NACE CP2 Exam Questions and Answers (Graded A)

oxidation - ANSWER-loss of electrons, positive charge atoms (anode) reduction - ANSWER-gain of electrons, negative charged atoms (cathode) forms of corrosion - ANSWER-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 - ANSWER-Low hydrogen, alkaline, easy polarization Low PH - ANSWER-high hydrogen, acidic, harder to polarize High oxygen/High Hydrogen concentration - ANSWER-harder to polarize, work at the cathode only, called cathodic depolarizers. factors affecting corrosion - ANSWER-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 - ANSWER-a. corrosion current reduces with polarization b. polarization increases with CP current applied c. Occurs at surface of both anode and cathode. shielding - ANSWER-a. shorted casings b. proximity of other structures c. reinforced concrete amphoteric materials - ANSWER-corrode at low and high pH on potential - ANSWER-native + polarization + IR drop polarized potential (off) - ANSWER-native + polarization kirchoff's laws - ANSWER-a. voltage (series circuits) b. current (parallel circuits) faraday's law - ANSWER-W=KIT relates weight loss of metal in a corrosion cell with time and current flow. rate of corrosion - ANSWER-directly proportional to current flow cathodic protection limitations - ANSWER--shielding issues -attenuation -interference -excessive current density -contact with other metals (shorts) measurement methods - ANSWER-true voltage reading voltmeter reading % error calculations effect of voltmeter resistance IR correction methods test stations - ANSWER-allow electrical connection to structure for potential measurements and bonds. Spans allow current and direction (IR drop). inistall during construction pH - ANSWER-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 - ANSWER-- 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 - ANSWER-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 - ANSWER-impressed current- higher current, better control sacrificial- no power required, less maintenance, easy to install, less land AC considerations - ANSWER-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 - ANSWER-CuCuSO4- most common, (soil, fresh water) AgAgCL - seawater Calomel- lab Hydrogen - standard (lab) zinc- stationary Cathodic Protection - ANSWER-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 - ANSWER-confine protection to protected structure thus limiting the current required for protection. use for stray current with care continuity requirement - ANSWER-bonds are necessary for continuity: mechanical fittings (compression, bell & spigot, rebar etc.) Impressed Current Anodes - ANSWER-graphite cast iron (high silicon chromium) mixed metal oxides scrap iron conductive polymer cylindrical, tubular soil..... backfilled in coke breeze current span - ANSWER-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 - ANSWER-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 - ANSWER-higher corrosion penetration rate large anode to small cathode ratio - ANSWER-lower corrosion penetration rate 4 elements of a corrosion cell - ANSWER-Anode Cathode Metallic Patch Electrolyte Faraday's law calculates - ANSWER-metal lost due to current over time the stability of a copper-copper sulfate reference electrode does not vary with - ANSWER-a saturated solution of the copper ions. if a digital meter displays a negative value while measure a voltage across a resistor, it indicates - ANSWER- Which of the following are depolarizers? i.Dissolved oxygen ii. increased electrolyte velocity iii. bacteria action iv. current pickup on the metal - ANSWER-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 - ANSWER-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 - ANSWER-All of the above Which of the following are types of polarization? i. concentration ii. activation iii. temperature iv. velocity - ANSWER-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 d siemans - ANSWER-b. volts the relationship between current and polarization is described by which of the following? i. Corrosion cell current decreases with increased polarization. ii. polarization increases with increased current. iii. current increases with increased polarization iv. polarization decreases with increased current - ANSWER-i and ii only a passivating material will exhibit which of the following properties? i. does not corrode at all ii. corrosion increases as current increases initially iii. corrosion current decreases after a certain point. iv. corrosion current increases beyond the passive region - ANSWER-ii, iii, and iv only In a standard emf series of metals, which of the following is represented? a. the potential of a metal is measured in a saturated solution of copper-sulfate b. current is measured between the most noble and most active metals. c. the potential is measured with respect to a standard hydrogen electrode. d. the total number of electrons is determined. - ANSWER-c. the potential is measured with respect to a standard hydrogen electrode if two different metals are connected together in an