The weld bead and its heat-affected zone are the weakest point of any cladded 316 heater. The weld crown is uneven, the grind is imperfect, and the film over a weld tends to be thinner and more stressed than on the rolled tube. Chloride and vapor concentrate at the edge of the film, and the weld toe is where blisters first appear. A single uniform coat over the whole bundle therefore leaves the weld line under-protected.
The fabricator applies a second coat over the welded length, building the film thicker at the toe and on the heat-affected band. The weld crown is ground to a smooth radius before coating, because a sharp corner pulls the PTFE film away and leaves a microscopic air pocket. The glass-fiber filled composite flows over the rounded toe during sintering, locking in. Material-selection service specifies the double-coated weld length on the drawing, extending well beyond the bead.
At each turnaround, the weld toe is the first place the inspector holiday-sparks. A blister or edge lift along the weld line is repaired immediately, because a breach there exposes the sensitized weld band to chloride. The smooth cladding surface also resists gasket cement buildup, and the flange face is coated rather than ground. Procurement requires the weld-length thickness measurement.
The weld itself must be properly done: full penetration, no undercut, and post-weld pickling to remove heat tint. A poor weld cannot be rescued by a thick film. The pre-treated composite covers a clean, passive weld, but it cannot bridge a crack or a lack of fusion. Fabrication discipline, radiographic inspection and pickling precede the second coat.
The result is a heater where the weld line is as protected as the tube body. Plants that had recurring weld-edge corrosion on single-coated heaters switched to the double-coated, ground-radius weld and stopped the failures. The weld toe, not the tube, is where the cladding earns its keep.
The fabrication detail that most often undoes the double-coated weld is a sharp weld toe. If the fabricator leaves an undercut or a sharp grind at the toe, the film bridges a microscopic gap, and vapor tracks into it. The material-selection service therefore requires the toe to be ground to a smooth radius, then pickled, then double-coated. The glass-fiber filled composite flows over the rounded toe during sintering, but it cannot stretch over an undercut. At each turnaround the inspector focuses the holiday wand on the weld length, and the weld toe is probed by hand. Plants that made this change report that their weld-edge failures dropped to near zero. The double coat is not extra thickness for its own sake; it is insurance against the one geometry the film cannot naturally cover. The ground radius, the double coat, and the careful inspection together turn the weld from the weak point into a protected length.
Procurement also specifies the weld-length thickness measurement, because a thin film over a weld is the most common hidden defect. The fabricator records the gauge reading at the toe and at the center of the weld length, and the inspector compares them against the drawing. The pre-treated composite wets the radius, but only if the radius is ground smooth and the film weight is adequate. The double coat, the radius, and the recorded thickness together turn the weld line into a protected length rather than a recurring failure point.
