Corresponding author: Olga M. Gulina ( omgulina18@mail.ru ) Academic editor: Yury Korovin
© 2021 Valery I. Baranenko, Olga M. Gulina, Nikolay L. Salnikov.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Baranenko VI, Gulina OM, Salnikov NL (2021) Influence of operating time on the corrosion in singlephase and twophase media. Nuclear Energy and Technology 7(2): 127132. https://doi.org/10.3897/nucet.7.69175

Modern foreign computer codes predict a linear growth in the pipeline wall thinning with time due to the process of flowaccelerated corrosion (FAC), i.e. erosioncorrosion wear (ECW). Linear timethinning dependence and corrosion rate constancy are not however typical of the NPP piping operating conditions. And the associated excessive conservatism of the residual life estimates leads to increased economic costs of repeated inspections. In domestic software tools, EKI02 and EKI03, the influence of operating time are taken into account by introducing the respective coefficient into the ChexalHorowitz model based on the yield of corrosion products into the coolant. The ECW intensity can be however reduced through improvements in operating conditions, preventive measures, improvements in water chemistry, etc., and the use of the dependences once obtained may turn out to be too conservative. Based on a large number of repeated measurements as well as on data from corrosion testers, it has been shown that the influence of time can be described by the function of a particular form, the coefficients of which differ for different units and component and subsystem types. This makes it possible to determine the ‘aging function’ based on inspection data, and then use it in a targeted way for particular components. It has been shown that such estimates are much less conservative.
Erosioncorrosion wear, corrosion rate estimation, pipeline wall thinning, ChexalHorowitz FlowAccelerated Corrosion Modelt
Erosioncorrosion wear (ECW) is a widespread type of damage to the NPP thermomechanical equipment and pipelines. Practically all components of the turbine plant condensate feed and steam lines at NPPs and heat power stations are exposed to ECW. Other types of metal wear occur in most cases in parallel with this process (
Early versions of the codes for the ECW calculation were developed at the USA’s Electric Power Research Institute (EPRI). In the 1980s and 1990s, CHEC, CHECNDE, CHECT, CHECMATE, CHECWORKS, and other codes were developed. It was assumed in the process of the code development that the thinning values were directly proportional to time (
A linear dependence of the thinning value on time and the constancy of the corrosion rate are not typical of the NPP pipeline operating conditions. At the same time, excessively conservative estimates lead to increased economic costs of repeated inspections, since, in this case, the predicted residual life is much shorter than with the decreasing ECW rate. Such approach to thinning estimates is naturally highly conservative. The question of whether time shall or shall not be taken into account remains open.
The purpose of the study is to investigate the problem based on the wall thickness inspection data, including the analysis of repeated measurements, and the data on the damaged pipelines, as well as to analyze the corrosion tester readings.
The analog for Russian software tools, EKI02 and EKI03, (the authors of this paper are the developers of these codes) is the CHECWORKS (ChexalHorowitz Engineering Corrosion Workstation) code (ASME code Case N480). The codes were developed using foreign experience and the results of domestic investigations (
W _{ECW} = C_{0} × F_{1}(T) × F_{2}(ХС) × F_{3}(υ) × F_{4}(O_{2}) × F_{5}(pH) × F_{6}(K_{k}) × F_{7}(α) × F_{8}(A) × F_{9}(τ), (1)
$\Delta S\left(\tau \right)={\int}_{\tau 0}^{\tau}{W}_{ECW}d\tau $, (2)
where W_{ECW} is the corrosion rate, mm/year; С_{0} is the coefficient equal to 1 mm/year; F_{1}(T) is the coefficient that corrects for temperature; F_{2}(ХС) is the coefficient that corrects for the metal composition; F_{3}(υ) is the coefficient defined by the medium rate; F_{4}(O_{2}) is the coefficient that corrects for the oxygen concentration; F_{5}(pH) is the coefficient that corrects for the рН value; F_{6}(K_{k}) is the coefficient that corrects for the pipeline geometry (Keller coefficient); F_{7}(α) is the coefficient that corrects for steam humidity (for a singlephase medium, F_{7}(α) = 1); F_{8}(А) is the coefficient that corrects for the amine used (ammonia, morpholine, ethanolamine); and F_{9}(τ) is the coefficient that corrects for the component operating time; ΔS (τ) – wall thinning for the estimated time interval (τ_{0}, τ).
