Failure of Tool Steel Pylon Attachment Stud
A failed H-11 tool steel pylon attachment stud was found during a routine walk-around inspection. The stud exhibited gross localized corrosion pitting at several different areas on its surface. Light general rust was also evident. Severe pitting occurred near the fracture location. The fracture face contained evidence of intergranular SCC as well as ductile dimples. The protective coating was found to be an inorganic water-base aluminide coating having a coating thickness of 7.5 to 13 micron (0.3 to 0.5 mil). The coating was of a nonuniform mottled nature. It was concluded that the failure of the pylon attachment stud was caused by general corrosion followed by SCC. The stud was not adequately protected against corrosion by the coating. It was recommended that the coating be applied to a thickness of 38 to 75 micron (1.5 to 3 mil) to provide long-time corrosion resistance. The coating must be either burnished or cured at 540 deg C (1000 deg F) to provide cathodic protection to the steel. Other coatings, such as cadmium or aluminum, were also recommended if a thinner coating is needed.