Dentists should be aware of the corrosion properties and biocompatibility of any alloy they use. In the absence of detailed data on corrosion for an alloy, use of high-noble and noble alloys of single-phase microstructure will minimize biologic risk. Patients with Ni allergy may have sensitivity for cobalt. Such patients should be tested to rule out allergy before giving Co-Cr restoration.

1. Base Metal Alloys

2. Contents
 Introduction to metals and alloys.
 History of alloys in dentistry.
 Classification of alloys
 Base metal alloys-
Classification
Composition
Properties
Manipulation
Biocompatibility
Advantages
Disadvantages
Clinical implications.

3. Metals


4. Metals
 A metal is an element that
ionizes positively in solution.
 During ionization, metals release
electrons.
 All the metals have specific
optical, physical, chemical,
thermal & electrical properties &
specific applications in dentistry.

5. Classification of Metals
 Metals used in dentistry are mainly classified as
1. Noble metals
2. Precious Metals
3. Predominantly base metals.

6. Structure of metals
A. Body Centered Cubic
B. Face centered cubic
C. Hexagonal close packed

7. Alloys
 Alloy is defined as a metal containing
two or more elements at least one of
which is a metal & all of which are
mutually soluble in the molten state.
 Use of pure metals is limited in
dentistry.They are commonly used in
the form of alloys to enhance their
properties.

8. History of alloys in dentistry
 At the beginning of 20th century the alloys
were predominantly gold based.
 In 1932 alloys were classified according to
their hardness in 4 types by ADA.
 Type I alloys- for inlays with little or no
occlusal contacts.
 Type II alloys- stronger, strength to tolerate
some occlusal contact such as with onlays.
 Type III alloys- were nearly twice as strong as
type I alloys and were the mainstay for full
crowns and fixed partial dentures.

9. History of alloys in dentistry
 In 1948,to reduce tarnish of alloys in the mouth
palladium based alloys were introduced.
 Alloys for removable partial dentures consisted
of type IV gold The type IV gold alloys contained
enough palladium and copper to provide
strength.
 In 1950s,special metal ceramic alloys were
introduced.
 In 1970s as gold became expensive cobalt-
chromium & nickel-chromium base metal alloys
became popular in dentistry.
 Today there is a wide variety of alloys available
in prosthodontics.

10. Classification of Alloys
1. According to use-
 Alloys for full metal crowns & bridges
 Alloys for metal ceramic restorations.
 Alloys for removable partial dentures.
 Alloys for implants.

11. Classification of Alloys
2. According to strength ( ADA classification No 5)
 Type 1- Soft
 Type 2 - Medium
 Type 3 – Hard
 Type 4 – Extra hard.

12. Classification of Alloys
3. According to nobility- ( ADA specification 1984.)
 High Noble metal alloys- more than 60 wt%
noble metals and gold >40%
 Noble metal alloys –more than 25 wt% noble
metals.
 Predominantly base metal alloys.-less than
25 wt% noble metals

13. Classification of Alloys
4. According to major element constitution
 Gold alloys
 Silver alloys
 Palladium alloys
 Nickel alloys.
 Cobalt alloys
 Titanium alloys.

14. Classification of Alloys
5. According to the no of elements present-
 Binary- Two elements
 Tertiary - Three elements.
 Quaternary- four elements & so on..

15. Classification of Alloys
6. According to the phase system-
 Single phase- composition is
homogenous through out.
Nobel metals.
 Multiple phase – areas of
different compositions.are
visible. Each phase has its
own grain structure.Base
metal alloys.

16. Classification of Alloys
 Eutectic alloys- when the 2 or more metals are not
soluble in the solid state.
 Solid solution- when the metals remain soluble in the
solid state.
 Intermetallic compound- After the solidification
elements react to form a new compound of specific
composition.

