NACE CP2 questions well answered latest 2023

NACE CP2 questions well answered latest 2023oxidation loss of electrons, positive charge atoms (anode) reduction gain of electrons, negative charged atoms (cathode) forms of corrosion uniform, crevice, galvanic (dissimilar metals) if two metals have to be couple then best to couple metals close together in the galvanic series, pitting, intergranular, selective leaching (dezincification, graphitization), velocity phenomena, environmental cracking (corrosion fatigue, hydrogen embrittlement, SCC (Carbonates/bicarbonates) High PH Low hydrogen, alkaline, easy polarization Low PH high hydrogen, acidic, harder to polarize High oxygen/High Hydrogen concentration harder to polarize, work at the cathode only, called cathodic depolarizers. factors affecting corrosion A. anything that affects polarization. (temp. etc...) B. concentration cells (oxygen, temp, etc.) In an oxygen concentration cell, what is the anode? (i.e. the area with more or less oxygen?) In a metal ion cell, which area is the anode (i.e. the area with the greater of lower concentration)? C. Anaerobic bacteria (MIC) Absence of oxygen, depolarizer polarization a. corrosion current reduces with polarization b. polarization increases with CP current applied c. Occurs at surface of both anode and cathode. shielding a. shorted casings b. proximity of other structures c. reinforced concrete amphoteric materials corrode at low and high pH on potential native + polarization + IR drop polarized potential (off) native + polarization kirchoff's laws a. voltage (series circuits) b. current (parallel circuits) faraday's law W=KIT relates weight loss of metal in a corrosion cell with time and current flow. rate of corrosion directly proportional to current flow cathodic protection limitations -shielding issues -attenuation -interference -excessive current density -contact with other metals (shorts) measurement methods true voltage reading voltmeter reading % error calculations effect of voltmeter resistance IR correction methods test stations allow electrical connection to structure for potential measurements and bonds. Spans allow current and direction (IR drop). inistall during construction pH measure of acidity(hydrogen ion concentration) high pH is alkaline low pH is acid pH is a log function therefore each increase in pH is a factor of 10. RECTIFIER - positive to anode -negative to structure - AC suppy -AC circuit breaker -Transformer (increase or decrease voltage) -AC fuse -rectifier bridge - diodes change AC to DC -Voltmeter and Ammeter -Surge Protection -Filters Esecondary = (Eprimary x Secondary turns) / primary turns Reference electrode location the reference electrode should be placed as close to the structure as practical. (underground piping or tanks- over the center of the structure, multiple readings along the structure as required) internal surfaces of storage tanks, waterfront and offshore structures (as close to the wall as possible) impressed current vs sacrificial impressed current- higher current, better control sacrificial- no power required, less maintenance, easy to install, less land AC considerations capacitance, inductance, and resistance hazardous AC potential - 15 Vac Safe method of connecting ground0 cable to ground then to pipe arcing hazards corrosion rates from AC (current density) switching or pulse rectifiers Reference cells CuCuSO4- most common, (soil, fresh water) AgAgCL - seawater Calomel- lab Hydrogen - standard (lab) zinc- stationary Cathodic Protection protect the cathode attempts to bring the cathode potential as negative as the anode potential NACE criteria- what are the exceptions? -high temperature -bacteria -sulfides -acid environments -dissimilar metals Isolation Requirement confine protection to protected structure thus limiting the current required for protection. use for stray current with care continuity requirement bonds are necessary for continuity: mechanical fittings (compression, bell & spigot, rebar etc.)