The corrosion behavior of electroless Ni-P coatings in concentrated KOH electrolyte

Ni-P has been widely used as a protective coating for many substrates. The corrosion resistance of Ni-P in neutral solutions such as NaCl, or acidic electrolytes such as HCl and H 2 SO 4 , has been extensively studied. However, the corrosion behavior of Ni-P coatings in caustic media, such as KOH, has received much less attention. Typically, corrosion behavior is studied through the use of electrochemical methods with corrosion rates determined from corrosion currents and potentials measured from Tafel curves. In this work, the corrosion rates of Ni-P coatings, with P concentrations varying from 2 to 11 wt%, in highly alkaline KOH (11 M) are obtained directly through electron microscopy measurements of cross sections and subsequent correlation with electrochemical data. Phosphorus concentration affects the corrosion rate; corrosion rate increases with increasing P content, peaks out at about 6–8 wt% P, and then decreases with any further increase in P content. This behavior is correlated to internal stress levels developed in the coatings.


Introduction
Ni-P coatings have been widely used to protect various kinds of surfaces, both metallic and non-metallic, due to their good wear resistance, high hardness, solderability, and corrosion resistance.Ni-P represents over 95% of industrial electroless coatings [1][2][3][4][5].In the last few decades, the mechanical properties of the coatings, such as hardness and wear resistance, have been extensively studied for as-deposited lms and after heat treatment.Researchers are now focusing on adding third, or even fourth elements, or nanoparticles to further improve the mechanical properties of Ni-P coatings [2].In contrast to mechanical properties, the literature available on the corrosion resistance of Ni-P coatings is not as widespread, with a general consensus that a high P content provides better corrosion resistance [4,5].
Most of the electrolytes used for Ni-P testing in the literature are neutral solutions (e.g., 3 or 3.5 wt% NaCl) or acidic solutions (e.g., HCl or H 2 SO 4 ) [6][7][8][9][10][11], with very few reports for alkaline environments [12,13].This is because many of the applications involve exposing Ni-P coatings to air with humidity or slightly acidic environments, such as pipelines for transporting petroleum products [14][15][16].Studies on the corrosion behavior of Ni-P in alkaline environments has become necessary, since Ni-P coatings are being increasingly applied as catalysts for electrodes [17] in Zn-air batteries (ZABs) or Zn-air ow batteries (ZAFBs).Nickel can assist with the oxygen evolution reaction (OER) during battery charging; however, Ni can also catalyze the hydrogen evolution reaction (HER), which is undesirable.The electrolytes in these Zn-based batteries are highly alkaline with concentrations as high as 45% KOH (11 M KOH) [18,19].It is necessary to evaluate the corrosion behavior of Ni-P under alkaline conditions, since corrosion can affect electrode lifetime and operation cost.This work aims to investigate the corrosion behavior of Ni-P coatings in an extremely caustic alkaline electrolyte (11 M KOH).Conventional electrochemical testing techniques and direct measurements of coating thickness and morphology changes are employed and compared.

Sample selection and preparation
Ni-P coatings can be electrolessly deposited from either alkaline electrolyte [20] or acidic electrolyte [21].For this work, electrolessly deposited samples were obtained from a commercial supplier.The details of the deposition process are proprietary; however, the Mg samples went through a pretreatment process that included alkaline cleaning, acid pickling, activation, and Zn immersion steps.Some of the coatings consisted of single Ni-P layers, while others were composed of multiple layers with Cu strike layers in some cases.Samples were chosen such that there were a range of P concentrations in the coatings, from 1.8 wt% to 11.9 wt% (Table 1).The concentrations were measured using a scanning electron microscope (SEM) coupled with energy dispersive x-ray (EDX) spectroscopy.Figure 1 shows a schematic drawing for a multilayer coating.For all multi-layer coatings, the thickness of Ni-P above the Cu strike (t NiP1 in Fig. 1) was ≥ 24 µm, while the total coating thickness exceeded 30 µm for all samples.The dashed line in the t NiP1 layer (Fig. 1) means that the P content above or below the line may be different.
For all samples in Table 1, the P content refers to the amount of P in the outermost Ni-P layer; i.e., the layer in contact with the electrolyte during testing.Dynamic polarization measurements were performed using a Biologic VSP-300 potentiostat; the scan speed was set at 1 mV/s with a potential range of -0.8 to 0.4 V.A Hg/HgO reference electrode was employed along with a graphite counter electrode.The corrosion potential E corr and corrosion current density I corr were determined from measured Tafel plots using EC-Lab.The area exposed to the corrosive medium (11 M KOH, 125 ml) was about 1 cm 2 , with non-exposed portions covered using epoxy.

Cycle testing
Galvanostatic cycling tests for the Ni-P coatings were carried out to simulate the HER and OER processes in a ZAFB cell with 11 M KOH electrolyte.The experimental settings were the same as those in Section 2.2.1.The cycling test parameters are listed in Table 2, where E we is the working electrode voltage.All cycling tests were carried out at room temperature and most samples were tested for 2000 cycles.One sample (Sample 9) was also tested for 10,000 cycles to measure any possible corrosion rate changes with extended cycling.

