Requirements for Corrosion Protection

To protect against corrosion, a good paint should meet the following requirements  

For zinc chromate, the inhibiting ion is CrOj, solubility relations being just right to release at least the minimum concentration of the ion ( 10 mole/liter) for optimum inhibition of steel. The solubility of zinc tetroxychromate is reported to be 2 X 10 mole/liter [3]. Lead chromate, on the other hand, is not nearly 

The minimum amount of zinc dust pigment required to provide cathodic protection depends on several factors, including Zn particle size, nature of the vehicle, and the amount of ZnO and other pigments that may be present [6], It probably also depends on the extent to which insulating coatings form on zinc particles before the paint is applied (age of paint). 

Provide Long Life at Low Cost. A reasonable cost of paint should be gauged by its performance. A paint system lasting 5 years justifies double the cost of paint if the more expensive paint provides 35% longer life or lasts short of 7 years (labor to paint cost ratio of 2 1). 

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Six iron anodes are required for corrosion protection of each condenser, each weighing 13 kg. Every outflow chamber contains 14 titanium rod anodes, with a platinum coating 5 /tm thick and weighing 0.73 g. The mass loss rate for the anodes is 10 kg A a for Fe (see Table 7-1) and 10 mg A a for Pt (see Table 7-3). A protection current density of 0.1 A m is assumed for the coated condenser surfaces and 1 A m for the copper alloy tubes. This corresponds to a protection current of 27 A. An automatic potential-control transformer-rectifier with a capacity of 125 A/10 V is installed for each main condenser. Potential control and monitoring are provided by fixed zinc reference electrodes. Figure 21-2 shows the anode arrangement in the inlet chamber [9]. 

The specification of these later coatings is covered by the German DIN 30670 standard for steel tubes and DIN 30674 for ductile iron pipes. These standards note that some 1 mm thickness of polyethylene is required for corrosion protection alone, but to improve the mechanical load-bearing capacity of the coating, total thicknesses of 1.8-3.0 mm, depending on pipe diameter, are to be specified. 

They must meet the same requirements for corrosion protection that apply to new USTs. 

Next, the liquor is pumped to the NH3-still preheater. At this point, the sulfide solution required for corrosion protection is added. The preheated process stream enters a stripper where excess C02 is removed prior to treatment with lime and distillation removal of the NH3. The hot liquor from the stripper flows to an agitated vessel, to which milk of lime is added to release NH3 ... 

A fully equivalent substitution for chromates has, to date, still not been found, however, and most of the comparative smdies of chromate-free corrosion protection solutions have led unfortunately, but undoubtedly, to the conclusion that chromates provide outstandingly better corrosion resistance than any other coatings yet known. In some industry branches, dire to the very high requirements for corrosion protection performance (for instance in aircraft), the use of chromates has been temporarily allowed (Velterop, 2003) whilst experts continue their intense efforts to develop a method of corrosion protection technology which is equivalent to or even better than the rrse of chromirrm compoimds. 

The principle of electrochemical corrosion protection processes is illustrated in Figs. 2-2 and 2-5. The necessary requirement for the protection process is the existence of a potential range in which corrosion reactions either do not occur or occur only at negligibly low rates. Unfortunately, it cannot be assumed that such a range always exists in electrochemical corrosion, since potential ranges for different types of corrosion overlap and because in addition theoretical protection ranges cannot be attained due to simultaneous disrupting reactions. 

All organic coatings show varying degrees of solubility and permeability for components of the corrosive medium, which can be described as permeation and ionic conductivity (see Sections 5.2.1 and 5.2.2). An absolute separation of protected object and medium is not possible because of these properties. Certain requirements have to be met for corrosion protection, which must also take account of electrochemical factors [1] (see Section 5.2). 

Nodal points of the platform require special attention for corrosion protection. Therefore the anodes have to be installed in the vicinity of these points, as indicated in Fig. 16-4. The spacing must be sufficiently large that the welded Joints of the nodes do not lie in the area of the lap Joints. The effort for calculating the optimal distribution with the lowest weight of anodes is considerable and has led to computer programs by which the anode distribution can be estimated [11]. 

Figure 20-9 shows the negative effect of uninsulated heating elements on corrosion protection. In a 250-liter tank, an electric tube heating element with a 0.05-m surface area was screwed into the upper third without electrical separation, and in the lower third a tinned copper tube heat exchanger with a 0.61 -m surface area was built in. The Cu heat exchanger was short-circuited for measurements, as required. For cathodic protection, a potential-controlled protection system with impressed current anodes was installed between the two heating elements. The measurements were carried out with two different samples of water with different conductivities. 

When coolers or condensers are shut-down but remain full of water, the amount of current required to maintain satisfactory cathodic protection is considerably reduced. If the current is not reduced over-protection occurs and excessive amounts of chlorine can be generated which would tend to accumulate in the upper section of the water boxes causing considerable corrosion, not only to the water boxes, but also possibly to the tubes. To ensure against this a stand-by condition should be included on the control panel which effectively reduces the current required under shut-down conditions. This control is effected by a limit switch fitted to the outlet valve of the condenser or cooler concerned. It is impossible to determine exact requirements for the protection of circulating water systems in advance and it is normal to adjust the current to provide protection during commissioning. 

Thermoplastics which are used for corrosion protection can be applied in coatings as thin as 0.025 mm by solution techniques and in excess of 5 mm by extrusion or plastisol dipping. They are used where environmental resistance, chemical resistance, abrasion resistance, sound deadening or cushioning are required. They are used in those market areas that necessitate metallic mechanical strength plus thermoplastic corrosion resistance. 

The modular principle is also frequently used in the design of the extruder screw. The screw comprises individual screw elements, which are threaded onto a shaft. The elements are positively fitted to the shaft to transmit the required torque. They are also attached together and axially clamped. The elements of a screw module are typically made of nitrided steel. Special designs are required for wear protection and corrosion protection depending on the respective requirements ... 

More efficient coal utilization can be realized with combined power plant cycles. For instance, the post combustion gases of a conventional combustor or an advanced MHD system can be further utilized to drive a gas or steam turbine. However, the sustained durability of downstream turbine or heat exchanger components requires minimal transport of corrosive fuel impurities. Control of mineral-derived impurities is also required for environmental protection. For the special case of open cycle-coal fired MHD systems, the thermodynamic activity of potassium is much higher in the seeded combustion gas (plasma) than in common coal minerals and slags. This results in the loss of plasma seed by slag absorption and is of critical concern to the economic feasibility of MHD. 

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