Reactions Metallic Piping

The reduction of aqueous chlorine (HOC1) to chloride by Fe° and other ZVMs [Eq. (5)] has long been known as a major contributor to the decay of residual chlorine disinfectant during distribution in drinking water supply systems that contain metal pipes (e.g., Ref. 82). This reaction can, however, be turned to advantage for the removal of excess residual chlorine, and a variety of proprietary formulations of granular ZVMs are available commercially for this purpose (e.g., KDF Fluid Treatment, Inc. Three Rivers, MI). This application is sometimes called dechlorination, but should not be confused with the dechlorination of organic contaminants, which is discussed below. 

CAUTION Sodium azide is highly toxic and environmentally damaging. There is also a possibility of forming sensitive explosive azides when certain metals - copper, mercnry and, possibly, zinc - are used to catalyse some of the following reactions, and also if they are flushed into drains where they can react with metal pipes. 

Buried metal pipes can be protected from corrosion by connecting them to blocks of metal such as magnesium, called sacrificial anodes, which corrode in preference to the pipe. A cell made from the redox couples Fe/ Fe + and Mg/Mg is a laboratory representation of a sacrificial anode, (a) Draw the cell diagram [not shown in the answers at the end of this book], (b) write the cell half-reactions and the cell reaction and (c) calculate the standard cell voltage. 

The rapid reaction of CIF and BrF with metals is the basis of the commercial use in cutting pipe in deep oil weUs (64—68). In this appHcation, the pipe is cut by the high temperature reaction of the halogen fluoride and the metal. 

General description. Porosity refers to cavities formed within the weld metal during the solidification process. Such cavities may form due to decreased solubility of a gas as the molten weld metal cools or due to gas-producing chemical reactions within the weld metal itself. At times, cavities can form a continuous channel through the weld metal (worm holes, piping), resulting in leaks (Case History 15.3). 

Corrosion of steel by carbonic acid is probably the most common problem in the post-boiler section, producing pipe grooving and general metal wastage, especially in threaded joints. This form of corrosion is not self-regulating and the reaction products can produce more carbon dioxide, thus perpetuating the corrosion problem. Typically, the condensate pH level is depressed to around 5.0 to 5.5. 

Tin finds widespread use because of its resistance to corrosion, or as foil or to provide protective coats/plates for other metals. Properties of lead which make industrial application attractive surround its soft, plastic nature permitting it to be rolled into sheets or extruded through dies. In the finely-divided state lead powder is pyrophoric in bulk form the rapidly-formed protective oxide layer inhibits further reaction. It dissolves slowly in mineral acids. Industrial uses include roofing material, piping, and vessel linings, e.g. for acid storage. 

In order to investigate the dependence of a fast reaction on the nature of the metal, Iwasita et al. [3] measured the kinetics of the [Ru(NH,3)6]2+/3+ couple on six different metals. Since this reaction is very fast, with rate constants of the order of 1 cm s-1, a turbulent pipe flow method (see Chapter 14) was used to achieve rapid mass transport. The results are summarized in Table 8.1 within the experimental accuracy both the rate constants and the transfer coefficients are independent of the nature of the metal. This remains true if the electrode surfaces axe modified by metal atoms deposited at underpotential [4]. It should be noted that the metals investigated have quite different chemical characteristics Pt, and Pd are transition metals Au, Ag, Cu are sd metals Hg and the adsorbates T1 and Pb are sp metals. The rate constant on mercury involved a greater error than the others... 

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