Zinc rich paint as anode system for cathodic protection (CP) of reinforced concrete structures and development of corrosion/CP monitoring probes

摘要

Since mid-80‟s cathodic protection (CP) has been recognised as the „‟only technique known to stop corrosion regardless of the levels of chloride contamination in concrete‟‟ (FHWA, 1982) and is proved to be the most cost effective means to extend the useful life of the structure.Cathodic protection is an electrochemical technique to stop/mitigate corrosion by supplying „current‟ from an external source in order to suppress the „internally generated‟ current flow due to corrosion processes. The „external‟ current source could be obtained simply by coupling the steel to another electrochemically more active metal, e.g. zinc; alternatively the „external‟ current may be derived from a mains operated low voltage DC power source, viz. transformer/rectifier unit. These two different approaches to supply „external‟ current to stop corrosion are generically termed as:„Sacrificial Anode Cathodic Protection (SACP)‟ system and„Impressed Current Cathodic Protection (ICCP)‟ system, respectively.Both approaches have proved to be feasible, but the impressed current CP system offers greater flexibility with regard to its ability to provide the necessary current in situations where concrete resistivity is relatively high and variable. The sacrificial anode system is most effective if the concrete resistivity is very low or the anode is placed in a very low resistivity environment such as soil with low resistivity, as the inherent driving voltage is low e.g. the potential difference between zinc and corroding steel in concrete is limited to approximately 0.7 volts.Other contra-distinction between the two approaches are that the design life of the sacrificial anode systems are usually range between 10 -15 years; on the other hand the design life of the ICCP systems could be well in excess of 60+ years ( depending on the type of anode system).Page 2Following the successful application of first CP system, based on impressed current CP (ICCP), on a bridge deck in California, USA 1973, the technology has advanced significantly, particularly the anode systems (which is the main arbiter of a CP system) to deliver the protection current efficiently providing adequate protection (i.e. meeting the criteria recommended in BS EN ISO 12696: 2012 and other International Standards). Most of the CP installations worldwide are operating in ICCP mode. However, due to the escalating cost of anode systems and associated external power supply as well as monitoring/control units for ICCP installation has led researchers to actively pursue different means of developing low cost anode systems. Researchers have mainly focused on sacrificial anode CP (SACP) systems, as SACP does not require an external power supply and control units, but the drawback to this anode system is that it has a shorter life span (usually 10 -15 years compared to 60+ years for ICCP anodes).This work describes the development of an ICCP anode system design utilising commercially available zinc rich paint (ZRP) as a primary anode material offering an innovative but considerably low cost alternative to currently used materials for ICCP anode systems. It also describes the development of a simple and low cost „multifunctional‟ probe for monitoring the performance of the installed CP system, among other functions, such as LPR measurements, macrocell corrosion current measurement, E-log I tests for assessing the current requirements for CP design. For these functions both laboratory investigations and field trial on real life structure were employed