Engineering Materials Week 12 Interacti
Engineering Materials Week 12-Interactivity
17.4 (a) From the galvanic series cite three metals or alloys that may be used to galvanically protect nickel in the active state.
Tin, lead and cast iron may be used.
(b) Sometimes galvanic corrosion is prevented by making an electrical contact between both metals in the couple and a third metal that is anodic to these other two. Using the galvanic series, name one metal that could be used to protect a copper-aluminum galvanic couple.
Zinc may be used.
17.6 Is Equation 17.23 equally valid for uniform corrosion and pitting? Why or why not?
Equation 17.23 is not equally valid for uniform corrosion and pitting. With pitting, the corrosion attack is very localized, and a pit may penetrate the entire thickness of a piece with very little material loss and a very small corrosion penetration rate.
17.8 From a molecular perspective, explain why increasing crosslinking and crystallinity of a polymeric material will enhance its resistance to swelling and dissolution. Would you expect crosslinking or crystallinity to have the greater influence? Justify your choice.
During swelling and dissolution of polymeric materials, the solute molecules diffuse to and occupy positions among the polymer macromolecules. Increasing both the degrees of crosslinking and crystallinity will enhance a polymer's resistance to these types of degradation since there will be a greater degree of intermolecular bonding between adjacent chains. For linear polymers that are highly crystalline, the intermolecular bonds are secondary ones and relatively weak in comparison to the strong covalent bonds associated with the crosslinks.
17.9 List three differences between the corrosion of metals and
(a) the corrosion of ceramics
Ceramic materials are more corrosion resistant than metals in most environments. Corrosion of ceramic materials is normally just a chemical dissolution process. Ceramics are more corrosion resistant at elevated temperatures.
(b) the
17.4 (a) From the galvanic series cite three metals or alloys that may be used to galvanically protect nickel in the active state.
Tin, lead and cast iron may be used.
(b) Sometimes galvanic corrosion is prevented by making an electrical contact between both metals in the couple and a third metal that is anodic to these other two. Using the galvanic series, name one metal that could be used to protect a copper-aluminum galvanic couple.
Zinc may be used.
17.6 Is Equation 17.23 equally valid for uniform corrosion and pitting? Why or why not?
Equation 17.23 is not equally valid for uniform corrosion and pitting. With pitting, the corrosion attack is very localized, and a pit may penetrate the entire thickness of a piece with very little material loss and a very small corrosion penetration rate.
17.8 From a molecular perspective, explain why increasing crosslinking and crystallinity of a polymeric material will enhance its resistance to swelling and dissolution. Would you expect crosslinking or crystallinity to have the greater influence? Justify your choice.
During swelling and dissolution of polymeric materials, the solute molecules diffuse to and occupy positions among the polymer macromolecules. Increasing both the degrees of crosslinking and crystallinity will enhance a polymer's resistance to these types of degradation since there will be a greater degree of intermolecular bonding between adjacent chains. For linear polymers that are highly crystalline, the intermolecular bonds are secondary ones and relatively weak in comparison to the strong covalent bonds associated with the crosslinks.
17.9 List three differences between the corrosion of metals and
(a) the corrosion of ceramics
Ceramic materials are more corrosion resistant than metals in most environments. Corrosion of ceramic materials is normally just a chemical dissolution process. Ceramics are more corrosion resistant at elevated temperatures.
(b) the