Oxidation Behaviour of Ni-CrY 2 O 3 Composite Coatings Synthesised by Sol Enhanced Pulse Electroplating

The objective of this study was to improve the oxidation resistance of electroplated Ni-Cr coatings by introducing a fine dispersion of yttria (Y2O3) nanoparticles to the coatings. A technique was developed to produce Y2O3 nanoparticles nm in size. Electroplating conditions were also investigated and it was found that pulse plating could produce high quality coatings without cracking. The Y2O3 nanoparticles were then co-deposited with Ni-Cr and formed a fine dispersion in the coating. The Ni-Cr-Y2O3 composite coatings had improved oxidation resistance compared with the conventional Ni-Cr coatings. It is believed that this is due to Y2O3 nanoparticles causing finer grains to occur, thereby improving oxide scale adhesion.


Introduction
By 2035, the world's energy consumption is expected to increase by ~50% from that in 2007 (Doeman, 2010).One important way of improving the efficiency of energy generation and usage is by increasing the high temperature oxidation resistance of materials.According to Carnot's equation, the higher the temperature of the hot reservoir (T H ) compared to that of the cold reservoir (T 0 ), the higher the thermal efficiency ( ) (Smith, Van Ness, & Abbott, 2005).The materials used for high temperature applications, such as Cr-containing steels, have been designed with desirable mechanical properties.However, their limited oxidation resistance restricts them to a maximum service temperature of 700 o C (Rahman, Chawla, Jayaganthan, Chandra, & Ambardar, 2010;Trinidade, Christ & Krupp, 2010).If a coating is applied to improve the oxidation resistance, such that the material can operate at 900 o C, then the Carnot efficiency increases from approximately 69% to 75% (assuming a T 0 of 25 o C).Such efficiency increases can lead to a significant reduction in fuel consumption and CO 2 emissions (Sloof, 2008;Young, 2008).
Oxidation resistance in Ni-Cr coatings is provided by the protective chromia (Cr 2 O 3 ) scale that forms on the surface.However, this scale has a tendency to crack and spall due to growth and thermal stresses.The thermal stress is due to the different thermal expansion of the oxide to the underlying coating and substrate (Sloof, 2008;Wang, Wang, Yu, Zhu, 2008).Previous studies have found that scale adhesion can be improved by the addition of small quantities of rare earth elements, such as yttrium, or their corresponding compounds.This is known as the rare earth effect (Buscail, et al., 2004; Hussey & Graham, 1996; Wang, et al., 2008).One of the theories to explain the rare earth effect is that oxides of rare earth elements act as nucleation sites, causing smaller grains to occur in the oxide scale.The rare earth oxides can also segregate to grain boundaries in the oxide scale and prevent the grains from growing too large in size (Whittle & Stringer, 1980).In either case, it is expected that these effects can be enhanced by adding the rare earths to the coatings as a fine dispersion of nanoparticle 2 54   oxides, rather than as an element.This is because the fine dispersion would increase the density of nucleation sites or the number of particles available for pinning grain boundaries.If added as an element, the size and agglomeration of the rare earth oxide particles produced during oxidation could not be controlled.Electroplated coatings do not need expensive equipment and are therefore much more cost-effective than physical or chemical vapour deposition methods (Gao, 2011).However, when solid nanoparticles are added to electroplating solutions, the particles are typically agglomerated.These agglomerates cannot be broken down by agitation or surfactants effectively.It is therefore difficult to fabricate composite coatings with a real nano-dispersion of particles.But this has recently been accomplished with Ni-TiO 2 composite coatings.This involved developing a method of adding TiO 2 nanoparticles (~10 nm) in the form of sol and co-deposited with Ni or other metals (Chen & Gao, 2010).For this project, this technique was extended to produce yttria (Y 2 O 3 ) nanoparticles for Ni-Cr-Y 2 O 3 composite coatings.While there are a number of elements that produce the rare earth effect, yttria was selected as a starting point to determine whether or not the oxidation resistance of electroplated coatings can be improved by adding in rare earth oxides as nanoparticles, rather than as an element in solution.
The hypothesis of this study was that if Y 2 O 3 nanoparticles are synthesised in sol and added to the electrolyte of Ni-Cr to produce Ni-Cr-Y 2 O 3 composite coatings, then the oxidation resistance is greater than if the Y was added as an element or agglomerated oxide particles.To test this hypothesis there were three specific aims.The first was to develop a repeatable method for synthesising Y 2 O 3 nanoparticles in sol.The second aim was to develop the electroplating process to deposit Ni-Cr coatings with good oxidation and wear resistance.The third aim was to compare the oxidation resistance of Ni-Cr coatings electroplated from electrolytes that had Y element, Y 2 O 3 powder and Y 2 O 3 sol added.

