MOCCA - Mitigation of cracking through advanced water chemistry

Corrosion problems in the PWR secondary circuit are mostly related to deposition of magnetite into steam generator (SG) and enrichment of impurities into crevices within the circuit. The enrichment is typically driven by boiling. Water entering the crevices within a SG (e.g. between tube and tubesheet or under a magnetite deposit on a straight tube) boils letting volatile species escape as steam and leaving non-volatile species (salts, lead, copper etc) in the small water volume of the crevice. After some time of operation, the crevice chemistry can become very aggressive (either acidic or basic) due to impurity enrichment.

This project focuses on advanced water chemistry tools by which the formation of magnetite particulates in the feed water line can be mitigated and their deposition into SG can be minimised. To that end, the mechanism of lead assisted stress corrosion cracking as a major threat to SG integrity is researched. In addition, substitutes for using hydrazine, a potentially cancerous chemical used in PWRs, are studied.

Specific goals in 2018

The use of hydrazine, an oxygen scavenger routinely used in PWRs/WWERs both during outage and power operation to ensure low oxygen concentration and thereby low corrosion rates, is under consideration because it’s negative effects on environment and health. In 2018, the target was to compare experimentally the effectiveness of several alternatives to hydrazine as oxygen scavengers. Especially alternatives which are active at low temperature, at T = 50^oC, i.e. during plant shut-down were of interest here.

One of the primary causes of SG corrosion damage is magnetite particle formation in the secondary side feed water line and further deposition of magnetite particles into SGs – thus, finding ways to mitigate deposition of magnetite into the SG is a major goal in this study. In 2018, the goal was to verify experimentally the surface charge of stainless steel (a major structural material used in SG internals) in PWR secondary side water as a function of temperature.

Another clearly established cause of SG corrosion damage is the lead assisted stress corrosion cracking, PbSCC. In 2018 the target was to finalise the work on PbSCC of carbon steel in alkaline crevice environment, representative of some steam generators.

International co-operation was continued in 2018 by e.g. attending the 17^th Nordic Corrosion Congress held in Copenhagen, Denmark, the Nuclear Plant Chemistry (NPC 2018) conference held in San Francisco, USA and by taking part in the European Co-operative Group on Corrosion Monitoring (ECG-COMON) work and the yearly meeting held in Ljubljana, Slovenia.

Alternatives for hydrazine

The primary requirements for an oxygen scavenger are strong reducing action and good passivation ability. Strong reducing action keeps oxygen levels entering the steam generator low, thus minimising the risk of SG tube degradation by localised corrosion modes. Good passivation ability, on the other hand, reduces propensity to flow assisted corrosion (FAC) in the feed water line and thereby reduces rate of delivery of corrosion particles into SG where they could form deposits and enhance susceptibility to localised corrosion modes.

Based on a previous literature study, the present investigation focused on the effectiveness of several hydrazine alternatives as oxygen scavengers. The conditions used were representative of a steam generator under revision period, i.e. T = 50^oC and alkaline pH. To evaluate the oxygen removal rate, measurements of dissolved oxygen concentration with time

in borate buffer solutions were performed and analysed with different concentrations of carbohydrazide, diethyl-hydroxylamine, methyl-ethyl-ketoxime and iso-ascorbic acid.

A comparison of the oxygen removal rate by i-AA and MEKO is shown as an example in Figure 2.41. Based on quantitative evaluation of the data, the following ranking of the studied compounds by oxygen scavenging ability at 50 °C and pH 9.2 can be proposed: hydrazine >

iso-ascorbic acid > carbohydrazide >> DEHA > MEKO, Table 2.5. This is the same ranking found earlier at 21^oC. The rate of oxygen removal by hydrazine was about twice higher than that of i-AA and CBH, and about seven and fifteen times higher than that of DEHA and MEKO, respectively.

The effect of the investigated additives on the corrosion of 22K carbon steel in borate buffer solution at 50 °C was also studied. Based on the data it can be concluded that all the additives increase the general corrosion rate of 22K steel (by stabilising Fe3O4 over FeOOH and Fe2O3) with respect to that in pure buffer solution, hydrazine having the strongest effect.

Figure 2.41 Evolution of the dissolved oxygen concentration with time in a borate buffer solution with the addition of different concentrations (in mmol dm^-3) of (a) i-AA and (b) DEHA at 50 °C. The rate of decrease of oxygen is clearly faster with i-AA than with DEHA.

Table 2.5. Oxygen removal rate at the highest initial scavenger concentration in mmol dm^-3 (in parentheses).

