Repair; Refurb; Retrofit
Fig 4. Laboratory analysis of alkali and chloride concentrations following the application of a CP system. The graph illustrates an increase in alkali and a reduction of chloride ions at the steel surface.
As previously mentioned, CP within the performance parameters of the standards is not written or aimed at always stopping corrosion and or repassivating steel but to at least maintain a low and non-destructive level of corrosion. As the current of an ICCP system is very low, it is logical that these secondary effects may take many years to be established. If however, enough change (current over time) is passed, re-passivation is possible as seen on many long term operating ICCP systems.
TWO STAGE FUSION TECHNOLOGY
It is clear that by modifying the way we look at the application of CP we can achieve steel passivation in a significantly reduced period. Subsequently, it would then be possible to maintain the passive state by delivering a much lower current density of the order of 0.2-2mA/m2
Fig 6. Schematic view of the Galvashield®
Fusion™ T2-100 unit. Capable of operating autonomously between an ICCP and Galvanic system
(as described in ISO 12696) as
cathodic prevention. Such a current density was shown to be easily achieved by galvanic anodes for a period of at least 19 years (Fig 5.). Unlike ICCP, a sacrificial metal is used to provide the DC current instead of an external power supply. As a consequence, the systems tend to be much simpler in nature and relatively maintenance free for their service life (20-30 years). As there are no associated power/rectifier, control system and complicated wiring, galvanic systems are extremely cost effective for smaller isolated schemes.
Fig 7. Graph illustrating the change applied between stages 1 and 2 of a Galvashield® Fusion™ system
One method of illustrating passivation of steel is via the measurement of the open circuit steel potential. For a CP system this is the natural potential of steel once the DC current has been switched off for an appropriate period of time (typically 24-48 hours). The data in figure 9 was taken from Groby Bridge in Leicestershire, UK. As can be seen, following the application of an initial high charge density (stage 1) over a relatively short period, steel re-passivation resulted as defined by the NACE standard.
Fig 5. illustrates the performance of a galvanic cathodic prevention scheme over a period of 19 years. The current density being between 0.2-2mA/m2
It is a logical step therefore, to enable passivation of the steel in one short treatment and then maintain it in a passive condition long-term in a second galvanic treatment and, therefore, end up with an effective and much simplified alternative CP system. Such a system would, by default, combine the benefits of both ICCP and Galvanic CP (GCP) without the associated limitations; temporary ICCP but long term GCP.
In keeping with this simplification ideology, Vector Corrosion Technologies have developed a product capable of achieving both of these treatments in a single unit (Fig 6.) without the need for external power, complicated zoning/wiring and or continuous control and maintenance. Galvashield® Fusion™ T2 has been developed over a number of years utilising innovative technologies to bring together the designable, high level performance of an ICCP system with the low maintenance, autonomous performance of a galvanic system.
The product automatically operates in two distinct stages. As can be seen in Figure 7, Stage 1 aims at increasing the alkalinity around the steel and repelling chloride from its surface and, therefore, passivating the steel. This is followed by a secondary GCP stage which operates at a cathodic prevention level of performance as defined in the ISO standard (0.2-2mA/m2
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Fig 8. Graph illustrating the passivating effect of the Stage 1 treatment from a Galvashield® Fusion™ system and the maintenance of that passivation in Stage 2
IN SUMMARY
By changing the way we look at the long-term protection offered by cathodic protection to reinforced concrete structures, the more we are able to develop innovative ways of mitigating corrosion to the benefit of the market as a whole. By concentrating upon steel passivation as the primary purpose of applying CP followed by the maintenance of that passivity, an alternative system and or methodology can be envisaged. Fusion™ technology has been specifically developed with this in mind. The modular system is self-contained and autonomous in performance delivery. This results in a system where there are no limitations on the size of the project from a commercial perspective. This makes the system extremely flexible by design and allows the benefits of ICCP to be utilised in areas where historically, only short term isolated repair techniques may have been used.
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