Dezincification of bronze impellers

[mapleleafs]

Has anyone experienced dezincification of pump impellers? We started our water treatment plant 5 years ago and now have to replace all our impellers on are vertical turbine pumps. The original material was a brass alloy 875 ( 15% Zinc ) and the manufacturer is recommending a Tin Alloy 903. We are also trying to find out why this occurred. Are water at the time of startup would have been ph 6.8 and have a chlorine residual of 1.0 mg/l free. The water is low in alkalinity ( 10 mg/l ) It appears that the damage was done in the first two years as the flows had dropped off by 12% during this time. We started using a zinc polyphosphate for corrosion control after the 2 years and pump production stayed the same as if the damage stopped occurring. Any comments would be appreciated.

 

[unclesyd]

Brass alloys with more than 15% Zinc are susceptible to dezincification and has been known to occur under the conditions listed below. This is not a complete list of conditions. Your alloy at 15% zinc can be quite susceptible and the recommendation to change alloys is good one. Usually a pump/valve manufacturer will know what alloys work in your area or can make a recommendation based on water chemistry and on-stream time of a pump or valve.

Oxygen and carbon dioxide in water.
Stagnant or slow moving waters.
Slightly acidic water, low in salt content and at ambient.
Low conductivity, minerals, + oxygen.
Waters with high chloride ion.
Neutral or alkaline waters, high in salt content and at or above room temperature.

 

[EdStainless]

If you are considering other alloys a NiBronze or NiALBronze would be a good move.

My guess is that the damage stopped because the stress on the now damaged impellers was low. The outer tips of the vanes go very fast.

Corrosion never sleeps, but it can be slowed down.