Cathodic protection exposures

Cathodic protection exposures One accelerated test involves subjecting the material to which the coating has been applied to various voltages of impressed current, often in excess of that normally required to achieve cathodic protection, with the aim of demonstrating the ability of the coatings to resist the disbonding effect of the cathodic current or to resist electroendosmosis (Fig. 14.30). 

The determination of polarisation curves of metals by means of constant potential devices has contributed greatly to the knowledge of corrosion processes and passivity. In addition to the use of the potentiostat in studying a variety of mechanisms involved in corrosion and passivity, it has been applied to alloy development, since it is an important tool in the accelerated testing of corrosion resistance. Dissolution under controlled potentials can also be a precise method for metallographic etching or in studies of the selective corrosion of various phases. The technique can be used for establishing optimum conditions of anodic and cathodic protection. Two of the more recent papers have touched on limitations in its application and differences between potentiostatic tests and exposure to chemical solutions. ... 

It is possible to determine the four contributions to the total material loss rate by the following experimental principles the total material loss rate Wt is determined by weighing the specimen before and after exposure under combined erosive and corrosive conditions. The sum of Wc and Wce (the corrosion components) can be measured by electrochemical methods during the same exposure (the methods described in Section 9.2 can also be used under erosive conditions). We is determined by weighing the specimen before and after exposure in special tests where corrosion is eliminated by cathodic protection (or possibly by oflier means) but otherwise under the same conditions as in the former experiments. Wc can be measured electrochemically in tests like the original ones but with all solid particles excluded. Finally, the synergy components, Wce and Wec, can be derived from Equation (7.9) and the mentioned experiments. 

The simplest probe is the reference electrode. These are designed for exposure in concrete for installation with cathodic protection systems (Section 7.3). 

Galvanic cathodic protection systems have been used extensively since the early 1990s in Florida on prestressed concrete bridge support piles in the sea. One of the reasons the galvanic system is used there is because concrete resistivity is low due to the marine exposure conditions. The Florida systems frequently incorporate a distributed anode of zinc fixed on the atmospherically exposed concrete and bulk zinc anodes in the water which pass current through the low resistance sea water to protect the submerged area as shown in Figure 7.4. 

Originally specified with a 10-15 year performance, the oldest systems on the Midland Links are over 15 years old and will be in need of major maintenance within a 20 year life. Some areas have performed badly due to excessive water exposure. Some poor quality products have been used and in some cases inadequate surface preparation has given problems. On the M4 elevated section cathodic protection trial a water-based and a solvent-based coating were applied back to back . This trial should be evaluated after five years of operation. 

The advantage of calcinm nitrite is that it can be added to the mix and has no serions effects on the design, construction and performance of the strnctnre other than its effect as a set accelerator. Mix design may require adjusting to include a retarder. Its disadvantage is that there mnst be enough to stop corrosion and it is consnmed with its exposure to chlorides. It is therefore important to calcnlate the chloride exposure for the life of the structure and add snfficient inhibitor. It does not inhibit the application of cathodic protection or chloride extraction in later life of the strnctnre if necessary. 

Coupon Tests. A weighed metal coupon shaped to conform to the outside surface of a buried pipe is attached by a brazed connecting cable, and both the cable and surface between coupon and pipe are overlaid with coal tar. After exposure to the soil for a period of weeks or months, the weight loss, if any, of the cleaned coupon is a measure of whether cathodic protection of the pipeline is complete. 

The cathode material is carbon steel in diaphragm cells, and nickel, often with a catalytic coating in membrane cells. As discussed in Section 4.6.6, exposure to anolyte containing active chlorine (CI2. HOCl, and OCl ) without cathodic protection is the primary reason for the corrosion of these components, unless the cathode coating is pore-free and noble metal based. Another species contributing to the corrosion of iron and nickel is the hydroxyl ion in the catholyte. 

The cathodic protection on damaged galvanized items depends on (a) the coating thickness, the width of the bared area or scratch, and (c) the local climate. Allowance must be made for any extra loss of zinc due to cathodic protection effects or due to exposure of more zinc surfaces to the environment. It also has been shown that coatings of Al/Zn alloys alone offer no or only poor cathodic protection. More or less the same applies to duplex systems, where the paint coat covers the zinc (-q) layer, thus preventing or retarding the liberation of the necessary ions. 

