ENGR 220 Final Exam Questions with Verified Solutions Rated A+ (Already Passed)

ENGR 220 Final Exam Questions with Verified Solutions Rated A+ (Already Passed) deterioration mechanism - metals - Answers loss of material via dissolution (corrosion), formation of oxide scales/films which may flake off (oxidation) deterioration mechanism - ceramics - Answers relatively deterioration resistant bc already oxidized, issues mainly at high temps and very extreme environments deterioration mechanism - polymers - Answers degradation (dissolve in some solvents...absorb and swell, UV and other radiation, heat, etc. can break covalent bonds what is corrosion? - Answers destructive, electrochemical attack of a material what is corrosion mechanism? - Answers actual atomic molecular or ionic transport process occurring at the interface between material and its environment what is metallic corrosion? - Answers electrochemical process - reaction involving transfer of electron from one material to another what is oxidation? - Answers metals lose or give up one or more valence electrons what is reduction? - Answers metals give up or gain one or more valance electrons why are ceramics immune to environmental degradation? - Answers many common ceramics are stable oxides delta v must be _______ for reaction to proceed spontaneously in direction assumed - Answers positive if delta v is negative..... - Answers negative arrow must be reversed what is the galvanic series? - Answers ranks relative reactivity of various metals/alloys in seawater --- practical series developed to rank relative nobility of coupled metals in specific environments at specific temps what happens when material passivates? - Answers formulation of highly adherent, very thin, protective oxide film on metal surface examples of inert metals - Answers Cr, Fe, Ni, Al, and many of their alloys pilling-bedford ratio - Answers determines level of protection pilling-bedford ratio (PBR) if less than 1 - Answers film = porous, less than 100% coverage, unprotective pilling-bedford ratio (PBR) if greater than 1 - Answers compressive internal stresses; 2-3 may crack and spall pilling-bedford ratio (PBR) if equal to 1 - Answers ideal for file being productive generalized attack/uniform corrosion assumptions - Answers - electrochemical corrosion is the only deterioration mechanism - anodic + cathodic reactions take place all over electrode surface with location of reactions changing places on the surface - no significant macroscopic differences in electrolyte concentration over the metal surface - metal fairly homogenous uniform attack corrosion - Answers oxidation and reduction reactions occur uniformly over surfaces (general "rusting") galvanic corrosion - Answers dissimilar metals physically joined, in the couple (Galvanic couple) more anodic one corrodes - ex. Zn, Mg very anodic crevice corrosion - Answers by geometry develops different concentration of dissolved species between 2 pieces of material (rivet hole, gasket) pitting corrosion - Answers gravity driven, downward propagation of small pits and holes selective leaching corrosion - Answers preferential corrosion of solid solutions - 1 element or constituent - depending on nature/environment exposed to 1 or more elements can be dissolved) process zinc goes through to be dissolved - Answers dezincification erosion corrosion - Answers mechanical abrasion of passivating layer and chemical attack (at pipe elbows/bends) stress corrosion - Answers corrosion at crack tips when a tensile stress is present intergranular corrosion - Answers corrosion along grain boundaries, often where special phases exist general attack/uniform corrosion - Answers - uniform over a surface, leading to relatively even thickness reduction - results in greater material mass loss bc occurs over entire surface - easily distinguishable as long as component is visible - effects can be predicted by taking observations overtime - internal corrosion makes observation more challenging for metallic corrosion to occur, which conditions must be satisfied? - Answers - at least one oxidation half reaction - at least one reduction half reaction most common type of localized corrosion? - Answers pitting pitting takes name from? - Answers - appearance of attacked surface - small volumes of metal removed by corrosion leading to formation of craters or "pits" which may go through entire thickness of metal when/where does pitting occur? - Answers on passivated metals and alloys in environments containing chloride, bromide, iodide, perchlorate ions why can pitting be dangerous? - Answers penetration occurs without clear warning bc pits very narrow or could be covered + pit growth difficult to predict localized corrosion: crevice - Answers - "pit" geometry created artificially by design - occurs as consequence of concentration differences of ions/dissolved gases in electrolyte solution + between 2 regions of same metal piece - occurs in cracks, crevices, recesses formed between mating surfaces of metal assemblies - can occur under dirt, scale and surface deposits or under loose-fitting washers/gaskets - stagnant solution with localized depletion of dissolved O2 (wide enough gap for liquid to penetrate but narrow enough for crevice solution to be stagnant) localized corrosion: galvanic - Answers - metallic contact occurs between more noble metal and less noble one, corrosion rate will decrease on more noble material + increase on latter - requires electrolytic connection between metals for this to occur - anodic (oxidation) - reaction centered on less noble - cathodic (reduction) - reaction centered on more noble galvanic series diagram (higher vs lower) - Answers open rectangles = low velocity/poorly aerated - corrosion of metal higher on list will be accelerated - corrosion of metal lower on list will be reduced - grouped metals/those with close potentials will have minimal galvanic interaction