Some methods of analysis and diagnostis for corrosion of material used in construction of various components of nuclear power plants

SOME METHODS OF ANALYSIS AND DIAGNOSTIS FOR CORROSION OF MATERIAL USED IN CONSTRUCTION OF VARIOUS COMPONENTS OF NUCLEAR POWER PLANTS

abstract

In Nuclear Power Plants it is necessary to ensure a longer and safely operating as difficult and expensive it is the maintenance of these very complex installations and equipments. In this regard, The Analysis and Diagnostic Laboratory Corroded Metal Components in Nuclear Facilities-LADICON; was authorized RENAR and CNCAN notified as a testing laboratory for nuclear-grade materials.

As part of the investigation and evaluation of corrosion behavior for these materials are used two types of test methods:

Longer corrosion tests such as: Autoclaving at high temperature and pressure in different chemical media-specific patterns in CNE and

Accelerated methods like: electrochemical techniques accelerated chemical tests, etc.

This paper presents some methods of analysis for materials corrosion; methods of assessment that corrosion of structural materials exposed to specific operating conditions and environments from CNE.

The electrochemical measurements show the following advantages:

a) Allowing a direct method to accelerate the corrosion processes without altering the environment,

b) It can be used as an indestructible tool for assessing the speed of corrosion and

c) Offers the possibility of conducting such investigations in - situ and ex- situ. Corroborating the environmental chemistry that was born on samples movies investigation results obtained by the methods above, it is possible to identify the types of corrosion  by the material and sometimes even those processes and mechanisms of corrosion

Introduction

Nuclear energy plants produce electricity through the fission of uranium, not the burning of classical fuels. Consequently, nuclear power plants do not pollute the air with nitrogen oxides, sulfur oxides, dust or greenhouse gases like carbon dioxide (Fig.1). More than 400 nuclear power plants are operating in 25 countries around the world today, supplying almost 17 percent of the world's electricity (Fig 2.). Many nations are building new nuclear energy plants to meet the needs of their growing populations and expanding economies, about 83 new nuclear energy plants being built now around the world

During operation of nuclear power stations (NPS) it was found that, even under normal operating conditions of the heat transfer circuits may happen some adverse effects due in particular to corrosion, erosion, hydrating and deposit of various corrosion products formed on the heat transfer surfaces.

The maintenance of a nuclear power plant (NPP) is highly complex and difficult due to its specific nature. Thus, unscheduled stops of nuclear plants due to their degradation by corrosion represent about 50% and in last 10 years they have operated average about 5% lesser.

Given a set of requirements such as:

- the increasing of operation life in safety of structural components;

- the reducing of the installation costs necessary to restore normal operating conditions after stopping due to the replacement of some corroded parts;

- the reducing of the radiation level in NPP systems;

- the avoidance of power diminishing in nuclear power plants;

- the reducing of the risk of losing of public confidence in those who operate nuclear facilities,

It is necessary to know and understand the degradation phenomena and processes of NPP structural components.

Analysis and diagnosis of corrosion processes leading to degradation of metal components related to nuclear power plants and the identification of solutions to reduce/ prevent these processes are particularly important, because the installations stopping in view to repair or replacement of some components suppose high costs, increases the exposure risk to radiation of operator personnel and the radiochemical contamination risk[1].

In the research department ' Nuclear materials and corrosion' of the SCN, there is the Analysis and Diagnosis Laboratory of Metallic Corroded Components from Nuclear Facilities, which is a laboratory accredited by RENAR according to ISO / IEC 17025:2005 - "Competence general requirements in case of testing and calibration laboratories"[2].

In this paper will be presented both some methods used in LADICON for the analysis of corroded components from nuclear installations and testing methods used to assess the corrosion of structural materials in similar conditions and environments with those in which worked the corroded components.

II. Main activities of the Analysis and Diagnosis Laboratory of Corroded Metallic Components from Nuclear Installations - LADICON

The corrosion experiments carried in LADICON out-reactor approached the generalized and localized corrosion of various components and structural materials using several methods and procedures in accordance with current standards or rules, namely:

- Long period chemical testing (static autoclavizing at those medium parameters that simulate the operation conditions of the NPS circuits);

- accelerated tests (using for example electrochemical methods).

II.1. Research activity

Referring to water chemistry and corrosion problems in NPP circuits, some research activities develop in the frame of Research Experimental Programs referring to the corrosion of structural materials from the primary circuit, the Steam Generator (SG) and the secondary circuit of nuclear power plants in normal and abnormal operating conditions. These research programs have emerged as a necessity in providing of technical and scientific support for the safe operation and increasing the life span of nuclear power plants from our country.

