Technical Position

SCC is a complex phenomenon that is affected by several factors in addition to chloride/fluoride. A tensile stress is required. Dissolved oxygen and temperature are very important. As the oxygen level and temperature decrease, the tendency for SCC decreases. Except for very alkaline

conditions (caustic solutions), the probability of SCC decreases as the pH increases. In CCW systems, many of these factors are such that the probability of SCC is low. This is discussed below and references are provided.

A.3.1 Factors Affecting SCC of Stainless Steel Materials

Under nuclear power plant CCW conditions of operation, SCC of Type 304 and 316 Stainless Steel is influenced by many interrelated factors. These factors include:

• Metal alloy composition • Temperature

• Stress Levels

• Chloride concentration

• Component surface conditions • pH

• Oxygen

• Other ionic species (such as sulfate, fluoride) • Microorganisms

• Corrosion inhibitors/chemical additives

The parameters that are controlled in a CCW system include pH, microorganisms, ionic contaminants, and inhibitors. The pH in the CCW system water is controlled by the use of chemical additives. Chemicals such as chromate, nitrite or molybdate (carbon steel corrosion control) and azoles (copper alloy corrosion control) are added to reduce corrosion in the systems. Biocides are added as needed to reduce microbe populations. Monitoring of other parameters (that is, chloride, fluoride, sulfate), and heat exchanger performance, when possible, and taking prompt corrective actions when necessary, will provide assurance that detrimental corrosion is mitigated.

Stress. CCW systems are designed such that stress levels do not exceed 33 % of material yield

strength. CCW system components will not exceed 90 % material yield strength even under accident conditions of operation.

By establishing and maintaining a CCW system chemistry control program, there can be

reasonable assurance that corrosion and SCC are under control in the CCW system components. It has been documented (Ref. 2,3,4) that with chloride concentration levels maintained below 200 ppm, SCC is not a major concern. Documented data also indicates that for an alkaline range (pH > 7.0), SCC is minimized (Ref. 6). Chemical additives, such as nitrite, molybdate, chromate, etc., provide a corrosion boundary for carbon steel to enhance the CCW components’ resistance to corrosion. Operating temperatures and component surface temperatures in the CCW for the majority of the system are <180°F. SCC is minimized at these temperatures.

Although some references indicate that fluoride concentrations in the CCW water are not detrimental to system components and will not cause significant corrosion or SCC. (Ref. 9), fluoride is considered a stainless steel corrodent for purposes of this document. For pure water CCW systems (BWR), soluble impurity concentrations are limited and controlled by the CCW water-conductivity limit. The GE specification for conductivity of a pure water CCW system is 3.0 µS/cm.

The GE specification for chloride concentration limit in a nitrite corrosion inhibitor solution is <10 ppm.(Ref. 10)

Sensitization. Sensitization is a state of reduced corrosion resistance of austenitic stainless steels

caused by a depletion of chromium in the grain boundary area. It occurs when high carbon containing stainless steels (~.08%C) are heated in the temperature range 950-1450°F. It is often observed after welding, in the heat- affected zone. Sensitized stainless steel is very susceptible to intergranular corrosion and SCC. Intergranular SCC of sensitized Type 304 stainless steel has been observed in the laboratory in pure water with 1-2 ppm oxygen in the temperature range 122 - 212°F (Ref. 6). However, sensitization can be avoided if low carbon grades of stainless steel (~.03%C) are used, for example, Types 304L and 316L. It is expected that these low carbon grades will be in place in the CCW systems under consideration and, hence, sensitization will not be an issue.

A.3.2 Failure Modes with Stainless Steel System Materials

SCC of Stainless Steel Tubing in Heat Exchangers

The closed cooling water system heat exchangers in nuclear power plants are normally

horizontal with the cooling water on the outside of the tubes. Experience indicates that there are three distinct failure modes of tubing when cooling water is on the shell side and that these are influenced by the metallurgy in use. The three failure modes are:

Position Paper on Impurity Concentration Limits for Closed Cooling Water Systems

Mechanism: The flow around the baffles on the shell side tends to deposit sediment and

entrained matter in the cooling water in the bottom of the heat exchanger, often to the extent that the lower tubes are covered. Under deposit pitting/crevice corrosion and/or MIC, are common failure modes for Type 304 and Type 316 stainless steel and copper alloy tubes, covered by sediment/crud in such heat exchangers. Highly alloyed stainless steel tubing (that is, 6% molybdenum) is resistant.

