title: investigations concerning chromium plating from ... · pdf filenodules (figure 3-4),...

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Authors and contributors: Juergen Fischer, Eric Nagel, Michael Mann

Date (written): 12-20-2012

Report number: 120808 JKF

Approved:

Title: Investigations concerning chromium plating from electrolytes containing chromium-III

chloride and ionic liquid.

1 Introduction 2 Literature review 3 Chromium plated sample from the University of Leicester, UK 4 Instrumentation and electrochemical cell setup 5 Investigating electrolytes made from choline chloride and chromium-III

chloride hexahydrate 6 Investigating electrolytes made from choline chloride, chromium-III

chloride hexahydrate and additions 6.1 Adding water 6.2 Changing the chromium to ionic liquid ratio in the electrolyte 6.3 Adding lithium chloride 6.4 Adding potassium chloride 7 Investigating electrolytes made with 1-ethyl-3-methylimidazolium chloride

([EMIm]Cl), anhydrous chromium chloride 7.1 Varying the current density 7.2 Adding lithium chloride and changing the current density 8 Summary, conclusions and recommendations for future research Attachments

A References

B Cr plating tests

AEM Center

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Investigations concerning chromium plating from electrolytes

containing chromium-III chloride and ionic liquid.

1 Introduction The investigations were done by the Advanced Engineered Materials Center (AEM Center) of the University of North Dakota (UND), Grand Forks, ND. The purpose of these experiments was to rapidly assess the feasibility of using ionic liquids to electroplate steel with chromium metal from a non-aqueous chromium-III plating bath. The chromium metal coating will function as a corrosion barrier, so the coating must be contiguous with no cracks; high thickness uniformity and minimal inclusion of impurities is also desired. Published literature [1] indicates crack-free coatings of chromium have been deposited using choline chloride and chromium (III) chloride hexahydrate eutectic mixtures. The experimental tasks are meant to use the published methodologies for electrodeposition of chromium with ionic liquids [1-4] as a base-line to develop a commercially useful process for plating chromium onto steel with an electrolyte containing chromium-III salt and ionic liquid. Later we decided to do some testing in 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) with anhydrous chromium chloride to avoid the possibility of introducing hydrogen embrittlement into the substrate.

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2 Literature review The references mentioned are listed in attachment A. A good overview of the aqueous chromium plating is given by M. Schlesinger and M. Paunovic [5]. The basic information about electrodeposition from ionic liquids can be found in a book from F. Endres [6]. Newer information about chromium plating with ionic liquids is provided in [1, 2, 4, 7 to 10]. The most promising approach seems to use a mixture of choline chloride, chromium-III chloride and water as published in 2010 by E. Smith et. al. [4]: The exact electrolyte composition is proprietary, but chromium(III) chloride hexahydrate, choline chloride and water are the only components. The bath water content is monitored by electrolyte conductivity. The plating temperature is kept at 40 oC using a cold water cooling coil and the current density is set at 600 mA/cm2. The plating time is generally 45 to 60 minutes. The current efficiency for black chromium can be up to 90 %, where for hard chromium it is in the region of 40 %. The overall current efficiency depends strongly on the composition of the electrolyte and the anode material. The pathways of lost current density are mostly known and well defined, but are related to the proprietary electrolyte composition and cannot be detailed. The amount of micro cracking found in the chromium layer varies with current density and plating time. The adherence of the chromium layer needs improvement. The absolute current density of 600 mA/cm2 and/or the general plating time seems very high for a current efficiency of 40 or 90 %. For chromium A chromium plated steel tube sample from the IONMET consortium was requested, received and analyzed (see Chapter 3). In 2007, P. Benaben [7] reported about a similar electrolyte: At about 40 oC the current density was 150 mA/cm2 and the deposition rate around 50 µm/hour. This suggests a current efficiency of 10 % (see Chapter 6.1 and 6.2). A. Abbott et. al. published 2004 the electrodeposition of chromium black from an electrolyte consisting of choline chloride, chromium chloride hexahydrate and lithium chloride (molar ratio = 1:2:2.8) at 60 oC and a very low current density of -0.345 mA/cm2 (see Chapter 6.3). S. Eugénio et. al. [8, 9, 10] published findings of precipitation of black chromium which consists of chromium oxide, chromium hydroxide and metallic chromium. The electrolyte consisted of 1-butyl-3-methylimidazolium tetrafluoroborate and chromium(III) chloride hexahydrate. The electrodeposition occurred in two steps involving Cr(III) and Cr(II) electroactive species. In 1997, M. Ali et. al. [11] investigated the electrodeposition of aluminum–chromium alloys from electrolytes made from chromium(II) chloride, aluminum chloride and N-(n-butylpyridinium chloride.

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3 Chromium plated sample from the University of Leicester, UK The AEM Center requested from the University of Leicester a steel sample plated with a crack free, 10 micrometer chromium layer, using their proprietary electrolyte consisting of choline chloride, chromium-III chloride and water for the plating. The sample piece from Leicester is shown in Figure 3-1 was evaluated with the scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). It is a copper tube with 1” outer diameter. The plated chromium layer is not well adhered to the surface and was partially flaked off (Figure 3-1). The chromium layer has a grey metallic shine, is 20 to 25 µm thick (Figure 3-2), shows cracks (Figure 3-3 and 3-4) and nodules (Figure 3-4), and the typical EDS surface analysis gives 42 at.% chromium with 35 at.% oxygen and carbon (Figure 3-5). In some of our experiments we came close to these values but we only produced “black chromium” powder, not metallic chromium.

Figure 3-1: 1” outer diameter copper tube plated with chromium by the University of Leicester, UK using an electrolyte probably containing chromium-III chloride, water and choline chloride.

Chromium coating flaked off and the oxidized copper beneath was revealed. Chromium plating with cracks (see next three SEM pictures and the following EDS analysis)

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Figure 3-2: SEM picture of chromium layer flakes with thickness measurements.

Figure 3-3 and 3-4: SEM pictures (100X and 1000X) of the middle region of the chromium plated sample shown in Figure 3-1.

Figure 3-5: Typical EDS spectrum with analysis of the chromium layer shown in Figure 3-3.

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4 Instrumentation and electrochemical cell setup All electrodeposition experiments were conducted in a nitrogen-filled glove box seen in Figure 4-1. The atmosphere in the glove box was maintained at less than 0.5 ppm H2O and O2.

Figure 4-1: Glove box containing an inert gas atmosphere; atmosphere maintained at <0.5 ppm H2O and O2. The electrochemical cell for the deposition experiments consisted of a 25 mL round-bottomed, three-necked flask, into which two chromium anodes and a cylindrical cathode were inserted. The cathode was made of copper, 420 steel, or nickel, depending on the experiment. A picture of the electrochemical cell setup is shown in Figures 4-2 and 4-3, while a close-up view of the setup used to ensure electrical contact with the rotating cathode is shown in Figure 4-4. The rotating cathode was used to agitate the electrolyte and ensure a constant availability of Cr-ions to the surface of the cathode.

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Figure 4-2: 50 mL electrochemical cell with temperature probes and electrical contacts.

Figure 4-3: Electrochemical cell setup

1. Stirrer motor (app. 600 rpm for all experiments) 2. Flexible coupling 3. Contact for the cathodic stirrer (see Figure 5.3 for details) 4. PTFE covered stirring rod in glass bearing 5. Cylindrical copper, steel or nickel cathode in 50 mL three neck round flask with PTFE stoppers, thermoelement and two chromium anodes with electrical platinum contact to rectifier 6. Aluminum bowl with copper shot on aluminum plates 7. Thermoelement controlled hotplate

1. Adapter with the PTFE covered stirrer. 2. Thermoelement in the electrolyte 3. Thermoelement in the copper shot to control the temperature 4. Electrical contacts for both anodes 5. Cathode screwed to the stirrer in the electrolyte

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Figure 4-4: Rotating cathode electrical connection setup The electrolyte used in the experiments consisted of approximately 25 mL of a choline chloride/chromium-III chloride hexahydrate mixture. All electrochemical cell measurements were carried out with a Solartron 1287 potentiostat/galvanostat controlled by CorrWare software. A pass-through fitting was employed to connect the instrument from outside the glove box to the electrochemical cell inside the glove box. A picture of the Solartron instrument is shown in Figure 4-5.

Figure 4-5: The Solartron 1287 potentiostat/galvanostat used for all electrodeposition experiments with pass-through fitting for the glove box. The electrodeposition experiments began by attempting to first deposit chromium onto a nickel cathode. Cathode material was later changed to 420 steel. Because 420 steel contains 13 w.% chromium we encountered complications during EDS analysis of the precipitated layer and switched to copper cathodes.

