# YOCO (You Only Coat Once)

Prepared for:Dr. David Drown

Prepared by:

Diane Edwards, Quinn Macpherson, Zach Campbell, Adam GrebilUniversity of Idaho

Advisors:

Dr. David DrownDr. Krishnan Raja

TABLE OF CONTENTS EXECUTIVE SUMMARY………………………………………………………………..2INTRODUCTION………………………………………………………………………….2BACKGROUND…………………………………………………………………………....3DESIGN ANALYSIS……………………………………………………………………….5

Figure 2: (Bench Scale Set-Up)……………………………………………………...5RESULTS/RECOMMENDATIONS FOR FUTURE STUDY……………………...…...5

Effect of Voltage…………………………………………………………………......7Effect of Plating Time……..………………………………………………………....8Weight Loss Test…….……………………………………………………………….9

PHYSICAL DELIVERABLES…………………………………………………………..…12CONCLUSIONS AND RECOMMENDATIONS………………………………………....13ACCNOLEGMENTS………………………………………………………………………..13

REFERENCES……………………………………………………………………………....14APPENDIX…………………………………………………………………………………..15

Safety and Treatment NH4F Exposure………………………………………………..15Safety and Treatment for NaOH Exposure…………………………………………...16

Electrical Chemical Testing…………………………………………………………..17 Theoretical Background on Electrochemical Testing Methods…………………………….20

Weight Loss Set-Up…………………………………………………………………..23Weight Loss Images with Scale………………………………………………………………24

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EXECUTIVE SUMMARYThe magnesium alloy Electron 21 was anodized in a solution of 1 M NaOH with 0.2 M

NH4F at 30 Volts for 1 hr. The anodized Elektron 21 samples (1 x ½ x ⅛ in) gained 0.0007-

0.0004 grams/in2 during anodization. Electrochemical tests of these samples in 3.5% NaCl

indicate improved the corrosion resistance. The anodized samples had an Icorr value of 4.2-17

µAmp/cm2 (3.8-15mpy), which is roughly 5 times better than bare samples which ranged from

Icorr values of 24-80 µAmp/cm2 (21-72mpy). Weight loss corrosion test yielded mixed results and

should be redone with a more accurate scale to improve accuracy. It can be concluded that

anodizing Elektron 21 in 1 M NaOH with 0.2 M NH4F only marginally increases corrosion

resistance. Further testing should be done before full scale operation in order to determine life

expectancy of the coating.

INTRODUCTION The scope of this project is to anodize the magnesium based alloy of Elektron 21 in order to

create a passive layer to reduce corrosion. The solution created to achieve this layer is 1 M

NaOH with 0.2 M NH4F which is a fluorine electrolyte solution. A wide range of parameters

were tested in order to find an optimum condition for anodization of this particular alloy.

BACKGROUND

Magnesium and its alloys represent an interesting alternative to aluminum for light

weight applications. Magnesium has a higher specific strength than both aluminum and stainless

steel. However, magnesium has a significant downside in that it has a very high corrosion rate

when compared to aluminum and stainless steel. While it is true that magnesium will form its

own oxide layer, the layer is porous and contains cracks and thin spots that are highly susceptible

to corrosion. Anodization forms an oxide layer that is expected to be both highly uniform and

able to adequately shield the magnesium alloy from the environment and significantly reduce

any corrosion.

Prior testing has been done extensively on AZ31D(4) and AZ91(5).

The approximate compositions of these alloys in mass percentage are as follows: (4,5,6)

AZ31D 3% Aluminum 1% Zinc

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0.3% Manganese Balance Magnesium

AZ91 9% Aluminum 1% Zinc 0.6% Silicon 0.2% Manganese Balance Magnesium

Elektron 21 3% Neodymium 1.5% Gadollinium 0.5% Zinc Saturated Zirconium Balance Magnesium

In this project the effectiveness of a fluoride based solution to anodize the magnesium

alloy Elektron 21 is explored. Initial testing of the solution was done on the alloy AZ31D. These

preliminary results were used to determine the best solution to use for Elektron 21.

