BY

Mustafa Jaleel Aziz

Master's student at the University of Kufa

Faculty of Engineering _ Department of Materials

Supervisor :. Asst. Prof. Dr. Ali Sabea Hammood

Corrosion of Titanium alloys and use HA

in coating of implant

Ti

Titanium is a lustrous transition metal with a

silver color, low density and high strength

Pure titanium melts at 1670oC and has a density

of 4.51 g /cm3. It is fairly abundant in nature,

constituting about 1% of Earth’s crust.

It is highly resistant to corrosion in sea water,

aqua regian and chlorine.

Ore

The principal ores of titanium are rutile, which is

98% to 99% TiO2, and ilmenite, which is a

combination of FeO and TiO2

Rutile is preferred as an ore because of its

higher Ti content.

Ti ore

Property

Ti is stiffer and stronger than aluminum

Ti’s coefficient of thermal expansion is relatively low among metals (Alloy Ti-6Al-4V 8.6*10-6(oc)-1)

It retains good strength at elevated temperatures

Pure titanium has excellent resistance to corrosion because it forms a thin adherent oxide coating (TiO2) and is used widely in the chemical industries.

Titanium alloys are considered biocompatible and bioactive.

These properties give rise to two principal

application areas for titanium:-

In the commercially pure state, Ti is used for corrosion

resistant components, such as marine components and

prosthetic implants.

Titanium alloys are used as high- strength components in

temperatures ranging 25oc -550oc, especially where its

excellent strength to weight ratio is exploited. E.g aircraft

and missile components.

Crystal structure

The crystal structure of titanium at ambient temperature

and pressure is close packed hexagonal (α) with a c/a ratio

of 1.587.

TITANIUM ALLOYS

The crystal structure of titanium at ambient temperature and pressure is close-packed hexagonal α phase . At about 890°C, the titanium undergoes an allotropic transformation to a body-centred cubic β phase which remains stable to the melting temperature.

Some alloying elements raise the alpha-to-beta transition temperature (i.e. alpha stabilizers) while others lower the transition temperature (i.e. beta stabilizers).

Aluminium, gallium, germanium, carbon, oxygen and nitrogen are alpha stabilizers.

Molybdenum, vanadium, tantalum, niobium, manganese, iron ,chromium, cobalt, nickel ,copper and silicon are beta stabilizers.

Major alloyed metals are Aluminium ,vanadium , chromium and Molybdenum.

Around 50% of titanium used as Ti- 6Al- 4V.

Figure.1 beta/alpha transformation

according

to Burgers relationship.

Figure.2 Lamellar microstructure of Ti-6Al-4V

(basket-weave).

Figure 3: Optical micrographs of (a) Ti-6Al-7Nb and (b) Ti-6Al-4V alloys.

TITANIUM ALLOYS

Figure 4. XRD spectrum of Ti-6Al-7Nb alloy.

Ti6Al4V alloy is widely used to manufacture implants and its chemical

composition is given in Table 1. The addition of alloying elements to titanium

enables it to have a wide range of properties because aluminium tends to

stabilize the alpha phase and vanadium tends to stabilize the beta phase,

lowering the temperature of the transformation from alpha to beta .

The alpha phase promotes good weldability, excellent strength characteristics

and oxidation resistance. The addition of controlled amounts of vanadium as a

beta -stabilizer causes the higher strength of beta-phase to persist below the

transformation temperature which results in a two-phase system. The -phase

can precipitate by an ageing heat treatment.

Ti- 6Al- 4V

Ti- 6Al- 4V

Table 2. Mechanical properties of Ti CP (ASTM F 67) and Ti6Al4V alloy

(ASTM F 136)

The modulus of elasticity of these materials is about 110 GPa. This is

much lower than stainless steels and Co-base alloys modulus (210

and 240 Gpa) respectively . When compared by specific strength

(strength/density) the titanium alloys exceed any other implant

materials.

Ti- 6Al- 4V

The Ti6Al4V alloy has some disadvantages:

1) Its elastic modulus, although low, is 4 to 6 times that of cortical

bone and has low wear resistance that is a problem in articulations

surfaces.

2) V can cause potential cytotoxicity and adverse tissue reactions.

3) Al ions from the alloy might cause long-term Alzheimer diseases.Briefly, a biocompatible titanium base alloy suitable for bone implant should

meet at least the following requirements :

- Potentially toxic elements, such as vanadium, cooper and tin, should be

avoided completely.

- Elements that may have potential toxicological problems, such as

chromium, nickel and aluminium, should be used only in minimum,

acceptable amounts

- The alloy should have high corrosion resistance.

