Propeller Nut - What happened to this Nut

I would agree with this. I would also add that these grades of stainless steel should not be used in submerged applications unless you are maintaining cathodic protection. In your case, it's a bit hard to see where your sacrifical anode is and how it's coupled to the system as it's not listed in your drawing and material list provided above. You may have difficulties with the geometry of that specific nut due to the contact surface that you have.

dj

 

Looking a bit more at your design, I'd recommend you change out your nuts from 316 to 17-4 PH in the H1000 condition, or higher. I would not use 17-4 PH in the more commonly found H900 condition. (I always hate throwing out problems without a solution).

dj

 

AISI 316 is not magnetic. Cast 316 or CF8M is slightly magnetic, but it won't be likely to be a cast nut.
Also, passivating stainless steel with nitric acid is simple and pretty cheap.

 

AISI 316 if cold worked is quite magnetic. Sorry for your misinfomation.

 

I'd suggest you purchase some 1/2 hard 316 alloy per ASTM 666 and stick a magnet on it.

 

Firstly, I stated GENERALLY
Secondly, I simply took a magnet and checked some bolts that I had in both 304 and 316, the 316 was not magnetic but the 304 was. While what you say is correct, that they can be or not be magnetic, in GENERAL TERMS, applied to
the bolts purchased regularly at marine and ordinary bolt stores, the 304 is GENERALLY magnetic.

 

The issue is not passivating the nut, it's maintaining the passive layer. That maintenance is the problem. That is where you need to make sure you have good cathodic protection on submerged parts of these alloys, or in some way maintain that passive layer once formed.

You can also passivate with citric acid. There is some indications that it may in fact produce better passivation in some cases. Take a look at the report by NASA:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110001362.pdf

In all cases, you must maintain the passive layer while in use.

dj

 

Yeah, sorry, I hear all the time people saying that "good" stainless steel is non-magnetic and "bad" stainless steel is magnetic. It's just not a conclusive test at all.

Your 304 bolts - have you checked to make sure that they are not 303? What happens a lot is that fasteners will be called "18-8" stainless steel, and if that's the case, you need to be careful. They may well be 303 stainless steel, a free machining variant that works really well in screw machines. Great in fresh water, well, good enough in fresh water, but 303 just doesn't stand up well in ocean environments.

304 actually performs better than most give it credit for in marine environments. But what I've seen happen is that it was thought that the bolt or screw or... was 304 but in fact it was 303. 303 will not perform in the marine environment as one would wish....

The other thing that often happens, is your 304 bolts are more likely to be cold worked than your 316 bolts. 316 is actually kinda hard on equipment to produce cold working, more so than 304. Cold working is done, of course, to increase the tensile strength of these otherwise non-heat treatable alloys. Very notable strength increases can be obtained this way.

So I didn't mean to offend. It's just a topic that drive me nuts as I hear it time and time again and it's just not correct.... I was in a conversation recently with a guy who was selling his old 316 standing rigging to a junk yard and they wouldn't give him 316 prices because the rigging was magnetic. They told him it couldn't be 316 due to that fact. Just bull....

dj

p.s. - othen threaded fasteners are produced through thread rolling machines - thread rolling induces cold working.

 

The ends were stamped 304 and 316

True but rarely available at a marine or bolt supply store.
U bolts in the automotive sector are almost always rolled. The easiest way to tell is to run the shank through your thumb and index finger and if it appears that the diameter at the threads is larger than the shank, then the threads are
99% sure to be rolled.

 

So, that schematic is of the propeller hub/shaft connection. I was just wondering if, with that protective cap over the bolt, there could have been a build up of heat. There doesn't seem to be any moving parts for friction.

Perhaps the propeller was installed in the water and with seawater trapped inside, a crevices or pit corrosion condition was setup with restricted oxygen flow beneath the cap.

-Will (Dragonfly)

 

I was just reviewing this thread and am not clear which surface on the nut you are seeing this degradation.


Is it on the outside towards the cap? Or is it in between the two nuts? The first would imply stray current corrosion, the second - failure of the passive oxide layer.

dj

 

Can't be stray current, that would have preferentially attacked the corners of the nut.

 

That's a good point, but if it's the outside surface of that nut, not the interface between the two, what mechanism do you suggest?

 

Is there any way to prevent this form corrosion so that we can change the design of the propeller nut & cone area.

 

It is a M30 Nut . The corrosion has happened between the thread area(Which come in contact with the shaft thread) and outside of the nut.