For steam generator tubing, which alloy family is most commonly used and what is the primary corrosion concern?

• A. High nickel content alloys have no risk
• B. ZIRLO alloys experience PWSCC
• C. Alloy 600 family (600/690) nickel-based alloys; PWSCC and corrosion
• D. Aluminum alloys are best for SG tubing

Answer: (C)

Steam generator tubing relies on nickel-based alloys because they provide reliable performance in the hot, high-purity primary-water environment and can tolerate the stresses from tube geometry and fabrication. The family includes Alloy 600 and its derivative Alloy 690. Alloy 690, with higher chromium content, offers even better resistance to corrosion and stress cracking than Alloy 600, but neither is free of risk.

The primary concern for these materials is primary water stress corrosion cracking (PWSCC). The combination of tensile stresses (from fabrication, welding, and temperature changes) and the aggressive, high-temperature primary water can initiate and drive cracks, especially at welds and bends. This is the main form of corrosion to watch for in steam generator tubes, and that's why the nickel-based 600/690 family is favored—while ongoing improvements reduce PWSCC risk, it remains the key worry.

Other materials don’t fit the application well: aluminum alloys don’t hold up in high-temperature reactor water, ZIRLO (a zirconium alloy) is used mainly for fuel cladding, not SG tubes, and stating that high-nickel alloys carry no risk ignores the reality that PWSCC can occur in these materials under the right stress and environmental conditions.