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Material Specification


(Harold Scadden) #1

What metal type are you using for the embedded nuts (4 each) on the T100 thrusters?

I am trying to see if I want to pop them and put new ones etc. to reduce galvanic corrosion to where I am mounting the thruster.

 


(Rusty) #2

Harold,

They are 304 stainless and we are having some issues with light corrosion in saltwater. Unfortunately, they are not available in 316SS so we will likely switch to aluminum or anodized aluminum. What would your thoughts be on that?

Best,

Rusty


(Harold Scadden) #3

What is the size etc. other than it is a M3 thread on those items? 304 is not the most ideal type of steel to have out in the water.

The pressure proof canister I told you I was working on … I am looking at a blend of two or three different types of metal for my area that the screws will hold the unit together. It is a REAL pain in the rear matching up materials. Now if someone was using screws made from 304 you shouldn’t have problems, but then again it boils down to the other types of metal around the item the thruster is mounted on. I almost want to say switch to helicoils and use Phosphor Bronze for the coil and then you can use 17-4PH screws etc. and not be as paranoid when it is around other metal combinations, more so Aluminum.

For what I am doing now with the thruster, I am not worried about long time use … I am just playing with a prototype, but for production is going to be a different story.

 


(Rusty) #4

Harold,

Here are the details of that part. We’re looking into alternative but the company does not make a 316 version.

Our screws are 316SS which is probably contributing to the corrosion.

Our best option right now is to switch to anodized aluminum. The other aluminum parts seem to be doing pretty well.

-Rusty


(Harold Scadden) #5

Ok … seen those but haven’t used them. Almost be better off going with a standard helicoil then your variety will go through the roof on what you can get.

You still have to drill / tap for the coil but it might be worth it.

 


(Paul) #6

Please, please, please do -not- switch to aluminum! Aluminum, anodized or otherwise, is much more susceptible to galvanic corrosion than either 304 or 316 stainless steel. See here: http://en.wikipedia.org/wiki/Galvanic_series The copper wires in the motor are much more susceptible to galvanic action than any of the stainless nuts in the housing.

Unless the ROV is in the water for extended period of time, a fresh water rinse (and allowing it to dry) at the end of every saltwater operation should eliminate almost any possibility of galvanic corrosion. A liberal coat of “Lanocote” wouldn’t hurt either.

In a worst case situation, add a sacrificial zinc anode to the ROV and, if the frame is metal, bond the zinc to the metal frame. But if you’re seeing electrolytic activity in the metal of your ROV then perhaps it’s time to check your wiring for possible leaks. Unless your ROV is immersed in saltwater for weeks on end, you really shouldn’t be seeing any electrolytic activity at all. Surface rust (which is common with stainless steel) perhaps, but not electrolysis.

Another thing to consider is whether you have unlike metals in contact. Allowing stainless steel to directly contact aluminum (or other metals) is just asking for trouble. The two metals need to be electrically isolated (with nylon washers and/or Lanocote).

All of these suggestions will work millions of times better than switching to aluminum, so please don’t switch!


(Harold Scadden) #7

Well here is the funny thing about Aluminum. 6061-T6 is stronger than 316SS and a ton lighter for making pressure proof canisters. The big thing is trying to keep your galvanic voltage differences to about 0.2 volts or less and you can use transition layers to knock down the corrosion.

You don’t need to have a bare wire in water to get ionic reaction. Just the e-field generated by current in the motors, cutting the flux lines of the earth and the list continues can lead to corrosion. I have had cable assemblies where the neoprene jacket pretty much dissolved in seawater because of how the ground plane was around the cable, the induced voltages on unused pairs and a bunch more fun to boot.

I have seen Aluminum used on a lot of underwater equipment because of weight displacement issues. If it is coated properly it can still last just fine. Seen plenty of pitting in 316SS, monel and k-monel too.

 


(Paul) #8

I agree with most of what you have to say Harold, but I have my doubts that the corrosion on the 304 SS nut was caused by electrolysis. I suspect it was more likely surface corrosion. Keep in mind that “stainless” steel is only corrosion resistant when exposed to air ( http://en.wikipedia.org/wiki/Stainless_steel ). In an anaerobic environment it will corrode (as witnessed by a number of sad sailors whose boat’s keel fell off due to corroded keel bolts). A touch of Lanocote might solve the problem and cost less than a penny (not including labor :wink: ).

Anodized aluminum is used in a lot of subsea applications, but I wouldn’t want it used in this particular case, especially if you suspect electrolytic activity. See the attached photo of an aluminum mast I recently surveyed. The anodized aluminum is severely pitted anywhere it was in contact with stainless steel. This could have been prevented by using an isolating barrier (electricians tape + lanocote is common practice).

@Rusty - if you do decide to switch to aluminum, could you please save 6 of the “old” thrusters for me please?



(Harold Scadden) #9

304 is a bad grade of steel to have in saltwater period and I take wikipedia with a grain of salt. I have been building systems for a long time with 316SS connectors on them for continuous submerged exposure in seawater. We do not depend on passivation to happen in air … we send it out for that process. One of the other things that is happening when you have it chemically passivated is surface carbon and other impurities are stripped away. You can see a different shine on the metal before and after the process.

The nut feature in the thruster happens to be quick and dirty to use. Less pain in the rear than other methods. I still would entertain using phosphor bronze in aluminum applications.

 


(Paul) #10

What about bronze? We use 316 stainless with bronze all the time (e.g. - boat shafts and props). Again, as long as the parts aren’t in saltwater for extended periods and you give it a freshwater rinse, it should work fine. Brass might also work, but some alloys are less corrosion resistant than others.

 

 


(Harold Scadden) #11

I thought a lot of the pleasure crafts still used Zinc’s for cathodic protection. Nevertheless, I currently have been using Naval Brass (C46400) for pressure proof penetrators to test connectorized cable assemblies. The material is holding up real well and I am mating with 316SS parts … that have been chemically passivated.

Now for the record, the medium is freshwater but that still contains a certain amount of crap in it. If you look up C46400 alloy you will find that they use it in pumps, propeller shafts and marine hardware a lot.

I chose it for the machinability and strength.

 


(Paul) #12

Yes, ships, boats, oil rigs, undersea pipelines, etc. all have zinc anodes of one shape or another for cathodic protection in salt water. That was one of my suggestions earlier in the thread. In fact, Outland Technologies (and other ROV manufacturers) who build their ROV frames out of aluminum have used zinc anodes to protect their robots from electrolysis.

Another possibility is that the stainless steel used by the Chinese manufacturer may be of an inferior alloy. “Around the docks” Chinese metal (be it stainless, brass, bronze or galvanized) has a pretty awful reputation. I have no personal experience with this particular manufacturer, but based on my other experiences with Chinese manufactured alloys (stainless steel chainplates in Chinese built boats for example) it certainly is a possibility worth investigating.


(Rusty) #13

Hey guys,

Just wanted to give an update here. We found a source for the threaded inserts in 316 Stainless Steel instead of 304 Stainless Steel so future batches will be more resistant to corrosion of the inserts.

Rusty


(Paul) #14

Outstanding! :slight_smile:


(Harold Scadden) #15

Awesome find. That should help longevity on the unit.