Welds crevice corrosion

Welds and mechanical joints can be areas where accelerated corrosion occurs for several reasons. Field coatings applied to joints are sometimes not of the same quality as shop-applied coatings on the remainder of the pipeline. This can lead to the environment penetrating to the metal and the formation of corrosion cells, Mechanical joints and rough welds can lead to crevice corrosion. Weld metallurgy also plays a role in corrosion. Welds containing high levels of sulfide inclusions can lead to localized corrosion of the weld metal. 

Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

In general, the higher the residual or applied metal stress, the more severe the corrosion at a given acidic pH. This explains why many heat exchanger tube ends are often attacked so severely (Fig. 7.1). Tube ends that have been rolled or welded often contain high residual stress. Further, crevices are sometimes present in which acidic species may concentrate (see Chap. 2, Crevice Corrosion ). Screens, rolled sheet metal, and other highly worked metals (not stress relieved) are also prone to attack. 

General description. In incomplete fusion, complete melting and fusion between the base metal and the weld metal or between individual weld beads does not occur (Fig. 15.8). Incomplete fusion that produces crevices or notches at surfaces can combine with environmental factors to induce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). See Fig. 15.9. 

Figure 15.9 Cross section of stainless steel weld showing crevice corrosion along a site of incomplete fusion. (Magnification 15x.)...

In addition to the form of attack described above, sensitized welds are prone to pitting, stress-corrosion cracking in certain environments (see Chap. 9), and crevice corrosion (see Chap. 2). 

For tanks and vessels, welded units are preferred over riveted or bolted designs. Fastener joints provide sites for crevice corrosion. Undrainable horizontal flat tops of tanks should be avoided unless proper drainage schemes are included in the design. Tank bottoms should be sloped toward... 

Most fabrication techniques have implications for corrosion performance. Riveted and folded seam constmction creates crevices as shown in Figure 53.8. Those materials that are susceptible to crevice corrosion should be fabricated using alternative techniques (e.g. welding). Care should be taken to avoid lack of penetration or lack of fusion, since these are sites for crevice corrosion to initiate. 

The second degree of freedom is to design-out crevices where possible, although it must be remembered that crevice corrosion can go on underneath deposits. Crevice corrosion at a butt weld with incomplete root penetration is a common case (Fig. 9.7a). Where internal inspection is not possible and crevice corrosion is recognised as likely, A"-radiography of each weld can be specified. 

Weld corrosion (Section 9.5) Crevice corrosion at butt welds due to poor penetration has already been discussed and was shown in Fig. 9.1(a). Conversely, if there is a large weld bead protruding in the pipe bore, erosion/cor-rosion can occur downstream due to the turbulence produced over the weld bead (Fig. 9.8). In either case, the fault probably lies in the incorrect spacing of the butts at welding. 

It is obvious that these physical defects are dangerous in their own right but it is also possible for them to lead to subsequent corrosion problems, e.g. pitting corrosion at superficial non-metallic inclusions and crevice corrosion at pores or cracks. Other weld irregularities which may give rise to crevices include the joint angle, the presence of backing strips and spatter (Fig. 9.29). Butt welds are to be preferred since these produce a crevice-free profile and, furthermore, allow ready removal of corrosive fluxes. 

Socket welded joints (para. GR-3.4.7) should be avoided where crevice corrosion or severe erosion may occur. 

The susceptibility of the piping material to crevice corrosion under backing rings, in threaded joints, in socket-welded joints, and in other stagnant, confined areas. 

All joints, including deck plate to structural members, shall be continuously seal-welded to prevent crevice corrosion. Stitch welding, top or bottom, is unacceptable. 

Nickel-based alloys withstand chlorides in the feed better than does stainless steel but under extreme conditions, pitting, SCC and crevice corrosion is present [19, 20, 21]. The corrosion takes place in non-annealed structures in the as welded condition. Performance of these alloys may be enhanced by post-weld solution annealing [22]. 

The nature of corrosion depends critically on both the environment and the material. Often corrosion is localized in particular areas because of non-uniformities in the material (near welds or corners) or non-uniformities in the environment such as shielded areas near gaskets (crevice corrosion). 

In the Uhde [986], [987] and Steinmiiller [989] concept the tubesheet is anchored to and supported by the tubes to withstand the differential pressure, which imposes some restriction on the tube length. Babcock-Borsig s [988], [990], [991] tubesheet is reinforced by stiffening plates on the back side (Figure 100). Both solutions have full-penetration tube-to-tubesheet welds for the tubes to prevent crevice corrosion. Struthers [992] reduces stress by making the tubesheet-to-shell connection flexible. 

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