Corrosion & Degradation of Materials

1. Define corrosion as it pertains to materials.

Solution;

Corrosion is the deterioration of a material resulting from chemical attack by its environment.

2. What are some of the factors that affect the corrosion of metals?

Solution;

Some of the factors that affect the corrosion of metals are: the temperature and concentration of the reactants and products of a chemical attack; the state of mechanical stress in the metal; the presence of erosion in the metal; and the energy state of the metal.

3. A standard galvanic cell has electrodes of zinc and tin. Which electrode is the anode? Which electrode corrodes? What is the emf of the cell?

Solution;

Zinc has a more negative Eo potential of –0.763, as compared to tin’s potential of –0.136, and will thus serve as the anode. The zinc corrodes in an oxidation reaction.

The emf of the cell is obtained by adding the half-cell reactions together:

Anode reaction: Zn → Zn²⁺ + 2e^- E^o = - 0.763 V

Cathode reaction: Sn²⁺ + 2e^- → Sn E^o = - 0.136 V

_________________

E^o_cell = - 0.627 V

4. Consider a magnesium-iron galvanic cell consisting of a magnesium electrode in a solution of 1 M MgSO4 and an iron electrode in a solution of 1 M FeSO4. Each electrode and its electrolyte are separated by a porous wall, and the whole cell is at 25°C. Both electrodes are connected with a copper wire.

i) Which electrode is the anode?

ii) Which electrode corrodes?

iii) In which direction will the electrons flow?

iv) In which direction will the anions in the solution move?

v) In which direction will the cations in the solution move?

vi) Write an equation for the half-cell reaction at the anode.

vii) Write an equation for the half-cell reaction at the cathode.

Solution:

i) The magnesium electrode is the anode.

ii) The magnesium electrode corrodes since the anode in a galvanic cell corrodes (oxidizes).

iii) The electrons will flow from the anode, Mg, to the cathode, Fe.

iv) The SO_4- anions will flow toward the magnesium anode.

v) The cations will flow toward the iron cathode.

vi) The oxidation reaction that occurs at the magnesium anode is: Mg ® Mg²⁺ + 2e⁻

vii) The reduction reaction that occurs at the iron anode is: Fe²⁺ + 2e⁻ ® Fe

5. Describe two methods by which cathodic protection can be used to protect steel pipe from corroding.

Solution:

Cathodic protection of steel pipe can be achieved by (i) an external DC power supply, or by (ii) galvanic coupling with a metal, such as magnesium, that is more anodic than the steel.

Cathodic protection of an underground tank by using impressed currents

Cathodic protection of an underground pipeline by using sacrificial Mg anode

Fundamentals of Metal, Steel, Alloy & Biometal

1. What are the three (3) basic types of stainless steels?

Solution

The three (3) basic types of stainless steels are ferritic, martensitic, austenitic.

2. What are some applications for ferritic and martensitic stainless steels?

Solution

Ferritic stainless steels are used mainly as general construction materials requiring good corrosion and/or heat resistance. Examples include range hoods, restaurant equipment and combustion chambers. Martensitic stainless steels have comparatively low corrosion resistance but high strength and are thus used in applications such as machine parts, pump shafts, bearings and bearing parts, cutlery, and surgical tools.

3. What are the cast irons? What is their basic range of composition?

Solution

Cast irons are a family of ferrous alloys intended to be cast into a desired shape rather than worked in the solid state. These alloys typically contain 2 to 4 percent C and 1 to 3 percent Si. Additional alloying elements may also be present to control or vary specific properties.

4. What are some of the properties of cast irons that make them important engineering materials? What are some of their applications?

Solution

Cast irons are easily melted and highly fluid and do not form undesirable surface films or shrink excessively; consequently, they make excellent casting irons. They also possess a wide range of strength and hardness values and can be alloyed to produce superior wear, abrasion, and wear resistance. In general, they are easy to machine. Their applications include engine cylinder blocks and gear boxes, connecting rods, valve and pump casings, gears, rollers, and pinions.

5. Why are ductile cast irons in general more ductile than gray cast irons?

Solution

Ductile cast irons are, in general, more ductile than gray cast irons because their spherical graphite nodules are surrounded by relatively ductile matrix regions which allow significant deformation without fracture. In contrast, the gray cast irons consist of an interlacing network of graphite flakes which can be fractured easily.

6. Why does the graphite form spherical nodules in ductile cast irons instead of graphite flakes as in gray cast irons?

Solution

Graphite forms spherical nodules in ductile cast irons because the levels of phosphorus and sulfur are reduced significantly compared to those in gray cast irons; these two alloying elements prevent the formation of nodules and thus promote the formation of graphite flakes.

