Welded joints, protection

In order to control the tightness of welded joints on various products and structures, a range of units and complexes for control of pipes, pipelines, tanks, protective casings were developed, in particular, a set of put-on vacuum chambers for sheet structures. 

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]. 

Therefore, the use of welding as a method of fabrication may modify the corrosion behaviour of an engineering structure, and this may be further aggravated by removal of protective systems applied before welding, whilst at the same time the use of such anti-corrosion coatings may lead to difficulties in obtaining satisfactorily welded joints ... 

Tubing with thicker walls, typically in the range of 0.080 to 0.170 in. (2 to 4 mm), is fabricated mainly from polyolefins and is used in to cover splices in telecom, CATV and electric-power industries. Often, such tubing is combined with mastic or hot melt that aids in forming an environmental barrier for the splice. Diameters of the heavy wall tubing may be up to 7 inches (178 mm) or even 12 to 24 inches (300 to 600 mm) when used as corrosion-protection sleeves on weld joints of gas and oil pipelines.92... 

A thermocouple is made by welding together at each end two wires made from different metals (Fig. 4.15). If one welded joint (called the hot junction) becomes hotter than the other joint (the cold junction), a small electrical potential develops between the joints. In IR spectroscopy, the cold junction is carefully screened in a protective box and kept at a constant temperature. The hot junction is exposed to the IR radiation, which increases the temperature of the junction. The potential difference generated in the wires is a function of the temperature difference between the junctions and, therefore, of the intensity of IR radiation falling on the hot junction. The response time of the thermocouple detector is slow thermocouples cannot be used as detectors for FTIR due to their slow response. 

As indicated in Table 10.18, a zinc-rich primer is often recommended. It can be an organic zinc-epoxy or an inorganic zinc-ethyl-silicate primer. Zinc-rich primers are also used as so-called shop primers, or prefabrication primers, for temporary protection of semi-manufactured steel goods. After fabrication, e.g. of welded steel structures, the shop primer surface must be cleaned (degreased), and possible shop primer defects and weld joints have to be blast cleaned and coated with a primer before the whole structure is painted. Iron oxide is also used as a pigment in some shop primers. These must not be overpainted with a zinc-rich paint. 

Where pipelines are laid in sections welded in the field, the area around the welded joint must be protected against corrosion in the same way as the rest of the pipe sections. While coating of these areas must be carried out in the field, it is important that the protection at these sites should be comparable with that of the factory-applied coating, in order not to produce weak spots in the system. 

Poly- vinylchloride (PVC) - Resistant to oils and water - Not resistant to fuels - Requires protective layer - Potential hidden problems around seals and penetrations - Base ground to be prepared well, ie remove stones, requires a layer of gravel and sand/ geotextile before the liner - Requires specialist installer to weld joints - Very Low - Burns readily if unprotected Medium... 

Axial movement may be a cause for pipe restriction. When this is the case, it is common to apply split collars around the outside diameter of the pipe with solvent-welded joints to protect the pipe from contact with concrete. It is recommended that the solvent-welded joints between the collars and the pipe exterior be allowed to dry for at least 48 hours prior to pouring concrete in the area. 

The values calculated using Eqs. (3-36) and (3-37) are only true for welded pipelines. Extension joints, fittings, and screwed or caulked joints can raise the longitudinal resistance of a pipeline considerably and therefore must be bridged over for cathodic protection. 

Two possible forms of membrane are hot applied mastic asphalt or bitumen/butyl rubber sheeting with welded or glued joints. The membrane under the floor slabs has to be lapped with that around the walls. It is essential that the membrane is protected during construction, and a typical arrangement is as shown in Figure 6.15. 

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