The basic reaction between magnesium and fluoride follows:

3 Mg+2 N H 4 F+4 H2O → 2MgO+Mg F2+2N H 4OH +3 H2

There are a number of surface coatings for magnesium alloys that have been previously tested.

Below is a list of the various coatings with accompanying coating conditions and corrosion

results where applicable and known.

Self-passivation of Magnesium: even though magnesium forms its own layer of oxide

hydroxide, it is a poor guard against corrosion because the Pilling-Bedworth ratio is 0.81

so the hydroxide doesn’t completely cover the surface, leaving cracks exposed to

corrosion.(7)

HAE – In this treatment the magnesium is anodized with an electrolyte of potassium

hydroxide (165g/L), aluminum hydroxide (35g/L), potassium fluoride (35g/L), sodium

phosphate, potassium manganate (20g/L), and sodium phosphate (35g/L). (7) Current: 20-

25 mA/cm2 AC for 8-60min.

Zhang et. al. performed electro-chemical corrosion tests on AZ91D treated with

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HAE and reports a polarization resistance of 7.63 kohms, a corrosion current of

1.496 micro-amp/cm2, and a corrosion potential of -1.630V vs. SHE. (5)

DOW 17 - In this treatment the magnesium is anodized with an electrolyte of ammonium

fluoride (240g/L), sodium dichromate (100g/L), and 85% phosphoric acid (90ml/L). (7)

Current: 5-50 mA/cm2 for 5-25min.

Zhang et. al. performed electro-chemical corrosion tests on AZ91D treated with

Dow 17 and reports a polarization resistance of 41.99 kohms, a corrosion current

of 1.694 micro-amp/cm2, and a corrosion potential of -1.577V vs. SHE. (5)

Zhang’s environmentally friendly coating – In this treatment the magnesium is anodized

with an electrolyte of potassium hydroxide, sodium carbonate, sodium silicate, sodium

borate.

Zhang et. al. performed electro-chemical corrosion tests on AZ91D anodized with

this electrolyte and reports a polarization resistance of 509.3 kohms, a corrosion

current of 0.184 micro-amp/cm2, and a corrosion potential of -1.379V vs. SHE. (5)

Zhu’s ethylene glycol electrolyte coating – Zhu et. al. report anodizing AZ31B

Mg alloy with an electrolyte composed of sodium hydroxide (40g/L), sodium

silicate (30.0g/L), sodium borate (10g/L), sodium citrate (10g/L). In addition

10g/L of ethylene glycol or polyethylene of various degrees of polymerization

were added respectively to test their effect.

The minimum corrosion rate occurred for a degree of polymerization of 1000 at

which the Ecorr = -1.222V vs. SHE and Icorr = 0.1232 micro-amp/cm2. (5)

Success has been seen using many different variations; however, the most significant

results were found using a basic electrolyte. (5) These anodization processes tend to be run at

higher voltages, this means that less time is needed; however, this results in cracking in the

surface layer. In addition to the basic electrolyte, a substance that would readily react with the

magnesium surface had to be added. Initial testing showed that a solution comprised of

ammonium fluoride in NaOH gave the most uniform layer formation. Solutions of ethylene

glycol and NaH2PO4 were also tested; however, they were not as effective as the sodium

hydroxide solution and were not used on the Elektron sample.

DESIGN ANALYSIS

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Most of the parameters adjusted during design optimization were chemical and electrical

rather than physical. Anodization was conducted in a 200 mL beaker as shown in Figure 2, with

a titanium plate as the cathode and the magnesium alloy as the anode. The magnesium alloy and

titanium plate were spaced 1 cm apart as per convention. The AZ31D samples were ⅞ inch

diameter circular samples and Elektron 21 samples were 1 inch by ½ inch rectangles. A resistor

was placed on the ground wire in order to measure the current using Ohm’s law. Knowing the

current – which often decreases exponentially with time - helped to determining optimal

anodization time. The solution was only reused three times in order to retain ample fluorine ion

concentration.

Bench Scale Setup

Figure 2: Bench Scale design consisting of a power supply, resistor, titanium

plate, alloy, stir bar, and 200 mL beaker.

