Underwater stainless and bronze bushing corrosion

[AnthonyJ777]

Forum,

I need help trying to determine why the SAE 660 bronze bushings in our underwater nuclear fuel handling tool are experiencing aggressive corrosion rates. The two pictures below are just two examples of some of the types of corrosion we're seeing. From my understanding the blue color is Patina, a corrosion specific for bronze/copper metals. The red color looks like a traditional corrosion on copper based metals.

My theory is that perhaps the bushing tolerance of the tool is too tight causing the chromium oxide layer of the stainless to wear off which then would be the catalyst to rust. After use, the iron particulates would get transferred onto the bronze bushing and begin to spread across the face of the bushing too? Does anyone have any experience with such a chemical reaction with underwater bushings in plants?

 

[bimr]

What is the water quality?

Water Quality and Corrosion:

 

[EdStainless]

What problem is this causing other than cosmetic?
What is the temperature and time frame?
What are the other alloys (than 660)?

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P.E. Metallurgy, consulting work welcomed

 

[AnthonyJ777]

Water quality depends on the site, but acts as a moderator for nuclear fuel reactivity in the primary system. It can consist of Boron, Iron, Silica, Magnesium, Calcium, Aluminum, Fluoride, Chloride, and Sulfate. Keep in mind the sulfate, chloride, flouride, and silica are in the range of 1-1200 ppb so rather small. The Boron on the other hand can be as high as 2600 ppm. The pool temperature ranges from 95f to 115f.

The problem is after several months of the tool installed in the water the bearings build up enough of a corrosive layer that the tool seizes up. The bushings are in contact with 303 SST parts.

 

[EdStainless]

To start with look at the 303 parts with a magnifying glass. You might have to clean them a lightly first.
Are they covered with little pits?
My hunch is that in this clean water the 303 is pitting, and the corrosion products are fouling the works.
Someone picked 303 because they wanted to save a few bucks on machining. And I'll wager that the SS parts weren't pickled after they were machined.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[AnthonyJ777]

You are correct with that assumption on selecting 303! Do you have a recommendation on magnification scale to make this pitting observation?

 

[EdStainless]

Something in the 5x-10x range should work. I use hand lenses.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[AnthonyJ777]

We investigated the 303 parts including wiping them clear of the corrosion and could not see any pitting or wear marks. The corrosion must have started on the bronze bushings and transferred to the SST parts during operation.