Basic mechanism of secondary side steam generator tube degradation

In a first observation of the damages reported, it results evident that there has been practically no damage of SG tubing in the free span sections of the tubing, where the tubes are in contact with the bulk water under turbulent regime, with few exceptions [3]. The basic mechanism for the occurrence of SG tube corrosion deserves attention since it gives the basis for a correct SG and plant design and the associated chemistry.

The steam generation causes all the non-volatile compounds present in traces in the feedwater flow to be concentrated (enriched) in the SG water, achieving concentrations quite higher than those of feedwater. To keep these concentrations at a reasonable level, the SGs are regularly purged by continuous blowdown. At a SG blowdown rate of for example 0.5% of the feedwater flow rate, the equilibrium concentrations of dissolved impurities in the bulk SG water are in principle expected to be therefore 200 times of the feedwater concentration for a non-volatile compound.

In fact:

Ingress by feedwater = elimination by blowdown + elimination by main steam [g/h]

Cfw Ffw = Cbd Fbd + Cms Fms ≈ Cbd Fbd [g/h]

Cbd = Cfw Ffw/Fbd = Cfw 200 [µg/kg]

Where:

Cfw is the impurity concentration in feedwater Ffw is the feedwater flow rate

Cbd is the impurity concentration in SG blowdown

Fbd is the SG blowdown flow rate

Cms is the impurity concentration in main steam Fms is the main steam flow rate.

That means, for the usual concentration of impurities in feedwater (in the rule below the limits of detection, i.e. quite below 1 µg/kg for most of the non-volatile impurities), the concentrations of these impurities in the SGs are expected to be few µg/kg only, provided that these substances are soluble and homogeneously distributed in the SG bulk water.

Owing to the SG tubing material properties, these expected concentrations in SG are far insufficient to cause any corrosion damages. This is widely supported by laboratory as well as the field experience, reporting no or negligible tube damage in the free span area of the tubes.

However, there is also a permanent ingress of corrosion products, mainly magnetite into the SGs, and deposits will be accumulated on the SG tube surfaces at top of tube sheet or in the tube to tube support crevices.

If the accumulated corrosion product deposits are sufficiently thick there is a risk of local enrichment of impurities beneath these deposits. This can occur at the tube sheet area within the flow steadied zone (sludge pile) and also at heavily encrusted tube to tube support device intersections. See Figure 4.13 [24] that represents the case at the tube sheet, as a matter of example. In fact, if thick corrosion product deposits accumulate in contact with the tube heat transfer surfaces, there will be a local overheating due to the additional resistance to heat transfer, causing the deposits to dry out. The surrounding water will keep the outer surface of the deposits wet, and this water will evaporate as it penetrates the deposit, creating a dry-to-wet interface where there will be a permanent boiling. This boiling will cause non-volatile impurities to concentrate at the wet/dry interface up to very high local concentrations. The concentration factors may achieve very high values, in the range of 10⁴ to 10⁶ times the bulk concentration.

These high impurity concentrations, associated to the locally higher temperatures (overheating) may be responsible for initiation of corrosion processes on the involved metal surfaces, especially at the SG tubing.

Figure 4.13. Impurity enrichment mechanism beneath deposits at the SG tube sheet (schematic).

Corrosion Products (Fe3O4)

Depending on the composition of the impurities in question, a series of physic-chemical changes will occur, like hydrolysis, precipitation of compounds after achievement the solubility product, etc.

These processes are very complex and strongly dependent on the composition rather than the concentration present in the bulk SG water. The composition of the bulk impurities will be given by the impurity sources, mainly:

 Ingress via make-up water (output quality of the water treatment plant),

 Condenser leaks (composition of cooling water),

 Consumables and auxiliary products which may enter into contact with inner surfaces (montage aids, for example),

 Impurities in the conditioning chemicals,

 Others.

As the concentration of the impurities in deposits increase, the pH can shift locally in these areas to acidic or alkaline conditions, entering in a pH range where initiation of corrosion phenomena cannot be longer excluded.

By maintaining of sufficiently reducing conditions, the occurrence of certain corrosion mechanisms will be excluded (like pitting), but certain forms of SG tube corrosion may still occur.

If these concentration mechanisms are considered, together with the redox condition of the system, a variety of possible corrosion mechanisms may appear [3, 13, 18, 19], including denting, SCC, IGA or the combination of both (IGA-SCC), pitting, among others, which are schematically shown in the Figure 4.14 below, where the possible corrosion mechanisms at different pH-redox conditions are graphically shown.

Worldwide performed investigations show clearly the SG tube material lack of stability against corrosion in the case of extremely acidic and/or alkaline conditions. Even with reducing conditions, stress corrosion cracking (SCC) of SG tubes cannot be excluded, if pHT values of < 5.0 or > 9.5 prevail (see Figure 4.15 for I800 SG tube material² and Figure 4.16 for Alloys 600MA/TT and 690TT).

That means that ensuring a local pHT value within the range of 5.0 up to 9.5 under reducing conditions would minimize the risk of stress corrosion cracking, according to this source. However, the safe pH range at operation temperature shall be considered to be roughly 4 to 8.5, based on available experience.

This pH cannot be measured, but only calculated. This calculation is strongly dependent on the input data, and the results should be interpreted as indicative only.

Computer aided programmes like the EPRI’s MULTEQ have been developed to model the on-going processes as impurities concentrate, departing of a known (pre-defined) composition and concentration of impurities present in the SG bulk water. For instance, it can be predicted how a condenser leak will modify the chemical environment in the enrichment areas, or also the influence of the make-up water quality.

From these calculations, it results evident that depending on the input composition, the local chemical environment at the enrichment zones may shift to aggressive environmental conditions or not, depending on which species are being concentrated. A possible output of such calculations is schematically shown in Figure 4.17.

2 In this figure, the redox potential is indicated by the author as ‘oxidation potential’. Negative values mean reducing conditions. SHE: Standard Hydrogen Electrode.

Figure 4.14. Illustration of SG tube corrosion in dependence of environmental conditions.

Figure 4.15. Stress corrosion cracking of I800 as a function of pH.

Figure 4.16. Stress corrosion cracking susceptibility of Alloy 600MA/TT and 690TT as a function of pHT [25].

Co: impurity concentration at the SG bulk water C: impurity concentration inside the deposit

Figure 4.17. Evolution of pHT as a function of the impurity enrichment factor C/Co.

A shift into alkaline conditions may be responsible for the initiation of caustic induced SCC. A shift to moderately acidic conditions implies a risk of IGA initiation. Strongly acidic conditions associated with insufficiently reducing conditions have been responsible for the initiation of denting³ in plants having carbon steel tube support plates.

Summarizing: Where corrosion products accumulate, local overheating may occur, increasing the local temperature and causing enrichment of the impurities dissolved in the SG bulk water up to corrosive levels. Corrosion is likely to occur, almost independently of the tube material.

There have been five ways selected by different suppliers to avoid the formation of aggressive local environments (see Figure 4.17):

1. Avoid the enrichment of impurities by avoiding at the maximum possible extent the