All experiments were conducted galvanostatically, with current densities varied between -0.345 mA cm-2 to -154 mA/cm2. Two chromium anodes and the copper, steel, or nickel cathode was weighed prior to the start of the experiment. The two 1.0 cm by 1.5 cm pure chromium anodes were positioned on opposite sides of the electrochemical cell,

1. End of flexible coupling 2. Copper leads for spring loaded carbon contact brushes 3. Contact brushes for the cathodic stirrer 4. Electrical contacts for brushes 5. Aluminum housing with holder

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with the 3 cm2 rotating cylindrical cathode in the center of the cell. The rate of rotation of the cylindrical cathode was 600 rpm. This equates to a cathode surface speed relative to the electrolyte of about 30 cm/s. The chromium anodes were connected to the working electrode of the Solartron, while the cathode was connected to the counter electrode. Reference electrodes were connected directly to the working and counter electrodes. In each experiment, the cell potential versus time was recorded.

After the various deposition time, the anodes and cathode were removed from the plating cell, removed from the glove box, and rinsed in ethanol and acetone. The mass of each anode and cathode was recorded, and the mass difference from before the experiment was noted. The surface of each cathode and anode were then analyzed in the scanning electron microscope (SEM).

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5 Investigating electrolytes made from choline chloride and chromium-III chloride hexahydrate

This chapter describes the investigation of electrodeposition of chromium from electrolytes made from choline chloride and chromium-III chloride hexahydrate. All plating experiments in this report were conducted galvanostatically, so as to compare the performance of the electrolyte at different current densities and concentrations. For the tests 1Cr and 2Cr a molar ratio of 3.1:1 for chromium-III chloride hexahydrate to choline chloride was used. In 8Cr the composition was 2:1. All the chromium plating experiments are listed in Attachment B. Table 5-1 lists the tests relevant for this chapter. In the first test we plated onto a nickel cylinder cathode before we switched to steel 420 and later to copper cylindrical cathodes. Pure chromium plates were the anodes in all reported plating attempts. Figure 5-1 provides information about the cell voltage versus plating time for the three test. The maximum voltage of the galvanostat (15 to 16 V) was reached fast if the current density was high. In the 1Cr-test run, the desired current density of -150 mA/cm2 was never reached. An average current density of -140 mA/cm2 was established under these conditions. At lower absolute current densities (10.34 mA/cm2 in average) the desired current density (15.00 mA/cm2) could be maintained for only the first 3000 seconds. It seems as though an insulating layer builds up on one or both electrodes under these conditions. At higher absolute current densities (>9 mA/cm2) the cathodes turn black (Figures 5-2 and 5-9). After plating, the electrodes were always rinsed with ethanol and acetone and then dried in the air. The 1Cr-cathode, which was plated with -140mA/cm2, is covered with a crust of black powder (Figures 5-3, 5-5, 5-10, 5-12) containing 35 at.% chromium and 55 at.% oxygen (Figure 5-4). As reported in Chapter 3, the EDS analysis of the chromium plated sample from the University of Leicester gave 42 at.% chromium and 35 at.% oxygen. Plating with an average of 10.3 mA/cm2 resulted in a crust with 13 at.% chromium and 68 at.% oxygen (Figure 5-11). A current density of -0.34 mA/cm2 resulted in corrosion of both electrodes (Table 5-1) but no plating on the copper cathode (Figure 5-15 till 5-18). The anode lost weight in all cases and looked etched. (Figure 5-6, 5-7, 5-13, 5-18, 5-19, 5-21). The EDS of the “pure” chromium anode surface showed 67 +- 3 at.% chromium with 26 +-2 at.% oxygen (Figure 5-8, 5-14, 5-20).

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Table 5-1: Chromium plating experiments 1Cr, 2Cr and 8Cr Chromium Plating Experiment

Number 1 Cr 2 Cr 8 Cr

Electrolyte compositionCrCl3*6H2O : Cholin

chloride = 3.10:1

CrCl3*6H2O : Cholin

chloride = 3.10:1

CrCl3*6H2O :

Cholin chloride =

2:1

Electrolyte history Newly prepared 1Cr New

Plating temperature oC 58-70oC (no cooling) 72-46 60

Stirrer setting 1-10 5 = 645-675 rpm 5 = 634-658 rpm 5

Start weight cathode g 2.8111 2.0039 2.2122

Start weight anode 1 g 2.1837 2.1450 2.2289

Start weight anode 2 g 2.6460 2.6134 1.8544

Plating time sec 1200 12000 7200

Area of cathode cm2 3.15568 2.67257 2.594484293

Current mA -441.66 -27.6343738 -0.8950978

Cathode material Nickel Type 420 steel (13%Cr) Copper

Current density mA/cm2 -139.96 -10.34 -0.35

Plating remark 1

At the beginning the

cell voltage was the

Solartron maximum

and the current too

low.

The cell voltage was

the Solartron

maximum and the

current too low.

positive voltage

was observed

Plating remark 2

The wanted current

was -473 mA. The

average current was

only -442 mA.

The wanted current

was-40.1 mA. The

average current was

only -27.6 mA.

End weight cathode g 2.8239 2.0270 2.212

End weight anode 1 g 2.1498 2.1255 2.2231

End weight anode 2 g 2.6174 2.5969 1.8499

Weight gain (cathode) g 0.0128 0.0231 -0.0002

Weight loss (anodes) g 0.0625 0.0360 0.0103

How did the electrolyte appearance change? no change no change no change

How did the cathode look like?Covered with black

powder

Unevenly covered

with black powderno plating

How did the anode look like? Partially etched Partially etched no change

Cathodic current efficiency % 13.44 38.78 -17.28

Anodic current efficiency % 65.65 60.43 889.71

Open circuit cell voltage V

Characteristic cell voltage level V -15.6 -16 ±0.03

Analysis of cathode surface in atomic %

% C 2.734 14.5 34.6

% N 2.687 3.2 0.0

% O 54.925 67.8 3.2

% Cl 4.504 1.0 0.1

% Fe 0.142 *Ni not Fe 0.2 0.0

% Cu 0 0.0 61.8

%Cr 35.0 13.4 0.2

Analysis of cathode surface 2 in atomic % n/a n/a n/a

% C n/a n/a n/a

% N n/a n/a n/a

% O n/a n/a n/a

% Cl n/a n/a n/a

% Fe n/a n/a n/a

% Cu 0 0.0 n/a

%Cr n/a n/a n/a

Analysis of cathode surface 3 in atomic % n/a n/a n/a

% C n/a n/a n/a

% N n/a n/a n/a

% O n/a n/a n/a

% Cl n/a n/a n/a

% Fe n/a n/a n/a

% Cu 0 0.0 n/a

%Cr n/a n/a n/a

Analysis of anode surface in atomic %

% C 5.7 2.3 2.851

% N 2.2 2.1 1.836

% O 27.0 26.8 24.885

% Cl 0.2 0.2 0.07

% Fe 0.243 *Ni not Fe 0.0 0

% Cu 0 0.0 0.233

%Cr 64.7 68.7 70.1

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Figure 5-1: Cell voltage versus plating time for the 1st test series

Figure 5-2: Electrodes after test 1Cr. Figure 5-3: 1Cr-cathode (100X)

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Figure 5-4: Spectrum with analysis of the 1Cr-cathode

Figure 5-5: 1000X SEM picture of the 1Cr-cathode

Figure 5-6: 1Cr-anode (100X) Figure 5-7: 1Cr-anode (1000X)

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Figure 5-8: Typical spectrum with analysis of the 1Cr-anode

Figure 5-9: 2Cr-cathode Figure 5-10: 100X SEM picture of the 2Cr-cathode

Figure 5-11: Typical spectrum with analysis of the 2Cr-cathode

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Figure 5-12: 2Cr-cathode crust (1000X) Figure 5-13: 2Cr-anode (100X)

Figure 5-14: Typical spectrum with analysis of the 2Cr-anode

Figure 5-15: 8Cr-cathode Figure 5-16: 100X SEM picture of the 8Cr-cathode

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Figure 5-17: Typical spectrum with analysis of the 8Cr-cathode

Figure 5-18: 8Cr cathode (1000X) Figure 5-19: 8Cr anode (100X)

Figure 5-20: Typical spectrum with analysis of the 8 Cr-anode

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Figure 5-21: 8 Cr anode (1000X) 6 Investigating electrolytes made from choline chloride, chromium-III

chloride hexahydrate and additions 6.1 Adding water This chapter describes the investigation of electrodeposition of chromium from electrolytes made from choline chloride, chromium-III chloride hexahydrate and additional water. All plating experiments in this chapter were conducted galvanostatically at -15 mA/cm2. The intent of this chapter is to compare the performance of the electrolyte at different water contents (2Cr till 4Cr) and different plating times (4Cr and 5Cr). A molar ratio of 3.1 to 1 for chromium-III chloride hexahydrate to choline chloride was used. For every mol chromium-III chloride hexahydrate 1.5 mol of water was added for 3Cr and 3 mol of water for 4Cr. 5Cr is a repeat of 4Cr with longer plating time. Unfortunately, the cathode for 4Cr, 5Cr and 6Cr was left for 65 hours in the electrolyte and lots of the plated substance fell off during that time. Only small amounts remained at the surface for EDS analysis. All the chromium plating experiments are listed in Attachment B. Table 6.1-1 lists the tests relevant for this chapter. We plated onto a steel 420 as cathodes and with pure chromium plates as anodes. Figure 6.1-1 informs about the cell voltage versus plating time for the four tests. As mentioned in Chapter 5, the desired absolute current density of 15 mA/cm2 was never reached in the 2Cr-test run. An average current density of -10.3 mA/cm2 was established under these conditions with a -16 V cell voltage limit. The higher the water content the lower the absolute cell voltage at 15 mA/cm2. The 5Cr-curve matches nicely with the 4Cr-curve like it should be because 5Cr was a repeat of 4Cr with fresh electrolyte and longer plating time. We decided to use the longer plating time because we saw the possibility that we create Cr(II) compounds at the electrodes. With longer plating times we tried to better the outcome of the plating.

