2Corrosion NACE defines Corrosion: Deterioration of a substance (usually a metal) or its properties because of a reaction with it’s environment.
3Pipeline CorrosionPrimarily concerned with the destruction of the metal by either chemical or electrochemical reaction with a given environment.
4Electrochemical Reactions Most metals are found as “ores” which are metallic oxides or salts.The ores are converted into the metals by refining, which involves adding energy.The more energy added during the refining process, the higher the energy level, or potential of the metal.
5The Corrosion CycleThe energy stored in the metal during the refining process is the driving force for corrosion.The refining-corrosion cycle is somewhat like rolling a ball up a hill and then watching it roll back down the hill the instant you release it.The job Corrosion Control is to prevent the ball from rolling back down, or at least to slow down the rate at which it rolls.
6Electromotive Force Series of Metals Most Energy Required for RefiningMagnesium VoltsAluminum VoltsZinc VoltsIron VoltsLead VoltsCopper VoltsSilver VoltsPlatinum VoltsGold VoltsGreatest Tendency to CorrodeLeast Energy Required for RefiningLeast Tendency to Corrode
8The Parts of the Corrosion Cell AnodeThe metal with the highest Energy level in an electrochemical cell. The anode is the location of the oxidation reaction, which means the anode looses energy (electrons) and is “corroded” as metal ions are released from the metal into the electrolyte by the loss of electrons that are needed to hold the ions.
9The Parts of the Corrosion Cell CathodeThe metal with the lowest energy level in an electrochemical cell. The cathode is the location of the reduction reaction, which means the cathode gains energy (electrons) and is “plated” with + ions that are present in the electrolyte. The cathode metal is un-reactive.
10The Parts of the Corrosion Cell Electron BridgeAny electric conducting pathway between the anode and cathode.
11The Parts of the Corrosion Cell ElectrolyteAn electrically conductive solution surrounding the anode and cathode in the electrochemical cell. The electrolyte is required to complete the electrical circuit.
12Rate of Corrosion Electrolyte Composition One ampere of current flowing for one year = 20 pounds of iron.Electrolyte CompositionpH: Increases if <5, or >12Conductivity: Increases with salinity.Dissolved Gases: O2, CO2, H2S,Potential difference between Anode and Cathode - EMFTemperature - “rule of thumb” every 10oC doubles the reaction rate.
13Types of CorrosionThere is a multitude of different types of corrosion, mostly resulting in different patterns and amounts of wall loss.PittingCrevice CorrosionConcentration and/or Differential Aeration CellsScale and sludge - deposits
14Types of CorrosionThere are also several types of environmental cracking that can occur and are directly related to corrosionStress Corrosion Cracking (SCC)Sulfide Stress Cracking (SSC)Hydrogen related embrittlement and cracking.Cracks are the most dangerous concern to pipeline integrity.
15Controlling Corrosion Eliminate any one of the four parts of the corrosion cell and no corrosion will occur.Electrolytecoatings, internal and external.Anode and Cathodematerial selectionCathodic Protection.
16Cathodic ProtectionThe goal of cathodic protection system is to polarize the pipe with an abundance of e-, which will be available to any e- acceptors that would otherwise steal e- from the pipe. In order to do this over a long distance the pipe must have a coating which will prevent the e- from jumping off at every available chance!
17Cathodic ProtectionThis is why pipe coatings are electric resistors first, physical protection to the pipe second. Thus a small pinhole in the coating will allow e- to escape from the pipe to the soil (electrolyte) and no metal is lost from the pipeline.
18Cathodic ProtectionIn fact, the charge on the pipeline will actually attract the ions that a cathode typically attracts and in the case of a pipeline in the ground this often results is the formation of deposits on the outside of the pipe; calcium (C++) forming calcareous deposits.
19Cathodic ProtectionElectricians refer to a CP “drain” on a pipeline system; this is where the Positive current is drained off of the system. Remember that positive current off of the pipeline is achieved by putting electrons on the pipeline! Rectifiers are always hooked up with the - side to the pipeline and the + side to an anode bed.
21Designing Cathodic Protection Systems Electrolyte Composition - Salt Water, Soil.Volume of metal to protect (milliamperes/sq ft).Quality of coating.Availability of electricity.Power Requirements = Size and # of Anodes
22Maintaining and Monitoring CP Annual adjustive surveys - Pipe to soil potential.Rectifier inspection and monitoring.Interference investigationStray AC currentcompeting CP systemsTelluric currentClose Space Surveys - On/Off Potentials
23Effective CPCommon to use -850 mV as a minimum value to ensure adequate CP.Lower or higher values may be required, but detailed engineering assessments and surveys are usually required to determine “spot specific” areas and adjust accordingly.Potentials of mV and lower can result in excessive H2 gas production.
24Coating SurveysCoating surveys are conducted to evaluate the coating integrity on buried pipelines.These surveys can be used to determine potential locations for corrosion, environmental cracking, or C.P. current loss.
25Coating SurveysBecause of the cost and time required to conduct these surveys they are often only completed in environmentally sensitive areas or in high temperature zones downstream of plants or boosters where coating damage is expected.
26Coating Surveys Utilize the Electrical resistivity of the coating. Apply a current and measure the attenuation of the current over a known distance to find areas of exceptionally high attenuation.
27Coating SurveysThe most advanced technology is to induce an alternating current on the pipeline, and then to measure the field strength along the length of the pipeline from above ground at regular intervals. The strength of the AC field above ground will attenuate at a logarithmic rate that can be calculated, and confirmed.
28Coating SurveysWhere non-logarithmic attenuations are measured, the location is investigated to determine if AC leakage to the soil or other buried facilities has resulted. If no interference can be determined, then that interval is identified as having faulty or compromised coating.