Summary NACE CP2 Study Guide Complete Solutions Verified

Summary NACE CP2 Study Guide Complete Solutions Verified oxidation 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.) Impressed Current Anodes graphite cast iron (high silicon chromium) mixed metal oxides scrap iron conductive polymer cylindrical, tubular soil..... backfilled in coke breeze current span current in a pipe can be measured by taking a voltage drop across a known length and either calibrating the section in question or using resistance from tables. unknown pipe size must be calibrated by using I/ deltamV or R= V/I using 4 wires Sacrificial anode types magnesium zinc cylindrical, bullet, ribbon, bracelet soil backfilled in gypsum, bentonite mixture to reduce contact resistance to soil and to retain moisture make sure to remove plastic shipping bag small anode to large cathode ratio higher corrosion penetration rate large anode to small cathode ratio lower corrosion penetration rate 4 elements of a corrosion cell Anode Cathode Metallic Patch Electrolyte Faraday's law calculates metal lost due to current over time the stability of a copper-copper sulfate reference electrode does not vary with a saturated solution of the copper ions. if a digital meter displays a negative value while measure a voltage across a resistor, it indicates Which of the following are depolarizers? i.Dissolved oxygen ii. increased electrolyte velocity iii. bacteria action iv. current pickup on the metal i, ii, iii only polarization occurs a. at the anode surface only b. at the cathode surface only c. at the anode and cathode surfaces d. in the metallic path C. at the anode and cathode surfaces Which of the following factors affect polarization? a. current b. ion concentration c. anode/cathode ratio d. temperature All of the above Which of the following are types of polarization? i. concentration ii. activation iii. temperature iv. velocity i and ii The measure of the electrochemical difference between two metals in an electrolyte is which of the following? a. amps b. volts c. ohms