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Table 6.1-1: Chromium plating experiments 2Cr till 5Cr Chromium Plating Experiment

Number 2 Cr 3 Cr 4 Cr 5 Cr


chloride = 3.10:1

CrCl3*7.5H2O : Cholin

chloride = 3.10:1

CrCl3*9H2O : Cholin

chloride = 3.10:1

CrCl3*9H2O : Cholin

chloride = 3.10:1

Electrolyte history 1Cr 1-2Cr and water 1-3Cr and water 1-4Cr

Plating temperature oC 72-46 48.0 50 50

Stirrer setting 1-10 5 = 634-658 rpm 5 5 5

Start weight cathode g 2.0039 1.6053 1.4488 1.567

Start weight anode 1 g 2.1450 1.9622 1.9291 1.8279


Plating time sec 12000 12000 12000 60300

Area of cathode cm2 2.67257 2.6246436 2.506991 2.5533043

Current mA -27.6343738 -39.37 -37.6048641 -38.29956445

Cathode material Type 420 steel (13%Cr) Type 420 steel (13%Cr) Type 420 steel (13%Cr) Type 420 steel (13%Cr)

Current density mA/cm2 -10.34 -15.00 -15.00 -15.00

Plating remark 1


the Solartron

maximum and the

current too low.

Magnetic stirrer in the

electrolyte caused

scratches on cathode.

Plating remark 2

The wanted current

was-40.1 mA. The

average current was

only -27.6 mA.

Cathode was looking

metallic grey but was

left for 65 hours in the

electrolyte. See 6Cr

remark.

End weight cathode g 2.0270 1.6561 1.444 1.7256

End weight anode 1 g 2.1255 1.9298 1.8488 1.712


Weight gain (cathode) g 0.0231 0.0508 -0.0048 0.1586

Weight loss (anodes) g 0.0360 0.0535 0.1267 0.2805

How did the electrolyte appearance change? no change no change no change no change

How did the cathode look like?Unevenly covered

with black powderDark. Black. Powdery

Looked metallic grey

after 65h in

electrolyte.

Unevenly covered

with black powder

How did the anode look like? Partially etched Etched. Etched. Etched.

Cathodic current efficiency % 38.78 59.86 -5.92 38.23

Anodic current efficiency % 60.43 63.04 156.30 67.61


Characteristic cell voltage level V -16 -5.3 -3.3 -3.5

Analysis of cathode surface in atomic % smooth, shiny outer crust

% C 14.5 12.8 15.1 19.7

% N 3.2 3.2 0.1 4.2

% O 67.8 64.8 5.7 50.8

% Cl 1.0 1.6 0.1 3.5

% Fe 0.2 0.2 62.9 -

% Cu 0.0 0.0 0.0 0.0

%Cr 13.4 17.4 16.1 21.8

Analysis of cathode surface 2 in atomic % n/a n/a black spot inner crust

% C n/a n/a 9.171 16.785

% N n/a n/a 2.227 3.556

% O n/a n/a 61.387 55.191

% Cl n/a n/a 3.291 2.07

% Fe n/a n/a 0.438 -

% Cu 0.0 0.0 0.0 0.0

%Cr n/a n/a 23.5 22.4

Analysis of cathode surface 3 in atomic % n/a n/a n/a n/a

% C n/a n/a n/a n/a

% N n/a n/a n/a n/a

% O n/a n/a n/a n/a

% Cl n/a n/a n/a n/a

% Fe n/a n/a n/a n/a

% Cu 0.0 0.0 0.0 0.0

%Cr n/a n/a n/a n/a


% C 2.3 2.7 0.0 4.8

% N 2.1 3.9 1.8 1.9

% O 26.8 30.3 20.1 20.0

% Cl 0.2 0.1 0.2 0.1

% Fe 0.0 0.3 0.1 -

% Cu 0.0 0.0 0.0 0.0

%Cr 68.7 62.7 77.8 73.2

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Figure 6.1-1: Cell voltage versus plating time for the 2nd test series

Figure 6.1-1: 3Cr-electrodes Figure 6.1-2: 3Cr-cathode crust (100X) The results regarding test 2Cr are reported in Chapter 5. Figure 6.1-1, 6.1-7, 6.1-16 shows the electrodes for 3Cr till 5Cr. All cathodes were covered with black powder after the plating attempts. The 3Cr cathode was partially touching a PTFE-covered stirring magnet which compressed the plating in this area. The 4Cr-cathode and the 5Cr-cathode lost most of the powder because they were allowed to sit for 65 hours (over the weekend) in the electrolyte after plating. During rinsing more of the powder came off. Nevertheless we found the remaining powder (Figure 6.2, -4, -5, -8,-10, -11) with the SEM and using EDS analysis we determined the chromium content (Figure 6.1-3, -12, -18). The chromium content in the precipitate increased with the water content in the electrolyte (Table 6.1-1) from 13 at.% to 28 at.% (Figure 6.1- 12). The anodes lost weight during plating. EDS analysis showed 63 till 78 at.% Cr on the anodes.

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Figure 6.1-3: Typical spectrum with analysis of the 3Cr-cathode

Figure 6.1-4: 3Cr-cathode crust (1000X) Figure 6.1-5: 3Cr-anode (100X)

Figure 6.1-6: Typical spectrum with analysis of the 3Cr-anode

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Figure 6.1-7: 4Cr-electrodes Figure 6.1-8: 4Cr-cathode (100X)

Figure 6.1-9: Typical spectrum with analysis of the 4Cr-cathode without crust

Figure 6.1-10: 4Cr-cathode with crust (100X) Figure 6.1-11: 4Cr-cathode crust

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Figure 6.1-12: Typical spectrum with analysis of the 4Cr-cathode precipitate

Figure 6.1-13: 4Cr-anode (100X) Figure 6.1-14: 4Cr-anode (1000X)

Figure 6.1-15: Typical spectrum with analysis of the 4Cr-anode

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Figure 6.1-16: 5Cr-cathode Figure 6.1-17: 5Cr-cathode crust (100X)

Figure 6.1-18: Typical spectrum with analysis of the 5Cr-cathode precipitate


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Figure 6.1-21: Typical spectrum with analysis of the 5Cr-anode in a heavily etched area with high current density (see Figure 6.1-20) 6.2 Changing the chromium to ionic liquid ratio in the electrolyte This chapter describes the investigation of electrodeposition of chromium from electrolytes made from choline chloride, chromium-III chloride, and water in a molar ratio of 1: 3.1: 27.9 and 1:2:18. All plating experiments in this chapter were conducted galvanostatically, at -15 mA/cm2, only for 7Cr -60 mA/cm2 was used. All the chromium plating experiments are listed in Attachment B. Table 6.2-1 lists the tests relevant for this chapter. Normally we plated onto steel 420 as cathodes, and with pure chromium plates as anodes. In test 7Cr a copper cathode was used. Figure 6.2-1 shows the cell voltage versus plating time for all four platings. In the previous chapter we discussed the results for 4Cr and 5Cr. The curve for 6Cr gives a higher absolute cell voltage suggesting that the absolute cell voltage increases with lower chromium chloride concentration. The curve for 7Cr in Figure 6.2-2 goes down to an absolute cell voltage twice that of 7Cr because of the four-fold increase in current density. The visual appearance of the 6Cr and 7Cr is documented in Figures 6.2-3 and -7. The crust is shown in Figures 6.2-4, -6, -11. The EDS analysis of the crust on the 6Cr 420 steel cathode, which itself contains around 14 at.% chromium at the surface, resulted in chromium contents from 25 to 40 at.% chromium. To avoid the uncertainty of the influence of the chromium content in the substrate, we decided to switch to copper cathodes. The 6Cr anode had 43 at.% chromium at the surface (Figure 6.2-9), which is a relatively low value. At a -60 mA/cm2 current density, we produced a crust with 34 at.% chromium on the 7Cr cathode. (Figure 6.2-12)

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chloride = 3.10:1

CrCl3*9H2O : Cholin

chloride = 3.10:1

CrCl3*9H2O : Cholin

chloride = 2:1

CrCl3*9H2O : Cholin

chloride = 2:1

Electrolyte history 1-3Cr and water 1-4Cr New 6 Cr

Plating temperature oC 50 50 50 40

Stirrer setting 1-10 5 5 5 5




Plating time sec 12000 60300 9000 1800


Current mA -37.6048641 -38.29956445 -37.872168 -168.26056

Cathode material Type 420 steel (13%Cr) Type 420 steel (13%Cr) Type 420 steel (13%Cr) Copper


Plating remark 1

Problems with the

electrical connection.

