Chapter 2 · 2018-06-12 · 17 Chapter 2 1. The Pourbaix diagram of manganese is given below. Mark each zone with Corrosion, Passivation, or Immunity. 2. Fill the blanks in the following

17

Chapter 2 1. The Pourbaix diagram of manganese is given below. Mark each zone with Corrosion,

Passivation, or Immunity.

2. Fill the blanks in the following description: The pH value can change in tissue that has been injured or infected.

o Normal tissue fluid has a pH of about 7.4 . o In a wound it can be as low as 3.5 . o In an infected wound the pH can increase to 9.0 .

3. Search on internet (Google, Pubmed and Web of Science are recommended) and find the pH values of the following organs of the body:

a. Stomach b. Lung c. Liver d. Small intestine

No standard answers, and it is entirely up to students to find out.

Corrosion

Passivation

Passivation

Passivation

Passivation

Passivation

Immune

Corrosion

Corrosion

18

4. Search on internet (Google, Pubmed and Web of Science are recommended) for the electrode potential of Co or Co-base alloys in sea water. Use this electrode potential as the first approximation to predict the corrosion potential of Co in the body, based on the Pourbaix diagram of Co below. Analyze what could happen if a cobalt prosthesis is exposed to the above three micro anatomic environments in Exercise 1, and cite the reference properly.

Pourbaix diagram of Co metal

Answer In sea water, Ecorr of Co-Cr alloy is -0.25V (SCE) = -0.008V (SHE). Ref:http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&ved=0CB0QFjAA&url=http%3A%2F%2Fstellite.co.uk%2FPortals%2F0%2FStellite%25206%2520Final.pdf&ei=c6cJVLenOM-zuAS4kID4CA&usg=AFQjCNFDqzcIxDd16MXzqTARtYsVwI3eHw&bvm=bv.74649129,d.dGc When pH = 3.5, or 7.4, Co2+ ion is stable, and thus corrosion is possible. When pH = 9.0, Co3O4 is stable and thus passivity is possible.

5. Read the corrosion potential of magnesium on Figure 2.5. The Pourbaix diagram of

magnesium is given below. Analyze the corrosion tendency of this metal in normal body fluid.

19

Answer The corrosion potential of magnesium in sea water is about -1.6. When pH = 3.5, or 7.4, Mg2+ ion is stable, and thus corrosion is possible. When pH = 9.0, Mg(OH)2 is stable and thus passivity is possible.

6. Compare the relative location of the following pairs of metals in the emf series and in the galvanic series for seawater.

(a) Zinc and chromium (use 316 stainless steels for chromium in the galvanic series for seawater)

(b) Platinum and titanium (c) Nickel and silver (d) Titanium and aluminium (use aluminium alloys for aluminium in the galvanic

series for seawater)

What does the relative position of these various pairs of metals tell you about the use of the emf series to predict possible galvanic corrosion in seawater?

Answer

(a) Zinc (-0.762) and Cr (-0.744), the two are close in emf series.

Zinc (-1.0) and Cr (-0.1), the two are distant in the galvanic series.

(b) Pt (+1.18) and Ti (-1.630), the two are distant in emf series.

Pt (+0.2) and Ti (0.0), the two are close in the galvanic series.

(c) Ti (-1.630) and Al (-1.662), the two are very close in emf series.

Ti (0.0) and Al (-0.3), the two are distant the galvanic series.

Hence, the emf series cannot be used reliably to predict the corrosion tendencies of coupled metals in other than standardized environments, such as sea water.

20

7. Avoid surface damage of metallic implants is strongly advised to orthopedists in surgical operations. Explain the reason behind this clinical good practice.

Answer

The corrosion resistance of metallic implants are achieved by passivation. That is, a thin oxide layer forms on the surface, which can protect the metal from anodic dissolution. Hence, surface perfection is important.

8. What long-term toxic effects could be caused by the release of nickel and chromium ions?

Answer

Metal allergy and cancer.