Results/Recommendations for Future Study

While optimizing the anodization process, several things were noticed that will be useful for

further research on the protection of magnesium alloys by anodizing. Prior to the arrival of the

Elektron 21 sample, magnesium AZ31 samples were anodized under varying anodizing

conditions and tested electrochemically. While too few samples were tested to reach definitive

conclusions about the optimal anodizing conditions, preliminary results were found that proved

useful for guiding further research.

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Comparison of Solutions

The solution that produced the best results was (0.2M NH4F + 1M NaOH). The majority

of the results in this report are based on anodizing using this solution. An ethylene glycol based

solution (3.8vol% H2O + 0.2 M NH4F in ethylene glycol) was used to anodize both AZ31 and

Elektron 21. In each case the ethylene glycol based solution showed visibly poor coatings with

Icorr values only marginally better than that of the as-received samples. Consequently, the

ethylene glycol was rejected for further testing.

Two variants of the (0.2M NH4F + 1M NaOH) solution were experimented with. The

first was to double the concentration of NH4F to 0.4M. Though increasing NH4F concentration

didn’t dramatically change the results, it did lead to an increased Icorr on the few samples for

which it was tested. The second was the addition of 0.5M NaH2PO4. When used to anodize both

alloys, the added NaH2PO4 dramatically increased anodizing current (for a set voltage) and led to

rapid buildup of a non-uniform porous oxide layer on the sample. Addition of NaOH to increase

the pH of the solutions with NaH2PO4 failed to mitigate the poor results.

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Figure 3: Image on the left is 0.5 M NaH2PO4 using Elektron 21 as the alloy; image on the right is 0.5 M NaH2PO4 using AZ31D as the alloy.

The SEM bare sample image of Elektron 21 in Figure 4 shows a two phase alloy. The

SEM anodized sample image shows that it does coat after anodization. SEM pictures were taken

of Elektron 21 at various steps. It was also observed after Echem testing where corrosion does

occur due to the salt solution used for testing. These images led to the conclusion that even

though coating was observed as in Figure 4, it did not stop corrosion completely. Figure 5 is

Elektron 21 as received with Echem as well as Elektron 21 anodized with Echem.

Effect of Voltage

The AZ31 was anodized at voltages ranging from 40V to 100V. Observing that the best

results when coating AZ31 were obtained at lower voltages, the Elektron 21 was tested at 20V,

35V, and 45V over which range no significant change in corrosion current was found.

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Figure 4: Image on the left is a polished bare Elektron 21 SEM image; image on the right is an anodized sample in 0.2M NH4F + 1M NaOH solution 30 volts 1 hr using Elektron 21 as the alloy.

Figure 5: Image on the left is a polished bare Elektron 21 SEM image with corrosion from Echem; image on the right is an anodized sample in 0.2M NH4F + 1M NaOH solution 30 volts 1 hr using Elektron 21 as the alloy with corrosion from Echem.

Figure 6: Effect of voltage on Icorr results

Based on the results obtained during testing, it is recommended that future research use

30-40V as a baseline value, though further optimization may be worthwhile.

Effect of plating time

Most AZ31 samples and all Elektron 21 samples were anodized for one hour. However,

a few of the AZ31 samples that were tested for longer periods suggest that increasing anodizing

time may lead to better results.

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Figure 7: Effect of time period of anodization on Icorr

Choice of an anodizing time is complicated in that the anodizing current does not proceed

uniformly with time. Sometimes the anodizing current decreased monotonically, other times it

oscillated with time. On a few Elektron samples, a very low current was obtained for the first 20

minutes after which the current spontaneously increased dramatically, coating the sample and

then returning to low level.

Weight Loss Tests The weight loss test is a simple, though tedious, method of test for measuring corrosion rate.

Samples are weighed, placed in a corrosive solution for a prescribed time, weighed again, and

deduce the weight of metal lost to corrosion. Because a surface oxide layers often absorb

moisture which can add error to the weight measurements the samples were dried in an

evacuated oven at 120C for three hours prior to weighing both before and after corrosion.