Crevice corrosion of stainless steel tubes at stainless steel tubesheets, from the shell side

Mechanism: The tube to tubesheet crevice is a tight stationary crevice where crevice corrosion is likely for Type 304 tubes, in Type 304 tubesheets, in waters with > 150-200 ppm chlorides. It is also likely for Type 316 tubes, in Type 316 tubesheets, in water with > 1000 ppm chlorides. For these alloys in clad tube- sheets, the carbon steel under the cladding is galvanically protective to stainless steel and prevents crevice corrosion, even when the chloride concentration exceeds the 150-200 ppm limits for Type 304, and the 1000 ppm limits for Type 316. For copper alloy metallurgy, crevice corrosion in solid or clad tubesheet-to- tube crevices is not a problem. Stress corrosion cracking of stainless steel tubes in stainless steel tubesheets

Mechanism: SCC is a location-specific failure mode. The residual stresses from rolling the tubes into the tubesheet are high enough for SCC to occur. Eight (8) ppm of oxygen is adequate. If chlorides were able to reach critical concentrations in the crevice, SCC might occur.

To avoid SCC, the following limits are proposed: chlorides < 10 ppm, tube inlet temperatures < 180F, oxygen < 8 ppm, pH > 7, and residual stress < 90% of yield.

Chloride Ion Concentration in the Crevice

Evaporation from hot surfaces is a leading mechanism for concentrating chlorides and initiating SCC. Stainless steel tubes in vertical heat exchangers, used in the chemical industry, suffer SCC in the vapor pocket just below the top tubesheet. Stainless steel downcomer piping suffers SCC just above the liquid level. Insulated stainless steel piping has suffered SCC when wet by rain or overhead leaks that filter through the insulation and evaporate from the hot surface. Time is a major factor. In many cases, 3 to 7 years are required for initiation of SCC from chloride

concentration on hot surfaces. The higher the chlorides in the water or moisture evaporated from hot surfaces, the shorter is the time for initiation of SCC. There are no well accepted

mathematically certain guidelines.

SCC has been encountered in vertical heat exchangers, in waters with as low as 14-ppm chlorides, although it took seven years for the SCC to occur (Reference 8).

However, in horizontal heat exchangers, with water on the shell side, the tubes are covered and there is no hot wall to evaporate water and concentrate chlorides. The literature mentions possible chloride ion concentration in crevices by occlusion and other mechanisms, so

concentration by other than evaporation cannot be ruled out. Experience, however, suggests that the proposed 10 ppm chloride upper limit for Type 304 or Type 316 tubes in solid tubesheets, in

horizontal heat exchangers with cooling water on the shell side, would be ultraconservative for both SCC and crevice corrosion.

Experience also suggests that it would be wise to include a prohibition against the use of carbon tetrachloride and other chlorinated solvents in degreasing after tube rolling operation. Use of chlorinated solvent degreasers has resulted in SCC failures of heat exchangers when placed in service.

For clad stainless steel tubesheets, the carbon steel would protect stainless steel from SCC as well as from crevice corrosion. SCC is not a problem with copper alloy tube and tubesheets. Summary

The proposed guidelines should include recommendations that:

• Periodic flushing out of sediment that collects in the bottom of such units is recommended. Well-maintained filters in closed cooling water systems are beneficial.

• The use of chlorinated solvents for degreasing after tube rolling operations be prohibited. • For Type 304 tubes in solid stainless steel tube sheets, and Type 316 tubes in solid stainless

steel tubesheets, the proposed 10-ppm chloride limit is believed to be ultraconservative insofar as prevention of SCC and crevice corrosion is concerned.

• For clad tube sheets, for more highly alloyed stainless steel tubes, and for copper alloy metallurgy, much higher chloride ion concentrations could certainly be tolerated.