9. Although magnesium is a macro-element in the body, what disease could be introduced by a long-term over-dose of magnesium in the body?

Answer

There is a risk of renal dysfunction with an overdose of magnesium

10. Search on internet and identify at least two trace elements that are not included in Table 2.4. Describe their biological roles in maintaining health, and their toxicity if over dosed.

No standard answer. It is entirely for students to find out.

21

Chapter 3 1. In Table 3.1, the yield strength is Not Applicable (N.A.) for ceramics and skin.

Explain the reasons.

Answer

Yield point is when deformation transits from elastic to plastic.

Ceramics are highly brittle such that they virtually show no plastic deformation before fracture.

Skin, on the other extreme end, is highly elastic, virtually elastic all the way until rupture with little plastic deformation.

2. The tensile engineering stress-strain curve of an alloy is given below. (a) What is the yield strength at a strain off-set of 0.002? (b) What is the ultimate tensile strength? (c) What is the elongation at break?

Answer

(a) 420 MPa

(b) 460 MPa

(c) 3.2%

Courtesy of CRC Press/Taylor & Francis Group

(a)Normal bone Osteoporosis

(b)

Figure 2.1(a) Iron-deficiency anemia; (b) boron-deficiency osteoporosis; (Continued)

002x001a.eps 002x001b.eps


(c) (d)

Figure 2.1 (Continued)(c) thyroid gland enlargement caused by iodine deficiency; and (d) premature hair graying due to lack of copper.

002x001c.eps


Ni

Control: Glass

Ti

PdIn

TaZrSn

104

103

102

100

10–1

SiCrAuAg

Bi

Co

Toxic Mo?

Toxic

VNi

Cu

CoFe

Al Mo

AuAg

PtTa

NbTi

Zr

Vital

304L

316LCo-Cr Alloy

Capsule

V, Cu, ZnCd, Hg

00

0.2

Coe

�ci

ent o

f �br

obla

stic

out

grow

th

Pola

riza

tion

resis

tanc

e (R

/Ωm

)

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.2 0.4 0.6 0.8Relative growth rate of L929 cells

Biocompatibility1.0 1.2 1.4 1.6

Fe

Toxi

c

SrMg

Al

(a) (b)

Figure 2.2(a) Cytotoxicity of some pure metals. (b) The relationship between polarization resistance and biocompatibility of pure metals, cobalt–chromium alloy, and stainless steels.

002x002.eps


Earth

Mining

Extractive metallurgy(casting) Pure metals

Alloys

Billets (or called ingots)

Ores: oxides, sulfides, silicates

Components

End-user products

Alloying metallurgy(casting)

�ermomechanical processingDur

ing

use

Cor

rosio

n

Shaping and forming

Machining, joining, and finishing

Customers

Figure 2.3Processing of metals.

002x003.eps


Magnesium

2+

2+

2+

2+ 2+

2+

2+

2+

2+2+2+

2+

2+

2+

2+

Water

Copper

Figure 2.4Electrical double layer (EDL) around metals in pure water, showing that an electrode potential exits across the metal/solution interface.

002x004.eps


ZincBerylium

Active

Noble

Aluminum alloysCadmium

Mild steel, cast ironLow alloy steel

Austenitic Ni cast ironAluminum bronzeNaval brass, yellow brass, red brass

TinCopperPd-Sn solder (50/50)Admiralty brass, aluminum brassManganese bronzeSilicon bronzeTin bronzes

Nickel silver90-10 Cu-Ni80-20 Cu-Ni

Lead70-30 Cu-NiNi-Al bronze

Ni-Cr alloy 600Silver braze alloys

NickelSilver

Stainless steel Type 304Ni-Cu alloys 400, K-500 (Monel)

Stainless steel Type 316Alloy 20Ni-Fe-Cr alloy 825

Ni-Cr-Mo-Cu-Si alloy BTitaniumNi-Cr-Mo alloy C

PlatinumGraphite

0.5 0.0 –.0.5E in V vs. SCE

–1.0 –1.5

Magnesium

Stainless steel Types 410, 416

Stainless steel Type 430

Figure 2.5The galvanic series in seawater. (Redrawn from LaQue, F.L.: Marine Corrosion, Chapter 6. 1975. Copyright Wiley-VCH Verlag GmbH & Co. KGaA, New York. Reproduced with permission.)

