Corrosion anodic protection

Riggs O L and Locke C E 1981 Anodic Protection Theory and Practice in the Prevention of Corrosion (New York Plenum)... 

Anodes. Lead—antimony (6—10 wt %) alloys containing 0.5—1.0 wt % arsenic have been used widely as anodes in copper, nickel, and chromium electrowinning and metal plating processes. Lead—antimony anodes have high strength and develop a corrosion-resistant protective layer of lead dioxide during use. Lead—antimony anodes are resistant to passivation when the current is frequendy intermpted. 

Plate and frame coolers using HasteUoy C-276 plates have been used successfuUy. Anodically protected plate coolers are available as weU as plate coolers with plates welded together to minimize gasketing. Another promising development is the introduction of plate coolers made of HasteUoy D205 (105). This aUoy has considerably better corrosion resistance to concentrated sulfuric acid at higher temperatures than does C-276. Because of the close clearance between plates, cooling water for plate coolers must be relatively clean. 

Anodic Inhibitors. Passivating or anodic inhibitors produce a large positive shift in the corrosion potential of a metal. There are two classes of anodic inhibitors which are used for metals and alloys where the anodic shift in potential promotes passivation, ie, anodic protection. The fkst class includes oxidking anions that can passivate a metal in the absence of oxygen. Chromate is a classical example of an oxidking anodic inhibitor for the passivation of steels. 

Fatigue life can be slightly lengthened by anodic protection or by passivation. In acids even passive stainless CrNi steels suffer corrosion fatigue [104]. Resistance can occur if the passive film itself has a fatigue strength (e.g., in neutral waters [105]). 

The fundamentals of this method of protection are dealt with in Section 2.3 and illustrated in Fig. 2-15. Corrosion protection for the stable-passive state is unnecessary because the material is sufficiently corrosion resistant for free corrosion conditions. If activation occurs due to a temporary disturbance, the material immediately returns to the stable passive state. This does not apply to the metastable passive state. In this case anodic protection is necessary to impose the return to the passive state. Anodic protection is also effective in the unstable passive state of the material but it must be permanently switched on, in contrast to the metastable passive state. 

A particular problem arises in the anodic protection of the gas space because the anodic protection does not act here and there is a danger of active corrosion. Thus these endangered areas have to be taken account of in the design of chemical... 

In sulfuric acid production involving heat recovery and recovery of waste sulfuric acid, acids of various concentrations at high temperatures can be dealt with. Corrosion damage has been observed, for example, in sulfuric acid coolers, which seriously impairs the availability of such installations. The use of anodic protection can prevent such damage. 

Carbon steels can be anodically protected in certain salt solutions. This involves mainly products of the fertilizer industry such as NH3, NH4NO3 and urea. Anodic protection is effective up to 90°C [26]. Corrosion in the gas space is suppressed by control of pH and maintenance of a surplus of NH3. 

Stress corrosion can arise in plain carbon and low-alloy steels if critical conditions of temperature, concentration and potential in hot alkali solutions are present (see Section 2.3.3). The critical potential range for stress corrosion is shown in Fig. 2-18. This potential range corresponds to the active/passive transition. Theoretically, anodic protection as well as cathodic protection would be possible (see Section 2.4) however, in the active condition, noticeable negligible dissolution of the steel occurs due to the formation of FeO ions. Therefore, the anodic protection method was chosen for protecting a water electrolysis plant operating with caustic potash solution against stress corrosion [30]. The protection current was provided by the electrolytic cells of the plant. 

Six caustic soda evaporators were anodically protected against stress corrosion in the aluminum industry in Germany in 1965 [27]. Each evaporator had an internal surface area of 2400 m. The transformer-rectifier had a capacity of 300 AJ 5 V and was operated intermittently for many years. Automatic switching on of the protection current only took place in case of need when the drop in potential reached... 

The safety, availability and capacity of production plants are predetermined by the quality of the materials and the corrosion protection measures in the essential areas both are major considerations in the initial planning. Even today, damage to equipment and tanks is often assumed to be unavoidable and the damaged components are routinely replaced. By carrying out damage analysis, which points the way to knowledge of prevention of damage, the availability and life of plants can be increased considerably. This particularly applies to the use of anodic protection. 

The use of corrosion-resistant materials and the application of corrosion protection measures are in many cases the reason that industrial plants and structures can be built at all. This is particularly so in pipeline technology. Without cathodic protection and without suitable coating as a precondition for the efficiency of cathodic protection, long-distance transport of oil and gas under high pressures would not be possible. Furthermore, anodic protection was the only protective measure to make possible the safe operation of alkali solution evaporators (see Section 21.5). 

Spiral-plate exchangers are fabricated from any material that can be cold worked and welded. Materials commonly used include carbo steel, stainless steel, nickel and nickel alloys, titanium, Hastelloys, and copper alloys. Baked phenolic-resin coatings are sometimes applied. Electrodes can also be wound into the assembly to anodically protect surfaces against corrosion. 

Anodic Protection-a technique for reducing corrosion of a metal surface via passing sufficient anodic current to it to cause its electrode potential to enter into the passive state. 

The corrosion resistance of unalloyed titanium in hydrochloric or sulfuric acids can be increased significantly by anodic protection, which maintains the oxide film so that the corrosion will be negligible even in severely reducing conditions. 

To protect stainless-steel equipment from chloride stress-corrosion cracking by triggering an anodic protection system when the measured potential falls to a value close to that known to correspond to stress-corroding conditions. 

To trigger off an anodic protection system for stainless-steel coolers cooling hot concentrated sulphuric acid when the potential moves towards that of active corrosion. 

Before considering the principles of this method, it is useful to distinguish between anodic protection and cathodic protection (when the latter is produced by an external e.m.f.). Both these techniques, which may be used to reduce the corrosion of metals in contact with electrolytes, depend upon the electrochemical mechanisms that result from changing the potential of a metal. The appropriate potential-pH diagram for the Fe-H20 system (Section 1.4) indicates the magnitude and direction of the changes in the potential of iron immersed in water (pH about 7) necessary to make it either passive or immune in the former case the stability of the metal depends on the formation of a protective film of metal oxide (passivation), whereas in the latter the metal itself is thermodynamically stable and egress of metal ions from the lattice into the solution is thus prevented. 

A further difference is that in anodic protection the corrosion rate (passivation current density) will always be finite, whereas ideally a completely cathodically protected metal should not corrode at all. Raising the potential... 

The majority of the applications of anodic protection involve the manufacture, storage and transport of sulphuric acid, more of which is produced world-wide than any other chemical. Oleum is 100% sulphuric acid containing additional dissolved sulphur trioxide. The corrosion rate of steel in 77-100% sulphuric acid is 500-1 000 my" at 24°C and up to 5 000 my at 100°C which indicates the necessity for additional protection. 

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