Failure analysis of a butterfly valve made with nickel aluminum Bronze (NAB) and manganese aluminum Bronze (MAB)

Butterfly valves installed at the seawater system used to cool the fresh water of cooling medium in an offshore oil platform type Floating Production Storage and Off-loading (FPSO) failed in an unexpected way after periods of three to six months of operation. When closed, the valve faces were kept in contact with water injection on one side, and with untreated seawater on the other side. Moreover, they were totally exposed to seawater when opened. One failed valve was sent for metallurgical failure analysis and the damage appeared in the valve body in the position of sealing. The body and the disc of the failed valve were made of a cast Nickel Aluminum Bronze alloy (NAB) type ASTM B148 UNS C95800, and the sealring attached to the disk was made of Ni superalloy 625 with polytetrafluoroethylene (PTFE). The investigations conducted in the failure analysis revealed that the material of the seat area was different from the material of the body, since a large ring of NAB was welded to the body, using a Manganese Nickel Bronze (MAB) alloy as feed metal. The failure was associated to three distinct problems. First, several welding defects were observed. Second, an adequate post weld heat treatment was not performed in the seat part, leading to the presence of β retained phase (β’) which decreases the corrosion reistance of NAB and MAB alloys. Third, galvanic corrosion also ocurred since the Ni superalloy 625 is nobler than the NAB, and they were in close contact. Additional corrosion tests revealed that NAB alloy was very susceptible to de-alloying in 3.5 %NaCl solution.