Pore density and pore areal fraction determination
Pore sizes in the Ni-P coatings were measured from SEM back scattered electron (BSE) images of polished cross section samples.Pore shapes were irregular, so to be consistent pore sizes were measured as the largest diameters and pore areal fractions were estimated based on the assumption that the pores were circular.Pore numbers and diameters were measured using ImageJ software and then converted to pore areal densities and pore areal fractions.

Scanning electron microscopy (SEM)
All samples were examined in cross section orientation, before and after electrochemical testing, using scanning electron microscopy (Tescan Vega3 SEM).The samples were cold mounted, followed by mechanical grinding, and then nal polishing using a 0.05 µm Al 2 O 3 suspension.The polished samples were coated with a thin layer of evaporated carbon prior to SEM examination.Ni-P coating thicknesses were measured from BSE images before and after electrochemical cycling from the same samples.BSE images provided atomic number contrast and allowed the various coating layers to be easily identi ed.
Composition analysis of the various layers was done using EDX microanalysis (X-MaxN 20, Oxford Instruments).BSE imaging and EDX analysis for all samples were done at the same accelerating potential (20 kV), the same working distance (14 mm), and the same magni cations.For each sample, ve BSE images were obtained and coating thicknesses were measured at ve locations per image, giving a total of 25 measured thicknesses for each sample.

Results and discussion
For corrosion testing in neutral electrolytes (e.g., 3.5 wt% NaCl) or mildly acidic electrolytes (water diluted HCl or H 2 SO 4 ), there is a consensus in the literature that the corrosion current I corr decreases as the P concentration in Ni-P increases [5][6][7][8][22][23][24][25].However, the situation for alkaline solutions is different.Based on polarization and electrochemical impedance spectroscopy (EIS) experiments, Zeller and Salvati [13] reported that for a 50% NaOH electrolyte a Ni-P coating with 5.0 wt% P had the lowest corrosion rate (0.08 nm/h).Coatings with 10.5 wt% P had the highest corrosion rate (0.30 nm/h), while Ni-P coatings with 2.0 wt% P had corrosion rates (0.13 nm/h) between the two extreme values.
The various Ni-P coatings were cycle tested.Polarization curves were obtained before and after cycle testing; examples for coating 11 are shown in Fig. 2. E corr and I corr values were extracted from the plots (Table 3) and I corr values were converted to corrosion rates (CR) based on Faraday's Law using Eq. ( 1) [26] and plotted as a function of P content in the outer Ni-P layer (Fig. 3).
where CR is given in nm/h and I corr in µA/cm 2 .K 1 is equal to 0.373 nm•g/(µA•cm•h), ρ is the density of the corroding metal (8.908 g/cm 3 for pure Ni was assumed), and EW is the dimensionless equivalent weight, which is given as W/n (W is the atomic weight of the element and n is the valence for the corroding metal, which is 2 for Ni).In all cases, there is an increase in corrosion rate for the post-cycled samples compared with the pre-cycled samples.In addition, although there is considerable scatter, the corrosion rate is highest at intermediate P compositions (~ 6 wt%) and reduced at lower and higher P concentrations.
I corr ρ The use of I corr to determine corrosion rate is an indirect method.A direct approach was taken by measuring the layer thicknesses of regions that were not exposed to the electrolyte and regions that were exposed to the electrolyte.The average corrosion rate is then just the difference in coating thickness divided by the cycle testing time.Examples of SEM BSE cross section images for non-cycled and cycled regions are shown in Fig. 4. It is clear that corrosion of the Ni-P coating is not localized, but occurs uniformly across the sample.The example in Fig. 4a corresponds to Sample 8 and represents a case where corrosion of the outer Ni-P layer is clearly apparent.The example in Fig. 4b indicates a case (Sample 11) where the amount of corrosion is low.For many of the samples, the Cu strike layer was utilized as a marker layer for the coating thickness measurements.
The thickness changes and corresponding average corrosion rates are provided in Table 4.The average corrosion rate as a function of P content in the outermost Ni-P layer is plotted in Fig. 5.The behavior is not linear, but exhibits a parabolic shape.The corrosion rates are lowest at the highest and lowest P compositions, with the highest rates at intermediate compositions in the 6-8 wt% P range.This behavior is similar qualitatively, but more obvious, to that shown in Fig. 3, where corrosion rates extracted from I corr values were plotted against P composition.To the authors' knowledge, this is the rst time that corrosion rate has been correlated with the P contents in Ni-P coatings through direct measurements of coating thickness changes under the same corrosion conditions, rather than just indirectly from corrosion currents.Although the results are qualitatively similar for both indirect (polarization curves) and direct measurements, the indirect measurements can, in some instances, provide corrosion rates signi cantly in excess of actual values (e.g., Samples 5, 6 and 10 -Table 3 and Table 4).In addition, inconsistencies can arise in the way that I corr values are obtained from polarization curves; e.g., determination of cathodic and anodic slopes.The work cited earlier in this section by Zeller and Salvati [13], for Ni-P coatings in NaOH electrolyte, reported an opposite corrosion dependence with P content.
The lowest corrosion rate was observed for intermediate P levels (e.g., 5.0 wt% P), although their rates were determined indirectly; i.e., through electrochemical measurements such as polarization electrochemical impedance spectroscopy.In addition, their electrolyte (NaOH) was different than that used in this work.Cycle testing was also done for 10,000 cycles for Sample 9 using the same set of parameters; the results are shown in Fig. 6. Figure 6a is from the unexposed region (no cycling), Fig. 6b is from the region tested for 2000 cycles, and Fig. 6c is from the region that underwent 10,000 cycles.The average thickness change after 2000 cycles is 2079 nm, while the average thickness change is 11,645 nm after 10,000 cycles.These values correspond to average corrosion rates of 44.0 nm/h and 49.3 nm/h, respectively, which indicate that the corrosion process does not vary signi cantly with time.
It is interesting to note that the parabolic shape of CR vs P content curve is very similar to that for coating stress vs P content reported by Duncan in 1996 [27] and shown in Fig. 7.In Fig. 7, the blue dots, showing stress vs P content, were reproduced from Fig. 2 of Duncan's work [27].The dotted line has been added by the current authors to better illustrate the trend for the distribution of stress vs P content.
To compare the stress distribution vs P content with the corrosion rate distribution vs P content, Fig. 5  To con rm that the corrosion rates of Ni-P coatings are indeed dependent on P content and not other factors, such as coating porosity, pore fraction and areal density were measured from SEM cross section images.The effect of porosity on Ni-P coating corrosion behavior in alkaline solutions has not been reported previously.The results, for 8 of the coatings, are presented in Fig. 8 and reveal no clear relationship between corrosion rate and pore density or pore fraction.One could argue that there is a weak trend indicating that corrosion rate decreases slightly as pore fraction increases.This behavior seems counterintuitive, as increased porosity would presumably lead to increased corrosion [31][32][33].
The pores for the Ni-P coatings in this work are likely isolated in nature; i.e., do not continuously propagate from the substrate/coating interface to the coating surface.As such, porosity does not enhance corrosion rates.To con rm the nature of the porosity, several solutions were analyzed using atomic absorption spectroscopy (AAS), before and after cycling testing.In all cases, Mg levels in the electrolyte were below the detectability limit of AAS (0.07 ppm Mg), indicating that no Mg dissolution occurred.Mg dissolution would require extension of pores from the coating surface to the Mg/coating interface.

