 |
|
 |
 |
Codes and Standards |
|
ANNEX J: CORROSION CONTROL OPTIONS FOR GUY ANCHORS IN DIRECT CONTACT WITH SOIL 1 INTRODUCTION Many guy anchors in direct contact with soil, designed in accordance with EIA/rlA Standards, have performed well without detrimental corrosion. However, depending on the required design life of the structure and on site-specific conditions, corrosion control measures, in addition to hot-dip galvanizing, may be required to prevent the premature deterioration of these types of anchors. Hot-dip galvanized materials have been proven to be very effective in resisting corrosion when in direct contact with soil. In a 10-year study involving 45 types of soils performed by the National Bureau of Standards, only one sample had some penetration of the base steel. A 13-year test in cinders ( one of the most corrosive subgrade environments), indicated that corrosion was effectively reduced, even though the zinc coating was destroyed within the fa'st two years. One theory for this behavior is that the alloy layer between the zinc and steel surface, formed during the hot-dip galvanizing process, results in a major source of protection. Also, in some soils, a protective layer of a zinc compound forms during the corrosion process, slowing the rate of corrosion. Despite the protective nature of hot-dip galvanized materials, there have been reports of unacceptable anchor corrosion occurring within 10 years after installation. Anchor inspections are imperative to determine if accelerated corrosion is occurring at a given site. Corrosion activity may vary widely across a site. Anchor corrosion could occur at one or more of the anchors at a site and could occur at any depth along a given anchor. Some of the site conditions which may result in accelerated corrosion are briefly described in this annex. Under these conditions, additional corrosion control measures should be considered. This annex is not intended to be a treatise on the subject of anchor corrosion but is provided to help owners become aware of the potential anchor corrosion problems and the importance of anchor inspections; and to encourage owners to pursue further information from appropriate specialists for both new and existing construction. A corrosion specialist may recommend methods to curtail or monitor corrosion discovered at existing sites or present options to consider for proposed sites. 2 TYPES OF CORROSION When a dissimilar metal is electrically connected to an anchor, a difference in potential exists between the two materials, ff the dissimilar metal is also in contact with a low resistivity soil, a complete circuit will exist. Current will flow from one metal to the other due to the electrical connection and remm through the soil completing the circuit. This naturally occurring phenomenon is why current is obtained from a battery when its terminals are electrically connected. Dissimilar metals behave in this manner because of the difference in potential each metal inherently has. Metals may be listed in order of their potential. Such a list is called a galvanic series. A galvanic series of commonly used metals and alloys is given in Table J1. When a complete circuit exists, corrosion occurs on the metal listed higher in the galvanic series. This is the location where current exits and travels through the soil towards the metal listed lower on the galvanic series. For example, if a large copper ground system in a conductive soil is directly or indirectly (through guys) electrically connected to a steel anchor, corrosion will occur on the anchor since steel is listed higher on the galvanic series than copper. The rate of corrosion will depend largely on the conductivity of the soil and the relative locations of the metals in the galvanic series. The higher the soil conductivity, and the further apart the metals are in the galvanic series, the faster the corrosion. Many other factors beyond the scope of this commentary could influence the rate of corrosion and result in accelerated anchor corrosion. Galvanic corrosion may also occur atvarious rates without the presence of a dissimilar metal when conditions along the surface of the anchor are not uniform. This situation may exist when the base of the anchor is embedded in concrete. The moist concrete, being much different than the soil surrounding the exposed portion of the anchor, will have a different potential. If the surrounding soil conductivity is high, accelerated corrosion of the anchor may occur. Backfill conditions with non-uniform composition, compaction, moisture content, porosity, etc., may result in similar localized differences in potential along the anchor. 2.2 Electrolytic Corrosion 3 CORROSION POTENTIAL OF SOIL The corrosion potential at a given site is a function of many variables. Fortunately, one of the most important variables, the conductivity of soil, may be determined by a geotechnical investigation. 3.1 Soil Conductivity The conductivity of a soil is usually determined by measuring resistivity. Resistivity is most often measured in units of ohm-centimeter (ohm-em). The lower the resistivity, the higher the conductivity. For example, salt water, a very corrosive environment, has a resistivity of approximately 25 ohm-em. Clean dry sand, which is usually a non-corrosive environment, may have a resistivity of more that 1,000,000 ohm-em. A soil with a resistivity below 2,000 ohm-em is generally considered to be highly corrosive. 3.2 Other Factors Soil resistivity may vary seasonably and is generally a function of mineral composition, moisture content and the concentration of dissolved salts. Clays and high moisture content soils generally have lower resistivity than sands or low moisture content soils. However, a dry sandy soil may become very aggressive upon an increase in moisture content if dissolved salts are present. Likewise, a wet soil may not be aggressive without the presence of dissolved salts. Temperature also affects resistivity values. The resistivity of a soil may measure very high if measured under near freezing conditions, yet be very aggressive under warmer conditions. Many other factors
influence the corrosion potential of soil to varying degrees. Some of
these factors are: drainage, soil porosity (aeration), acidity or alkalinity
(ph), certain chemical properties, the metabolic activities of certain
micro-organisms, adjacent and/or cathodically protected structures. These
factors may also vary seasonably or vary due to other activities at a
site, such as the doping of soil to increase the effectiveness of a grounding
system. Due to the many possible factors involved, it may not always be
possible to determine the controlling factor when aecelerat~l corrosion
