Corrosion and Cathodic Protection © 2007 Spectrum eXternal Line Inspection Technologies Inc.

Corrosion NACE defines Corrosion: Deterioration of a substance (usually a metal) or its properties because of a reaction with it’s environment.

Pipeline Corrosion Primarily concerned with the destruction of the metal by either chemical or electrochemical reaction with a given environment.

Electrochemical 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.

The Corrosion Cycle The 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.

Electromotive Force Series of Metals Most Energy Required for Refining Magnesium -2.37 Volts Aluminum -1.66 Volts Zinc -0.76 Volts Iron -0.44 Volts Lead -0.13 Volts Copper +0.34 Volts Silver +0.80 Volts Platinum +1.20 Volts Gold +1.60 Volts Greatest Tendency to Corrode Least Energy Required for Refining Least Tendency to Corrode

The Electric Circuit of Corrosion

The Parts of the Corrosion Cell Anode The 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.

The Parts of the Corrosion Cell Cathode The 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.

The Parts of the Corrosion Cell Electron Bridge Any electric conducting pathway between the anode and cathode.

The Parts of the Corrosion Cell Electrolyte An electrically conductive solution surrounding the anode and cathode in the electrochemical cell. The electrolyte is required to complete the electrical circuit.

Rate of Corrosion Electrolyte Composition One ampere of current flowing for one year = 20 pounds of iron. Electrolyte Composition pH: Increases if <5, or >12 Conductivity: Increases with salinity. Dissolved Gases: O2, CO2, H2S, Potential difference between Anode and Cathode - EMF Temperature - “rule of thumb” every 10oC doubles the reaction rate.

Types of Corrosion There is a multitude of different types of corrosion, mostly resulting in different patterns and amounts of wall loss. Pitting Crevice Corrosion Concentration and/or Differential Aeration Cells Scale and sludge - deposits

Types of Corrosion There are also several types of environmental cracking that can occur and are directly related to corrosion Stress Corrosion Cracking (SCC) Sulfide Stress Cracking (SSC) Hydrogen related embrittlement and cracking. Cracks are the most dangerous concern to pipeline integrity.

Controlling Corrosion Eliminate any one of the four parts of the corrosion cell and no corrosion will occur. Electrolyte coatings, internal and external. Anode and Cathode material selection Cathodic Protection.

Cathodic Protection The 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!

Cathodic Protection This 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.

Cathodic Protection In 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.

Cathodic Protection Electricians 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.

Cathodic Protection

Designing 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

Maintaining and Monitoring CP Annual adjustive surveys - Pipe to soil potential. Rectifier inspection and monitoring. Interference investigation Stray AC current competing CP systems Telluric current Close Space Surveys - On/Off Potentials

Effective CP Common 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 -1500 mV and lower can result in excessive H2 gas production.

Coating Surveys Coating 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.

Coating Surveys Because 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.

Coating 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.

Coating Surveys The 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.

Coating Surveys Where 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.

Coating Survey Report