Experimental Procedure
The method for synthesising nanoparticles of Y 2 O 3 in sol was developed to fulfil the first aim of this project and is discussed in Section 3.1.The final method used was to add 9 mL of 1 mol/L NaOH dropwise at 0.1 mL/min to 40 mL of 1.8 mol/L Y 3+ solution under 600 rpm magnetic stirring.A water bath was used to keep the temperature constant at ~20 o C. The sol, containing 0.04 mol/L of Y 2 O 3 , was added to the electroplating solution discussed in Section 3.2 in place of 49 mL of distilled water.It was necessary to leave this solution at room temperature for two days in order for the Y-O complexes to form Y 2 O 3 particles and the Cr 3+ to complex with glycine.Afterwards, the solution was stored at 4 o C to reduce nanoparticle growth to a very low rate.
Transmission electron microscopy (TEM) was used to characterise the nanoparticles formed in the sol.The instrument used was a Philips CM12 microscope with an accelerating voltage of 120 kV.A droplet (<1 μL) of Y 2 O 3 sol was put onto a copper mesh grid and dried at 40 o C for 10 minutes.
The substrates for electroplating were 8 mm by 15 mm rectangular 316 stainless steel samples.These were ground with SiC paper to P1200 grit and cleaned in ethanol.They were then degreased by immersion in xylene for 30 minutes, followed by pickling for 15 minutes in 30% HCl.In DC plating, the current required was set on the power supply.In pulse plating, the forward and reverse currents and pulse durations were set.The pH and temperature of the electrolyte were also controlled at 2.5 and 25 o C respectively.
Oxidation testing was performed at 900 o C (see Section 1.) in a vertical oxidation furnace.Six quartz crucibles were used to contain the samples and spalled oxides.For each crucible, the total mass and the mass of the crucible only was measured and recorded.This weighing was repeated after 1, 2,3,20,40,60,80,100,120,140,160,180 and 200 hours of oxidation.Spallation is the mass that is lost from the coating and was calculated from the change in mass of the crucible over the time of oxidation.The net mass was the difference between the total mass and crucible mass.The change in net mass over the time of oxidation gave the mass gain.
The wear resistance of the electrodeposited coatings was measured using a Nanovea Tribometer.The sample was oscillated linearly at a rate of 100 rpm for 1510 revolutions under a 6 mm diameter alumina ball with a 2 N load applied.The amplitude was set to ~10 mm.Optical microscopy was used to measure the width of the wear tracks, from which the volume loss was calculated.
The surface morphology and cross-sections of the Ni-Cr coatings were analysed using an XL30S FEG scanning electron microscope that was also equipped with an energy dispersive spectroscope (EDS).X-ray Diffraction was done using a D8 Advance Bruker AXS diffractometer, which used collimated and monochromated Cu-K α X-rays (λ = 0.15406 nm) produced at 40 kV and 40 mA (Seal, 2011).Diffracted X-rays were collected at scanning angles (2θ) of 40 o to 76 o with a scanning interval of 0.020 o and step time of 1 s.The peaks from the collected spectra were matched against a library of peaks using DIFFRAC plus to determine the compounds present.

