Corrosion of Stainless in Marine Environment

[adamgnt]

Hello all,

I have frequented this site often this week searching for my answers before starting a thread. But everyone does such an awesome job wih quality replies I thought I'd just dive in.

My issue is some pitting corrosion I am experiencing with a 316 stainless jaw that holds onto a composite clevis pin in a loaded marine environment. In the two examples I have, the system had been 6 months underwater in coastal New England. The corrosion is only evident at the area where the jaw is experiencing it load and the rest of the framework has resisted. Out of the possible choices I cannot determine if it is due to crevice corrosion, (the water may be stagnant around the contact area?) S.C.C. (I was under the impression this only occured at higher loads and warmer temperatures?), or the fact that the motion of the jaw slightly rotating due to currents is maybe rubbing off the passivation layer? Also, there are already sacrificial zinc anodes present elsewhere on the device due to the presence of aluminum and stainless.

Some suggestions have been made to coat the whole system with some coating (I've heard thermoplastics adhere better to stainless than thermosets) but I am not convinced it will help. One opinion states that if the exposed surface area of the cathode (the untouched steel) is reduced, then there will be less electron availabilty to drive the corrosion of the anode (the pitted area next to the pin). I can see the logic but I am curious to hear what this community has to offer for insight. Wide open to anything that anyone would like to add!

Adam

 

[rconner]

Fyi

http://www.eng-tips.com/viewthread.cfm?qid=231863&page=2

(It doesn’t sound/look like your application is necessarily “crevice free” as per EdStainless’s post on that thread)

 

[EdStainless]

Combination of fretting and crevice corrosion. You don't want me to go into the technical explanation of the combination of wear and corrosion.
You don't have SCC. If the sun shines directly on this then you might. If that happens you will only have pieces left.
Solutions
1. Us an alloy that will resist crevice corrosion for the hook. Either a 6% Mo superaustenitic (AL-6XN) or a supperduplex (Zeron 100).
I would make the entire unit from these alloys, you can mix and match.
The rust staining is indicative of crevice corrosion in many locations of the assembly. You need to eliminate crevices, weld instead of bolting and make sure that it can drain.

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Plymouth Tube

 

[adamgnt]

Thank you for the excellent advice Ed. From your other posts, it is clear you are an asset to this forum.

I will have to look into the costs associated with implenting these alloys (machining difficulty? different material fasteners?). You suggest fabrication of the entire unit, Is this to prevent future galavanic complications by adding too many new metals to the same environment? What implications would occur if only we replaced only the hook with a super-resistant alloy?

Am I barking up the wrong tree with the "reduction of exposed surface area" approach? I agree that fretting is a major player in this problem, and any coating will be sort lived in the contact area under the hook. However, if the majority of the other stainless was sealed from the water, would this affect the strength of the galvanic affect? Or is this point mute since crevice corrosion is mainly due to the lack of oxygen which is replenishing the passivation layer? On a side note, chemical passivation has also been suggested as a solution. Thoughts?

Again thank you so much for sharing your knowledge and tangible experience with the community.

 

[EdStainless]

I would at least go with the hook and the pivot in a higher alloy. All that you can do is try to move teh corrosion to a less critical area.

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Plymouth Tube

 

[SJones]

You are barking up an adjacent tree with the theory of the coating. It would be appropriate if there was a bimetallic factor at the location of corrosion but, from the picture, there isn't. For bimetallic corrosion, the cathode is coated to reduce the anode:cathode area ratio and control corrosion current density on the anode, i.e. the aluminium. However, coating will really help the cathodic protection, assuming that it has been designed correctly, by creating much better current distribution. I would expect correctly designed and applied CP to have prevented the observed corrosion.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[HEC]

referring to Ed's post, the fretting corrosion can be reduced or eliminated by reducing the local surface stresses. A minor redesign such that the bearing area between the pin and hook is increased will improve the situation.

Mark Hutton

 

[strider6]

My opinion is that the problem is the wrong material selection. 316 is not a material to be used in continuous exposure to Seawater. I mean you can use it but you've to expect serious problems. CP can help but i'll not use any coating, if the CP is well designed then it will protect the material under CP.

http://www.force.dk/NR/rdonlyres/46D7DBAD-B123-46C1-863C-BCB029C6835E/1638/03083.pdf

S

Corrosion Prevention & Corrosion Control

 

[EdStainless]

adamgnt and HEC, Reducing contact stress is why I suggest making some parts out of higher alloy, so that you have the stress on the 316 in a location where it is easier to manage.
I have seen some wear/corrosion testing data. For medium grade alloys (316, 317, 2205) when they are under wear conditions the corrosion potential is just like carbon steel.

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Plymouth Tube

 

[HEC]

Ed,

So the higher alloy will provide beter resistance to corrosion as well as higher strength to prevent the "spalling".

Mark Hutton

 

[adamgnt]

Thank you everyone for your input. I knew this forum would be could a good place for intelligent discussion and hopefully someone else will gain from what we accomplish in this thread.

Ed, I agree that we want to move the local stress point away from the stainless contact surface, so we came up with the idea of placing a high strength polymer insert along the area where the corrosion is most evident. The more of the original fixturing and material we keep in place (i.e. keep the stainless hook) the faster the transition to the new product will be, and I am reticent about introducing a additional metal into to the system. I'm thinking this plastic on plastic interface should eliminate any oxidized rust particles from further accelerating the frettting or spalling. I'm thinking a small (maybe 0.1" thick with a key for placement) insert of engineering plastic with high hardness (to prevent cradling the clevis pin) such as Ultem or Torlon, could be an low cost solution to the problem. You can see a rough sketch attached where I took 2 secs to throw in a general insert.

Keep the responses coming guys, this project is just slightly out of my experience area. If anyone has fluid dynamics questions down the road I'll be happy to return the favor haha

 

[EdStainless]

How will you bond the insert to the hook in order to eliminate the possibility of moisture ever getting under it?

I would go with a thermoset insert to minimize the risk of creep.
I would mold it to the hook.
I would still make the hook from a 6% Mo superaustenitic or a superduplex alloy.

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Plymouth Tube

 

[adamgnt]

Am I incorrect in feeling a marine grade adhesive (epoxy? polymer based?) would keep this in place since it's not experincing a true shear load? I am indeed looking at thermosets, preferably one with a higher hardness than the glass filled nylon covering on the pin. Any cradling could compromise the hooks ability to release the pin once it is allowed to swing freely. I am currently looking for appropriate equations to better document my "gut feeling" on the thickness necessary (I don't feel we need much). If anyone knows off the top of their head where to find what I'm looking for, I would love a point in the right direction.

Thanks
--Adam