002x005.eps


Oxygen

(b)

(a)

Water

Hydrogen

–2–1.2

–0.8

–0.4

0

Eh/V 0.4

0.8

1.2

1.6

0 4 8pH

12 16

Figure 2.6Pourbaix diagram of water.Two lines:Below line (a)—water is unstable and must decompose to H2Above line (a)—water is stable and any H2 present is oxidized to H+ or H2OAbove line (b)—water is unstable and must oxidize to give O2Below line (b)—water is stable and any dissolved O2 is reduced to H2OThree regions:Upper: H2O electrolyzed anodically to O2Lower: H2O electrolyzed cathodically to H2Middle: H2O stable and will not decompose

002x006.eps


2.0

0

–1.5

–1.0

–0.5

0.0

0.5

1.0

1.5

4 8pH

(a)

(Immunity)

Cu

Cu2OCuO2

–2

(Passivity)CuO (C

orro

sion)

Cu+2

(Corrosion)

a

E in

V v

s. SH

E

b

12 16

2.3

0.0

–1.60 7

(b)pH

Cu stable(immunity)

Passivation

14

Corrosion

Corrosion

Figure 2.7Pourbaix diagram of copper. (a) The Pourbaix diagram of copper superimposed by diagram for water. (b) Three regions: corrosion, passivation, and immunity. (From the WikimediaCommons, http://commons.wikimedia.org/.)In regions where

• Cu2+orCuO22−ionisstable,corrosionispossible

• CopperoxideCu2OorcopperhydroxideCu(OH)2isstable,passivityispossible• Cuisstable,thermodynamicallyimmunetocorrosion

002x007a.eps 002x007b.eps


AgO(Passivity)

AgO–

Ag(Immunity)

a

b

Ag+

(Corrosion)

2.0

1.5

1.0

0.5

0.0

E in

V v

s. SH

E

–0.5

–1.0

–1.5

0 4 8

pH(a)12 16

(Corrosion)

pH(b)0

Fe(Immunity)

a

b

HFeO2–

(corrosion)

Fe3O4

Fe2O3

Fe+2

Fe+3

(cor

rosio

n)

(Pas

sivi

ty)

–1.5

–1.0

E in

V v

s. SH

E

–0.5

0.0

0.5

1.0

1.5

2.0

4 8 12 16

pH(c)

a

b

Ti+2

(Corrosion)

Ti (immunity)

Ti+3

TiO2

Ti2O3

TiO

(Passivity)

2.0

1.0

0.0

–1.0

E in

V v

s. SH

E

–2.0

40 8 12 16pH(d)

(Immunity)

Cu

Cu2O

CuOCu+2

(Corrosion) CuO2–2

(Cor

rosio

n)

(Passivity)

a

bE

in V

vs.

SHE

0

–1.5

–1.0

–0.5

0.0

0.5

1.0

1.5

2.0

4 8 12 16

Figure 2.8Pourbaix diagrams of silver (Ag), iron (Fe), titanium (Ti), and copper (Cu). (a) Ecorr = −0.12 V (SCE) = 0.122 V (SHE). When pH = 3.5, 7.4 or 9.0, Ag is stable. Immunity is possible. (b) Ecorr = −0.466 V (SCE) = −0.224 V (SHE) [7], when pH = 3.5 or 7.4, Fe+2 is stable, Corrosion is possible; when pH = 9.0, Fe2O3 is stable, Passivity is possible. (c) Ecorr = 0 V (SCE) = 0.242 V (SHE), when pH = 3.5, 7.4 or 9.0, TiO2

is stable, Passivity is possible. (d) Ecorr = −0.3 V (SCE) = −0.058 V (SHE), when pH = 3.5 or 7.4, Cu is stable, Immunity is possible. When pH = 9.0, Cu2O is stable, Passivity is possible.

