Instant-off potential

To this concern. Figure 20.5 shows the results of application of cathodic prevention to slabs subjected to ponding with a NaCl solution [43]. After about 700 days, initiation of rebar corrosion was detected in the control slab (in the free corrosion condition) at spots where the chloride content at the steel surface had reached more than about 1 % by mass of cement. From about that point in time, the slab receiving a very low current density of 0.4 mA/m showed a similar (instant-off) potential as the control slab and its 4-hour decay values became lower than 100 mV. The slab receiving 0.8 mA/m showed lower decays from about... 

Figure 20.5 Instant-off potential of non corroding steel cathodically polarized with current densities typical of cathodic prevention and chloride content in the control slab. Specimens exposed to saturated NaCI solution alternating...

The question is, how can we show that this has happened In cathodic protection of steel in soil or water it is usual to do this by achieving a potential of -770 mV or -850 mV against a copper/copper sulphate half cell on the surface as the system is switched off (the instant off potential). However, these criteria are not appropriate for steel in atmospherically exposed concrete for a nnmber of theoretical and practical reasons. Two of the practical reasons are the difficulty in accurately measuring an absolute potential over a nnmber of years when reference electrodes calibration may drift, and the fact that if an absolnte minimum (or maximum negative) potential is achieved then some parts of the structure will be overprotected as the corrosion environment varies so rapidly and severely across a high resistance electrolyte like concrete. 

NACE RP290 It is a shorter document than BSEN 12696 and is specifically concerned with control criteria which are at the front of the document in Section 2. The first criterion mentioned is 100 mV polarization development or decay to be measured between the rest , equilibrium , or natural corrosion potential and the instant off potential measured O.l-l.O seconds after switching off the system. The instant off potential is required to remove the iR drop ... 

The criteria (Section 8.6 of the standard) start with a requirement that no (instant off) potential should exceed a limit of 1,100mV with respect to Ag/AgCl/0.5M KCl for reinforced concrete and —900mV for prestressed concrete. This is aimed at minimizing hydrogen evolution with a wider margin for prestressed steel where the consequences could be more extreme as discussed in Section 7.2.1. 

An instant off potential more negative than -720 mV vs. Ag/AgCl/0.5 M KCl - this criterion is the conventional one for buried and submerged structures. 

This has lead to a number of criteria because of the practicalities of measuring potential shifts. The potential shift can be measured vhen switching on the system and switching it off. It must be measured as an instant off potential when the system is on so that the cathodic protection current, or iR drop as it is called, does not interfere (Figure 6.22), This means that the half cell potential, without the effect of the cathodic protection current, is measured between 0.1 and 1 s after. switching off the current. 

It is therefore advisable to set an limit of -1100 mV vs. CSE for the instant off potential for all systems. This is more as a convenient upper limit for reinforced concrete structure.s,but as a rigid limit in the case of prestressing steel if exposed to the cathodic protection current. 

When several rectifiers protect a structure, it is necessary that all rectifiers be interrupted at the exact same instant in order to obtain meaningful measurements. Pipeline operators usually specify that at least two rectifiers ahead of the survey team and two rectifiers behind the survey team have to be interrupted in a fully synchronized manner. The amount of time between current interruption and depolarization can vary from a fraction of a second to several seconds, depending upon details of the structure. In addition, capacitive spikes that occur shortly after current is interrupted may mask the instant-off potential. Measurements made with a recording voltmeter are preferred as they can be subsequently analyzed to determine the real instant-off potential [16]. 

Figure 11.4 Measurement of instant-OFF potentials, by interrupting the CP current supply (schematic).

Several measurements can be made after a coupon-type corrosion sensor has been attached to a cathodically protected pipeline. on potentials measured on the coupon are in principle more accurate than those measured on a buried pipe, if a suitable reference electrode is installed in close proximity to the coupon. The potentials recorded with a coupon sensor may still contain a significant IR drop error, but this error is lower than that of surface on potential measurements. Instant-OFF potentials can be measured conveniently by interrupting the coupon bond wire at a test post. Similarly, longer-term depolarization measurements can be performed on the coupon without depolarizing the entire buried structure. Measurement of current flow to or from the coupon and its direction can also be determined, for example, by using a shunt resistor in the bond wire. Importantly, it is also possible to determine corrosion rates from the coupon. Electrical resistance sensors provide an option for in situ corrosion rate measurements as an alternative to weight loss coupons. 

In an impressed-current cathodic protection system the power source has a substantial capacity to deliver current and it is possible to change the state of polarisation of the structure by altering that current. Thus effective control of the system depends on credible potential measurements. Since the current output from any given anode is substantial, the possibility of an IR error which may reach many hundreds of millivolts in any potential measurements made is high. Thus the instant-off technique (or some other means of avoiding IR error) is essential to effective system management. 

By contrast a cathodic protection system based on sacrificial anodes is designed from the outset to achieve the required protection potential. If this is not achieved in practice there is no control function that can be exercised to improve the situation. Some remodelling of the system will be required. Moreover, the currents from each current source (the sacrificial anodes) is modest so that field gradients in the environment are not significant. It is at once clear that potential measurements are less significant in this case and instant-off measurements are neither necessary nor possible. 

Therefore the 100-150 mV shift in potential is often measured as a 100 mV shift from instant off to a period typically four hours later. This is measured at an actively corroding (anodic) location. Measurements are made throughout the four hour period so that the depolarization curve can be plotted. If depolarization continues at four hours then it is reasonable to expect that the 100-150 mV criterion has been achieved. 

A potential decay over a maximum of 24 h of at least 100 mV from instant off - this is very similar to the NACE depolarization criterion but has a time period specified. 

An important consideration in the potential readings is the ohmic (IR) drop error that is included in the potential measurements when a CP system is operational. A commonly used method to correct for the IR drop, often called making instant-off measurements, can be a... 

For an SACP system, only current-on potentials were measured and recorded at the location of each reference electrode embedded in submerged areas. There was no provision or facility provided to interrupt the current flowing between the sacrificial anode and the steel reinforcement, to measure instant-off steel potentials. 

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