The latest corrosion assessment tool from Testconsult is the BGCMap™ system. It has been designed for the Electrical Power Industry and in particular for engineers responsible for the assessment and maintenance of steel transmission towers and poles. However, it is equally at home used on any steel structure such as lighting columns.
The system uses the Linear Polarisation Resistance (LPR) technique to determine the potential for corrosion and the rate of corrosion of steel components buried below ground. It provides a rapid way of assessing the potential life of steelwork, and can help develop effective maintenance schedules. The equipment and software is purpose designed to provide the following advantages:
- Fast and easy to use on site
- One man operation
- Robust, reliable equipment
- Accurate location using GPS technology
- User definable terms in software
- Transmission line terminology
- Data export to client database
The Principle
Corrosion of a steel element embedded in the ground is a chemical reaction, whereby iron, oxygen and water react. The reaction can be characterised by the following formula:
2Fe +O2 +2H2O > 2Fe (OH)2
The metallic reaction releases electrons whereas the oxygen/water part of the reaction takes in electrons.
Metallic (anodic reaction) 2Fe > 2Fe2+ + 4e
Oxygen/water (cathodic reaction) 2H2O + O2 + 4e > 4OH
The corrosion current is the total flow of electrons from the steel column to the soil (oxygen and water ) and if it were possible to measure this current then calculation of the number of iron molecules converted to rust every year and thus the rate of corrosion would be simple and direct. However, there are many thousands of anodic and supporting cathodic sites at microscopically adjacent points on the metal surface and inserting an ammeter probe in these circuits is not possible.
However, any corroding electrode will have a potential, Ecorr (normally measured with respect to a Copper/Copper Sulphate half cell). Over a region either side of this potential, the potential/current response of the system is approximately linear, the slope of the line being the Polarisation Resistance. Hence the name of the technique, Linear Polarisation Resistance measurement, LPR.
Polarisation Resistance, Rp = dE/dI = B/Icorr
where : B = Ba x Bb/ 2.3 (Ba + Bb ) and Ba and Bb are cathodic Tafel constants
By assuming certain values, or bands for the Tafel constants, the corrosion current, Icorr , can be calculated and thus the corrosion rate. Alternatively, the Polarisation resistance can be used directly as a comparative value.
Hardware
The BGCMap™ system consists of:
- Data collection unit & carry case
- Copper/Copper Sulfate Half Cell
- Connection cables and heavy duty clips
- AC Charging Adapter - 110/240VAC
- DC Charging Cable – 12VDC
- Serial or USB download cable
- User Manual
- BGCMap™ Analysis software
The data collection unit is lightweight, portable and waterproof with a transflective screen for easy daylight viewing. The unit is driven from a simple menu system which prompts the operator for inputs and is powered by re-chargeable batteries which give approx 8 hours use. The asset nomenclature is designed for the power line industry and can be described in terms of Structure, Component, Line, Segment, Circuit and Street. The operator is also able to enter details of the Inspector, Weather and Soil conditions. The unit also contains a GPS module to enable test locations to be verified and recorded.
The Unit will automatically apply a current to the structure and determine ECorr. The operator can then chose the best fit line to measure Rp.
The connectors are all top quality waterproof stainless design, made by lemo. Each connector has a different keyway to avoid mis-connection. The unit has a wipe clean waterproof keypad with tactile keys and full alpha-numeric functions. Data transfer is by serial/usb port to a PC
The complete kit is supplied in a rugged, orange case constructed of structural resin and fitted with a neoprene “O” ring. These cases are virtually indestructible and fully waterproof.
BGCMap™ Software & Reporting
he BGCMap software operates under windows and follows all the usual file handling and printing protocols. Files are downloaded from the site unit, and are automatically entered into a report table. From the table each file can be viewed independently and site data reviewed. The ‘best fit’ slope can be chosen to determine the Polarisation Resistance (Rp).
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