A parameter has been introduced into the EKI02 and EKI03 codes to take into account the influence of operating time on the corrosion rate and the wall thinning value which allows estimating the ECW and thinning rate while staying within the ‘reasonable conservatism’ limits.
The time parameter introduced into the EKI02 and EKI03 codes allows one to calculate
It was assumed in determining the dependence which describes the influence of operating time on corrosion rate, that the concentration of iron in feedwater depends on the escape of iron in the process of the secondary circuit equipment and piping ECW.
The content of iron reflects just indirectly the ECW processes, since there are more factors such as contribution of downtime corrosion to the corrosion processes, measures to remove corrosion products from equipment, and others. Naturally, the model simplification at the expense of such factors affects the code calculation accuracy. Currently, there is a sufficient array of data accumulated on the intensity of the ECW process in the pipeline components at NPPs with different reactor facilities. Analyzing such data makes it possible to propose a methodology to estimate the individual dependence of the ECW rate on time and to use it to predict the thinning of the pipeline component and to calculate its residual life.
The corrosion rate is estimated based on inspection data as follows (
W _{ecw} = (S_{nom} – S_{min})/τ, (3)
where W_{ecw} is the ECW rate based on inservice inspection data, mm/year; S_{nom} is the nominal wall thickness value, mm; S_{min} is the minimum wall thickness value based on inservice inspection data, mm; and τ is the time between the component operation start and the inspection date.
In the process of followup measurements, pointwise estimates of the ECW rate, obtained using formula (3), are approximated by the function of the form f (τ) = aτ –^{b}. The corrosion rate calculation results for feedwater pipelines (FWP) and steam lines (SP) are presented in Figs
Feedwater pipeline ECW rate obtained from inspection data and its approximation
No.  Date, year  τ, years  W _{ecw}, mm/year  W _{trend} ^{fw}, mm/year 

1  1984  3  0.900  0.772 
2  1988  7  0.400  0.396 
3  1989  8  0.238  0.357 
4  1995  14  0.286  0.230 
5  2001  20  0.180  0.174 
6  2001  20  0.185  0.174 
7  2005  24  0.158  0.146 
8  2006  25  0.136  0.141 
9  2007  26  0.154  0.142 
10  2007  26  0.112  0.142 
11  2008  27  0.148  0.138 
12  2009  28  0.154  0.134 
No.  Date, year  τ, years  W _{ecw}, mm/year  W _{trend} ^{sl}, mm/year 

1  1985  4  0.700  0.882 
2  1989  8  0.575  0.381 
3  1995  14  0.200  0.194 
4  1999  18  0.120  0.143 
5  2000  19  0.158  0.134 
6  2005  24  0.096  0.101 
7  2006  25  0.092  0.096 
For feedwater, the dependence for the ECW rate is
W _{trend} ^{fw} = 1.725· τ ^{–0.75},
and for steam lines, it is
W _{trend} ^{sl} = 3.307·τ^{–1.10}.
The following dependence is used to determine the pipeline operating time, Δτ, until the minimum allowable thickness (residual life) is reached
Δτ = (S_{min} – S_{all})/W_{ecw},
where Δτ is the residual life; and S_{all} is the minimum allowable thickness value, mm.
The smooth lines in Figs
Fig.
Corrosion testers (CT) are used to calculate the metal corrosion rate (corrosion penetration depth) for different components and pipelines. Testers compute the corrosion penetration rate value as the ratio of the loss of weight to the metal surface area and density. In other words, this is a characteristic of the metal general corrosion in a medium.