Bad connection

between rod and

cathode?

lots of gas evolution

Plating remark 2

Cathode was looking




remark.

Cathode was covered

with black powdery

crust which

disappeared after 65

hours in the

electrolyte.




Weight gain (cathode) g -0.0048 0.1586 -0.0056 0.0079



How did the cathode look like?


after 65h in

electrolyte.

Unevenly covered

with black powder

Covered with

powdery black crust.


after 65h in

Dark. Black. Powdery

How did the anode look like? Etched. Etched. Etched. Etched.

Cathodic current efficiency % -5.92 38.23 -9.15 14.52



Characteristic cell voltage level V -3.3 -3.5 -3.6 -7.3

Analysis of cathode surface in atomic % smooth, shiny outer crust smooth

% C 15.1 19.7 20.6 8.8

% N 0.1 4.2 0.0 2.3

% O 5.7 50.8 23.6 49.7

% Cl 0.1 3.5 0.1 5.0

% Fe 62.9 - 38.5 0.0

% Cu 0.0 0.0 0.0 0.5

%Cr 16.1 21.8 17.2 33.7

Analysis of cathode surface 2 in atomic % black spot inner crust smooth n/a

% C 9.171 16.785 15.6 n/a

% N 2.227 3.556 0.0 n/a

% O 61.387 55.191 22.8 n/a

% Cl 3.291 2.07 0.1 n/a

% Fe 0.438 - 43.1 n/a

% Cu 0.0 0.0 0.0 n/a

%Cr 23.5 22.4 18.4 n/a

Analysis of cathode surface 3 in atomic % n/a n/a crust n/a

% C n/a n/a 4.6 n/a

% N n/a n/a 3.5 n/a

% O n/a n/a 47.7 n/a

% Cl n/a n/a 3.1 n/a

% Fe n/a n/a 0.5 n/a

% Cu 0.0 0.0 0.0 n/a

%Cr n/a n/a 40.5 n/a


% C 0.0 4.8 11.9 n/a

% N 1.8 1.9 5.5 n/a

% O 20.1 20.0 39.5 n/a

% Cl 0.2 0.1 0.3 n/a

% Fe 0.1 - 0.2 n/a

% Cu 0.0 0.0 0.0 n/a

%Cr 77.8 73.2 42.6 n/a

Analysis of crystals in electrolyte

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Figure 6.2-1: Cell voltage versus plating time for the 3rd test series

Figure 6.2-2: Cell voltage versus plating time for the 4th test series

27

Figure 6.2-3: 6Cr-cathode with some Figure 6.2-4: 6Cr-cathode (100X) with remaining plating leftover plated crust

Figure 6.2-5: Spectrum with analysis of the 6Cr-cathode remaining crust shown in

Figure 6.2-4


28

Figure 6.2-8: Spectrum with analysis of the 6Cr-cathode remaining crust shown in

Figure 6.2-6

Figure 6.2-9: Spectrum with analysis of the 6Cr-anode

Figure 6.2-10: 7Cr-cathode Figure 6.2-11: 7Cr-cathode (100X)

29

Figure 6.2-12: Spectrum with analysis of the 7Cr-cathode precipitation 6.3 Adding lithium chloride After test 5Cr, it was decided to finalize the test series with additional water in the electrolyte and to investigate the effect of lithium chloride as an addition. The basis for this decision was the publication from A. Abott et. al. [2]. They reported “that thick adherent, crack free deposits of chromium black can be electrodeposited from an ionic liquid containing Cr (III)”. An electrolyte containing choline chloride and chromium(III) chloride hexahydrate (molar ratio = 1 : 2) with 15 wt.% lithium chloride was used with copper cathodes at 60 oC for 2 hours at a current density of -0.345 mA/cm2. We used a copper cathode for our tests and similar process parameters. This chapter describes the investigation of electrodeposition of chromium from electrolytes made from choline chloride, chromium-III chloride, water, and lithium chloride in a molar ratio of 1:2:12:0 and 1:2:12:2.8. All plating experiments in this chapter were conducted galvanostatically at varying current densities. The intent of this chapter is to compare the performance of the electrolyte with and without the addition of lithium chloride (8Cr and 9Cr) in the 5th test series, as well as the effect of current density on plating solutions containing LiCl in the 6th test series. In the 5th test series we decided to use the low current density of -0.345 mA/cm2 recommended in the literature [2] and got no plating. All the chromium plating experiments are listed in Attachment B. Table 6.3-1 lists the tests relevant for this chapter. Copper cathodes were used in all experiments presented in this chapter. Figure 6.3-1 shows the cell voltage versus plating time for 8Cr and 9Cr. Because of the obvious influence of corrosion (see current efficiency in Table 6.3-1) the cell voltage is close to 0. The 8Cr curve documents some contact problems. No precipitate was found on the cathode (Figure 6.3-3).

30



Electrolyte composition

CrCl3*6H2O :

Cholin chloride =

2:1

CrCl3*6H2O :

Cholin chloride =

2:1 and 15 w.% LiCl

CrCl3*6H2O : Cholin

chloride = 2:1 and

15 w.% LiCl

CrCl3*6H2O : Cholin

chloride = 2:1 and

15 w.% LiCl

Electrolyte history New 8 Cr 8-9Cr 8-10Cr

Plating temperature oC 60 60 60 60

Stirrer setting 1-10 5 5 5 5




Plating time sec 7200 7200 7200 7200


Current mA -0.8950978 -0.940682706 -8.794788103 -28.58676

Cathode material Copper Copper Copper Copper


Plating remark 1positive voltage

was observed

large amounts of

carbon from

brushes present

Plating remark 2




Weight gain (cathode) g -0.0002 -0.0001 0.0010 0.0020



How did the cathode look like? no plating no plating Black. Powdery

black w/ copper

showing through

How did the anode look like? no change no change Partially etchedno change / stained

Cathodic current efficiency % -17.28 -8.22 8.79 5.41



Characteristic cell voltage level V ±0.03 -0.08 -4.1 range: -5 -> -9

Analysis of cathode surface in atomic % crust

% C 34.6 27.7 10.2 7.9

% N 0.0 0.0 3.1 3.3

% O 3.2 2.9 56.8 61.4

% Cl 0.1 0.2 1.1 2.6

% Fe 0.0 0.0 0.0 0.0

% Cu 61.8 68.8 3.3 0.1

%Cr 0.2 0.4 25.4 20.2

Analysis of cathode surface 2 in atomic % n/a n/a n/a smooth

% C n/a n/a n/a 11.569

% N n/a n/a n/a 3.828

% O n/a n/a n/a 57.59

% Cl n/a n/a n/a 1.522

% Fe n/a n/a n/a 0

% Cu n/a n/a n/a 19.004

%Cr n/a n/a n/a 6.5


% C n/a n/a n/a n/a

% N n/a n/a n/a n/a

% O n/a n/a n/a n/a



% Cu n/a n/a n/a n/a

%Cr n/a n/a n/a n/a


% C 2.851 n/a 3.2 1.7

% N 1.836 n/a 3.3 4.4

% O 24.885 n/a 33.0 30.0

% Cl 0.07 n/a 0.4 0.1

% Fe 0 n/a 0.0 0.0

% Cu 0.233 n/a 0.2 0.3

%Cr 70.1 n/a 59.9 63.6

31



32

Figure 6.3-3: 8Cr-cathode Figure 6.3-4: 9Cr-cathode


Figure 6.3-7: Spectrum with analysis of the 10Cr-cathode surface in Figure 6.3-6

33

Figure 6.3-8: 10Cr-cathode (1000X) Figure 6.3-9: 10Cr-anode (100X)

Figure 6.3-10: Spectrum with analysis of the marked 10Cr-cathode precipitation in Figure 6.3-8

Figure 6.3-11: Spectrum with analysis of the 10Cr-anode in Figure 6.3-9

34



Figure 6.3-16: Spectrum with analysis of the marked 11Cr-cathode precipitation in Figure 6.3-13

