Final corrosion

Question 1

concentration cell corrosion aka crevice corrosion

how can this be a benefit

Answer 1

a semipermeable film covers a metal surface, causing the region just beneath the film to act as a local anode and adjacent surfaces as cathodes (or .1mm gap)
*rapid local destruction causing pit and then crack

Question 2

pitting corrosion?
similar to?
fast or slow?
what is extra susceptible?

Answer 2

a type of corrosion which occurs on smooth surfaces and its self-initiating

• similar to crevice, but attack is different
• extended iniation period, penetrate rapidly
• chromium-passivate metal are very susceptible even with a smooth surface (smoothly amalgam is good)

Question 3

hydrogen damage

Answer 3

gas accumulates in voids deep within the bulk of a metal and exerts a hydrostatic pressure (hydrogen embrittlement)
*important to keep amalgam dry

Question 4

galvanic corrosion

what acts as cathode?

Answer 4

corrosion caused by direct contact of dissimilar metals in an electrolyte
*gold acts as cathode

Question 5

dry corrosion

Answer 5

corrosion caused by the direct combination of the metallic and non metallic elements

Question 6

how can concentration cell corrosion aka crevice corrosion be a benefit?

Answer 6

with dental amalgam. gap between alloy and cavity wall becomes filled with corrosion products which effectively seals the margins

Question 7

cathodes verses anodes

Answer 7

cathodes= + =adjacent surfaces
anodes= - = pit surface

Question 8

grain boundaries are more or less reactive than grain centers? this causes?
*what is important about stainless steeles

Answer 8

grain boundaries are chemically more reactive

• permits grain boundaries to act as a local ANODE
• C diffuses towards grain boundary when heated thus sensitizing stainless steels, important not to heat and hold at sensitizing temps

Question 9

stress corrosion cracking

Answer 9

caused by simultaneous tensile stess plux specific corrosive medium, accelerated by increasing temps or change in state

Question 10

corrosion fatigue

Answer 10

accelerated by low stress and long time lapses between stress cycles, allow electrolyte penetration

Question 11

types corrosion protection

Answer 11

1) alloy with noble metal- lower rate but expensive and hinder mechanical properties
2) paint surface- would work if paint exists, any hole creates localizes attack
3) plate surface of reactive metal with gold - BAD IDEA bc plate becomes a huge cathode when scratched (scratch is anode)
4) use passiveating metals- COMMON, active metal that form protective film to separate electrolyte and metal

Question 12

types of passivating metals

Answer 12

1) Cr- key to base metals, stainless steel
2) Ta- offers some passiveation in certain stainless steels
3) Al- used as temporary crowns
4) Ti- implants, stable oxide, good biological properties

Question 13

what are 3 factors that affect galvanic raet?

Answer 13

1) environment- a metal with a lesser resistance to a given environment becomes the anodic member (can change)
2) distance- close=increase; far away=decrease corrosion (depend on surrounding electrolytes)
3) surface area- IMPORTANT. can use gold to close endodontic access of base metal crown

Question 14

better to have a large or small anode or cathode?

Answer 14

LARGE anode and small cathode

Question 15

do uniform or non-uniform corrosion occur in the mouth?

Answer 15

non-uniform (typically localized)

**uniform cannot happen in mouth because it must have homogeneous environment

Question 16

conduction pathway

Answer 16

electrolytes carry ions from the anode TO the cathode

Question 17

corrosion verses tarnish

Answer 17

• tarnish= formation of a surface layer which alters appearance of metal
• corrosion= deteriation of metal by reaction with its environment

Question 18

chemical (dry) verse electrochemical (wet) corrosion?

most common in dentistry?

Answer 18

dry= direct combination or metallic and nonmetallic elemetns
wet= requires fluid electrolyte *COMMON indentisty*