Chloride-induced stress corrosion cracking happens when chloride ions disrupt the protective passive film on stainless steels, allowing cracks to grow in a humid, warm environment like what forms under insulation in CUI conditions. Austenitic stainless steels have a high chromium-nickel content that gives them excellent corrosion resistance in many environments, but that same protective film can be attacked by chlorides when there is tensile stress and moisture under insulation. In these conditions, cracking can initiate and propagate along grain boundaries or through the grain structure, often without noticeable overall metal loss, making it a common failure mode for austenitic stainless steels in CUI. Copper alloys, aluminum, and carbon steel don't exhibit chloride SCC as routinely or predictably under CUI. Copper alloys may suffer other chloride-related effects, aluminum can pit or corrode, and carbon steel is more prone to general corrosion or hydrogen-related issues rather than classic chloride SCC, especially under insulation.

In harsh service conditions, primary lines are built for maximum reliability, so seamless pipe is preferred. It is produced without a welded seam, eliminating the weld bead and heat-affected zone that can become initiation points for cracks or corrosion under high pressure, temperature, or aggressive fluids. This gives more uniform strength, better toughness, and higher resistance to failure, which is crucial for critical piping. Welded pipes, including ERW, have seams that can be vulnerable in demanding service, and cast iron is brittle and generally not suitable for high‑pressure primary lines. For these reasons, seamless pipe is commonly specified for primary SAMPLElines.

The main idea here is how often you should schedule external inspections based on the pipe class. API 570 uses a risk-based framework that assigns longer intervals to lower-risk classes. For Class 3 piping, the upper limit for external inspections is 10 years. This reflects that Class 3 piping is considered less critical than Class 1 or 2, so the external inspection interval can be longer, provided there are no other factors driving a shorter schedule (like prior corrosion, coating damage, harsh service, or elevated corrosion rates). In practice, if everything is in normal condition, you would not exceed a 10-year external inspection interval for Class 3. If any adverse conditions are present, the interval should be shortened accordingly.

Martensitic stainless steels are the heat-treatable part of the stainless family. They contain enough carbon that, when quenched, the structure transforms to martensite, giving high hardness and strength while offering moderate corrosion resistance. This group sits in the 400-series, and a well-known example is the martensitic grade used for valve trim and cutlery. In contrast, the 300-series (like 316) are austenitic and cannot be hardened by heat treatment, and some 400-series grades are ferritic rather than martensitic. Therefore, the martensitic subset is represented by the 410-series.