Corrosion Process of Incoloy-800 in High Pressure and Temperature Aqueous Environment

The Steam Generators (SG), equipment that ensures the connection between the primary and secondary circuits, creates several safety problems during operation, mainly due to corrosion and mechanical damages. To provide information about the corrosion behaviour of the structural materials from CANDU SG under normal and abnormal conditions of operation and to identify the failure types produced by the corrosion were performed corrosion experiments consisting in chemical accelerated tests, static autoclaving and electrochemical methods. The gravimetric method, optical metallographic microscopy, XRD and EDS analysis, as well as electrochemical measurements have been used to evaluate the corrosion behavior of the steam generator tubes material (Incoloy-800).

Water chemistry can contribute to steam generator degradation in a CANDU nuclear plant.Materials of construction and associated failure mode susceptibility can be more closely correlated to long-term steam generator degradation.Experimental results concerning the corrosion of the structural materials from the CANDU steam generator were published in our previous papers [1][2][3][4][5][6].Approximately 27 forms of tubing steam generator problems have been reported industry-wide [7][8][9][10] that consist in: secondary side IGA/SCC (Intergranular Attack/ Stress Corrosion Cracking) in crevice, denting at tubesheet surface, ID SCC (Inside Diameter Stress Corrosion Cracking) -expanded region, caustic IGA under sludge, sludge accumulation, phosphate wastage under sludge, secondary loose parts damage, pitting open span, pitting under support plate deposits, tube damage at supports, improper heat treatment, support plate clogging, acid wastage, denting at TSP(Tube Support Plate), sulphur/ sulphate attack, secondary IGA/SCC (Intergranular Attack/ Stress Corrosion Cracking) at TSP, high cycle fatigue, wastage, deposits AVBs (Antivibrations Bars), U-bend fretting, failures due to improperly installed AVBs, surface fouling, TSP (from carbon steel) deterioration, primary SCC-tight bends, impingement erosion, lead induced IGA, secondary IGA/SCC at tube supports, sulphate/silicate hideout etc.
Table 1 presents the main types of CANDU steam generator tubing degradations.
In this paper new results obtained in the frame of our experimental programs about Incoloy-800 corrosion will be presented.
The goals of the experimental program referring of the most important steam generator material (Iy-800 alloy) consisted in: the assessment of the corrosion kinetics, corrosion testing simulation devices of tube-tubesheet joint with and without deposits in normal and abnormal steam generator operation conditions, chemical cleaning of deposits placed in the simulation crevice devices and the assessment of corrosion intensity at the operation resuming after the chemical cleaning, concentration of impurities and corrosion products on simulated defects and the influence on the corrosion of tubes and tubesheet materials, the achievement of correlation between the presence of deposits and the intensity of crevice corrosion at tube-tubesheet joint.The experimental results were used for mathematical modeling of the impurities concentration process inside crevice.

Experimental part
Corrosion experiments have been carried out on the Iy-800 samples by autoclaving at the parameters specific for the secondary circuit of the CANDU steam generator: temperature 260 o C and pressure 5.1MPa.The testing environment used was demineralised water containing morpholine and cyclohexylamine, pH = 9.5 (AVT -All Volatile Treatment).

Table 1 THE MAIN TYPES OF CANDU STEAM GENERATOR TUBING DEGRADATIONS
The testing periods of time were 650, 2050 and 3550 h.The specimens used were cut from Incoloy-800, Iy-800 cylinder used as steam generator tube (samples of 15.9mm outside diameter and 1.13mm wall thickness which were sectioned on the diameter into 15mm long pieces).Experimental works included: autoclaving (PROLABO autoclaves having 1 liter capacity), gravimetric analyses (analytical balance SHIMADZU AUW 220D), optical microscopically analyses (NEOPHOT microscope), electrochemical measurements, potentiodynamic polarization and electrochemical impedance spectroscopy, (PAR PRINCETON MODEL 273 potentiostat) and X-ray diffraction (PHILIPS diffractometer).