17. General Requirements of
casting alloys
1 Tarnish & corrosion resistant.
2 Sufficient strength.
3 Biocompatible- non-toxic & non-allergic.
4 Easy to melt, cast, cut , grind.& finish.
5 Good flow so as to duplicate details
properly.
6 Should have minimal shrinkage on cooling
after casting.
7 Easy to solder.

18. BASE METAL ALLOYS
 Alloys which contain little or no noble
metals are called base metal alloys.
 Classification- On the basis of uses-
 1 For removable partial denture
 2.For fixed partial denture- for full
metal & for metal ceramic systems.
 3.For surgical implants.

19. Classification of Base metal alloys
for full metal restorations
 On the basis of composition-
 Nickel –chromium- with or without berillium
 Nickel-high chromium-high molybdenum alloys
 Cobalt-chromium (high % of cr)
 Cobalt-chromium (low% of cr)
 Titanium alloys.

20. Ni-Cr Alloys
 Nickel Chromium alloys
(Lunorium,Ticonium)
 Composition- Basic elements-
 Nickel- 61 to 81%
 Chromium- 11 to 27% ( >16% high Cr)
 Molybdenum- 2 to 9% (> 6% high
molybdenum)

21. Composition of Ni-Cr alloys
 Minor additions-
 Beryllium- 0.5 to 2.0%
 Iron- 0.1 to 0.5%
 Silicon- 0.2 to 2.8%
 Copper- 0.1 to 1.6%
 Manganese- 0.1 to 3.0 wt%
 Cobalt- 0.4 to 0.5wt%
 Tin- 1.25 wt%

22. Properties of Ni based alloys-
1 Color- White
2 Melting range- 1155 to 1300 0C
3 Density- 7.8 to 8.4 gm/cm3 light in weight
4 Castability- Technique sensitive.
5 Hardness & Workability- ranges from 175 to
360 VHN Much harder than nobel metal
alloys. Difficult to cut, grind, polish. In the
mouth chair side time required for occlusal
adjustments is more. Cutting & removing a
defective crown also becomes difficult .

23. Properties-contd
6 Yeild strength- 310 to 828 Mpa. Stronger than
gold-palladium alloys.
7 Modulus of elasticity- ranges from 150 to 210
Mpa. They twice as stiff as the gold alloys.
So coping thickness can be kept 0.2 to0.3
mm where as gold requires min 0.3 to 0.5
mm
8 Percent elongation- ranges from 10 to 28%
This gives an indication of the ductility

24. Properties
9 Porcelain bonding- These alloys form an adequate
oxide layer which is essential for porcelain bonding.
10 Sag resistance- they are stable at porcelain firing
temperature.. They have a higher sag resistance.
11 Tarnish & corrosion resistance.- Good resistance to
tarnish & corrosion.Due to the property of
passivation
12 Casting shrinkage- they have a higher casting
shrinkage as compared to gold alloys.

25. Cobalt chromium alloys
 Trade name- Vitallium
 Supplied in small pallets.
Applications
 Cast partial removable denture framework.
 Crown and bridge.
 Metal Ceramics

26. Composition of Co-Cr Alloys
 Cobalt- 55to 60 wt%
 Chromium- 25 to 27 wt%
 Nickel- 0 to 20 wt%
 Molybdenum- 0 to 7 wt%
 Iron- 0 to 5 wt%
 Carbon up to 0.5 wt %

27. Properties of cobalt chromium
alloys-
 They have replaced type 4 gold alloys for
making RPDs because of their lower cost &
good mechanical properties.
 Density- 8 to 9 gm/cm3.Half that of gold
alloys
 Fusion temperature- 1250oC to 1480oC
According to fusion temp is classified in to
two groups-
 Type 1-high fusing- liquidus temp is greater
than 1300oC
 Type 2- low fusing- liquidus temperature less
than 1300oC

28. Properties of cobalt chromium
alloys
 Modulus of elasticity- 225 MPa Good
stiffness. Thus casting can be thinner,
thereby, decreasing the weight of rpd.
 Hardness- 432 VHN. Thus, cutting,grinding
& finishing becomes difficult.
 Tarnish & corrosion resistance- Chromium
oxide layer formation prevents tarnish &
corrosion.
 Hydrochlorite or other chlorine
compounds that may be present in the
denture cleansing solutions will cause
corrosion of all chromium based alloys.

