**1. Introduction**

Nuclear power plants steam generators (NPPs SGs) are normally vertical cylindrical vessels made up of inverted U-tubes and with steam-water separators at the top of the device. Boiling occurs on the shell side named the secondary side while on the primary side, inside of the tubes, the coolant never reaches the state of steam (310–330°C and 15 MPa). Most NPPs PWR have between two and four SGs.

Actually, most steam generator tubes are fabricated with thermally treated Alloy 690 even though is still supplied with replacement Alloy 800NG tubing. In this manner, progressively replacing Alloy 600 MA has been performed due to many failures caused by stress corrosion cracking (SCC) in this alloy. **Figure 1** shows a Fe-Cr-Ni ternary diagram where is shown the composition of the alloys used in nuclear power plants.

Alloy 690 is a solid substitutional alloy consisting of approximately 60% nickel, 30% chromium, and 10% iron. The concentration limits of these elements as well as

#### **Figure 1.**

*Fe-Cr-Ni ternary diagram for 400°C for NPPs SGs [1].*


#### **Table 1.**

*Designations for alloy 690.*

the rest of the constituent elements present slight differences depending on the manufacturer or institution that sets the specification. **Table 1** presents various designations and standards for this alloy. The chemical composition of alloy 690 is listed in **Table 2**.

The control of carbon within the limits established according to **Table 2** is essential to obtain the desired corrosion resistance and mechanical properties. In general, a maximum limit is established, although in some specifications a minimum limit is indicated [2]. Carbon range between 0.015 and 0.025% is the optimum to obtain a microstructure with inter and intragranular carbide precipitation. The lower carbon limit (0.015%) is necessary to ensure enough carbon is available for correct carbide precipitation. The upper limit (0.025%) is specified to ensure that most of the carbides formed can be dissolved during mill-annealing heat treatment [3]. Limits have also been specified for sulfur and phosphorus since these elements could segregate at grain boundaries and have a detrimental effect on corrosion resistance. In addition, sulfur decreases the ability of these alloys to be hot worked, so it is necessary to keep its level at low values.

On the other hand, Alloy 800 is an alloy closer to titanium stabilized stainless steel. This alloy has been used for industrial applications with slight differences in its composition and with different thermal treatments to ensure optimal behavior in the


**Table 2.**

*Composition limits, %. ASTM designation B163 or ASME SB163: "Standard Specification for Seamless Nickel and Nickel Alloy Condenser and Heat-Exchanger Tubes."*

operating environment. In the nuclear industry, the composition and heat treatment of alloy 800 has been optimized, establishing a clear difference between the standard alloy and its modified version named modified alloy 800, **Table 3**.

The compositional variations introduced in the alloy 800 have important repercussions in terms of the corrosion resistance of this material (**Table 4**). For example, the reduction of the carbon content to 0.03% and the stabilization ratio Ti/C>12 and Ti/C +N>8 have been established to ensure good resistance to sensitization and, therefore, resistance to intergranular corrosion in conditions representative of acid-sulfate chemistry. In addition, the total amount of aluminum and titanium is important. Al+Ti contents above 0.55%, produce the precipitation of γ´phase (Ni3(Al, Ti)) when the material is subjected to thermal treatments between 500 and 700°C. This gamma phase and the M23C6 type carbides produce an increase in creep resistance with a clear decrease in ductility [4, 5]. Increasing the minimum chromium level to 20% results in good pitting corrosion resistance. On the other hand, increasing the minimum nickel content to 32% is intended to improve resistance to transgranular stress corrosion cracking.

Most of the failures that occur in NPPs SGs tubes involve various mechanisms, including:



**Table 3.** *Designations for alloy 800.*


**Table 4.**

*Composition limits, %. ASTM designation B163 or ASME SB163: "Standard Specification for Seamless Nickel and Nickel Alloy Condenser and Heat-Exchanger Tubes."*


Most nuclear power plants operate under AVT (all-volatile treatment) conditions on the secondary side of the steam generators. Common volatile conditioning agents are ammonia, amines, and hydrazine (or hydrazine substitutes). With AVT, the pH of the feed water ranges from 8.8 to 9.8.

During normal operation of the SGs, nonvolatile compounds rise to a concentration level higher than the feed water. For this reason, the SGs are periodically purged using continuous blowdown. Although this operation should be sufficient to avoid corrosion damage, the permanent ingress of corrosion products produces deposits accumulation on the tube-sheet increasing the risk of the initiation corrosion process. Physical and chemical changes may occur depending on the impurities level in the deposits or the sludge composition, such as solubility product, precipitation of compounds, hydrolysis, etc [6]. Impurity sources can be treatment plant quality, cooling water (condenser leaks), repairs and impurities in conditioning chemicals, etc. The insulating effect of a deposit of corrosion products can cause overheating of the tube metal and subsequent failure. Because they are generally porous, corrosion product deposits can also provide sites for boiler water concentration and thus the potential for caustic attack. In general, the concentration of species due to the presence of crevices and sludge accumulation gives rise to acidic or basic environments far away from normal secondary chemical conditions.

For this reason, many nuclear plants periodically perform sludge removal by tubesheet lancing (SL) and inner bundle lancing (IBL) on the steam generator's secondary side. Lancing uses high-pressure jets to mechanically remove sludge from the tubesheet surface, and in this way, impurities concentration is eliminated and denting formation is avoided.
