Local corrosion tubes

Note that localized corrosion having the appearance illustrated in Figs. 12.18 through 12.20 could be associated with brief exposure to a strong acid. In this case, however, all available information indicated that the tubes had never been exposed to an acid of any type. Cavitation was caused by high-frequency vibration of the tubes. The vibration apparently induced a threshold cavitation intensity such that rough or irregular surfaces produced cavitation bubbles, and smooth internal surfaces did not. 

Dents in tubing can induce erosion failures, especially in soft metals such as copper and brass. Welding and improper heat treatment of stainless steel can lead to localized corrosion or cracking through a change in the microstructure, such as sensitization. Another form of defect is the inadvertent substitution of an improper material. 

Microstructural examinations revealed V-shaped openings along the tube seam, some extending into as much as 50% of the tube wall thickness. The incompletely closed seam provided a crevice in which differential concentration cells developed (see Chap. 2, Crevice Corrosion ). The resulting localized corrosion caused the observed pits. 

Chemical removal of deposits and corrosion products revealed the appearance of the groove (Fig. 14.5). The crevice formed by the incompletely fused weld seam fostered the establishment of differential concentration cells (see Chap. 2). This resulted in localized corrosion and eventual perforation through the greatly thinned tube wall at the bottom of the crevice. The tubercle, which is composed of corrosion products, is a simple result of the corrosion process occurring locally within the crevice. 

Oxygen corrosion, occurring as General etch corrosionAmiform rate corrosion (less common form) Localized corrosion (takes several forms and common where waterside conditions are less than ideal) Rare in correctly treated boilers but can affect drum waterline and tubes. More common in idle and low-load boilers, where water chemistry is unbalanced, under high MU conditions and after poor chemical cleaning. Also in peak-load boilers, especially where deposition can occur. Localized corrosion can be very serious, causing metal failure. 

Additional means of reducing corrosion were the use of prepared steel laths, which were introduced into the FW system and would preferentially rust, leaving boiler tubes unscathed. Sacrificial zinc sheet anodes also were employed, but neither method ensured complete protection of the overall boiler system, and uniform general corrosion was often replaced by insidious localized corrosion. 

In two test rigs materials like Inconel 625, Haynes 214, Hastelloy C-276, Nicrofer 5923 and 6025 were tested in aqueous solutions containing 0.5 mol/kg oxygen and 0.05 to 0.5 mol/kg HC1. One of the rigs offers the possibility to test five tubes in parallel at pressures between 240 to 400 bars and at temperatures up to 600 °C. Practically all types of material showed heavy local corrosion after some tens of hours. More encouraging results were obtained for oxide ceramics, small samples of which are presently under test. 

Remarkably enough, statements of high chemical stability under harsh corrosive environments such as pH < 3 or pH > 9 are not substantiated in the literature by reliable measurements on membrane systems and much more work is needed here. Problems that occur with the bursting pressure of support tubes after long-term usage might indicate local corrosion at the contact points between the ceramic particles making up the microstructure. 

Organic matter in the water may affect corrosion in different ways. Substances that exist suspended or dissolved may be deposited and form more or less voluminous layers in regions with low flow velocity. This may give a reduced average corrosion rate, but increased and sometimes heavy localized corrosion, i.e. deposit corrosion. In hot tubes, such layers may also result in local overheating and crack formation. 

During 2012, after approx. 3 years in operation, economizer tubes ruptured/thiimed out at inlet in all the four HRSGs. Figure. 3 4 shows ruptured/leaky tube location at inlet header. Figure. 5 6 shows close view of ruptured tube. Figure. 7 shows cut section of localized corrosion at tube bends. 

EVS were put to work on real systems on several occasions in the oil and gas industries for two main reasons. The 1st reason was the critical nature of many operations associated with the transport of gas and other petroleum products, and the 2nd is the predictability of localized corrosion of steel, the main material used by the oil and gas industry. Meany has, for example, reported four detailed cases where extreme value distribution proved to be an adequate representation of corrosion problems in underground piping and power plemt condenser tubing [15]. In another study, data from water injection pipeline systems and from the published literature were used to simulate the sample functions of pit growth on metal surfaces [Id]. It was concluded that maximum pit depths were adequately characterized by extreme value distribution, that a Gaussian distribution could model corrosion rates for water injection systems, and that an exponential pipeline leak growth model was appropriate for all operation regimes. 

Corrosion problems in nuclear plants have almost always been associated with localized corrosion phenomena or environmentally assisted cracking. For example, steam generator tubes in pressurized water reactors (PWRs) have experienced wastage and thinning, SCC, intergranular attack. 

Copper alloys (Admiralty, copper-nickel) and austenitic stainless steels are the most commonly used materials for feedwater heater tubing based upon their resistance to general and localized corrosion, erosion-corrosion, and SCC, and adequate heat transfer performance [1,2]. Carbon and low-alloy steels are most often used for the shells of such heaters for economy and availability. 

Local weistage of the stainless steel cladding from composite tubes at ports in the waterwall (particularly primary and secondary air ports) has been attributed to hydroxide condensation [223,224]. The localized corrosion of stainless steel cladding from composite tubes at air ports of recovery boilers has been shown by deposit analysis to be a cycUcal mechanism. Laboratory tests in molten salt supported the theory that this corrosion is caused by molten NaOH [225]. 

Brushing and chemically cleaning the tube to examine tube surface for general and localized corrosion. 

They can be used to measure the thickness of the wall of a pipe and situate pitting or local corrosion in two dimensions. Possible applications for probing tube bundle heat exchangers (e.g. the Probolog). 

An example of shallow-pit corrosion is given in Fig. 1-34. This corrosion damage was observed in an open recirculation cooling system after 18 months of operation. The tube, made of structural steel (St 37), showed several local attacks by pitting with a local corrosion rate of up to 3 mm year" . Iron sulfide was detected in the corrosion products. This indicates the presence of anaerobic sulfate-reducing bacteria (SRB). Under strictly anaerobic conditions the bacteria reduced the sulfates to sulfides leading to enhanced corrosivity of the medium (Weber and Knopf, 1994). 

Figure 1-34. Localized corrosion of a structural steel tube by SRB.

Caustic corrosion of unalloyed and low-alloy steel is encountered in some unusual situations. For example, in boilers traces of sodium hydroxide can become concentrated and cause local corrosion and caustic embrittlement. This occurs usually in boiler tubes that alternate between wet and dry conditions or in which deposits form. Boiler feed water permeates the deposits and evaporates. This causes concentration of the caustic material, to up to several percent, which is enough to destroy the protective magnetite and/or to initiate caustic embrittlement (Effertz et al., 1982 Hersleb, 1982). 

In situ pH and corrosion potential Boiler tubes Susceptibility to general and localized corrosion... 

An insidious aspect of carburization is its nonuniform nature. Just as for other forms of localized corrosion, it is extremely difficult to predict and model localized carburization damage. As a rule of thumb, carburization problems only occur at temperatures above 815°C, because of unfavorable kinetics at lower temperatures. Carburization is therefore not a common occurrence in most refining operations because of the relatively low tube temperatures of most refinery-fired heaters. 

Localized corrosion frequently observed in oil-well tubing in which a circumferential attack is observed near a region of metal upset. ... 

---