29. Manipulation of base metal alloys by
investment casting method.
1.Ethyl silicate or phosphate bonded
investments should be used. Thermal
expansion of these investments
compensates, in part, for the high linear
casting shrinkage of nickel and cobalt
chromium alloys.
2.Adequate spruing and mold venting should be
carried out. Casting accuracy depends upon
sprue length, number, attachment design,
shape, location and direction.

30. Manipulation
3.Burnout temperature range -732c to 927c.
815c being the most commonly used.
4.An induction melting unit provides the most
reliable means for attaining proper melting
and casting temperatures.
5.Excessive temperatures and overheating
should be avoided in order to prevent
porosity in the casting.

31. Manipulation
 After the casting, molds should be set aside
and bench cooled to room temperature before
further handling.
 High speed laboratory engines and special
abrasive discs and stones should be used for
sprue removing and finishing.

32. Biological considerations and
precautions during casting
 Care should be taken to avoid inhalation of
nickel containing dust by wearing a face mask
to avoid lung and nasal neoplastic diseases.
 Work areas must be well equipped with
adequate air-exchange systems and should be
kept free from dust.
 Finishing and polishing procedures should be
carried out properly behind the transparent
barrier.

33. Biological considerations and
precautions during casting
 Beryllium which is present in many
base metal alloys is a potentially toxic
substance and can cause berylliosis
(chemical pneumonitis) due to
inhalation of beryllium containing dust
or fumes. Can give flu like symptoms &
granulomas in the lungs.
 Adequate exhaust and filtration
system should be used.
 Beryllium containing alloys can cause
contact dermatitis. Gloves should be
used to.

34. Biocompatibility for the patients
 Nickel may produce allergic reactions in the
form of localized gingival inflammation.
 It is also a potential carcinogen. 10 to 20 % of
the population is hypersensitive to nickel.
 The release of Ni ions in to the surrounding
tissues due to corrosion is a primary concern
.
 Women demonstrate higher sensitivity to Ni
than men because of sensitization caused by
jewelry containing Ni. ( J of Prosthet D
1986,56,4,507-9)
 Reduced pH can significantly increase Ni
release from the tested alloy.

35. Allergic reactions of metal alloys
Inflammotory allergic
Reaction Following Insertion
of a base metal Fixed Partial
Denture:‘
J Prosthod 1997;6:144-148
Patients with metal allergies
have been shown to have
external skin lesions.The
patient’s arm following a
test with nickel, chrome,
molybdenum, and beryllium
metal alloy.

36. Corrosion
 Corrosion of alloys occurs when elements in the alloy ionize. Thus, the
elements that are initially uncharged inside the alloy lose electrons and
become positively charged ions as they are released into solution.

37. Corrosion
Corrosion can be measured by,
(1) looking at alloy for visible effects of loss of
atoms.
(2) measuring current flow from loss of
electrons, or
(3) measuring released elements themselves.
Most biocompatibility risks of casting alloys are
associated with issue of elemental release.

38. Corrosion
J Prosthet Dent 2002;87:351-63.
Severe corrosion in this
specimen markedly
compromised physical
integrity and unacceptable
surface.
When corrosion is severe,
ions from alloy can be
released in sufficient
quantities to discolor
adjacent tissues

39. Corrosion of base metal alloys
Depends on –
 Phase system- Total mass released from the
multiple-phase alloy is 30times (69 g) more
than from the single-phase alloy (1.9 g).
 Liability-certain elements have an inherently
higher tendency to be released from dental
alloys, regardless of alloy composition.
 Surrounding environment-Reduced Ph
increases corrosion of Ni alloys
 Chromium Content- Less than 20% increases
corrosion

40. Liability
 Copper, nickel, and gallium are liable
elements.
 Cadmium and zinc also have relatively high
liability and will tend to be easily released.
 Gold, palladium, and platinum have low
liabilities and are unlikely to be released at
high levels.
However, elemental liabilities are not
absolute.