Because corrosion products, which are often heavier than the base metal, can remain attached to

the surface it is not uncommon for weight loss tests to increase rather than decrease. After

weighing the corroded samples, they were brushed with a soft bristle toothbrush to remove

loosely attached corrosion product and reweighted.

As the accuracy of the initial weight measurements for most of the samples was done on a

scale with accuracy estimated to be +/- 2 mg, results must be taken with a grain of salt.

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Images and tables below are weight loss data for 98 as well as 24 hour intervals.

Figure 8 (right): 98 hour aerated weight loss. The three samples on the top are as received samples. The three samples on the bottom are anodized samples at 30 volts for 1 hour in a solution of 0.2M NH4F + 1M NaOH. Left to right: above solution, at the water line, fully submerged.

Above are the changes from initial weight. Positive values represent weight gain.

Figure 9 (left): 98 hour non-aerated weight loss. The three samples on the top are the as received sampled. The three samples on the bottom are anodized samples at 30 volts for 1 hour with a solution of 0.2M NH4F + 1M NaOH. Left to right: above solution, at the water line, fully submerged.

Above are the changes from initial weight. Positive values represent weight gain.

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Weight Change 98hr, aerated (mg)

abovewater line

submerged

as received, before brushing 5 -1.8 -3.4as received, after brushing 2.8 -4.2 -5.7anodized, before brushing 2.9 -3.9 1.3

anodized, after brushing 1 -5.7 -3.4

Weight Change 98hr, non-aerated (mg)abov

ewater line

submerged

as received, before brushing 2.1 -2.8 -7.5as received, after brushing 0.1 -7.3 -13.1anodized, before brushing 2.6 2.4 -1.7

anodized, after brushing 1.1 0.9 -8.8

Figure 10 (right): 24 hour aerated weight loss. The three samples on the left side are as received sampled. The three samples on the right are anodized samples at 30 volts for 1 hour with a solution of 0.2M NH4F + 1M NaOH.

Above are the changes from initial weight. Positive values represent weight gain.

Figure 11(left): 24 hour non-aerated weight loss. The three samples on the left side are as received sampled. The three samples on the right are anodized samples at 30 volts for 1 hour with a solution of 0.2M NH4F + 1M NaOH.

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Weight Change 24hr, aerated (mg)abov

ewater line

submerged

as received, before brushing 1.7 0.4 1.8as received, after brushing -0.3 -1.1 -0.1anodized, before brushing 2.8 0 0.1

anodized, after brushing 1.4 -2.5 -2.2

Weight Change 24hr, non-aerated (mg)abov

ewater line

submerged

as received, before brushing 1.3 2.3 2.5as received, after brushing -0.2 0.4 0.5anodized, before brushing 2 4.5 2.7

anodized, after brushing 0.2 1.5 1.4

Above are the changes from initial weight. Positive values represent weight gain.The weight loss was inconsistent on which conditions (fully-submerged, half-submerged,

and none-submerged) were gaining or losing weight. This may be the result of the poor accuracy the scale used to take the initial weight measurements.

Non-Uniformities of Elektron 21

Porosity was observed on several of the Elektron 21 samples after polishing. The pores

were visible to the naked eye, and they were easily observed under low magnification. Pores can

have detrimental effects to the corrosion protection of the samples. It is recommended that future

studies check for porosity in the samples before use. Porosity may be sample specific. During

the course of this project porosity was only observed on samples that were obtained from the

outer edge of the larger sample.

Comparison of Tafel tests for AZ31 and Elektron 21

The corrosion resistance of Elektron 21 vs. AZ31 is of some interest for assessing the

advantage of Elektron 21 as an alloy. This comparison is difficult as several of the electro-

chemical tests performed on AZ31 were done before testing procedures were perfected.

However, based on the limited data, it appears that Elektron 21 has a corrosion rate roughly five

times lower than that of AZ31.

De-aeration during electrochemical testing

Electro-chemical tests were performed on AZ31 both with and without de-aeration by

bubbling nitrogen through the salt water corrosion solution. Based on two repetitions each (the

number of samples tested with at least half way reliable results) it appears that the de-aeration

reduces corrosion current by a factor nearly of an order of magnitude.

