101 Boiler and HRSG Tube Failures 101 BOILER AND HRSG TUBE FAILURES LESSON 2: Corrosion Fatigue R. Barry Dooley and Albert Bursik INTRODUCTION Fatigue damage occurs in general when a boiler tube is subject to repeat cyclic or fluctuating loading although the stress produced is below the material yield strength. The types of fatigue damage include, e.g., corrosion, thermal, mechanical, vibration, and creep fatigue. It is important to determine which form of fatigue is active, because measures to avoid repeat failures differ as the case arises. In this lesson, the focus is exclusively on corrosion fatigue. Corrosion fatigue occurs by the combined synergistic actions of cyclic loading and a corrosive environment. It is a discontinuous process with crack initiation and growth during transient periods. The excessive stresses may be caused during boiler operation by the restraint at tube attachments and by load changes (in particular during cold starts or forced cools) or during shutdown or restart of circulation boilers by thermal stratification of water along the tube length. Poor water chemistry and its excursions influence both initiation and propagation of corrosion fatigue. The key issue here is the breakdown of the protective magnetite layer. The most decisive chemistry parameter is the ph (low ph excursions). FEATURES OF FAILURES Figure 1 shows a typical multiple array of corrosion fatigue cracks initiated from the inside surface along the neutral axis of an economizer tube. The initiation sites of these cracks are associated with surface defects like pits or other discontinuities. The wide cracks have irregular profiles and are filled with iron oxides. Figure 1: Multiple array of corrosion fatigue cracks. IDENTIFICATION Pinhole thick-edged leaks are by far the most predominant form of corrosion fatigue failures. In much fewer cases, corrosion fatigue emanates as a long thick-edged crack. Note that not all BTF with a thick-edged fracture surface result from corrosion fatigue. Thick-edged fractures also occur when thermal fatigue, mechanical fatigue, low temperature creep cracking, circumferential cracking, and hydrogen damage are active. The most important physical feature of a corrosion fatigue failure is multiple parallel cracks initiated on the inside of the tube. Upon metallurgical examination the cracks are transgranular as they propagate through the tube wall. In Figure 2, corrosion fatigue failure of a low pressure economizer tube of an HRSG is depicted. Corrosion fatigue cracks may have different appearances: pinhole leak thick-edged crack thick-edged blow-out or rupture The pinhole leak caused by corrosion fatigue may be confused with a mechanical fatigue crack. In contrast to corrosion fatigue, the mechanical fatigue cracks initiate on the outside surface and are associated with welds or weld discontinuities (e.g., the toe of a weld). 586
101 Boiler and HRSG Tube Failures Figure 2: Corrosion fatigue of a low pressure economizer tube (HRSG). Thick-edged cracks are generally associated with attachments. However, they may be of considerable length and extend beyond the attachment. The thick-edged rupture is characterized by cracking down both sides of the tube along the weld lines of the membrane. This relatively rarely occurring form may cause catastrophic damage (an entire tube section fails) and is a serious safety issue if it occurs on the cold side of the tube and in high traffic areas. LOCATION OF FAILURES Conventional Boilers In waterwalls, the predominant locations are those at which large stresses develop during transient operating conditions as thermal expansion is constrained by tube attachments (Figure 3). Typical locations include attachments in windbox casing, buckstay attachments and scallop bar attachments. In economizer tubing, locations at bends, welds with the potential for high residual stresses (e.g., fin welds), and locations at attachments are most threatened. Endangered locations are, for example, lug mounted tiebars connected to tubes. Corrosion fatigue is often initi- Figure 3: Typical multiple array of corrosion fatigue cracks (initiated from the inner surface at an attachment). 587
101 Boiler and HRSG Tube Failures ated at the end of membranes where the membrane stops because of a tight bend in the tube or where the tubes are bent to form an opening such as for sootblowers or a mandoor. Figures 4 and 5 show some typical examples. Heat Recovery Steam Generators As in conventional boilers, the most likely failure locations are at welds, at bends, and at attachments. These are locations where significant thermal stresses may develop because of restrained thermal expansion. Jeopardized are also tubeto-header connections where due to significant thickness transients local thermal stresses may develop (a thin tube changes temperature more rapidly than a thick header). Figure 4: Typical scallop bar attachment on the waterwall surrounding a burner. Large transients or temperature differences may have