To determine the nature of the CT corrosion wear at VVER440 NPPs, there were processed the corrosion penetration rate values for 27 CTs installed on components with a singlephase medium in three VVER440 NPP units with the exposure time between 7.9 to 61.3 thousand hours (Table
No.  Exposure, h  W _{pen}, μm/year  No.  Exposure, h  W _{pen}, μm/year 

1  7896  5  14  18456  2 
2  8568  3  15  19128  1.2 
3  8736  4  16  24960  2 
4  8784  4  17  25848  0.6 
5  8856  3  18  26304  3 
6  8928  4.1  19  33168  1.5 
7  9528  3  20  35208  2.5 
8  10224  3.2  21  35808  2 
9  17208  2  22  41472  1.1 
10  17640  2  23  43416  1.1 
11  17784  3  24  51624  0.97 
12  17904  2  25  51672  0.6 
13  18336  1  26  61224  0.9 
27  61296  0.85 
Fig.
It should be noted that processing and interpretation of inspection data are one of the most complex issues (
The corrosion penetration rate was calculated for CTs of carbon and highchromium steels installed in pipelines at RBMK1000 NPP units. While carbonsteel CTs are subject to corrosive wear during operation in an aqueous medium, highchromium steels and alloys corrode electrochemically which is defined by the presence of the electrode potential.
For steel 20, the exposure time in different pipelines at NPP 1 and NPP 2 is from 10 to 113 thousand hours. The corrosion penetration rate values lie in a range of 1 to 11.5 μm/year.
In a general form, the equations to calculate the corrosion penetration rate W_{pen} are reduced to the form
W _{pen} = С_{o}×τ^{n}, μm/year,
where С_{o} is the coefficient; τ is the CT exposure time, years; and n is the power index.
The average error of the corrosion penetration rare calculation, according to the trend with respect to the inspection data, does not exceed 10%.
The W_{pen} diagram for the saturated steam pipeline CTs is presented in Fig.
Fig.
A similar function describes the dependence of the corrosion penetration rate for CTs in saturated steam pipelines.
W _{pen} ^{ss} = 0.4126×τ ^{0.921}.
Data from corrosion testers in stainlesssteel pipelines at NPP 2, also with the RBMK1000 reactor, is presented in Table
Exposure time, thsd h  18  26  40  93  133 
W _{pen}, μm/year  0.20  0.17  0.13  0.10  0.08 
Calculation of confidence intervals for the trend:
Δ_{s},_{t} = t_{n}_{,β} ·(S^{2}/n)^{1/2},
where Δ_{s},_{t} is the confidence interval value; β is the confidence probability (β = 0.95); n is the number of measurements (sample size = 5); t_{n}_{,β} is Student’s inverted distribution (for n = 5 and β = 0.95, the value t_{n}_{,β} = 2.776); and S^{2} is the sampling variance:
$S=\sqrt{\sum _{i=1}^{n}{\left(f\left({t}_{i}\right){y}_{i}\right)}^{2}/(n1)}$,
where f (t_{i}) is the rate value according to the trend at the time t_{i}; y_{i} is the rate values according to the experimental data at the time t_{i}; and t_{i} is the exposure time. Hence, we get the following results: S = 0.0052; Δ_{s},_{t} = 0.0064 (Fig.
In nuclear power, the corrosive impacts of the medium on the material of structural components in conditions of operation are taken into account using allowance С_{2} which characterizes the influence of the medium on the structural material in operating conditions (
The greatest value of the corrosion penetration depth for an exposure time of 60 years, predicted from the obtained functional dependences, is equal to 2.64 μm, that is, much below 0.1 mm.
The experience of operation, repeated measurements, and an analysis of corrosion tester data show a substantial reduction in the ECW rate with time. Therefore, ignoring the effects of time on the ECW rate estimation leads to excessive conservatism in estimating the residual life.
Apart from the time function built in the EKI02 and EKI03 codes, one can use the dependence obtained from repeated measurements on components of a particular NPP. It has been shown that this dependence for different pipelines is the same: f (τ) = aτ –^{b}.
It has been shown that, for a time span of up to 20 to 25 years, the time function used in the EKI02 and EKI03 codes leads to more conservative residual life values than the functions obtained based on inspection results; the difference is nullified with longer operating times.
The processing of data for corrosion testers in one and twophase media has shown the same dependence for the corrosion penetration rate as for pipelines. However, unlike the ECW rate (local effect), these values characterize general corrosion.
Using the functional dependences obtained based on inspection data makes it possible to reduce the conservatism in calculating the ECW and general corrosion rate during the specified life period and to predict adequately the values of these characteristics for the extended life period.