A Case 1 Failure of Aluminum Alloy Spacers by Galvanic Attack

Description of the problem Several rebuilt hydraulic actuators had been in storage for three years. At each joint, there was an aluminum alloy spacer and a vellum gasket. The mounting flanges of the steel actuators were nickel plated. While assembling the actuators, a lubricant containing molybdenum sulfide had been applied to the gasket to serve as a sealant. The galvanic attack occurred on the aluminum alloy spacer [2].

Identification The three major components: aluminum spacers, vellum gasket and actuators housing were examined in a laboratory. The following were the conclusions of the examination:

1. Corrosion had penetrated to a depth of 64 ?m in some areas of the aluminum spacers which had been stained badly.

2. The vellum gasket was electronically conductive.

3. A potential of 0.1 V was measured between the aluminum alloy spacers and the actuator housing.

4. No potential was detected after breaking contact between aluminum spacer and the actuator housing.

5. Deposits of molybdenum sulfide were found on the aluminum surface of aluminum spacers.

Conclusion Galvanic corrosion was the reason for failure. Molybdenum disulfide acted as an electrolyte between the aluminum spacers and the nickel plated steel actuator housing. In order to remedy the situation, dry vellum gaskets were used and the use of molybdenum disulfide was discontinued.

B Case 2 Failure of Hot Water Galvanized Pipes in a Housing Area

Description of the problem In a housing society...