Corrosion control structures

The Institute has many-year experience of investigations and developments in the field of NDT. These are, mainly, developments which allowed creation of a series of eddy current flaw detectors for various applications. The Institute has traditionally studied the physico-mechanical properties of materials, their stressed-strained state, fracture mechanics and developed on this basis the procedures and instruments which measure the properties and predict the behaviour of materials. Quite important are also developments of technologies and equipment for control of thickness and adhesion of thin protective coatings on various bases, corrosion control of underground pipelines by indirect method, acoustic emission control of hydrogen and corrosion cracking in structural materials, etc. 

Cathodic Protection This electrochemical method of corrosion control has found wide application in the protection of carbon steel underground structures such as pipe lines and tanks from external soil corrosion. It is also widely used in water systems to protect ship hulls, offshore structures, and water-storage tanks. 

Thorough assessment of the service environment and a review of options for corrosion control must be made. In severe, humid environments it is sometimes more economical to use a relatively cheap structural material and apply additional protection, rather than use costly corrosion-resistant ones. In relatively dry environments many materials can be used without special protection, even when pollutants are present. 

A wide variety of protective coating types and systems is available for corrosion control on external and internal surfaces of structural and process plant in marine and offshore engineering. These are discussed in detail elsewhere in this text, and the purpose here is to highlight the critical importance of certain design and related operational aspects which affect both the selection and performance of protective coating systems. The following design considerations should be made ... 

Table 0.1 shows the enormous scope of corrosion control, and serves to emphasise the fact that it is just as important to avoid certain features in the design of a structure as to apply a particular protective scheme, and it is also apparent from Method I that many of the factors that determine the choice of a metal or a particular protective scheme are outside the realm of metallic corrosion. 

Finally, it is necessary to point out that although a particular method of corrosion control may be quite effective for the structure under consideration it can introduce unforeseen corrosion hazards elsewhere. Perhaps the best example is provided by cathodic protection in which stray currents (interaction) result in the corrosion of an adjacent unprotected structure or of steel-reinforcement bars embedded in concrete a further hazard is when the cathodically protected steel is fastened with high-strength steel bolts, since cathodic protection of the tatter could result in hydrogen absorption and hydrogen cracking. 

Protective coatings of paint or plastic resins are a familiar means of corrosion. control by cutting off access of water, air, and electrolytes to the structure. The coating must, however, be complete and remain intact to be protective. Local penetration of the coating will generally create an... 

Phosphonates are surface-active chelants and are widely used in industry for scale and corrosion control, and in cleaning agents, dispersants, and other applications. The annual sales of phosphonates in the U S A amount to over two billion dollars per year [8]. Phosphonates inhibit calcium scale formation by interfering with the crystal structure of calcium salts, making the crystals softer so that they are more easily removed by washing - the threshold effect [9]. A key application is controlling scale particularly in hard-to-reach places such as laundry and dishwashing machines. 

These types of polymers are similar to phosphonates and some actually are phosphonates, despite attempts to confuse the issue by the use of alternative names. They tend to exhibit varying degrees of both deposit-control and corrosion-control properties. Several PCA polymers were introduced in the 1980s and further developed in the 1990s and were termed phosphinocarboxylic acids. Most were, in fact, acrylic acid/organic phosphate polymers, but with a C P C bond (the phosphonates have a C P O bond). In the 1990s some low phosphorus content phosphonocarboxylic acid PCAs have been introduced. Structures for the different chemical PCAs, types 16 and 4, are shown in Figure 5.5. 

The polymeric materials are used in (i) barrier applications (ii) self-supporting structures (tanks, piping, valves and pumps) (ii) column internals, seals, gaskets, adhesives and caulkings. An overview of polymeric materials used in corrosion control applications is given in Figure 4.8. 

Silicate Modifications. A method has been described in which silicate minerals are simultaneously acid-leached and trimethylsilyl end-blocked to yield specific trimethylsilyl silicates having the same silicate structure as the mineral from which these were derived (12). Olivine, hemimorphite, sodalite, natrolite, laumontite, and sodium silicates are converted to TMS derivatives of orthosilicates, pyrosilicates, cyclic polysilicates, etc, making it possible to classify the minerals according to their silicate structure. The same technique is used to analyze the siloxanol structure of aqueous solutions of vinyltrimethoxysilane (13). Certain anionic siliconates stabilize solutions of alkali silicates to give stable solutions in water or alcohols at any pH (14). Such silicate—siliconate mixtures are used as corrosion inhibitors in glycol antifreeze (15) (see Antifreezes and deicing fluids Corrosion and CORROSION CONTROL). 

Corrosion control is one of the demonstrated uses of silicates in detergents since builders may be imagined as breaking the structural integrity (and increasing the rate of corrosion) of the hydrous oxide films that protect metals such as aluminum in contact with water. Other metals (e.g. zinc) and combinations of materials are sometimes attacked by alkaline detergents. 

A. Sagues, R. G. Powers, Corrosion and corrosion control of concrete structures in Florida. What can be learned , Proc. Int. Conf. Repair of Concrete Structures. From Theory to Practice in a Marine Environment, Svolvear (Norway), 28-30 May 1997, p. 49. 

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