Scavenger Oxygen removal rate at T = 50^oC / mmol dm^-3s^-1 Carbohydrazide 0.020 (0.034)

DEHA 0.0054 (0.46)

MEKO 0.0028 (0.75)

Iso-ascorbic acid 0.025 (0.45)

Hydrazine 0.043 (0.32)

Magnetite and stainless steel surface charge

During previous years in the current project we have studied the effect of ammonia, morpholine, ethanolamine and octadecylamine (ODA) on the surface charge of magnetite particles. This year, the target was to study the effect of temperature on the surface charge of

stainless steel. However, due to equipment malfunction and failure, this target was not reached.

Mechanism of lead assisted stress corrosion cracking of carbon steel

Lead (Pb) has been detected in effectively all tube-support samples, crevice deposits and surface scales removed from steam generators. Typical concentrations seen are 100 to 500 ppm but in some plants, concentrations as high as 2,000 to 10,000 ppm have been detected.

The cracking susceptibility is believed to have a strong dependence on the redox-potential of the crevice environment. Redox-potential, on the other hand, is affected by the amount of e.g.

copper oxide in the crevice solution.

The PWR steam generator tube materials considered to be most resistive towards stress corrosion cracking (SCC), i.e. Alloy 600TT, Alloy 800 and Alloy 690 have each been shown to be susceptible to SCC enhanced by the presence of lead (PbSCC). In case of VVER-type PWRs, the steam generator tubing is most often stainless steel which has a relatively low susceptibility to PbSCC. However, the VVER steam generator body material, carbon steel, has been very little studied, in spite of wall-through cracking insidents found both in Russia and Czech Republic.

The technique for studying SCC in this project was to perform slow strain rate tests (SSRT) in which a tensile specimen is loaded at a constant strain rate until fracture occurs. Susceptibility to SCC is deduced from the reduction in fracture strain as compared to that in an inert environment (e.g. air at the same temperature) and additionally from the morphology of the fracture surface.

Figure 2.42. Slow strain rate testing (SSRT) results: (a) at open circuit and at several anodic potentials in acidic crevice solution, (b) at open circuit in alkaline crevice solution with and without Pb addition.

Figure 2.43. Micrographs of cracked SSRT specimens in air (above, left), acidic crevice solution without (above, right) and with Pb at open circuit (middle, left) and at -0.42 V(middle, right), (below) corresponding micrographs in alkaline crevice solution at open circuit without (left) and with Pb (right).

Figure 2.42a shows a comparison of stress-strain curves of of carbon steel 22K in crevice solution at T = 278^oC with and without Pb, at corrosion potential and at slightly elevated potentials. Surprisingly, addition of 100 ppm Pb at corrosion potential is seen to increase the fracture strain from 22% to 28%, i.e. make the material more ductile. However, increasing potential (simulating slightly oxidising environment produced by e.g. oxygen inleakage into the SG) results in a dramatic reduction of the fracture strain to values at and below 10%. Figure 2.42b shows similar curves measured in alkaline crevice solution. Without Pb, the fracture strain is 28.0% and with Pb 29.2%, indicating a rather small susceptibility to SCC under the alkaline conditions. The fracture surfaces shown in Figure 2.43 corroborate the information found in the SSRT in that the percentage of ductile fracture surface (i.e. area with no marks of SCC) is clearly higher for alkaline solution than for acidic solution, with or without Pb.

Figure 2.44. Current density vs. potential curve for carbon steel 22K in simulated alkaline SG crevice environment at 278 °C with and without the addition of 0.45 mmol kg^-1(100 ppm) Pb as PbO.

The current density - voltage -curve in the alkaline crevice environment without lead showed a slowly increasing trend as a function of potential, indicating a passive surface, Figure 2.44.

The rate of increase goes slightly up above -0.35 VSHE, at which potential Fe3O4 (magnetite) is expected to transform to Fe2O3 (heamatite). The curve in the presence of Pb, on the other hand, shows an active peak around -0.62 VSHE, followed by passivation and another increase in current density for potentials higher than -0.4 VSHE. Based on these results it can be assumed that close to the corrosion potential (-0.7 VSHE) Pb²⁺ ions are reduced on the steel to metallic Pb. At potentials more positive than the corrosion potential, Pb starts to dissolve as PbCl⁺ accounting for the active peak in the current density - potential -curve in Figure 2.44.

The dissolution of lead is most probably accompanied with Fe dissolution leading to an unstable passivation of the surface. The reduction in the SCC susceptibility in presence of Pb (as noted by higher fracture strain in Figure 2.43 and higher portion of ductile area in Figure 2.44) is proposed to result from this unstable passivation, which at lest partially prevents localisation of corrosion necessary for SCC to occur.