Effect of various parameters

The variability of anodizing results was possibly due to impurities or segregation of

alloying elements. The amount of impurities was reduced by cleaning the magnesium clips each

use, as well as polishing thoroughly, and putting the clip through an ultra-sonic cleaner. The use

of the magnesium clips were useful during weight loss by maximizing the surface area exposed

to the solution. Early it was observed that a 600 grit polish was leaving deep valleys in the

samples that were susceptible to corrosion. This was reduced by increasing the final polishing

step to 1500 grit.

PHYSICAL DELIVERABLES

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Aside from the technical knowledge obtained in this study, there are also physical products

that Dr. Raja and his graduate students may find useful for continued research.

Echem tester: The electrochemical corrosion testing apparatus can be used for

electrochemical corrosion testing of other samples whether they are magnesium alloys or not.

The description of the use of the electrochemical set up is attached as an appendix to this report.

In addition to the single electrochemical testing apparatus, which can be used with or without

duration, an additional two testers are planned.

Weight loss setup: this loss setup which consists of the plumbing and containers set up to

bubble air through beakers for weight loss tests. The description of this setup and its use is

attached as an appendix.

Samples: There is leftover Elektron 21 and Elektron 79 that should be sufficient for

further studies if Dr. Raja's lab should decide to pursue them further.

Other minor items include: purchase bottles and beakers, magnesium clamps that are

useful for anodizing magnesium samples for weight loss tests, etc.

CONCLUSIONS AND RECOMMENDATIONS

The magnesium alloy, Elektron 21, was anodized at 30 Volts for 1 hr in a solution of 1 M

NaOH with 0.2 M NH4F resulted in a coating that slowed corrosion marginally. It was found that

samples with a thinner coating, and thus less weight gain, resulted in better electrochemical

results as well as losing less weight during weight loss. It is recommended that further testing be

done to determine duration of anodization, as well as why the coating happens simultaneously at

different times throughout anodization. The electrochemical Tafel tests performed corrosion rate

on anodized samples to be about 1/5 of that on un-anodized samples. Specifically, anodized

samples which had an Icorr value of 4-17 µAmp/cm2 (3.8-15mpy) where bare sample Icorr values

were 24-80 µAmp/cm2 (21-72mpy). Further testing should be done on the microstructure of

Elektron 21 as well as continuing weight loss testing using a more accurate scale.

ACCNOLEGMENTS

We thank Dr. Raja for his guidance, lab space, and resources. We thank Elektron

Corporation for providing samples. We also thank the Stewart and Kalyan for the help in the

lab. We would also like to thank Margret, Gail, Dr. Drown, Charles, and Dmac for their

assistance.

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REFERENCES

3) Gagnon, Steve. "The Element Magnesium." It's Elemental -. Thomas Jefferson National

Accelerator Facility - Office of Science Education , n.d. Web. 5 Mar. 2014.

<http://education.jlab.org/itselemental/ele012.html>.

4) Martin Jönsson, Dan Persson and Rolf Gubner. “The initial steps of atmospheric corrosion on

magnesium alloy AZ91D”. Journal of the Electrochemical Society, vol. 154, C684-C691 (2007).

5) Fang Zhu, Jinwei Wang, Shanghua Li, and Jin Zhang. “Preparation and characterization of

anodic films on AZ31B Mg alloy formed in the silicate electrolytes with ethylene glycol

oligomers as additives.” Applied Surface Science, vol 258, (2012)

6) "Elektron 21 Data Sheet." Magnesium-Elektron. Luxfer Group Company, 1 May 2006. Web.

5 Mar. 2014. <http://www.magnesium-elektron.com/data/downloads/ds455.pdf>.

7) Horst E. Friedrich, and Barry L. Mordike. Magnesium Technology: Metallurgy, Design Data, Applications. Springer (2006).

10) Korn, Derek. "Bringing Anodizing In-House." Modern Machine Shop. Modern Machine

Shop, 27 July 2008. Web. 05 Mar. 2014. <http://www.mmsonline.com/articles/bringing-

anodizing-in-house>.