35

Figure 6.3-17: Spectrum with analysis of the 11Cr-anode in Figure 6.3-14 In Figure 6.3-2 we see how the curve for the cell voltage versus plating time changes with the current density. Figures 6.3-4, -5, -12 record how the amount of precipitate on the copper cathode increases. The chromium content of the black powder (Figure 6.3-6, -8, -13, -14) is 25 at.% for 10Cr (Figure 6.3-10) and 20 at.% for 11Cr (Figure 6.3-16) combined with huge amounts of oxygen. The high oxygen content could be a result of the moisture in the rinsing liquids, the handling in air and/or the huge surface area of the powdery precipitate. The anodes (Figures 6.3-9, -15) show 60 to 64 at.% chromium at their surface with some oxygen (Figures 6.3-10, -14). 6.4 Adding potassium chloride This chapter describes the investigation of electrodeposition of chromium from electrolytes made from choline chloride, chromium-III chloride hexahydrate, and lithium chloride or potassium chloride. The intention was to create electrolytes in a molar ratio of 1:2:12:2.8 which results in an addition of 15 wt.% of lithium chloride and 26.38 wt.% of potassium chloride. The potassium chloride solution became saturated approximately at a concentration of 20 to 25 wt.%. All plating experiments in this chapter were conducted galvanostatically at -10 mA/cm2. The intent of this chapter is to compare the performance of the electrolyte with the addition of lithium chloride to that of the addition of potassium chloride. For the tests a molar ratio of 2 to 1 for chromium-III chloride hexahydrate to choline chloride was used. For every mol of chromium-III chloride hexahydrate, 2.8 mol of lithium chloride or 2.4 mol of potassium chloride was added for 11Cr and 12Cr, respectively. All the chromium plating experiments are listed in Attachment B. Table 6.4-1 lists the tests relevant for this chapter. Copper cathodes were used in all experiments presented

36

in this chapter. Figure 6.4-1 documents the cell voltage versus plating curves for the 7th test series and shows problems with the contact in test 12Cr. It is evident that the absolute cell voltage is much smaller with potassium chloride, the precipitate is increased (Figures 6.4-2, -3, -4), it contains more chromium (31 at.% see Figure 6.4-6) and the anodes (Figure 6.4-5) are cleaner with 76 at.% chromium (Figure 6.4-7). The results for 11Cr were shown in chapter 6.3. After these tests we decided to continue the experiments with entirely water free chromium chloride electrolytes. The solubility of chromium(III) chloride was tested in several ionic liquids, but only EMIm Cl showed promising results. The tests are described in chapter 7.



37

Table 6.4-1: Chromium plating experiments 11Cr and 12Cr Chromium Plating Experiment

Number 11 Cr 12 Cr

Electrolyte composition

CrCl3*6H2O : Cholin

chloride : LiCl = 2:1:

2.8

CrCl3*6H2O : Cholin

chloride : KCl = 2:1:2.5

Electrolyte history 8-10Cr Fresh

Plating temperature oC 60 60

Stirrer setting 1-10 5 5

Start weight cathode g 2.5046 1.9826

Start weight anode 1 g 1.8095 1.7892


Plating time sec 7200 7200

Area of cathode cm2 2.858676 2.725959

Current mA -28.58676 -27.25959

Cathode material Copper Copper

Current density mA/cm2 -10.00 -10.00

Plating remark 1

Plating remark 2

End weight cathode g 2.5066 1.9894

End weight anode 1 g 1.7938 1.7576


Weight gain (cathode) g 0.0020 0.0068

Weight loss (anodes) g 0.0256 0.0667

How did the electrolyte appearance change? no change no change


black w/ copper

showing through

Green&black

How did the anode look like?no change / stained no change / stained

Cathodic current efficiency % 5.41 19.29

Anodic current efficiency % 69.24 189.18


Characteristic cell voltage level V range: -5 -> -9 -1.75

Analysis of cathode surface in atomic % crust nodules

% C 7.9 7.9

% N 3.3 2.5

% O 61.4 49.9

% Cl 2.6 7.7

% Fe 0.0 0.0

% Cu 0.1 1.2

%Cr 20.2 30.9

Analysis of cathode surface 2 in atomic % smooth smooth-some plating

% C 11.569 6.6

% N 3.828 1.9

% O 57.59 54.0

% Cl 1.522 10.7

% Fe 0 0.0

% Cu 19.004 6.6

%Cr 6.5 20.3

Analysis of cathode surface 3 in atomic % n/a smooth-little/no plating

% C n/a 9.9

% N n/a 1.4

% O n/a 41.3

% Cl n/a 7.0

% Fe n/a 0.0

% Cu n/a 31.7

%Cr n/a 8.7


% C 1.7 0.5

% N 4.4 2.7

% O 30.0 20.4

% Cl 0.1 0.1

% Fe 0.0 0.0

% Cu 0.3 0.2

%Cr 63.6 76.1

38


Figure 6.4-6: Spectrum with analysis of the 11Cr-cathode precipitation in Figure 6.4-4

Figure 6.4-7: Spectrum with analysis of the 11Cr-anode in Figure 6.4-5

39

7 Investigating electrolytes made with 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl), anhydrous chromium chloride

Because it was decided to investigate electrolytes formulated without any water, we tried unsuccessfully to dissolve anhydrous chromium chloride in [MMPIm][TFSI] and choline chloride. [MMPIm][TFSI] showed virtually no solubility for chromium chloride. At 320 oC the choline chloride decomposed without dissolving chromium chloride, but we were able to create electrolytes with 10 wt.% chromium chloride in [EMIm]Cl. 7.1 Varying the current density This chapter describes the investigation of electrodeposition of chromium from electrolytes made from anhydrous chromium(III) chloride and 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) in a molar ratio of 1:9.7. Both plating experiments in this chapter were conducted galvanostatically at -10 mA/cm2 and -25 mA/cm2 respectively. We used copper cathodes rotated at 660 rpm and a plating temperature of 120 oC. The intent of this chapter is to compare the plating performance of this electrolyte at different current densities. All the chromium plating experiments are listed in Attachment B. Table 7.1-1 lists the two tests relevant for this chapter. Figure 7.1-1 shows the cell voltage versus plating time for the 8th test series. The curve for 13Cr reveals some contact problems. Curve 14Cr seems ideal. As expected, there are increases in the voltage with increased current density. At 25 mA/cm2 and 120 oC, the absolute cell voltage decreases from 1.8 V and levels out at around 1.45 V. The visual appearance of the 13Cr- and the 14Cr-cathode is given in Figures 7.1-2 and -8. The 13Cr-cathode is covered with a thin grey layer (Figures 7.1-3 and -4). The EDS analysis (Figure 7.1-4) gives only 10 at.% chromium and 42 at.% copper besides oxygen and carbon. The black powdery layer on the 14Cr-cathode (Figures 7.1-9 and -11) shows an even lower chromium content in the EDS analysis (Figure 7.1-10): 3 at.% chromium and 46 at.% copper besides oxygen and carbon. Because the cathodic current efficiency was so low (Table 7.1-1), it is assumed that corrosion is occurring besides the plating and/or that instead of plating occurring, soluble Cr(II)-salts were produced. To eliminate the effect of the substrate on the analysis we scrapped the black precipitate off the 14Cr-cathode (Figure 7.1-14) and analyzed the black powder without the copper substrate. The EDS-analysis (Figure 7.1-15) gave us 64 at.% copper and 3 at.% chromium besides oxygen and carbon. Both pairs of anodes (Figures 7.1-6, -7 and -12, -13) are nicely etched. Because the anodic current efficiency was too high (Table 7.1-1), it is assumed that corrosion is occurring along with the electrochemical oxidation and/or that soluble Cr(II)-salts were produced. In the 9th test series we investigated the effects of a lithium chloride addition and in the 10th test series a longer plating time.

40

Table 7.1-1: Chromium plating experiments 13Cr and 14Cr Number 13 Cr 14 Cr

Electrolyte compositionCrCl3 : EMImCl =

1:9.7 = molar ratio

CrCl3 : EMImCl =

1:9.7 = molar

ratio

Electrolyte history Fresh 13Cr

Plating temperature oC 120 120

Stirrer setting 1-10 5 5

Start weight cathode g 3.3775 2.0708



Plating time sec 7200 7200

Area of cathode cm2 3.3115 2.61534468

Current mA -33.115 -65.3836117

Cathode material Copper Copper

Current density mA/cm2 -10.00 -25.00

Plating remark 1Good looking

graph

Plating remark 2

End weight cathode g 3.3607 2.0718



Weight gain (cathode) g -0.0168 0.0010

Weight loss (anodes) g 0.1165 0.1505

How did the electrolyte appearance change?no change purple to

brownish-black


copper with some

grey

powdery black

How did the anode look like?no change / stained very shiny, some

dark spots

Cathodic current efficiency % -39.23 1.18

Anodic current efficiency % 272.01 177.97


Characteristic cell voltage level V -0.8 range: -1.8 -> -1.47


% C 12.7 17.0

% N 0.9 0.0

% O 34.3 32.8

% Cl 0.2 1.0

% Fe 0.0 0.0

% Cu 42.5 46.2

%Cr 9.6 3.0

Analysis of cathode surface 2 in atomic % n/a n/a

% C n/a n/a

% N n/a n/a

% O n/a n/a

% Cl n/a n/a

% Fe n/a n/a

% Cu n/a n/a

%Cr n/a n/a

Analysis of cathode surface 3 in atomic % n/a n/a

% C n/a n/a

% N n/a n/a

% O n/a n/a

% Cl n/a n/a

% Fe n/a n/a

% Cu n/a n/a

%Cr n/a n/a


% C 1.5 0.0

% N 2.3 4.4

% O 20.1 31.9

% Cl 0.2 0.2

% Fe 0.0 0.0

% Cu 0.7 0.4

%Cr 75.2 63.2

41


Figure 7.1-2: Picture of electrodes after 13Cr-test Figure 7.1-3: SEM picture of the 13Cr-cathode (100X)