electrolyte, the metal with the a. most negative potential is the cathode b. most positive potential is the cathode c. positive potential will corrode d. highest valence is the anode. - ANSWER-b. the most positive potential is the cathode in a practical galvanic series of metals, which of the following is represented? a. the potential between two different metals in the series is measured. b. the current is measured between two of the metals in the series. c. the electrolyte contains metal ions of that metal. d. the potential of metals in a common electrolyte - ANSWER-d. the potential of metals in a common electrolyte the nernst equation - ANSWER-calculates the potential of a metal in solutions of different metal ion concentrations a corrosion cell can form under which of the following conditions? a. a difference in temperature where teh higher temperature is the cathode. b. a difference in oxygen where the highest concentration of oxygen is at the anode. c. a difference in metals where the anode is at the most noble metal. d. a difference in salt concentration where the anode is at the highest ion concentration - ANSWER-d. a difference in salt concentration where the anode is at the highest ion concentration when bare metal passes under a paved road surface, a. an oxygen concentration corrosion cell can develop with the metal under the pavement being the anode b. an oxygen concentration cell can develop with the metal outside the pavement being the anode c. a temperature corrosion cell can develop with the metal outside the pavement being the anode. d. a high salt concentration at the edge of the pavement with the metal under the pavement being the anode. - ANSWER-a. an oxygen concentration corrosion cell can develop with the metal under the pavement being the anode an environment where cathodic protection is NOT effective is a. liquid pherically exposed surface d. steel in concrete - ANSWER-c. atmospherically exposed surface the cathodic protection criteria for steel includes which of the following? I. a potential of -850 mVCSE or more negative with curent applied after IR drop is considered II. a polarized potential of -850mVCSE or more negative III. polarization of 100 mV or greater IV. a polarized potential of -790 mv AgAgCl or more negative - ANSWER-all of the above when cathodic protection is applied to a corroding steel surface, it shifts the corrosion cell potential - a. no change in potential b. cathode more negative c. cathode more positive d. anode more positive - ANSWER-b. cathode more negative cathodic protection polarizes a. the potential of the structure anode to the structure cathode potential b. the potential of the structure cathode to the structure anode potential. c. the potential of the structure cathode to the CP anode potential. d. the cp anode potential to the structure cathode potential. - ANSWER-b. the potential of the structure cathode to the structure anode potential. the current requirement for cathodic protection of a structure will increase with a a. decrease in pH b. decrease in temperature c. decrease in oxygen d. decrease in velocity - ANSWER-a. decrease in pH if lead, copper, steel, and aluminum are connected together in water, which will corrode the most? - ANSWER-aluminum in a galvanic anode system, the driving voltage is due to the a. electrolyte itself to structure couple in the electrolyte ction to the structure d. anode backfill - ANSWER-anode to structure couple in the electrolyte which is NOT a primary reaction at the anode? a. metal oxidation b. oxygen evolution ide evolution d. hydrogen evolution - ANSWER-d. hydrogen evolution which of the following will decrease cathodic protection current requirements? a. increased pH b. increased oxygen c. increased temperature d. increased agitation - ANSWER-a. increased pH a galvanic anode system is NOT normally suitable in which of the following conditions? a. when a small amount of current is required. high soil resisitivity localized protection interference mitigation. - ANSWER-b. in high soil resistivity The purpose of the galvanic anode backfill is NOT to a. prevent passivation of the anode b. create uniform corrosion of the anode c. passivate the anode de a low resistivity environment around the anode. - ANSWER-c. passivate the anode the elements of a galvanic cathodic protection system in soil includes which of the following? i. active material ii. noble structure iii. carbon backfill iv. wiring - ANSWER-I, II, and IV only an impressed current system is NOT normally practical under which of the following conditions a. with very small current requirements b. for protection of poorly coated and bare structures c. in high soil resistivity d. inside water storage tanks - ANSWER-a. with very small current requirements Rectifier components could include which of the following? I. Transformer II. Photovaltaic cells III. thermopile IV. silicon diodes - ANSWER-I and IV only Which of the following DC power supplies are commonly used to charge batteries that in turn provide cathodic protection current?