Industrial Application of the

Electropolishing Process of

Stainless Steel in Ionic Liquids

UNIVERSITA DEGLI STUDI DI FERRARA

ISTITUTO NAZIONALE DI FISICA NUCLEARE

LABORATORI NAZIONALI DI LEGNARO

Supervisor internal: Prof. G. Fiorentini

Supervisor external: Prof. V. Palmieri Student: Vlada Pastushenko

2013

Outline

Chemical polishing

Passivation

Corrosion studies:

Test on Atmospheric corrosion

Corrosion test in Seawater

Analysis of anodic passivation curves

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2013 3

Chemical polishing

2013 4Initial surface Etched surface

Solution №1 Retrying the Solution №1

with increasing

concentration

Etched surface

Chemical polishing (first solution)

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Solution №2

Initial surface Etched surface

Retrying the Solution №2

with increasing

concentration

Etched surface

Chemical polishing ( second solution)

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Next Step:

Passivation

Stainless steels are corrosion-resistant by nature, which might suggest thatpassivating them would be unnecessary.

However, stainless steels are not completely impervious to rusting.

One common mode of corrosion in corrosion-resistant steels is when small spotson the surface begin to rust because grain boundaries or embedded bits of foreignmatter allow water molecules to oxidize some of the iron in those spots despitethe alloying chromium.

Some grades of stainless steel are especially resistant to rouging; parts made fromthem may therefore forgo any passivation step, depending on engineeringdecisions.

Natural passivation of steel

The most common methods oxidation of steel

Chemical oxidation

• in alkaline solutions (NaOH)

• in acidic solutions (HNO3)

High-temperature oxidation (>500°C)

Anodizing

May include the use of hazardous acids and bases, high temperatures and produce

colored oxide films, mostly black

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Alternative solutions

for passivation of steel

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10

• Alternative solution emits no NOx vapors that can be harmful to theatmosphere. Nitrogen oxides aid in the production of smog.

• There is also no need for hazardous waste removal

• The formulations remove the free iron and iron oxides without removingsignificant amounts of nickel, chromium or other heavy metals

• Waste removal costs are reduced or eliminated. Raw materials are storedas nonhazardous chemicals

Why ?

2013

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№ of samples 0 1 2 3 4

Conditions of the passivation

Initial surface

Without passivation

Solution for passivation №1

Solution for passivation №2

Solution for passivation №3

Passivation

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Test on Atmospheric corrosion

№ of samples 0 1 2 3 4

Solution for thepassivation

Initial surfaceWithout

passivationSolution for

passivation №1Solution for

passivation №2Solution for

passivation №3

Before After one week outside

The result on the atmospheric corrosionfor electropolished in IL samples

The result on the atmospheric corrosionfor chemically etched samples

№ of samples 1 2 3 4

Solution for thepassivation

Solution for passivation №2

Withoutpassivation

Solution for passivation №1

Solution for passivation №3

After etching

After 2 weeks outside

41 2 3

41 2 3

After etching

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Corrosion test in Seawater

Seawater test

We prepared solution – 30g/l of

sea salt, heated it to 50°C. Air is

supplied into the solution to saturate it

with oxygen.

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2 3 410

Electropolished samples in IL Chemical etched samples

№ of samples 0 1 2 3 4

Solution for thepassivation

Without passivation

Solution for passivation №2

Solution for passivation №2

Solution for passivation №1

Solution for passivation №3

The result after 40 hours in the salt bath

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Analysis of

the anodic passivation curves

∆E

Ideal anodic passivation curve

Trying to analyze passive layer on stainless steel, we measured voltage dependence on current (not taking in account

surface area). As we’ve already passivated surface we obtained only part of graphic (red line), started from passivation area

and reached the over passivation when began the gas formation (O2↑)

Anodic passivation curve for the stainless steel in 0,5M H2SO4

This graph we obtained for passivated steel after electropolishing. We provide series of measurement to get some statistic

data, and compare it.

∆E

O2↑

0,5 M H2SO4

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5 6

Cu

rre

nt,

A

Voltage, V

Average values of the passivation techniques

Solution for passivation № 2 Solution for passivation № 3 Solution for passivation № 1 Without passivation

We built the average graphs for the all passivation methods, samples polished electrochemically in IL

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1

1,5

2

2,5

3

3,5

4

Solution for passivation № 2

Without passivation Solution for passivation № 3

Solution for passivation № 1

∆E,

V

Solutions for passivation

Voltage breaking point for current - 0,1 A

Average voltage breaking point for steelchemically etched and passivated in different solutions

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Summary:

Alternative electrolytes for chemical polishing the thick layer of scale on steel were notas effective as electrochemical polishing in Ionic liquids;

After descaling steel should be protected from further corrosion by passivation, so westudied alternative solutions for passivation steel;

To control the quality of passive layer on steel we carried out different corrosion tests;

All solutions for passivation showed approximately the same results, some testsshowed that passivation in Solution № 2 gives more stronger oxide;

But also Solution № 1 and Solution № 3 we can easily use for passivation.

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Thank you for attention!