41. Current issues on base metal toxicity
 Nickel and cobalt have been measured in
tongue, gingiva and other oral tissues in
patients with removal partial dentures.Levels
are low.
 Systemic presence of elements is not
significant.
 Corrosion in the mouth can cause local
toxicity, allergy and carcinogenesis
 The amount of release from any alloy is
directly proportional to the number of
castings present in the mouth.
 Nickel released from nickel-based prostheses
may approach the 400 g/day

42. Carcinogenicity of base metals
 Mutagenicity describes an alteration of the
base pair sequence of DNA (a mutation).
 Carcinogenicity means that alterations in the
DNA have caused a cell to grow and divide
inappropriately. Carcinogenicity results from
several mutations.
 Cobalt and Nickel possibly carcinogenic
 Cr is not mutagenic but carcinogenic
 Cu mutagenic but not carcinogenic

43. Comparison of Co-cr to Ni-cr alloys
( DCNA-2007,51,3,603-627)
Property Co Cr Ni Cr
Elastic modulus
and strehgth
More Less
Ductility Less More
Corrosion
resistance
More Less
Sensitivity Less More

44. Advantages of base metal alloys
as full metal restorations
1.Lighter in weight
2.Good physical and mechanical properties
3.Good corrosion resistance because of passivating effect.
4. Less expensive.-economical.

45. Disadvantages of base metal alloys as
full metal restorations
1 Technique sensitive.
2 Complexity in production of dental appliance.
Laboratory labor is more costly.
3 High fusing temperatures.
4 Extremely hard. So requires special
equipments for finishing. More chair side time
5 Biocompatibility issues. ( Ni and Be )
6 High hardness can cause wear of opposing
restorations or natural teeth contacting the
restoration.

46. Alloys for metal ceramic system.
 Ability to produce surface oxides.
 High melting temperature than ceramic firing
temperature.
 Compatibility of coefficient of thermal
expansion
 Adequate stiffness and strength of the alloy is
important for fixed bridges and posterior
crowns.
 High sag resistance since copings are thin to
prevent distortion.

47. Base metals for metal ceramic systems.
1. Nickel chromium with or without beryllium
2. Cobalt chromium alloys
 Molybdenum is added to decrease the coefficient of
thermal expansion.
 Beryllium improves castability and provides better
bond between ceramic and metal.
(J Prosthet Dent 1998;80:570-4.).
 Ni-Cr-Be alloys are considered to have superior
properties for metal ceramic systems.
 Co-Cr alloys are used if patients are sensitive to Ni
and biocompatibility issues of beryllium are of
concern.

48. Ceramic metal bonding.
 Bond results due to diffusion between the
surface oxides on the alloy and in the
ceramic. Gives good chemical bonding.
 Surface roughness of the alloy provides
mechanical bonding. Airborne particle
abrasion of the coping enhances bonding.
 Ceramic should wet the metal surface and
fuse without voids to prevent
debonding.(contact angel 60 degrees or less)

49. Bond of base metals with ceramics
(J Prosthet Dent 2005;93:64-9.)
A, Ni-Cr alloy; B, Metal-
porcelain interaction
region; C, Porcelain
A, Co-Cr alloy; B, Metal-
porcelain interaction
region; C, Porcelain
Optical microscope
photograph of the fracture
surface for a representative
metal-ceramic
specimen.

50. Advantages of base metals as metal
ceramic alloys.
 They are economical.
 Thin copings can be produced.
 Have high sag resistance.
 Low density permits more castings per ounce.
 Can be etched.