occurs. 3.3 Geotechnical Investigations When a geotechnical investigation
is performed, as a minimum, the local soil resistivity and the type and
concentration of dissolved salts should be established. With this information,
together with a description of all existing and/or proposed construction,
a corrosion specialist should be able to recommend various corrosion control
measures to be considered. Additional site testing may be required by
the corrosion specialist in order to properly design 4 OPTIONS FOR CORROSION CONTROL None of the following options for corrosion control eliminate the need for proper monitoring and maintenance over the life of the structure. 4.1 Site Modifications Improving drainage or placing an impermeable layer of soil at an anchor location may be beneficial in reducing the rate of corrosion. Under some situations it may be possible to backf'fil around an anchor with a high resistivity soil. Adding chemicals to neutralize existing corrosive soils or to mitigate the actions of micro-organisms may also be an alternative. Care must be taken to ensure that the required structural capacity of an anchor support is maintained during excavations and to avoid contaminating the local soil with toxic substances. Relocating an anchor may also be a reasonable alternative if the cause or possibility of accelerated corrosion at a site is known to be a localized, isolated condition. If copper ground rods serve as grounding for an anchor, replacing them with galvanized steel rods would reduce galvanic corrosion by eliminating the presence of a dissimilar metal. Special attention should be paid to the ground lead and its connection to a galvanized rod, particularly when the connection is placed below grade. Isolation of anchors from the structure using guy insulators may help to reduce the transmission of stray currents from outside sources and therefore 'minimize electrolytic corrosion. Galvanic corrosion due to the presence of copper ground rods would be eliminated if the ground wires were connected on the tower side of the isolation point. Isolation may also increase the efficiency of sacrificial anodes described in 4.4. Bonding the anchors to adjacent cathodicaUy protected pipelines or structures may protect the anchors as opposed to subjecting them to possible electrolytic corrosion. This should only be done in accordance with recommendations from a corrosion specialist. 4.2 Protective Coatings Many types of organic and inorganic protective coatings are available. The effectiveness of a coating is highly dependent upon the preparation of the anchor surface, the method of application and the vulnerability of the coating to damage during construction. Protective coatings may be particularly effective when used in conjunction with a cathodic protection system described in 4.4. 4.3 Concrete Encasement Direct contact with soil may be avoided by encasing an anchor with reinforced concrete over the entire embedded length of an anchor. The encasement should extend a minimum of 6 inches above grade. When a concrete anchor block is used with an anchor, the reinforcing in the concrete encasement must be properly developed into the anchor block to prevent excessive cracking. Sulfate resisting concrete mix designs should be used for all concrete below grade when soluble sulfates exist in the soil or groundwater. 4.4 Cathodic Protection For both galvanic and electrolytic corrosion, corrosion occurs when current flows from the anchor into the surrounding soil. The objective of cathodic protection is to reverse the direction of current, resulting in current flowing to the anchor instead of away from it, thus preventing corrosion of the anchor. This may be accomplished by installing galvanic anodes or by introducing an impressed current. By electrically connecting a metal (galvanic anode) listed higher on the galvanic series and burying it in close proximity, current can be forced to flow to the protected item from the anode. This will result in corrosion of the installed metal anode instead of the item to be v protected. For this reason, the installed metal is called a sacrificial anode and also why these anodes must be periodically inspected to make sure they have not corroded away beyond use. AdditiOnal sacrificial anode material may eventually have to be added. A common sacrificial anode used is magnesium packaged in a prepared bacld'fil mixture to enhance its conductivity with soil. The number, size, type and location of galvanic anodes should be determined by a corrosion specialist and must be adequate to ensure current flows in the correct direction, overcoming the effects of all other influences at the site. The effectiveness of an installed system should be periodically monitored over the life of the structure by a corrosion specialist. This may be done by measuring the potential of the protected anchor with respect to a reference electrode placed in the ground. A large enough negative potential indicates that current is flowing to the anchors as desired for corrosion control. Under certain circumstances, installing enough galvanic anodes to ensure current will flow in the desired direction may not be feasible or economical. Using an impressed current with an anode may be required under these circumstances. The impressed current requires the use of a reliable power source to produce the desired current. The positive terminal of the power source is connected to the anode resulting in current traveling from the anode, through the soil to the anchor, overcoming the effects of all other influences. Since current would be entering the anchor from the soil, corrosion of the anchor would be controlled. The voltage of the power source, the size, location and type of anode required, and the possible effects on adjacent structures should be determined by a corrosion specialist. Overprotection may result in accelerated corrosion of surrounding structures and may also damage the anchor or anchor coating as a result of high current forming undesirable chemical compounds and/or hydrogen gas at the anchor. 5 REFERENCES Uhlig, H. H., "The Corrosion Handbook", John Wiley & Sons, NY, 1948. Uhlig, H. H., Revie, R. W., "Corrosion and Corrosion Control", Third Edition, John Wiley & Sons, NY, 1985. Wilson, C. L., Oates, J. A., "Corrosion and the Maintenance Engineer", Hart Publishing Company, NY, 1968. Husock, B., "Fundamentals of Cathodic Protection", HARCO Technologies Corporation, Medina, Ohio.
GALVANIC SERIES OF COMMONLY USED METALS AND ALLOYS MAGNESIUM ZINC ALUMINUM STEEL, IRON LEAD, TIN BRASS, COPPER, BRONZE SILVER GRAPHITE |
Copyright © Preferred Tank & Tower Inc., 2008