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The structures that receive cathodic protection can be classified into three groups. First group consists of those that are buried in soil. This includes oil and gas transporting pipelines buried in soil, petroleum storage tanks, tower footings and cables. The second category consists of structures that are immersed in water both marine and fresh water such as ship hulls, barge hulls, Locks, dams, gates, piers, piling and offshore structures. The third group comprises of structures that contain the electrolyte within. Its members include cold oil tankers, water tanks and settling basins, sewage disposal plants and water box coolers.
Cathodic protection of buried structures
Pipelines are utilized to transport valuable products like oil, gases and water. Successful application of cathodic protection depends upon the selection, design, installation and maintenance of the system. Before designing cathodic protection adequate field data must be gathered, analyzed and evaluated.
Field-tests
Nature and conditions of the soil are reflected by field measurements like soil resistivity, hydrogen ion activity (pH) and redox potential. To understand the nature of the pipeline, potential measurements, coatings, resistance meaningful design current requirement tests must be conducted.
Soil resistivity
Soil is not an uniform electrolyte. Its resistivity varies from place to place. Marshy soils may have low resistivity where as the resistivity may be several thousand ohms in the case of rocky soils. Corrosivity is always an inverse function of soil resistivity. At low resistant areas, current flow is favoured and increases the probability of anodic dissolution. It is quite natural at highly resistant areas corrosion problems will not be severe.
Probability of corrosion activity for steel exposed to soils of varying activity
Moisture greatly affects the soil resistivity. A typical clay with 5% moisture can have resistivity 100000 Ohm-Cm when the moisture content is increased the same soil can have resistivity 7000 Ohm-Cm. Corrosion activity may be severe during raining season. Soil resistivity data should not be collected when the season is abnormally dry.
The Wenner's four-pin technique is the most commonly used method for measuring soil resistivity.
Hydrogen ion activity (pH)
pH of the soil is the measure of hydrogen ion activity.
pH = -log10 [aH+]
aH+ = activity of hydrogen ion
pH of soils may vary in the range of 3.5 to 10. The neutral soils can have pH of 6.5 to 7.5. Alkaline soils can have pH in the range of 7.5 to 10. Acidic soils can have pH 3.5 to 6.
Lower the value of pH indicates higher the measure of corrosion. The soil pH can be determined from the underground water in the area of interest. When ground water is not available pH can be measured by making a solution by mixing 1 volume of soil with 1 volume of distilled water. Antimony and copper/copper sulphate electrode can be used to obtain insitu pH values.
pH = 0.018E sb -1.54.
Redox potential and microbiological activity
Soil may become aggressive if they support microbiological activity in presence of and in the absence of oxygen. Commonly observed bacteria are the sulphate reducing type (Desulpho Vibrio desulphuricans). These bacteria consumes hydrogen and reduces sulphates to sulphide.
S042- + 4H2 S2- + 4H2O
The product formed is hydrogen sulphide and it reacts with steel and produces black coloured FeS. Under anorebic conditions the reactions occurring may be put in the form of
Fe + S042- + 4H2O 3Fe (OH)2 + FeS + 20H-
It is explicit that hydrogen is consumed and H2S is formed. The acceleration of corrosion by bacteria is two folds. The hydrogen consumption accelerate corrosion by decreasing cathodic polarisation. Iron sulphide formation increases the corrosion rate by increasing the corrosivity of the environment.
Bacteria thrive by consuming certain tape adhesives. Kraft papers and fillers used in felt pipeline wrappers. The conditions favorable for promoting bacteriological activity include the temperature range about 75 to 95°F and pH range of about 5.5 to 8.5. The existence of bacteriological activity can be qualitatively established by placing few drops of hydrochloric acid on the corrosion products. The evolution of H2S indicate the existence of anaerobic bacteriological activity.
Otherwise the bacteriological existence may be established by Redox potential measurement. This can be done by placing a redox probe into freshly digged soil at the pipeline depth and measuring the potential between a clean platinum surface and saturated calomel reference electrode.
Caution should be taken in using the above data. The lower values indicates only that the soil can support bacteriological activity but it does not indicate that anaerobic bacteria are present. More than 400 mV may indicate the non-existence of bacteriological activity but it does not reflect other forms of corrosion do not occur.
Besides, sulphate reducing bacteria sulphur oxidising bacteria (Thiobacillus, thioxidants) can exist in aerated environment. These bacteria during their metabolic activity consumes oxygen and oxides sulphides to sulfates generally as Sulfuric acid (H2SO4) They can produce sulphuric acids having concentrations as high as 10% (i.e. pH 0.5).
Structure to electrolyte potential
Structure to electrolyte potential surveys are effective in analysing the corrosion activity of an existing pipeline. High input impedance voltmeters are employed to measure the structure to electrolyte potential measurements. The potential survey are conducted along the pipelines at incremental distances with reference electrode.
Data must be interpreted incorporating the IR drop values in the measured values, occurring at the point of reference electrode and the underground structure. IR drop is a problem in highly resistant soil and areas covered with concrete. By wetting the soil IR drop can be minimized.
The more negative pipeline potentials corresponds to regions of low electrolyte resistivity for bare and coated structures. The anodic areas corresponding to more negative potential will occur at more negative potentials will occur at river crossing and under areas of clay.
At low resistant areas the large potential difference would correspond to areas of corrosion activity. At high resistive areas this may not be a problem. The potential data must be interpreted in conjunction with soil resistivity data.
More detailed information can be obtained on galvanic attack, stray current corrosion by measuring the potential also along the uncoated pipelines that are lateral to the structure. (e.g. 20 to 50 feet normal to the structure).
In general
1. When the lateral potentials are less negative than those measured over the structure anodic areas will occur at locations along the pipelines.
2. When anodic areas coincide with regions of high negative potential normal corrosion activity is probably taking place stray current and galvanic corrosion should not be suspected.
3. When anodic areas coincide with low negative potential corrosion activity is probably caused by stray current or galvanic attack.
General interpretation of pipe to soil potential measurements :
1. Newer pipelines show more negative potential than older pipeline.
2. In acidic soil potentials are more negative than in alkaline soil.
3. Well-coated structures show more negative potentials than uncoated structures.
4. More negative potentials correspond to locations of low resistivity for uncoated structures.
CURRENT REQUIRED FOR PROTECTION
The current requirement for cathodic protection depends on the corrosion rate and surface area of the metal exposed. The current requirement may be estimated from corrosion current density in specific environments
In practice it is better to estimate the current density requirement by constructing temporary anode beds and applying current from D.C. source.
Coating resistance
Cathodic protection is always employed as a complimentary to coating. Coatings reduce the amount of current to be impressed for cathodic protection. Coatings range include coal tar and asphalt enamels, mastics, waxes, greases poly vinyl chloride and polyethylene tapes thermosetting epoxy resins and epoxy coaltar. A perfect coating may have coating resistance of the order of 1012 Ohm/ft2. But coatings are seldom perfect. They develop holidays and deteriorate with time.
Coating damages are caused by chemicals present in soil bacteriological activity penetration by tree roots thermal and mechanical stress.