Corrosion Mechanisms of Tubes from the Candu High Pressure Feed Water Heaters

NUCLEAR – 2013

CORROSION MECHANISMS OF TUBES FROM THE CANDU HIGH PRESSURE FEED WATER HEATERS

M. FULGER*, M. MIHALACHE*, L. VELCIU*, I. VITELARU**

*Institute for Nuclear Research Pitesti, Romania, [email protected] ** Cernavoda Nuclear power Plant, Romania

ABSTRACT

This study has been carried out to investigate the corrosion mechanism developed on stainless steel tubes from HP Feed Water Heater in a CANDU NPP. With this goal in mind, specimens of the failed tubes removed from HP Feed Water Heater were analyzed in laboratory by different methods (visual examination, scanning electron microscopy (SEM) and optical microscopy). The corrosion deposits from the surfaces of tubes were examined by energy dispersive X-ray spectrometry method (EDX) to identify the chemical composition. The results of the laboratory tests showed that the tubes failed by OD (outside diameter) chloride induced stress corrosion cracking mechanism. Stress corrosion cracking (SCC) is a form of slow crack growth that occurs when a susceptible alloy is stressed in a specific corrosive environment. Cracks were independent of tube support locations and welds. The corrosion mechanism and possible causes identified are presented, followed by conclusions and recommendations for corrosion minimization

Key words: HP feed water heater, stress corrosion cracking, SEM, EDS

Introduction

The Feedwater Heating System uses extraction steam to preheat the feedwater in order to optimize thermodynamic efficiency and to raise the temperature at the desired value for admission in the steam generators. Feedwater heaters normally operate as counterflow heat exchangers. Malfunctioning of feedwater heaters can significantly affect power plant efficiency by increasing heat rate and/or decreasing generation capacity. These heaters can fail due to vibration, flashing of drain flow, inadequate level control, steam impingement, erosion, and/or corrosion. Failures have been identified in the following major physical locations: tubes, condensing, and drain cooler zones [1]. The major mechanisms identified to affect the feed water heaters are as follows: Uniform or General Corrosion - characterized by a chemical or electrochemical reaction that proceeds uniformly over the entire exposed surface or a substantial portion of that surface. The metal becomes progressively thinner and eventually fails because of the stress loadings imposed on it. Crevice Corrosion and Pitting are partly discussed together because they are mechanically similar and partly because, in feed water heater tubes, they are often phenomenologically similar. The term "pitting" should be used to describe the corrosion that follows the local breakdown of a protective (e.g., passive)

125 ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████

NUCLEAR – 2013

█ ███████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████ film on a metal surface. Crevice corrosion, on the other hand, is a localized corrosion of a metal surface adjacent to an area shielded from full exposure to the environment. Crevice corrosion may occur, under porous scales, corrosion products, mud or debris. When the creviced areas are small, the resulting localized corrosion may resemble pitting attack. In pitting and crevice corrosion, chlorides play a critical role, tubes presenting localized corrosion when the environment contain more than 10000 ppm chlorides. [2]. Galvanic Corrosion - is the accelerated corrosion of a metal that occurs because of an electrical contact with more noble metal in a corrosive solution. The less resistance metal is described as anodic and the more resistance metal as cathodic.[2,3]. Erosion Corrosion or water side "impingement attack" is a form of localized corrosion that occurs on the water side of the tubes in areas where the turbulence intensity at the metal surface is high enough to cause mechanical disruption of the protective oxide film. Stress Corrosion Cracking (SCC) is a form of slow crack growth that occurs when a susceptible alloy is stressed in a specific corrosive environment. Generaly, in feed water heaters, most of the SCC failures are initiated on the steam side of the stainless stel tubes, though a limited number of SCC failures are reported in water side[2,3]. This study has been carried out to investigate the degradation mechanism developed on plugged tubes removed from Cernavoda Unit 1 HP feed water heater after 15 years of operation. The specifications of the tubes and operation parameters were as follows: - On the outer side of tubes, flow steam and condense with an inlet temperature of 190oC, outlet temperature of 151.2o C and pressure of 15 bar. - On the inner side of tubes, flow demineralized water with inlet temperature of 145.7oC, outlet temperature of de 167.2o C and pressure of 90 bar. - The tubes are made of SA 213 TP 304 stainless steel with 1.25mm wall thickness. The heat exchange surface of the feed water heater contains a number of 1200 U-tubes with a radiant surface of 2013m2. The total length of the feed water heater is nearly 15 m with tubing supports at 1.5m distance.