42

Figure 7.1-4: Spectrum with analysis of the 13Cr-cathode

Figure 7.1-5: 1000X SEM picture of the 13Cr cathode

Figure 7.1-6: 100X SEM picture of the 13Cr Figure 7.1-7: 1000X SEM picture of the 13Cr anode anode

43

Figure 7.1-8: Electrodes after the 14Cr-test Figure 7.1-9: 14Cr-cathode (100X)



44

Figure 7.1-13: 14Cr anode (1000X) Figure 7.1-14: Powder from 14Cr-cathode

Figure 7.1-15: Spectrum with analysis of the 14Cr-powder from the 14Cr-cathode (Figure 7.1-14)

45

7.2 Adding Lithium Chloride and Changing the Current Density This chapter describes the investigation of electrodeposition of chromium from electrolytes made from anhydrous chromium-III chloride, lithium chloride, and 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) in a molar ratio of 1:1:9.7. All plating experiments in this chapter were conducted galvanostatically at 120 oC with varying current densities and plating times. The intent of this chapter is to compare the plating performance of this electrolyte at different current densities and plating times. A molar ratio of 1 to 9.7 for anhydrous chromium-III chloride to [EMIm]Cl was used for the tests. For every mol of anhydrous chromium-III chloride, 1 mol of lithium chloride was added. All the chromium plating experiments are listed in Attachment B. Table 7.2-1 lists the tests relevant for this chapter. A copper cathode was used in 15Cr while nickel cathodes were used in 16-20Cr. The addition of lithium chloride to the 13-14Cr-electrolyte resulted in a higher absolute cell voltage (Figure 7.2-1) and no plating (Figures 7.2-5 and -6), meaning no chromium (Figure 7.2-7) at the cathode. The anodes were nicely etched (Figures 7.2-5, -8, -9, -10). Because of the possibility that we created soluble Cr(II)-compounds, in test 16/17Cr we plated for 17.6 hours at 8.53 mA/cm2 onto a nickel rod hanging on a platinum wire and moved in a circle at 300 rpm by a magnetic field. The plating documented in Figure 7.2-2 was interrupted after 800 seconds for a few minutes and had contact trouble after 42,000 seconds. The electrolyte turned from violet to dark brown during the plating. The electrodes are shown in Figure 7.2-11, the resulting black plating at the cathode in Figures 7.2-12, -14, -15 and the analysis with 23 and 32 at.% chromium in Figure 7.2-13 and -16 . The appearance of the 16/17Cr-anodes is shown in Figures 7.2-11, -17, -18, and the analysis in Figure 7.2-19. We ran test 18/19Cr like 16/17Cr, but with lower absolute current density (5 mA/cm2). As recorded in Figure 7.2-3, we had similar contact problems as with 16/17Cr. The cell absolute voltage was higher than in the previous test with a higher current density. Most of the black precipitate fell off the cathode (Figures 7.2-20, -21, -23, -24) The analysis of the surface showed up to 15 at.% chromium (Figures 7.2-22, -25). In test 20Cr, we ran the highest absolute current density of this test series, 13.15 mA/cm2. To avoid problems with electrical resistance we attached the platinum wires of anodes and the copper wire of the cathode with silver epoxy to the electrodes and protected this area with PTFE foil to avoid contact with the electrolyte. Figure 7.2-4 shows again an unstable cell voltage. The plating at the cathode was a thin, black, non-adhering foil shown in Figures 7.2-26 and -27, consisting of mostly carbon which points to electrolyte destruction. The anodes turned dark during plating. We found 63 at.% chromium at their surface (Figure 7.2-31). More research is required to make more conclusive statements. One test is not sufficient to draw decent conclusions.

46

Table 7.2-1: Chromium plating experiments 15Cr till 20Cr Number 15 Cr 16+17 Cr 18+19 Cr 20 Cr

Electrolyte compositionCrCl3 : LiCl : EMImCl =

1:1:9.7 = molar ratio

CrCl3 : LiCl : EMImCl =






Electrolyte history Fresh New New New

Plating temperature oC 120 120 100-117 125-137

Stirrer setting 1-10 5 magn. stir. 300 rpm magn. stir. 300 rpm magn. stir. 300 rpm




Plating time sec 7200 63403 121000 57900


Current mA -25.7459801 -57.41 -34 -80.72

Cathode material Copper Nickel rod as cathode Nickel rod as cathode


Plating remark 1

Contact problems

during the first

minutes and the last

hours. Electrolyte

turned dark brown.

Run with lots of

problems. Anode fell

off the wire. Lsot

contacts. Electrolyte

turned brown.

Contacts with silver

epoxy secured and

area covered with

PTFE foil.

Plating remark 2

Potentiostat shut

down before the end

of the test.

Only app. One third

of the time the run

was normal. Plating

was mostly rinsed off

the cathode.

Cell voltage jumped

from 5 to 16 V - back

and forth - after 6400

seconds.




Weight gain (cathode) g -0.0035 0.0164 0.0012 -0.0532


How did the electrolyte appearance change?slightly darker Turned from pink to

dark brown.

Turned from pink to

dark brown.

Turned from pink to

dark brown.


copper w/

discoloration

Ni-rod turned black Ni-rod turned black

but lost crust during

rinsing.

Ni-rod turned black

but lost some foil

during rinsing.

How did the anode look like?darker, etched etched darker, etched dark

Cathodic current efficiency % -10.51 2.51 0.16 -6.34


Open circuit cell voltage V -0.225 -0.235 0.3416

Characteristic cell voltage level V -1.6 -1.50 -1.5 till 2.5 -5


% C 12.8 32.5 43.3 94.2

% N 0.0 1.7 0.0 0.0

% O 6.4 41.4 25.9 0.2

% Cl 0.0 0.7 1.2 5.2

% Fe 0.0 - 20.8 at.% Ni 0.2 at.% Ni

% Cu 80.3 0.6 at.% Ni 1.3 0.0

%Cr 0.4 23.0 7.5 0.1


% C n/a n/a n/a n/a

% N n/a n/a n/a n/a

% O n/a n/a n/a n/a




%Cr n/a n/a n/a n/a


% C n/a n/a n/a n/a

% N n/a n/a n/a n/a

% O n/a n/a n/a n/a




%Cr n/a n/a n/a n/a


% C 0.2 0.2 4.8

% N 0.1 3.5 2.5

% O 18.9 31.9 27.0

% Cl 0.2 0.2 0.2

% Fe 0.0 0.2 at.% Ni 0.3 at.% Ni

% Cu 0.2 0.3 0.6

%Cr 75.8 63.6 64.6

47


Figure 7.2-2: Cell voltage versus plating time for test 16/17Cr

48

Figure 7.2-3: Cell voltage versus plating time for test 18/19Cr

Figure 7.2-4: Cell voltage versus plating time for test 20Cr

49

Figure 7.2-5: Electrodes after test 15Cr Figure 7.2-6: 15Cr-cathode (100X)


Figure 7.2-8: 15 Cr-anode (100X) Figure 7.2-9: 15Cr-anode (1000X)

50

Figure 7.2-10: Spectrum with analysis of the 15Cr-anode

Figure 7.2-11: 16/17Cr-electrodes Figure 7.2-12: 16/17 cathode (100X SEM picture)

Figure 7.2-13: Spectrum with analysis of the 16/17Cr-cathode

51

Figure 7.2-14: 16-17Cr cathode (1000X) Figure 7.2-15: 16-17Cr cathode (1000X)

Figure 7.2-16: Spectrum with analysis of the cathode area shown in Figure 7.2-15

Figure 7.2-17: 16/17Cr-anode (100X) Figure 7.2-18: 16/17Cr-anode (1000X)

52

Figure 7.2-19: Spectrum with analysis of the cathode area shown in Figure 7.2-17

Figure 7.2-20: Cathode after 18/19Cr-test Figure 7.2-21: 18/19Cr-cathode (100X)

Figure 7.2-22: Spectrum with analysis of the 18/19Cr-cathode

53

Figure 7.2-2-23: 18/19Cr-cathode (1000X) Figure 7.2-2-24: 18/19Cr-cathode (1000X)

Figure 7.2-25: Spectrum with analysis of black crust in the marked area in Figure 7.2-24

Figure 7.2-26: 20Cr-electrodes Figure 7.2-27: 20Cr-cathode (35X)

54


Figure 7.2-29: 20Cr-anode (100X) Figure 7.2-30: 20Cr-anode (1000X)

Figure 7.2-31: Spectrum with analysis of the 20Cr-anode (Figure 7.2-29)

