Corrosion failure of aluminum fittings in a chiller system

[driegel]

I have had an aluminum fitting fail in the plumbing of a chiller system used to cool the water for the cooling chamber of an annealing oven. The water is untreated city water that is recirculated and fresh water is added when the level drops or when the chiller cannot cool the water below a set temperature. Normal water temps are 100-130 degrees F.

What could be causing this corrosion? I have heard that dissolved oxygen could be the cause as well as galvanic corrosion as the system included aluminum, copper, brass and mild steel.

Can anyone offer any assistance before I spring for lab testing of the water and failed fitting?

 

[CoryPad]

What does the fitting screw into? How did it fail? I see the corrosion products inside the fitting, but did it fracture? Pit and leak? Other?

If the fitting screws directly into steel, brass or copper then galvanic corrosion is extremely likely to be a problem.

 

[driegel]

The threaded portion of the fitting fractured when a maintenance man tugged on the hose it was connected to.

The fitting that failed was screwed into brass. There were six of the aluminum fittings in the system, some screwed into brass and some into galvanized steel, and all showed nearly the same degree of corrosion. Also, the brass float rod for the makeup water was nearly corroded in two. The valve is threaded into a plastic fitting.

All six aluminum fittings have been replaced with stainless steel because that's the default reaction to corrosion for those of us who don't normally think about such things.

My main concern is actually for the copper piping and coils in the chiller and the mild steel water jackets of other heat exchangers associated with the oven. These fittings are cheap and easy to replace as a maintenance item compared to the equipment itself.

 

[driegel]

Also, the fitting on the right in the photo has been bead blasted to remove the scale so we are looking at the exposed aluminum.