51. Disadvantages of base metals as metal
ceramic alloys.
 Nickel and beryllium content can have
potentially harmful effects on technicians and
patients.
 Oxide layer if not of optimum thickness can
cause bond failure. (Be controls oxide formation)
 High hardness can cause wear of opposing teeth.
 Proper melting and casting requires skill.

52. Alloys for removable cast partial dentures
 Alloys used for cast partial dentures are-
1 Cobalt Chromium
2 Cobalt chromium nickel ( 19% nickel)
3 Nickel chromium ( Lower carbon level)
4 Titanium.

53. Disadvantages of base metals for
denture frameworks.
 Allergic responses especially to nickel
have been observed.
 But most adverse tissue reactions are
because of improper design or poor
fit.
 Adjustments during insertion are
difficult because of hardness of alloy
 Clasps cast from non ductile base
metals can break in service.
 Hardness of alloy can cause wear of
opposing dentition.

54. Base metal alloys as surgical alloys
 Presently titanium is the most widely used
alloy for implants.
 But chromium containing alloys are also used
in bone plates, screws, intermedullary bars,
splints and metallic obturators.
 Surgical ticonium and vitallium can be used
as surgical alloys.
 But their ionization in the solution leading to
corrosion and post operative pain,swelling,
necrosis make their use limited in surgery
though they are economical.
 Stainless steel alloys are used in making
healing collars, transfer copings and implant
analogs.

55. Clinical Implications

56. Effect of bleaching
Journal of Oral Rehabilitation 2002 29; 1014–1019
 The application of 10% carbamide peroxide
solution in night guard bleaching regime can
cause corrosion of Ni–Cr alloys
 However, if the average application time of
bleaching agents is short it is beneficial and can
reduce elemental release.

57. Effect of bleaching agents
Time behavior of the alloys in 10% carbamide peroxide
solution. (a) Gold base alloys. (b) Base
metal alloys.
Journal of Oral Rehabilitation 2002, 29; 1014–1019

58. Effect of tooth brushing on elemental release
J Prosthod 1999;8:245-257.
 Tooth brushing may produce increased
elemental release because the surface layers
on alloys that inhibit elemental release arc very
thin .
 The Ni-Cr alloy was the most susceptible to
elemental release when toothpastes and acids
were combined with brushing, whereas the
noble and high-noble alloys were more
resistant to such conditions.

59. Effect of Ph on base metal alloys.
Journal of Oral Rehabilitation 2000; 27; 563–575
 Exposure of Ni-based alloys to relevant
levels of reduced pH can significantly
increase Ni release from the tested alloy.
 High-noble and noble alloys were resistant to
this effect and probably release a lower
amount of elements into the body locally
and systemically compared with Ni-based
alloys.

60. The effects of recasting on the
cytotoxicity of base metal alloys.
J Prosthet Dent 2005;93:158-63.
 Recasting increases the element release and
cytotoxicity of base metal alloys.
 The cytotoxicity of the Co-Cr recast alloys is
more than the Ni-Cr alloys.
 The release of Cu remarkably increases after
recasting.

61. Effect of Proteins on base metal alloys
 Proteins can bind to metal ions and transport
them away from the interface
 Proteins can also get absorbed on the surface of
alloy decreasing its passivating effect.
 Proteins can thus decrease corrosion resistance
of alloys
 But the amount of the released elements was well
below the dietary intake levels of these elements.

62. Effect of Preconditioning on
base metal alloys
J Prosthet Dent 1999;81:591-6.
 Preconditioning of casting alloys in a
biologic solution like saline, cell culture
medium, and saline with 3% BSA solution
(bovine serum albumin) decreases
subsequent cytotoxicity.
 Preconditioning casting alloys removed
liable (easily released) elements initially,
which in most cases led to a decrease in
alloy cytotoxicity.
 Conditioning before cementation could
decrease exposure of healing oral tissues
to potentially harmful elements.