55

8 Summary, conclusions and recommendations for future research The investigations regarding chromium plating from electrolytes containing chromium(III) chloride in ionic liquid were done in a technical setup with chromium anodes, a good bath movement and a variety of materials for cathodes. In the literature we found that the most promising electrolyte for hard chromium is a proprietary mixture of chromium(III) chloride, choline chloride and water. We started our investigations with chromium(III) chloride hexahydrate and choline chloride mixed in a molar ration of 3.1 to 1. The black precipitate contained more chromium (35 at.%) at higher absolute current densities (140 mA/cm2). In the layer was a high amount of oxygen (around 58 at.%). The absolute cell voltage was extremely high (16 V), even if the absolute current density was only 10 mA/cm2. At very low absolute current densities (0.35 mA/cm2) the plating was overpowered by the corrosion occurring at the electrodes. The anodes dissolved during the plating process. Adding more water to this kind of electrolyte lowered the cell voltage. Also here we saw that a higher current density gives us a higher chromium content in the black precipitate, but also leads to more gas evolution during plating. Future research could investigate the effect of the addition of more water and/or higher current densities with these electrolytes. It was decided to explore the effect of the addition of lithium and potassium chloride on the process. We found that both additions improved the process but potassium chloride additions are more promising than lithium chloride additions due to the superior absolute cell voltage drops and chromium content of the plated layer . Future research should investigate how far the chromium content can be increased in the plated layer with electrolytes saturated with potassium chloride and possibly with less than six water molecules per chromium ion. It was observed that water condensed in the cold parts of the plating cell when mixtures of choline chloride and chromium(III) hexahydrate were heated up. To create an electrolyte with less water we can pull a vacuum on the electrolyte and remove some of the water or we try to add anhydrous chromium(III) chloride. Water in the electrolyte increases the possibility of introducing hydrogen embrittlement into the substrate during plating. We decided to investigate water-free electrolytes from [EMIm] Cl and 10 wt.% anhydrous chromium(III) chloride. We saw very little or no plating during our first tests with a high copper content in the layer produced at the copper cathode. The chromium concentration was below 10 at.% on the cathode. Long term plating experiments with nickel cathodes resulted in high absolute cell voltages at some point of time during plating, which was first attributed to bad contacts, but the last test showed a thin carbon rich film on the cathode pointing to electrolyte destruction. More research in this area would focus on electrolytes with other ionic liquids and additions like potassium chloride.

56

Attachment A: References [1] A. Abbott, G. Capper, D. Davies, R. Rasheed (2004) Chem. Eur. J. 10, 3769. [2] A. Abbott, G. Capper, D. Davies, R. Rasheed, J. Archer, and C. John, (2004)

Trans. Inst. Met. Finish. 82, 14. [3] Tabereaux, A. T. (1994) JOM, 42, 30-34. [4] E. L. Smith, C. Fullarton, R. Harris, S. Saleem, A. P. Abbott, K. S. Ryder: Metal

finishing with ionic liquids: scale-up and pilot plants from IONMET consortium (2010) Institute of Metal Finishing.

[5] M. Schlesinger, M. Paunovic: Modern electroplating, 5th edition (Chapter 7: Electrodeposition of chromium) (2010) John Wiley & Sons, Inc.

[6] F. Endres, D. MacFarlane, A. Abbott: Electrodeposition from ionic liquids. (2008) Wiley-VCH Verlag GmbH & Co. KGaA, ISBN: 978-3-527-31565-9.

[7] IONMET – New ionic liquid solvent technology to transform metal finishing – products and processes Chapter 3: P. Benaben : Hard and decorative coating http://www.ionmet.eu/fileadmin/ionmet/Dissemination/IONMET_brochure_200711.pdf

[8] S. Eugénio, C. Rangel, R. Vilar: Departamento de Engenharia de Materiais, Instituto Superio Técnico, Universidade Técnica de Lisboa, Portugal

http://repositorio.lneg.pt/bitstream/10400.9/529/1/ELECTRODEPOSITSONIA.pdf [9] S. Eugénio, C. Rangel, R. Vilar, S. Quaresma: Electrochemical aspects of black

chromium electrodeposition from 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid (2011) Electrochimica Acta 56 10347-10352

[10] S. Eugénio, C. Rangel, R. Vilar, A. M. Botelho do Rego: Electrodeposition of black chromium spectrally selective coatings from a Cr(III)-ionic liquid solution (2011) Thin solid Films 519 1845-1850

[11] M. Ali, A. Nishikata, T. Tsuru: Electrochimica Acta 42 (1997) 2347

http://www.ionmet.eu/fileadmin/ionmet/Dissemination/IONMET_brochure_200711.pdf

http://repositorio.lneg.pt/bitstream/10400.9/529/1/ELECTRODEPOSITSONIA.pdf

57

Attachment B: Cr-plating tests Chromium Plating Experiment

Number 1 Cr 2 Cr 3 Cr 4 Cr 5 Cr 6 Cr


chloride = 3.10:1

CrCl3*6H2O : Cholin

chloride = 3.10:1

CrCl3*7.5H2O : Cholin

chloride = 3.10:1

CrCl3*9H2O : Cholin

chloride = 3.10:1

CrCl3*9H2O : Cholin

chloride = 3.10:1

CrCl3*9H2O : Cholin

chloride = 2:1

Electrolyte history Newly prepared 1Cr 1-2Cr and water 1-3Cr and water 1-4Cr New

Plating temperature oC 58-70oC (no cooling) 72-46 48.0 50 50 50

Stirrer setting 1-10 5 = 645-675 rpm 5 = 634-658 rpm 5 5 5 5

Start weight cathode g 2.8111 2.0039 1.6053 1.4488 1.567 1.458

Start weight anode 1 g 2.1837 2.1450 1.9622 1.9291 1.8279 2.2828


Plating time sec 1200 12000 12000 12000 60300 9000

Area of cathode cm2 3.15568 2.67257 2.6246436 2.506991 2.5533043 2.524112

Current mA -441.66 -27.6343738 -39.37 -37.6048641 -38.29956445 -37.872168

Cathode material Nickel Type 420 steel (13%Cr) Type 420 steel (13%Cr) Type 420 steel (13%Cr) Type 420 steel (13%Cr) Type 420 steel (13%Cr)

Current density mA/cm2 -139.96 -10.34 -15.00 -15.00 -15.00 -15.00

Plating remark 1

At the beginning the

cell voltage was the

Solartron maximum

and the current too

low.


the Solartron

maximum and the

current too low.

Magnetic stirrer in the

electrolyte caused

scratches on cathode.

Problems with the

electrical connection.

Bad connection

between rod and

cathode?

Plating remark 2

The wanted current

was -473 mA. The

average current was

only -442 mA.

The wanted current

was-40.1 mA. The

average current was

only -27.6 mA.

Cathode was looking




remark.

Cathode was covered

with black powdery

crust which

disappeared after 65

hours in the

electrolyte.

End weight cathode g 2.8239 2.0270 1.6561 1.444 1.7256 1.4524

End weight anode 1 g 2.1498 2.1255 1.9298 1.8488 1.712 2.2524


Weight gain (cathode) g 0.0128 0.0231 0.0508 -0.0048 0.1586 -0.0056

Weight loss (anodes) g 0.0625 0.0360 0.0535 0.1267 0.2805 0.0572

How did the electrolyte appearance change? no change no change no change no change no change no change

How did the cathode look like?Covered with black

powder

Unevenly covered

with black powderDark. Black. Powdery


after 65h in

electrolyte.

Unevenly covered

with black powder

Covered with

powdery black crust.


after 65h in

How did the anode look like? Partially etched Partially etched Etched. Etched. Etched. Etched.

