Cathodic protection from wires

Prestressed concrete pipelines occasionally require cathodic protection. Protection must be done carefully to avoid damage to the prestressing wire from hydrogen embrittlement or SCC [75]. 

There is a CPA Technical paper on stray currents (CPA Monograph 10, 2002). These can arise from the effect of a cathodic protection system on isolated steel in the protected structure. This can be trivial such as nails or tie wire left in the concrete during casting. It can be more significant lengths... 

Zinc ribbon has been used as a grounding electrode to dissipate induced ac from a pipeline (Kuir, 1979). In addition, zinc ribbon is used as a ground mat to equalize the potential of the earth at a valve, cathodic protection test station, or other appurtenance of the pipeline. The ribbon is installed about 0.5 m underground in the form of a flat, horizontal, pancake coil with a nominal diameter of 2.5 m or more. One end of the wire core of the zinc ribbon protects the workers who are standing on the earth surface, touching the pipeline valve or test station, by minimizing the potential difference between the pipeline and earth. 

In the process droplets of semimolten zinc are sprayed from a special gun that is fed with either wire or powder onto a grit-blasted surface. The semimolten droplets coalesce with some zinc oxide present at each interface between droplets. Electrical continuity is maintained both throughout the coating and with the iron substrate so that full cathodic protection can be obtained since the zinc oxide forms only a small percentage of the coating. 

Because of the Mears effect (wire corrodes faster per unit of area than more massive materials) galvanized wire corrodes some 10-80% faster than galvanized sheet. However, the life of rope made from galvanized steel wires is greater than the life of the individual wire. This is explained by the fact that the parts of the wire that lie on the outside are corroded more rapidly and when the zinc film is penetrated in those regions, the uncorroded zinc inside the rope provides cathodic protection for the outer regions. 

The Alaskan oil pipeline, shown in Figure 2.7, is an example of steel that is cathodically protected. However, instead of coating, zinc is connected to the pipe by a wire. The zinc will oxidize before the iron in the steel does. As the zinc anode corrodes, it gives electrons to the cathode, the steel, and as the zinc anode does so, it prevents the steel from corroding. As the zinc dissolves, it needs to be replaced. As long as zinc metal that is able to corrode is present, the steel will be protected from corrosion. 

OxyTech Systems manufactures and licenses the MDC series of monopolar diaphragm cells (Fig. 39). The MDC cells feature woven steel wire cathode screen tubes open at both ends, which are welded into thick steel tube sheets at each end. The tubes, tube sheets, and the outer steel cathode shell form the catholyte chamber of the cell (Fig. 40). Copper is bonded, rather than welded, to the rectangular cathode shell on the two long sides parallel to the tube sheets. Copper connectors attached at the ends of the bonded copper side plates complete the encompassing of the cathode with copper. Anodes are connected to a copper patented cell base, which is protected from the anolyte by a rubber cover or a titanium base cover (TIBAC) [126], Orientation of the cathode tubes is parallel to the cell circuit, the opposite of a Hooker-type cell. This arrangement accommodates thermal expansion of the cell and circuit without changing the anode-to-cathode alignment. 

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. 

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