Yttria Nanoparticle Synthesis
The yttria nanoparticle synthesis method developed was given in Section 2. Evidence was obtained to determine whether the sol of well-dispersed Y 2 O 3 nanoparticles was produced, and the reaction mechanism was studied.

Method for Nanoparticle Synthesis
From the previous work undertaken on the synthesis of nanoparticles in sol, the key feature of successful results was avoiding equilibrium conditions to prevent the particles from growing too large (Gao, 2009).To fulfil this, the solution of Y 3+ was kept at room temperature using a water bath, and the solution agitated at 600 rpm.The setup used for this controlled hydrolysis and condensation technique is shown in Figure 1 and the method for producing 0.04 mol/L Y 2 O 3 sol given in Section 2. It is desirable to maximise the concentration of Y 2 O 3 produced, as this is expected to maximise the improvement in oxidation resistance.However, this would require increasing the OH -concentration.If [OH -] is too high, then precipitation occurs.From our preliminary experiments, it was found that this was mitigated by reducing the droplet size and addition rate, and by increasing the concentration of Y 3+ .0.2 mol/L Y 2 O 3 was the most concentrated sol produced in these trials.This was done by adding 6.7 mL of 2.9 mol/L NaOH at 0.1 mL/min to 20 mL of 2.2 mol/L Y 3+ .However, it was found that the amount of nitric acid needed to solubilise the high concentration of yttria precursor meant that the pH was too low (~0) for uniform Ni-Cr coatings to be produced.Hence, 0.04 mol/L Y 2 O 3 sol was used in further investigations.
One of the advantages of producing nanoparticles in sol is that the particles are well dispersed.By contrast, when solid powder is added to water, they tend to agglomerate due to the high surface energy.To show this difference, both yttria sol and a mixture of yttria powder were prepared.The Y 2 O 3 sol was colourless, whereas the powder mixed in water was opaque.The opacity would be due to the agglomerates of yttria powder scattering light.For the sol to be colourless, the particles in the original sol must have been well-dispersed in order for the particles to be small enough to not scatter light.This supports the conclusion that the sol contained a dispersion of nanoparticles rather than agglomerates.
To measure the particle size, TEM micrographs were taken of a dried droplet of yttria sol (Figure 2).Image analysis was used to determine the mean equivalent circle diameter of the particles, which was found to be nm for the nanoparticles from sol.The agglomeration of particles shown in Figure 2 is due to the drying of the sol.This shows that well-dispersed nanoparticles of yttria in sol were indeed produced by the technique developed.The technique is advantageous over other sol-gel techniques, which involve precipitating the Y 2 O 3 out as a solid powder, invariably leading to their agglomeration (Cheng, 1999).