Results and discussions
The goal of the work consists in the assessment of the kinetics of corrosion for the Incoloy-800 -material of the steam generator tubes-and experimental results processing in the purpose of including in a future database.Some examples of experimental results for the testing of the Incoloy-800 samples for different time intervals in demineralised water environment with pH=9.5 (AVTmorpholine and cyclohexylamine) at specifical parameters (260 0 C and 5.1MPa) are presented in the figures 1, 2. The Incoloy-800 tubes samples tested 240 h, in demineralised water (pH=9.5)visually present a shiny aspect colored in yellow-brown shades, while in steam they have a radiant aspect colored in blue-green shades on a brown light background.Continuing the testing duration of the samples, their aspect becomes dark-brown to brown for those tested in water and light-grey with bluish shades for those tested in steam.The oxides occurred on the Incoloy-800 samples after the 1200 and 2400 h testing are uniform, continuous, adherent and very thin films.By EDS examinations there were determined the thickness of the oxides layer (0.50µm).Also EDS analysis proved the chemical composition, which consists of Fe 3 O 4 and NiO.There were also performed corrosion kinetics studies by assessment of loss of metal by corrosion, corrosion rate, the total corrosion products, the adherent corrosion products, the released corrosion products, the release rate of corrosion products and the release rate of metal.Figures 3 -9 show the evolution during testing time of the studied parameters.
The nickel-based alloys (Incoloy-800) are currently used as corrosion resistant materials in the nuclear industry because their corrosion rates are quite low.This behavior is due to the protective character of the oxide film formed on their surface by the contact with the pressurized hightemperature water environment.
Nevertheless, oxidation processes or deposition of corrosion products can promote the development of particular corrosion problems.These phenomena result from changes in the structure of the oxide films throughout The corrosion rate, release rate of corrosion products and release rate of metals decrease when the exposure time increase.
The loss of metal by corrosion, total corrosion products, the adherent corrosion products and released corrosion products increases when the time of exposure increases.
The corrosion potential takes great values when the surfaces are covered with more protective oxide layers formed during a long time of exposure (150 days).Some mitigation measures for steam generator tubing degradation were established taking into consideration the operation experience of the worldwide nuclear power plants.These methods are the following: Design measures for corrosion prevention and control: minimize number of crevices, minimize concentrating action at unavoidable crevices, eliminate very high quality dry-out regions, optimize recirculation ratio, modify flow distribution for improved management of sludge, increase blowdown capacity, design for improved accessibility for inspection and maintenance, design tube expansion technique for most favourable stress distribution in tube at tubesheet.
Systems measures for corrosion prevention and control: condensate polishing systems (full flow or side arm), magnetic filter systems for feedwater, systems for detecting and location of condenser leaks, balance of plant systems, materials selection for condensers feedwater, heaters, moistures separators reheaters, drain routing for heaters and moistures separators reheaters, optimization of turbine plant chemistr y, analytical systems for continuous monitoring of feedwater, shutdown heat recovery systems and improve make-up water systems.
Water chemistry control and treatment measures for corrosion prevention and control: all volatile treatment (specification for impurity limits), amines for condensate pH control (morpholine, cyclohexylamine), oxygen scavengers (hydrazine, catalyzed hydrazine), inhibitors for denting (crevice neutralizers, boric acid), chelating treatments (continuous or intermittent) and lay-up treatment.
Operating practice and maintenance measures for corrosion prevention and control: condenser maintenance, air in-leakage control practices, sludge removal by water jet lancing, chemical cleaning, intermittent high blowdown rate, periodic tubing inspections, inspections of internals, tubes removals and examinations and also preventive plugging and sleeving operations.

Conclusions
The oxide layer obtained on the Incoloy-800 samples after the 1200 and 2400 h testing are uniform, continuous, adherent and very thin.
By EDS analysis and optical microscopic examinations it was determined the thickness of the oxides layer (0.50µm).Also the chemical composition assessed by RDX analysis consists of Fe 3 O 4 and NiO.
The corrosion rate, release rate of corrosion products and release rate of metals decrease for a long period of exposure time.
The loss of metal by corrosion, total corrosion products, the adherent corrosion products and released corrosion products increase when the time of exposure increase.
The corrosion potential take great values when the surfaces are covered with more protective oxide layers formed during a long time of exposure (150 days).
If the chemical parameters of the cooling water on the secondary side of the steam generator are kept at the imposed values, the corrosion is not a problem for the Incoloy-800 tubing, but the corrosion, especially localized corrosion, appears in the case of the presence of impurities and this process has a great contribution in the ageing management of the steam generator like nuclear power plant key component.