practice quizzes Flashcards
(26 cards)
what is corrosion
deterioration of a metal due to chemical reactions, often involving oxidation
what is electrochemical corrosion
involves redox reactions between a metal and an electrolyte (water + salt) where the metal loses electrons (oxidizes) at the anode and gains electrons (reduces) at the cathode
why does iron rust more quickly in salt water than fresh water
salt increases conductivity, speeding up electron transfer and accelerating corrosion
what is passivation
formation of a stable, protective oxide layer on a metals surface that reduced further corrosion
what is a protective oxide layer
thin, adherent oxide that blocks oxygen and moisture, slowing corrosion
what is galvanic corrosion
occurs when two different metals are electrically connected in a corrosive environment, the more reactive metal (anode) corrodes faster
what is crevice corrosion
occurs in tight gaps (under gaskets) where oxygen depletion leads to aggressive localized attack
what is stress corrosion cracking
growth of cracks due to combined stress and corrosion, especially in environments like chlorides and on stainless steel
what is high temperature oxidation
oxidation of metals at elevated temperatures, forming metal oxides. rate depends on temperature, oxygen, and oxide layer properties
what is parabolic oxidation behavior
oxidation rate slows over time because the oxide layer becomes thicker and blocks further oxygen diffusion. it follows parabolic rate law. mass gain squared = k times time
how can galvanizing prevent corrosion
coating iron/steel with zinc. zinc corrodes first (sacrificial anode) protecting the underlying metal
how does cathodic protection work
an external anode (sacrificial metal like zinc) corrodes instead of the protected metal, supplying electrons to prevent oxidation
why are coating like paint used
they form a barrier to water, oxygen, and salts, preventing corrosion initiation
why is aluminum corrosion resistant
it forms a stable thin layer Al2O3 that adheres well and prevents further oxidation
what properties make a metal good at resisting high-temp oxidation
forms a dense, slow-growing oxide. layer must adhere well and not fall off. Al2O3, Cr2O3
what is oxidation? what is reduction
OIL, RIG , electrons
which metal corrodes in a Cu-Zn couple
zinc corrodes because it is less noble (more reactive) than copper
how do temperature and pH affect corrosion
higher temps and acidic pH generally increase corrosion rates
why is design important in corrosion prevention
poor design traps moisture (crevices), mixes incompatible metals (galvanic corrosion) or creates stress (stress corrosion cracking SCC)
if pt markers end up on the surface of the oxide layer after oxidation of Ta, does it indicate inward or outward oxide growth
if the marker is found at the surface of the oxide, it means the oxide grew outward, away from the metal. this happens when metal cations diffuse outward through the growing oxide and react with oxygen at the gas/oxide interface. if it had grown inward, the marker would remain buried withing or at the oxide/metal interface
cobalt oxidation at 800c features
two-phase scale, growth controlled by metal cation diffusion, has metal vacancies and positive hole carriers called p-type oxide. ions diffuse outward to react with oxygen at the surface, so cation diffusion limits the growth rate. parabolic kinetics - oxide thickening slows down over time because diffusion becomes harder through a growing layer
chromium oxidation at 1100c
Cr2O3 becomes unstable, single-phase scale, weight gain decreases
what is the correct equation for standard Gibbs free energy change
change G = change H - T change S
expresses the driving force of a reaction (change G) as the balance between enthalpy change (change H, heat absorbed or released) and the entropy term (T change S, disorder contribution). if change G is less than zero, the reaction is spontaneous
entropy explanation
a negative entropy value shows that the system becomes more ordered, gas to solid, so entropy decreases