Conclusions
The corrosion behavior of 12 Ni-P coatings, electrolessly deposited on Mg substrates in highly alkaline KOH (11 M) electrolytes, was investigated using cyclic testing where samples were exposed to oxygen evolution reaction and hydrogen evolution reaction conditions.Corrosion rates were determined both indirectly through I corr measurements from polarization curves and directly through SEM measurements of cross section samples.Corrosion rates were found to correlate with the P content in the Ni-P coatings, which appeared to correspond to stress levels in the coatings.Corrosion rates were lowest for P concentrations below ~ 3.5 wt% and above ~ 11 wt%, which correlates with compressive stresses within the coatings.Schematic drawing for coating layers.Note that not all samples have this number of layers.Speci c details for coating layers for each sample are provided in Table 1.
Figure 2 Potentiodynamic polarization curves before and after cycle testing for Sample 11.
Page 14/16 Relationship between calculated corrosion rates (CR) from I corr and P concentration for electroless Ni-P coatings.The blue circles represent corrosion rates before cycle testing, while the orange circles represent corrosion rates after cycle testing.The insets show results from Sample 10, which had signi cantly higher pore density and pore fraction.
There is no obvious relationship between corrosion rate and pore density/pore fraction.

Figures
Figures

Figure 4 a
Figure 4

Figure 5 Relationship
Figure 5

Figure 7 Relationship
Figure 7

Table 3 E
corr and I corr before GCPL and after GCPL testing, as well as corresponding corrosion rates (CR)

Table 4
is superimposed on Fig.7(shown as orange dots and orange dashed curve).As pointed out by Duncan, at P concentrations below ~ 3.5 wt% or above ~ 11 wt% the internal stress within the coatings is compressive, while coatings with compositions between these values exhibit tensile stresses.The corrosion rate vs P distribution essentially follows Duncan's distribution of stress vs P content.Generally, tensile stresses are not favorable in terms of corrosion resistance [28-30], since tensile loads may open cracks or weaken coating coverage and may also accelerate metallic ion dissolution into the electrolyte.On the other hand, compressive stresses in coatings can inhibit crack initiation or crack growth, thereby reducing corrosion rates.If coating stress is unavoidable, then compressive stresses are preferred relative to tensile stresses.As such, coatings with P levels either below 3.5 wt% or above 11 wt% are favorable for Ni-P coatings utilized in 11 M KOH electrolytes.