002x008a.eps

Last Lecture

• A biomaterial is a biocompatible material, which is used to replace or assist part of an organ or its tissue, while in intimate contact with living tissue.

• No harm to the host body defines biocompatibility. Biocompatibility is standing in the first place of consideration.

• Four types of Biomaterials: metallic, ceramic, polymeric & composite

Question 1

Part IBiomaterials Science

Chapter 2Toxicity and Corrosion of

Materials

Recommend Reading

Chapter 2 of

Part I BIOMATERIALS Chap 2 Toxicity & Corrosion

Introduction to Corrosion Science, by E

McCafferty. Springer (Excellent Book)

Objectives of the Lecture

• Describe the concept of trace elements and understand their biological roles and toxicities.

• Predict the corrosion tendency of metals in body fluid using galvanic series.

• Describe the corrosive nature of body fluid.

• Be able to read Pourbaix diagrams.

• Predict the possible events when metals are immersed in the body fluid using galvanic series and Pourbaix diagrams.

• Describe the strategies to minimise corrosion/toxicity of metallic implants in the body.


Elements in the Body

Element O C H N Ca P K S Na Cl Mg Trace Elements

Wt.% 65 18.5 9.5 3.3 1.5 1.0 0.4 0.3 0.2 0.2 0.1 <0.01% At.% 26 10 62 1.5 0.5

• A trace element is a chemical element that is needed in extremely low quantities for the proper growth, development, and physiology of the body. A trace element is also referred to as a micronutrient.


Macro-elements in the body


Macro

Elements

Roles

O, C, H, N in water and the molecular structures of proteins

Ca Structure of bone and teeth; role in cell signalling, metabolism,

tissue maintenance

P Structure of bone and teeth. Required for ATP, the energy

carrier.

Mg Important in bone structure.

Na Major electrolyte of blood and extracellular fluid.

K Major electrolyte of blood and intracellular fluid.

Cl Major electrolyte of blood and extracellular and intracellular

fluid.

S Element of the essential amino acids methionine and cysteine.

List of Trace Elements

• Barium • Beryllium • Boron • Cadmium • Caesium • Chromium

• Lithium • Manganese• Molybdenum • Nickel • Selenium

• Strontium • Tungsten • Zinc

• Cobalt • Copper • Iodine • Fluorine• Iron

Many metal elements exist in the body as trace elements.

Question 2


Functions of Some Trace Elements

• Iron: Deficiencies of Iron can cause anaemia.

• Boron: Deficiencies of boron can contribute to osteoporosis.

• Iodine: Deficiencies of iodine can cause a thyroid imbalance.

• Copper: Deficiencies of copper can cause premature hair greying, sterility and premature wrinkling of the skin.


Toxic Examples –Co, Cr & Ni

• Although Co and Ni is an essential element for life in minute amounts, at higher levels of exposure it shows mutagenic and carcinogenic effects.

• In 1966, the addition of cobalt compounds to stabilize beer foam in Canada led to cardiomyopathy, which came to be known as beer drinker's cardiomyopathy.

• After nickel and chromium, cobalt is a major cause of contact dermatitis.

Erin Brockovich


Biocompatibility of Trace Elements

• Most trace elements can be tolerated by the body in minute amounts, but cannot be tolerated in large amounts in the body, although they are essentials in cell function (e.g. Fe) and vitamin B12 (e.g. Co), for instance.


Metallic Structure-Properties

• Metallic valency gives following properties

Conductive (thermally and electrically)

Ductile (non-brittle, safe to be used in structure)

Strong (good combination of strength and ductility)

Corrosion

Hence, corrosion can introduce toxicity, and thus is a major concern!


Considerations in Designing Metallic Implants

• Primarily

Priority choices of elements are those either native in the body (such as Fe, Ti and Cr) or inert (such as Au).

Design corrosion resistant alloys.