63. Effect of soldering on base metals
J Prosthet Dent 2005;94:435-9
 Soldering or brazing may be required
to add proximal contacts, repair
casting voids, and connect
attachments or retainers,repair of
cpd.
 Soldering is a process in which a
molten filler metal wets and fills the
gap between the parent metal
surfaces
 Soldered joints may enhance corrosion
but have significant tensile strength.

64. Effect of soldering on metal ceramic
bond strength.
J Prosthet Dent 2005;94:435-9.
 It is found that solder does not change
porcelain bonding properties.
 But though soldering alloy did not decrease
the metal-ceramic bond strength of porcelain,
fixed partial denture metal frameworks may
be prone to failure when soldered.

65. Effect of welding on corrosion
resistance and tensile strength
J Prosthet Dent 2006;96:273-82
 Welding is a joining process in which parent
metals fuse and form the joint with or
without a filler alloy. Laser welding is
commonly performed.
 Laser-welded joints exhibit excellent
corrosion resistance, but their tensile
strength is limited due to the shallow weld
penetration depth.

66. Etched cast restorations
J Prosthet Dent 2005;93:1-7.
 Electrochemical etching of
Ni-Cr alloys helps in retaining
resin bonded metal fixed
bridges.
 The surface to be etched is
cleaned with an air abrasive
with 50 mm particle size
aluminum oxide.
 The etching occurs when a
current passes through the
acid and turns the etched
alloy surface black .

67. Summary
Base metal
casting
alloys
Removable
partial
denture
Co Cr, Co Cr
Ni,
Ni Cr, Ti.
Fixed
partial denture
Co Cr,
Titanium,
Ni Cr.
With beryllium
Without
Beryllium
Surgical implants
Co Cr molyb,
Ni Cr Co. Ti

68. Conclusion
 Dentists should be aware of the corrosion
properties and biocompatibility of any alloy
they use.
 In the absence of detailed data on corrosion for
an alloy, use of high-noble and noble alloys of
single-phase microstructure will minimize
biologic risk.
 Patients with Ni allergy may have sensitivity for
cobalt. Such patients should be tested to rule
out allergy before giving Co-Cr restoration.

69. References.
 Phillip’s Science of Dental Materials
 Craig’s Restorative Dental materials.
 Dental Materials and their selection- William
O’Brien
 Introduction to metal ceramic technology. Naylor
 Allergic Reaction Following Insertion of a Pd-Cu-Au
Fixed Partial Denture: Paul A. Hamen, ‘J Prosthod
1997;6:144-148
 Accelerated toxicity testing of casting alloys and
reduction of intraoral release of elements Steven
K. Nelson . J Prosthet Dent 1999;81:715-20.
 Dental Materials DCNA-2007,51,3,603-627

70. References
 Shear bond strengths of a ceramic system to
alternative metal alloys Renata Marques J Prosthet
Dent 2005;93:64-9.
 In vitro evaluation of the effect of a current
bleaching agent on the electrochemical corrosion
of dental alloys SENAY CANAY Journal of Oral
Rehabilitation 2002 29; 1014–1019
 Accelerated toxicity testing of casting alloys and
reduction of intraoral release of elements, Steven
K. Nelson J Prosthet Dent 1999;81:715-20.
 The effects of recasting on the cytotoxicity of
base metal alloys Ahmad S. J Prosthet Dent
2005;93:158-63.

71. References
 Effect of welding on corrosion resistance and
tensile stregth. Rok Zupancic J Prosthet Dent
2006;96:273- 82
 Effect of soldering on the metal-ceramic
bond strength of an Ni-Cr base alloy Ioannis
Nikellis J Prosthet Dent 2005;94:435-9.
 Effect of Ph on metal alloys.Journal of Oral
Rehabilitation 2000; 27; 563–575.
 Effect of tooth brushing on elemental
release J Prosthod 7999;8:245-257.
 Recent advances in etched cast restorations
Gerald Barrack,J Prosthet Dent 2005;93:1-7.