Experimental

To investigate the damage mechanism of tubes the following analyses have been performed: Visual examination - preliminary visual examination of the degraded components with extensive photographic documentations precedes any mechanical testing or any metallurgical examination. A photographic record is useful for subsequent reference or comparison Metallography analysis includes a macroscopic examination of the surface of the selected specimen followed by a microscopic examination of etched and unetched material as well as transverse and longitudinal cross sections. Has been used a metallographic microscope OLYMPUS GX 71 type which permits magnifications in the range of x 12.5 - x 2000. For microscopic examination which include analysis of damaged surfaces, secondary cracks, abnormalities, origin of fracture, and direction of the crack growth, has been used a TESCAN VEGA II LMU electron microscope operating up to 30 kV. This device is equipped with 3 detectors: a secondary electron (SE) detector, a back-scattered electron (BSE) detector and energy dispersive X – ray spectrometer (EDS). Chemical analysis of the corrosion products composition- may follow the metallurgical analysis for surface characterization, depending on the investigation.

126 ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████

NUCLEAR – 2013

Results and discussions

Visual examination has been made on the entire length of tubes and was recorded photographs for a better comparison. After visual examination, for an easier manipulation in further analyses, the tubes have been cut in pieces of 2 cm of length (Figure 1). At the first visualization the information was following: - the tube noted T1 is curved at one extremity which indicates the proximity to the U–bend (Figure 1). - on the both tubes surfaces there are scratches probably resulted from manipulation and marks resulted from friction with supports of tubes . - only on the one side of both tubes have been remarked red oxides and hard deposits formed above a continuous black oxide (magnetite) while on the reverse side there is a magnetite layer maintained almost intact (Figure 2).

T1 T2 Figure 3 Photographs showing the tubes T1 and T2 segmented for analysis

T1 T2 Figure 4 Photographs showing the degraded side and reverse outside of tubes T1 and T2

The first conclusion was that the degraded faces of tubes were up warded being subjected to small droplets of water which eroded tubes while the reverse sides were safely, just dry steam was in contact with its.

Metallography analysis Using an optical microscope there were visible corrosion products deposits covering the most of the outside diameter (OD) surface as shown in Figure 3. On the inside surface of tubes there were not observed deposits, only an adherent oxide layer. Scanning the tubes out side surfaces at higher magnifications has been put in evidence small pits coated by red oxides (Figure 4). In Figure 5 are shown the morphologies of the inside surfaces of tubes which appeared covered with a black oxide. The spalling of the oxides occured owing to flowing of internal fluid. Another disturbances of the internal surfaces not been observed.

127 ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████

NUCLEAR – 2013

a) b) Figure 5 (a) Red oxydes and hard corrosion deposits visualized on outside surface and (b) an aderent oxide layer on the inside surface (magnification x40)

For a better visualization of the tubes outside surfaces were imposed a cleaning, so, has been performed a brushing of oxides after that could be observed pits with different sizes occured on the entire surface (Figure 6). Conclusion was that the corosion products formed on the tubes surfaces initiated crevices where agressive agents from environment concentrated and in time degraded the metal by pitting corrosion. Because the brushing process could not remove very well oxides, the next operation was descalling of samples using an acidic solution (15%HNO3, 3% HF, 3% HCl and water) for 30 seconds at temperature of 75o C. After descaling, on the samples surfaces have been relieved many pits and longitudinal cracks, as in Figure 7.

T1 T2 Figure 6 Out side surface morphology of the tubes (magnification x50)

T1 T2 Figure 7 Inside surface morphology of the tubes (magnification x 100)

128 ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████

NUCLEAR – 2013

T1 T2 Figure 8 Out side surfaces after oxides brushing (magnification x50)

T1 T2 Figure 9 Pits and longitudinal cracks on the tubes surfaces after descalling (x50)

For microstructural examination, the samples were electrolytic etched in 10% oxalic acid solution. Microstructural examination relieved that the parent materials for the tubes has a single phase grain structure characteristic of 304 austenitic stainless steel (Figures 8-9). There are not visible carbides at grain boundaries, therefore the metal was not sensitized but can be observed many inclusions formed probably in casting process of alloy.