To follow the effects of water chemistry on the corrosion and radioactivity generating by some structural materials and to demonstrate the influence of transport activity in circuits from Cernavoda NPS. At Cernavoda NPS there is a system of 4 autoclaves in the 'bypass' of the primary circuit. In Y1 and Y4 autoclaves existing at the exit of the reactor is determined and studied the corrosion at 310^oC, after the outlet of heat agent from fuel channels; in Y2 and Y3 autoclaves, placed at the inlet in reactor, is determined the corrosion at 265^oC, in operation conditions of secondary circuit after heat output from Steam Generator. Using some devices having an adequate geometry, supplementary to general corrosion can be studied and other corrosion types such as: stress corrosion cracking, galvanic corrosion, crevice corrosion, corrosion of heat affected zones (HAZ), etc.

Using both the samples tested in autoclaves placed in primary circuit bypass and those obtained from the decommissioned components, can execute some studies on contaminated surfaces which follow the optimization of decontamination and descaling procedures of surfaces covered with several deposits and can be tested new better structural materials, in view of their adoption as replaceable materials.

Using all these inlet data and experimental results, was created a data basis entitled "Corrosion" that includes all data describing the corrosion behavior of some structural materials using the results of experiments executed out-reactor. In data basis "CNE Corozi Test" are included the data referring to the characterization and post-testing examinations of samples exposed in autoclaves system Y1-Y4, after several operation periods of CANDU-6 reactor.

II.2. Analysis and diagnosis activity

Considering that the analysis and diagnosis activity of corroded metal components from nuclear installations is extremely important for maintaining of nuclear safety, it shall be accomplished in conformity with the legislation of the National Commission for the Control of Nuclear Activities -CNCAN.

Consequently, the notification of this testing laboratory LADICON was made by CNCAN - Bucharest (notification no. Li-04-2008) for the following analysis and evaluation methods:

. the gravimetric analysis of corrosion coupons;

. metallographic microscopy of corrosion coupons and respectively of parts taken from corroded metal components in nuclear installations;

. determination of corrosion behavior of structural materials using electrochemical methods.

It is known that most metal components and in particular those made of carbon steel suffer some corrosion processes in aqueous environment at high temperatures and pressures, similarly conditions existing in primary and secondary circuits of CANDU NPP. From this resulted the importance of execution of corrosion tests in those conditions that simulate the best the conditions from those circuits - primary and secondary -, in view of prognosis of the advancement direction of corrosion and of detecting and understanding of corrosion mechanisms.

III.1. Investigation of corrosion behavior of materials using the autoclavizing in aqueous solutions at high temperatures and pressures

Corrosion testing of coupons using the autoclaving exposure implies the coupons exposure taken from different structural materials in aqueous medium or steam at high temperatures and pressures for limited periods of time. The working parameters and testing environment should be similar to operation conditions in which have been corroded the samples studied.

The main equipments used in this type of corrosion testing are[3][4]:

- one liter Prolabo autoclave: maximum working temperature 400^oC ( 5^oC the measurement error) and maximum pressure 20 MPa ( 0.5 MPa the measurement error);

- Autoclaves Baskerville of 5litres: working max temperature ( 5^oC the measurement error) and maximum pressure 20 MPa ( 0.5 MPa the measurement error) (Fig. 3.)

- Autoclave Baskerville of 13litres: working maximum temperature ( 5^oC the measurement error) and maximum pressure 20 MPa ( 0.5 MPa the measurement error).

- A decontamination (descalling) installation in dynamic regime;

- one liter electrochemical autoclave which allows electrochemical measurements such as: open circuit corrosion potential, chronoamperometrie, voltammetry, etc, at specific parameters of nuclear power plant circuits (max.temp. 300⁰C, pressure 100atm)[5]. (Fig.4.)

III.2. Application of gravimetry at samples corrosion evaluation

Gravimetry is a quantitative analytical method based on the mass measuring of the corrosion coupons before and after their exposure a well-defined period of time in different environments. The operations implied in gravimetric analysis differ from one type of material to another. Based on initial weighing and those recorded, for example, after autoclaving, it can determine the weight variation - ΔG - in mg which shall be reported to the area calculated in dm² .

Fig.5. Weight loss function of autoclavization periods curves corresponding to carbon steel samples autoclavized in three types of solutions

On the basis of the variation in weight determined as ΔG/S (mg/dm²), reported at the exposure time expressed in days, it can estimate the corrosion rate in mg/dm² day (Fig.5). Based on gravimetric determinations as variations of weight gains may be made and an estimation of the thickness of superficial compounds, provided knowledge of type compound and its density.