The effectiveness of cathodic protection probably depends on the composition of the outer portion of the zinc layer. Pure zinc is more anodic than iron-zinc alloys, so bare steel can more readily be protected by zinc than by the alloy. The degree of cathodic protection depends largely on the exposure conditions. 

A special type of coating for steel is used where exposure to crude oil and seawater is encountered, as in oil tankers. This involves an inorganic paint containing zinc powder which gives cathodic protection against localized corrosion. This has already been discussed in connection with the uses of lithium silicates in Chapter 2. Some further points are as follows. 

FIG. 3—Potential profile on pipe subject to exposure from cathodic protection Interference [37]. (Reprinted from Corrosion. Copyright by NACE International. All Rights Reserved by NACE reprinted with permission.)... 

Cathodic protection is used widely for the protection of submerged steel in waterfront structures. It also can provide considerable benefit in the intertidal zone and can even reduce the usually high corrosion rate experienced at the boundary between the intertidal zone and the splash and spray zone. Cathodic protection also is used to prevent corrosion of the soil side of steel in marine structures such as sheet steel bulkheads. Cathodic protection also is effective in the control of the corrosion of reinforcing steel in concrete in all exposure zones in waterfiont structures. Particularly for impressed current systems, it is important to select materials for the cathodic protection system components such as rectifiers and junction boxes with consideration of the environment to which they will be exposed. When considering cathodic protection, periodic inspection and maintenance is required for proper system operation. The costs for inspection and maintenance must be considered in the overall cost of cathodic protection. While there are no specific standards for cathodic protection of piers and docks, information in NACE RP0176 (Corrosion Control of Fixed Offshore Platforms Associated with Petroleum Production) and NACE RP-0187 (Design Considerations for Corrosion Control of Reinforcing Steel in Concrete) contain information that is applicable to marine piers and docks. 

The steel XlOCrAl (1.4713) with about 7% Cr shows, under exposure in the immersion zone and in the tidal zone, pronounced rusting, shallow pit corrosion and crevice corrosion under bolt washers. The cathodic protection effect of iron or zinc anodes is also unsatisfactory [99]. 

Some of the samples were cathodically protected by contact with iron or zinc anodes. The corrosion potential was measured at intervals throughout the entire exposure duration and the depth of the crevice corrosion sites was determined. Table 67 shows the corrosion depth results in stagnating seawater and Table 68 shows the results in flowing seawater. 

Only the highly molybdeniferous materials Hastelloy C-276 (DIN-Mat. No. 2.4819), MP 35N (DIN-Mat. No. 2.4999), Inconel 625 (DIN-Mat. No. 2.4856) and TIA16V4 (DIN-Mat. No. 3.7165) proved completely corrosion-resistant under all test conditions. The materials Monel K-500 (DIN-Mat. No. 2.4375), Inconel 617 (DIN-Mat. No. 2.4663) and Incoloy 825 (DIN-Mat. No. 2.4858) also show sufficient resistance in stagnant seawater without cathodic protection. In all materials, cathodic protection with both iron and with zinc anodes in stagnant and flowing seawater reduced local corrosion to levels under 0.075 mm after exposure for 1.6 years. This also applies to the materials exposed to free corrosion conditions that were perforated after this time period. 

Coatings must be considered for aU applications of steel. Cathodic protection should be considered for steel pipe where soil or groundwater resistivity is less than 10,000 t2-cm, and where steel win be in contact with process streams. Cathodic protection of steel is strongly recommended where resistivity is less than 5000 Q-cm. For aU exposures, steel should be electrically isolated from dissimilar metals to prevent the formation of unfavorable galvanic corrosion ceUs. In areas where abrasive materials are hkely to damage coatings, cathodic protection by impressed current or galvanic anodes may be desirable. 

Bare or galvanized steel is subject to corrosion when exposed to aggressive fluids. Corrosion is most severe in the splash zone, where readily available oxygen hastens the corrosion process. Submerged steel shoiUd be coated with a material suitable for use in the anticipated exposure. Where there are concerns regarding the corrosion of steel in contact with process streams, cathodic protection should be provided for steel structures considered to be in a corrosive exposure. This type of corrosion control should be incorporated along with suitable coatings. 

Coating and cathodic protection should be considered for stainless steel in soil and water. Stainless steel may be used in most atmospheric exposures and may also be used as hardware for connection to steel. Stainless steel should not be used for complex structures with overlapping bolted connections in soil or fluid exposures. Bolted connections of this type in soil or fluid exposures can experience very rapid crevice corrosion. 

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