- The alloy should have, at least, the following desirable mechanical

properties: low modulus, high strength and good smooth and notched

fatigue strength

Ti- 6Al- 4V as implant material in human body

Biocompatibility of a biomaterial is defined as their ability to successfully fulfill a

specific application, with an appropriate response of the host. That is, the

biocompatibility means more than the fact that a material is not harmful in the

body.

A biocompatible material may be considered "inert" if there is no reaction with

tissue and the material is stable for indefinite periods of time

Biomaterials must fulfill the following requirements:

a) corrosion resistance

b) biocompatibility

c) favorable mechanical properties, e.g. Young’s Modulus, similar to that of

the bone, fatigue strength according to the intended application

d) processability (casting, deformation, powder metallurgy, machinability,

welding,

brazing, etc.)

e) availability (low prices)

Corrosion of Ti- 6Al- 4V in human body

The most corrosion resistant biocompatible metallic

biomaterials are the special metals namely-titanium,

niobium, tantalum and their alloys, followed by cobalt based

alloys and finally the stainless steel grades.

Commercially pure titanium and its alloys are known for

their use in medical application owing to their good

corrosion resistance, biocompatibility and bioactivity in the

human body .

The most commonly used implant/ prostheses material

used today has been summarized below in Table 3 with

their common names, UNS, ASTM, ISO and alloy

designations .

Table 3. UNS, ASTM, ISO and alloy designations for titanium base

biomaterials

Types of corrosion in implants

Types of corrosion in implants

Figure 5.. Failure analysis of implants Ti6Al4V and 316L

steel

Types of corrosion in titanium implant

A uniform regular removal of the metal from the surface is usually the most

common mode of corrosion. The corrosive environment in aqueous body fluids

like phosphate buffer saline (PBS), ringer’s lactate (RL), normal saline (NS) etc

may take the mottled form, severely roughened metal surface that resembles

localized attack. This uneven localized attack results from variations in the

corrosion rate of localized surface patches due to localized masking of metal

surfaces by process scales, corrosion products, food lodgment and surrounding

and adjacent superstructures.

When titanium is in the fully passive condition, corrosion rates are typically less

than 0.02 mm/yr (0.8 mil/yr) and well below the 0.13 mm/yr (5 mils/yr) maximum

corrosion rate commonly accepted for biomaterial design and application. This

minimal acceptable corrosion rate is primarily due to the finite +4 oxidation of

titanium alloys owing to the formation of adherent TiO2 film although the surface

oxide is more complex than a single TiO2 oxide over their surface.

General corrosion becomes a concern at high temperatures in highly

acidic environments owing to consumption of hot, spicy and sticky

foods. In strong and/or hot reducing acids (plaque deposits) the oxide

film of titanium can deteriorate and dissolve, and the unprotected metal

is oxidized to the violet colored soluble trivalent ion (Ti3+) in acid

solutions which is further converted to pale yellow Ti4+ ion in presence

of oxidizing species which on further hydrolysis may form insoluble

TiO2 precipitates/ scales and inhibiting subsequent corrosion.

Uniform corrosion for titanium implants can be determined from weight

loss data (increase or decrease in weight depending upon the

environment and by products in accordance to ASTM G1 & G31),

dimensional changes (shape, size, appearance and texture) and

electrochemical methods (anodic and cathodic polarization, cyclic

voltammetry and electrochemical impedance measurements).

Types of corrosion in titanium implant

Corrosion rates in millimeters per year for titanium alloys can be calculated from

weight loss data as under:

Corrosion rate (mm/yr) = kw/dAt

where d is the titanium implant alloy density (in grams per cubic centimeter

which is approximately 4.51g/cc for c.p.Ti), A is the sample surface area (in

square centimeters), t is the exposure time (in hours), and W is the weight

change (in grams).

Corrosion rates in millimeters per year can be calculated from electrochemical

measurements on the other hand using the equation:

Corrosion rate (mm/yr) = 0.129 icorr *EW /d

where icorr is the measured corrosion current density (in milliamps per square

centimeter), d is titanium alloy density (in grams per cubic centimeter), and EW

is the equivalent weight for titanium. The equivalent weight for titanium is

approximately 16 under reducing acid conditions and 12 under oxidizing

conditions depending upon the number of valence electrons involved. The

value of icorr is typically determined from Tafel slope extrapolation or linear

Uniform corrosion (Generalized corrosion)

Figure 6. Open-circuit potential of Ti-6Al-4V alloys after 5 d immersion in artificial

saliva without and with fluorine ion.

Figure 7 Polarization curves of Ti-6Al-4V alloy immersed in artificial saliva with and

without fluorine ion for 5 d.

Table 4 . Parameters of polarization curves of Ti-6Al-4V alloys immersed in solutions

without and with NaF for 5 d.