11) Cole Parmer. “Large-Capacity Stainless Steel Ultrasonic Cleaner, 40 Hz, 21 gal; 230VAC”

17 Apr. 2014 <http://www.coleparmer.com>

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APPENDIX

Safety and Treatment for NH4F

AMMONIUM FLUORIDE

Neutral ammonium fluorideNH4FMolecular mass: 37.0

TYPES OF HAZARD/ EXPOSURE

ACUTE HAZARDS/SYMPTOMS

PREVENTION FIRST AID/FIRE FIGHTING

FIRE Not combustible. Gives off irritating or toxic fumes (or gases) in a fire.

In case of fire in the surroundings: water in large amounts to knock down acid vapors, then use appropriate

EXPLOSION

EXPOSURE PREVENT DISPERSION OF DUST!

Inhalation Cough. Sore throat. Local exhaust or breathing protection.

Fresh air, rest. Refer for medical attention.

Skin Redness. Protective gloves. Remove contaminated clothes. Rinse skin with plenty of water or shower.

Eyes Redness. Pain. Face shield, or eye protection in combination with breathing protection if powder.

First rinse with plenty of water for several minutes (remove contact lenses if easily possible), then take to a doctor.

Ingestion Diarrhea. Nausea. Vomiting. Abdominal pain. Burning sensation. Shock or collapse.

Do not eat, drink, or smoke during work.

Rinse mouth. Do NOT induce vomiting. Give plenty of water to drink. Refer for medical attention.

SPILLAGE DISPOSAL PACKAGING & LABELLING

Sweep spilled substance into dry plastic containers. Carefully collect remainder, then remove to safe place. Personal protection: P3 filter respirator for toxic particles. Do NOT let this chemical enter the environment.

T Symbol Do not transport with food.R: 23/24/25 S: (1/2-)26-45UN Hazard Class: 6.1UN Pack Group: III

EMERGENCY RESPONSE STORAGETransport Emergency Card: TEC (R)-61GT5-III NFPA Code: H3; F0; R0

Separated from incompatible materials, food and feedstuffs.Dry. Well closed.

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Safety and Treatment for NaOH

Sodium Hydroxide

Sodium HydroxideNaOHMolecular mass: 40.0

TYPES OF HAZARD/ EXPOSURE

ACUTE HAZARDS/SYMPTOMS

PREVENTION FIRST AID/FIRE FIGHTING

FIRE Not combustible. Colorless and odorless

In case of fire in the surroundings: water in large amounts to knock down acid vapors, then use appropriate extinguishing agent.EXPLOSION

EXPOSURE PREVENT DISPERSION OF DUST!

Inhalation Cough. Sore throat. Local exhaust or breathing protection.

Fresh air, rest. Refer for medical attention.

Skin Redness. Burns. Protective gloves. Remove contaminated clothes. Rinse skin with plenty of water or shower.

Eyes Redness. Pain. Permanent eye damage.

Face shield, or eye protection in combination with breathing protection if powder.

First rinse with plenty of water for several minutes (remove contact lenses if easily possible), then take to a doctor.

Ingestion Diarrhea. Nausea. Vomiting. Abdominal pain. Burning sensation. Shock or collapse.

Do not eat, drink, or smoke during work.

Rinse mouth. Do NOT induce vomiting. Give plenty of water to drink. Refer for medical attention.

SPILLAGE DISPOSAL PACKAGING & LABELLING

Sweep spilled substance into dry plastic containers. Carefully collect remainder, then remove to safe place. Personal protection: P3 filter respirator for toxic particles. Do NOT let this chemical enter the environment.

C Symbol Do not transport with foodR: 35 S: 26/37/39/45UN Hazard Class: 8UN Pack Group: II

EMERGENCY RESPONSE STORAGECAS# 497-19-8: VZ4050000 Separated from incompatible materials, food and feedstuffs.

Dry. Well closed. Avoid contact with water.

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Electrical Chemical Testing

An electrical chemical testing apparatus was constructed to measure corrosion potential, polarization resistance, and TAFEL plots. A diagram and associated picture are shown below.