Mechanism for Nanoparticle Synthesis
For the sol-gel method, when a hydrolysing agent is added to the sol, the sol undergoes hydrolysis and condensation reactions (Barlier, Bounor-Legaré, Boiteux, Davenas, & Léonard, 2008).It is therefore expected that when OH -is added to Y 3+ solution, the reactions occur by paths a (hydrolysis) and b (condensation) of Figure 3.The Y -O complex formed after condensation would complex further with OH -and Y 3+ until the atomic ratio of Y to O is 2:3.
The greater the amount of hydroxide added to the Y 3+ mixture, the more Y 2 O 3 nanoparticles that are produced.But if excessive hydroxide is added, then the forward reaction rate along path c in Figure 3 would be increased, leading to the precipitation of Y 3+ as Y(OH) 3 and formation of a gel.However, it was found that when OH -was added dropwise at a rate of 0.1 mL/min, white precipitates initially formed but disappeared with agitation.This would have been because agitation broke apart the precipitates and exposed them to excess Y 3+ , which would have attracted the OH -to complex in a similar way to path a of Figure 3.
Five experiments were conducted to investigate the veracity of the proposed reaction mechanism.The 'base conditions' were 20 mL of 0.2 mol/L Y 3+ , 600 rpm agitation, and 50 μL droplets of 0.2 mol/L NaOH added at 0.1 mL/min.All of these variables were changed one at a time, except for the volume of the Y 3+ solution.From Figures 4 and 6, it can be seen that the droplet rate, droplet size and agitation rate did not significantly change the volume of NaOH added for the sol-gel transition to occur.It can also be seen that the ratio of OH -to Y 3+ at the sol-gel transition was approximately 0.7.This is about half of the expected ratio of 1.5 (ratio of O to Y in Y 2 O 3 ).
However, the fact that this ratio was consistently obtained in Figures 4, 5 B and 6 makes it unlikely that it was an experimental error.
The reason for the sol-gel transition point consistently being half of that expected is most likely because when the OH -content became too large, path c in Figure 3 became favourable compared with path b, leading to precipitation.The main discrepancy to this is in Figure 5 A, when the [OH -]: [Y 3+ ] ratio almost increased to 1.5.However, this is mainly due to the volume added to reach the sol-gel transition point being very small (1 mL).It was difficult to judge precisely where the transition point occurred.Further trials with concentrations below 3 mol/L of NaOH should be done to reliably determine whether the [OH -]:[Y 3+ ] changes from ~0.7:1.

Electroplating Process
Before Y 2 O 3 nanoparticles could be added to the Ni-Cr coatings, the conditions for electroplating Ni-Cr coatings with good oxidation and wear resistance first needed to be established.The factors that affect the electroplated coating quality and Cr content are pH, temperature, Cr 3+ content in the electrolyte and current density.From literature, the reported optimum pH value for Ni-Cr deposition is 2.5, temperature 25 o C, Cr 3+ content 75 g/L and current density 160 mA/cm 2 (Xu, et al., 2007).The optimum pH and temperature values were confirmed from preliminary trials.The optimum Cr 3+ content in the bath was found to be 80 g/L, which was very close to literature.This value was found by analysing the EDS spectra of the coating cross-sections; 80 g/L of Cr 3+ in the bath allowed the Cr content in the coating to exceed 20 wt.%, which is the minimum needed for protective chromia scale to cover the coating (Sloof, 2008).The effects of the deposition technique on the coating quality are discussed in Sections 3.2.1 and 3.2.2.

DC vs. Pulse Plating
Oxidation tests were done at 900 o C to determine the best coating technique.The effective current density here was 70 mA/cm 2 .All direct current (DC) plated Ni-Cr coatings spalled off in oxidation testing at 900 o C.This can be seen from the negative mass gains in Figure 7. From Figure 8, there appears to be cracks running throughout the DC plated coating.According to Lin et al., cracks are commonly observed on electrodeposited Cr and Cr alloy coatings.This was attributed to the unstable chromium hydrides forming and decomposing to evolve hydrogen gas.Cracks result from the large tensile stresses generated by the 15% decrease in volume when the hydrogen gas is released (Lin, Hsu & Chang, 1992).The evolution of hydrogen gas would also lead to significant porosity in the coating, which would weaken the bonds between the coating and substrate.Consequently, when thermal stresses are applied during oxidation testing, the coating would readily spall off.
It is reported that pulse plating can overcome cracking in Ni-Cr coatings (Lin, et al., 1992).This involves a periodic alternation in current direction.The reverse currents cause the hydrides to decompose (Lin, et al., 1992).
From the optical micrograph of the pulse plated Ni-Cr coating shown in Figure 9, it can be seen that the coating is crack-free.Also, from the preliminary oxidation test results shown in Figure 7, it can be seen that the pulse plated Ni-Cr coating remained intact throughout the oxidation test.The parabolic rate constant ( mg 2 .cm - .hr - ) for the pulse plated Ni-Cr was lower than that of the uncoated stainless steel ( mg 2 .cm - .hr - ), indicating that the pulse plated Ni-Cr coating was adherent and more protective.It can be seen that pulse plating can overcome cracking in the Ni-Cr coatings and was therefore used in all subsequent experiments.