Figure 8. Surface morphologies of Ti-6Al-4V alloys immersed in solutions without (A)

and with 1000 ppm NaF and (C) magnified view of (B).

Figure 9. Optical micrographs of (a) CP titanium and (b) Ti-15Al (c) Ti-6Al-4V (d) Ti-6Al 4Nb.

The microstructures were obtained on the area used for electrochemical testing in simulated body

fluid solution at 37°C and 7.4 pH

The combination of corrosive fluid (saliva

with several enzymes and food particles)

and high velocity in the oral environment

results in erosion-corrosion or fretting. It is

responsible for most of the metal release in

tissue. Conjoint action of chemical

(enzymes and proteins) and mechanical

wear (mastication) during function further

aggravates the attack].In general during the

passive

environments, the hard and tenacious TiO2

surface film over the metal surface provides

a superb barrier to erosion-corrosion. For

this reason titanium alloys can withstand

flowing water velocity as high as 30 m/s

with little or no metal loss. The ability of the

oxide film to repair

itself when damaged and the intrinsic

hardness of titanium alloys both contribute

to their excellent resistance to erosion-

Fretting Corrosion / erosion corrosion

The typically low concentrations of organic material in oral cavity is of little

importance but continuous exposures to local changes around the implant during

function can lead to finite removal of the metal as well as the cementing material

between the implant and superstructure there by not only promoting erosion

corrosion but crevice and galvanic corrosion as well.

Fretting Corrosion / erosion corrosion

Titanium alloys exhibit relatively high resistance to fluids containing suspended

solids. Critical velocities for excessive metal removal depend upon the

concentration, shape, size, hardness of the suspended particles, fluid

impingement angle, local turbulence, and titanium alloy properties.

Figure10. SEM micrographs of circular wear marks on Ti-6Al-4V sample after the

friction test(severe deformation and plastic flow). Arrows indicate the sliding direction.

HYDROXYAPATITE AND THEIR USE

AS COATING MATERIAL IN

IMPLANTS

Uses include bone graft substitution and coatings on metallic

implants

The most important bioceramic materials for its unique

bioactivity and stability

strong chemical bonds with surrounding bone

Unlike the other calcium phosphates, HAp does not break

down under physiological conditions stable at physiological pH

Why HAp?

A) Titanium implant B) Titanium implant coated with HAp

PHYSICAL PROPERTIES OF

HYDROXYAPATITE MATERIALS

The hydroxyapatite is known to be very brittle ,like most ceramics.

The color of apatite depends on the type of apatite,but is usually

green,gray,red,brown,blue,violet or colorless.

The mineral may be transparent or opaque.

Most apatite is fluorescent in UV RAY.

Compressive strength is dependent on density and porosity.

HA is reported to be similar that of the human tooth enamel.

Degrades in pH 2.0 solution.

Applications of CaP in dentistry

1) Replacement for bony and periodontal defects & alveolar ridge

2) Tissue engineering systems

3) Bioactive coating on metallic osseous implants

4) Filler for reinforcing dental resins

5) Repair of mechanical bifurcation perforation

6) Apical barrier formation

7) Pulp capping

Advantages of nanosized HAp

1. Nanosized HAp has higher surface area and surface roughness

resulting in superior surface functional properties of nanosized

HAp compared to its microphase counterpart.

2. Mimic the bone mineral in composition and structure.

3. Promote osteointegration and subsequent bone tissue formation.

4. The best material to use for bone replacement and regeneration.

5. Enhanced resorbability and much higher bioactivity than

micron-sized ceramics.

6. Capability of decreasing apoptotic cell death and hence

improving cell proliferation and cellular activity related to bone

growth.

7. Improved cell proliferation and differentiation.

8. Better cell adhesion and cell-matrix interactions.

How to synthesis of Hap?

Solid-state synthesis of HAp

Mechanochemical method

Conventional chemical precipitation method

Sol-gel method

gelation

Combustion method

Synthes

is of

HAp

from

biogeni

c

sources

Fish bone is a form of waste generated from the fish crackers processing

industries that contain the highest content of calcium. In terms of food and

nutrition, fish bones are rich in calcium, phosphorus and carbonate

needed by human. Involvement of the community to use leftover fish bone

to produce hydroxyapatite is a way to improve society and reduce

pollution. In addition, hydroxyapatite can also be used as a bone

replacement implants, heart valves, hip extension and other implants in

the human body

Fish bone as HA

• Fish bones contain the highest amount of calcium.

• The main components of fish bones are Calcium, Phosphorous and

Carbonates; these play a crucial role in fulfilling the nutritional calcium

requirement of humans

• Approximately 80% of fishermen’s wives have no steady income. Thus,

it is important that an alternative means of income be created for them.