Figure: E-chem tester setup (electrical chemical testing)

Components:

- Solution container – The container for holding the corrosive solution during testing is a rectangular Teflon block with a cavity milled into it. It is designed with a hole in one side with an o-ring against which the sample seals. The sample is held in place by a set screw which is mounted in the horizontal brace. The aluminum brace is held in place by set screws.

- * Lid – A clear polycarbonate lid separates the solution from the atmosphere. Holes for a

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reference electrode, counter electrode, and nitrogen lines. The lid is held in place with set screws.

- *Gasket – A rubber gasket seals the lid to the solution container. Vacuum grease is sometimes needed to obtain a good seal but should be used sparingly and with caution to avoid contamination.

- Sample – The E-chem tester is designed to test samples of a large range of sizes and shapes. Samples must have a flat side large enough to cover a 1cm2 hole, must be electrically conductive, and mechanically rigid (so they can be held by a single set screw).

- Reference Electrode – The reference electrodes used are Ag/AgCl/sat. KCl reference electrodes constructed from a disposable pipette.

- Counter electrode – The counter electrode used is a platinum wire extending from a thin disposable pipette.

- Gamry Meter – The Gamry E-chem testing unit performs all power supply and testing needed for echem tests.

- *De-aeration container/bubbler – In order to de-aerate the corrosion solution prior to putting in contact with the sample. In order to transfer the de-aerated solution from the de-aeration container to the e-chem tester, the nitrogen pressure on the bubbler must be kept below one atmosphere gage.

*Not required when de-aeration is not required.

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E-chem testing procedure:

- Clean and rinse system with deionized water.

- Place sample O-ring in groove.

- Clamp sample and connection tab in place with set screw. The foil connection tab can be clamped on top the sample for ease of electrical connection.

- *Fill bubbler with corrosion solution.

- *Spread vacuum grease on gasket. Apply sparingly and carefully to avoid contamination.

- *Place gasket and lid on e-chem continuer and clamp in place with set screws.

- Mount reference electrode. In the de-aeration setup, this is done by placing it in the slanted hole. In the non-deaerated case the reference electrode is held with a clamp stand. The tip of the reference electrode should be within a centimeter of the sample.

- Mount counter electrode. In the de-aeration setup this is done by wrapping it in parafilm wax paper to form a seal and placing it in the non slanted hole. In the non-deaerated case the counter electrode was held with a clamp stand.

- Connect all five wires with alligator clamps on the Gamry meter to their proper locations – there is a color key on the mouse pad for this.

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- Turn on the Gamry.

- *Connect all nitrogen tubes: Regulator on tank to deaerator, de-aerator to e-chem tester, e-chem tester to flask of water to act as an air trap.

- *Start Nitrogen flow. Be sure to regulate the pressure at well under one atmosphere. Use needle valve to control flow. Continue to de-aerate for 30min before bringing the solution in contact with the sample.

- Fill e-chem tester container with corrosive solution. In the case of the de-aerated solution this is done by dumping the solution down the nitrogen line, in the case of the non-deaerated solution the solution is simply poured in (Note: if e-chem tester is on jack this is easier.)

- Start data acquisition program.

- Start open circuit potential test. The routine is found in under experiment>>DC Corrosion>>

- Start polarization resistance test. The routine is found in under experiment>>DC Corrosion>>

- Start TAFEL test. The routine is found in under experiment>>DC Corrosion>>

- *After test completed turn off nitrogen supply.

- Turn off Gamry.

- Disassemble and clean.

Theoretical Background on Electrochemical Testing Methods

The theoretical background behind electrochemical corrosion testing and data analysis is as

follows. The reader may refer to any introductory corrosion text (for example, Denny A. Jones,

Principles and Prevention of Corrosion. (1996)) for a more detailed presentation.

The corrosion reaction is a combination of anodic and cathodic half reactions. For example:

The rates of these half reactions are assumed to be governed by the Arrhenius rate constants.

