Galvanic Corrosion Through a Spiral Wound Gasket
Galvanic Corrosion Through a Spiral Wound Gasket

The elbow pictured to the left was removed from a pump volute. The pump was a cantilevered pump in submerged service and it had been in service for six years. During that period of time it had been removed for various reasons. However, this time there appeared to be a great degree of metal missing in the gasket region. The gasket specification recommended by the OEM was a fiber gasket, but the service it was used in called for a spiral wound gasket shown in Figure 2. The gasket that came out of service was never recovered and therefore a critical piece of evidence was lost. However, the corrosion pattern was pretty obvious and a new gasket was positioned exactly as the corrosion pattern indicated. The position of the old gasket is shown in Figure 2 with the new gasket. When looking at the corrosion pattern in Figure 3 and then looking at the offset position of the gasket in Figure 2 one might be quick to think that turbulence eroded the region of greatest metal loss.

However, when a ruler was placed over the flange face (Figure 4, left) it revealed that the flange face nearest the inner diameter of the pipe elbow was still there. Only the metal directly underneath the spiral wound portion of the gasket was missing. Erosion was ruled out.

The next possibility was crevice corrosion. Crevice corrosion requires very tight clearances where stagnant conditions can exist. It is not uncommon for some gaskets to wick and allow favorable conditions for crevice corrosion to initiate. When looking at Figure 5 there were deposits that accumulated between the retaining ring and the chrome-steel flange face. The imprint of the retaining ring remained on the deposits. There were clear signs of green oxides where chromium reacted. When the deposits were removed, as shown in Figure 6, the flange face was still in good condition. Crevice corrosion is not preferential in where it attacks, yet there was a clear demarcation line between the attacked region (under the spiral windings) and the retaining ring that did not come into contact with the flange (as evidenced by the accumulation of deposits shown in both figures).

If the cause for the metal removal on the flange face wasn t erosion or crevice corrosion, then what was it? It was certain that there was evidence of a corrosion mechanism. There were definite signs of corrosion byproducts in Figure 3. The green regions were chromium oxides. A close up view of the lower right region in Figure 3 can be seen by clicking on it. The elbow was a 29% chromium-iron material. The gasket was a 316 stainless steel spiral wound gasket with a 316 stainless steel retaining ring. If it were galvanic corrosion then a clear circuit path would have to exist between the flange face and the gasket. http://www.sealmaterial.com.cn/


spiral wound gasket