• The implementation of this project will involve the local community, and

will help raise their economic standing via the production of calcium-

filled chocolate from waste materials i.e. fish bones.

PRODUCT BENEFITS

Fish bone as source of HA

Figure11. Field emission-scanning microscopy images of nHA in fish bone (salmon) at

different magnifications. (A) 500; (B,C) 1000 and (D) 2500

Figure 12. High Resolution Transmission Electron Microscopy (HR-TEM) micrographs

demonstrating the appearance of the obtained nHA crystals at different scale bars: (A) 200

nm; (B) 100 nm and (C) 50 nm from fish bones after alkali treatment; (D) The corresponding

selective area diffraction data of nHA.

Figure 13 a) SEM image of the cross section of an eggshell. b) Unit cell of

CaCO3

Eggshell source of HA

a b

Eggshell source of HA

Figure 14. Microwave conversion of eggshells into a flower-like hydroxyapatite

nanostructure.

Figure 15 . Scanning Electron Micrograph of a porous sample, made from HAp

powders.

COMPARISON BETWEEN HUMAN TOOTH

ENAMEL AND STOICHOMETRIC

HYDROXYAPATITE

CONSTITUENT HUMAN TOOTH

ENAMEL(w/t%)

Ca 36.40

P 17.80

OH -----

CO2 2.05

H2O <4.00

ORGANIC 0.39

MOLAR RATIO

Ca/P 1.58

CONSTITUENT STOICHIOMET

RIC HA(w/t%)

Ca 39.90

P 18.50

OH 3.38

CO2 ----

H2O ----

ORGANIC ----

MOLAR RATIO

Ca/P 1.67

HYDROXYAPATITE COATING METHODOLOGY

The Hydroxyapatite is coated by PLASMA SPRAYING

PROCESS.

PLASMA SPRAYING PROCESS

1.The gas stream (90%ARGON,10%HYDROGEN) carry the

hydroxyapatite powder.

2.The gas stream passes through an electrical plasma produced by a

low voltage, high current electrical discharge.

3.Then the semi molten hydroxyapatite powder are sprayed onto to

the titanium substrate.

4.Where they solidify.

HYDROXYAPATITE COATING

METHODOLOGY

HYDROXYAPATITE COATED

IMPLANT

ADVANTAGES OF HYDROXYAPATITE

COATING

Hydroxyapatite coatings are considered biocompatible

because this compound found naturally in bone.

HAp does not break under physiological conditions

Hydroxyapatite coating by plasma spraying process in

significantly low cost.

In fact, it is thermodynamically stable at physiological pH

and actively takes part in bone bonding. This property has

been exploited for rapid bone repair after major trauma or

surgery.

CLINICAL SUCCESS OF HYDROXYAPATITE

COATED IMPLANT

A successful implant is-----

1.Clinically immobile.

2.Absence of symptoms such as pain , infection , neuropathy ,

violation of mandibular canal during restorative function.

CASE OF SUCCESSFUL HYDROXYAPATITE COATED

IMPLANTS

FAILURE CASE OF HYDROXYAPATITE

COATED IMPLANTS

References

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hydroxyapatite with diverse structures. Acta Biomaterialia 2013; 9: 7591–621.

2. Anitha P, Pandya HM. Comprehensive review of preparation methodologies of nano

hydroxyapatite. Journal of Environmental Nanotechnology 2014; 3: 101–21.

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Solution 6 (2011) 5499-5509.

4- Rahul.B, Shaily M. B, Brajendra M and David L. O. Corrosion in Titanium Dental

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2003 ; 6-7.

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Corrosion of Ti-6Al-4V Alloy in Artificial Saliva 2015 ;10 ; 7453-7464.

References

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Body Fluid Environment Vol 18 (2), January 2005 .

8- Mamoun . F, Mohamed .L, Omar .A, Leila .D, Ahlem .T, Hadda .R . Tribological behavior of

Ti-6Al-4V and Ti-6Al-7Nb Alloys for Total Hip Prosthesis. vol 2014 .

9- Idris .A, Hamzat I.T, Bashir A. M, Haruna .S, Hindatu .Y, Mohammed N .J , and Sulaiman .M.

From Garbage to Biomaterials: An Overview on Egg Shell Based Hydroxyapatite. Vol 2014

10- Jayachandran. V, Baboucarr. L, Panchanathan . M, Kyong-Hwa .K, Elna P. C ,

Sukumaran .A, Dong . G. K, and Se-Kwon Kim. Isolation and Characterization of Nano-

Hydroxyapatite from Salmon Fish Bone. 2015 (8) 5426- 5439.