III. 3. Electrochemical methods

It is known that the electrochemical parameters can furnish some data referring to the corrosion susceptibility of structural materials used in a NPS. The main electrochemical methods are: the variation of open circuit corrosion potential function of time (Fig. 6), the linear and cyclic polarization, the potentiodynamic, potentiostatic, galvanostatic and electrochemical impedance spectroscopy (EIS) method (Fig.7). The last method (EIS) implies the evaluation of the performance of superficial films deposited on the surface of metals and has the advantage of not accelerate the electrochemical reactions from the interface metal/ solution[8].

To execute the electrochemical measurements is used a Princeton Model 2273 electrochemical system (Fig.8 and 9) which includes: a potentiostat / galvanostat, the electrochemical cell itself (Fig. 9) and a Windows XP computer for data processing[9].

The electrochemical impedance spectroscopy method (EIS) is widely used in corrosion investigation because it has the following advantages[10]:

- uses very small signals which do not disturb the properties of electrodes;

- it can be studied the corrosion processes and determined the corrosion rates in fluid having very low conductivity, in which the traditional methods cannot be used;

- executing a single measurement, can be obtained at least two electrical characteristics of superficial films: electrical double layer capacity and polarization resistance.

III.4. Metallographic analysis

Microscopic or metallographic analysis consists in the microscopic examination of metallic materials. It is an indispensable method in the study of materials microstructure, allowing the determination of following parameters: the determination of microscopic non-metallic inclusions in metals and alloys, study of modifications of the crystalline grains, the determination of their distribution and morphology of crystallites after size, etc.

The microscope Olympus GX 71 (Fig.10) is an optical instrument used at magnifications ranging from (x12. 5 - x2000). The microscopic images reflect the morphology of superficial layers of oxides and the deposition of corrosion products formed on samples. For acquisition, processing and archiving of images, the microscope is equipped with a working PC and software [6].

This analysis method provides information referring to the existing grains and crystalline phases, their distribution and nature, the crystal sizes, the morphology of hydrides, etc. It also may reveal the morphology of corroded surface, the appearance in section of the superficial films and could determine both the thickness and uniformity and continuity of superficial films [7].

Conclusions

1. Activity in the LADICON is divided in two directions: research referring to water chemistry and corrosion in NPP circuits and corrosion behavior diagnosis and analysis of corroded metal components of nuclear installations.

2. The corrosion experiments dealing with generalized and localized corrosion using long periods chemical tests in static autoclaves and respectively the accelerated tests such as electrochemical tests. Evaluation of corrosion behavior is made by gravimetric calculations and surface analysis techniques.

3. The electrochemical impedance spectroscopy (EIS) method is used at the study of corrosion processes and analysis, of adherence properties and porosity of the films formed as result of corrosion[11].

4. Gravimetric analysis allows the determination of corrosion rates, of the releasing of the corrosion products or metallic ions during corrosion, as the total quantity of whole formed products by corrosion, adherents and released products in corrosive environment.

5. The metallographic analysis is a complex interesting method consisting in macroscopic and microscopic examination of metallic materials.

6. To investigate and evaluate the corrosion behaviors of structural materials in view of investigate the reasons of corrosive degradation are currently used some electrochemical methods such as: the cyclic, potentiodynamic and linear polarization, the variation in time of open circuit potential and the electrochemical impedance spectroscopy.

References:

[1] I. Pirvan s.a, "Studiul cerintelor din reglementari si descrierea metodelor de analiza si caracterizare la coroziune generalizata si localizata a componentelor metalice corodate" R.I. 7721/ 2006

[2] M.Fulger  " Manualul Calitatii MC LADICON, Ed5, act 0 /2009

Li-TH-06 - Instructiuni de lucru la autoclava Baskerville de 5l;

[4] Li-TH-08 - Instructiuni de operare pentru autoclavele Prolabo de 1l.

[5] LI-TH-203 - Operarea autoclavei electrochimice,

[6] ASTM E 3/2007: Standard Methods of Preparation of Metallographic Specimens

[7] ASTM E 7/2003: Standard Definitions of Terms Relating to Metallographic

Scully J.R., "Corrosion Tests and Standards: Application and Interpretation" Ed.R.Baboian (1995);

[9] "Handbook on Corrosion Testing and Evaluation" Ed.Ailor W.H. (1971);

[10] Scully J.R., "Corrosion Tests and Standards: Application and Interpretation" Ed.R.Baboian (1995);

[11] "Handbook on Corrosion Testing and Evaluation" Ed.Ailor W.H. (1971);