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Anodic reaction Cathodic reaction

where ΔG*a and ΔG*b are the activation energies. At steady state corrosion when no applied

voltage is present:

where a is the conversion rate to mass, moles or in whatever unit r is expressed. Each half

reaction rate can be expressed as a current density.

Now assuming a bias voltage is applied, then because voltage is a measure of the chemical

potential of electrons, applying the voltage (multiplied by the appropriate constant) adds to the Δ

G. Some fraction, α, of the applied voltage will be applied to the uphill portion of the energy

barrier, while the remainder will be applied to the downhill portion - and will therefore have no

effect on reaction rate.

where F is Faraday's constant, and n is the number of electrons transferred in the reaction (two in

the above example reaction). The difference in the reaction rates results in a net current density,

i, that flows though the power supply, which creates the bias voltage.

When η≈0 both exponentials must be considered. When the applied voltage differs significantly

from 0 in either the positive or negative direction, one of the two terms will dominate.

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ra rcicorr a

n F

icorr K'a e

G* a

R T K'c e

G* c

R T

ia icorr e

a n F

R T

ic icorr e

c n F

R T

i ic ia icorr e

c n F

R T e

a n F

R T

0i icorr e

c n F

R T

0i icorr e

a n F

R T

logi

icorr

a/c n F

ln 10( ) R T

This equation is the basis for Tafel plots, which have absolute value of the log of applied current

on the horizontal axis and voltage verses the SHE on the vertical axis. According to this

equation, the Tafel plot should make a horizontal V shape with linear branches. As a result of

the aforementioned approximation should not expect the data to fit the V shape near its vertex

where η =0.

The coordinates of vertex of the V need to be known because icorr is proportional to corrosion

rate and Ecorr (the potential of the corrosion reaction when no external load is applied). In theory

this can be done by extrapolating the branches of the V and finding where they intersect. In

practice, the upper (anodic) branch of the V is often difficult to fit with a straight line so the

vertex is found by extrapolating the lower (cathoic) branch upward to Ecorr where the plot

approaches the vertical axis.

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The portion of the cathoic branch that is sufficiently linear to be used for Tafel interpolation is a

judgment call that inevitably adds a bit of subjectivity to Tafel results. The Tafel plots for the

Elektron 21 samples had a single clearly linear region. The Tafel plots for the AZ31 were more

difficult to analyze then those for the Elektron 21 because they typically exhibited two linear

regions.

Another method for determining icorr is the Polarization Resistance. Consider again equation

Taking the derivative with respect to the η .

Evaluating at E.corr i.e. η =0.

The derivative of the liner IV curve where it crosses zero applied current is easy to determine

from a quick voltage sweep. Its value is referred to as the polarization resistance. While the

values of α .c and α .a are unknown, they are typically on the order of ~0.5 each. They rarely

make a practically significant difference to icorr, which can range over several orders of

magnitude. The constants a/c can be estimated from the slopes of the branches of the Tafel plot.

All of the polarization resistance measurements generally gave icorr values very similar to the

icorr values from Tafel Analysis.

Weight Loss Setup

Weight loss corrosion tests were performed on anodized

and un-anodized samples. The corrosion conditions were as

follows:

Corrosion Solution: 3.5% NaCl in de-ionized water

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i ic ia icorr e

c n F

R T e

a n F

R T

id

dicorr

c n F

R T e

c n F

R T a n F

R Te

a n F

R T

id

dicorr

c n F

R T

a n F

R T

icorrR Tn F

1c a

i

Temperature: 23C

Volume solution/Surface area of sample: >30ml/cm2

Samples were tested in all combinations of the following conditions:

Treatment: polished (1500 grit), polished and anodized

Weight loss time: 24hr, 98hr

Aeration: stagnated and sealed, bubbled with air (see figure)

Submersion: Fully submerged, half submerged, above solution

An image containing each weight loss sample at the various parameters is on the following page.

Where the image is arranged as top three being as received samples and bottom being anodized

samples of Elektron 21. The first two groups are for 98 hour weight loss aerated and non-aerated

samples respectively. The last two groups are for 24 hour weight loss aerated and non-aerated

samples respectively.

All weight loss samples with scale for reference.

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