7. How Steel & Cast Iron Differ?

Solution

Steel	Cast Iron
§ Iron with C still in solution	§ Iron which some of the C has precipitate out & appears as flakes
§ C content; 1.6 ~ 2.0%	§ C content; 2.0 ~ 6.0%C
§ Ductile compare to C. iron	§ Brittle compare to steel
§ High strength	§ Poor Strength
§ Hard to machine	§ Easy to machine
§ Hard to control casting	§ Easy to cast
§ Low damping capacity	§ Good Damping Capacity

8. What are some of the important properties of unalloyed copper that make it an important industrial metal?

Solution

Properties of unalloyed copper, which are important to industrial applications, include high thermal and electrical conductivity, good corrosion resistance, ease of fabrication, medium tensile strength, controllable annealing properties, and general soldering and joining characteristics.

9. What are some of the properties that make aluminum an extremely useful engineering material?

Solution

Aluminum is an extremely useful engineering material due to its low density (2.70 g/cm3), good corrosion resistance, good strength when alloyed, high thermal & electrical conductivities and low cost.

10. What are some of the properties that make aluminum to be high prospect for transportation materials?

Solution:

To be used as engineering materials for transportation to reduce fuel assumption because its specific strength, which is quantified by the tensile strength-specific gravity ratio. Its TS is inferior to a more dense material (such as steel), on weight basis it will able to sustain a larger load.

11. What advantages do magnesium alloys have as engineering materials?

Solution:

As engineering materials, the primary advantage of magnesium alloys is their lightness; magnesium has the low density value of 1.74 g/cm3.

12. Why are titanium and its alloys of special engineering importance for aerospace applications?

Solution:

Titanium and its alloys are of special engineering importance for aerospace applications because of their high strength-to-weight ratios.

13. Define a refractory metals. Name the metal elements that are considered to be refractory elements?

Solution:

Refractory metals are metals with exceptionally high melting points; above 2450^oC for example; Niobium (columbium) (Nb), Tantalum (Ta), Molybdenum (Mo) and Tungsten (W)

14. Define a biometals. Name the metal/ alloys that are considered to be biometals and give examples.

Solution:

Biometals is a metal alloys that replace damaged biological tissues, restore function, and are constantly or intermittently in contact with body fluids.

Biometals Group	Example
Stainless Steel (SS)	§ 316 L SS
Cobalt-based alloys	§ Co-28 Cr-6 Mo cast alloy § Co-20 Cr-15 W-10 Ni wrought alloy § Co-35 Ni-20 Cr-10 Mo wrought alloy § Co-28 Cr-6 Mo cast alloy
Ti alloys	§ Pure Ti § α Ti alloy (Ti-Al, Sn, Zr) § α-β Ti alloy (Ti-6 Al-4 V, Ti-6 Al-7 Nb

15. Metals/ alloys used as surgical instruments are not classified as a biometals. Briefly discuss this issue.

Solution:

Because metals/ alloys used as surgical instruments are used in external prosthesis. In addition this instrument is not exposed to body fluids in continuous or intermittent manner

16. A striker from Liverpool FC team was badly injured resulted by hard tackling from Manchester United FC player. After getting some x-ray scanning by orthopaedic medical officer, it was found that there are few knee bone fracture and crack. The footballer has to take 9 month rest for a surgical operation on his knee to replace a damaging bone in order to restore it function back to normal. As a biomedical engineer you are responded to make some recommendations on the suitable materials for used as bone joint and fixation devices.

i) Taking into consideration the function of knee in a footballer profession, analyse the factors that may affect on the biometal performance.

ii) Recommend the suitable materials that would satisfy your requirements.

Solution:

i) Considering knee function for a footballer that may experience million cycles of stress on his knee bone in daily activity. Thus, the metal/ alloys should be able to endure large & variable (cyclic) stress in the highly corrosive environment of the human body.

The internal knee environment is highly corrosive and could degrade the implant materials, resulting in the release of harmful ions or molecules. Thus the characteristic of a biometal is biocompatibility. Thus the additional characteristic for knee implant is chemical stability, corrosion resistance, non carcinogenic & nontoxic when used in the human body.

ii) Biometal that may satisfy the requirement must be strong, fatigue & wear resistance and biocompatibility in a highly corrosive environment such as;

a) Stainless steel

b) Cobalt based alloys

c) Titanium alloys