The Effect of Current Density
Mechanical and oxidation properties were used to assess the effect of the applied current densities on the coating quality.From the trials with DC plating, it was found that using 160 mA/cm 2 (Section 3.2) was too high because the coatings were cracked and not adherent.On the other hand, when the current density was too low (< 50 mA/cm 2 ), there was no deposition.The optimum current density was found to be ~80 mA/cm 2 .Based on this, three current density patterns (Table 1) were studied in detail through oxidation tests.The pulse pattern consisted of a forward period of 5 ms and reverse period of 3 ms (Lin, et al., 1992).
Figure 10 shows that when the forward current density was larger than the reverse current density (Pattern C), spallation occurred, similar to DC plating.One possibility is that the driving off of hydrogen gas by the reverse pulses was inadequate, leading to a reduction in the adhesion of the coating to the substrate.Thus, the two desirable current density patterns were those of Patterns A or B. Wear testing was used to decide on which to use since it indicates at the adhesion of the coating.Figure 11 indicates that when plating with a larger reverse current density (Pattern B), the wear resistance was larger and also higher than uncoated stainless steel.These results were because the reverse currents have a tendency to dissolve more Ni, leading to a higher Cr composition (Lin, et al., 1992).As Cr has a higher hardness than Ni, the Ni-Cr coating was harder overall.
From these studies, the optimum forward pulse current density for Ni-Cr electroplating was found to be 320 mA/cm 2 , with a period of 5 ms.The optimum reverse current density was 340 mA/cm 2 , with a period of 3 ms.The electrolyte composition was also developed as part of the second aim and is given in Table 2.

Comparison of Oxidation Resistance
Ni-Cr-Y 2 O 3 composite coatings were produced by immersing the steel substrates in the Ni-Cr bath with Y 2 O 3 sol added to it and electrodepositing the coating onto the substrates (see Section 2.).In order to fulfil the third aim, the oxidation behaviour of the Ni-Cr-Y 2 O 3 coatings was compared with Ni-Cr, Ni-Cr electroplated with Y 3+ solution in the electrolyte (Ni-Cr-Y), and Ni-Cr electroplated with 0.04 mol/L Y 2 O 3 added as powder to the bath.
As the parabolic model indicates that the scale is protective, linear regression between the square of mass gain per unit area (t s 2) and time of oxidation (t) was used to obtain the rate constants of the fitted parabolic model (Figure 12).These are given in Table 3.It can be seen that in all cases, the variability captured by the parabolic models exceeds 85% (from the Multiple-R 2 values), indicating that the parabolic model is suitable for making predictions.The p-values for the parabolic rate constants are all below 0.001, which provides very strong evidence against their being zero.Overall, these indicate that the parabolic models are suitable for the mass gains of the samples during oxidation, and the scale on all coatings were stable.Also, a lower parabolic rate constant usually indicates at an improved oxidation resistance (Young, 2008).Ni-Cr-Y has the lowest parabolic rate constant.This is in agreement with literature: adding Y improves the oxidation resistance of the alloy through the rare earth effect (Hussey, Sproule, & Graham, 1993;Whittle & Stringer, 1980).But Ni-Cr coatings with Y 2 O 3 added as sol and powder both had slightly higher parabolic rate constants than Ni-Cr.
However, an important determinant of oxidation resistance is the adhesion of the scale to the coating.Adhesion is indicated by oxide scale spallation; the more scale that spalls off, the poorer the adhesion (Haynes, Pint, Porter, & Wright, 2003).The spallation results are shown in Figure 13 Two of the main theories to explain how the rare earth effect reduces spallation are the oxide pegging model and grain refinement by the solute-drag effect (Whittle & Stringer, 1980).The most likely explanation for the reduction in spallation with the Ni-Cr-Y and Ni-Cr-Y 2 O 3 coatings is grain refinement.XRD analysis showed that the oxidised samples had magnetite, chromia, NiCr 2 O 4 and chromium-iron oxides present.But only the magnetite peaks did not overlap in the diffractograms, meaning that only these could be analysed to compare the oxide grain size of the different coatings.From Figure 15 and Table 4, it can be seen that the ~50 nm magnetite grain size on Ni-Cr is reduced by the addition of Y and even more by the addition of Y 2 O 3 sol, in which the grain size is ~30 nm.Assuming that the trends in the grain size of magnetite are the same across all other crystals (especially Cr 2 O 3 ), the Ni-Cr-Y 2 O 3 composites would have formed the smallest chromia grains.
Grain refinement would be due to Y 2 O 3 nanoparticles either acting as nucleation sites for grains in the scale, or due to the pinning effect of the grain boundaries, which prevents them from growing (Whittle & Stringer, 1980).The Y in Ni-Cr-Y would have oxidised to form Y 2 O 3 and thereby resulted in grain refinement.But the Y 2 O 3 in Ni-Cr-Y 2 O 3 would have been present from the start to nucleate small grains and pin grain boundaries, leading to further grain refinement.Ni-Cr-Y 2 O 3 composite coatings should therefore have better oxidation resistance than Ni-Cr-Y coatings.This is because Ni-Cr-Y 2 O 3 coatings have a higher grain boundary density, which should lead to the greater relief of thermal stresses during oxidation testing.