different causes, e.g., uneven distribution of gas flow and non-uniform pressure drop in the individual tube sections due to design failures. All Steam Generators Corrosion fatigue cracks may also develop in steam-touched tubing when fluid of a significantly lower (or higher) temperature than the tubing is introduced. This may occur during stand-by due to inappropriate boiler layup. Figure 5: MECHANISMS OF FAILURE Locations of pinhole corrosion fatigue leaks associated with the attachment for the drip shield on a boiler. The synergistic effects of stress and environment cause corrosion fatigue. Corrosion fatigue is also known by a number of other names designating basically the same mechanism, for example stress-assisted cracking or stress-assisted pitting. Sometimes the latter is aligned along original extrusion marks on the tube inner surface. Stress corrosion cracking is not the same mechanism as it requires a continuous application of stress and is most often a continuous cracking process. It is essential in addressing the root cause of corrosion fatigue that the importance of both the stress and the envi- 588 ronmental components be identified. Most often corrosion fatigue is driven by the application of a stress imposed by the system or restraints (attachments etc. as above). In some cases the cycle chemistry has an influence, but it is always minor compared to the stress, which is required in the description of the mechanism to produce a strain on the inner surface that is great enough to crack (initiate corrosion fatigue) and continue to crack (repetitive initiation) the protective oxide layer (magnetite) on the inner surface of the tube.
101 Boiler and HRSG Tube Failures Figure 6: Typical corrosion fatigue cracks illustrating the discontinuous nature of the cracks (bulges along the crack length). Breakdown of Magnetite Protective Layer The use of carbon steel or low-alloyed steel materials thermodynamically instable in water at operating temperatures for boiler components exposed to high temperatures and pressures is only possible because a protective oxide layer is formed on the waterside surface of the tube. This protective layer consists mainly of magnetite (Fe 3 O 4 ). When the imposed strain is greater than the fracture strain of the oxide (magnetite), the oxide will crack in a regular array. This cracking then allows boiler water (or evaporator water in HRSGs) to come in touch with the tube surface. This then causes more magnetite to grow on this exposed surface at a relatively fast rate (parabolic growth law). This oxide will remain in place until the next application of strain greater than the fracture strain. This will crack the newly formed magnetite at the bottom of the corrosion fatigue crack. The cracks grow by a repetition of this process (called repetitive crack initiation); see Figure 6. Rupture of the magnetite film acts as a stress concentrator. Generally, it is recommended to keep the strain level in the magnetite layer below 0.2 % in tension to avoid film rupture. Addressing Root Causes of Corrosion Fatigue Three issues are important when evaluating corrosion fatigue occurrence on a particular boiler and its root causes: "Geography" of failures and damage It is important to find out where in the boiler corrosion fatigue failures have occurred and in what boiler areas non-destructive evaluations have indicated damage by the corrosion fatigue mechanisms. This "geography" helps to identify locations at which detailed monitoring should be carried out. 589
101 Boiler and HRSG Tube Failures "History" of failures Comparison of the number of failures in different operation periods is also important. Each operation period might have a number of hot, warm, or cold starts, unit trips, two-shift cycles or forced cools. In this way, it becomes obvious which of the operating spaces might be driving the corrosion fatigue mechanism. Operating space It is important to delineate all the different types of operating space which have been used on the boiler. With knowledge of these three factors, we can then move ahead to address the root cause of the corrosion fatigue problem on that particular boiler. This requires monitoring (temperature, strain, and waterwall movement) selected endangered locations (recognized from the geography) through all the operating spaces identified by the history of failures. Excessive Stresses/Strains Restraint Stresses Breakdown of the magnetite layer is probable at locations at which excessive strains may be developed (geography and history) during particular operation spaces. It is vital for any short-term and long-term actions targeting corrosion fatigue failures to identify the critical regions (geography) which often exist at tube attachments. Redesigning tube attachments in order to increase the flexibility at the connection and/or improvement of weld profiles by grinding may be important; however, any measures taken should ensure that the fracture strain of the magnetite will not be