Figure 10 Austenitic microstructure of T1 sample Figure 11 Austenitic microstructure of T2 sample (after electrolitic etching) (x500) (after electrolitic etching) (x500)

129 ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████

NUCLEAR – 2013

Following the procedure SCN, LI-CQ-02 Ed 1 act.1 [4] was determined Vickers micro hardness (MHV) using a pressing force of 100gf (MHV 0.1). The results were 175 units HV for tube T1 and 167 HV units for tube T2. Compared to characteristic value for 304 SS (ASME standard No.213 indicate a 200 Vickers hardness maximum value) there is a noticeable decrease of material micro hardness for this two tubes. Analysis by SEM of the tubes surface revealed the presence of a double-layered oxide film containing an inner black layer uniform and adherent, of iron oxide (magnetite) over which there is another layer consists of neddle shaped crystals (Figure 10). The oxide's thickness measured by SEM was between 12.9 and 20 µm (Figure 11).

Figure 12 SEM image of outer oxide layer with neddle Figure 13 SEM image of outer oxide layer thicknes shape crystals inclusions(x5000) (x2000)

Also, cracks have been observed in the oxide outer layer by SEM analysis (Figure 12). These cracks formed in the oxide could allow impurities from steam to penetrate the base metal. Another SEM images, achieved into pits filled with oxides, highlighted the presence of same neddle shaped crystals (Figure 13). Through subsequent EDS analysis these crystals were identified to be chloride crystals.

Figure 14 SEM image of the cracks in the oxide Figure 15 SEM image of the oxides identified (x2000) inside of pit (x5000)

130 ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████

NUCLEAR – 2013

For the next step of analysis a transverse cross section through the longitudinal oriented cracks has been performed. Figure 14 shows secondary electrons SEM image of one longitudinal crack which was identified previous through metallograpy analysis on the outside surface of tube T2.

a) b) Figure 16 Secondary electron SEM image of longitudinal crack identified in section T2_3 a) (x200) b) (x700)

Note that the crack is branched, and pass both at grain boundary as through the grains. Cracks that run across the grain boundaries are called ―transgranular‖ and those that follow the grain boundary and are termed ―intergranular‖. This type of branched crack usually occurs on steel 304 when chlorides are present in operation environment. The crack has initiated from a surface pit and propagated in the longitudinal direction and to the inside close to half of the tube wall's thickness.

Energy dispersive X- ray spectroscopy analysis

The chemical composition of the corrosion products within the corroded area of the outside diameter surface was analyzed using a scanning electron microscope (SEM) equipped with an energy dispersive x- ray spectrometer (EDS). An analysis in points of the corrosion deposits covering much of the outer side surface was realized and the resulted spectrum is shown in Figure 15. From the EDS spectrum, resulted that the deposits contain: iron (Fe), chromium (Cr), nickel (Ni), oxygen (O), plus few amounts of impurities, such as chloride (Cl), sulfur (S), calcium (Ca), and kalium (K). Detection of these impurities leads to the conclusion that inside of the heater unpurified water infiltrations occurred, which allowed an accumulation and concentration of impurities under the corrosion deposits.

131 ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████

NUCLEAR – 2013

Point conc. Fe Cr Ni Si S Ca Cl K O (weight %) P9 (%) 60.25 6.61 2.04 1.34 1.35 0.46 0.75 0.63 26.58 P10 (%) 10.82 0.82 0.44 0.35 0.81 0.16 0.43 1.92 84.25 P11 (%) 72.46 3.1 1.48 1.32 1.12 0.45 0.53 0.4 19.14 Figure 17 EDS analysis in points showing the presence of impurities Cl, Ca, S si K in oxides(P10,P11) and inside of pit( P9)