Conclusions
Electroplating has been used to produce oxidation resistant Ni-Cr coatings with a fine dispersion of Y 2 O 3 nanoparticles.The processing, microstructure and properties of the composite coatings were studied in detail.The main results are presented below: Y 2 O 3 nanoparticles were synthesised in sol by controlled hydrolysis and condensation.Nanoparticles of nm were formed.
It is believed that Y 2 O 3 nanoparticles form by OH -complexing with Y 3+ through a series of hydrolysis and condensation reactions, producing Y-O-Y complexes.These produce Y 2 O 3 nanoparticles after aging due to further complexing.
Pulse plating with a forward period of 5 ms and reverse period of 3 ms electrodeposited crack-free Ni-Cr coatings onto the 316 stainless steel substrate.
In studies on current density, it was found that the most wear and oxidation resistant Ni-Cr coatings occurred when the forward current density was 320 mA/cm 2 and reverse current density 340 mA/cm 2 ; the overall current density was 70 mA/cm 2 .There is evidence to suggest that adding Y 2 O 3 nanoparticles via sol improves the oxidation resistance of Ni-Cr coatings than when Y is added as an element.Extended oxidation tests need to be done to provide further support to this conclusion.Table 1 specifies the forward (i f ), reverse (i r ) and effective (i effective ) current densities for the various current patterns that were used to electroplate Ni-Cr.The samples for each pattern were oxidation tested at 900 to determine which pattern to use.*NaF was added as a brightener to reduce the density of carbides on the surface of the coatings and thereby reduce the darkness of the coatings.

