What kind of corrosion does this look like? (Stainless steel bolt in chlorinated water)

[ajz]

Hey there,

We have a gate valve submerged in chlorinated water (5-10 ppm hypochlorite); about 20 years old. Operators found the valve hand wheel to not be actuating the physical gate. Investigation (which required a diving crew to access the submerged valve) showed that the bolts which mate the stem portion of the valve to the bonnet were obliterated. Im not sure of the precise technical terms so I've included a diagram below.

We are suspicious of the exact mode of failure because we would like to prevent future occurrences.

Based on visual inspection, is anyone familiar with what failure mode this might be? The bolt (per spec) is 304 stainless steel. It was encased in a big glob of bitumen. I would think 5-10 ppm hypochlorite would not be sufficient to cause this level of attack, especially on a stainless steel bolt, however a lot can happen in 20 years. The case material of the valve is epoxy coated bronze, and I don't know how much physical contact there might have been. I dont know if any other materials were in contact with the bolt.

Any advice is appreciated. Thank you!

 

[EdStainless]

You can be assured that there was contact with the bronze.
It is very odd to see general corrosion like that on stainless.
But your description of being encapsulated may explain it.
As the stainless started to corrode the corrosion products were trapped.
This solution then slowly becomes both oxygen depleted and more acidic.
Eventually it becomes strong enough to begin just dissolving the stainless.
It is likely that if the bolts had been left filly exposed they would have been fine.
If I was replacing these I would use duplex 2205 hardware. It is more corrosion resistant and stronger.

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P.E. Metallurgy, Plymouth Tube

 

[ajz]

Thank you Ed,

I'm a bit rusty on my knowledge of corrosion (pun intended). Do you have any comment on the difference between "active" and "passivated" stainless steel, and whether or not those effects could have had an effect? For instance, did we likely receive a passivated stainless steel bolt that could have resisted the corrosion potential, but by being coated in the bitumen, the fastener became de-passivated/activated, and thus made much more vulnurable to corrosion?

I noticed stainless steel had multiple values on corrosion charts based on it being passive or active, and that each of these potentials are on opposite sides of the corrosion potential for bronze.

 

[Dhurjati Sen]

@ajz

The SS 304 bolts have given you a life of 20 years. What about the other parts of the valve, like the seat and the trim?

It would be fine to put back identical SS 304 bolts.

 

[ajz]

What about the other parts of the valve, like the seat and the trim?

I will be receiving a report from the divers who performed the inspection within a week or two. They packed up so, if found necessary, we will replace the valve under different job. Since I haven't heard anything about the other bolts, I presume they're in relatively OK condition. That is reinforced by the idea that the bitumen tar might have been a major reason for the stainless steel bolt's demise. I don't think any of the other bolts were covered in the stuff.

Edit: I should add that the bolt was replaced in kind.

 

[EdStainless]

All stainless moves from passive to active when corrosion is taking place.
The covering of the bolt likely drove this by depriving the surface of the oxygen that it needed to repassivate and also by lowering the pH.
HCl is about the only environment that will cause general corrosion in stainless steels.
I am sure that someone thought that covering it was being helpful, but not in this case.


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P.E. Metallurgy, Plymouth Tube

 

[racookpe1978]

From the cutaway image (blue circle) the square-head bolt appears to be external to the process fluid, right? So, internal corrosion of valve parts would be a separate problem - possibly a problem that has not actually occurred.

However, what was the condition of the rest of valve bonnet and valve body external surfaces?

Were any other external surfaces also covered in the asphalt (bitumen) compound? Where the other 3 (?) bonnet bolts covered in bitumen? If so, were they in good or terrible condition, or were they also replaced when the photo'd bolt was replaced by the divers?