Disscusions

Analysis of the tube sections indicated that the degradation was related to chloride induced stress corrosion cracking (CLSCC). CLSCC initiates from sites of localised pitting or crevice corrosion This type of cracking most likely occurred as a result of very small amounts of chlorides depositing on the OD of the tubes over a long period of operation. Degradation occurred in the condensing zone as a result of the relatively high steam velocities in this area where the tube wall temperature is cycled. In particular, when a unit ramps down, the tubes remain relatively hot drying the outside of tubes after they have been in a wet condensing area. This drying, then washing and drying cycle will tend to concentrate minors amounts of chlorides on the tube OD in this zone. The environmental conditions were met. The susceptible material in this case was austenitic stainless steel which contains nickel. The susceptibility of austenitic stainless steels to CLSCC depends on a range of environmental variables that include chloride concentration, temperature and pH. Other variables include, for example, stress level, surface finish and the metallurgical condition of the steel. Chlorides identified indicate that this mechanism could exist. The stress level includes residual stresses or service stresses. Residual stress could occur from a manufacturing process or during installation. Service stress is caused by an expansion stress, hoop stress, or bending stress. A susceptible environment carries temperatures greater than 60 degree Celsius and the presence of oxygen. For feed water heaters this would be in the form of dissolved oxygen as a result of air in leakage through the surface condenser. For stainless steel materials, this includes the presence of concentrated chlorides [5, 6]. Chlorides enter the FW system when there are condenser tube failures or problems with the demineralizer.

Conclusions

This paper emphasized the degradation mechanism of plugged tubes removed from CANDU HP feedwater heater, after 15 years of operation. The final conclusion is that the degradation mechanism of tubes from high-pressure feedwater heater from Cernavoda Unit 1 was chloride corrosion cracking (CLSCC) and was according to the following scenario:

132 ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████

NUCLEAR – 2013

- on the tubes surface were deposited corrosion products, which have acted as crevices where fluid was stagnant leading in time to impurities concentration. - impurities containing aggressive ions (chloride, sulfur mainly identified by EDX) attacked the oxide film and lead to localized corrosion (pitting). - due to the operating stress, residual stress and lower resistance of 304 steel at chlorides, from pits were initiated branched cracks which penetrated the tubes wall leading to leaks.

Recommends

- identification of any leaks from condensers to avoid entry of aggressive environment; - a rigorous chemical control of environment to any deviation of water specification from recommended values and to avoid local concentration of aggressive species in areas with restricted circulation; - tubes must be inspected regularly, especially in problem areas: in the cooler drain, near the inlet of steam around plugging tubes and random; - adequate facilities for drainage during shutdown, removal of suspended solids, corrosion product removal by ion exchange and removing deposits; - residual stress elimination by heat treatments applied to tubes before use; - a rigorous control of tubes surfaces before to installation for elimination of surface defects.

References

[1] G. Bereznai, G. Harvel ―Introduction to CANDU systems and operation‖ Courses Systems and Nuclear Science, at the University of Ontario Institute of Technology, Canada 2011 [2] Rafaa Abbas Al-Baldawi, Mohammed Najm Abdullah „Tube damage mechanism and analysis in feedwater heaters‖ Journal of Engineering and Development, Vol. 16, No.1, March 2012 ISSN 1813- 7822 , p 261-272 [3] Thomas J. Muldoon, ―Stress Corrosion Cracking (SCC) of 304SS Tubes at Outlet of the Desuperheating Zone in Feedwater Heaters‖ - Engineering Manager American Exchanger Services, Inc. Hartford, WI [4] *** „Pregatirea cupoanelor din materiale structurale (otel inox, otel carbon, aliaje de nichel) pentru expunerea lor in autoclavele de la CNE Cernavoda‖. Procedura SCN, LI-TH-33, ed.1, act.1 [5] Mehrooz Zamanzadeh, Edward S. Larkin, and George T. Bayer „Failure Analysis And Investigation Methods For Boiler Tube Failures‖ CORROSION 2007 Paper No. 07450 Waterside Boiler Tube Failure Symposium [6] R Parrott H Pitts ―Chloride stress corrosion cracking in austenitic stainless steel‖, RR902 Research Report, HSE Book, London, 2011

133 ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████