Spallation results show that
**Glycine was added as a complexing agent to allow chromium deposition.
The final volume of the electrolyte was 88 mL: 80 mL distilled water, 8 mL of 2 g/L sodium dodecyl sulfate.In Table 3, the parabolic rate constants were determined by linear regression between the square of mass gain and time of oxidation.The uncertainties are two standard deviations for the rate constant.The variability captured by the model refers to the Multiple-R 2 from the F-test, from which the p-values were also obtained.In Table 4, the FWHM was obtained from Figure 14 from the {440} peaks of Magnetite.However, the FWHM of Ni-Cr with powder needed to be obtained from the {400} peak, as the {440} peak had too much overlap with nickel chromium oxide for that particular sample.The Miller indices were determined using Bragg's law and the magnetite have a cubic unit cell and lattice parameter of 8.44 Å.The line broadening due to the diffractometer was found to be 0.137 o .Scherrer's constant was taken as 0.9.      Figure 9 shows that with pulse plating, no cracks could be detected from optical micrographs take at 20 times magnification on the objective lens.
. The fitted model for the spallation per unit area (s) of Ni-Cr was ; for Ni-Cr deposited with Y 2 O 3 powder was .The F-statistic p-values were both <0.001, indicating at very strong evidence against the proportionality constants being zero.The p-values for the F-statistics of linear models fitted to the spallation of Ni-Cr-Y and Ni-Cr-Y 2 O 3 were both ~0.1, indicating at no evidence against spallation changing with time from zero.This indicates that the oxidation resistance of Ni-Cr-Y and Ni-Cr-Y 2 O 3 coatings were significantly better than the other two coatings, due to the much lower spallation.The improvement in oxidation resistance of Ni-Cr-Y 2 O 3 composite coatings is also supported by the SEM micrographs in Figures 14 A to D. These show that the Ni-Cr, Ni-Cr electroplated with Y 2 O 3 powder andNi-Cr-Y coatings all had oxide scale with some degree of spallation.By contrast, the Ni-Cr-Y 2 O 3 coating had an oxide scale that was compact with no spallation.These SEM micrographs also show that although no significant spallation of Ni-Cr-Y coatings was measured, some spallation did actually occur.This indicates that the Ni-Cr-Y 2 O 3 composite coatings have better oxidation resistance than Ni-Cr-Y coatings.
Ni-Cr-Y and Ni-Cr-Y 2 O 3 coatings possess better oxidation resistance than Ni-Cr and Ni-Cr electroplated with Y 2 O 3 powder.Results indicate that this was because of the enhanced scale plasticity due to the finely dispersed Y 2 O 3 nanoparticles refining the grain size.

Figure
Figure 1. S ey features of nts the nanopar instant gelifica

Figure 2 Figure 3 .
Figure 2. TEM by analysing t nt.This indica m in the TEM ands of the cop

Figure 4 .Figure 5 .
Figure 4. Volume of NaOH added to reach sol-gel transition and ratio of n(OH -) at gelification to n(Y 3+ ) in solution as a function of rate of OH-addition (A) and size of droplets (B)Figure4shows that the volume of 0.2 mol/L NaOH added to gelify 20 mL of 0.2 mol/L Y 3+ is independent of droplet rate or droplet size, as the ratio of the amount of OH -added at gelification to the amount of Y 3+ present in the solution is approximately constant in both cases.

Figure 6 .Figure 7 .
Figure 6.Volume of NaOH added to reach sol-gel transition and ratio of amount of NaOH added at the transition point to the amount of Y 3+ in the solution as a function of agitation of the solutionFigure6shows that both the volume of 0.2 mol/L NaOH needed to gelify 20 mL of 0.2 mol/L Y 3+ is independent of agitation, as the ratio of the amount of OH -added at gelification to the amount of Y 3+ originally present is approximately constant.

Figure 8 .Figure 9 .
Figure 8. Optical micrograph with arrows pointing at the crack running across the DC plated Ni-Cr coating Figure 8 shows that the DC plated Ni-Cr coatings tended to have cracks running through them.One of these is indicated by the black arrows.

Figure 10 .
Figure 10.Oxidation kinetics of the three studied current density patterns for pulse plating Figure 10 shows that the mass gain of Ni-Cr pulse plated by Pattern C is significantly negative, indicating at significant spallation.But Ni-Cr pulse plated by both Pattern A and B have positive mass gains, indicating at adherent coatings.

Table 1 .
Current density patterns studied in oxidation testing

Table 2 .
Final electrolyte composition for Ni-Cr plating

Table 3 .
Comparison of parabolic rate constants and goodness of fit of parabolic models to mass gain curves

Table 4 .
Calculation of magnetite grain size using line broadening