reached at that location with the new design. Note that often not the design but a change in the operation space is required to avoid extensive strains and breakdown of the protective oxide layer. In the case of HRSG, the approach is parallel to that used at conventional plants. The focus here should be on the operating space and the thermal transient which is responsible for protective oxide layer breakdown. Water Chemistry It is a matter of common knowledge that boilers that have had boiler water purity problems suffer more often from corrosion fatigue failures than those operated with correct chemistry. Units operated on older or incorrect phosphate treatments experiencing hideout and hideout return have been at risk. Large swings in the boiler water ph and ph depressions during shutdown and early startup are in particular harmful. However, they are detrimental only if they occur at the same time that the strain is highest during whichever operating space is applicable. This is an important caveat because chemistry excursions can occur at any time, but only if they coincide with the application of a strain which is great enough to crack the oxide will they initiate and reinitiate corrosion fatigue. Figure 7 demonstrates that a low boiler water ph de - creases the number of cycles to initiation of corrosion fatigue cracks. Not only the ph depression but also a high level of oxygen during inadequate boiler layup may contribute to aggravation of corrosion fatigue damage since oxygenated stagnant water promotes pitting and the formation of corrosion fatigue initiation centers. Subcooling in Natural Circulation Boilers High strains have occurred during stratification of water along the length of boiler tubing during shutdown and restart of natural circulation boilers. This requires that the boiler be monitored at the top and bottom to determine the stratification amount during particular operating spaces (which in this case may include shutdown). Operational Aspects Unit operation can have a significant effect on the occurrence of corrosion fatigue failures. Based on the evaluation of the history of failures, the operating spaces, which might drive the corrosion fatigue mechanism, should be identified. In this way, it becomes obvious which of the operating spaces might be driving the corrosion fatigue mechanism. This could be a hot, warm or cold start, it could be the shutdown for any of these, and it could be a forced cool, or a trip or any other operating space. It is vital that an organization recognizes the full extent of the operating spaces through which a unit has operated. Figure 7: The influence of ph depression on the initiation of corrosion fatigue cracks in boiler water with a very low level of dissolved oxygen (< 5 µg kg 1 oxygen). Dooley, B. R., Paul, L., Proc., International Water Conference, 1995 (Pittsburgh. PA, U.S.A.). Engineers' Society of Western Pennsylvania, Pittsburgh, PA, U.S.A., 56, 146-151 (Paper #95-17). 590
101 Boiler and HRSG Tube Failures Chemical Cleaning The use of inhibited hydrochloric acid as a chemical clean solvent may aggravate corrosion fatigue in comparison with other solvents such as ammoniated citric or ethylenediaminetetraacetic acids or hydroxyacetic-formic acid. PROBABILITY OF CORROSION FATIGUE FAILURES For estimating the probability of corrosion fatigue failures, the stress level, environmental conditions, and the influence of operation mode have to be evaluated. Two of these factors of influence, the stress level and the operation mode, are typically a standard part of any analysis of root causes. Their influence on the probability of corrosion fatigue failures is most important and should not be disregarded in the root cause analysis. Corrosion fatigue, however, occurs by the combined synergistic actions of cyclic loading and an adverse environment. Note that the environment or cycle chemistry is only of a minor influence compared to the stress or strain. For this reason, it is hardly possible to numerically express the probability of corrosion fatigue as a function of improper environmental conditions. However, some of the risk-aggravating factors are: phosphate hideout under phosphate boiler water alkalinity control chemistry excursions resulting in hydrogen damage or caustic gouging boiler water ph < 8 (at 25 C) during startup at the point of reaching operation pressure (sampling point: blowdown or downcomer) Any of these factors are detrimental only if they occur at the same time that the strain is highest during whichever operating space is applicable. For this reason, the approach is to conduct a root cause analysis, which involves monitoring typical locations across a range of operating spaces as already discussed. Chemical cleans with inhibited hydrochloric acid and improper or no corrosion protection during boiler shutdown promote pitting and in this way the formation of corrosion fatigue initiation centers. Figures 1 6 courtesy of Structural Integrity Associates, Inc. 591