore count in the microdiscontinuous chromium layer. Unfortunately, it has been virtually impossible to readily measure the electrochemical potential of the separate nickel layers in a multilayer nickel deposit, especially on a finished part. The individual nickel potentials are ordinarily determined by obtaining a foil or other nickel sample from the specific nickel plating bath used to plate the nickel layer in question, and measuring the open circuit potential.10 After the electrochemical potential is determined for a sample of nickel metal from each of the baths used to plate a multilayer nickel deposit, the potential difference between layers is calculated.
Because of the frequent need to measure the thickness of the individual nickel layers in multilayer nickel deposits, experiments were carried out to determine the feasibility of making the measurements using a coulometric method, with the hope that the time-consuming microscopic procedure could be avoided. During the development of a relatively simple and rapid test, it was found that the test procedure simultaneously permitted the measurement of the electrochemical potential difference between the nickel layers as well as the relative deposit polarities. (Simultaneous Thickness and Electrochemical Potential suggests the acronym "STEP" as a descriptive name for the test.)
Development of procedure and equipment
Thickness testers based on the anodic dissolution of the deposit are essentially cells in which the reaction of interest is at the anode. Knowing the area being deplated, the stripping current and time, plus the electrochemical equivalent weight and density of the metal being stripped, it is possible to calculate the deposit thickness using Faraday's Laws, provided the current efficiency at the anode is known.
If the plated metal being stripped is more easily dissolved in a particular electrolyte than the substrate under the plate, a relatively large voltage change is observed across the deplating cell when stripping is completed. This property was exploited to build a device which amplified the voltage change and actuated a relay which, in turn, stopped an electric timer which had been started at the beginning of the deplating operation.11 With this device, the deplating time and deposit thickness could be measured. A commercial thickness tester* makes use of this principle. This and similar testers will rapidly measure the total deposit thickness of most commonly plated metals with an accuracy of ±10%.12-14
By monitoring the potential changes across a deplating cell and recording the voltage as a function of time on a chart recorder, thicknesses of a sulfamate nickel deposit over a Watts nickel deposit have been measured.15 Because the power supplies used in coulometric stripping devices are designed to provide constant current but variable voltage, small voltage changes produced by differences in the electrochemical activity of the various nickel layers in multilayer deposits can be easily obscured by voltage changes originating in the power supply. Consequently, measuring voltage variations across a deplating cell is not a reliable way of measuring the thickness of individual nickel layers. A device for determining the thickness of tin and tin-iron alloy deposits on ferrous substrates has recorded the potential difference developed between the deposit being stripped and a reference electrode.16 This greatly reduced the problem of voltage variation mentioned above. Although this procedure apparently had not previously been exploited for measuring thickness or potential differences between semibright and bright nickel layers, the author discovered that it could be useful for this purpose.
To make the measurements described in this paper, a regulated, constant-current power supply and a sensitive millivoltmeter and/ or chart recorder are required in addition to a suitable stripping cell. The millivoltmeter and the chart recorder should have an input impedance of at least one megohm to prevent loading of the anode-reference electrode couple. The deplating cell contains the electrolyte, a reference electrode, the cathode, an anode of known area and provision for solution agitation at the anode to prevent concentration polarization. Rather than constructing such a device, it is easier and more convenient to modify the aforementioned, commercially available thickness tester, which is fully transistorized and provides the constant current source, a timer calibrated in deposit thickness units and a deplating cell that meets the above requirements.
Although this tester is designed to automatically turn the current off and stop the thickness counter when the deposit has been stripped to the substrate, it is best to keep the current on after the deposit is stripped in order to prevent premature termination of the test when stripping multilayer deposits with relatively large interlayer potential differences. To defeat the automatic shut-off feature of the device, a toggle switch was wired across the "Start" button and used to start and stop each test (The original "Start" switch may be used in its normal mode when the toggle switch is off.). Figure l shows how the toggle switch was installed.