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CorrosionManagement | September/October 2009TECHNICALARTICLEvariation between different areas of thesandbox. It is also possible that the formationof the passive film on the steel section, thedesired consequence of applying CP, doesnot occur evenly leading to some variability.Despite these errors the boundary elementmethod provides sufficiently accurate resultsto produce potential and current distributionmaps for the surface of buried steel elementsand identify areas of excessive or inadequatepolarisation, as shown in Figure 7.5. Stray current effectsFigure 6: Comparison of experimental andmodelled potential line scan for steel beamin sand.Figure 7: Potential distribution on the surface of a steel beam subject to cathodic protection,as modelled by the boundary element method.Figure 8: Sand box test to assess stray current effects.Steel-framed masonry buildings contain avariety of metallic elements. In addition tothe frame itself, metal window frames, drainpipes and fixings such as wall ties and clampsare commonly encountered. Generallyelectrical continuity between structuralmembers is rarely a problem [9] since thestructural connections are typically boltedor riveted. However, other elements aremore likely to be electrically discontinuousand this must be taken into account whendesigning a CP systems. Failure to ensure theelectrical continuity of all metallic elementscould result in stray current interactionsbetween the various elements of thestructure, resulting in accelerated corrosionof the discontinuous items.By employing the boundary elementmethod, it has been possible to modelthe effect of discontinuous steel on straycurrent corrosion and the results have beencompared with weight loss measurementsfrom sand box tests, as shown in Figure 8. Themodel predicts the steel between the anodeand the steel section will pick up currenton the face nearest the anode and releasecurrent on the face nearest the steel section,the latter resulting in a loss of metal. For theother bar, while there is some pick up, thereis relatively little loss and consequently littleor no corrosion [10]. Estimated weight lossesbased on the modelled currents comparedwell with actual weight loss measurementsobtained from the bars, demonstrating thevalidity of the boundary element methodfor assessing the effects of stray current ondiscontinuous metallic items.6. Design implicationsHaving demonstrated the adequacy of themodel, it has subsequently been employedto assist in the detailed design of cathodicprotection systems for historically significantsteel framed structures. The model can assistin the optimisation of anode locations intwo ways. Firstly, it can help identify the bestlocations for anodes so as to achieve fullprotection from the least number of anodes,12