Cathodic current efficiency % 13.44 38.78 59.86 -5.92 38.23 -9.15

Anodic current efficiency % 65.65 60.43 63.04 156.30 67.61 93.42


Characteristic cell voltage level V -15.6 -16 -5.3 -3.3 -3.5 -3.6

Analysis of cathode surface in atomic % smooth, shiny outer crust smooth

% C 2.734 14.5 12.8 15.1 19.7 20.6

% N 2.687 3.2 3.2 0.1 4.2 0.0

% O 54.925 67.8 64.8 5.7 50.8 23.6

% Cl 4.504 1.0 1.6 0.1 3.5 0.1

% Fe 0.142 *Ni not Fe 0.2 0.2 62.9 - 38.5

% Cu 0 0.0 0.0 0.0 0.0 0.0

%Cr 35.0 13.4 17.4 16.1 21.8 17.2

Analysis of cathode surface 2 in atomic % n/a n/a n/a black spot inner crust smooth

% C n/a n/a n/a 9.171 16.785 15.6

% N n/a n/a n/a 2.227 3.556 0.0

% O n/a n/a n/a 61.387 55.191 22.8

% Cl n/a n/a n/a 3.291 2.07 0.1

% Fe n/a n/a n/a 0.438 - 43.1

% Cu 0 0.0 0.0 0.0 0.0 0.0

%Cr n/a n/a n/a 23.5 22.4 18.4

Analysis of cathode surface 3 in atomic % n/a n/a n/a n/a n/a crust

% C n/a n/a n/a n/a n/a 4.6

% N n/a n/a n/a n/a n/a 3.5

% O n/a n/a n/a n/a n/a 47.7

% Cl n/a n/a n/a n/a n/a 3.1

% Fe n/a n/a n/a n/a n/a 0.5

% Cu 0 0.0 0.0 0.0 0.0 0.0

%Cr n/a n/a n/a n/a n/a 40.5


% C 5.7 2.3 2.7 0.0 4.8 11.9

% N 2.2 2.1 3.9 1.8 1.9 5.5

% O 27.0 26.8 30.3 20.1 20.0 39.5

% Cl 0.2 0.2 0.1 0.2 0.1 0.3

% Fe 0.243 *Ni not Fe 0.0 0.3 0.1 - 0.2

% Cu 0 0.0 0.0 0.0 0.0 0.0

%Cr 64.7 68.7 62.7 77.8 73.2 42.6

58

Chromium Plating Experiment

Number 7 Cr 8 Cr 9 Cr 10 Cr 11 Cr 12 Cr


chloride = 2:1

CrCl3*6H2O :

Cholin chloride =

2:1

CrCl3*6H2O :

Cholin chloride =

2:1 and 15 w.% LiCl

CrCl3*6H2O : Cholin

chloride = 2:1 and

15 w.% LiCl

CrCl3*6H2O : Cholin

chloride : LiCl = 2:1:

2.8

CrCl3*6H2O : Cholin

chloride : KCl = 2:1:2.5

Electrolyte history 6 Cr New 8 Cr 8-9Cr 8-10Cr Fresh

Plating temperature oC 40 60 60 60 60 60

Stirrer setting 1-10 5 5 5 5 5 5




Plating time sec 1800 7200 7200 7200 7200 7200


Current mA -168.26056 -0.8950978 -0.940682706 -8.794788103 -28.58676 -27.25959

Cathode material Copper Copper Copper Copper Copper Copper


Plating remark 1 lots of gas evolutionpositive voltage

was observed

large amounts of

carbon from

brushes present

Plating remark 2




Weight gain (cathode) g 0.0079 -0.0002 -0.0001 0.0010 0.0020 0.0068


How did the electrolyte appearance change? no change no change no change no change no change no change

How did the cathode look like? Dark. Black. Powdery no plating no plating Black. Powdery

black w/ copper

showing through

Green&black

How did the anode look like? Etched. no change no change Partially etchedno change / stained no change / stained

Cathodic current efficiency % 14.52 -17.28 -8.22 8.79 5.41 19.29



Characteristic cell voltage level V -7.3 ±0.03 -0.08 -4.1 range: -5 -> -9 -1.75

Analysis of cathode surface in atomic % crust nodules

% C 8.8 34.6 27.7 10.2 7.9 7.9

% N 2.3 0.0 0.0 3.1 3.3 2.5

% O 49.7 3.2 2.9 56.8 61.4 49.9

% Cl 5.0 0.1 0.2 1.1 2.6 7.7

% Fe 0.0 0.0 0.0 0.0 0.0 0.0

% Cu 0.5 61.8 68.8 3.3 0.1 1.2

%Cr 33.7 0.2 0.4 25.4 20.2 30.9

Analysis of cathode surface 2 in atomic % n/a n/a n/a n/a smooth smooth-some plating

% C n/a n/a n/a n/a 11.569 6.6

% N n/a n/a n/a n/a 3.828 1.9

% O n/a n/a n/a n/a 57.59 54.0

% Cl n/a n/a n/a n/a 1.522 10.7

% Fe n/a n/a n/a n/a 0 0.0

% Cu n/a n/a n/a n/a 19.004 6.6

%Cr n/a n/a n/a n/a 6.5 20.3

Analysis of cathode surface 3 in atomic % n/a n/a n/a n/a n/a smooth-little/no plating

% C n/a n/a n/a n/a n/a 9.9

% N n/a n/a n/a n/a n/a 1.4

% O n/a n/a n/a n/a n/a 41.3

% Cl n/a n/a n/a n/a n/a 7.0

% Fe n/a n/a n/a n/a n/a 0.0

% Cu n/a n/a n/a n/a n/a 31.7

%Cr n/a n/a n/a n/a n/a 8.7


% C n/a 2.851 n/a 3.2 1.7 0.5

% N n/a 1.836 n/a 3.3 4.4 2.7

% O n/a 24.885 n/a 33.0 30.0 20.4

% Cl n/a 0.07 n/a 0.4 0.1 0.1

% Fe n/a 0 n/a 0.0 0.0 0.0

% Cu n/a 0.233 n/a 0.2 0.3 0.2

%Cr n/a 70.1 n/a 59.9 63.6 76.1

59

Chromium Plating Experiment

Number 13 Cr 14 Cr 15 Cr 16+17 Cr 18+19 Cr 20 Cr

Electrolyte compositionCrCl3 : EMImCl =

1:9.7 = molar ratio

CrCl3 : EMImCl =

1:9.7 = molar

ratio









Electrolyte history Fresh 13Cr Fresh New New New

Plating temperature oC 120 120 120 120 100-117 125-137

Stirrer setting 1-10 5 5 5 magn. stir. 300 rpm magn. stir. 300 rpm magn. stir. 300 rpm




Plating time sec 7200 7200 7200 63403 121000 57900


Current mA -33.115 -65.3836117 -25.7459801 -57.41 -34 -80.72

Cathode material Copper Copper Copper Nickel rod as cathode Nickel rod as cathode Nickel rod as cathode


Plating remark 1Good looking

graph

Contact problems

during the first

minutes and the last

hours. Electrolyte

turned dark brown.

Run with lots of

problems. Anode fell

off the wire. Lsot

contacts. Electrolyte

turned brown.

Contacts with silver

epoxy secured and

area covered with

PTFE foil.

Plating remark 2

Potentiostat shut

down before the end

of the test.

Only app. One third

of the time the run

was normal. Plating

was mostly rinsed off

the cathode.

Cell voltage jumped

from 5 to 16 V - back

and forth - after 6400

seconds.




Weight gain (cathode) g -0.0168 0.0010 -0.0035 0.0164 0.0012 -0.0532


How did the electrolyte appearance change?no change purple to

brownish-black

slightly darker Turned from pink to

dark brown.

Turned from pink to

dark brown.

Turned from pink to

dark brown.


copper with some

grey

powdery black copper w/

discoloration

Ni-rod turned black Ni-rod turned black

but lost crust during

rinsing.

Ni-rod turned black

but lost some foil

during rinsing.

How did the anode look like?no change / stained very shiny, some

dark spots

darker, etched etched darker, etched dark

Cathodic current efficiency % -39.23 1.18 -10.51 2.51 0.16 -6.34


Open circuit cell voltage V -0.225 -0.235 0.3416

Characteristic cell voltage level V -0.8 range: -1.8 -> -1.47 -1.6 -1.50 -1.5 till 2.5 -5


% C 12.7 17.0 12.8 32.5 43.3 94.2

% N 0.9 0.0 0.0 1.7 0.0 0.0

% O 34.3 32.8 6.4 41.4 25.9 0.2

% Cl 0.2 1.0 0.0 0.7 1.2 5.2

% Fe 0.0 0.0 0.0 - 20.8 at.% Ni 0.2 at.% Ni

% Cu 42.5 46.2 80.3 0.6 at.% Ni 1.3 0.0

%Cr 9.6 3.0 0.4 23.0 7.5 0.1

Analysis of cathode surface 2 in atomic % n/a n/a n/a n/a n/a n/a

% C n/a n/a n/a n/a n/a n/a

% N n/a n/a n/a n/a n/a n/a

% O n/a n/a n/a n/a n/a n/a

% Cl n/a n/a n/a n/a n/a n/a

% Fe n/a n/a n/a n/a n/a n/a

% Cu n/a n/a n/a n/a n/a n/a

%Cr n/a n/a n/a n/a n/a n/a

Analysis of cathode surface 3 in atomic % n/a n/a n/a n/a n/a n/a

% C n/a n/a n/a n/a n/a n/a

% N n/a n/a n/a n/a n/a n/a

% O n/a n/a n/a n/a n/a n/a

% Cl n/a n/a n/a n/a n/a n/a

% Fe n/a n/a n/a n/a n/a n/a

% Cu n/a n/a n/a n/a n/a n/a

%Cr n/a n/a n/a n/a n/a n/a


% C 1.5 0.0 0.2 0.2 4.8

% N 2.3 4.4 0.1 3.5 2.5

% O 20.1 31.9 18.9 31.9 27.0

% Cl 0.2 0.2 0.2 0.2 0.2

% Fe 0.0 0.0 0.0 0.2 at.% Ni 0.3 at.% Ni

% Cu 0.7 0.4 0.2 0.3 0.6

%Cr 75.2 63.2 75.8 63.6 64.6

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