• Secondarily

Other desired properties.


Questions 3&4

Corrosion of Metallic Implants

• Tissue fluid in the human body contains water, dissolved oxygen (O), proteins, and various ions such as Na+, Cl- and OH-. As a result, the human body presents an aggressive environment to metals used for implantation.

• Corrosion resistance of a metallic implant material is consequently the most important aspect of its biocompatibility.


Why does corrosion occur?

• The lowest free energy state of many metals in an oxygenated and hydrated environment is that of their oxide, which is the natural state of elements in ores.

Ores


Processing of MetalsEarth

Ores: oxides, sulfides, silicates

Billets (or called ingots)

Mining

End-user productsCustomers

Components

Pure metals

Alloys

Extractive Metallurgy(Casting)

Alloying Metallurgy(Casting)

Machining, Joining and Finishing

Shaping and Forming

Thermomechanical Processing

Du

rin

g u

se

Co

rro

sio

n


How does corrosion occur?

• Corrosion occurs when metal atoms become ionised and go into solution, or combine with oxygen or other species in solution to form a compound that flakes off or dissolves.

• Ionisation is the key step of corrosion mechanism.


How does ionisation occurs?


Electrode Potential E

• The difference in electric potentials across a metal/solution interface is commonly referred to as an electrode potential.

• Theoretically, the electrode potential can be measured as voltage and used to indicate the tendency of electrons to flow away, i.e. the tendency of the metal to be ionised.


Standard Electrode Potentials (Relative Corrosion Tendency of Metals)

• The tendency of elements to corrode can be quantified by the standard electrode potential(also known as standard electrochemical series).

• The lower the potential value, the higher the tendency to corrode.

http://www.csupomona.edu/~seskandari/physiology/physiological_calculators/nernst_potential.html

http://en.wikipedia.org/wiki/Table_of_standard_electrode_potentialshttp://www.engr.ku.edu/~rhale/ae510/corrosion/sld021.htm


Two Standard Electrodes

• The standard hydrogen electrode (SHE) is universally accepted as the primary standard. Under these standard conditions, the electrode potential of hydrogen is arbitrarily defined as Eo = 0.000V.

• The saturated calomel electrode (SCE) is another widely used reference electrode, based on mercury.

• Conversion between SHE and SCEE vs. SHE = E vs. SCE + 0.242


Electromotive Force Series In

cre

asin

g

co

rro

sio

n t

en

den

cy

Decre

asin

g

co

rro

sio

n t

en

den

cy


Limitations of emf Series

• The emf series applies to pure metals in their own ions at unit activity.

• The relative ranking of metals in the emf series is not necessarily the same in other aqueous solutions (such as physiological fluids). Thus, the emf series cannot be used reliably to predict the corrosion tendencies of coupled metals in other environments.

• The emf series applies to pure metals only and not to metallic alloys

• The relative ranking of metals in the emf series gives corrosion tendencies but provides no information on corrosion rates.


Corrosion Potential Ecorr

• Any metal or alloy placed in a corrosive environment has its own electrode potential, called the corrosion potential Ecorr.

• In principle, you should use corrosion potentials to predict corrosion tendency, rather than standard electrode potentials.


Galvanic Series (Seawater)

Note that the electrode potentials in the galvanic series are measured relative to a saturated calomel electrode (SCE), whereas standard electrode potentials are always referred to as the standard hydrogen electrode (SHE). The conversion between the electrode potentials measured against the two reference electrodes is given by E vs. SHE = E vs. SCE + 0.242


Use of Galvanic Series

• Sea water is similar to the body fluid!

• So, the galvanic series for seawater can be used as a first approximation, although data in the body solution itself should be used if available.


Question 5

Galvanic Corrosion

• When two dissimilar metals are immersed in an electrolyte and electrically connected, one metal corrodes preferentially to another, a process called Galvanic corrosion. In Galvanic corrosion, the anodic metal will have a higher corrosion rate in the couple than in the freely corroding (uncoupled) condition. Galvanic corrosion is usually not a desired occurrence. It can be minimized by a number of methods


Minimization of Galvanic Corrosion

• Select combinations of metals as near to each other as possible in the galvanic series.

