Compared with Alloy 600, Alloy 690 in steam generator tubing provides is primarily due to:

• A. Higher corrosion resistance overall
• B. Superior resistance to primary water stress corrosion cracking
• C. Lower cost
• D. Lower mechanical strength

Answer: (B)

In this context, the most important degradation mechanism for steam generator tubing in a PWR is primary water stress corrosion cracking under the high-temperature, oxidizing conditions of primary water. Alloy 690 contains significantly more chromium than Alloy 600, which leads to the formation of a more stable and protective chromium-rich oxide layer on the surface. This protective film acts as a barrier, reducing the initiation and propagation of cracks under tensile stresses in the primary circuit. The higher chromium content, along with low carbon content that minimizes sensitization, makes Alloy 690 much more resistant to PWSCC than Alloy 600. While alloy 690 may also have other advantages, its primary benefit for steam generator tubing is superior resistance to primary water stress corrosion cracking, not merely overall corrosion resistance, cost, or strength.