• Insulate the contact between dissimilar metals whenever possible.

• Apply organic coatings.

• Avoid the unfavourable area effect of having a small anode coupled to a large cathode.

• Install a third metal which is anodic to both metals in the galvanic couple (“sacrificial” anode).


Corrosion Tendency of Metals at Aqueous Environments

• Some metals become covered with passivating film (oxide film, e.g. Al2O3 ), which protects the metal from further attack.

• Tendency to corrode of an element depends on both relative electric potential and pH of the environment.


Pourbaix Diagrams –Corrosion Phase Diagrams


Two lines:Below line {a} – water is unstable and must decompose to H2

Above line {a} – water is stable and any H2 present is oxidised to H+ or H2O Above line {b} – water is unstable and must oxidize to give O2

Below line {b} – water is stable and any dissolved O2 is reduced to H2O

To Read Pourbaix Diagrams

(Immunity)

Oxygen

Hydrogen

Water

Pourbaix diagram is similar to phase diagram. A phase diagram tells you the gas, liquid, solid state of materials when (T, P and X) change. A Pourbaix diagram tells you the ionic state of the material when (Voltage and pH) vary.


pH Values in the Body

• Different parts of the body have different pH values and oxygen concentrations. Consequently, a metal that performs well (is immune or passive) in one part of the body may suffer an unacceptable amount of corrosion in another.

• pH can change dramatically in tissue that has been injured or infected. Normal tissue fluid has a pH of about 7.4.

In a wound it can be as low as 3.5.

In an infected wound the pH can increase to 9.0.


Corrosion Tendency of Ag in body

• Ecorr = -0.12 (SCE) = 0.122 (SHE). When pH = 3.5, 7.4 or 9.0, Ag is stable. Immunity is possible.


• Ecorr = 0.242 (SHE) When pH = 3.5, 7.4 or 9.0, TiO2 is stable. Passivity is possible. Actually, Ti remains passive under physiological conditions. Corrosion currents in normal saline are very slow: 10-8 A/ cm2. Ti implants remain unchanged virtually in appearance. Ti offers superior corrosion resistance but is not as stiff or strong as steel.Hence, a perfect surface is important.


Corrosion Tendency of Ti in body

Minimisation of Corrosion1. Use appropriate metals (native or inert

elements of the body).2. Design alloys to minimise corrosion.

3. Avoid implantation of different types of metal in the same region.

4. In the manufacturing process, provide matched parts from the same batch of the same variant of a given alloy.

5. In surgery, avoid contact between metal tools and the implant, unless special care in taken (to avoid surface damage).


Question 6

Highlights of the Lecture

• Most metal elements exist in the body as trace elements. Trace elements can be tolerated by the body in minute amounts, but cannot be tolerated in large amounts in the body.

• Corrosion Resistance is standing in the first place of consideration in design of metallic biomaterials, in alignment with the requirement on biocompatibility of biomaterials.



• The difference in electric potentials across a metal/solution interface is commonly referred to as an electrode potential, E.

• Conversion between SHE and SCE

E vs. SHE = E vs. SCE + 0.242

• Any metal or alloy placed in a corrosive environment has its own electrode potential, called the corrosion potential, Ecorr.

• The galvanic series is the corrosion potentials in seawater.



• pH can change dramatically in tissue that has been injured or infected. Normal tissue fluid has a pH of about 7.4.

In a wound it can be as low as 3.5.

In an infected wound the pH can increase to 9.0.

• Strategies to minimise corrosion

Corrosion resistant materials

Avoid dissimilar metals

Avoid surface damage


Next Lecture

Mechanical Properties

Documents

Chapter 2 · 2018-06-12 · 17 Chapter 2 1. The Pourbaix diagram of manganese is given below. Mark each zone with Corrosion, Passivation, or Immunity. 2. Fill the blanks in the following